Hydrogeologic Characterization of the Cox/San Andreas Oasis, California

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The Bureau of Land Management (BLM) manages the Dos Palmas Oasis complex on the northeastern side of the Salton Sea for the maintainence of threatened and endangered species. This Oasis complex represents a rare area of riparian/wetland habitat in the midst of an extremely arid desert region. Anthropogenic development of water resources during the 1900s depleted natural groundwater supplies feeding the Oasis, and subsequent water supply for the Oasis complex provided by seepage from the Coachella Canal was lost following a 2006 project to prevent seepage losses by constructing a lined canal adjacent to the original canal channel. Concern that the reduction in canal seepage would cut off the water supply sustaining the Dos Palmas Oasis complex led to an agreement by BLM and the Coachella Valley Water Management District to spread water in the unlined canal channel to provide seepage upgradient of the Oasis complex. 

The Dos Palmas Oasis complex

The Dos Palmas Oasis complex including the proposed study areas of the Cox Oasis and the San Andreas Oasis

However, recent investigations show declining groundwater discharge to the the Cox Oasis and San Andreas Oasis since the lining of the canal. The concern is that mitigation procedures to maintain flows to the Dos Palmas oasis complex are unsuccessful as a result of the distal location and possible hydrogeologic conditions at the Cox Oasis and San Andreas Oasis. The geometry of the layered aquifer sequence at the oases and connection with other parts of the complex is unknown. Additionally, there is a linament visible in the surface landscape located approximately 1,100 m northeast of the Cox Oasis that may be a fault disrupting hydrogeologic layering and groundwater flow across this feature. In order to understand how to mitigate declines in groundwater supply to the Cox Oasis and the San Andreas Oasis, it is necessary to better understand the hydrogeologic structure.

The primary objective of this study is to identify the geometry and thickness of layered aquitards and permeable aquifers potentially supplying groundwater to the Cox oasis and San Andreas oasis. A secondary objective is to determine the change in hydrogeologic layering across the lineament located northeast of the the Cox and San Andreas oases.

This project will use a high-resolution shallow seismic-reflection (HRSR) study coupled with auger drilling to to determine the lithology in the area, locate possible faults, help determine the structure controlling the oases, and determine the extent of the aquifers associated with the oases. The study plans to collect about 3 kilometers of seismic data. Surface vibrations will be induced at regularly spaced intervals (2 meters) along a transect and seismic waves created by the surface vibrations will travel through the subsurface, reflect off of physical boundaries such as a changes in lithology, and recorded at the surface by sensitive "geophones" placed along a cable at the surface. Prior to seismic data collection, four test holes will be drilled at selected locations along the seismic lines to provide ground-truth data for comparison with the seismic data. After completion of the drilling of each test hole, electromagnetic (EM) resistivity borehole logs will be collected. Following drilling and logging, one (1) 2-inch PVC monitoring well will be placed in each test hole. The four single completion monitoring wells will be screened in the upper or middle confined aquifer, depending upon site conditions. Generally, seismic data and test holes will be collected along a northeast to southwest line extending through Cox and San Andreas Oases. The northeast end of the line will be located slightly northwest of where the line crosses the lineament. One test hole will be located northeast of the lineament. The southwest end of the line will be near San Andreas Oasis and one test hole will be located there. Another test hole will be located near Cox Oasis. The exact placement of the seismic lines and test holes will depend upon site access and may be constrained by the location of existing roads or tracks.

The proposed hydrogeology study fits within the USGS Strategic Science Plan objectives of conducting a water census of the United States by quantifying and forecasting freshwater resources. Specifically, the proposed analysis addresses three Water Resources Mission Area and USGS priorities: (1) water availability, by contributing to better understanding of sources and movement of water, (2) geologic and watershed characterization, through examination of the hydrologic structure by surficial geophysical methods and test hole drilling (3) forecasts of likely outcomes of water availability, water quality, and aquatic ecosystem health due to changes in land use and land cover, natural and engineered infrastructure, water use, and climate. Results of the study are expected to have transfer value for characterizing layered alluvial aquifers and interactions with desert oases elsewhere in arid portions of the southwestern United States.

Results from this study will allow BLM to better manage the Dos Palmas Oasis complex. Information on the lithology and hydrogeologic structure of the areas surrounding the Cox Oasis and Andreas Oasis may be used to help determine the causes of declining groundwater discharges and the best mitigation measures to increase flow of groundwater to the oases.