2009-10-16_PERMIT FILE - M2009082 (11)

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ENVIRONMENTAL GEOLOGY No. 6-G ,7'30' EXPLANATION Generalizod probable yields in gallons per minute <25 tf-x.00 f00 - 1000 >1000 dtij3 DISCUSSION The yields of water wells shown on this rap are approximate. These yields may vary substantially as a result of local changes in aquifer characteristics and the coy>struction of the well. Although ground water is available throughout most of the study area, the yields will differ significantly depending on the geologic unit. The shallow alluvial (unconfined) aquifers, found along the major stream courses, yield the largest quantities of water (up to 2000 qpm), and the bed- rock (confined) aquifers yield smaller quantities (generally 0 to 15 gpm). The potential yield of a wolf tapping an unconfined aquifer depends on the hydraulic conductivity (the ability to transmit water) and the saturated thickness of the aquifer. As a result, the highest yields are obtained from the saturated, highly permeable, thick sand and gravel deposits in the major valley fills. Yields generally decrease as the distance from the major streams increases. Those lower yields are caused by a decrease in saturated thickness and, to a lesser degree, a decrease in hydraulic conductivity. Similarly, potential yields from bedrock aquifers depend on hydraulic condur.- tivity, thickness of aquifer, and hydraulic gradient. In general terms this means that a sandstone will be a better aquifer than a shale due to the greater capacity of the sandstone to transmit water. The recharge regime for unconfined and confined aquifers differs as do many of their other hydrologic characteristics. Unconfined aquifers are recharged vertically from the ground surface and are thus directly linked to surface hydrologic conditions. In the study area, the alluvial aquifers respond very quickly to seasonal variations of surface water and to precipitation, with the most important change apixirently related to the irrigation season. For example, the water table rises markedly during the summer months as a result of seepage from irrigation ditches and of the Irrigation of cropland. Bedrock aquifers obtain water in two ways. First, where the bedrock units crop out west of the study area, water percolates directly into the bedrock. Second, water percolates through the overlying material ovor /arge:3rcas. Th_eratorw, rec.harce to confined aquifers occurs very slowly with no short-term, direct link to the surface water conditions. REFERENCES Briscoe, 11. J., Jr., 1972, Stratigraphy and aquifer characteristics of the Fox Hills sandstone in the Greeley area, Weld County, Colorado: Colorado School xulc•s, master Sc•l- Thesis T-1456, 78 p., 7 pls. Colorado Division of Water Resources, 1971, Well data: Colorado Div. Water Resources unpub. data. Hershey, L. A., and Schneider, P. A., Jr., 1964, Ground-water inves-