WSP02811

• File Number: 8200.766
• Description: Gunnison River General
• State: CO
• Basin: Gunnison
• Water Division: 4
• Date: 1/1/1991
• Author: USGS
• Title: Reconnaissance Investigation of Water Quality-Bottom Sediment-and Biota Associated with Irrigation Drainage - Gunnison and Uncompahgre River Basins and Sweitzer Lake - West Central Colorado
• Water Supply Pro - Doc Type: Report/Study

Salinity studies of the Uncompahgre Project area (U.S. Bureau of
Reclamation, 1984) determined that most of the increase in salt load (equi-
valent to dissolved-solids load) in the Uncompahgre River through the study
area was caused by surface water entering the shallow ground-water system and
dissolving salts from the soil and underlying shale. The shallow ground water
then flows through weathered and fractured shale or through sand and gravel
layers and discharges into canals, into natural drainages, or directly into
the Gunnison River or the Uncompahgre River. A large percentage of the
recharge to the ground-water system is from deep percolation of applied irri-
gation water and distribution-system losses (U.S. Bureau of Reclamation,
1984). Although only about 38 percent of the irrigated area served by the
Uncompahgre Project is located on the east side of the valley, 64 percent of
the salt-load increase came from project lands on the east side (U.S. Bureau
of Reclamation, 1984). Virtually all of the salt loading from irrigated areas
on the east side of the valley was caused by surface water dissolving salt
from adobe (Mancos) soils or from weathered and fractured shale layers in the
Mancos Shale. The water in the Mancos Shale had an estimated salinity concen-
tration (or dissolved-solids concentration) of 4,500 mg/L (U.S. Bureau of
Reclamation, 1984).

PREVIOUS INVESTIGATIONS

Water-quality data were collected in the Uncompahgre Project area pri-
marily to study and monitor salinity; much of the long-term data available
are major-ion and dissolved-solids data for stream sites. Trace-element data
were collected at two sites on the Gunnison River and two sites on the
Uncompahgre River. There are no data available for pesticide concentrations
in the study area. Ground-water use in the project area is quite limited;
consequently, few water-quality analyses are available. Geochemical data for
soils were available for irrigated areas near Delta and Grand Junction. The
few biological data were collected from Sweitzer Lake, Ridgway Reservoir, and
the Uncompahgre River downstream from Ridgway Reservoir.

Water-Quality Data

Streams

The U.S. Geological Survey collected major-ion and dissolved-solids data
from 1959-63 at sites on the Gunnison River between the North Fork confluence
and Delta. The only trace-element data collected were for boron. Mean dis-
solved-solids concentration (80 samples) was 488 mg/L, and boron concentra-
tions ranged from 50 to 150 ~g/L (8 samples). The Colorado Department of
Health collected trace-element data for the Gunnison and Uncompahgre Rivers
at Delta and for the Uncompahgre River at Ridgway; these data are summarized
in table 1. The overall collection period for the data listed in table 1 is
1968-88. The data were retrieved from the storage and retrieval (STORET) part
of the U.S. Environmental Protection Agency's Water Quality Control
Information System. The longest term and most comprehensive water-quality
data-collection site in the Gunnison River basin is at the gaging station near
Grand Junction (station 09152500 in fig. 1). The U.S. Geological Survey has

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