Geoscience Reference
In-Depth Information
Chapter 17
Contaminant-Induced Irreversible
Changes in Groundwater Chemistry
Chemical contaminants disposed of on the land surface may be transported to the
groundwater, interacting with the solid phase or largely bypassing the vadose zone
by migrating rapidly through preferential flow paths. The rate and magnitude of
transport in preferential flow paths are governed by the properties of the solid
phase and the contaminants themselves.
The background chemistry of groundwater is defined by the properties of the
aquifer solid phase and by natural processes that occur in the groundwater envi-
ronment. Groundwater chemistry may, however, be altered by chemicals origi-
nating from anthropogenic activities, via areal source (diffusive) or point source
contamination. Because many groundwater aquifers are open systems, the extent
and persistence of groundwater contamination are governed largely by the low rate
of groundwater flow; the slow movement of contaminants results in relatively low
mixing rates and dilution. Slow groundwater flow and contaminant mixing rates
allow contamination to remain localized over long periods of time (Pye and
Patrick
1983
), while the effect of biological processes on natural attenuation of
contaminants decreases (Hemme et al.
2010
).
A review of groundwater remediation efforts in the United States, using various
techniques, shows that of 1,700 polluted sites on the US EPA's national priority
list, only 363 have reached drinking water quality (Ehlers and Kavanaugh
2013
).
These authors also quote a US National Research Council Committee's report on
contaminated groundwater sites, which notes that persistent, significant limitations
will prevent achievement of drinking water standards in many contaminated
aquifers over a time scale of at least 50-100 years. As a consequence, the com-
position and concentration of groundwater contaminants often remain close to
their initial state. Under such conditions, we can consider groundwater contami-
nation as irreversible at least on a human lifetime scale.
In this chapter, we present examples of several specific irreversible changes in
the
chemistry
of
groundwater
which
result
from
anthropogenically
induced
contamination.
