Page 87 - Environmental Control in Petroleum Engineering

74 Environmental Control in Petroleum Engineering

the substance is called teratogenic. If the mutation results in cancer,
the substance is called carcinogenic. As a rule, nearly all carcinogens
are also mutagens, but not all mutagens are carcinogens.
For nonmutagenic or noncarcinogenic substances, a threshold dose
is assumed to exist, below which there is no toxic effect. The threshold
dose depends on the ability of the organism to detoxify and excrete
the substance and repair any damage through normal biological pro-
cesses. If an organism is exposed to a dose higher than one that can
be repaired by normal biological processes, then toxic impact will
occur. The magnitude of this impact will increase as the dose increases
over the threshold dose. Although some substances are toxic in high
concentrations, they may be essential in low concentrations for normal
biological processes. These required substances include trace minerals
and heavy metals commonly found in petroleum operations. Bioassays
are generally not able to determine this information. For carcinogenic
or mutagenic substances, however, it is assumed that there is no
threshold dose. The impact is assumed to increase with the dose over
all exposure levels.
Another significant limitation to bioassays is the time it takes for
results to be obtained. Bioassays typically take two to three weeks to
be completed. A related difficulty is that these tests are normally
conducted off-site, which requires shipping of the fluid samples and
delays in starting the tests. These delays can affect test results because
the fluid chemistry can change over time. All of these difficulties
prevent on-site decisions from being made about the fluid system,
particularly drilling muds. They can result in a drilling mud from an
offshore platform being shipped to shore for more expensive disposal,
when it could legally be discharged overboard. A considerable effort
is underway to develop more rapid bioassays for drilling fluids,
particularly those that can be performed on-site.
One potentially valuable method for rapid toxicity characterization
is the Microtox method (Hoskin and Strohl, 1993). In this method, a
marine luminescent bacterium, Photobacterium phosphoreum, is used.
These bacteria emit light as part of their metabolic processes. Exposure
to a toxic substance interferes with these processes and results in a
reduction in their light output. An advantage of the Microtox method
is that the test is conducted in 15 minutes. A related process for
measuring the toxicity of materials is the cumulative bioluminescence
of Pyrocystis lunula (Wojtanowicz et al., 1992). The correlation