aerial photograph of Montebello Forebay spreading grounds
Montebello Forebay spreading grounds, where stormwater, imported water, and recycled water have been actively recharged since the late 1950s

Key Issues

  • Extensive reliance on recycled water for irrigation, industrial uses, seawater intrusion protection, and replenishment
  • Plans to increase reliance on recycled water
  • Elevated TDS and chloride in coastal areas due to historical seawater intrusion

Solutions

  • Mixing model methodology developed by Todd Groundwater to predict future groundwater quality approved by LARWQCB
  • SNMP analysis shows that use of advanced treated recycled water for replenishment significantly improves groundwater quality with respect to TDS and chloride
  • SNMP provides technical support for streamlined permitting or recycled water projects

Salt and Nutrient Management Plan

Central Basin and West Coast Basin
Los Angeles County

The Central Basin and West Coast Basin in Los Angeles County have some of the most significant recycled water recharge projects in the country, some in operation since the late 1950s.

Background

Two important groundwater basins in Southern California are the Central Basin and West Coast Basin, located in southern Los Angeles County. Currently, groundwater in this area meets approximately 40% of the overall water supply needs of nearly 4 million residents plus businesses in the 43 cities overlying the basins.

Groundwater supply is augmented by imported water and recycled water. While long an important source, imported water supplies have become more uncertain and expensive. Accordingly, recycled water has become a crucial component of the area’s water supply portfolio.

Because of significant historical over-pumping of groundwater, seawater intruded along some coastal areas. To halt seawater intrusion and replenish the basins, three seawater intrusion barrier have been constructed. Currently, a blend of imported water and an increasing portion of advanced treated recycled water is injected at the barriers.

Additional replenishment is conducted with the most significant managed aquifer recharge in the Montebello Forebay Spreading Grounds, where stormwater, imported water, and tertiary-treated recycled water are recharged.

Findings

Seeking to encourage water recycling, the Water Replenishment District of Southern California and other stakeholders embarked on a Salt and Nutrient Management Plan (SNMP) and retained Todd Groundwater to prepare the plan. A cornerstone of the SNMP is a mixing model to incorporate existing groundwater quality with future salt and nutrient loading.

The model was used predict the groundwater quality impacts of future basin operations including increased use of recycled water for both non-potable irrigation uses as well as indirect potable replenishment at the seawater intrusion barriers and the Montebello Forebay Spreading Grounds. The Los Angeles Regional Water Quality Control Board (LARWQCB) reviewed and approved the mixing model methodology. The SNMP analysis found that use of advanced treated recycled water significantly improves groundwater quality with respect to salts (total dissolved solids [TDS] and chloride), and has essentially no impact on nitrate concentrations, which are found at very low levels in the basins due to minimal nitrate loading sources.

In accordance with the overall goals of the State Water Resources Control Board’s Recycled Water Policy (which requires preparation of SNMPs for all basins in California) the Central Basin/West Coast Basin SNMP analysis provides technical support for streamlined permitting of recycled water projects.