|
5,000
<br />Cc
<br />wF
<br />Z W 4,500
<br />�zCl)
<br />�UW
<br />Z CC U 4,000
<br />OU d W
<br />0 U) w
<br />1Z ZCC
<br />W WO 3,500
<br />aW
<br />Cn U) N
<br />F5 9 ¢ 3,000
<br />w�
<br />�2Z
<br />2,500
<br />GROWING WINTER -
<br />SEASON STORAGE
<br />SEASON
<br />Figure 10. Specific conductance for the
<br />Arkansas River at Lamar, 1964 -94.
<br />During the growing season, the
<br />median specific conductance decreased
<br />from 3,400 µS /cm in 1964 -79 to
<br />2,995 µS /cm in 1980 -94 (fig. 10). During
<br />the winter - storage season, the median
<br />specific conductance decreased from
<br />4,900 µS /cm in 1964 -79 to 4,375 µS /cm in
<br />1980 -94 (fig. 10). In addition to increased
<br />winter streamflow downstream from the
<br />reservoir and an associated increase in the
<br />dilution potential of the river, the same
<br />factors that affected specific conductance
<br />at the site downstream from John Martin
<br />Reservoir probably caused the changes in
<br />specific conductance at Lamar.
<br />CONCLUSIONS
<br />An analysis of historical specific -
<br />conductance data indicates that specific
<br />conductance in the Arkansas River changed
<br />as the result of main -stem reservoir opera-
<br />tions. Specific conductance upstream from
<br />Pueblo and at Las Animas tended to
<br />decrease following the construction of
<br />Pueblo Reservoir. Likewise, specific con-
<br />ductance downstream from John Martin
<br />Reservoir and at Lamar decreased after
<br />implementation of the 1980 John Martin
<br />Reservoir operating plan. The decrease in
<br />specific conductance at the site upstream
<br />from Pueblo is beneficial from a municipal
<br />drinking- water - supply perspective because
<br />streamflow in this area provides drinking
<br />water to the greater Pueblo area. Although
<br />specific conductance increased at the site
<br />near Avondale, the increase after 1974 was
<br />not large enough to change the salinity haz-
<br />ard for irrigated agriculture. Although spe-
<br />cific conductance decreased at Las Animas,
<br />downstream from John Martin Reservoir,
<br />and at Lamar, the decrease was not large
<br />enough to change the salinity hazard for
<br />irrigated agriculture. The salinity hazard
<br />at all three sites remained high to very
<br />high (greater than 2,250 µS /cm).
<br />REFERENCES
<br />Abbott, P.O., 1985, Description of water -
<br />systems operations in the Arkansas
<br />River Basin, Colorado: U.S. Geologi-
<br />cal Survey Water- Resources Investiga-
<br />tions Report 85 -4092, 67 p.
<br />Arkansas River Compact Administration,
<br />1980, Thirty- second annual report:
<br />Lamar, Colorado, 50 p.
<br />Cain, Doug, 1987, Relations of specific
<br />conductance to streamflow and
<br />selected water - quality characteristics
<br />of the Arkansas River Basin,
<br />Colorado: U.S. Geological Survey
<br />Water- Resources Investigations
<br />Report 87 -4041, 93 p.
<br />Lewis, M.E., and Brendle, D.L., 1998,
<br />Relations of streamflow and specific -
<br />conductance trends to reservoir opera-
<br />tions in the lower Arkansas River,
<br />southeastern Colorado: U.S. Geologi-
<br />cal Survey Water- Resources Investiga-
<br />tions Report 97 -4239, 48 p.
<br />Miles, D.L., 1977, Salinity in the Arkan-
<br />sas Valley of Colorado: Denver,
<br />U.S. Environmental Protection
<br />Agency and Colorado State
<br />University, Interagency Agree-
<br />ment, EPA– IAG –D4 -0544, 80 p.
<br />Richards, L.A., ed., 1954, Diagnosis and
<br />improvement of saline and alkali
<br />soils: Washington, D.C., U.S. Depart-
<br />ment of Agriculture Handbook 60,
<br />160 p.
<br />U.S. Department of the Interior, 1994,
<br />Salinity update: Denver, Bureau of
<br />Reclamation, 17 p.
<br />U.S. Environmental Protection Agency,
<br />1986, Secondary maximum contami-
<br />nant levels (section 143.3 of part 143,
<br />National secondary drinking -water
<br />regulations): U.S. Code of Federal
<br />Regulations, Title 40, Parts 100 -149,
<br />p. 587 -590.
<br />This study was conducted in coopera-
<br />tion with the Colorado Springs Utilities;
<br />Pueblo Board of Water Works; South-
<br />eastern Colorado Water Conservancy
<br />District; Pueblo County, Department of
<br />Planning and Development; city of Aurora,
<br />Department of Utilities; St. Charles Mesa
<br />Water District; Upper Arkansas Area
<br />Council of Governments; Upper Arkansas
<br />Water Conservancy District; city of Pueblo,
<br />Department of Utilities; Pueblo West
<br />Metropolitan District; Fremont Sanitation
<br />District; and the cities of Rocky Ford,
<br />Las Animas, and Lamar.
<br />— Michael E. Lewis,
<br />Water Resources Division,
<br />Pueblo, CO
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