Tritium Helium Age Dating LLNL

**This data only represents the tritium-helium data, not all data presented in the paper referenced. GRIDCODE = years The state-wide data set was subjected to geostatistical analysis to reveal patterns related to recharge, groundwater residence time, and large-scale groundwater management. Variables of interest were mapped by means of kriging prediction. Noble gas derived parameters (recharge temperature, excess air, terrigenic helium-4, terrigenic helium isotope ratio and tritiogenic helium-3) were calculated using the unfractionated excess air (UA) model with a recharge elevation assumed to be the well head elevation. The local terrigenic helium isotope ratio was derived from a geostatistical interpolation and subsequently used in separating the tritiogenic helium component in each sample. In total, 1630 3H/3He ages are presented, in addition to 977 samples with less than 1 pCi/L (and assigned an age of 60 years). Young 3H/3He ages delineating areas of groundwater recharge are found along the major, perennial rivers in the Central Valley, in the forebay areas of major coastal basins dominated by managed aquifer recharge, along the northern California coast, and along the eastern slopes of the Sierra Nevada. In large portions of southeastern California, tritium-free, fossil groundwater predominates, while modern groundwater abstracted in this area is found only near distributaries of the Colorado River, which has recharged under distinctly different conditions. Tritium concentrations in the Central Valley are largely determined by the mixing of premodern (3H free) groundwater with modern groundwater (containing 3H) and not by decay of atmospheric tritium. In basins with managed aquifer recharge operations, tritium concentrations are more strongly determined by the initial tritium concentrations in specific historical recharge periods than by mixing of modern and pre-modern groundwater. Noble gas recharge temperatures range from 25 °C in the Mojave Desert to below 10 °C in the Sierra Nevada. Because of strong elevation gradients, local anomalies in groundwater recharge temperatures are difficult to distinguish on the map of noble gas recharge temperatures. However, managed aquifer recharge operations have a major impact on signatures of recharge, including patterns in stable isotopes and noble gas recharge temperatures. The depth to tritium-free groundwater typically varies between 100 and 400 ft. Several areas, including Santa Clara River Valley, San Gabriel Basin, portions of the Mojave Basin, the Los Angeles physiographic basin, and the area between Davis and Sacramento, stand out as having a very deep boundary between modern and pre-modern groundwater. On the other hand, tritium-free groundwater is found at shallow depths in the Volcanic Highlands of the North San Francisco Study Unit and in the Indian Wells area of the Basin and Range province.