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Chapter 3
Water Sources: Groundwater
EXAMPLE 3.10 DETERMINATION OF NET GROUNDWATER FLOW TO A LAKE
Determine the net groundwater flow to a lake G, which equals to the groundwater inflow to the lake G , minus the groundwater
in
outflow G . Use a water budget for year 2012 in which the rainfall (precipitation) R was 44 in. (1,117.6 mm), the evaporation E was
out
55 in. (1,397.0 mm), the surface-water inflow Q was 1.2 in. (30.5 mm), the surface-water outflow Q
in
the change in water storage ΔS was –3 in. (–76.2 mm), and the evapotranspiration E was in negligible amount.
Solution 1 (US Customary System):
G = G − G
=ΔS − R + E + E − Q + Q
in
out
=−3 − 44 + 55 + 0 − 1.2 + 170
= 176.8 in. for the year. t in t out out was 170 in. (4,318.0 mm),
Solution 2 (SI System):
G = G − G out =ΔS − R + E + E − Q + Q out
in
in
t
=−76.2 − 1,117.6 + 1,397.0 + 0 − 30.5 + 4,318.0
= 4,490.7 mm for the year.
where ΔS = change in water storage; G in = groundwa- direct hydrologic connection to it, are more subject to con-
ter inflow; G out = groundwater outflow; E = evaporation; tamination than are deeper lying artesian aquifers.
R = rainfall or precipitation; E = evapotranspiration; The discharge of wastes into streams has had both direct
t
Q out = stream outflow; Q = stream inflow; and G = ground- and indirect effects on the quality of groundwater. The pol-
in
water flow to a lake. luted rivers that cross recharge areas of artesian aquifers
tapped by wells have affected the quality of their discharge.
The aquifers that are replenished by infiltration from polluted
3.19 GROUNDWATER QUALITY streams will eventually be contaminated by soluble chemical
MANAGEMENT wastes carried in the stream. Induced contamination of an
aquifer can result when the cone of depression of a discharg-
In a majority of cases when polluted water has been drawn ing well intersects a polluted river. This is frequently the
from wells, the contamination was introduced at the well site, case in coastal areas in wells located near streams containing
indicating faulty construction. There are, however, numer- brackish water. Artificial recharge with river water of poorer
ous examples of contamination of groundwater caused by quality than that found in the aquifer will ultimately result in
disposal of wastes. Once groundwater is contaminated, the the deterioration of the quality of groundwater.
impairment of the groundwater resource is long lasting and
recovery is extremely slow.
To predict where the contaminating fluids will go 3.19.1 Biological Contamination
requires a three-dimensional geologic, hydrodynamic, and
geochemical analysis. The rate and extent of the spread of Because of increasing numbers of septic tanks and growing
pollution are controlled by (a) the characteristics of the source use of effluents from wastewater treatment plants for artifi-
of pollution, (b) the nature of rock formations in the unsatu- cial recharge of aquifers, the potential for contamination of
rated and saturated zones, and (c) the physical and chemical groundwater by bacteria and viruses needs to be considered.
properties of the contaminant. The phenomena governing the Filtration through granular material improves the biologi-
disposition of the contaminant are capillary attraction, decay, cal quality of water. A 10 ft (3 m) downward percolation
adsorption, dispersion, and diffusion. in fine sand is capable of removing all bacteria from water.
There have been numerous examples of contamination The length of time bacteria and viruses may survive and the
of groundwater by wastes allowed to seep into the ground, distances they may travel through specific rock materials in
wastes discharged into pits and ponds, and leaks from holding different subsurface environmental conditions are uncertain.
tanks and sewers. The safe distance from a polluting source They seem to behave in a manner similar to the degrad-
of this type is determined to a large degree by the velocity able and adsorbable contaminants. Under favorable condi-
of percolation through the unsaturated zone and by the lat- tions some bacteria and viruses may survive up to at least
eral movement once the contamination reaches groundwater. 5 years in the underground environment. However, the dis-
Water table aquifers, being near the surface and having a tances traveled in both the saturated and unsaturated media