WSP03721

• File Number: 8276.300
• Description: Lower Gunnison Basin Unit - Colorado River Basin Salinity Control Project
• State: CO
• Basin: Colorado Mainstem
• Water Division: 4
• Date: 12/1/1989
• Title: Colorado River Water Quality Improvement Program Lower Gunnison Basin Unit North Fork Area Preliminary Findings Report December 1989
• Water Supply Pro - Doc Type: Report/Study

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CHAPTER I

INTRODUCTIGl

Problems and Needs

In the study area, an estimated total of about 422,000 tons of salt are
picked up annually and conveyed to the Colorado River system. Most salt
pickup is attributable to conveyance system seepage and deep percolation as
water passes through the weathered and fractured shales of the
Mancos Formation. Consumptive use of water and salt pickup by ground-water
return flows leave higher salt concentrations in water returning to the river.
1\dditional salt sources in the study area include saline water flowing from
abandoned oil and gas wells and salt from soils derived from Dakota Sandstone.
The frontispiece map depicts the location of salt sources in the unit area.

Information based on 9 years of record shows that the inflow water
quality varies from 50 milligrams per liter (mg/L) total dissolved solids
(TDS) in the mountain streams to 84 mg/L TDS in the North Fork of the
Gunnison River at Somerset gauge and 176 mg/L TOS in the Gunnison River above
the confluence with the North Fork. Outflow water quality of the
Gunnison River as it leaves the unit area averages 398 mg/L TOS, while
tributaries in the saline areas are as high as 7,000 mg/L TOS for individual
samples.

Ground-water quality varies from l,OOO to 12,000 mg/L TOS below irrigated
areas. Records confirm that TOS concentration in an old unplugged gas well
near Austin has reached 30,416 mg/L TOS, the highest recorded concentration in
the study area.

Salt reduction estimates for this preliminary findings memorandum were
obtained from available Bureau of Reclamation (Reclamation) and U.S.
Geological Survey (USGS) gauging station data in the study area. This
information was correlated with data from the Grand Valley unit,
Colorado River Basin Salinity Control project, and Lower Gunnison Basin unit,
Colorado River Water Quality Improvement Program. In or near the unit area,
the USGS has 11 gauging stations that monitor daily flows of the
Gunnison River and many of the major tributaries. At the North Fork of the
Gunnison River near Somerset, USGS also collects water quality data. In'
addition to the USGS gauging network, Reclamation maintains a variety of
gauging stations ranging from monthly grab sample sites to daily flow
stations.

Salt loading from the Cottonwood Creek, Unnamed Gulch, Alum Gulch, and
North Delta Canal watersheds was estimated with summer and winter grab sample
runs for wetted perimeters and water quality on the canals and laterals.
Seepage rates were assumed to be similar to those found in the Grand Valley
unit. Ground-water quality was assumed to be similar to that found in the
Lower Gunnison Basin unit for similar geologic formations. The salinity
estimates for the remaining watersheds were based on a proration of remaining
salt contributions from the off-farm sources.

Many identified ground-water seeps, springs, and abandoned oil and gas
wells are believed to be adding to the salt loading of the area. Salt loading
estimates of the abandoned oil and gas wells were determined from grab samples
obtained in the summer of 1985. Of six abandoned test wells located on the
south bank of the Gunnison River, one near Austin intermittently spews saline

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