Evaluating the Geohydrology in the San Gorgonio Pass Area

The San Gorgonio Pass Water Agency (SGPWA) is responsible for operating and maintaining the State Water Project within the SGPWA service area, which includes parts of the Coachella Valley-San Gorgonio Pass groundwater subbasin, the Santa Ana Valley-San Timoteo groundwater subbasin, and the Santa Ana Valley-Yucaipa groundwater subbasin. The Coachella Valley-San Gorgonio Pass groundwater subbasin is located within San Gorgonio Pass; the other two groundwater basins are to the west.

SGPWA imports supplemental water to protect and enhance local water supplies. A goal of the agency is to support the region’s quality of life through sustainable water management. A key part of this undertaking includes partnering with the U.S. Geological Survey (USGS) to monitor and evaluate the area’s aquifer system.

Background

Over the last 27 years, the USGS, in cooperation with the SGPWA, has monitored groundwater levels and quality at over 100 public and private wells, and has installed 13 multiple-well monitoring sites.  

As a result of this work, areas where gaps exist in groundwater data and subsurface geologic data were identified To address these gaps, it was proposed that the groundwater monitoring network be expanded through the addition of four new multiple-well monitoring sites, and new geophysical surveys be conducted in selected areas.

Data from new multiple-well monitoring sites will provide:

• Critical hydrologic, hydrogeologic, and geochemical data
• Information about changes in groundwater levels
• Information about the vertical and lateral hydraulic gradients

The figure below shows the locations of the groundwater monitoring network and planned multiple-well monitoring sites.

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Objectives

In order to characterize the area’s aquifer system, it is necessary to improve the understanding of the hydrogeology, geochemistry, and subsurface geology. The current groundwater monitoring network provides valuable data on groundwater levels and quality. The planned expansion of the monitoring network, and new geophysical surveys will help further define the storage properties of the Coachella Valley–San Gorgonio Pass groundwater subbasin, and the locations of subsurface features, such as faults, that may influence groundwater movement.

The results of this evaluation will provide information to help SGPWA find the best approach for continued groundwater management, including local and imported water-supply options.

One such option is using surface spreading for managed aquifer recharge. In surface spreading, imported water is applied to the land’s surface, typically in large ponds. The water then percolates through subsurface materials to the aquifer below.

Information will also help to support ongoing management decisions related to source, quantity, and quality of groundwater resources in the area. This effort will consider the impact of imported State Water Project water that is surface spread as part of aquifer storage and recovery activity. More specifically, this multi-year study will:

1. Continue long-term monitoring of sites in the groundwater-level and groundwater-quality networks. 
2. Complete geophysical surveys to help determine the location of potential faults near proposed aquifer storage and recovery sites within the SGPWA service area.
3. Install four new multiple-well monitoring sites in areas that have known data gaps.
4. Publish an interpretive report that includes recent and historical data related to the groundwater-level and groundwater-quality networks, results of the gravity surveys, and analysis of the data collected during the drilling and construction of the new monitoring sites. The interpretations in the report will help improve the understanding of the hydrogeology of the study area.

Timeline and products

The approach will be divided into five tasks. The anticipated timeline for these tasks spans from the fall of 2023 to the summer of 2027.

Task 1- Groundwater-level monitoring 
Task 2- Groundwater-quality monitoring
Task 3- Geophysical surveys
Task 4- Installation of multiple-well monitoring sites
Task 5- Data documentation, reporting, interpretation, and publication

An  interpretive report will be produced as part of this work. The groundwater-level and groundwater-quality data collected from wells in the monitoring network, as well as data from the proposed monitoring wells, will be publicly available on the National Water Information System website. Geophysical and lithologic logs for the proposed monitoring sites will be publicly available on the USGS GeoLog Archiver. Raw data from the gravity survey will also be published in USGS ScienceBase.

Relevance and benefits

This study meets a need to understand the sources and processes that control concentrations of contaminants in SGPWA groundwater basins. USGS expertise in the following areas contributes to addressing that need.

• Surface water-groundwater connectivity
• Geochemical evaluation of water and nutrient source identification
• Groundwater age dating