Page 113 - Environmental Control in Petroleum Engineering

100 Environmental Control in Petroleum Engineering

water flea for a variety of pure salts are on the order of 1,000 mg/1
(Mount et al., 1993). Studies indicate that a concentration of 230 mg/1
for total dissolved solids may be sufficient to protect warm water
species in natural streams. No significant change in macro invertebrate
behavior was observed below a level of 565 mg/1 (Vickers, 1990).
Because the salinity of many produced waters is greater than that
of marine waters, the environmental impact of high salt concentrations is
also of concern regarding marine organisms. Highly saline water has a
higher density than seawater and will segregate to the bottom of any
surface waters. This density gradient inhibits the mixing and dilution
of the very salty water. This segregation is only a problem in shallow
estuaries and marshes that allow little dilution (St. Pe et al,, 1990).
The impact of a saline brine spill in a saltwater marsh was observed
in 1989 following a spill of about 35 million gallons of brine (Bozzo
et al., 1990). The salinity of the brine varied between 0 and 274 parts
per thousand (ppt), with 17 million gallons having a salinity over 220
ppt. In comparison, seawater has a salinity of 35 ppt. Following the
spill, vegetation in areas with poor drainage and along drainage
channels was completely killed. Flushing from rainwater, turbulent
mixing from nearby barge traffic, and tidal events lowered the salinity
in the soils around the spill to ambient levels within a few months.
Salt-tolerant plants began growing in the areas where the salt had
killed the less tolerant plants. The following year, vegetation in all
areas except those most severely affected showed signs of recovery,

3.4 HEAVY METALS

The heavy metals encountered in drilling and production activities
are related to a variety of environmental concerns, depending on the
metal and its concentration. At very low concentrations, some metals
are essential to healthy cellular activity. Essential metals include
chromium, cobalt, copper, iodine, iron, manganese, molybdenum,
nickel, selenium, silicon, vanadium, and zinc (Valkovic, 1978). At high
concentrations, however, metals can be toxic. Because most concentra-
tions encountered during drilling and production are relatively low, the
environmental impact is generally observed only after chronic exposure.
The environmental impact of heavy metals is manifested primarily
through their interaction with enzymes in animal cells. Enzymes are
complex proteins that catalyze specific biochemical reactions. Heavy