STATISTICAL AND SPATIAL ANALYSIS OF INORGANIC ANIONS, CATIONS AND RADIONUCLIDES IN CALIFORNIA’S GROUNDWATER: ESTABLISHING A BACKGROUND AND ANALYZING SPATIAL VARIABILITY

California’s regional geological environments, such as aquifers with a granitic basement or sedimentary basins with oil reservoirs, constrain the groundwater’s chemical composition. The geological complexity of California generates aquifer systems that contain a wide range of sediments, recharge mechanisms, dissolved oxygen quantities and additional regional to sub-regional geochemical differences. A spatial analysis of ten hydrologic regions provides regional average distributions of chemical constituents with respect to mixing of deeply sourced waters associated with organic sedimentary deposits and shallow groundwater used for drinking water supply. In addition, the analysis provides a representation of the patterns in background concentrations that could aid in the determination of anthropogenic effects on groundwater chemistry, including oil and gas production.

The statistical and spatial analysis was conducted through inputting and interpreting a large query of supply well data into a geographic information system data frame. The primary constituents of interest are radionuclides: radium and radon and related anions and cations. Sedimentary organic deposits typically contain high concentrations of naturally occurring radioactive materials, such as ²³²Th and ²³⁸U. Furthermore, groundwater that interacts with the sedimentary organic deposits generally has high activities of ²³²Th and ²³⁸U decay products ²²⁸Ra and ²²⁶Ra. Previous studies have indicated that the ratio of ²²⁸Ra/²²⁶Ra in groundwater correlates with the ratio of ²³²Th/²³⁸U in the aquifer materials providing evidence for the source of radium in groundwater. Comparing the hydrogeologic setting and the groundwater chemistry demonstrates the usefulness of the ²²⁸Ra/²²⁶Ra ratio in the determination of the mixing of groundwater between deep-seated aquifers with shallow drinking water supply aquifers. Establishing a background database of certain anions, cations and radionuclides associated with sedimentary organic material will provide local, regional and state authorities and water-resource managers with information that could prove useful when examining future effects of conventional or non-conventional energy resource development.