GROUNDWATER-SURFACE WATER INTERACTIONS AT THE SALTON SEA, CALIFORNIA, AND IMPLICATIONS FOR WETLAND WATER BUDGETS

Groundwater-surface water interactions were assessed along the shoreline of the Salton Sea in southern California in order to evaluate water sources to newly forming wetlands. Tritium samples collected from springs and wetlands around the Salton Sea had concentrations between 0.11 and 6.80 tritium units (TU). San Felipe Creek, Bombay Beach and a CO2 spring near Niland all had concentrations less than 1 TU, suggesting these waters were dominated by pre-1950 groundwater with a small component of younger groundwater. A spring discharging from fractured volcanic rock near one wetland had a tritium concentration of 1.82 TU, warm temperatures above 50 C, low oxygen and high specific conductance in excess of 100,000 µS/cm and may be a mixture of irrigation return in the perched aquifer mixing with deeper geothermal water. Some springs that rapidly respond within 5 to 10 days to changes in upgradient irrigation had concentrations between 4.40 and 5.30 TU, compared to an estimated concentration of 5.02 TU in Colorado River water based on a decay-correction from the last measured tritium concentrations above Imperial Dam, California in 1984. Springs near the Alamo River had the highest concentrations between 6.31 and 6.80 TU, suggesting these sites may have a component of older groundwater mixing with younger irrigation return. Salinity and stable isotope measurements in some shoreline wetlands suggest discharging groundwater with a signature potentially similar to Salton Sea water in the 1980s. These measurements in combination with tritium concentrations in the area suggest these wetlands may have a small component of water sourced either from older bank storage, or a mixture of perched and deeper aquifer groundwater. The Salton Sea shoreline on the east and west away from agricultural areas has active groundwater discharge deposits. We hypothesize these discharge deposits are forming from a mixture of discharge from bank storage, and discharge from the older local groundwater system. We estimate the groundwater discharge component to some wetlands in the south to be between 10 and 20%, which may change how irrigation water moves through the wetlands. Future management of shoreline wetlands will need to adapt to potential changes in groundwater-surface water interactions that may occur due to changes in water management in the region.