Important Geologic Water Bering Units
To establish a better understanding of the hyfrology in Boyd County and around the LLRW site specifically, an explaination of geologic units and their water-bering properties is necessary.
The deepest formation, the Dakota Sandstone, underlies the entire county and is a source of water to some wells. The qualilty of this water is unsuitable for domestic use, but is used for livestock purposes. The Greenhorn Limestone, Graneros Shale, and the Carlile Shale overlie the Dakota Sandstone, but do not yield water. The Cordell Sandstone Member lies above th Carlile Shale, and yields water to only the eastern third of the county. It is suitable for livestock and domestic use. The Niobrara Formation is not recognized as a water source to wells throughout the county, but the Missouiri River Valley prooduces significant yields. This is due to fractures and solution channels produced by weathering of the exposed formation. The Pierre Shale overlies the Niobrara which limits this water source availability to where presently exposed. These water sources are at depths from 600 ft. to 1200 ft. below the surface.
The thickest formation found throughout the county is the Pierre Shale. This layer is above the aformentioned formations and is approximately 800 ft. thick. Due to its high impermeability, the Pierre Shale prevents water from passing through it. The LLRW site ws chosen in part, because of the shale thickness and impermeable characteristics. The shale would prevent contamination from the LLRW site to lower water sources.
Above the Pierre Shale lies silt beds of Miocene age and the
Ogallala Formation, occurring on uplands in the western, central
and northeastern parts of the county. A layer of undifferentiated
clay, silt, sand and gravel overlie the Ogallala, and occur over
the entire county except on steep slopes and escarpments. Undifferentiated
deposits have high infiltration and permeability and can be a
good sources of water. These layers contain high water quality
for livestock and domestic use, but low yields limit its use.
Therefore, the water is generally used for livestock.
The LLRW site is located within the Ponca Creek drainage basin where undifferentiated deposits occur. This layer is referred to as the Contact Zone according to the Draft Saftey Evaluation Report (DSER), and ranges from 10-40 ft. thick. According to title 194 of the DSER, requires that the proposed disposal site provide sufficient depth to the water table that groundwater intrusion will not occur. U.S. Ecology responded that nearly all groundwater has been supplied by precipitation falling upon the site, and that recharge to the water table is significant only during major precipitation events. In very wet years, based on hydrographs of 1992 and 1993, the water table remains near the surface, and recedes quickly in response to decreased precipitation. Vernon Sauders report of the hydrology of Boyd County compares seasonal precipitation vs. low seasonal runoff as an indication that during the summer most precipitation is lost to evapotranspiration and what is not lost to the atmosphere is absorbed and extracted by vegetation. Intense, short phases of rainfall might saturate the upper surface materials, but evapotranspiration is rapid when this type of precipitation occurs and would quickly remove excess water. Dr. Stewart Taylor's testimony in (court decision), stated that groundwater was 8-10 ft. below the surface 43% of the time annually, 6-8 ft. below the surface 34% of the time annually, and 2-4 ft. below the surface only 6% of the time annually. The average depth to groundwater was 10-12 ft. 8% of the time annually and no average depths of 0-2 ft. below the surface occurred.
Although yields are very low, the water-bering deposits above the Pierre Shale are considered shallow aquifers. In the upland areas, these aquifers are very thin, but change rapidly in short distances due to low fill places on the suface. The water table is not continuous throughout Boyd County, but saturation is usually found within the undifferentiated silt, sand and gravel beds. The Ogallala Aquifer which encompasses moat to the state of Nebraska does not exist in Boyd County.
The most likely water pathway from the proposed site is Ponca Creek. It is located 2.5 miles north and 150 ft. below the site location. Ponca Creek and its tributaries drain about 62% of Boyd County. As previously mentioned, the site would have been built on the Ponca Creek drainage basin. The drainge area of Ponca Creek consists mainly of the Pierre Shale, and its flow is a result of overland runoff and negligibly from groundwater seepage (Sauders 1974). Title 194 of the DSER, require that the upstream drainage areas be minimized. The DSER reported that the LLRW Program had a concern regarding why a drainage basin analysis had not been performed for the northeastern portion of the original 320-acre site. When the site area was reduced, this was no longer a concern.
The water pathways can be obvious at times of large precipitation
events, where the small amount of runoff discharges into shallow
upland depressions or is impounded in stock ponds around the upland
margin (Sauders 1974). These small areas of ponded water might
be inferred as "wetlands".