Water and the Hydrogeology of Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 107

3.7.2 Anthropogenic Groundwater Influences
Documentation of anthropogenic influences on groundwater in urban areas is a recent
phenomenon (Barber 1997). The late arrival of this technique occurred because the connec-
tion between anthropogenic processes at the surface and shallow groundwater in urban
areas is complex and difficult to investigate—or was simply not thought to be important.
Some anthropogenic influences on groundwater include (Howard 1997; Sharp 1997)
• Excessive groundwater withdrawal resulting in saltwater intrusion along coastal
areas.
• Significant decreases in groundwater surface elevations in arid regions or any
region where a consistently high volume of groundwater withdrawal occurs.
• Decreasing natural groundwater recharge resulting from urban development and
the construction of impervious surfaces.
• Decreasing residence time of water in shallow aquifers in urban areas through
construction and de-watering efforts.
• Decreasing residence time of water in deeper aquifers through excessive
withdrawal.
• Increasing anthropogenic groundwater recharge resulting from leaking sewers,
septic tanks, and injection wells.
• Ground surface subsidence resulting from excessive groundwater withdrawal;
an event frequently evident in urban regions where groundwater withdrawal is
excessive.
• Flooding in urban areas due to the increase in impervious surfaces as a by-prod-
uct of urban development.
• Excessive sewer leakage resulting in groundwater quality degradation and poten-
tial flooding.
• Increased potential for land subsidence and landslides in areas near dams and
reservoirs due to rising groundwater levels.
• Direct and indirect discharges of contaminants to groundwater through acciden-
tal releases, leaking sewers, septic systems, injection wells, and residential, com-
mercial, and industrial discharges.
3.7.2.1 Saltwater Intrusion
Fresh groundwater in the United States is surrounded by saline groundwater both verti-
cally and laterally. This is especially evident along the coastlines as noted in Section 3.6.4,
although much of the interior of the country is underlain by deep saline aquifers (Alley
et al. 1999). This juxtaposition of freshwater and saltwater creates the potential for salt-
water intrusion and rendering the freshwater nonpotable. This scenario can occur from
installing and pumping a water well placed too close to a saltwater source and is shown
in Figure 3.51 (Alley et al. 1999).
3.7.2.2 Changes in Groundwater Flow
As established in Section 3.5.8, a minimum of three monitoring wells are necessary to
estimate the direction of groundwater flow. Because urban areas are complex, using the