GRA SoCAL BRANCH ONLINE MEETING: Developing and Implementing a Robust, Deep Nested Groundwater Monitoring Program in Southern Los Angeles County, California

June 24, 2020 @ 4:30 pm – 5:45 pm

The Water Replenishment District of Southern California (WRD) manages groundwater for nearly four million residents in 43 cities of southern Los Angeles County. The 420 square mile service area uses about 220,000 acre-feet per year (AFY) of groundwater, which equates to over 40 percent of the total demand for water. WRD ensures that a reliable supply of high quality groundwater is available through its clean water projects, water supply programs, and effective management principles.

For years, water supply well data was used to evaluate the overall water quality within two adjudicated groundwater basins in southern Los Angeles County (Basin). However, using data from long screen intervals common to the water supply well industry often results in a blended water quality from multiple aquifers compared to actual water quality and a limited understanding of the various aquifers present within the Basin. In the 1990s, WRD began an effort to better understand the water quality of each aquifer and continues to implement a robust groundwater monitoring program using over 335 deep nested groundwater monitoring wells installed in collaboration with the United States Geological Survey (USGS). Groundwater samples are collected semiannually from each major aquifer with screen intervals installed up to 2,900 feet below ground surface (ft bgs). More than 100 water quality constituents are analyzed, resulting in over 60,000 individual data points to help track water quality throughout the Basin. Groundwater elevation data are also continuously tracked using pressure transducers and key monitoring wells are used to evaluate water level trends throughout the basin including a monthly assessment of storage change within the Basin. Advanced geophysical logging tools were also used during drilling to collect additional geologic data to better understand subsurface soils and when combined with oil industry geophysics, a sequence stratigraphy model was developed to better understand the structure of the basin.

This talk will present an overview of our groundwater monitoring program and the benefits of having a robust, depth specific monitoring network for evaluating water quality conditions in a complex basin and how additional data resulted in a better overall understanding and water management for two adjudicated basins in southern Los Angeles County.