Carbonate Evolution of Groundwater Contaminated by Highly Saline Produced Water and Hydrocarbons

Abstract

The physical properties and the major ionic concentration and stable carbon isotope composition ( &#948;13CDIC) of dissolved inorganic carbon (DIC) was measured in groundwater at an abandoned oil production site near Lake Skiatook, Osage County, Oklahoma. The shallow aquifer is contaminated with highly saline produced water and hydrocarbons that were transported through a trench and stored in an unlined earthen pit. Results show the groundwater to have high concentrations of Cl- and Na<super+ from the mixing of produced water with natural groundwater. The produced water also contributed moderately high concentrations of Ca2+ and Mg2+, but extensive weathering and ion exchange has added significant quantities of Ca2+ and Mg2+ to the groundwater. Weathering of aquifer carbonates and dissolution of CO2(g) from root respiration and hydrocarbon degradation has altered the carbonate chemistry, adding DIC to the groundwater with an average &#948;13CDIC value of approximately -12 /. Continued weathering and precipitation due to dedolomitization has added isotopically enriched DIC to the groundwater and has shifted the &#948;13CDIC to higher than -12 /, indicating greater influence from carbonate dissolutions on the isotopic value of many groundwater samples. Residual hydrocarbons still exist throughout the site and are being degraded, further adding CO2 to the groundwater and weathering aquifer minerals. It is concluded that the addition of high concentrations of ions and degradation of residual hydrocarbons has altered the saturation state of carbonate minerals calcite and dolomite and dissolution and precipitation of these minerals alters the DIC concentration in the groundwater and enriches the groundwater in 13C.