GROUNDWATER - Water Quality Engineering in Natural Systems - page 141

5

GROUNDWATER

5.1 INTRODUCTION 5.2 CONTAMINANT SOURCES

Groundwater is a major source of drinking water in
the United States, supplying approximately 40% of
public water utilities and accounting for almost all of
the water supply to rural households. Approximately
50% of the U.S. population relies on groundwater
sources for drinking water. The direct use of groundwa-
ter for drinking is the reason why drinking-water stan-
dards are usually applied to groundwater, and the
reason why groundwater contamination is such a sensi-
tive issue. A typical groundwater contamination sce-
nario is shown in Figure 5.1, where the contaminant
source is located on the ground surface and the con-
taminant plume is migrating toward a water supply well.
Groundwater frequently contributes the baseflow to
rivers and streams, a condition that occurs when the
river stage is lower than the adjacent water table. Under
these conditions, groundwater inflow can contaminate
rivers and streams.
Regulations associated with groundwater and well-
head protection programs require engineers to predict
the fate and transport of contaminants released either
directly into the groundwater or on land surfaces above
the groundwater. These quantitative predictions are
used to assess the impact of existing or potential con-
taminant sources on groundwater quality, to design
systems to mitigate any deleterious effects, and to design
systems to remediate contaminated groundwater.

Common sources of groundwater contamination are
septic tanks, leaking underground storage tanks
(LUSTs), land application of wastewater, irrigation and
irrigation return flow, solid waste disposal sites (i.e.,
landfills), waste disposal injection wells, and hazardous
chemicals in agriculture. Contaminants introduced into
groundwater from these sources include petroleum
products, volatile organic compounds, nitrates, pesti-
cides, and metals.

5.2.1 Septic Tanks

Septic tanks discharge pathogenic microorganisms, syn-
thetic organic chemicals, nutrients (such as nitrogen and
phosphorus), and other contaminants directly into the
groundwater and have the potential to cause illness if
drinking-water sources are too close to the septic tanks.
A typical two-chamber septic tank is shown in Figure
5.2. In addition to siting concerns, for septic tanks to
work properly, it is important that a zone of unsaturated
soil exist between the leach bed and the water table so
that the effluent from the septic tank does not enter the
groundwater directly.
The discharge from septic tank systems is commonly
estimated as 280 L/capita·d (75 gal/capita·d), and this
effluent typically contains 40-80 mg/L of nitrogen, 10-
30 mg/L of phosphorus, and 200-400 mg/L of BOD 5 .