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3.19 Groundwater Quality Management 107
3. The vertical position of each aquifer and the hydraulic characteristics of the over-
lying and underlying beds
4. The effect of proposed withdrawals on the recharge and discharge characteristics
of the aquifer

Thus it is evident that the safe yield of an aquifer is not necessarily a fixed quantity, and it
is not strictly a characteristic of the groundwater aquifer. It is a variable quantity dependent
on natural hydrogeologic conditions and on recharge and discharge regimes. Safe yield has
been defined in a variety of ways, each definition placing emphasis on a particular aspect
of groundwater resource development. These include, within economic limits, (a) develop-
ment to the extent that withdrawals balance recharge and (b) development to the extent that
change in the quality of groundwater allows.

3.19 GROUNDWATER QUALITY MANAGEMENT
In a majority of cases when polluted water has been drawn from wells, the contamination
was introduced at the well site, indicating faulty construction. There are, however, numer-
ous examples of contamination of groundwater caused by disposal of wastes. Once
groundwater is contaminated, the impairment of the groundwater resource is long lasting,
and recovery is extremely slow.
To predict where the contaminating fluids will go requires a three-dimensional geo-
logic, hydrodynamic, and geochemical analysis. The rate and extent of the spread of pollu-
tion are controlled by (a) the characteristics of the source of pollution, (b) the nature of
rock formations in the unsaturated and saturated zones, and (c) the physical and chemical
properties of the contaminant. The phenomena governing the disposition of the contami-
nant are capillary attraction, decay, adsorption, dispersion, and diffusion.
There have been numerous examples of contamination of groundwater by wastes al-
lowed to seep into the ground, wastes discharged into pits and ponds, and leaks from holding
tanks and sewers. The safe distance from a polluting source of this type is determined to a
large degree by the velocity of percolation through the unsaturated zone and by the lateral
movement once the contamination reaches groundwater. Water table aquifers, being near
the surface and having a direct hydrologic connection to it, are more subject to contamina-
tion than are deeper lying artesian aquifers.
The discharge of wastes into streams has had both direct and indirect effects on the
quality of groundwater. The polluted rivers that cross recharge areas of artesian aquifers
tapped by wells have affected the quality of their discharge. The aquifers that are replen-
ished by infiltration from polluted streams will eventually be contaminated by soluble
chemical wastes carried in the stream. Induced contamination of an aquifer can result
when the cone of depression of a discharging well intersects a polluted river. This is fre-
quently the case in coastal areas in wells located near streams containing brackish water.
Artificial recharge with river water of poorer quality than that found in the aquifer will
ultimately result in the deterioration of the quality of groundwater.

3.19.1 Biological Contamination
Because of increasing numbers of septic tanks and growing use of effluents from waste-
water treatment plants for artificial recharge of aquifers, the potential for contamination
of groundwater by bacteria and viruses needs to be considered. Filtration through gran-
ular material improves the biological quality of water. A 10-ft (3-m) downward perco-
lation in fine sand is capable of removing all bacteria from water. The length of time