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Hydrogeologic framework of the Big Lost River Basin, south-central Idaho, chap. A of Zinsser, L.M., ed., Characterization of water resources in the Big Lost River Basin, south-central Idaho

Surface-water and groundwater resources in the Big Lost River Basin of south-central Idaho are extensively interconnected; this interchange affects and is affected by waterresource management for irrigated agriculture and other uses in the basin. Concerns from water users regarding declining groundwater levels, declining streamflows, and drought helped motivate an updated evaluation of water resources in the Big Lost River Basin. The hydrogeologic framework presented in this report provides a conceptual basis for understanding groundwater resources in the Big Lost River Basin and comprises three major parts: (1) conceptual description of four hydrogeologic units, (2) development of a three-dimensional hydrogeologic framework model representing the spatial distribution of the hydrogeologic units, and (3) a description of groundwater occurrence and movement. This hydrogeologic framework represents the first of three planned reports describing water resources in the Big Lost River Basin; subsequent reports are intended to present a groundwater budget for the basin and to describe the results of a series of events measuring gains to and losses from streamflow in the Big Lost River. This report was prepared by the U.S. Geological Survey in cooperation with the Idaho Department of Water Resources. The Big Lost River Basin has four hydrogeologic units. First, the most heavily used hydrogeologic unit is the Quaternary unconsolidated sediments, which are further subdivided by grain size or other distinguishing characteristics into six subunits (boulders, gravel, sand, clay, cemented sediments, and soil), and which comprise the alluvial aquifer. Second, Paleozoic sedimentary rocks, primarily carbonates with some siliciclastic rocks, represent the major bedrock aquifer and contribute subsurface recharge at the margins of the alluvial aquifer. Third, Tertiary volcanic rocks, primarily andesite and dacite with lesser tuff, are locally important to water production, particularly in faulted and fractured zones. Fourth, Quaternary basalt rocks contain at least three water-bearing zones at the southern end of the Big Lost River Valley. The three-dimensional hydrogeologic framework model describes the spatial distribution and general thicknesses of the hydrogeologic units and subunits and yields new insights into controls on water movement in the basin. The Quaternary unconsolidated sediments hydrogeologic unit that comprises the basin-fill alluvial aquifer is highly spatially heterogeneous. The Paleozoic sedimentary rocks hydrogeologic unit occurs at the valley margins and underlies tributaries throughout the basin, whereas the Tertiary volcanic rocks hydrogeologic unit primarily occurs in uplands in the western half of the basin. The Quaternary basalt hydrogeologic unit consists of multiple basalt flows that are interbedded with the Quaternary unconsolidated sediments unit in the southern end of the Big Lost River Valley. Subunits within the Quaternary unconsolidated sediments unit generally are coarse (sands and gravels) and well-sorted with high hydraulic conductivity along the central axis of the Big Lost River Valley. Quaternary unconsolidated sediments subunits at the margins of the valley tend to be more vertically discontinuous and contain more clay, which creates locally confining layers in some areas. A laterally continuous clay subunit is present about 50 to 100 feet below the land surface around the town of Arco, Idaho, and causes confining conditions in some areas. Groundwater flow generally mimics topography although wide spacing between long-term groundwater-level monitoring wells limits detailed interpretation. The alluvial aquifer is most heavily used (primarily by irrigation and domestic pumping) within 250 feet of the land surface although deeper bedrock wells occur at the valley margins and in the basalt flows around Arco. Potentiometric-surface altitudes vary substantially interannually and seasonally between Darlington, Idaho, and Arco; annual precipitation, irrigation demand, and recharge can result in raising or lowering of potentiometric surface altitudes over the irrigation season and likely affect streamflow in the Big Lost River. Historical surface-water losses occur in areas where the valley widens and substrates coarsen; historical surface-water gains occur in areas where the valley narrows, streamflow and irrigation recharge the alluvial aquifer, and confining layers occur in the subsurface. Vertical hydraulic connectivity between multiple waterbearing zones in the southern end of the Big Lost River Valley is unclear but continued monitoring of wells at multiple depths and additional aquifer tests could help improve understanding of hydraulic connectivity in this area. In summary, this hydrogeologic framework provides an updated conceptual model for groundwater resources and a three-dimensional, data-derived hydrogeologic framework model used as a tool to represent the hydrogeologic units and subunits in the basin. The model shows the spatial distribution 2 Hydrogeologic Framework of the Big Lost River Basin, South-Central Idaho of hydrogeologic units and subunits and, combined with groundwater and surface-water data, illustrates their effect on groundwater movement and surface-water/groundwater interactions throughout the basin. Insights gained from this updated hydrogeologic framework will help inform current waterresource management in the Big Lost River Basin and could also be used to develop a potential future groundwater model.