RECONNAISSANCE OF 3He/4He IN CALIFORNIA GROUNDWATER

Helium isotopes were measured in 1535 groundwater samples collected for the GAMA Program, a collaboration of the California Water Boards, USGS, and LLNL. Analyses of this extensive dataset provide a means to investigate the influence of geologic age, lithology, and tectonic structures on the spatial variability of ³He/⁴He in California groundwaters. Previous work identified mechanisms that introduce ³He (magmatism, seismicity) and ⁴He (erosion, in situ and deep crustal production). Preliminary data analyses show that ³He/⁴He, corrected for air-entrainment, ranged from 0.01 R_A to 5.10 R_A (R_A is the ³He/⁴He of air 1.4 x 10^-6). To identify sources of ³He and explain variations in the ³He/⁴He statewide, the geology mapped at the well and the well’s proximity to faults were investigated.

Nonparametric statistics were used to test for correlations between ³He/⁴He and potential explanatory factors. The correlation between ³He/⁴He and the categorical explanatory factors: rock type (sedimentary, plutonic, volcanic), geologic Era (Cenozoic, Mesozoic, pre-Mesozoic), Cenozoic age (Holocene, Pleistocene, Tertiary), and Cenozoic sediment type (alluvium, glacial, marine, non-marine, sand, volcanic) were evaluated with the Wilcoxon rank-sum test. The Spearman’s method was used to establish correlation between the continuous variables ³He/⁴He and the distance of a well to the nearest fault.

Statistical tests demonstrate that ³He/⁴He had a significant positive correlation to the proximity of a well to faults. There also are significant differences in ³He/⁴He between the following categorical factors: higher in volcanic than in plutonic rocks; higher in plutonic than in sedimentary rocks; higher in Cenozoic than Mesozoic rocks; and higher in pre-Mesozoic than Mesozoic rocks.

These results, higher ³He/⁴He in wells located in volcanics and rocks of Cenozoic age, may be explained by mantle ³He from relatively recent magmatic activity. The increase of ³He/⁴He near faults suggests seismic activity as a source of ³He. Other statistical comparisons revealed no significant correlations.

Identifying the factors that explain the spatial variations in ³He/⁴He is the first step towards characterizing groundwater affected by seismic activity, ³He from tritium decay, long residence times, and crustal degassing.