Water Resources in the Desert Southwest - Design with the Desert: Conservation and Sustainable Development - page 84

and age of groundwater. 36 Although many inorganic constituents and organic compounds
are regulated in drinking water,* here we will focus on four of the most commonly cited
indicators of water quality: pH, hardness, salinity, and total dissolved solids (TDS).
pH of water determines much of its chemistry and suitability as a domestic supply.
Typical waters in the western United States range in pH from slightly acid (around pH = 6)
to highly basic (pH>10) or neutral (pH ≈ 7.0). Rainwater tends to be slightly acidic to
very acidic, depending upon the amount of certain pollutants in the atmosphere, but its
chemistry changes immediately upon contact with soils and vegetation. Throughout much
of this region, pH is buffered by calcium carbonate and various salts that are dissolved as
rainwater passes over rocks, through soils, and into aquifers, which raises the pH to 7-8 or
higher. Hardness is a measure of the carbonate and bicarbonate content of water, whereas
salinity is a measure of salt content.
Hardness is particularly important to the buildup of calcium carbonate (“scale”) in pipes
and plumbing fixtures in buildings. The carbonates that comprise hardness generally are
highest in rivers and aquifers emanating from limestone and dolomite bedrock, sandstones
with calcareous cement, or old soils bearing significant caliche (pedogenic carbonate).
Salinity, which includes all salts but generally is dominated by sodium chloride, has many
sources but is highest in sedimentary units—particularly shales and evaporites—that
either accumulated salts under marine conditions or evaporation of freshwater lakes. Water
high in salinity affects human health, can corrode pipes and water-delivery systems, and
eventually will degrade agricultural lands subject to irrigation.
Perhaps the most important measure of water quality is total dissolved solids, or the
amount of inorganic and organic compounds carried in solution. Generally, half to nearly
two-thirds of the aquifers in the southwestern United States contain less than 500 parts
per million (ppm) of TDS, and 70% of aquifers had less than 1,000 ppm, but some brackish
aquifers can reach 10,000 ppm. 37 TDS in surface water varies considerably with the amount
of runoff and the size of rivers; dissolved solids tend to be higher during drought periods
and in larger rivers, such as the Colorado.
A good example of the impact of water quality on the urban environment is the effect
of CAP water deliveries on Tucson in the 1990s. Colorado River water delivered in CAP
canals and mixed with some surface water in central Arizona has significantly higher
TDS than groundwater in the Tucson basin, as well as different chemical constituents. †
Old pipes in some houses, which were conditioned to the lower TDS groundwater, burst
after short exposure to CAP water, in part because the different water chemistry caused
dissolution of compounds lining those old pipes. The problem appears to have been solved
by mixing groundwater and CAP water (blended water), ‡ thereby making a compromise
water chemistry that doesn't impact domestic users.
4.9 Riparian Vegetation
Riparian vegetation is a substantial environmental resource in the southwestern United
States that both provides a major esthetic element, provides ecosystem services for aquatic
ecosystems, decreases flow velocities and flood peak discharges in headwater areas, but also
can increase potential flood hazard in urban areas. Riparian vegetation generally increased
* http://www.epa.gov/waterscience/standards/ (accessed June 23, 2009).
† http://www.cap-az.com/operations/water-quality (accessed June 11, 2009).
‡ http://www.uswaternews.com/archives/arcquality/ttucwat11.html (accessed June 23, 2009).