Todd Engineers

In the early half of the 20th century, groundwater pumping in the West and Central basins in Los Angeles County lowered groundwater levels to over 100 feet below sea level, resulting in seawater intrusion. For over 50 years the West Coast Basin Barrier Project (WCBBP) has been operated to protect groundwater from seawater intrusion along the coastal area. The WCBBP includes a series of injection wells, which form a pressure ridge to repel seawater.

Key Issues
Recycled water travel time and percentage in groundwater Permitting support for recycled water injection in seawater barrier

The WCBBP originally injected imported potable water, but since 1995, the West Basin Municipal Water District (District) has been injecting a potable/recycled water blend into the barrier. The recycled water undergoes advanced treatment including microfiltration, reverse osmosis, and disinfection. Currently, the maximum allowable recycled water content (RWC) in injected water is 75 percent. The Regional Water Quality Control Board has authorized the District to inject 100 percent recycled water, but prior to proceeding to 100 percent recycled water, the District must satisfy California Department of Public Health (CDPH) requirements for verifying travel time through, and RWC in, one or more of the three West Coast Basin aquifers.

Since 2008, Todd Engineers has been conducting an intrinsic tracer study to:

1. determine if travel time and recycled water content could be verified in one or more compliance monitoring wells located between the WCBBP and a drinking water supply well,
2. confirm whether CDPH requirements for 100 percent recycled water injection have been satisfied, and
3. assess the effectiveness of the WCBBP in combating seawater intrusion.

We evaluated intrinsic water quality parameters (sodium, calcium, chloride, and sulfate ions) using Brine Differentiation Plots (BDP) -- a powerful tool generally used to identify different brine source waters. BDPs were used to determine relative amounts of three dominant parameters over time for water sources in WCBBP compliance monitoring wells:

1. baseline groundwater prior to recycled water injection,
2. blended WCBBP water, and
3. seawater.

Estimated blended water content in groundwater was further apportioned to its two basic components to estimate the RWC in groundwater.

Our studies showed that BDPs are useful in verifying injected water travel time in monitoring wells close to the WCBBP. BDPs were also effective in verifying both WCBBP blended injection water content and RWC in monitoring wells and in identifying seawater influence. The District is using our key findings to bring the WCBBP into compliance with CDPH requirements. The project is continuing with a field experiment that will cover an 18 month period with 100 percent recycled water injection.