Civil Engineering Reference
In-Depth Information
be the maximum that economics, land ownership, geology, and topography will permit.
If possible, a well site would be located at an elevation higher than that of any potential
source of contamination. The direction of groundwater flow does not always follow
the slope of the land surface, so the slope of the water table should be determined
from observation wells.
Groundwater quality in aquifer systems varies spatially and may range from good
to unacceptable. Water-quality problems vary from high concentrations of dissolved
solids to small amounts of trace elements, organics, and pathogens that exceed drinking
water standards. In some cases water-quality problems become apparent only after a
water well is drilled, constructed, developed, and tested. However, test wells may be
used to predict the quality of water that will be obtained from permanent, high-capacity
wells. Other chapters of this topic address the significance of various contaminants
and techniques for their removal. A few contaminants are more frequently of concern
in groundwater than in surface water, including iron, manganese, fluoride, and nitrates.
Iron can cause staining of plumbing fixtures and clothes and may encrust well
screens and pipes. Concentrations greater than 0.5 mg / L are usually troublesome.
Water may pick up iron from contact with the well casing, pump parts, and piping.
The more corrosive the water, the more metal it will dissolve from the iron surfaces
with which it comes in contact. Water standing in a well that has been idle will have
a higher iron content than the water in a water-bearing formation.
Upon contact with air, dissolved ferrous iron changes to the ferric state and pre-
cipitates. The resulting iron hydroxide and iron oxide are commonly called rust.
Well water containing iron in appreciable amounts may be completely clear and
colorless when first pumped. If it stands for a time, oxygen from the air oxidizes the
dissolved iron. The water grows cloudy and will yield a deposit of a rust-colored
material.
Iron-bearing waters also favor the growth of iron bacteria, such as crenothrix. These
growths can form so abundantly in water mains, recirculating systems, and other places
that they exert a marked clogging action.
Manganese resembles iron in its chemical behavior. Because manganese is less
abundant in rock materials than iron, its occurrence in water is less common than that
of iron. Manganese occurs in groundwater as soluble manganous bicarbonate, which
changes to insoluble manganese hydroxide when it reacts with oxygen of the air. The
stains caused by manganese are more annoying and harder to remove than those caused
by iron.
Fluoride in groundwater may be derived from fluorite, the principal fluoride mineral
of igneous rocks, or from any of a considerable number of complex fluoride-bearing
minerals. Volcanic or fumarolic gases may also contain fluoride and may be the source
of fluoride in water. Too much fluoride in the water has been shown to be associated
with the dental defect known as mottled enamel. This may appear on the teeth of
children who drink water containing too much fluoride during the period when per-
manent teeth are formed. Conversely, small concentrations of fluoride are beneficial
and help to prevent tooth decay. The desirable fluoride content varies with air tem-
perature, as shown in Chapter 20.
Nitrate in concentrations greater than 45 mg / L (as NO
3
) is undesirable in water
used for domestic purposes because of the possible toxic effect that it may have on
young infants. Nitrates are transported through soil without significant degradation or
adsorption. Groundwater under heavily fertilized areas may have very high (more than
