GROUNDWATER AT WIPP 2005 EPA WIPP RECERTIFICATION FACT SHEET No. 5 What is Groundwater? Groundwater is water that exists under the Earth's surface. Depending on the types of rock present, groundwater can be stored in different quantities, and move at different speeds. In general, groundwater travels through dense rock exceptionally slowly, as slow as a few feet over hundreds and even thousands of years. Groundwater moves more quickly through porous rocks where cracks, or fractures, exist. Such rocks are said to be permeable to groundwater. Ground Surface Feet Meters 500- -200 1000- - 1500_ -400 2000_ -600 2500- -800 3000- - 3500- -1000 4000- -1200 4500- -1400 The Salado Formation contains thick layers of salt, thin layers of anhydrites, and some clay. The Salado Anhydrite units are located above arid below the zone where waste is being placed. (You may see them referred to as marker beds 138 and 139.) Even though the Salado has very low permeability to groundwater, it is still mod- eled (via the marker beds) as a potential pathway for radioactive releases. Cross section showing major geologic units above and below the WIPP repository. Both groundwater units were included in DOE's calcula- tions for the Certification Performance Assessment. Both units were found to have only very minor contributions to releases from the WIPP repository. This is due to the large amount of time it would take radionuclides to travel through them, and the limited amount of radionuclides expected to reach them. Dewey Lake >¦ Red Beds > Rustler Formation \ fagenta Salado, Formation I I I I I I.I I I I l\ Tamarisk -JLyjTjjM II " .^Ciilebra II 'I \ 1 11 I1iffy \ Los Medanos ]/ NJ I I I | | IK McNutt Potash Zone Castile Formation Bell Canyon Formation DOE scientists have studied groundwater flow and conditions at the WIPP site to determine potential pathways for radioactive releases. DOE's Certification Application identified two geologic units that are potential groundwater pathways for radioactive releases to the environ- ment: 1. The Culebra Dolomite unit in the Rustler Formation 2. The Salado Anhydrite units within the salt bed and close to the waste. Dockum Group \ WIPP Repository How Does Groundwater Affect WIPP? Although none of the geologic units above the sea repository are highly permeable, the most per- meable unit is the Culebra Dolomite. Because the Culebra is located over 1,000 feet above the WIPP repository, radioactive releases into this rock unit would only be possible if a borehole was drilled through the repository. United States Environmental Protection Agency I Office of Air and Radiation (6608J) I EPA 402-F-05-007 I June 2005 www.epa.gov/radiation/wipp ------- Monitoring of Groundwater at WIPP What's New in the Recertification Application? • As part of the overall monitoring plan for WIPP, DOE maintains an extensive groundwater monitoring program. • EPA conducts annual inspections of the DOE's ground- water monitoring program and receives and analyzes periodic reports on groundwater data from DOE. • Since 1998, DOE has reported overall increases in water levels in the wells that monitor the Culebra Dolomite unit. • DOE is drilling new wells to enhance monitoring of the Culebra Dolomite unit. • EPA has identified the need for DOE to increase under- standing of the hydrologic characteristics of the geologic units above the repository, such as the Magenta. Appearance of Water in the Exhaust Shaft In 1995 DOE observed water in the air exhaust shaft at a depth of about 80 feet below the ground surface. No water was observed in this zone when the shaft was cre- ated. The appearance of water in the exhaust shaft is of concern because it may impact air sampling activities. See Issue Paper #4, Air Monitoring at WIPP-Station A, for more information. The Recertification Application documents changes in the water levels of the Culebra Dolomite wells. These changes have been incorporated into groundwater modeling. Results show travel times for water flow that are two to three times longer than they were estimated to be in 1996. These changes will not significantly affect release levels. See Chapters 2 and 6 of the Recertification Application for more information on groundwater. EPA will focus on the changes and updates listed above during their technical review of the Recertification Applica- tion. EPA will also conduct independent modeling to verify DOE's conclusions about groundwater modeling. DOE conducted an investigation into the source of the water by drilling 15 wells around the site surface facili- ties and monitoring the occurrence of water. One of the 15 wells was found to be dry, while water was encountered at 50-60 feet below ground surface in the other 14 wells. The highest water levels in the test wells were near the salt stor- age evaporation pond. Water from this area flows outward to other areas, including the exhaust shaft. DOE believes that the source of the water is from two sources: 1. Runoff of rainfall into and infiltration from the retention ponds located to the south of the WIPP surface facili- ties, and 2. Infiltration of water from the salt storage area, the salt storage evaporation pond, or remnants of the drilling and tailings pit used during the construction of the WIPP salt shaft. DOE is nearing completion on a project to line all reten- tion ponds to control water infiltration. This will cut off the source of the water to the air exhaust shaft. Fact Sheets in this Series: Recertification Overview Fact Sheet No. 1 Public Involvement Fact Sheet No. 2 Performance Assessment Fact Sheet No. 3 TRU Waste Inventory Fact Sheet No. 4 Groundwater Fact Sheet No. 5 Karst Fact Sheet No. 6 United States Environmental Protection Agency I Office of Air and Radiation (6608J) I EPA 402-F-05-007 I June 2005 www.epa.gov/radiation/wipp ------- |