Page 98 - Environmental Control in Petroleum Engineering

The Impact of Drilling and Production Operations 85

hydrocarbons has been found to vary considerably and generalizations
cannot be easily made. Factors that affect toxicity include molecular
weight, hydrocarbon family, the organism exposed to the hydrocarbon,
and life-cycle stage of the organism exposed (egg, larva, juvenile, or
adult). For mixtures of hydrocarbons, such as crude oil, the toxicity
also depends on the history of the exposure.
For hydrocarbons of a similar type (the same family), the toxicity
tends to increase with decreasing molecular weight. Smaller molecules
tend to be more toxic than large molecules. Light crude oils and
refined products tend to be more toxic than those of heavy crude
oils, because heavy crude oils have a higher average molecular weight.
For similar molecular weight hydrocarbons, the toxicity varies with
family. The toxicity of hydrocarbon families generally increases in the
following order: alkanes, alkenes, cycloparaffins, aromatics, and poly-
aromatic hydrocarbons.
Some of the least toxic hydrocarbons include dodecane and higher
paraffins. In fact, these high molecular weight paraffins are used in
cooking, food preparation, and candles. The most toxic hydrocarbons
are the low-boiling-point aromatics, particularly benzene, toluene,
ethylbenzene, and xylene. Because of their similar properties, these
four aromatic molecules are commonly referred to as BTEX. The most
toxic hydrocarbons also tend to have a high solubility in water. A high
solubility makes a molecule more accessible for uptake by plants
and animals.
The toxicity of a given hydrocarbon varies considerably with the
organism exposed. Factors that also affect the toxicity to a particular
organism include the general health of the organism and whether the
organism is already stressed. Stress factors include water salinity,
temperature, and food abundance. The toxicity of crude oil to some
fish can be twice as high in seawater as in fresh water. The toxicity
of a particular hydrocarbon also appears to increase with decreasing
temperature. Synergistic effects from the presence of other toxins can
also significantly alter the toxicity of specific hydrocarbons.
The toxicities (LC 50) for a variety of aromatic and polyaro-
matic hydrocarbons are shown in Tables 3-4a and 3-4b (National
Research Council, 1985). The LC 50 values for many aromatic hydro-
carbons are less than about 5 ppm, although some have values as high
as 28 ppm. From these tables, it can also be seen that the toxicity
is higher (lower LC 50) for higher molecular weight polyaromatic