F.1.0 INTRODUCTION
This appendix describes the approach and results of the groundwater impact analysis for the Tank Waste Remediation System (TWRS) Environmental Impact Statement (EIS) alternatives. The purpose of groundwater modeling is to calculate concentrations of contaminants in groundwater from the various TWRS EIS alternatives. Using these calculations, potential human health risk and ecological risk can be estimated and compared between the alternatives. The potential sources of groundwater contamination are the waste tanks, the proposed low-activity waste (LAW) disposal facility, the proposed cesium-137 (Cs-137) and strontium-90 (Sr-90) capsule disposal facility, and the Effluent Treatment Facility (ETF). The groundwater exposure pathway is downward through the vadose zone underlying the potential sources; and laterally through the unconfined aquifer immediately underlying the vadose zone to the Columbia River. The tanks, LAW disposal facility, and Cs-137 and Sr-90 capsule storage areas are all located on the 200 Area Plateau (Figure F.1.0.1). The groundwater assessment has been performed using a combination of screening techniques and numerical modeling. The groundwater modeling results predict contaminant concentrations in the groundwater associated with selected alternatives from the present to 10,000 years from the present. The groundwater assessments provided in this appendix required several assumptions to address uncertainties. The major assumptions and uncertainties are 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 are associated with the following:
- 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 and vadose zone thickness due to climate change;
- Uncertainty in vadose zone transport due to use of one-dimensional flow and transport simulation; and
- Uncertainty due to calculation of releases during retrieval.
A discussion of these major assumptions and uncertainties and results of a limited parameter sensitive analysis are provided in Volume Five, Appendix K.
The modeling results were used to predict human risk and ecological risk associated with each modeled alternative (Volume Three, Appendix D). Calculated concentrations of five indicator contaminants are compared with drinking water standards. Contaminants that may have been previously released from existing facilities, such as the tanks, were not considered in this assessment because they are not within the scope of this EIS. The physical area of potential groundwater impacts, also called the area of interest (AOI), is the unconfined aquifer bounded laterally by the Rattlesnake Hills in the west and southwest, by the Columbia River in the north and east, and by the Yakima River to the south (Figure F.1.0.2).
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