Toxicity Testing With Bioluminescence
Authors: Sabata R.W. , Dewailly E.L. 
Published: October 2008

Quenching the natural bioluminescence of a common marine organism, Pyrocystis lunula, may provide a less expensive, more precise, and faster measurement of toxicity. Limitations on discharges of toxic substances have been imposed by Environmental Protection Agency (EPA) and various state agencies. The new method has been applied to agricultural pesticides and is being tested for effectiveness on industrial point-source discharges. Scientific validation of this method for drilling fluids is presentlyunder experimental investigation at Louisiana State University. Bioluminescence testing of drilling fluids is expected to correlate well with the EPA LC-50 protocol, which uses the shrimp Mysidopsis bahia, and promises to be more repeatable and reliable.

BACKGROUND

Regulatory

Each of the 10 Environmental Protection Agency (EPA) Regions is empowered to impose toxicity testing requirements on permits it issues under the National Pollutant Discharge Elimination System (NPDES) (1). Overlain on this permitting system is a 1986 EPA general permit for offshore discharges during oil and gas drilling operations in the Gulf of Mexico. Central to the requirements there under is the need to determine the concentration of toxins indrilling fluids before discharging them into the sea. Toxicity levels of basic drilling muds and common additives first appeared in 1986 (2), and the list was supplemented in 1989 (3). This information is helpful in concocting benign muds; nevertheless, the mud still must be subjected, to the EPA toxicity test protocol (4). Offloading toxic mud and barging it to an onshore hazardous waste site can cost as much as $400,000. Non-compliance can cost $25,000 a day in fines.

Bioassay

Mysidopsis bahia.

This is the specific name of a small shrimp used in the Mysid LC-50 toxicity test, which follows the EPA protocol. The testing procedure may be outlined in simple terms. A sample of drilling mud is diluted with seawater and allowed to settle into 3 distinct phases: filtrate, solid, and suspended-particle phase (SPP, Figure 1). A number of different SPP concentrations, from 0% (control) to 100%, are prepared in glass beakers, and 20 Mysid shrimp are placed in each beaker. After 96 hours, the live and dead shrimp in each beaker are counted. The results may be plotted on a graph of concentrations mortality (Figure 2). From this graph, the lethal concentration sufficient to kill 50% (LCSO) of the shrimp may be computed: thelower the number, the greater the toxicity. Actual calculation of LC-50 is done with mathematical-statistical formulae. Maximum allowable LC-50 for discharging muds is 3%, or 30,000 ppm (parts per million). Testing must be done when drilling commences, when it ends, and at least once monthly.

Other existing bioassays.

Other organisms and procedures, including a 48hourMysid LC-50 test, are being used to monitor mud toxicity to minimize the possibility of failing the protocol test. All of these have such limitations in precision or application as to disqualify them from operator confidence and EPA approval. Operators publicly (5) have expressed the need for a more reliable toxicity test.