Hydrogeologic framework of the Boise Aquifer system, southwestern Idaho

The City of Boise relies upon the underlying groundwater resource (38,000 acre-feet/year) for 90% of its public water-supply. Basin-fill sediments which comprise this system of aquifers are divisible into five distinct hydrogeologic settings which differ on the basis of sediment type, geophysical log character, and hydraulic properties. A large buried alluvial-fan/fan-delta complex (the Boise Fan) occupies the eastern head of the basin. Down-valley gradations in sediment type show a general increase in unit thickness and sediment color more typically gray; reflecting transition to the lake environment of deposition. Basinward (northwestward), the ancient fan materials grade into lake/fan transitional sediments which grade to predominantly lake sediment which grade to gray mudstones and fine sand layers of the deep lake environment. The depth to which drinking-water aquifers extend is limited by an underlying sequence of relatively impermeable volcanic rocks. Specific capacities of efficient wells, 400--1,200 feet deep and open to 80--100 feet of sand are highest in the lake/fan transition and the lacustrine aquifers of central Boise, lowest for the Boise Fan and intermediate for the deep artesian sand aquifers of west Boise. As a result of screen and filter-pack design based upon attention to sampling drill cuttings, sieve analysis of sands,more » and geophysical log location of aquifers, efficiency and productivity of new wells has been greatly increased. Groundwater recharge to the deeper aquifers is via permeable surface gravels. Increased groundwater withdrawals have possibly accelerated recharge by increasing vertical hydraulic gradients. Overbored wells with continuous surface-to-depth gravel packs, wells open to multiple aquifers, and improperly abandoned wells with deteriorating casing are also conduits for polluted shallow groundwater to enter the deeper aquifers.« less