Southwest Hydrology: The Resource of Semi-Arid Hydrology Volume 4 Number 2

• Description: Southwest Hydrology: The Resource of Semi-Arid Hydrology Volume 4 Number 2
• State: CO
• Date: 3/1/2005
• Title: Southwest Hydrology: The Resource of Semi-Arid Hydrology Volume 4 Number 2
• Water Supply Pro - Doc Type: Publication

Effects of the Drought on
Water Quality i in .ake Mead
Peggy Roefer, Kim Zikmund, Torn Maher, and Jim LaBounty — Southern Nevada Water Authority
ne of the most glaring effects of the
ongoing drought on the Colorado
River system is dramatically
reduced reservoir levels. But lower
reservoir levels mean all the constituents
in the water become concentrated,
affecting water quality. How have the
changes in water quality in Lake Mead
influenced drinking water quality and
efficiency of drinking water treatment for
Southern Nevada?
Lake Mead is the major reservoir on
the Colorado River below Lake Powell
and the Grand Canyon, formed by the
construction of Hoover Dam in 1935.
The lake's maximum surface area is
approximately 247 square miles and the
maximum surface elevation is 1,221.4 feet
above mean sea level. It extends east 110
miles from Hoover Dam to the west end
of the Grand Canyon and has a shoreline
length of approximately 550 miles. Lake
Mead contains three large basins: Boulder,
Virgin, and Gregg (see map). Sources
of water to Lake Mead are the Colorado
River (97 percent), the Muddy and
Virgin Rivers (1.5 percent) and the Las
Vegas Wash (1.5 percent). At maximum
capacity, Lake Mead holds approximately
28.5 million acre -feet of the 60 million
acre -feet of storage on the Colorado
River system. This storage declined from
24 million acre -feet in January 2000 to
approximately 14 million acre -feet at the
end of 2004.
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26 • March /April 2005 • Southwest Hydrology
Las Vegas Valley's Water Supply
Approximately 90 percent of the domestic
water supply for the Las Vegas Valley
is withdrawn from the Boulder Basin at
Saddle Island. This supply is pumped
through two intake structures to two
different treatment facilities, Alfred Merritt
Smith Water Treatment Facility and River
Mountains Water Treatment Facility. Both
facilities treat the water by predisinfection
with ozone, the addition of ferric chloride,
flash and rapid mixing, flocculation,
direct filtration, the addition of zinc
orthophosphate to inhibit corrosion, and
post - chlorination. The two water treatment
plants are capable of processing 750
million gallons of water per day.
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ctll 117c conwdllent. 111o1
iv ere e hi t1to I crier l ndel'
171,uher volmll"s twu'mw
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water g1talil1,
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Three water quality parameters that have
been influenced by drought in Lake Mead
are total dissolved solids (TDS), bromide,
and total organic carbon (TOC). The chart
below shows the average concentration of
each in Boulder Basin, Lake Mead. These
constituents have increased primarily due
to concentration from evaporation.
Because TDS affects the palatability
of a drinking water source, the U.S.
Environmental Protection Agency has set
a secondary aesthetic standard for drinking