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

Abstract

Summary The offshore oil and gas industry has moved toward the use of synthetic-based drilling fluids (SBFs), changing potential exposure scenarios for discharged cuttings when compared to those of water-based drilling fluids (WBFs). Unlike WBFs, SBFs sorb 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 (EPA) in addition to the existing water-column test to define a bestavailable-technology (BAT) limit. Inclusion of a sediment-toxicity test for National Pollutant Discharge Elimination System (NPDES) compliance was precedent-setting and unique. To fulfill the US EPA requirements, an interindustry research group worked with the 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 (C16–C18 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. Before the use of this test, there were concerns that false positive results could lead to incorrectly identified noncompliance events, limiting the use of SBF technology. Consequently, initial application allowed the use of a variability factor (K-factor) in determining the ratio. After use of the compliance test was initiated, research was continued to reduce test variability and minimize false positives. This research included (1) analysis of NPDES-compliance data (500+tests), (2) two interlaboratory testing programs, (3) analysis of reference-fluid data from one commercial laboratory, and (4) refinements to test sediment type and effects on 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.