The U.S. Bureau of Land Management recognized a need for baseline hydrologic data and an understanding of the hydrologic system in the L.C. Holding coal-lease tract in order to determine the potential effects of mining on the water resources of the area. The potential impacts of mining on the hydrology of the area are of concern because Zion National Park is less than 3 miles from the westernmost part of the lease tract. Much of the water that passes through the Park either originates in the lease tract or traverses it. Runoff from melting snowpacks and summer thundershowers contributes most of the flow to perennial streams in the area. Base flow is sustained by spring discharge and diffuse seepage. Regional groundwater movement is southward. Most of the geologic formations in the study area contain aquifers. The water table of the regional aquifer is about 870 ft deep in the Navajo Sandstone. Groundwater issuing from the Navajo Sandstone on the east side of Zion Canyon has specific conductance values several times larger than groundwater from the west side, indicating recharge to the Navajo from the overlying strata, which contain water of larger specific conductance. Potential effects of mining in the area include: (1) increased dissolved-solids concen- trations and decreased pH values in both surface and groundwater; (2) dewatering of aquifers, causing a decrease or cessation of flow to some springs; and (3) land subsidence and associated subsidence fractures.
