Predicted Impacts From Offshore Produced-Water Discharges on Hypoxia in the Gulf of Mexico

Abstract

Abstract Summer hypoxia (dissolved oxygen < 2 mg/L) in the bottom waters of the northern Gulf of Mexico has received considerable scientific and policy attention because of potential ecological and economic impacts. This hypoxic zone forms off the Louisiana coast each summer and has increased from an average of 8,300 km2 in 1985–1992 to over 16,000 km2 in 1993–2001, reaching a record 22,000 km2 in 2002. The almost 3-fold increase in nitrogen load from the Mississippi River Basin (MRB) to the Gulf since the middle of the last century is the primary external driver for hypoxia. A goal of the 2001 Federal Action Plan is to reduce the 5-year running average size of the hypoxic zone to below 5,000 km2 by 2015. After the Action Plan was developed, a new question arose as to whether sources other than the MRB may also contribute significant quantities of oxygen-demanding substances. One very visible potential source is the hundreds of offshore oil and gas platforms located within or near the hypoxic zone, many of which discharge varying volumes of produced water. The objectives of this study were to assess the incremental impacts of produced water discharges on dissolved oxygen in the northern Gulf of Mexico, and to evaluate the significance of these discharges relative to loadings from the MRB. Predictive simulations were conducted with three existing models of Gulf hypoxia using produced water loads from an industry study. Scenarios were designed that addressed loading uncertainties, settleability of suspended constituents, and different assumptions on delivery locations for the produced water loads. Model results correspond to the incremental impacts of produced water loads, relative to the original model results which included only loads from the MRB. The predicted incremental impacts of produced water loads on dissolved oxygen in the northern Gulf of Mexico from all three models were small. Even considering the predicted ranges between lower- and upper-bound results, these impacts are likely to be within the errors of measurement for bottom water dissolved oxygen and hypoxic area at the spatial scale of the entire hypoxic zone.