Hydrogeologic Framework of the Boise Aquifer System Ada County, Idaho
The city of Boise relies upon the underlying groundwater resource for 90 percent of its public water-supply. The cold-water aquifers are saturated sedimentary deposits of rivers and lakes that existed 9-to-2 million years ago. The 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 head of the basin. Erratic and spiky signatures of natural gamma and electrical resistivity logs are typical of these thinly bedded and complexly intercalated sand, gravel, and silty mud deposits. Color is characteristically the yellowish brown of oxidized iron; reflecting the subaerial depositional environment of an alluvial fan. Down-valley gradations in sediment type show a general increase in unit thickness (smoother and more gradual deflections of geophysical log-response), and sediment color more typically gray; reflecting transition to the lake environment of deposition. Basinward, the ancient fan materials grade into lake/fan transitional sediments which grade to predominantly lake sediments which grade to gray mudstones and fine sand layers of the deep lake environment. These translate into hydrogeologic settings respectively named: the Lake-to-Fan Transition, Central Boise Lacustrine Aquifers, and the Deep Artesian Lacustrine Sands of West Boise. A fifth setting are the Lake Margin Sands comprised of a wedge-shaped sand "sheet" of fluvial and lake margin sediments which thin away from the mountain front and may be separated from the other hydrogeologic settings by an erosional unconformity located 400-to-600 feet below the present day surface.
Specific capacities of efficient wells open to 80-to-100 feet of sand are highest in the Lake-to-Fan Transition and the Central Boise Lacustrine Aquifers (25-to-40 gallons per minute per foot (gpm/ft)), lowest for the Boise Fan (8-to-15 gpm/ft), and intermediate for the Deep Artesian Sand Aquifers of West Boise (15-to-20 gpm/ft) within the Boise Fan aquifer. As a result of screen and filter-pack design based upon careful attention to sampling drill cuttings, sieve analysis of sands, and geophysical log location of aquifers, efficiency and productivity of new wells has been greatly increased. For example, recent wells in the Boise Fan setting, completed with carefully placed screen and filter-packs, have resulted in production wells near sites where previous wells were considered unsuccessful.
The degree to which the waters of the Boise aquifers are confined generally increases down-valley. Piezometric levels, inferred from static water-levels of wells, in east Boise change very little as wells are deepened and often the static levels decline slightly with depth. In west Boise the static levels in wells rise significantly as wells are deepened, and many are flowing artesian wells with heads up to 10 feet above surface. Sub-surface mapping of aquifer units and overlapping hydraulic well-test data suggest that hydraulic continuity exists across the system. Below east Boise, this hydraulic interconnection is most direct and inter-aquifer leakage is measurable over pumping periods of hours-to-several days. The subsurface of west Boise has the greatest degree of separation between aquifer units and leakage is more difficult to measure.
The system of semi-confined and unconfined cold-water aquifers beneath Boise has boundaries related to lateral changes in the types and occurrence of lake and river sediments, and to crustal faulting. Interbedded sand, silt, and claystone of the upper Miocene and early Pliocene Idaho Group are the primary water-yielding strata. Production is mostly from the upper 500 feet in east and north-central Boise, and from as deep as 900 feet in south, west, and southeast Boise. The depths to which drinking-water aquifers extend is limited by an underlying sequence of relatively impermeable volcanic rocks identified by seismic exploration and deep-well data. In addition, a boundary to the available "cold" (less than 85 degrees F) water aquifers of the Boise area is defined by state law as the 85-degree isotherm. Water from deep wells may have temperatures in excess of 85 degrees F, and appropriation of this "low-temperature geothermal" groundwater is currently restricted by the Idaho Code.
Increasingly, we are discovering instances of local contamination of the surface-gravel aquifer. The present floodplain of the Boise River and the flight of river-terrace benches south of the river are mantled by a "sheet" of river gravels 30-to-100 feet thick. Groundwater recharge to the deeper aquifers is via these porous and permeable, saturated surface gravels. Increased groundwater withdrawals, commensurate with population growth, have possibly accelerated recharge (and so too, the downward movement of surface pollutants) 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. It is recommended that filter-packs be separated with bentonite-based seals within the formation/casing annulus and that surface casing be sealed by being driven into a low-permeability unit.
Continued urbanization of flood-irrigated agricultural land and lining/sealing of the larger canals is beginning to cause a decrease in the amount of water available as replenishment to the groundwater reservoirs. Reduction of recharge, coupled with increased pumpage will result in lowered water-levels in wells. Water-levels in wells of the east Boise area are presently in decline and have lowered, on average, over 40-feet during the last 20 years. A possible remedy to declining water-levels in the upper basin is artificial recharge to the Boise Fan aquifer.