IDENTIFYING THE SOURCE OF SALINE GROUND WATER CONTAMINATION USING GEOCHEMICAL DATA AND MODELING

The source of salinization of an unconfined aquifer is determined by a combination of graphical methods, comparative statistics, mass-balance mixing models, and geochemical computer models. Based on graphical and statistical analyses, water samples from the site are divided into two groups – Group 1 samples are chemically similar to the natural ground water chemistry of the region, and Group 2 samples are chemically similar to oil-field brines injected into deeper formations for disposal and petroleum recovery enhancement. The spatial distribution of the two groups and the results of statistical analyses support the similarity between the Group 2 and the salt-water disposal (SWD) samples and are consistent with the hypothesis that the Group 2 samples were contaminated by SWD brine. Bivariant plots and mixing models suggest that the Group 2 samples are a combination of brine contamination and subsequent reaction with mineral phases present in the aquifer. Three conceptual models for salinization of the Group 2 wells – evaporation of the natural ground water with no contamination; release of SWD brine directly into the aquifer; and a deep release of brine modified by mineral reactions in the subsurface – are developed and tested using geochemical models. The results indicate that the best model for salinization of the Group 2 samples is a deep release of brine, equilibration with minerals in the underlying formation; and cross-formational flow of the modified brine into the aquifer with subsequent mineral phase reactions.