ANALYSIS OF STABLE ISOTOPES AND RADIOISOTOPES ASSESS GROUNDWATER FLOW AT DOS PALMAS PRESERVE AND VICINITY, SALTON SEA AREA, CALIFORNIA

Water from the Colorado River has been diverted to the Coachella Valley, California through both lined and unlined sections of the Coachella Canal since 1949. A portion of the canal that was unlined previous to 2006 contributed leakage and a significant increase in flow in springs and artesian wells below the canal. In December 2006, a project to line a major unlined portion of the canal was completed. Prior to the completion of the lining, samples were collected from several springs and well locations downgradient from the Coachella Canal to assess their geochemical and isotopic signature for comparison to native and groundwater sources. Three distinct groups of water were identified based on analysis of stable isotopes. One group consisted of nearly pure canal water with δ18O ranging from -11.3 to -11.7 and δD ranging from -84 to -95. A second group consisted of nearly pure native groundwater with δ18O ranging from -7.3 to -8.7 and δD ranging from -59.5 to -71. A third group consisted of various mixtures of canal and native groundwater with δ18O ranging from -8.7 to -11.1 and δD ranging from -80 to -91. Radioisotopes were also analyzed in canal water and groundwater. Canal water contained about 6.0 Tritium Units (TU) and about 90 percent modern carbon (PMC). Groundwater at and near discharge springs showing nearly a canal water stable isotope signature had nearly the same radioisotope signature as canal water, suggesting a short groundwater residence time during transit from the canal to the springs. Groundwater at and near discharge springs showing a native groundwater signature had no measurable tritium and less than 10 PMC, indicating a relatively old water. Samples were collected again from several of the downgradient wells and springs in August 2008 after lining of the Canal was completed, and isotope analysis was used to monitor how this has affected the flow and composition of the water. These samples nearly all had δ18O values ranging from -11.2 to -11.9 and δD values ranging between -91 and -96, which correlates with the composition of nearly pure canal water. This indicates that the canal lining has not yet had any noticeable effect yet on the isotopic composition of the flow in downgradient springs and wells.