Development of a Revised Hydrogeologic Conceptual Model of the Indian Wells Valley Groundwater Basin
The geology and structure of the Basin has been well-studied using geophysics, gravity and magnetics, deep boreholes and monitoring wells. Indian Wells Valley is a structural half-graben produced by faulting, primarily along the Sierra Nevada frontal fault and Argus frontal fault, with the basement generally tilted downward to the west. The basin structure is further defined by the Garlock left lateral strike slip fault that bounds the basin along part of it southern boundary. Additional major mapped faults in the basin include the Little Lake and Airport. There are numerous smaller faults making the basin geology and structure complex. The deepest area of the valley (based on drilling data) is in the west-central area with basement encountered at approximately 6,500 feet below land surface.
The basement and highlands of the basin are of late-Cretaceous igneous and metamorphic rocks. Surficial geology in the basin generally consists of alluvium, lacustrine and playa deposits, sand dunes, and consolidated rock. The lower-most alluvial materials are of early Tertiary age, consisting of compact, consolidated alluvium derived from the basement rocks, which include some lacustrine beds containing pyroclastic materials and minor volcanics. Lacustrine and playa lake deposits as much as 800 feet thick underlain by alluvium dominate the north central portion of the basin, and alluvium dominates the western portion of the basin.
Two principal aquifer units have been identified as the shallow and deep (or main regional) aquifers. The shallow aquifer extends from land surface through the sand dune deposits, younger lacustrine and playa deposits, and shallow alluvium to approximately 400 to 500 feet below ground surface. Water quality is generally poor in the shallow aquifer with total dissolved solids (TDS) greater than 1,000 mg/L. The base of the shallow aquifer is not well defined, but has been estimated from 1,950 ft above mean sea level (msl) at its western edge to 1,850 ft above msl near China Lake.
The shallow and deep aquifer are separated by an intermediate hydrogeologic unit consisting mainly of low permeability lacustrine and playa clays, but containing sand stringers that create transmissive water bearing zones that can be highly productive. The unit acts as a confining bed to deeper, productive water bearing zones, but also can be screened by wells and considered part of the deeper aquifer.
The deep aquifer is semiconfined to confined in the eastern portion of the basin by silt and clay from the overlying lacustrine and playa deposits, but otherwise mostly unconfined. The medium-to-coarse grained alluvial and fluvial sands and gravels have an estimated saturated thickness of 1,000 ft and are the main source of water to the Basin producing adequate flow rates and TDS less than 1,000 mg/L.
Groundwater replenishment occurs dominantly from Sierra Nevada Mountain snowmelt and mountain block recharge. Additional smaller sources of recharge include inflows from the adjacent Rose Valley, Coso Valley and El Paso subbasins/subareas. Groundwater flows from the southwestern El Paso subarea to the northeast, the Rose Valley to the southeast, from the Sierra Nevada to the west, towards China Lake and to two pumping centers. Discharge primarily occurs through groundwater pumping and the China, Mirror and Satellite playa lakes located in the east-central portion of the Basin.
The basin is part of the Stanford Groundwater Architecture Project and was surveyed with aerial electromagnetics in late 2017, initially conducted to provide improved information on water quality and lithology to support a brackish groundwater resources feasibility study. An extensive effort to comprehensively assemble all pertinent hydrogeologic data began in late 2017, with all pertinent data stored in the GeoGIS data management system, which was completed in spring 2018. An updated hydrogeologic conceptual model is being developed with GeoGIS and GeoScene3D, to be completed in July 2018, and results will be presented along with the workflow and challenges to complete the project.