POTENTIAL IMPACT FROM PRODUCED WATER AT OIL AND GAS SITES IN LOUISIANA

Formation water from older oil and gas production ponds has been released into shallow groundwater at numerous locations in Louisiana. Formation water may have chloride concentrations in excess of 30,000 mg/L. The EPA secondary drinking water standard for chloride is 250 mg/L; surface water standards may be as low as 10 mg/L. Therefore, there is a concern that the migration of shallow impacted groundwater may degrade drinking water aquifers or surface water bodies.

We evaluated a former oil and gas production site using a groundwater model to assess the risk of chloride migration in groundwater to potential receptors. The site geology was a Holocene alluvial setting with interbedded clay and silt layers to a depth of approximately 100 feet. The groundwater model and risk evaluation accounted for two groundwater risk pathways: (1) protection of surface water with a standard of 60 mg/L and a point of exposure approximately 5000 feet away, and (2) protection of an underlying aquifer with a secondary EPA MCL of 250 mg/L.

A coupled MODFLOW and MT3D model was used to evaluate the site. The model projected that groundwater concentrations could exceed the surface water protection standard at the point of exposure after about 5,000 years. The underlying aquifer was predicted not to become impacted. Regulatory policy for site closure required that the site be monitored for five years to verify the that the model predictions and site conditions were protective. The results of monitoring showed that chloride concentrations in all wells were declining and that the predicted chloride migration was conservative relative to observed concentrations.

The site received regulatory closure on the basis of the model predictions and monitoring results. The evaluation indicated that shallow chloride plumes may not be significant threats to surface water or deeper aquifers in fine-grained alluvial settings. However, although the model did not account for density effects, modeled conditions appeared to be more conservative than observed site concentrations. The results from this site and experience from others indicate that chlorides in fine-grained alluvium may tend to disperse more than advect, and that standard groundwater transport models may be overly conservative.