IDENTIFICATION OF GROUND WATER ORIGINS IN THE SALTON SEA AREA USING ENVIRONMENTAL ISOTOPES

Our study of the northern half of the Salton Sea focused on the isotope hydrology of the area. Using stable isotopes of oxygen and hydrogen, along with tritium and carbon-14, we investigated the sources of groundwater units near the Salton Sea shoreline. Groundwater is currently being utilized to control dust on the exposed playa deposits as the shoreline recedes. The analysis identified contributions from imported Colorado River water, local fossil water over 9,000 years old, and more recent local water less than 1,000 years old. In the northern shoreline area near the thermal hot springs, groundwater isotope values plot parallel to and just below the meteoric water line, ranging from -8.25 to -9.25‰ δ¹⁸O. These geothermal waters are very old, containing about 5% modern carbon and no detectable tritium. Deeper groundwater in the northwestern area near Salton City exhibits slightly lighter δ¹⁸O values, ranging from -9.0 to -10.25‰, and is similarly ancient. These lighter isotopic signatures likely originate from recharge in the San Jacinto Mountains and alluvial fan deposits to the west. In contrast, groundwater in the eastern Salton Sea region in the Wister Sector near Niland, is primarily sourced from Colorado River water introduced mostly through irrigation. Isotopic values in this area follow an evaporation trend consistent with modern Colorado River water and are distinct from the older groundwater found in the northern and northwestern regions. An evaporation trajectory originating from Colorado River water is evident in the groundwater of the Niland area. A unique feature in the area is the Niland moving mud pond, characterized by high TDS levels (~17,500 mg/L) and enrichment in carbon dioxide and other gases. Isotopic analyses indicate that the mud pond is also sourced from Colorado River water. The pond has shown lateral movement toward the Salton Sea, likely driven by hydraulic gradients influenced by the lake’s recession. Our hydrogeological model suggests that groundwater at the Niland moving mud pond was recharged during historical events, such as the 1906 flood, or more recently through seepage from unlined canals and agricultural fields. Groundwater issuing from the mud pond interacts with ancient playa evaporites and absorbed magmatic gases from deep subsurface sources before surfacing at the pond.