Justin T. Kulongoski Avner Vengosh Isabelle M. Cozzarelli Matthew K. Landon Yousif K. Kharaka Janice M. Gillespie Tracy Davis Peter B. McMahon 2018 <p><span>Chemical and isotopic data for water co-extracted with&nbsp;hydrocarbons&nbsp;in&nbsp;oil and gas fields&nbsp;are commonly used to examine the source of the&nbsp;formation water&nbsp;and possible impacts on groundwater in areas of oil and gas development. Understanding the geochemical variability of oil-field water could help to evaluate its origin and delineate possible contamination of shallow&nbsp;aquifers&nbsp;in cases where oil-field water is released to the environment. Here we report geochemical and multiple isotope (H, C, O, Sr, Ra) data from 22&nbsp;oil wells, three sources of produced water that are disposed of in injection wells, and two surface disposal ponds in four oil fields in the southern San Joaquin Valley, California (Fruitvale, Lost Hills, North and South Belridge). Correlations between Cl and δ</span>¹⁸<span>O, as well as other ions, and gradual increases in&nbsp;salinity&nbsp;with depth, indicate dilution of one or more saline end-members by&nbsp;meteoric water. The saline end-members, represented by deep samples (610 m–2621 m) in three oil-bearing zones, are characterized by Na</span><img src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/prod/0d8da9e38aea1a6beec2ab8709ca87d9392372c6/entities/sbnd" alt="" data-mce-src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/prod/0d8da9e38aea1a6beec2ab8709ca87d9392372c6/entities/sbnd"><span>Cl composition, near-seawater Cl concentrations (median 20,000 mg/L), enriched δ</span>¹⁸<span>O</span><img src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/prod/0d8da9e38aea1a6beec2ab8709ca87d9392372c6/entities/sbnd" alt="" data-mce-src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/prod/0d8da9e38aea1a6beec2ab8709ca87d9392372c6/entities/sbnd"><span>H</span>₂<span>O (median 3.4‰), high&nbsp;ammonium(up to 460 mg-N/L), and relatively high&nbsp;radium&nbsp;activity (</span>²²⁶<span>Ra+</span>²²⁸<span>Ra = 12.3 Bq/L). The deepest sample has low Na/Cl (0.74), high Ca/Mg (5.0), and low&nbsp;</span>⁸⁷<span>Sr/</span>⁸⁶<span>Sr (0.7063), whereas the shallower samples have higher Na/Cl (0.86–1.2), Ca/Mg near 1, and higher&nbsp;</span>⁸⁷<span>Sr/</span>⁸⁶<span>Sr (∼0.7083). The data are consistent with an original seawater source being modified by various depth and&nbsp;lithology&nbsp;dependent diagenetic processes. Dilution by meteoric water occurs naturally on the east side of the valley, and in association with&nbsp;water-injectionactivities on the west side. Meteoric-water flushing, particularly on the east side, results in lower solute concentrations (minimum total dissolved solids 2730 mg/L) and total radium (minimum 0.27 Bq/L) in oil-field water, and promotes&nbsp;biodegradation&nbsp;of&nbsp;dissolved organic carbon&nbsp;and&nbsp;hydrocarbon gases&nbsp;like&nbsp;propane.&nbsp;Acetate&nbsp;concentrations and δ</span>¹³<span>C of&nbsp;dissolved inorganic carbon&nbsp;indicate biogenic&nbsp;methane&nbsp;production occurs in some shallow oil zones. Natural and human processes produce substantial variability in the&nbsp;geochemistry&nbsp;of oil-field water that should be considered when evaluating mixing between oil-field waters and groundwater. The variability could result in uncertainty as to detecting the potential source and impact of oil-field water on groundwater.</span></p> application/pdf 10.1016/j.apgeochem.2018.09.015 en Elsevier Regional patterns in the geochemistry of oil-field water, southern San Joaquin Valley, California, USA article