Page 88 - Environmental Control in Petroleum Engineering

The Impact of Drilling and Production Operations 75

between these two bioluminescence methods with each other and with
mandated mysid shrimp toxicity assays, however, has not been good,
Part of this poor correlation is from the poor reproducibility of the
mysid shrimp toxicity tests between different laboratories. Mysid
shrimp assays are discussed below in the section on drilling fluids,
Another rapid toxicity assay that has been studied is the fertiliza-
tion rate of sea urchins. In this test, sea urchin sperm and eggs are
collected, exposed to the substance being tested, and then combined.
The fraction of eggs fertilized is then measured. Like the biolumi–
nescence tests, the fertilization assays correlate poorly with the mysid
shrimp toxicity tests (American Petroleum Institute, 1989f).
Another approach to toxicity testing is to develop correlations
between chemical characteristics of the substance and the bioassays.
One such correlation was attempted for a high-weight lignosulfonate
drilling fluid (American Petroleum Institute, 1985c). The chemical
characteristics studied include pH, redox potential, sulfide concen-
tration, dissolved chromium, saturated aliphatic hydrocarbons, non-
volatile aromatic hydrocarbons, .straight-chain alkanes, volatile aro-
matic hydrocarbons, unidentified volatile hydrocarbons, and bacterial
activity. Dissolved chromium was found to correlate the best with the
mysid shrimp toxicity for the mud liquids. Unidentified volatile
hydrocarbons correlated the best for the toxicity of the solids. These
correlations, however, were poor, accounting for only about 20% of
the toxic effects.
Although bioassays are conducted on animals, the results are often
used to determine acceptable levels for human exposure. Animal
toxicity data are extrapolated to humans, using factors like body weight
and a variety of safety factors. Based on these extrapolations, a variety
of human health and safety guidelines have been developed.
Human safety guidelines include threshold limit values—time
weighted average (TLV-TWA), threshold limit values—short-term
exposure (TLV-STEL), threshold limit value—ceilings (TLV-C), and
reference dose (RfD). The TLV-TWA is the time-weighted average
concentration for a normal 8-hour workday and 40-hour workweek to
which nearly all workers can be chronically exposed without adverse
effects. The TLV-STEL is the highest short-term exposure to which a
worker can be exposed without the worker experiencing irrita-
tion, chronic or irreversible tissue change, or narcosis at a level that
impairs judgment or work efficiency. The TLV-C is the concentration