Page 16 - Environmental Control in Petroleum Engineering

Introduction to Environmental Control in the Petroleum Industry 5

although other times have been used. Common test organisms include
mysid shrimp or sheepshead minnows for marine waters and fathead
minnows or rainbow trout for fresh waters.
The concentration that is lethal to half of the exposed population
during the test is called LC 50. High values of LC 50 mean that high
concentrations of the substance are required for lethal effects to be
observed, and this indicates a low toxicity. A related measure of
toxicity is the concentration at which half of the exposed organisms
exhibit sublethal effects; this concentration is called EC 50. Another
measure of toxicity is the no observable effect concentration (NOEC),
the concentration below which no effects are observed.
The environmental impact of hydrocarbons in water varies consider-
ably (National Research Council, 1985). The toxicity of aromatic
hydrocarbons is relatively high, while that of straight-chain paraffins
is relatively low. LC 50 values for the most common aromatic hydro-
carbons found in the petroleum industry (benzene, toluene, xylene, and
ethylbenzene) are on the order of 10 ppm. Hydrocarbon concentrations
of less than 1 mg/1 in water have been shown to have a sublethal
impacts on some marine organisms. High molecular weight paraffins,
on the other hand, are essentially nontoxic. Chronic exposures of entire
ecosystems to hydrocarbons, either from natural seeps or from petro-
leum facilities, have shown no long- or intermediate-term impact; the
ecosystems have all recovered when the source of hydrocarbons was
removed. No evidence of irrevocable damage to marine resources on
a broad oceanic scale, by either chronic inputs or occasional major
oil spills, has been observed. Although there are short-term impacts
from major, spills, the marine resources can and do recover.
Other effects of hydrocarbons include stunted plant growth if the
hydrocarbon concentration in contaminated soil is above about 1% by
weight. Lower concentrations, however, can enhance plant growth
(Deuel, 1990). Hydrocarbons can also impact higher organisms that
may become exposed following an accidental release. Marine animals
that use hair or feathers for insulation can die of hypothermia if coated
with oil. Coated animals can also ingest fatal quantities of hydro-
carbons during washing and grooming activities.
The high dissolved salt concentration of most produced water can
also impact the environment. Typical dissolved salt concentrations for
produced water range between 50,000 and 150,000 ppm. By compari-
son, the salt concentration in seawater is about 35,000 ppm. Dissolved