5.2 WATER RESOURCES
5.2.1 Groundwater
The following is a summary of the potential impacts to groundwater as described in Volume Four, Appendix F. Groundwater would be impacted by all of the EIS alternatives. Groundwater impacts were analyzed by comparing the impacts for each alternative with drinking water standards for key contaminants that have high carcinogenicity, toxicity, and mobility in the groundwater. The environmental impacts of these and other potential groundwater contaminants also were used to analyze human and biological health risk (Section 5.11).
The No Action and Long-Term Management alternatives would result in exceedances of drinking water standards for carbon-14, iodine-129, technetium-99, uranium-238, and nitrate in groundwater. Of all the alternatives, these alternatives would exceed the drinking water standards by the greatest magnitude. The exceedance of the standards in groundwater would occur within a period of 500 years. The In Situ Fill and Cap alternative would exceed the limits of current drinking water standards for the same contaminants with the exception of carbon-14, but most of the exceedances would be delayed for 2,500 years.
The ex situ alternatives and the In Situ Vitrification alternative would include measures to reduce the rate of release of radionuclides, which would result in lower peak concentrations at the water table. These measures would result in groundwater impacts that would occur mostly after 500 years. The ex situ alternatives and the In Situ Vitrification alternative would comply with current drinking water standards for uranium-238. Without considering contamination from the LAW vaults, alternatives that removed waste from the tanks would not meet the limits of current drinking water standards in groundwater. The calculated exceedances mostly would be attributable to the assumption that 1 percent of all tank waste would remain in the tanks after retrieval. There would be no groundwater releases during Phase 1 of the Phased Implementation alternative and hence no groundwater impacts; however, there would be groundwater releases and impacts from the Phased Implementation Total alternative.
The amount and type of waste that would remain in the tanks after retrieval is uncertain. The Hanford Federal Facility Agreement and Consent Order (Tri-Party Agreement) (Ecology et al. 1994) set a goal of no more than 1 percent residuals, and the ex situ alternatives have been developed to attempt to achieve that goal. However, achieving this level of tank waste retrieval could require extensive effort and cost, and it may not be practicable to achieve 99 percent retrieval. Conversely, the contaminants that were not recovered would be likely to be insoluble in water, because substantial quantities of water would be used in an attempt to dissolve or suspend the waste during retrieval. Because neither of these issues can be resolved, a conservative assumption was made to bound the impacts of the residual waste. For purposes of this analysis, it was assumed that 99 percent retrieval would be achieved, but that the residual waste in the tanks would contain 1 percent of all the contaminants including the water soluble contaminants. There are a total of 177 million (1.77E+08) curies (Ci) in the tanks (Volume Two, Appendix A). Retrieval of 99 percent of the tank contents as part of the ex situ alternatives would leave 1.77E+06 Ci remaining for potential dissolution by groundwater. Existing groundwater contamination would be in addition to these 1.77E+06 Ci, but is not in the scope of the EIS.
The groundwater assessments provided in this section required several assumptions to address uncertainties. The major assumptions and uncertainties were related to either the natural system (i.e., an understanding and ability to assign vadose zone and aquifer parameter values) or uncertainties inherent to the assessment approach.
The major assumptions and uncertainties were as follows:
- The rates of infiltration into natural ground and through a cap;
- Distribution coefficient (Kd) of contaminants;
- Uncertainty in future groundwater flow direction due to decay of groundwater mounds onsite;
- Uncertainty in future groundwater flow direction due to future land use (e.g., irrigation and groundwater withdrawal);
- Uncertainty in future groundwater flow direction and vadose zone thickness due to climate change;
- Uncertainty in future groundwater flow direction due to changes in land use;
- Uncertainty in vadose zone transport due to use of one-dimensional flow and transport simulation; and
- Uncertainty due to calculation of releases during retrieval.
DOE has a system of monitoring wells called drywells installed in the vicinity of each waste tank. The depth of these drywells varies but they do not extend to the water table of the unconfined aquifer. These drywells were installed as a way of detecting gamma emissions and serve as an indirect means of detecting or confirming waste tank leaks and mobilization of existing contamination in the vadose zone by other water sources such as potable water line leaks. Until recently, the gamma emissions that were detected were indicative of undifferentiated radioisotopes. Such emissions have been detected in many of the drywells at depths ranging from ground surface to up to 38 m (125 ft) belowground surface. Recent improvements in the borehole logging detection equipment have resulted in the identification of specific gamma-emitting radioisotopes. Thus, previously characterized gross gamma contamination is now specifically linked to several radioisotopes. The most prevalent radioisotope detected was cesium-137 while other gamma-emitting radionuclides such as carbon-60, europium-152, and europium-154 were generally found near the surface and are believed to be the result of spills (Brodeur 1996).
The transport of cesium-137 in the vadose zone sediments at the Hanford Site is believed to be greatly retarded due to adsorption. Cesium would not be expected to be found at depths of up to 38 m (125 ft) if it were being transported via interstitial flow through the sediment pore spaces and under ambient conditions that include neutral pH and infiltration rates ranging from 2 mm/yr (5.48E-06 m/day) to 10 cm/yr (2.74E-04 m/day). The detection of cesium-137 at this depth raises several questions concerning the active transport mechanisms. These questions and others are being addressed by DOE in a RCRA Groundwater Assessment of the S and SX Tank Farms (Caggiano 1996). The improved borehole logging detection equipment provides information on the specific contaminant in the vicinity of the drywells, but there is still uncertainty on the lateral distribution of these contaminants within the vadose zone.
The most recent vadose zone characterization information is for the SX Tank Farm. Ten of the 15 tanks in the SX Tank Farm are assumed or verified as leaking as discussed in Volume Five, Appendix K. Ninety-five drywells ranging in depth from 23 to 38 m (75 to 125 ft) from ground surface were logged with the improved logging system in the SX Tank Farm. The most abundant and highest-concentration radionuclide detected was cesium-137, which was detected in virtually every borehole (Brodeur 1996). Cesium-137 was detected in several drywells at the following depths: 23 m (75 ft) in drywells 41-09-03 and 41-08-07; 32 m (105 ft) in 41-09-04; 27 m (90 ft) in 41-11-10, and 38 m (125 ft) in 41-12-02.
Other gamma-emitting radionuclides detected include cobalt-60, europium-152, and europium-154, which were generally found near the surface and are believed to be the result of spills (Brodeur 1996). Cobalt-60 was found in drywell 41-14-06 only. It was detected at a depth of 17 to 23 m (55 to 76 ft) belowground surface. The data were insufficient to conclude whether relatively immobile contaminants such as cesium-137 would be found dispersed laterally within the vadose zone (i.e., at observed concentrations laterally several meters from the drywells) at the depths of over 30 m (100 ft) based on ambient conditions and vadose zone contaminant transport via advective flow in interstitial pore spaces. Vadose zone contaminant transport mechanisms, such as discussed in Volume Five, Section K.4.1.3, in addition to interstitial transport through the pore spaces could be active. The viability of any other potential transport mechanism has not yet been demonstrated but is one of the objectives of the ongoing investigations.
A discussion of these major assumptions and uncertainties is provided in Volume Five, Sections K.4.1 and K.4.2 and results of a limited parameter sensitivity analysis are summarized in Section 5.2.1.3 and provided in Volume Five, Section K.4.2.
Vadose zone, groundwater flow, and contaminant transport were simulated for each alternative with a combined flow and transport model called VAM2D (Huyakorn et al. 1991). The groundwater impact of interest area is shown in Figure 5.2.1. The analysis approach is summarized briefly in the following text and additional details are provided in Volume Four, Appendix F.
The approach for assessing the impact to the groundwater system is illustrated in Figure 5.2.2. In the source characterization step shown in the top of Figure 5.2.2, the 177 tanks were aggregated to eight source areas, and the contaminants were placed in groups based on their mobility in the vadose zone and unconfined aquifer. The next step, vadose zone modeling shown in the center of the figure, required the development of a conceptual model for each of the source areas. Then, as described in Volume Four, Section F.2.3.1.3, the vadose zone flow field was established based on steady-state flow simulations for an ambient infiltration of 5.0 centimeters (cm)/year (2.0 inches [in.]/year).
Figure 5.2.1 Area of Interest for Groundwater Impact Assessment
Figure 5.2.2 Groundwater Impacts Assessment Approach
Contaminant transport through the vadose zone then was simulated for each source from which results were processed for use by the groundwater model.
The groundwater modeling step, shown in the bottom of Figure 5.2.2, required the development of a conceptual model of the unconfined aquifer. Then a steady-state flow field, which is one of the principal bases for the groundwater impacts assessment, was developed using December 1979 sitewide water level measurements because it was determined (Wurstner-Devary 1993) that this data set was most representative of steady-state conditions. Using this data set also meant that the mounding from U Pond and B Pond would be evident. The mounding was recognized as a present-day condition that could dissipate over the next several decades with changes in the Site waste management practices. With the mounds in place, the vadose zone would be thinner in the 200 West and 200 East Areas and contaminant travel times would be faster to the groundwater. The travel time in the unconfined aquifer to the Columbia River would not be materially affected by the groundwater mounds compared to the vadose zone travel time. The approach based on the December 1979 water level data provides reasonable results for each alternative, especially in light of the uncertainties of land use and waste disposal practices and how these practices would affect the present groundwater mounds. Future land use such as irrigation to the west of the Site and on the Site, uncertainty in the depth of contamination in the unconfined aquifer, and climate change.
The groundwater model then was used to predict contaminant transport given the results from the vadose zone modeling as inputs. The groundwater results then were processed as appropriate for radioactive decay, initial concentration, and aquifer thickness. The final processed data then were plotted in various ways to show contaminant concentration versus time at selected points and contaminant concentration distribution on the Site for selected times in the future (e.g., 300, 500, 2,500, 5,000, and 10,000 years from present).
5.2.1.1 Source Characterization
Source characterization involved: determining the level of analysis for each alternative (screening of alternatives), aggregating the many potential sources into common source areas, grouping contaminants into categories based on their mobility, and developing the source term (i.e., mass flux and fluid flux release as a function of time) for each source area.
Screening of Alternatives and Waste Facilities
Screening was performed to exclude alternatives or waste treatment or storage facilities that had little or no potential for impacting groundwater from rigorous numerical modeling. The following sections provide the rationale for screening each alternative and for inclusion or exclusion of each from detailed groundwater modeling. Vadose zone and groundwater flow and transport simulations were used to analyze the groundwater impacts of those alternatives identified through screening as having the potential to impact groundwater.
No Action Alternative (Tank Waste)
This alternative potentially would impact the groundwater because no remediation would be performed, and all waste would remain in the tanks. During the 100-year institutional control period, tank waste management operations would continue. Waste releases to the vadose zone for both the DSTs and SSTs would occur primarily after the end of the institutional control period.
Long-Term Management Alternative
This alternative potentially would impact groundwater because no remediation would be performed and all waste would remain in the tanks. During the 100-year institutional control period, tank waste management operations would continue and the DSTs would be replaced twice during the 100-year institutional control period. Waste releases to the vadose zone would occur primarily after the end of the institutional control period for the SSTs and 100 years after the end of the institutional control period for the DSTs.
In Situ Fill and Cap Alternative
Under this alternative, the tanks would be filled with gravel, and a Hanford Barrier would be placed over the tanks. Potential releases to the groundwater system that would occur with the In Situ Fill and Cap alternative are associated with the contaminants in the waste tanks. The form of the waste and inventory are identical to the No Action alternative, and the total mass of waste entering the vadose zone and ultimately reaching the groundwater would be the same as for the No Action alternative. However, the release would occur at a slower rate because the Hanford Barrier would reduce the rate of infiltration into the tanks and the rate of migration of the waste downward into the vadose zone. While the gravel fill would structurally stabilize the tanks by supporting the tank domes, it otherwise would not help to reduce infiltration or retard contaminant transport.
In Situ Vitrification Alternative
Under this alternative, all tank waste would be vitrified in situ. A Hanford Barrier then would be placed over the tanks. Potential releases to the groundwater system from dissolution of the vitrified mass would be associated with the contaminants in the waste tanks, but the form of the waste and inventory would differ from the No Action alternative. Materials for making glass would be added to the waste, and the organic and other volatile materials present in the No Action alternative inventory would be destroyed or vaporized.
Ex Situ Intermediate Separations Alternative
Under this alternative, waste would be retrieved from the tanks, high-level waste (HLW) would be separated from the LAW, and both HLW and LAW would be vitrified. The HLW then would be shipped to a potential geologic repository and the LAW would be disposed of onsite in near-surface vaults. A Hanford Barrier would be placed over the tanks and LAW vaults. Potential releases to the groundwater would be associated with releases 1) during retrieval from the waste tanks; 2) from residuals remaining in the tanks; and 3) from the onsite LAW vaults.
Ex Situ No Separations Alternative
Under this alternative, waste would be retrieved from the tanks, vitrified or calcined, and shipped to the potential geologic repository for disposal. A Hanford Barrier would be placed over the tanks. Potential releases to the groundwater system would be associated with releases 1) during retrieval from the waste tanks; and 2) from residuals remaining in the tanks. The vitrified or calcined waste would not have a potential groundwater impact because all waste would be shipped offsite for disposal. The groundwater impacts for this alternative would be the same as those estimated for the retrieval and residual releases for the Ex Situ Intermediate Separations alternative.
Ex Situ Extensive Separations Alternative
This alternative would be similar to the Ex Situ Intermediate Separations alternative, with the difference being that a more extensive separations process would be implemented to remove a greater percentage of the HLW from the LAW waste. Under this alternative, waste would be retrieved from the tanks, HLW would be separated from the LAW, and both HLW and LAW would be vitrified. The extensive separations process would result in a smaller amount of contaminant source in the LAW vaults. A Hanford Barrier would be placed over the tanks and the LAW vaults. Potential releases to the groundwater system would be associated with releases 1) during retrieval from the waste tanks; 2) from residuals in the tanks; and 3) from the LAW vaults. Groundwater impacts associated with retrieval and residual releases would be similar to the Ex Situ Intermediate Separations alternative. However, the groundwater impacts of releases from the LAW vaults would be lower than those from the Ex Situ Intermediate Separations alternative LAW vaults because the source term is smaller.
Ex Situ/In Situ Combination 1 Alternative
Under this alternative, 107 tanks would be remediated in the manner described for the In Situ Fill and Cap alternative, and 70 tanks (60 SSTs and 10 DSTs) would be remediated in the manner described for the Ex Situ Intermediate Separations alternative. Releases to groundwater associated with the waste remediated under the In Situ Fill and Cap part of the alternative would occur as described previously. These tanks would contain disproportionately large amounts of low-mobility low-solubility contaminants. Tanks selected for waste retrieval and ex situ vitrification would contain approximately 90 percent of the high-mobility, high-solubility, high human health risk contaminants (i.e., technetium-99, carbon-14, iodine-129, and uranium-238).
Ex Situ/In Situ Combination 2 Alternative
Under this alternative, 25 tanks would be selected for retrieval and the remaining 152 tanks would be remediated in situ. The retrieved waste would be separated into LAW and HLW. The LAW would be placed into shallow subsurface LAW burial vaults in the 200 East Area and the HLW would be shipped offsite for disposal at the potential geologic repository. A Hanford Barrier would be placed over the tanks and vaults. This alternative was designed for the ex situ treatment of the largest contributors to long-term risk (i.e., technetium-99, carbon-14, iodine-129, and uranium-238) while limiting the total amount of waste to be retrieved and processed. Approximately 30 percent of the total waste volume in the tanks would be retrieved. The tank waste retrieved would contain approximately 85 percent of the technetium-99, 80 percent of the carbon-14, 50 percent of the uranium-238, and 80 percent of the iodine-129.
Phased Implementation Alternative
Phase 1
Under the first phase of this alternative, waste from the DSTs would be retrieved, vitrified, and stored temporarily onsite. There would be no groundwater impacts under this phase because 1) releases of waste would not occur during retrieval from DSTs; and 2) the storage of the vitrified waste would be temporary and under controlled conditions so there would be no liquid releases.
Phase 2
In the second phase of this alternative, the remainder of the tank waste would be retrieved and treated in the same way as in the Ex Situ Intermediate Separations alternative. Potential releases to the groundwater for Phase 2 of the Phased Implementation alternative would be similar to those calculated for the retrieval from SSTs for the Ex Situ Intermediate Separations alternative.
Effluent Treatment Facility
This facility would be common to all of the alternatives. It potentially would impact groundwater because treated effluent from the Effluent Treatment Facility would be discharged to a State-approved land disposal site located immediately north of the 200 West Area. The Effluent Treatment Facility wastewater originates as process evaporator condensate. All tank alternatives would contribute wastewater to the State-approved land disposal site either through periodic operations of the 242-A Evaporator, DST retanking campaigns, or as liquid effluent collected from the process facility. The State-approved land disposal site consists of a piping manifold used to infiltrate treated effluent into vadose zone soil and deeper groundwater beneath the disposal site. The primary contaminant present in the treated effluent would be tritium, with other organic, inorganic, and radiologic contaminants having been removed during the treatment process (Volume Two, Appendix B). Waste releases to the vadose zone beneath the State-approved land disposal site would occur only during the operations phase of each alternative.
The effects of treated effluent disposal on groundwater were simulated as entering the uppermost aquifer beneath the State-approved land disposal site at a projected rate of 570 liters per minute (L/min) (150 gallons per minute [gal/min]) over an area of 8,350 m2 (90,000 ft2). Tritium concentrations in the treated effluent entering the groundwater system were assumed to be 2.1E-05 Ci/L (2.1E+07 pCi/L) with a half-life of 12.3 years. The simulation results indicated that disposal of treated effluent would have little effect on the local direction of groundwater movement beneath the State-approved land disposal site. Groundwater flow directions resume their northeasterly regional flow direction at a point approximately 300 m (980 ft) downgradient of the disposal site. It is estimated that it would take 100 years for tritium in the uppermost aquifer to travel between the disposal site and the Columbia River. Maximum tritium concentrations at the riverbank before dilution in the Columbia River were calculated to be 1.4E-08 Ci/L (1.4E+04 pCi/L), which is below the Federal drinking water standard of 2.0E-08 Ci/L (20,000 pCi/L) (Jacobs 1996). No further groundwater analysis was conducted for the effects of treated effluent disposal.
No Action Alternative (Capsules)
This alternative would not impact groundwater. Cesium and strontium capsules would be maintained and stored temporarily in the Waste Encapsulation and Storage Facility (WESF) basins for a period of approximately 10 years, until further remediation measures have been selected. Therefore, no groundwater analysis was necessary.
Onsite Disposal Alternative
Under this alternative, the capsules would be placed in 0.3-m (1.0-ft) canisters surrounded by a 0.76-m (2.5-ft)-diameter sand backfill. There would be 672 drywells on a 5-m (16-ft) center-to-center spacing with a 30-m (100-ft) buffer around the facility. The drywell depth would be 4.6 m (15 ft) belowground.
Both cesium and strontium are relatively immobile in groundwater systems at the Hanford Site. The result of this immobility would mean that no measurable amount of either cesium or strontium would reach the groundwater within the 10,000-year period of interest. In addition, cesium-137 decays to barium-137, a stable isotope that likewise is immobile in groundwater systems. Strontium-90 decays to zirconium-90, which also is stable and immobile in groundwater systems. No groundwater analysis was conducted for this alternative because no impacts would be expected from the capsule contents or their decay products.
Overpack and Ship Alternative
Under this alternative, capsules would be removed from temporary storage, overpacked, and shipped offsite. No release of liquid would occur. No groundwater assessment was necessary because there would be no release of contaminants to the vadose zone or the groundwater.
Vitrify with Tank Waste Alternative
Under this alternative, capsules would be removed from temporary storage and vitrified with the HLW. Releases of liquid would be accounted for in the ex situ alternatives. No groundwater assessment was necessary because there would be no release of contaminants to the vadose zone or groundwater in excess of those for the ex situ alternatives.
Aggregate Source Areas
The 179 potential sources (i.e., each of the 177 tanks and the proposed LAW disposal vaults) were aggregated into nine discrete source areas based on waste inventory and proximity. The criteria used for these groupings are as follows.
- The proposed LAW disposal facility was considered one source area, though there could be as many as 41 vaults. Vault spacing was assumed to be approximately 30 m (100 ft) over a continuous area of up to 9.4 ha (23 ac). The vaults would be covered with one continuous Hanford Barrier.
- The tank sources were grouped into eight source areas, three in the 200 West Area and five in the 200 East Area.
Contaminant Groups
The tanks contain more than 100 radioactive and nonradioactive contaminants that potentially could impact groundwater. The approach used for this analysis was to group the contaminants based on their mobility in the vadose zone and underlying unconfined aquifer. Contaminant groupings were used rather than the individual mobility of each contaminant primarily because of the uncertainty involved in determining the mobility of individual contaminants. The groups were selected based on relatively narrow ranges of mobility, and contaminants were placed in the more mobile group if there was uncertainty about which group they should be placed in.
Some of the contaminants, such as iodine and technetium, would move at the rate of water whether in the vadose zone or underlying groundwater. The movement of other contaminants in water, such as americium and cesium, would be slowed or retarded by interaction with soil and rock. The VAM2D flow and transport model accounted for the retardation of contaminant movement with the parameter Kd, which is the distribution coefficient (mL/g). This parameter is a measure of sorption and is the ratio of the quantity of the adsorbate adsorbed per gram of solid to the amount of adsorbate remaining in solution (Kaplan et al. 1994). Values of Kd for the contaminants range from 0 mL/g (in which the contaminant's movement in water is not retarded) to more than 100 mL/g (in which the contaminant moves much slower than water).
The waste inventory was grouped and modeled according to each contaminant's reported or assumed Kd. The contaminant groups, based on mobility and examples of common or potential constituents of concern, are described in the following text. A complete listing of tank waste constituents by Kd is provided in Volume Four, Appendix F. The waste inventory groups used for modeling included the following:
- Group 1 - Contaminants were modeled as nonsorbing (i.e., Kd = 0). Contaminant movement would be unretarded in water. Contaminant Kd values in this group ranged from 0 to 0.99 mL/g and included all the isotopes of carbon, iodine, technetium, uranium, and nitrate;
- Group 2 - Contaminants were modeled as slightly sorbing (i.e., Kd = 1). Contaminant Kd values in this group ranged from 1 to 9.9 mL/g and included all the isotopes of americium, nickel, and chromium;
- Group 3 - Contaminants were modeled as moderately sorbing (i.e., Kd = 10). Contaminant Kd values in this group ranged from 10 to 49.9 mL/g and included all the isotopes of lead, plutonium, strontium, and thorium; and
- Group 4 - Contaminants were modeled as strongly sorbing (i.e., Kd = 50). Contaminant Kd values in this group were 50 mL/g or greater and included all the isotopes cesium, rubidium, and thallium.
Source Terms
The numerical modeling used to analyze groundwater impacts required understanding and quantifying when, what, and how many (mass or activity) contaminants would be released. The quantification of this information is the source term and includes the water flux into the vadose zone, which results from precipitation infiltrating the waste and mass or activity solubilized from dissolution of waste in the tanks. A detailed description of the source term and the rates of release of contaminants into the groundwater are contained in Volume Four, Appendix F.
5.2.1.2 Results
Groundwater beneath the 200 Areas and in plumes leading from the 200 Areas toward the Columbia River currently is contaminated with hazardous chemicals and radionuclides at levels greatly exceeding Federal drinking water standards. Drinking water standards typically are applied to treated water and are used here for comparison. For radionuclides, the drinking water standard (40 Code of Federal Regulations [CFR] 141.16) is based on a calculated dose equivalent to 4 millirem (mrem)/year to an internal organ, except for uranium that has a standard of 0.02 mg/L based on total uranium (i.e., all isotopes). Hazardous chemical contaminants present at levels exceeding drinking water standards include nitrates, cyanide, fluoride, chromium, chloroform, carbon tetrachloride, trichloroethylene, and techrachloroethylene. Radiological contaminants include iodine-129, tritium, cesium-137, plutonium-239 and 240, and strontium-90.
The groundwater beneath the 200 Areas is severely contaminated at levels that substantially exceed drinking water standards for several constituents. For example, iodine-129 is present at levels that exceed standards by up to 20 times. Groundwater use restrictions have been implemented to prevent use of the contaminated groundwater. Implementing any of the TWRS alternatives would add contaminants to groundwater but in concentrations expected to be less than the current levels of contamination observed in groundwater beneath the 200 Areas. Groundwater impacts calculated for each alternative are described briefly in the following subsections.
No Action Alternative (Tank Waste)
Groundwater impacts would be essentially the same as at present for the remainder of the 100-year period of institutional control, with the exception of slightly increased contaminant levels due to additional SSTs that could develop leaks. The long-term effects of this alternative are discussed below.
Because the waste would remain in the tanks, the No Action alternative eventually would result in the long-term dissolution and release of the total waste inventory from the 177 tanks into the vadose zone. The contaminants ultimately would pass through the vadose zone and reach the groundwater in the underlying unconfined aquifer within the 10,000-year period of analysis. Once in the aquifer, the contaminants would move relatively quickly through the aquifer and discharge to the Columbia River. The calculated contaminant concentrations in the groundwater are described in the following sections.
For the Kd Group 1 (Kd = 0) contaminants (fast-moving contaminants), the vadose simulation results calculated first arrival of contaminants at the vadose zone/groundwater interface from approximately 130 to 150 years from the present for the SSTs and DSTs. Peak concentration at the vadose zone/groundwater interface would be reached approximately 210 to 260 years from the present.
For the Kd Group 2 (Kd = 1), the vadose simulation results calculated contaminant first arrival at the groundwater approximately 1,000 to 1,400 years from the present. The average time of first arrival for the three source areas in the 200 West Area would be approximately 1,300 years from the present, while the average time of first arrival for the five source areas in the 200 East Area would be approximately 1,200 years from the present. The longer average time to first arrival for source areas in the 200 West Area is consistent with the thicker vadose zone in the 200 West Area.
For the Kd Group 3 and 4 (Kd = 10 and 50), first arrival would occur late (i.e., beyond the 10,000-year period of analysis). For this reason, simulation results were not reported for these Kd groups.
Two time frames were selected to illustrate the contaminant distribution in the unconfined aquifer. The calculated nitrate distribution in the groundwater at 300 years from the present is shown in Figure 5.2.3. Nitrate has an assumed Kd equal to zero and thus would move at the same velocity as the groundwater. Figure 5.2.4 provides the calculated distribution of bismuth in the groundwater at 5,000 years from the present. Bismuth is in Kd Group 2 (Kd = 1). Bismuth would move through the groundwater system at a slower velocity than water. Maximum contaminant concentrations for the five indicator contaminants at selected time periods are provided in Table 5.2.1 with the drinking water standards for comparison.
Drinking water standards for carbon-14, iodine-129, technetium-99, nitrate, and uranium-238 all would be exceeded at the 300- and 500-year times. Contaminant concentrations would decrease by 500 years but still would exceed the drinking water standards. By 2,500 years from the present, contaminant levels would be well below applicable drinking water standards as the contaminants are flushed through the system.
Long-Term Management Alternative
Under the Long-Term Management alternative, the first retanking of the DSTs would begin 50 years from the present. As a result, there would be no short-term contaminant releases to the vadose zone and groundwater in addition to those already existing. Groundwater impacts essentially would be the same as at present for the remainder of the 100-year period of institutional control, with the possible exception of slightly increased contaminant levels due to additional SSTs that begin to develop leaks. Leaks would be very small because saltwell pumping of the SSTs would reduce the amount of liquids available for release and because leaks would not be expected from the outer tanks of the DSTs.
Long-term impacts from the Long-Term Management alternative would be similar to the No Action alternative except impacts from the DSTs would be delayed by up to 100 years. The Long-Term Management alternative would result in the release of the total waste inventory from the 177 tanks into the vadose zone. The contaminants ultimately would pass through the vadose zone and reach the groundwater in the underlying unconfined aquifer within the 10,000-year period of analysis.
Once in the aquifer, the contaminants would move quickly through the aquifer and discharge to the Columbia River. The difference between this alternative and the No Action alternative is that the DSTs were assumed to last 100 years longer under the Long-Term Management alternative.
For the Kd Group 1 (Kd = 0) contaminants, the vadose zone simulation results calculated first arrival of contaminants at the vadose zone/groundwater interface approximately 140 to 250 years from present. Peak concentration at the vadose zone/groundwater interface would be reached approximately 210 to 350 years from present.
For the Kd Group 2 (Kd = 1), the vadose simulation results in calculated contaminant first arrival at the groundwater approximately 1,000 to 1,500 years from the present. The average time of first arrival for the three source areas in the 200 West Area would be approximately 1,300 years from the present, while the average time of first arrival for the five source areas in the 200 East Area would be approximately 1,200 years. The longer average time to first arrival for source areas in the 200 West Area is consistent with the thicker vadose zone in the 200 West Area.
For the Kd Group 3 and 4 (Kd = 10 and 50), first arrival would occur late (i.e., beyond the 10,000-year period of analysis). For this reason, simulation results were not reported for these Kd groups.
Two time frames were selected to illustrate the contaminant distribution in the unconfined aquifer. The estimated nitrate distribution in the groundwater at 300 years from the present is shown in Figure 5.2.5. Nitrate has an assumed Kd equal to zero, and thus would move at the velocity of groundwater.
Figure 5.2.6 provides the calculated distribution of bismuth in the groundwater at 5,000 years from the present. Bismuth is in Kd Group 2 (Kd = 1). Bismuth would move through the groundwater system at a much slower velocity than water. Maximum contaminant concentrations for the five indicator contaminants at selected time periods are provided in Table 5.2.2 with drinking water standards for comparison.
Drinking water standards for carbon-14, iodine-129, technetium-99, nitrate, and uranium-238 all would be exceeded at the 300- and 500-year times. Contaminant concentrations would decrease by 500 years but still exceed the drinking water standards. By 2,500 years, contaminant levels would be below current drinking water standards.
In Situ Fill and Cap Alternative
There would be no contaminant losses to the vadose zone and groundwater under the In Situ Fill and Cap alternative during remediation in addition to those already existing, because retrieval of the waste would not be performed; therefore, retrieval activities would not cause increased leaks from the tanks. Groundwater impacts essentially would be the same as for the No Action alternative for the 100-year period of institutional control. The long-term effects of this alternative would commence after the 100-year period of institutional control and are discussed in the following paragraphs.
The waste that remained in the tanks under the In Situ Fill and Cap alternative eventually would result in the long-term dissolution and release of the complete inventory from the 177 tanks into the vadose zone. This complete release was considered to be a bounding condition for the EIS. However, only the most mobile contaminants, those modeled as Kd = 0, were calculated to reach the groundwater within the period of analysis. The source would be the same as for the No Action alternative. The major difference between these alternatives is that a Hanford Barrier would be constructed over the tanks, which would result in a lower infiltration rate and lower contaminant release rate to the vadose zone compared to the No Action alternative. For the In Situ Fill and Cap and all other tank waste alternatives, except No Action, Long-Term Management, and Ex Situ/In Situ Combination 1 and 2 alternatives, only the contaminants modeled as Kd = 0 would reach the groundwater within the period of analysis. For this reason, simulations were not reported for Kd Groups 2, 3, and 4 for the remaining alternatives except the Ex Situ/In Situ Combination 1 and 2 alternatives.
Once in the aquifer, the contaminants would move relatively quickly through the aquifer and discharge to the Columbia River. Contaminant first arrival at the vadose zone/groundwater interface was calculated to occur approximately 2,300 to 3,400 years from the present. Peak groundwater concentrations in the aquifer would be similar to those calculated for the No Action alternative but would occur approximately 4,100 to 6,300 years from present.
The average time to first arrival and peak concentration for the five source areas in the 200 East Area would be approximately 2,500 and 5,200 years, respectively. The average time to first arrival and peak concentration for the three source areas in the 200 West Area would be approximately 3,300 and 5,200 years, respectively.
The calculated peak concentrations for each of the eight areas at the vadose zone/groundwater interface were similar in magnitude to those calculated for the No Action alternative. As with the No Action alternative, contaminant levels would reach or nearly reach steady-state conditions with maximum concentrations near 400,000 g/m3 for all source areas except one.
The calculated nitrate distribution in the groundwater at 5,000 years from the present is shown in Figure 5.2.7. Nitrate has an assumed Kd equal to zero and thus would move at the velocity of groundwater. Nitrate concentration in the groundwater would reach steady-state conditions at approximately 5,800 years and would continue at those concentration levels for approximately 1,500 years. The nitrate concentrations shown in Figure 5.2.7 were based on an initial source concentration of 360,000 g/m3 calculated in the upper 6 m (20 ft) of the aquifer.
Maximum contaminant concentrations for the five indicator contaminants at selected time periods are provided in Table 5.2.3 with the drinking water standards for comparison, where available.
Contaminants would not reach groundwater from the sources at earlier time periods during 300 or 500 years from present. Very low levels were calculated for the 2,500-year period. Current drinking water standards exceedances were calculated for iodine-129, uranium, and technetium-99 from 5,000 years through 10,000 years from the present. Nitrate concentrations would exceed drinking water standards at approximately 5,000 years but would decrease to below the standard before 10,000 years.
In Situ Vitrification Alternative
There would be no contaminant losses to the vadose zone and groundwater under the In Situ Vitrification alternative during remediation because the waste would be immobilized by vitrification, and the resulting glass would leach extremely slowly. The long-term effects of this alternative would commence after remediation and are discussed in the following paragraphs.
The In Situ Vitrification alternative would result in the long-term partial release of the tank inventory from the 177 tanks into the vadose zone over the period of interest (10,000 years). Only the most mobile contaminants, those modeled as Kd equal to zero, were calculated to reach the groundwater within the period of analysis. The source would be similar to the alternatives previously described but the release rates would be very low. This would result in a release of contaminants at a constant concentration for several thousand years from each vitrified tank farm.
Contaminant first arrival at the vadose zone/groundwater interface was calculated to occur approximately 2,400 to 3,400 years from the present. Peak concentration at the vadose zone/groundwater interface would reach steady-state conditions with a concentration of 400 mg/L between approximately 6,200 and 7,500 years and remain at that concentration for the remainder of the period of analysis. Compared to the No Action alternative, this alternative would have a much longer calculated time to first arrival and a lower peak concentration at the vadose zone/groundwater interface, primarily because of the lower infiltration rate through the Hanford Barrier and the low solubility of the vitrified waste. The calculated peak concentration for each of the eight source areas at the vadose zone/groundwater interface also would be much lower. The time of first arrival would be affected by the material properties of the strata as well as the distance of travel (vadose zone thickness).
Figure 5.2.8 presents the calculated uranium-238 distribution in the groundwater at 5,000 years from the present. Uranium-238 was assumed to have a Kd equal to zero and thus would move at the velocity of groundwater. The uranium-238 concentrations would reach steady-state conditions at approximately 5,000 years and continue at those concentration levels throughout the 10,000-year period of analysis. The uranium-238 concentrations shown in Figure 5.2.8 represent calculated concentrations in the upper 6 m (20 ft) of the aquifer. Maximum contaminant concentrations for the five indicator contaminants at selected time periods are provided in Table 5.2.4 with the drinking water standards for comparison, where available. Calculated contaminant concentrations all would be below drinking water standards for all of the times shown in Table 5.2.4. Iodine-129, nitrate, and carbon-14 were not in the source term because iodine-129 and carbon-14 would be volatilized during the vitrification process, and nitrate would be converted to volatile nitrogen oxides.
Ex Situ Intermediate Separations Alternative
There would be contamination releases to the vadose zone under the Ex Situ Intermediate Separations alternative during remediation that would be caused by releases from the SSTs during retrieval. There would be no contaminant releases from the DSTs. However, the contaminants released by losses during retrieval would travel very slowly in the vadose zone, requiring approximately 1,100 years to reach the groundwater. Groundwater modeling did not distinguish between contaminants from retrieval releases and contaminants from residual waste left in the tank. The net result was that contaminants from retrieval releases would become intermingled with contaminants from residual waste left in the tanks. The modeling results shown in Volume Four, Appendix F show only the arrival of one group of contaminants at the boundary between the vadose zone and the groundwater aquifers. The long-term effects of the Ex Situ Intermediate Separations alternative are discussed in the following paragraphs.
This alternative would result in the long-term release of contaminants to the vadose zone from 1) waste from the 149 SSTs associated with retrieval operations (retrieval from DSTs do not result in releases); 2) residual waste left in the tanks (for all tanks); and 3) the LAW disposal facility.
Only the most mobile contaminants, those modeled as Kd Group 1 (Kd = 0), were calculated to reach the groundwater within the period of analysis. The contaminants modeled as Kd Group 1 (Kd = 0) would reach the vadose zone/groundwater interface approximately 1,100 to 3,400 years from present. Compared to the No Action alternative, the mass of contaminants that would be released from the tanks would be relatively small (i.e., less than 2 percent of the mass released under the No Action alternative).
Peak contaminant concentrations at the vadose zone/groundwater interface for the tank source areas would be approximately 3,600 to 5,100 years from present. Peak contaminant concentrations at the vadose zone/groundwater interface for the LAW disposal facility would be reached at approximately 6,600 years and would remain at about that concentration for the remainder of the period of analysis. Compared to the No Action alternative, this alternative would have a much longer time to first arrival and peak contaminant concentrations at the vadose zone/groundwater interface, primarily because of the lower infiltration rate through the Hanford Barrier and the lower corrosion rate of the vitrified waste in the LAW disposal facility.
The calculated nitrate concentration in the groundwater from the tank sources at 5,000 years from the present is shown in Figure 5.2.9. The nitrate concentrations shown in Figure 5.2.9 were adjusted for an assumed initial source concentration of 360,000 g/m3 of nitrate and represent calculated concentrations in the upper 6 m (20 ft) of the aquifer. Figure 5.2.10 presents the calculated uranium-238 concentrations in the groundwater from the tank and LAW disposal facility at 5,000 years from the present. Both nitrate and uranium-238 have an assumed Kd equal to zero and thus would move at the velocity of groundwater. Post processing of the modeling results is explained in Volume Four, Appendix F. There would be an exceedance at the current drinking water standard for uranium-238 at 5,000 years from present. No additional contaminants would exceed current groundwater standards.
Contaminants would not have reached the groundwater from the tank sources at the two earlier time periods of analysis (e.g., 300 and 500 years from the present). At 2,500 years from the present, contaminants would not have yet reached groundwater from the LAW disposal facility. Maximum concentrations for the five indicator contaminants at selected time periods are provided in Table 5.2.5 for comparison with the drinking water standards. The current drinking water standards for any of the indicator contaminants would not be exceeded by releases from the tank sources associated with waste retrieval and residuals nor the LAW disposal source, except for a slight exceedance of uranium at 5,000 years for the tank sources. Because the maximums for tank sources and LAW vaults occur at different locations, they are not additive.
Ex Situ No Separations Alternative
There would be contaminant releases to the vadose zone under the Ex Situ No Separations alternative during remediation that would be caused by releases from the SSTs during retrieval. There would be no contaminant releases from the DSTs. Because the retrieval process is the same as that for the Ex Situ Intermediate Separations alternative, the effects of these retrieval releases also would be the same. The following paragraphs discuss the long-term effects of groundwater contaminants.
The long-term impact to groundwater associated with the Ex Situ No Separations alternative would be a result of waste retrieval from the SSTs and residual waste remaining in both the SSTs and DSTs. These impacts would be the same as calculated for the Ex Situ Intermediate Separations alternative for the tank sources only, as illustrated in Figures 5.2.9 and 5.2.10. All retrieved waste would be processed and transported to the potential geologic repository. Maximum calculated concentrations for the five indicator contaminants at selected time frames are provided in Table 5.2.6.
Contaminants would not reach the groundwater until approximately 1,100 years from the present. Levels of contaminants would remain low. There would be an exceedance of the current drinking water standard for uranium-238 at 5,000 years from the present.
Ex Situ Extensive Separations Alternative
There would be contaminant losses to the vadose zone under the Ex Situ Extensive Separations alternative during remediation that would be caused by losses from the SSTs during retrieval. There would be no contaminant losses from the DSTs. Because the retrieval process would be the same as that for the Ex Situ Intermediate Separations alternative, the effects of these retrieval losses also would be the same. The following paragraphs discuss the long-term effects of the groundwater contaminants.
Long-term groundwater impacts for the Ex Situ Extensive Separations alternative would result from tank sources (waste retrieval releases from SSTs and residual waste releases from SSTs and DSTs) and the LAW disposal vaults. The groundwater impacts associated with the tank sources would be similar to those calculated for the Ex Situ Intermediate Separations alternative (Figures 5.2.9 and 5.2.10). The groundwater impacts associated with releases from the LAW disposal facility would be reduced from those calculated for releases from the LAW disposal vaults under the Ex Situ Intermediate Separations alternative because a greater amount of the HLW would be removed during the separations process. Maximum concentrations in groundwater of the five indicator contaminants from tank sources and of technetium-99 and uranium-238 from the LAW disposal vaults are shown at selected times in Table 5.2.7.
Contaminants would not reach the groundwater until approximately 1,100 years from the present. Levels of contaminants would remain low. There would be a slight exceedance of current drinking water standards for uranium-238 at 5,000 years from the present.
Ex Situ/In Situ Combination 1 Alternative
There would be contaminant losses to the vadose zone under the Ex Situ/In Situ Combination 1 alternative during remediation that would be caused by losses from the SSTs during retrieval. Because the retrieval process would recover waste from 60 SSTs instead of 149, the retrieval losses would be proportionately less. However, the contaminants released by losses during retrieval would travel very slowly in the vadose zone, eventually becoming indistinguishable from the contaminants caused by the residue remaining in the tanks after retrieval. There would be no contaminant losses from the DSTs during remediation. The long-term effects of groundwater contaminants are discussed in the following paragraphs.
The two major components that would result in long-term releases to the vadose zone under this alternative are 1) tank sources from retrieval losses and releases from tanks remediated in situ; and 2) releases from the LAW vault. The scenarios for these components would include all of the assumptions stated for the In Situ Fill and Cap and Ex Situ Intermediate Separations alternatives. The residual waste assumed to be 1 percent of the initial inventory, which could be left in the tanks after retrieval, was added to the inventory of tanks that would be remediated in situ. As with both the In Situ Fill and Cap and Ex Situ Intermediate Separations alternatives, only the most mobile contaminants, those modeled as Kd equal to zero, were calculated to reach groundwater within the period of analysis.
The objective of this alternative is to reduce the number of tanks in which the waste is processed ex situ and simultaneously achieve low groundwater concentrations of the high-risk contaminants technetium-99, carbon-14, iodine-129, and uranium-238. These contaminants all are mobile and are in Kd Group 1. They, along with the other contaminants in Kd Group 1, were calculated to reach the groundwater of the unconfined aquifer within the period of analysis. The distribution of uranium-238 in groundwater, the most abundant of the uranium isotopes in the tank waste, is presented in this section for 5,000 years from the present. Also, a tabulation of maximum concentrations of indicator contaminants in the unconfined aquifer in Kd Group 1 is provided.
Peak contaminant concentrations at the vadose zone/ groundwater interface for the tank source areas would be reached approximately 3,600 to 5,100 years from the present. Peak contaminant concentrations at the vadose zone/groundwater interface for the LAW disposal facility would be reached approximately 6,600 years from the present and would remain at that concentration for the remainder of the period of analysis.
Once in the aquifer, the contaminants would discharge to the Columbia River. Contaminant concentrations in the aquifer would be approximately 10 times lower than those calculated for the No Action alternative, primarily as a result of lower contaminant inventory and a lower infiltration rate due to the Hanford Barrier, which would be constructed over the tanks remediated in situ and the LAW vault.
The calculated uranium-238 concentrations in the groundwater from the tank sources and from the LAW disposal facility at 5,000 years from the present are provided in Figures 5.2.11 and 5.2.12, respectively. These concentrations represent calculated concentrations in the upper 6 m (20 ft) of the aquifer. Maximum concentrations for the five indicator contaminants at selected time periods are provided in Table 5.2.8, with the drinking water standard for comparison for both the tank sources and LAW disposal source. Nitrate, iodine-129, technetium-99, and uranium-238 were calculated to exceed the current drinking water standard at 5,000 years.
Ex Situ/In Situ Combination 2 Alternative
There would be contaminant losses to the vadose zone under the Ex Situ/In Situ Combination 2 alternative from 1) retrieval losses from the 13 SSTs; 2) losses from the residual waste that would remain in the 25 tanks from which waste would be retrieved (13 SSTs and 12 DSTs); 3) releases from the 152 waste tanks that would be remediated in situ with the fill and cap technology; and 4) releases from the LAW vaults. Only the most mobile contaminants, those modeled as Kd equal to zero, were calculated to reach the groundwater within the period of interest. The long-term impacts of these releases on the groundwater are discussed in the following paragraphs.
The calculated groundwater impact from retrieval losses from the 13 SSTs would be proportionally less than the calculated impacts associated with retrieval losses for Ex Situ/In Situ Combination 1 alternative in which the waste from 60 SSTs would be retrieved. To calculate the impact from waste retrieval, the predicted contaminant concentration of each contaminant from the Ex Situ/In Situ Combination 1 alternative was multiplied by the ratio of the mass of this alternative (on a contaminant by contaminant basis) over the mass of that constituent that would be released during retrieval for the Ex Situ/In Situ Combination 1 alternative.
Simulations of contaminant fate and transport as described for the In Situ Fill and Cap alternative were performed to calculate groundwater impacts from the 152 tanks remediated in situ and as with the retrieval impacts, only the most mobile contaminants would reach groundwater within the period of interest. The calculated groundwater impacts from the LAW vault would also be proportionate to the calculated impacts from the LAW vault in the Ex Situ/In Situ Combination 1 alternative. The impacts were therefore calculated by scaling the results from the Ex Situ/In Situ Combination 1 alternative as described in the previous text.
This alternative would be a variation of the Ex Situ/In Situ Combination 1 alternative. The objective of this alternative would be to reduce the quantity of the long-term risk contributors (i.e., carbon-14, iodine-129, technetium-99 and uranium-238) in tanks remediated in situ while also reducing the amount of overall tank waste that would be retrieved. The contaminants that would be long-term contributors to risk, along with the other in Kd Group 1 (Kd), were calculated to reach the groundwater of the unconfined aquifer within the period of interest. The calculated distribution of uranium-238 in groundwater, the most abundant of the uranium isotopes in the tank waste, is presented in this section for 5,000 years from present for both tank sources and LAW vault sources.
Peak contaminant concentrations at the vadose zone/groundwater interface for the tank releases (both from retrieval releases and releases from tanks remediated in situ) would be reached approximately 3,600 to 5,100 years from the present. Peak contaminant concentrations at the vadose zone/groundwater interface for the LAW disposal facility would be reached at approximately 6,600 years and would remain at that concentration for the remainder of the period of interest.
Once in the aquifer, the contaminants would discharge to the Columbia River. Contaminant concentrations in the aquifer would be approximately 10 times lower than those for the No Action alternative, primarily as a result of lower contaminant inventory and lower infiltration rate due to the Hanford Barrier, which would be constructed over the tanks from which waste would be retrieved, those tanks that would be remediated in situ, and the LAW vault.
The calculated uranium-238 concentrations in the groundwater from tank sources and from the LAW disposal facility at 5,000 years from the present are provided in Figures 5.2.13 and 5.2.14, respectively. These concentrations represent calculated concentrations in the upper 6 m (20 ft) of the unconfined aquifer. Maximum concentrations for the five indicator contaminants at selected time periods are provided in Table 5.2.9, with the drinking water standard for comparison. As with the Ex Situ/In Situ Combination 1 alternative, nitrate, iodine-129, technetium-99, and uranium-238 were calculated to exceed the current drinking water standard at 5,000 years.
Phased Implementation Alternative
Phase 1
There would be no groundwater impacts from the first phase of this alternative. Nearly all of the waste would be retrieved from the DSTs, and there would be no retrieval losses.
Phase 2
The short-term and long-term groundwater impacts from the second phase of this alternative would be identical to those for the Ex Situ Intermediate Separations alternative. There would be contaminant releases to the vadose zone under the second phase of this alternative during remediation, which would be caused by losses from the SSTs during retrieval. As explained in Volume Four, Appendix F, the impacts of the losses during retrieval would be approximately half of the impacts caused by the residual waste left in the tanks after retrieval. However, the contaminants released by losses during retrieval would travel very slowly in the vadose zone, requiring approximately 1,100 years to reach the groundwater. The net result would be that contaminants from retrieval losses would become intermingled with contaminants from residual waste left in the tanks. Groundwater modeling did not distinguish between the two sources of contaminants. The long-term effects of the second phase of the Phased Implementation alternative would be the same as those discussed for the Ex Situ Intermediate Separations alternative.
Cesium and Strontium Capsules
None of the cesium and strontium capsule alternatives would result in substantive groundwater impacts, as described in Section 5.2.1.1.
5.2.1.3 Parameter Sensitivity
Parameter sensitivity was investigated for the effect of 1) higher glass surface areas for the In Situ Vitrification alternative; 2) changing the performance period of the Hanford Barrier from 1,000 years to 500 years; 3) the eventual decay of the potentiometric head resulting from groundwater mounding related to the discharge to the Hanford Site ponds; 4) the effect of variations in infiltration rate; and 5) the effect of variations in distribution coefficient (Kd). Further information concerning parameter sensitivity is presented in Volume Five, Sections K.4.5.1 and K.4.5.2 .
In Situ Vitrification Surface Area
To investigate the sensitivity of the calculated results to the surface area of the glass produced by in situ vitrification, additional groundwater modeling was performed, based on the assumption that the glass surface area had increased by a factor of two. This would represent the case in which extensive cracking of the waste form had occurred. The additional modeling showed that the calculated contaminant concentrations were indistinguishable from those calculated by the base case analysis.
500-Year Versus 1,000-Year Hanford Barrier
The base case for modeling infiltration through the Hanford Barrier assumed the Barrier would not degrade for 1,000 years. Additional modeling was performed to investigate the situation in which the Hanford Barrier placed over the tanks would degrade 500 years after placement rather than 1,000 years. In the additional modeling, the water flux through the cap was assumed to increase from 0.05 cm/year to 0.1 cm/year (0.02 to 0.04 in./year) after 500 years. A comparison of the calculated nitrate concentrations for the two durations (500 versus 1,000 years) showed that the times of arrival of nitrate in the groundwater and the peak nitrate concentrations were almost identical. A comparison of calculated uranium-238 concentrations in groundwater at 10,000 years from the present indicates that for the 500-year cap, calculated uranium-238 concentrations would be lower by a factor ranging from 5 to 10. This would be due to the higher water flux through the 500-year cap, which would allow uranium-238 to move faster and be flushed from the groundwater system. With the 500-year cap, the contaminants would have already moved through the system. The conclusion is that the maximum contaminant concentrations in groundwater would be nearly unaffected, but the time of contaminant arrival to the groundwater would be proportional to the infiltration rate through the cap.
Groundwater Mounds
To investigate the sensitivity of the calculated contaminant concentrations in the unconfined aquifer, additional modeling for this saturated system was performed, which was based on calculated future groundwater levels where the present-day groundwater mounds associated with Site waste water disposal had dissipated. The additional modeling was conducted using conditions associated with the Ex Situ Intermediate Separations alternative (e.g., source term, infiltration rate, and Hanford Barrier performance). The data indicated that contaminant transport from the waste tanks and LAW vault would be in a slightly more easterly direction compared to that calculated for the base case flow field.
Variations in Infiltration Rate
To investigate the sensitivity of the calculated results to the assumed initial infiltration rate, additional vadose zone modeling was performed based on doubling the initial infiltration to 10 cm/year (4 in./year) from 5 cm/year (2 in./year) for the In Situ Fill and Cap alternative. The additional modeling showed that the calculated contaminant concentrations at the vadose zone/groundwater interface were indistinguishable from those calculated by the base case analysis. The infiltration-limiting effects of the cap was believed to be the controlling factor. Thus, these results would apply to the other alternatives that use a cap.
Variations in Kd
Sensitivity of contaminant travel time through the vadose zone to various Kd values was evaluated by varying Kd and calculating the arrival time for the 1WSS source area and the Ex Situ Intermediate Separations alternative. The additional modeling showed that at this source area and conditions for this alternative, contaminants with Kd values equal to or greater than approximately 0.125 mL/g would not reach the groundwater within the 10,000-year period of interest.
5.2.2 Surface Water
This section describes potential impacts on surface waters from liquid effluent discharges, seeps of contaminated groundwater, and alterations of surface water drainage systems.
5.2.2.1 Water Discharges
Although each tank waste and capsule alternative would generate liquid effluent, the effluent would not be discharged to surface waters, and thus there would be no direct impacts to any surface waters. Liquid currently in the tanks, or added to the tanks for purposes such as diluting waste so it could be pumped, ultimately would be removed to the extent possible under all alternatives. This liquid would be sent to an evaporator. Condensed water from the evaporator would be sent to the Effluent Treatment Facility in the 200 East Area. The water then would be treated in the Effluent Treatment Facility with a variety of systems, including evaporation, to meet applicable regulatory standards, and would ultimately be discharged through the Effluent Treatment Facility to the State-approved land disposal facility site, a subsurface drain field near the north-central part of the 200 West Area.
5.2.2.2 Groundwater Discharges
All of the tank waste alternatives would result in some contaminants being leached into the groundwater beginning in approximately 140 years for the No Action alternative to 3,400 years for the In Situ Vitrification alternative, as described in Section 5.2.1. Previously existing contaminants in the soils and vadose zone beneath the tanks from past tank leaks and spills also would migrate to the groundwater, but these are not in the scope of this EIS. Once contaminants reached the groundwater, they would eventually discharge into the Columbia River through seeps (springs) on the Columbia River bank or into the river through the river bed where the river intersects the unconfined aquifer. The present level of nitrate contamination in the unconfined aquifer (Volume Five, Appendix I) is approximately 20 mg/L at the river east of the 200 Areas. This concentration of nitrate in the unconfined aquifer has resulted in negligible changes in nitrate concentrations in the Columbia River and indicates that impacts to the Columbia River from any of the alternatives would be low. To verify this estimate, a mixing calculation for the water that would enter the Columbia River from the tank waste activities is described in the following text.
The analysis involved dividing the river into segments and then calculating the contaminant concentration in each segment based on inflow from the unconfined aquifer. Segments that were 1 kilometer (km) (0.6 mile [mi]) long were developed, and flow in the Columbia River was adjusted for each segment based on flux from the groundwater model at each node along the river. A water flux to or from the river then was assigned as part of the groundwater model calibration process. Contaminant mass entering the river was calculated from nitrate in groundwater at the 300-year time frame for the Long-Term Management alternative (Figure 5.2.5). This contaminant and alternative were selected because nitrate is the most abundant contaminant in the tank waste, is the contaminant most likely to impact the river, and was calculated to exceed drinking water standards in the unconfined aquifer for the No Action, Long-Term Management, and In Situ Fill and Cap alternatives. The 300-year time frame and the Long-Term Management alternative have the highest calculated nitrate concentrations in the unconfined aquifer at the river.
Columbia River Characteristics
The Columbia River flows through the northern and eastern portions of the Hanford Site for over 100 km (62 mi) and is hydraulically connected to the unconfined aquifer (Figure 5.2.1). This hydraulic connection allows river water to recharge to the unconfined aquifer along some reaches, notably in the vicinity of D Reactor. Groundwater discharges to the river at other locations, such as the reach around B Reactor and east of the Hanford Site. Both the groundwater discharge rate and nitrate concentrations vary along the approximate 105-km (65-mi) length of the Columbia River encompassed within the groundwater model. The minimum 7-day duration mean flow and the median flow rates were used in this analysis to bracket the calculated nitrate concentrations in the Columbia River.
The groundwater model, which encompasses approximately 100 km (62 mi) of the river located within the Hanford Site and a total of approximately 105 km (65 mi) of the river, calculated a net groundwater discharge to the river of approximately 0.51 m3/sec (18 cubic feet [ft3]/sec). This represents 0.022 percent of the median river flow.
Water quality information for the Columbia River was obtained from a U.S. Geological Survey water quality monitoring station at the Vernita Bridge, located near the western boundary of the Hanford Site where the Columbia River enters the Site. During the 1994 water year (October 1, 1993 to September 30, 1995), the combined nitrate and nitrite concentrations in the river at that location were less than the analytical detection limit (0.05 mg/L) on three occasions, 0.05 mg/L on one occasion, and 0.06 mg/L on one occasion (USGS 1994). Nitrate typically accounts for all but a small amount of the total nitrate and nitrite concentration.
Impacts Analysis
To determine how concentrations of a contaminant such as nitrate would vary along the river, the river reach was divided into many short segments in which complete mixing of the groundwater discharge with the river flow was assumed to occur. The resultant river concentration after mixing then was assumed to be the concentration for the river influent to the next downstream segment.
For the Long-Term Management alternative, an initial analysis was performed for nitrate, the chemical pollutant identified as having the greatest potential adverse impact, with river flow rates of 594 m3/sec (21,000 ft3/sec) (minimum 7-day duration mean flow rate) and 2,300 m3/sec (81,000 ft3/sec) (median flow rate). Background nitrate concentrations at the upstream end of the Columbia River Reach through the Hanford Site were assumed to be 0.05 mg/L, which is typical of present concentrations. The results are shown in Figure 5.2.15. Groundwater calculated to be contaminated with nitrate would first enter the river near B Reactor (Figure 5.2.5), and concentrations of nitrate in the river would increase slightly to approximately 0.06 mg/L for the minimum 7-day duration mean flow rate. The nitrate levels would remain at 0.06 mg/L until additional nitrate-contaminated groundwater was calculated to enter the river east of the 200 Areas (where nitrate concentrations ultimately were calculated to reach approximately 0.177 mg/L in the river), which would be approximately 0.12 mg/L above background. Nitrate concentrations in the river were much lower for the median flow rate, reaching a maximum of approximately 0.08 mg/L, which would be 0.03 mg/L above the 0.05 mg/L background nitrate concentrations.
Nitrate is a chemical contaminant belonging to Group 1, where Kd=0. The other members of Group 1 would behave in groundwater in a similar manner, because all of them were considered nonsorbing and their movement in groundwater would be retarded. As discussed in Volume Four, Appendix F, there are other members of Group 1. The radiological constituents in Group 1 include carbon-14, iodine-129, technetium-99, and uranium-238. These radioisotopes would move with nitrate in the groundwater and enter the Columbia River at the same locations. The concentrations of the radioisotopes would be less than that of nitrate in proportion to the amount of radioisotope that is present in the tanks. As is discussed in Volume Four, Appendix F, the tank contents were assumed to be released in proportion to the release of the most abundant tank consistent, which is nitrate.
Therefore for the Long-Term Management alternative, the calculated concentrations of Group 1 constituents of concern would be as follows: carbon-14=3 .09E-09 g/L; iodine=2.0E-07 g/L; technetium-99=1.0E-06 g/L; and uranium-238=1.0E-03 g/L. These concentrations would be in addition to background concentrations. As explained in Volume Four, Appendix F, these calculated concentrations would be expected at approximately 500 years from the present.
Contaminants entering the Columbia River could be observed from time to time at localized higher concentrations than would be calculated by the mixing model. However, the concentration of pollutants discharged to the river rapidly would become completely mixed with the river flow from several mechanisms. The factors controlling the rate of mixing or length of mixing zone would be: turbulence of the river flow, which depends on velocity of flow; irregularities in the stream channel, including bends; and the width of the river. Secondary currents created by channel irregularities and bends also would result in rapid mixing. These mechanisms would result in the rapid mixing of groundwater discharged from the unconfined aquifer to the Columbia River. Therefore, the contaminants in the groundwater from the tank waste alternatives would be rapidly diluted on discharging into the Columbia River. There would be a slight increase in the contaminant levels, but drinking water standards would not be exceeded.
5.2.2.3 Surface Water Drainage Systems
All facilities would be constructed on relatively flat, semi-arid terrain, which slopes gently to the northeast. No major drainage features are present. While each of the tank waste alternatives would result in slightly altered localized drainage patterns, the area around all temporary structures and all permanent facilities would be designed to conform with the surrounding terrain. Small increases in surface water runoff during heavy precipitation events or rapid snow melt would occur from temporary structures, but there would be no flooding of drainage systems.
The capsule alternatives would not alter surface water drainage systems with the exception of the Onsite Disposal alternative. Under this alternative, 3.8 ha (8.4 ac) of nearly flat and level terrain would be graded almost completely flat and level for the drywell disposal facility. Only enough slope would be provided to allow for runoff during heavy precipitation or rapid snow melt. The facility would be graded to conform with the surrounding terrain. Small decreases in surface water runoff during heavy precipitation or rapid snow melt would occur.
The three potential borrow sites would be constructed on gently sloping semi-arid terrain with no major drainage features. Slightly altered localized drainage patterns would occur during borrow site operations. Small increases in surface water runoff during heavy precipitation events or rapid snow melt would occur from the altered terrain, but there would be no flooding of any drainage system.
Measures would be taken to minimize any increases in runoff during operations of the borrow sites. Following operations, the borrow sites would be recontoured to conform with the surrounding terrain.
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