Determination of the geohydrologic characteristics of the boundary between the Mojave River and Antelope Valley groundwater basins, California

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The Mojave Basin Area was adjudicated in 1993 and the Mojave Water Agency (MWA) was appointed as Watermaster to ensure that water rights are allocated according to the Court Judgment (Riverside County Superior Court, 1996). Established in 1960, the southwestern boundary of MWA’s management area is not a hydrologic boundary but instead coincides roughly with the boundary between San Bernardino and Los Angeles Counties. This political boundary separates the Mojave River groundwater basin to the east and the Antelope Valley groundwater basin to the west.

Location of study area - Mojave Desert, CA

Location of study area - Mojave Desert, CA

Detailed geophysical, geological, and hydrological data in this area are sparse and no coordinated hydrogeologic study has been done to define a location of a potential hydrologic boundary or the characteristics of the groundwater system in the vicinity of the boundary. Previous studies in these two groundwater basins have been conducted separately, including groundwater-flow models in both basins. In the previous modeling studies, the boundaries are not juxtaposed and are different in discretization and location. At present, groundwater pumping by at least one stipulating party to the Court Judgment from the Mojave groundwater basin has begun pumping from wells on the eastern edge of the Antelope Valley groundwater basin. As the Antelope Valley groundwater basin also moves toward a final judgment for adjudication, several other entities, including the County of Los Angeles and the Antelope Valley-East Kern Water Agency (AVEK), have an interest in determining the source of the groundwater in this area, if groundwater moves between these adjacent basins, and how the potential exchange of water will affect future water production rights.

The objectives of this study are to: (1) determine groundwater flow directions in the boundary region between the Mojave River and Antelope Valley groundwater basins to evaluate if a groundwater divide exists, (2) determine the thickness of the unconsolidated deposits and depth to bedrock at the groundwater boundary, (3) determine the quality and recharge sources of the groundwater in the area, and (4) update existing models in the Mojave River and Antelope Valley groundwater basins to accurately simulate the shared boundary.

The proposed study will be done in five main tasks: 1) reconnaissance water–level and well-data collection, conduct surface-gravity geophysics, and produce an interim report documenting collected field data; 2) install and document the subsurface geology from drilling two monitoring sites; 3) collect water-quality data to evaluate the source and water chemistry of the groundwater in the area; 4) update the boundary conditions used in the two most recent groundwater-flow models in the basins; and 5) document all data and conclusions from the project in a USGS Scientific Investigations Report.

The proposed analysis addresses three Water Resources Mission Area and USGS priorities: (1) water availability, by contributing to better understanding of hydrogeologic constraints on water availability, and (2) geologic and watershed characterization, through examination of the hydrologic structure by surficial geophysical methods and test-hole drilling, and (3) land management by providing water-quantity and -quality information for future land-management decisions for a poorly-understood area.