Antelope Valley Adjudication
Los Angeles County Public Works (LACPW) provides infrastructure and essential services to more than 10 million people across a 4,000-square-mile service area. The Antelope Valley Groundwater Basin is located in the western Mojave Desert and covers 1,580 square miles of Los Angeles, Kern, and San Bernardino counties.
For six decades, groundwater was indiscriminately pumped from the Antelope Valley Groundwater Basin beyond the level of replenishment. The inevitable result was subsidence to the point that in the 1990s a crack appeared in a runway at Edwards Air Force Base. In 1999, a complaint was filed in Riverside County Superior Court, resulting in a case that led to a judgment in 2016.
The case involved more than 4,000 parties, including public water suppliers, landowners, small pumping and non-pumping property owners, and federal and state governments.
LACPW engaged Best Best and Krieger (BBK) to represent them in the case. BBK engaged Geoscience to provide the technical expertise to build a case for LACPW’s right among the 4,000 parties.
The adjudication involved four phases.
Define the boundaries of the basin. Each of the 4,000+ parties made their case for basin boundaries. Based on alluvial and bedrock geologic contacts, Geoscience clearly delineated the boundary of the Antelope Valley.
Consider the hydraulic connections between entities throughout the basin. Geoscience evaluated subsurface geologic conditions, hydraulic continuity, and groundwater movement in the Antelope Valley Area of Adjudication.
Then, using the original USGS MODFLOW model, Geoscience analyzed water level elevations and flow vectors in selected years, quantified groundwater underflow from the West Antelope Subbasin to the Lancaster Subbasin, and evaluated the impacts of no-flow from the West Antelope area.
Establish the safe yield of pumping from the aquifer without crossing the limits for each party. As overpumping was the culprit for subsidence, the primary goal of the complaint was to right-size safe yield limits across all parties.
Geoscience provided a technical review of the Phase 3 Antelope Valley Expert Report and updated and recalibrated the USGS MOD-1 model to create MOD-2.
Using the updated model, Geoscience verified the sustainable yield of 110,000 acre-ft/yr that was outlined in the Antelope Valley Expert Report. Based on this confirmation, a physical solution was developed that included ramping down groundwater pumping to the native safe yield and creating a management and monitoring system under the guidance of a Watermaster. In addition, Geoscience evaluated the effect of outside pumping on recharge in the basin.
Quantify total groundwater production for the basin. Geoscience evaluated the modified USGS model and found that the assumed recharge from mountain front runoff and groundwater pumping was underestimated compared to values established in the Antelope Valley Summary Export Report. Consequently, Geoscience recalibrated the model with revised pumping, natural recharge, and return flow in the MOD-2 model.
Geoscience provided expert witness testimony and oversaw efforts to update and refine the USGS Antelope Valley Groundwater Model as part of the adjudication process. The refined model was used along with other evidence to settle the longstanding dispute, which was recently approved by the court. The judgment identified the state of overdraft in the basin, established respective water rights among groundwater producers, and ordered a ramp down of production to the native basin safe yield.
“The refined model was used along with other evidence to settle the longstanding dispute”
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