Development, Verification, and Improvement of a Sediment Toxicity Test for Regulatory Compliance

Abstract

Abstract The offshore oil and gas industry has moved toward the use of synthetic-based drilling fluids (SBF), changing potential exposure scenarios for discharged cuttings when compared to those of water-based drilling fluids (WBF). Unlike WBF, SBF sorbs predominately to particles in the cuttings and are not dispersed extensively into the water column, therefore, a sediment toxicity test was required by the US Environmental Protection Agency (US EPA) in addition to the existing water column test to define a best available technology (BAT) limit. Inclusion of a sediment toxicity test for NPDES compliance was precedent setting and unique. In order to fulfill the US EPA requirements, an inter-industry research group worked with EPA to develop a suitable test that met the technology-based discharge standard. Toxicity of discharged field drilling fluid is compared to a reference SBF (C1618 internal olefin) and, for compliance, the ratio of the reference drilling fluid median lethal concentration (LC50) to the field mud LC50 must be ≤1.0. Prior to its use, there were concerns that false positive results could cause incorrectly identified non-compliance events, limiting the use of SBF technology. Consequently, initial application allowed the use of a variability (K) factor in determining the ratio. After initiation as a compliance test, research was continued to reduce test variability and minimize false positives. That research included: 1) analysis of NPDES compliance data (500+ tests), 2) two inter-laboratory testing programs, 3) analysis of reference fluid data from one commercial laboratory, and 4) refinements to test sediment type and animal health. The results of these efforts to date are reviewed in this paper and are used to identify potential improvements in the application of the test as a regulatory tool.