Groundwater - Mathematics and Statistics for the Environment - page 581

21.1.1 g roundWater q uality
Genera lly, groundwater possesses high chemical, bacteriological, and physical quality. When pumped
from an aquifer composed of a mixture of sand and gravel, groundwater is often used without filtra-
tion (i.e., if not directly influenced by surface water). It can also be used without disinfection if it has
a low coliform count. However (as pointed out earlier), groundwater can become contaminated when,
for example, septic systems fail, saltwater intrudes, wastes are disposed of improperly, chemicals are
improperly stockpiled, underground storage tanks leak, hazardous materials are spilled, fertilizers
and pesticides are misapplied, and when mines are improperly abandoned. When groundwater is
removed from its underground water-bearing stratum via a well, water flows toward the center of the
well. In a water table aquifer, this movement will cause the water table to sag toward the well. This
sag is called the cone of depression . The shape and size of the cone are dependent on the relation-
ship between the pumping rate and the rate at which water can move toward the well. If the rate is
high, the cone will be shallow and its growth will stabilize. The area that is included in the cone of
depression is called the zone of influence; ; any contamination in this zone will be drawn into the well.

21.1.2 g roundWater under the d ireCt i nFluenCe oF s urFaCe W ater
Groundwater under the direct influence of surface water (GUDISW) is not classified as a ground-
water supply. When a supply is designated as GUDISW, the state's surface water rules apply to the
source rather than the groundwater rules. The Surface Water Treatment Rule of the Safe Drinking
Water Act requires each site to determine which groundwater supplies are influenced by surface
water (i.e., when surface water can infiltrate a groundwater supply and could contaminate it with
Giardia , viruses, turbidity, or organic material from the surface water source). To determine whether
a groundwater supply is under the direct influence of surface water, the U.S. Environmental Protection
Agency (USEPA) has developed procedures that focus on significant and relatively rapid shifts in
water quality characteristics such as turbidity, temperature, and pH. When these shifts can be closely
correlated with rainfall or other surface water conditions or when certain indicator organisms associ-
ated with surface water are found, the source is said to be under the direct influence of surface water.

21.2 AQUIFER PARAMETERS
Certain aquifer parameters are relevant to determining the available volume of water and the ease
of its withdrawal. We identify and define these relevant parameters in this section.

21.2.1 a quiFer p orosity
Aquifer porosity is defined as the ratio of the volume of voids (open spaces) in the soil to the total
volume. Simply stated, porosity is the volume of open space and is often determined using the fol-
lowing equation:

Φ= V
V

void
total

(21.1)

Two basic types of porosity are common: primary , formed at the time the rock was deposited,
and secondary , formed later (e.g., by dissolution of carbonate in caverns). Well-sorted materials
tend to have higher porosities than poorly sorted materials. Fine-grained sediments tend to have
higher porosities than coarse-grained sediments, although they are often poorly connected. Some
typical values of porosity are clay, 55%; fine sand, 45%; sand and gravel, 20%; sandstone, 15%;
and limestone, 15%. The interconnected or effective porosity (Φ e ) is the most important in hydrol-
ogy and is always ≤ Φ.