Environmental Isotopes in Time Series Resolve Issues of Imported Water Recharge in Basin Fill Aquifer
Analysis of stable isotopes identifies distinct groups of water; one of nearly pure canal water with del 18O ranging from -11.3 to -11.9 and del 2H ranging from -84 to -95. A second of nearly pure native groundwater with del 18O ranging from -7.3 to -8.7 and del 2H ranging from -59.5 to -71. A third of mixtures of canal and native groundwater with del 18O ranging from -8.7 to -11.1 and del 2H ranging from -80 to -91. Most of the waters at the preserve are derived from Colorado River-fed canal water. Tritium and Carbon-14 support these interpretations. With lining of the canal, flow at the wetlands has decreased.
After canal lining a wetland mitigation project used artificial recharge to try to maintain adequate flow at springs. Artificial recharge is no more than 10-15% of the canal leakage that occurred before the canal was lined. A previous study in the late 1980s suggested that the San Andreas Springs at Dos Palmas Preserve was not connected to canal recharge, based on lack of tritium in flows. Regulators used this information to make plans for wetland mitigation. By 2003, our sampling showed that flow at San Andreas Spring was almost pure canal water, with bomb tritium detected. Canal water had not arrived in the late 1980s but had arrived by 2003. Our study points to the limitations of interpretations based on sampling in a moment in time, when a major retrofit to a groundwater basin is completed.