Report on the feasibility of using isotopes to source and age-date groundwater in Orange County water district`s Forebay region

From March to September 1995, the Isotope Sciences Division of Lawrence Livermore National Laboratory performed isotopic measurements on water in the Orange County Forebay region. The goal was to test the applicability of isotope techniques for determining the current groundwater flow paths and flow rates in the OCWD spreading facilities. Successful results could then be used to predict the fate of proposed reclaimed waste water recharge. Stable isotope measurements in surface waters and groundwaters in the Forebay region of the Orange County groundwater basin provided a general source indicator. The data defined three general groups: (1) groundwater derived from recharged Santa Ana River water (SAR),(2) groundwater resulting from a mixture of recharged Colorado River water and the SAR, and (3) groundwater recharged from the Santiago basin area. In the first group of data, recharge directly from the SAR flow was not readily distinguishable from groundwater recharged via the spreading ponds. Some groundwater samples from Forebay wells showed significant temporal variability in stable isotope values, while others remained constant throughout the study period. The temporal changes in the groundwater stable isotope signatures are believed to be controlled by similar variations in the stable isotope signatures of the surface water recharge. With further sampling, these seasonal isotopic variations may provide a viable tracer for young (