Page 102 - Environmental Control in Petroleum Engineering

The Impact of Drilling and Production Operations 81

exposure to hydrocarbons than the young. For all of the species
included in this table, however, the LC 50 values are below 10 ppm.
indicating a high toxicity at all life-cycle stages.
An important factor affecting the toxicity of crude oils is their
history before any organisms are exposed. Because the most toxic
hydrocarbons are also the most volatile, they rapidly evaporate from
a release site. Within a few days after a crude oil release, only higher
molecular weight hydrocarbons remain, so the toxicity of the remain-
ing crude oil is lower. Hydrocarbons in water also tend to adsorb onto
suspended sediments, making them much less bioavailable to marine
organisms than hydrocarbons in solution or dispersion in water. This
further lowers the toxicity of released crude oil. If the sediments
accumulate on the bottom of the sea, they can accumulate in estuarine
organisms. The accumulation and metabolism of these compounds,
however, vary with species (American Petroleum Institute, 1989e).

Impact of Crude Oil on Marine Animals

The actual impact of hydrocarbon exposure on marine animals is
more complex than simple bioassay tests reveal. Oil at sublethal
concentrations can significantly alter the behavior and development of
marine organisms. These effects, however, are difficult to quantify. The
problem of determining sublethal toxicity is further compounded
because different species have different reactions and there is mixed
effect when multiple toxins are present. Although there is a tremendous
amount of scatter in the data, most threshold concentrations of crude
oil in water for effects to be observed for eggs, embryos, and larvae
of marine fish are between 0.01 and 5 mg/1 (National Research
Council, 1985).
Behavioral changes from exposure to hydrocarbons are primarily
those involving motility, while in higher organisms, changes affect
avoidance, burrowing, feeding, and reproductive activities. Behavioral
changes in feeding have been observed at hydrocarbon concentrations
as low as a few microgm/1. Other measures of sublethal effects include
changes in respiration, the ratio of oxygen consumed to nitrogen
excreted, biochemical enzyme assays, and cellular activity. The respira-
tory rate following exposure is usually reduced, although in some
cases, it is increased. The level of exposure for respiratory impact for
fish and planktonic crustaceans in the laboratory is less than 1 mg/1.