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F.3.0 PREDICTED CONTAMINANT CONCENTRATIONS

This section describes the potential impacts to the groundwater aquifer from the TWRS alternatives. The discussion includes the calculated movement of contaminants through the vadose zone and unconfined aquifer. Results are presented for five time periods; 300, 500, 2,500, 5,000, and 10,000 years from the present for the primary contributor to human health risk, C-14, I-129, Tc-99, U-238, and nitrate.

F.3.1 NO ACTION ALTERNATIVE (TANK WASTE)

The No Action alternative would result in the release of the total waste inventory from the 177 tanks into the vadose zone. The contaminants in K_d groups 1 and 2, modeled as K_d equals zero and one, respectively, ultimately pass through the vadose zone and reach groundwater in the underlying unconfined aquifer, within the 10,000-year period of interest. Once in the aquifer, the contaminants in K_d group 1 move relatively quickly through the aquifer and discharge to the Columbia River. The results of contaminant transport modeling through the vadose zone and groundwater are discussed in the following sections.

F.3.1.1 Vadose Zone

The scenario for this alterative includes the following major assumptions:

• Infiltration is 5.0 cm/year (1.36E-04 m/day) initially and throughout the period of interest;
• Contaminant release for the five SST source areas and the three DST source areas is assumed to begin at the end of institutional control in the year 2095; and
• The initial unit concentration assumed in modeling for K_d groups 1 and 2 (K_d equals zero and one) is 400,000 mg/L.

For K_d equals zero, the vadose modeling results predict contaminant first arrival at the vadose zone/groundwater interface at approximately 130 to 150 years (Figure F.3.1.1). (Note: All figures and tables follow the text.) Peak concentration at the vadose zone/groundwater interface is reached at times varying from approximately 210 to 260 years.

Contaminant concentrations from four of the five SST source areas (1WSS, 2WSS, 1ESS, and 4ESS) reach or nearly reach steady-state and the maximum possible (400,000 mg/L) concentration. The vadose zone in the 200 West Area is generally thinner by 5 to 20 m (16 to 65 ft), compared to that in the 200 East Area. The flatter shape of the peak of the time/concentration curves for the 200 West Area sites compared to the 200 East Area sites indicates that peak concentrations calculated at the groundwater-vadose zone interface are relatively sensitive to vadose zone thickness. The fifth SST source area, 2ESS, is located in 200 East. First arrival of contaminants at the water table from this source area is similar to that from the other SST sources but the peak concentration is much lower, at approximately 28,000 mg/L^.. This occurs because the contaminant mass and corresponding release period (Table F.2.2.10) for the 2ESS source area is generally one or more orders of magnitude less than the other source areas.

For K_d equals one, contaminant first arrival at the groundwater varies from approximately 1,020 to 1,380 years (Figure F.3.1.2).

For contaminant groups 3 and 4 (K_d equals 10 and 50), first arrival occurs very late (i.e., beyond the 10,000 period of interest). For this reason, modeling results are not reported for these K_d groups.

F.3.1.2 Groundwater

Contaminants in K_d groups 1 and 2, modeled as K_d equals zero and one, respectively, are calculated to reach the groundwater of the unconfined aquifer within the period of interest. Two time frames were selected to illustrate the contaminant distribution in the unconfined aquifer. Figure F.3.1.3 presents the calculated nitrate distribution in the groundwater at 300 years from the present. Nitrate has assumed K_d equal to zero and thus moves at the velocity of groundwater. The time versus calculated concentration of nitrate at selected locations within the aquifer is provided in Figure F.3.1.4. Figure F.3.1.4 indicates that nitrate has moved completely through the groundwater system (i.e., nitrate concentrations in groundwater have fallen to approximately zero) prior to approximately 900 years from the present. The nitrate concentrations shown in Figures F.3.1.3 and F.3.1.4 have been adjusted for an assumed initial source concentration of 360,000 mg/L of nitrate and represent calculated concentrations in the upper 6 m (20 ft) of the aquifer.

Figure F.3.1.5 provides the calculated distribution of bismuth in the groundwater at 5,000 years from the present. Bismuth is representative of elements in K_d group 2 (modeled as K_d equals one). Bismuth moves through the groundwater system at a much slower velocity than water, as illustrated in Figure F.3.1.6. This figure shows that for the selected observation nodes within the aquifer, it takes bismuth over 8,000 years from first arrival until its concentration drops back to near zero. Time versus concentration for observation nodes 13767 and 23585 exhibit a bimodal pattern (Figure F.3.1.6). These two observation points are located along the Columbia River approximately due east of the 200 Areas, and southeast of the 200 Areas, respectively. Both of these locations receive contaminants from tank sources in both 200 East and 200 West Areas. The bimodal pattern is due to contaminants reaching the Columbia River from the 200 East Area sources first, followed by contaminants from 200 West Area sources. The other two observation nodes (25647 and 29076) do not exhibit the bimodal pattern because the primary source of contaminants to these points originates only from 200 West Area sources. These two observation nodes are located between Gable Butte and Gable Mountain and along the Columbia River near the B Reactor, respectively. The bismuth concentrations shown in Figures F.3.1.5 and F.3.1.6 have been adjusted for their initial source concentrations shown in Table F.2.2.11, and represent calculated concentrations in the upper 6 m (20 ft) of the aquifer. Also provided in Table F.3.1.1 are the calculated maximum concentrations of the contaminants in K_d groups 1 and 2 in groundwater at five periods of interest ranging from 300 to 10,000 years from the present. The values presented in this table have been adjusted for their initial source concentration and represent calculated concentrations in the upper 6 m (20 ft) of the aquifer. Figures F.3.1.7 through F.3.1.20 are provided to illustrate the distribution of Tc-99, I-129, C-14, U-238, and nitrate in the unconfined aquifer at time frames from 300 through 2,500 years from the present. These figures represent calculated concentrations in the upper 6 m (20 ft) of the aquifer.

F.3.2 LONG-TERM MANAGEMENT ALTERNATIVE

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 in K_d groups 1 and 2, modeled as K_d equals zero and one, respectively, ultimately pass through the vadose zone and reach the groundwater in the underlying unconfined aquifer within the 10,000-year period of interest. Once in the aquifer, the contaminants in K_d group 1 move relatively quickly through the aquifer and discharge to the Columbia River. The results of contaminant transport modeling through the vadose zone and groundwater are discussed in the following sections.

F.3.2.1 Vadose Zone

The scenario for this alternative includes the following major assumptions:

• Infiltration is 5.0 cm/year (1.36E-04 m/day) initially and throughout the period of interest;
• Contaminant release for the five SST source areas is assumed to begin at the end of institutional control in the year 2095;
• Contaminant releases from the three DST source areas are assumed to begin 100 years after the end of institutional control in the year 2195; and
• The initial unit concentration assumed in modeling for K_d groups 1 and 2 (K_d equals zero and one) is 400,000 mg/L.

For K_d equals zero, the vadose modeling results predict contaminant first arrival at the vadose zone/groundwater interface at times varying from approximately 140 to 150 years for the SSTs and from approximately 230 to 250 years for the DSTs (Figure F.3.2.1). The difference between the first arrival times for the two tank types corresponds well to the release scenario assumed for SSTs and DSTs. Peak concentration at the vadose zone/groundwater interface is reached at times varying from approximately 210 to 350 years.

Contaminant concentrations from four of the five SST source areas (1WSS, 2WSS, 1ESS, and 4ESS) reach or nearly reach steady-state and the maximum possible (400,000 mg/L) concentration. The vadose zone in the 200 West Area is generally thinner by 5 to 20 m (16 to 66 ft), compared to that in the 200 East Area. The flatter shape of the peak of the time and concentration curves for the 200 West Area sites compared to the 200 East Area sites indicate that peak concentrations calculated at the groundwater-vadose zone interface are relatively sensitive to vadose-zone thickness. The fifth SST source area, 2ESS, is located in the 200 East Area. First arrival of contaminants at the water table from this source area is similar to that from the other SST sources, but the peak concentration is much lower at approximately 28,000 mg/L. This occurs because the contaminant mass and corresponding release period (Table F.2.2.12) for the 2ESS source area is generally one or more orders of magnitude less than the other source areas.

For K_d equals one, the vadose modeling results predict contaminant first arrival at the groundwater at times varying from approximately 1,020 to 1,470 years (Figure F.3.2.2). The time lag between first arrival of contaminants from SST source areas compared to DST source areas that was observed for K_d equals zero is not apparent for the K_d equals one simulations. This lack of contrast occurs because as the K_d increases, contaminant transport becomes increasingly more sensitive to the distance of travel (i.e., vadose zone thickness). This is illustrated by comparing the average time of first arrival at the groundwater between sources in the 200 Areas. The average time of first arrival to groundwater for the three source areas in the 200 West Area is approximately 1,290 years while the average time of first arrival for the five source areas in the 200 East Area is approximately 1,180 years. The longer average time to first arrival to groundwater for source areas in the 200 West Area is consistent with the thicker vadose zone in the 200 West Area. Another observation apparent from the vadose modeling is that as the K_d increases, peak concentrations in groundwater decrease and duration increases for the period from first arrival until contaminant concentrations decrease back to zero. This is readily observed by comparing Figures F.3.2.1 and F.3.2.2.

For contaminant groups three and four (K_d equals 10 and 50), first arrival occurs beyond the 10,000-year period of interest. For this reason, modeling results are not reported for these K_d groups.

F.3.2.2 Groundwater

Contaminants in K_d groups 1 and 2, modeled as K_d equals zero and one, respectively, are calculated to reach the groundwater in the unconfined aquifer within the period of interest. In the following discussion, nitrate representing a contaminant with a K_d equal to zero, and bismuth, representing a contaminant with a K_d equal to one, are used to illustrate general groundwater flow and contaminant transport in the unconfined aquifer. At the end of this subsection, additional isoconcentration maps are provided for Tc-99, I-129, C-14, U-238, and nitrate for 300 to 2,500 years from the present. These maps are provided because these contaminants exceed drinking water standards or human health advisories or have the potential to create substantial human health risk from groundwater use onsite.

Figure F.3.2.3 presents the calculated nitrate distribution in the groundwater at 300 years from the present. Nitrate has an assumed value of K_d equal to zero and thus moves at the velocity of groundwater. The time versus calculated concentration of nitrate at selected locations within the aquifer are provided in Figure F.3.2.4. Figure F.3.2.4 indicates that nitrate has moved through the groundwater system (i.e., nitrate concentrations in groundwater have fallen to approximately zero) prior to approximately 900 years from the present. The nitrate concentrations shown in Figures F.3.2.3 and F.3.2.4 have been adjusted for an assumed initial source concentration of 360,000 mg/L of nitrate and represent calculated concentrations in the upper 6 m (20 ft) of the aquifer.

Figure F.3.2.5 provides the calculated distribution of bismuth in the groundwater at 5,000 years from present. Bismuth is in K_d group 2 (modeled at K_d equals one). Bismuth moves through the groundwater system at a much slower velocity than nitrate, as illustrated in Figure F.3.2.6. This figure shows that for the selected observation nodes within the aquifer, it takes bismuth approximately 7,500 years from first arrival until its concentration drops back to nearly zero. Time versus concentration for observation nodes 13767 and 23585 on Figure F.3.2.6 exhibit a bimodal pattern. Observation nodes 13767 and 23585 are located along the Columbia River approximately due east of the 200 Areas and southeast of the 200 Areas, respectively (Figure F.3.2.5). Both of these locations receive contaminants from tank sources in the 200 Areas. The bimodal pattern is due to contaminants first reaching the Columbia River from the 200 East Area sources and then followed by contaminants from the 200 West Area sources. The other two observation nodes (25647 and 29076) do not exhibit the bimodal pattern because the primary source of contaminants to these points originates from the 200 West Area sources only. Observation nodes 25647 and 29076 are located between Gable Butte and Gable Mountain and along the Columbia River near the 100 North Area, respectively. The bismuth concentrations shown in Figures F.3.2.5 and F.3.2.6 have been adjusted for their initial source concentrations shown on Table F.2.2.11 and represent calculated concentrations in the upper 6 m (20 ft) of the aquifer.

Maximum contaminant concentrations in the groundwater for each of the contaminants in K_d groups one and two are provided in Table F.3.2.1 for 300, 500, 2,500, 5,000, and 10,000 years from 1995. The values presented in this table have been adjusted for their initial source concentration and represent calculated concentrations in the upper 6 m (20 ft) of the aquifer. These selected times represent the times of concern for the risk assessment (Volume Three, Appendix D). Figures F.3.2.7 through F.3.2.20 are provided to illustrate the distribution of Tc-99, I-129, C-14, U-238, and nitrate in the unconfined aquifer at time frames from 300 through 2,500 years from the present.

F.3.3 IN SITU FILL AND CAP ALTERNATIVE

Under the In Situ Fill and Cap alternative, the complete inventory from the 177 tanks would be released into the vadose zone. Only the most mobile contaminants, those modeled as K_d equals zero, are calculated to reach the groundwater within the period of interest. The contaminant source is the same as for the No Action and Long-Term Management alternatives. The major difference between these alternatives is that a Hanford Barrier would be constructed over the tanks in the In Situ Fill and Cap alternative, which would result in a lower infiltration rate and mass flux to the vadose zone. Also, the tanks would be filled with sand and gravel to structurally stabilize the domes. Once in the aquifer, the contaminants move relatively quickly and then discharge to the Columbia River. Peak groundwater concentrations in the aquifer would be at least an order of magnitude lower than those calculated for the Long-Term Management alternative, primarily as a result of a lower infiltration rate due to the Hanford Barrier. The results of contaminant transport modeling through the vadose zone and groundwater are discussed in the following sections.

F.3.3.1 Vadose Zone

The scenario for this alternative includes the following major assumptions:

• The initial vadose zone flow field is based on an infiltration rate of 5.0 cm/year(1.36E-04 m/day);
• The infiltration rate is assumed to decrease to 0.5 cm/year (1.36E-05 m/day)in response to Hanford Site activities and decreases again to 0.05 cm/year (1.36E-06 m/day)after the Hanford Barrier is installed; the Hanford Barrier is assumed to lose integrity 1,000 years later, causing infiltration to increase to 0.1 cm/year (2.74E-06 m/day) throughout the remainder of the 10,000-year period of interest;
• Contaminant release for the eight tank source areas is assumed to begin 500 years after the Hanford Barrier is installed (NRC 1994);
• The initial unit concentration assumed in modeling is 400,000 mg/L; and
• The initial contaminant inventory and concentrations are the same as for the No Action alternative.

Contaminant first arrival at the vadose zone and groundwater interface is calculated to occur at times varying from approximately 2,330 to 3,380 years (Figure F.3.3.1). Peak concentration at the vadose zone and groundwater interface is reached at times varying from approximately 4,080 to 6,300 years. This alternative, compared to the No Action and Long-Term Management alternatives, has a much longer calculated time to first arrival and peak concentration at the vadose zone and groundwater interface due to the lower infiltration rate through the Hanford Barrier. The calculated peak concentration for each of the eight source areas at the vadose zone and groundwater interface is of a similar magnitude to that calculated for the Long-Term Management alternative. As with the No Action alternative, contaminant levels reach or nearly reach steady-state conditions with maximum concentrations of 400,000 mg/L for all source areas except site 2ESS.

F.3.3.2 Groundwater

Contaminants in K_d group 1 are calculated to reach the groundwater of the unconfined aquifer within the period of interest. In the following discussion, nitrate representing a contaminant with a K_d equal to zero is used to illustrate general groundwater flow and contaminant transport in the unconfined aquifer. At the end of this subsection, addition isoconcentration maps are provided for Tc-99, I-129, C-14, U-238, and nitrate at 2,500 through 10,000 years from the present.

Figure F.3.3.2 presents the calculated nitrate distribution in the groundwater at 5,000 years from the present. Nitrate has an assumed value of K_d equal to zero and thus moves at the velocity of groundwater. The time versus concentration of nitrate at selected observation nodes is provided in Figure F.3.3.3. Figure F.3.3.3 indicates that nitrate concentrations reach peak concentration at approximately 5,500 years and continue at those concentration levels for approximately 1,500 years for nodes 13767 and 23585. For nodes 25647 and 29076, peak concentration is reached at about 5,000 years and continues for approximately 3,000 years. This is because nodes 25647 and 29076 would receive contaminants in groundwater from the 200 West Area sources only, and the average longevity of contaminant release into the vadose zone is approximately twice as long for the 200 West Area sites. Table F.2.2.13 provides release duration and mass for this alternative. The nitrate concentrations shown in Figures F.3.3.2 and F.3.3.3 have been adjusted for an assumed initial source concentration of 360,000 mg/L of nitrate and represent calculated concentrations in the upper 6 m (20 ft) of the aquifer.

Contaminants have not yet reached groundwater from the sources from earlier time periods of interest (e.g., 300 and 500 years from the present). Maximum contaminant concentrations in the groundwater for each of the contaminants in K_d group 1 are provided in Table F.3.3.1 for 2,500, 5,000, and 10,000 years from 1995. Figures F.3.3.4 through F.3.3.17 are provided to illustrate the distribution of Tc-99, I-129, C-14, U-238, and nitrate in the unconfined aquifer at time frames from 2,500 through 10,000 years from the present.

The sensitivity of the In Situ Fill and Cap alternative to changing the operating infiltration rate through the Hanford Barrier from 0.05 to 0.1 cm/year (1.36E-06 to 2.74E-06 m/day) after 500 rather than 1,000 years was evaluated. This assumes that the Hanford Barrier degrades after 500 years. Results of this parameter sensitivity analysis suggest that peak concentrations of contaminants in groundwater remains the same but occur slightly earlier for the 500 year Hanford Barrier. Additional discussion of the parameter sensitivity is provided in Section F.4.3.5.

F.3.4 IN SITU VITRIFICATION ALTERNATIVE

The In Situ Vitrification alternative results in the partial release of the initial inventory from the 177 tanks into the vadose zone over the period of interest. Not all of the tank waste is released over the 10,000 year period of interest because of the low glass corrosion rate coupled with a cap over the tanks. Only the most mobile contaminants, those modeled as K_d equal to zero, are calculated to reach the groundwater within the period of interest. Because the source is relatively large and the release rates are relatively low, contaminants are released at a constant concentration for thousands of years from each source area.

Once in the aquifer, the contaminants move relatively quickly and discharge to the Columbia River. Contaminant concentrations in the aquifer reach a constant level for much of the period of interest because of the long, constant concentration discharge of contaminants from the vadose zone.

F.3.4.1 Vadose Zone

The scenario for this alternative includes the following major assumptions:

• The initial vadose zone flow field is based on an infiltration rate of 5.0 cm/year (1.36E-04 m/day);
• In response to Hanford Site activities, the infiltration rate is assumed to decrease to 0.5 cm/year (1.36E-05 m/day) and then 0.05 cm/year (1.36E-06 m/day) after the Hanford Barrier has been installed. The Hanford Barrier is assumed to lose some integrity 1,000 years later, which would cause infiltration to increase to 0.1 cm/year (2.74E-06 m/day) throughout the remainder of the 10,000-year period of interest;
• Contaminant release for the eight tank source areas is assumed to begin 500 years after the Hanford Barrier is installed; and
• The initial unit concentration assumed in modeling is 400 mg/L.

Contaminant first arrival at the vadose zone and groundwater interface is calculated to occur at times varying from approximately 2,350 to 3,410 years (Figure F.3.4.1). Peak concentration at the vadose zone and groundwater interface reach steady-state conditions with a concentration of 400 mg/L between approximately 6,250 to 7,500 years from the present and remain at that concentration for the remainder of the period of interest. This alternative, compared to the No Action and Long-Term Management alternatives, has a longer calculated time to first arrival and peak concentration at the vadose zone and groundwater interface primarily because of the lower infiltration rate through the Hanford Barrier. The calculated peak concentration for each of the eight source areas at the vadose zone and groundwater interface would be lower. This is because the initial source concentrations are three orders of magnitude less than the source concentrations for the No Action and Long-Term Management alternatives.

F.3.4.2 Groundwater

Contaminants in K_d group one are calculated to reach the groundwater of the unconfined aquifer within the period of interest. The distribution of two contaminants, Tc-99 and U-238, in groundwater at selected time frames are provided to illustrate the impact of this alternative. Figures F.3.4.2 and F.3.4.3 provide the predicted distribution of Tc-99 and U-238 respectively for 5,000 years from the present. Variations in the distribution are due to variations in the inventory of each contaminant at the eight source areas. The time versus concentration of U-238 in the unconfined aquifer at selected locations is provided in Figure F.3.4.4 where U-238 can be observed to reach steady-state conditions at approximately 6,000 years. The U-238 concentrations actually drop slightly after approximately 6,000 years because of radioactive decay. A stable contaminant such as sodium would continue at its peak or steady-state concentrations beyond the 10,000 year period of interest.

Maximum contaminant concentrations in the groundwater for each of the contaminants in K_d group 1

(K_d = 0) are provided in Table F.3.4.1 for 5,000 and 10,000 years from 1995. Contaminants for earlier time periods (e.g., 300, 500, and 2,500 years) are not presented because they would not have reached groundwater within this time period. The predicted distribution of U-238 in the unconfined aquifer 10,000 years from the present is provided in Figure F.3.4.5. The U-238 and Tc-99 concentrations shown in Figures F.3.4.2 through F.3.4.5 have been adjusted for their assumed initial source concentration and represent the calculated concentrations in the upper 6 m (20 ft) of the unconfined aquifer.

F.3.5 EX SITU INTERMEDIATE SEPARATIONS ALTERNATIVE

The Ex Situ Intermediate Separations alternative would release contaminants to the vadose zone from the 177 tanks associated with retrieval operations (from SSTs only), residual waste left in the tanks (for all tanks), and releases from the LAW disposal facility. Only the most mobile contaminants, those modeled as K_d equal to zero, are calculated to reach the groundwater within the period of interest. Compared to the No Action and Long-Term Management alternatives, the mass of contaminants released from the tanks would be relatively small.

Once in the aquifer, the contaminants would move relatively quickly and discharge to the Columbia River. Contaminant concentrations from the tank source areas have a relatively sharp peak. Contaminant concentrations from the LAW disposal facility would reach peak concentrations in the groundwater approximately 6,610 years from the present and remain at their peak concentration for the remainder of the period of interest. The peak concentrations in the groundwater from the LAW facility are over two orders of magnitude less than the peaks associated with contaminants from the tank sources.

F.3.5.1 Vadose Zone

The scenario for this alternative includes the following major assumptions:

• The initial vadose zone flow field is based on an infiltration rate of 5.0 cm/year (1.36E-04 m/day) for tank source areas and the LAW source area;
• In response to remediation activities, the infiltration rate is assumed to decrease to 0.5 cm/year (1.36E-05 m/day)when retrieval activities start and to 0.05 cm/year (1.36E-06 m/day) after the Hanford Barrier is installed at tank source areas and the LAW source area. The Hanford Barrier is assumed to lose some integrity 1,000 years later, which would cause infiltration to increase to 0.1 cm/year (2.74E-06 m/day) throughout the remainder of the 10,000-year period of interest for tank sources, and the LAW disposal facility.
• Contaminant release for the five SST source areas is assumed to occur during two periods: first during retrieval when the infiltration rate is 0.5 cm/yr and then from residual materials 500 years after Hanford Barrier construction when the infiltration rate is 0.05 cm/yr (1.36E-06 m/day) (NRC 1994).
• Contaminant release for the three DST source areas is assumed to result from releases from residual material 500 years after barrier construction.
• Contaminant release for the LAW facility is assumed to begin 500 years after the Hanford Barrier is constructed over the vaults (NRC 1994).
• For the tank source areas the initial unit concentration calculated is 400,000 mg/L.
• For the LAW source area the initial unit concentration calculated is 100,000 mg/L.
• For the tank source areas the initial contaminant concentrations would be the same as for the No Action and Long-Term Management alternatives. For the LAW disposal facility the initial concentrations are provided in Table F.2.2.17.

Contaminant first arrival at the groundwater is calculated to occur at times varying from approximately 1,070 to 3,420 years from the tank source areas and 3,320 years from the LAW facility. The comparatively early arrival time of over 1,000 years is related to vadose zone migration of contaminants released during retrieval when the infiltration rate is relatively high (0.5 cm/ year [1.36E-05 m/day]). Concentration versus time at the vadose zone and groundwater interface for the unit contaminant releases from the eight source areas and the LAW facility are illustrated in Figure F.3.5.1. The initial source concentration for the eight source areas and the LAW facility are 400,000 and 100,000 mg/L, respectively. It was necessary to use a different constituent to represent vadose zone concentrations for the LAW disposal facility because nitrate is not present in the vitrified waste source.

Peak contaminant concentrations at the vadose zone and groundwater interface for the tank source areas would be reached at times varying from 3,630 to 5,110 years. Peak contaminant concentrations at the vadose zone and groundwater interface for the LAW facility would be reached at approximately 6,610 years and remain at that concentration for the remainder of the period of interest. Compared to the Long-Term Management alternative, this alternative has a much longer time to first arrival and peak contaminant concentrations at the vadose zone and groundwater interface primarily because of the lower infiltration rate through the Hanford Barrier and the low corrosion rate of the vitrified waste in the LAW facility.

The calculated peak concentration for each of the eight source areas at the vadose zone and groundwater interface is lower that for the No Action and Long-Term Management alternatives for K_d equals zero by approximately an order of magnitude. Contaminants in K_d groups 2 through 4, modeled as K_d equals 1.0, 10.0, and 50.0 mL/g, did not reach the groundwater within the period of interest.

F.3.5.2 Groundwater

Contaminants in K_d group 1 are calculated to reach the groundwater of the unconfined aquifer within the period of interest. Figure F.3.5.2 presents the calculated nitrate distribution in the groundwater from the tank sources 5,000 years from the present. Nitrate has an assumed K_d equal to zero and thus moves at the velocity of groundwater.

The time versus concentration of nitrate from the tank sources at selected observation nodes are provided in Figure F.3.5.3. Time versus concentration for observation nodes 13767 and 23585 on Figure F.3.5.3 exhibit a bimodal pattern. Both of these locations receive contaminants from tank sources in the 200 Areas. The bimodal pattern is due to contaminants reaching the Columbia River first from the 200 East Area sources followed by contaminants from the 200 West Area sources. The other two observation nodes (25647 and 29076) do not exhibit the bimodal pattern because the primary source of contaminants to these points originate only from the 200 West Area sources. Figure F.3.5.3 indicates that nitrate has moved completely through the groundwater system (i.e., nitrate concentrations in groundwater have fallen to approximately zero) prior to approximately 7,000 years from the present. The nitrate concentrations shown in Figures F.3.5.2 and F.3.5.3 have been adjusted for an assumed initial source concentration of 360,000 mg/L of nitrate and represent calculated concentrations in the upper 6 m (20 feet) of the aquifer.

The calculated concentrations of U-238 versus time in the unconfined aquifer for tank sources only and the LAW vault source at selected observation nodes are provided in Figures F.3.5.4 and F.3.5.5, respectively. Time versus concentration for observation nodes 13767 and 23585 on Figure F.3.5.5 indicates that U-238 concentrations reach steady-state conditions at approximately 7,000 years and continue at those concentration levels throughout the remainder of the time period of interest. Because the LAW burial facility is located in the 200 East Area, observation nodes 25647 and 29076 remain at a concentration of zero throughout the time period of interest. This is because groundwater does not flow from the 200 East Area towards these nodes. Figure F.3.5.6 provides the predicted distribution of U-238 in the unconfined aquifer at 5,000 years from the present from both tank and LAW vault sources combined. The distribution of this contaminant is much the same as calculated for nitrate at the same time frame. The contaminant concentrations have been adjusted for their assumed initial source concentrations, radioactive decay where applicable, and represent calculated concentrations in the upper 6 m (20 ft) of the aquifer.

Maximum contaminant concentrations in the groundwater for each of the contaminants in K_d group one from the tank sources are provided in Table F.3.5.1 for 2,500, 5,000, and 10,000 years from the present. Contaminants have not yet reached groundwater from the tank sources at earlier time periods of interest (e.g., 300, 500, and 2,500 years from the present). Maximum contaminant concentrations in the groundwater for each of the contaminants in K_d group 1 from the LAW disposal site are provided in Table F.3.5.2 for 5,000 and 10,000 years from the present. Contaminants have not yet reached groundwater from the LAW disposal sources at earlier time periods of interest.

F.3.6 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. Groundwater impacts would result from potential releases to the groundwater system 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 they 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 Ex Situ Intermediate Separations alternative. The calculated distribution of nitrate in the unconfined aquifer 5,000 years from the present is illustrated in Figure F.3.5.2. The calculated U-238 concentrations in groundwater from the tank sources at 5,000 years is illustrated in Figure F.3.6.1.

F.3.7 EX SITU EXTENSIVE SEPARATIONS ALTERNATIVE

This alternative is similar to the Ex Situ Intermediate Separations alternative, with the only difference being that a more extensive separations process would be used. 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 associated with the LAW vaults. A Hanford Barrier would be placed over the tanks and LAW vaults. Potential groundwater impacts would result from contaminant releases to the groundwater system 1) during tank waste retrieval; 2) from residuals in the tanks; and 3) from the LAW vaults. Groundwater impacts associated with retrieval and residual releases would be the same as for the Ex Situ Intermediate Separations alternative. This alternative would include extensive waste separation processes, but there would still be some contribution of U-238 from releases associated with the LAW vaults. Figure F.3.7.1 shows the calculated U-238 concentrations in groundwater 5,000 years from the present for both tank and vault sources.

F.3.8 EX SITU/IN SITU COMBINATION 1 ALTERNATIVE

The tank waste Ex Situ/In Situ Combination alternative would remediate 107 tanks in situ by filling and capping the tanks using the methods described under the In Situ Fill and Cap alternative. The waste in the remaining 70 tanks (60 SSTs and 10 DSTs) would be retrieved and treated using methods described under the Ex Situ Intermediate Separations alternative. The LAW from these tanks would be disposed of in a LAW vault. The HLW would be shipped to a potential geologic repository. As with both the In Situ Fill and Cap and Ex Situ Intermediate Separations alternatives, only the most mobile contaminants, those modeled as K_d equal to zero, are calculated to reach groundwater within the period of interest.

Once in the aquifer, the contaminants move relatively quickly through the aquifer and discharge to the Columbia River. Peak groundwater concentrations in the aquifer would be at least an order of magnitude lower than those calculated for the No Action alternative, primarily as a result of a lower infiltration rate due to the Hanford Barrier, which is constructed over the tanks remediated in situ and the LAW vault. The results of contaminant transport modeling through the vadose zone and groundwater are discussed in the following sections.

F.3.8.1 Vadose Zone

The two major components resulting in releases to the vadose zone are 1) tank sources from retrieval releases and releases from tanks remediated in situ; and 2) releases from the LAW vault. The scenarios for these components include all of the assumptions stated for the In Situ Fill and Cap and Ex Situ Intermediate Separations alternatives. For purposed of analysis, the residual that may be left in the tanks after retrieval (assumed to be 1 percent of the initial inventory from the retrieved tanks) is assumed to be additive to the inventory of tanks that are remediated in situ.

F.3.8.2 Groundwater

One of the objectives of this alterative is to reduce the number of tanks in which the waste is processed ex situ and yet achieve low calculated groundwater concentrations of the high-risk contaminants Tc-99, C-14, I-129, and uranium. These contaminants are all mobile and are in K_d group 1. They, along with several other contaminants in K_d group 1, are calculated to reach the groundwater in the unconfined aquifer within the period of interest. The distributions of Tc-99, C-14, I-129, and U-238 in the unconfined aquifer (U-238 being the most abundant of the tank waste uranium isotopes) are presented in this section for 5,000 and 10,000 years from the present. Although contaminant first arrival for tank sources occurs before 2,500 years, concentrations are approximately one order of magnitude lower than those predicted at 5,000 years. Therefore, contaminant distribution maps were not prepared for the 2,500-year period of interest.

Figures F.3.8.1 through F.3.8.3 present the calculated distributions of Tc-99, I-129, and U-238 in the groundwater at 5,000 years from the present from the tank sources remediated in situ. These calculated concentrations are from approximately 5 to 10 times lower than the concentrations calculated for the In Situ Fill and Cap alternatives. Concentrations of Tc-99 and U-238 for the LAW vault source at 5,000 years from the present are shown on Figures F.3.8.4 and F.3.8.5, respectively. Contaminant concentrations from the LAW vault source are 100 to 1,000 times lower than from tank sources. I-129 is not shown for the LAW vaults because it is not in the vault inventory.

Figures F.3.8.6 through F.3.8.8 present the calculated distributions of Tc-99, I-129, and U-238 in the groundwater at 10,000 years from the present for the tank sources remediated in situ. These calculated concentrations are from approximately 5 to 1,000 times lower than the concentrations calculated for the In Situ Fill and Cap alternative. Concentrations of Tc-99 and U-238 for the LAW vault source at 10,000 years from the present are shown on Figures F.3.8.9 and F.3.8.10, respectively. As with the 5,000 year time frame, contaminant concentrations from the LAW vault source are 100 to 1,000 times lower than from the tank sources. Maximum contaminant concentrations in the groundwater for each of the contaminants in K_d group 1 are provided in Tables F.3.8.1 through F.3.8.3 for the tank retrieval, in situ tank remediation, and LAW vaults contributions, respectively. The maximums calculated in these tables are not additive on a one-to-one basis because the maximums for the three components of the alternative occur at a different location within the unconfined aquifer.

F.3.9 EX SITU/IN SITU COMBINATION 2 ALTERNATIVE

One objective of this alternative is to further reduce the number of tanks in which the waste is processed ex situ and yet achieve a high retrieval of the long-term contributors to risk (i.e., C-14, I-129, Tc-99, and U-238). These contaminants are all mobile and are in K_d group 1.

F.3.9.1 Vadose Zone

Only the most mobile contaminants, those in K_d group 1 (K_d = 0) and include C-14, I-129, Tc-99, and U-238 are calculated to reach the groundwater in the unconfined aquifer within the period of interest. The distributions of Tc-99, I-129, and U-238 in the unconfined aquifer are presented in this section for 5,000 and 10,000 years from the present. Although contaminant first arrival for tank sources occurs before 2,500 years, concentrations are approximately one order of magnitude lower than those predicted at 5,000 years. Therefore, contaminant distribution maps were not prepared for the 2,500-year period of interest.

F.3.9.2 Groundwater

Figures F.3.9.1 through F.3.9.3 present the calculated distributions of Tc-99, I-129, and U-238 in the groundwater at 5,000 years from the present from the tank sources. These calculated concentrations would be at or slightly greater than those calculated for the Ex Situ/In Situ Combination 1 Alternative. As indicated by the note on these figures, the contaminants contributed by retrieval (ex situ) are very small. By retrieving from fewer (as compared to the Ex Situ/In Situ Combination 1 alternative) tanks in the Ex Situ/In Situ Combination 2 alternative, the retrieval contribution becomes less while the residual portion actually becomes greater. Concentrations of Tc-99 and U-238 for the LAW vault source at 5,000 years from the present shown on Figures F.3.9.4 and F.3.9.5, respectively. contaminant concentrations from the LAW vault source are 100 to 1,000 times lower than from tank sources. I-129 is not shown for the LAW vaults because it is not in the vault inventory.

Figures F.3.9.6 through F.3.9.8 present the calculated distributions of Tc-99, I-129, and U-238 in the groundwater at 10,000 years from the present for the tank sources. Similar to the situation at 5,000 years, the most significant portion of the contamination results from the tank remediated in Situ. Contributions from retrieval are minimal. Concentrations of Tc-99 and U-238 for the LAW vault source at 10,000 years from the present are shown on Figure F.3.9.9 and F.3.9.10, respectively.

Maximum contaminant concentrations in the groundwater for each of the contaminants in K_d group 1 are provided in Tables F.3.9.1 through F.3.9.3 for the tank retrieval, in situ tank remediation, and LAW vaults components, respectively.

F.3. 10 PHASED IMPLEMENTATION ALTERNATIVE

Phase 1

There are no groundwater impacts associated with the first phase of this alternative. Waste retrieval only occurs in the DSTs, and there are no releases assumed to come from these tanks. The retrieved waste is vitrified and shipped to an onsite repository.

Total Alternative

The contaminant concentrations for this alternative would be the same as those for the Ex Situ Intermediate Separations alternative, discussed in Section F.3.5. Maximum contaminant concentrations in the groundwater for each of the contaminants in K_d Group 1 from the tank sources are provided in Table 3. 10 .1 for 2,500, 5,000, and 10,000 years from the present. Contaminants have not yet reached groundwater from the tank sources at earlier time periods. Maximum contaminant concentrations in the groundwater for each of the contaminants in K_d Group 1 from the LAW disposal site are provided in Table F.3. 10 .2 for 5,000 and 10,000 years from the present. Contaminants have not yet reached groundwater from the LAW sources at earlier time periods.

F.3. 11 EFFLUENT TREATMENT FACILITY

The effects of disposal on groundwater were simulated as entering the uppermost aquifer beneath the SALDS at a projected rate of 568 L/min (150 gal/min) over an area of 8,350 m² (90,000 ft²) for 125 years. Tritium concentrations in the treated effluent entering the groundwater system were assumed to be 2.1E-05 Ci/L (21µCi/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 for groundwater movement beneath the SALDS. Groundwater flow directions resume their northeasterly regional flow direction at a point approximately 300 m (980 ft) downgradient of the disposal site. A residence time of 100 years for tritium in the uppermost aquifer was obtained as the travel time for tritium between the disposal site and the Columbia River. Maximum tritium concentrations at the riverbank prior to dilution in the Columbia River are calculated to be 1.4E-08 Ci/L, which is below the Federal drinking water standard of 2.0E-08 Ci/L (20,000 pCi/L) (Jacobs 1996).

F.3. 12 SUMMARY OF RESULTS

F.3.12.1 Observed Contamination Concentrations

Currently, hazardous chemicals and radionuclides at levels that exceed Federal drinking water standards are present in groundwater beneath the 200 Areas and in plumes emanating from the 200 Areas that are moving toward the Columbia River. Hazardous chemical contaminants observed to exceed drinking water standards include nitrates, cyanide, fluoride, Cr, chloroform, carbon tetrachloride, trichloroethylene, and tetrachloroethylene (Dresel et al. 1994). Radiological contaminants include I-129, tritium, Cs-137, Pu-239 and 240, Tc-99, and Sr-90. Generally, the groundwater beneath the 200 Areas is severely contaminated at levels that substantially exceed drinking water standards. For example, I-129 is present at levels that exceed standards by up to 20 times (Dresel et al. 1994). Groundwater-use restrictions have been implemented to prevent current and future uses of contaminated groundwater. Implementing any of the TWRS alternatives would add contaminants to the groundwater. However, peak concentrations from the alternatives would result in less risk than that derived from existing contaminant distributions in groundwater.

F.3.12.2 Calculated Contaminant Concentrations

Table F.3. 12 .1 compares the maximum calculated contaminant concentration in the groundwater for the alternatives. These calculated contaminant concentrations are for five representative contaminants at five selected times within the unconfined aquifer beneath the Hanford Site. Federal drinking water standards are provided as a basis of comparison. The contaminants shown on Table F.3. 12 .1 were selected as indicators, based on the criteria of 1) mobility in the environment; 2) persistence (e.g., long half-life); and 3) high human toxicity. Many other contaminants are calculated to be released for each alternative and this information is carried forward to the human health risk assessment (Section 5.11). The following observations are based on data presented in Table F.3.12 .1.

Calculated contaminant concentrations would be highest at 300 and 500 years for the No Action and Long-Term Management alternatives compared to other alternatives. The tank inventory would be released faster than any of the other alternatives because there would be no engineered barriers such as the Hanford Barrier to reduce infiltration, nor would there be any effort to stabilize the waste. For the these two alternatives, the maximum calculated contaminant concentrations would drop several orders of magnitude by 2,500 years because all K_d group 1 contaminants would have passed through the groundwater system. The contaminant concentrations would be lowest for these two alternatives at 5,000- and 10,000-years because most of the mass released from the tanks would have currently passed through the groundwater system and discharged into the Columbia River prior to 5,000 years from the present.

At 2,500 years from the present, contaminants in the groundwater associated with the all of the alternatives, except the Phased Implementation alternative, would be evident but at lower maximum calculated concentration (e.g., by at least by a factor of 100 for nitrate) compared to the No Action alternative and Long-Term Management alternative. At this point in time, the concentration of all of the contaminants in the K_d group 2 for the No Action and Long-Term Management alternatives would have peaked in groundwater, and contaminant concentrations would be dropping. Conversely, source concentrations from the In Situ Fill and Cap and the Ex Situ Intermediate Separations alternatives would still be increasing and would peak between 2,500 and 5,000 years from the present.

The earliest arrival of contaminants in the groundwater associated with the In Situ Vitrification alternative would be 2,500 years from the present, and would peak between 2,500 and 5,000 years from the present.

The levels of contaminant concentrations for all of the Ex Situ alternatives would be low at all times. There would be only slight exceedances of drinking water standards. Under all of the alternatives that include placing waste in onsite LAW vaults, the concentrations of contaminants in the groundwater would be within drinking water standards for the contaminants of concern.