7983

• UCREFRP Catalog Number: 7983
• Author: U.S. Department of the Interior.
• Title: Quality of Water, Colorado River Basin.
• USFW Year: 1993.
• USFW - Doc Type: Progress Report No. 16,
• Copyright Material: NO

CAUSES OF SALINITY 11

Consumptive Use of Water as a
Source of Salinity

Addition of salts to the river system is not the
only cause of increased salinity in the Colorado
River Basin. The consumption (depletion) of
water reduces the dilution of saline inflows to
the river system, increasing the concentration of
salinity. Water use is evaluated as part of
Reclamation's responsibilities in managing the
river system. The Colorado River Basin,
Consumptive Uses and Losses Report [4]
summarizes water use in the Basin.

With the exception of the Central Arizona
Project (CAP), the Lower Basin has already
developed most of its water supply. CAP is the
last major development to deplete water from
the Lower Colorado River (approximately
1.5 mafper year). Consumptive use by
agriculture is the single largest cause of
depletions of the Colorado River. Exports,
reservoir evaporation, and M&I uses also
account for lesser but significant depletions.
Table 2 summarizes the Colorado River Basin
uses for 1981-85, including tributaries to the
Colorado River in the Lower Basin.

Table 2.-Water use in the Colorado River Basin (1981-85)

Type of use

Upper basin use
(1,000 af/yr)

Lower basin use
(1 ,000 af/yr)

Reservoir evaporation
and channel losses
Agriculture (within basin)
M&I
Fish, wildlife, and
recreational
Transbasin exports

812
2,312
203

1,255
5,101
841

o
669

30
4,063

Totals

3,996

11 ,290

Most of the exports from the Upper Basin are
made at higher elevations where the salinity
concentrations are very low. This loss of high
quality water results in the remaining flows
downstream becoming more concentrated.

Water exported from the Upper Basin during the
period 1941-72 averaged about 360,000 acre-feet

per year. Completion of such large projects as
the Colorado-Big Thompson, Duchesne Tunnel,
Roberts Tunnel, and more recent projects
increased exports to about 727,000 acre-feet per
year for 1976-80, with a peak in 1978 of
852,000 acre-feet.

Energy Exploration and
Development

Many of the geologic formations of the Colorado
River Basin were deposited in marine (salt
water) or brackish water environments.
Sulfates and sodium chloride are prevalent salts
in most of these formations. Many of the
formations were deposited in drier periods and
are capable of transmitting water, but these
aquifers are frequently sandwiched between
hundreds or even thousands of feet of
impermeable shales (aquicludes). These
aquifers are, therefore, static and often saline.
Many static and saline aquifers are present in
the Colorado River Basin. When a path of flow
is provided by drilling or mining, these aquifers
are mobilized, and brackish or saline waters flow
back to the surface.

The development of energy resources,
specifically coal, oil and gas, and oil shale, in the
Colorado River Basin may contribute significant
quantities of salt to the Colorado River. Salinity
can be increased either by dissolution of
minerals or consumption of good quality water.
The location of fossil fuels is associated with
marine-derived formations. Any disturbance of
these saline materials will increase the contact
surfaces allowing for the dissolutions of
previously unavailable soluble minerals.

Salinity increases associated with the mining of
coal can be attributed to leaching of coal spoil
materials, discharge of saline ground waters,
and increased erosion resulting from surface
disturbing activities. Spoil materials have a
greater permeability than undisturbed
overburden, allowing most of the rain falling on
the spoils to infiltrate instead of running off.
The water percolates through the spoils,
dissolving soluble minerals.

Studies[5-7] conducted on mining spoils in
northwestern Colorado indicate that the