Geology Reference
In-Depth Information
232
III. Introduction to Water Resources and Contamination
brines; specifically we will look at a case where
groundwater contamination has led to the deteriora-
tion of surface-water quality. A generalized diagram
showing movement of groundwater into a stream is
shown in Figure 15.5.
The water table usually lies at a depth of a few
feet and follows the general topography of the land
surface; that is, the water table is at a higher elevation
under hills than it is in nearby low-lying areas.
Groundwater moves in the direction of the water-table
gradient, from higher pressure to lower pressure,
which often may mean from higher elevation to lower
elevation. Where the land surface is lower than the
water table, such as at a swamp, lake, or stream,
groundwater will flow onto the land surface.
Rainwater has a low mineral content, but as it
slowly infiltrates soils and bedrock and perhaps flows
great distances, its mineral content increases. The types
and concentration of constituents in groundwater
reflect the composition of the soils and rocks through
which the water has moved. The naturally occurring
concentrations of elements in groundwater are called
background concentrations.
If a water-soluble contami-
nant is allowed to infiltrate the ground, it will increase
the concentrations of elements and compounds in the
groundwater and may contaminate it so it is unusable.
In some parts of the world, an indirect but signif-
icant cause of surface-water contamination is disposal
of oil-field brines. The brines, which are highly con-
centrated solutions consisting largely of sodium chlo-
ride (NaCl), are pumped from the ground with the oil.
The mixture is routed through a separator which
removes the oil from the brine. The oil flows to storage
tanks while the brine most commonly is discharged to
an unlined holding or evaporation pond (see Part A of
this exercise) or is pumped back into the bedrock. In
some climates where ponds are used, only a very small
part of the brine evaporates; most of it infiltrates.
Brines sterilize soil, kill vegetation, and create an
undesirable taste in water. The U.S. Environmental
Protection Agency has recommended that drinking
water should contain no more than 250 mg/L of chlo-
ride because higher concentrations cause a salty taste.
Even with careful regulation of the extractive
industries, higher background readings of some
components can be expected in some areas due to nat-
ural weathering and erosion. Before there was any
human exploitation of oil in North America, natural
brine seeps and springs degraded water quality in
some areas.
Alum Creek Basin, Ohio
Low relief and a relatively high water table character-
ize this watershed in central Ohio. Much of the agricul-
tural land is artificially drained. The region is underlain
by a clayey glacial till that in many places contains thin
layers of gravel. The till ranges from a few inches to
several tens of feet in thickness. Oil-bearing reservoirs
underlie much of the region at an average depth of
about 3,500 feet.
Sixty-five water samples were collected in late
autumn from streams in the upper part of the basin.
Because there was no rain or surface runoff for several
days prior to collection, and because groundwater dis-
charges to the streams, the stream data represent the
quality of the groundwater. The samples were analyzed
to determine chloride ion concentrations. Chloride is a
common constituent in oil field brines (Table 15.2).
Stream sampling sites and locations of existing or aban-
doned gas or oil wells, including dry holes, are shown
in Figure 15.6.
QUESTIONS 15, PART B
1. Using the data in Table 15.2 and Figure 15.6, construct a
surface-water quality map. Assume that the chloride content
at a station reflects the quality between that site and the next
upstream station. Mark in blue the stream reaches that con-
tain 25 mg/L or less of chloride. Use brown for reaches that
contain more than 25 but less than 50 mg/L, and red for
reaches that exceed 50 mg/L. A map of this type not only
presents an obvious picture of the change in water quality
from one area to the next, but it also can be used to deter-
mine the major source areas of contamination. Use only
those data representing sample sites 1-41. Sites 42-65 on the
other streams can be examined if you are interested or if the
instructor assigns them. You may substitute other colors or
patterns for the colors suggested above.
2.
Briefly describe the quality of the water in Alum Creek
basin using the map you completed in Question 1.
3. What areas are the major sources of chloride contamina-
tion in the drainage basin?
4. Why did the chloride concentration decrease between sites
21 and 25?
FIGURE 15.5 Generalized diagram illustrating contamination
of groundwater and surface water by pit disposal of oil field
brines (Pettyjohn, 1972, p. 168).
