T237 Noble and Trace Gas Geochemistry: Practical Applications and Current Research

Wednesday, 3 November 2010
8:00 AM - 12:00 PM
Room 207

RECONNAISSANCE OF 3He/4He IN CALIFORNIA GROUNDWATER

Paper number: T237-3
Program: Technical Sessions
Time: 8:30 AM – 8:45 AM

Authors

Abstract

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 RA to 5.10 RA (RA 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.