Water runoff from valleys to the north follows drainages such as the Big Lost River, Little Lost River and Birch Creek to enter an extensive groundwater system beneath the INEEL. The unsaturated zone and aquifer are composed of a 500-m thick sequence of discontinuous volcanic and sedimentary deposits of Pleistocene age. Basalt lava is porous along fracture systems and in rubble zones between flows. The geologically young basalt of the aquifer has filled up ancestral canyons of the Snake River and tributaries, causing the water to seep underground rather than flowing through canyons (canyons would be cut if flow velocity and volume increased, either due to lowering of the base level or increase in rainfall). Individual basalt lava flows in the aquifer are characterized by large zones of irregular fractures and voids creating high hydraulic conductivities.
Stratigraphic units underlying the INEEL include mainly basalt and sediment. Basalt units, which make up about 85 percent of the volume of deposits in most areas, include individual flows, flow groups, and supergroups (Lanphere et al., 1994; Anderson et al., 1996; Welhan et al., 1997; Wetmore et al., 1997). A basalt flow group is a sequence of basalt flows such as the Hells Half Acre and Wapi lava fields that erupted within a relatively short period of time from a single fissure or series of vents related to a common magmatic system (Kuntz et al., 1980, 1994). These units represent eruption episodes that lasted from days to years. A basalt supergroup is a group of flows that cannot be separated into individual flow groups on the basis of available well data, usually because most wells involved were not cored for paleomagnetic, petrographic and geochemical studies. These units represent multiple eruptive events over thousands of years.
The upper surface of the aquifer has flat and steep areas, like terraces and steps. The upper surface is steep, from 6,200 to 4800 feet from Island Park to Ashton, then flat to Mud Lake, then steep from 4750 to 4600 feet at the eastern edge of the INEEL. Then it is quite flat across much of the INEEL, at elevation about 4500 feet. There is a drop near the Great Rift in the American Falls area (from 4400 to 4150 ft), a flat through much of Minidoka County, and a final step, from 4050 ft to 3100 ft, from about Burley west to Hagerman.Irrigation water that is diverted from the Snake and not transpired by crops seeps back into the aquifer, and increases the flow from the springs. The flow from springs in the Snake River Plain aquifer increased from about 4,000 cubic feet per second in 1902 to a maximum of 7,000 cubic feet per second in 1955, and then declined to between 6000 cubic feet per second with the drilling of deep wells on the Snake River Plain, starting in 1960. The more water that leaks out of canals in the Magic Valley, the more pressure is in the aquifer system. Some irrigation is from ground water via wells, and this depletes the aquifer, at least by the amount that is evaporated or transpired by plants.
Liquid-waste disposal has resulted in detectable concentrations of several waste constituents in water in the Snake River Plain aquifer underlying the INEEL (Barraclough and Jensen, 1976; Bartholomay et al., 1995). Detailed stratigraphic studies using outcrops, cores, and geophysical logs are being conducted to evaluate the relations between the geologic framework and the movement of water and waste in the unsaturated zone and aquifer.
Complex controls by the basalt/sediment stratigraphy on the distribution of waste plumes have been identified near injection wells and waste ponds at the INEEL. Packer tests indicate stratigraphic control of an injected tritium plume in basalt flows of contrasting thickness at the Idaho Chemical Processing Plant (Morin et al., 1993; Frederick and Johnson, 1996). The formation of perched groundwater zones at the Test Reactor Area is controlled by the distribution of basalt and thick sediment layers beneath waste ponds (Cecil et al., 1991). The distribution of basalt shields and inferred dikes associated with a volcanic rift zone may control an injected plume of trichloroethylene that is perpendicular to regional groundwater flow directions at Test Area North (Hughes et al., 1997a).
Overall however, the aquifer is quite clean, basically because it is large and the amount of pollution is small. Agricultural sources of pollution are the biggest problem. There is no evidence that any leakage from the INEEL has damaged the quality of the aquifer in any real way. The traces of radionuclides present in the aquifer on the INEEL are diluted beyond detection down gradient to the southwest. With the cessation of injection of waste back into the aquifer in about 1975, the plume of Tritium, that is radiogenic hydrogen, which formerly extended off the INEEL near the Big Southern Butte, has retreated so it is almost totally within the INEEL now. This pollution business is, however, a political issue.