Geology topics

Research about Gunnison River Basin

Source-linked reports with geographic coverage including Gunnison River Basin.

3 recordsLinked to original sources

USGS2016

Response of selenium concentrations in groundwater to seasonal canal leakage, lower Gunnison River Basin, Colorado, 2013

Selenium is a water-quality concern in the lower Gunnison River Basin because irrigation water interacting with seleniferous soils derived from the Mancos Shale Formation has mobilized selenium and increased its concentrations in surface water. Understanding the occurrence of elevated selenium concentrations in groundwater is necessary because groundwater discharge is an important source of selenium in surface water in the basin. In 2013, the U.S. Geological Survey, in cooperation with the Bureau of Reclamation and the Colorado Water Conservation Board, began a study to understand how changes in groundwater levels attributed to canal leakage affected the concentrations and speciation of dissolved selenium in groundwater. The purpose of this report is to characterize the groundwater adjacent to an unlined leaky canal. Two locations, near the East Canal (W-N1 and W-N2) and farther from the East Canal (W-M1 and W-M2), were selected for nested monitoring well installations. The pressure exerted by changes in canal stage was more readily transferred to the deep groundwater measured in the W-N1 near the canal than the shallow groundwater at the W-N2 well. No definitive relation could be made between canal water-level elevation and water-level elevations in monitoring wells farther from the canal (W-M1 and W-M2). Water flowing through the East Canal before the irrigation season had much higher selenium concentrations (140 micrograms per liter) than water in the canal during the irrigation season (3.02 micrograms per liter). Total selenium concentrations in the monitoring wells near the canal initially increased to 51.8 micrograms per liter in W-N1 and 1.66 micrograms per liter in W-N2. The initial increase in groundwater selenium concentrations presumably resulted from the dissolution of salts in the unsaturated zone by rising groundwater levels associated with canal leakage. The subsequent decrease in total selenium concentrations resulted from a combination of dilution by canal leakage and selenium reduction processes. Total selenium concentrations in monitoring wells located farther from the canal were not directly affected by canal leakage. Selenite/total selenium mass ratios in the East Canal samples ranged from about 0.02 to 0.13, indicating that about 2 to 13 percent of the total selenium in canal samples was composed of selenite. The increase in total selenium at W-N1 from before the irrigation season to the early irrigation season was accompanied by a decrease in the percentage of selenite from about 10 to 1 percent, indicating that selenate was added to the groundwater. A nitrate pulse occurred with the selenate pulse in W-N1 at the beginning of the irrigation season but apparently dissipated to a low enough concentration during the irrigation season to allow for selenate reduction to occur, as indicated by the relatively high percentages of selenite in W-N1 during the late irrigation season. W-N2 generally contained higher percentages of selenite than W-N1. Percentages of selenite in W-M1 did not change in response to filling the canal and generally composed less than 1 percent of the total selenium in that well. The predominance of selenate in W-M1, and apparent lack of selenate reduction, cannot be explained by a lack of anoxic conditions in the groundwater because all the available dissolved-oxygen data indicate that concentrations were less than 0.5 milligrams per liter. The most likely explanation for the lack of selenate reduction in W-M1 is that the exceptionally high concentrations of nitrate in the groundwater (about 340 to 390 milligrams per liter as nitrogen) inhibited selenate reduction. These high nitrate concentrations presumably come from the Mancos Shale and its weathering products because there was no evidence for a human source of nitrate at the lower Gunnison River Basin wetland. The high concentrations of selenate in W-M1 may persist and eventually discharge to surface water unless nitrate concentrations are reduced to low enough levels to permit substantial selenate reduction to occur. Well W-M2 contained relatively low concentrations of total selenium and high percentages of selenite before and at the onset of the irrigation season. An increase in total selenium concentration associated with a drying and wetting period later in the summer was accompanied by a decrease in the percentage of selenite to near 0 percent, indicating that selenate was added to the groundwater. This pattern is consistent with the examples of increasing concentrations of total selenium in the other wells and presumably resulted from the dissolution of selenate-bearing salts in the unsaturated zone by rising water levels in W-M2.

Colorado↗

USGS2014

Characterization of salinity loads and selenium loads in the Smith Fork Creek region of the Lower Gunnison River Basin, western Colorado, 2008-2009

The lower Gunnison River Basin of the Colorado River Basin has elevated salinity and selenium levels. The Colorado River Basin Salinity Control Act of June 24, 1974 (Public Law 93–320, amended by Public Law 98–569), authorized investigation of the Lower Gunnison Basin Unit Salinity Control Project by the U.S. Department of the Interior. The Bureau of Reclamation (Reclamation) and the Natural Resources Conservation Service are responsible for assessing and implementing measures to reduce salinity and selenium loading in the Colorado River Basin. Cost-sharing programs help farmers, ranchers, and canal companies improve the efficiency of water delivery systems and irrigation practices. The delivery systems (irrigation canals) have been identified as potential sources of seepage, which can contribute to salinity loading. Reclamation wants to identify seepage from irrigation systems in order to maximize the effectiveness of the various salinity-control methods, such as polyacrylamide lining and piping of irrigation canals programs. The U.S. Geological Survey, in cooperation with Reclamation, developed a study to characterize the salinity and selenium loading of seven subbasins in the Smith Fork Creek region and identify where control efforts can be maximized to reduce salinity and selenium loading. Total salinity loads ranged from 27.9±19.1 tons per year (t/yr) to 87,500±80,500 t/yr. The four natural subbasins—BkKm, RCG1, RCG2, and SF1—had total salinity loads of 27.9±19.1 t/yr, 371±248 t/yr, 2,180±1,590 t/yr, and 4,200±2,720 t/yr, respectively. The agriculturally influenced sites had salinity loads that ranged from 7,580±6,900 t/yr to 87,500±80,500 t/yr. Salinity loads for the subbasins AL1, B1, CK1, SF2, and SF3 were 7,580±6,900 t/yr; 28,300±26,700 t/yr; 48,700±36,100 t/yr; 87,500±80,900 t/yr; and 52,200±31,800 t/yr, respectively. The agricultural salinity load was separated into three components: tail water, deep percolation, and canal seepage. Annual tail-water salinity loads ranged from 48.0 to 2,750 tons in the Smith Fork Creek region. The largest tail-water salinity load was in subbasin SF3, and the lowest salinity load from tail water was in subbasin R1. The remaining four agricultural subbasins—AL1, B1, CK1, and SF2—had tail-water loads of 285 t/yr, 180 t/yr, 333 t/yr, and 1,700 t/yr, respectively. The deep percolation component of the agricultural salinity load ranged from 3,300 t/yr in subbasin AL1 to 51,800 t/yr in subbasin SF2. Subbasins R1, B1, CK1, and SF3 had deep percolation salinity loads of 4,940 t/yr, 15,200 t/yr, 21,200 t/yr, and 23,600 t/yr, respectively. The canal seepage component of the agricultural salinity load ranged from 1,100 t/yr in subbasin AL1 to 15,300 t/yr in subbasin CK1. Subbasins B1, R1, SF2, and SF3 had canal seepage salinity loads of 6,610 t/yr, 3,890 t/yr, 9,430 t/yr, and 12,100 t/yr, respectively. Four natural subbasins—RCG1, RCG2, SF1, and BkKm—were used to calculate natural salinity yields for the remaining subbasins. The appropriate salinity yield was applied to the corresponding number of acres and resulted in a natural salinity load for each subbasin. The annual salinity yields for the Dakota Sandstone and Burro Canyon Formation, Mancos Shale, and crystalline geologies are 0.217 tons per acre (t/acre), 0.113 t/acre, and 0.151 t/acre, respectively. Three of the four natural subbasins had little to no selenium load based on the measured data and calculated selenium loads. Subbasins RCG1 and RCG2 had surface-water selenium loads of 0.106±0.024 pounds (lb) and 0.00 lb, respectively. Subbasin BkKm did not have an estimated surface-water selenium load because of the lack of any water-quality samples during the study period. The subbasin designated by site CK1 had the highest selenium load with 135±38.7 lb, and the next highest subbasins in decreasing order are B1, SF3, AL1, SF1, and R1 with selenium loads of 69.6±28.4 lb, 56.5±23.8 lb, 30.5±16.6 lb, 26.8±6.95 lb, and 15.6±27.7 lb, respectively.

Colorado↗

USGS2012

Surface-water salinity in the Gunnison River Basin, Colorado, water years 1989 through 2007

Elevated levels of dissolved solids in water (salinity) can result in numerous and costly issues for agricultural, industrial, and municipal water users. The Colorado River Basin Salinity Control Act of 1974 (Public Law 93-320) authorized planning and construction of salinity-control projects in the Colorado River Basin. One of the first projects was the Lower Gunnison Unit, a project to mitigate salinity in the Lower Gunnison and Uncompahgre River Basins. In cooperation with the Bureau of Reclamation (USBR), the U.S. Geological Survey conducted a study to quantify changes in salinity in the Gunnison River Basin. Trends in salinity concentration and load during the period water years (WY) 1989 through 2004 (1989-2004) were determined for 15 selected streamflow-gaging stations in the Gunnison River Basin. Additionally, trends in salinity concentration and load during the period WY1989 through 2007 (1989-2007) were determined for 5 of the 15 sites for which sufficient data were available. Trend results also were used to identify regions in the Lower Gunnison River Basin (downstream from the Gunnison Tunnel) where the largest changes in salinity loads occur. Additional sources of salinity, including residential development (urbanization), changes in land cover, and natural sources, were estimated within the context of the trend results. The trend results and salinity loads estimated from trends testing also were compared to USBR and Natural Resources Conservation Service (NRCS) estimates of off-farm and on-farm salinity reduction from salinity-control projects in the basin. Finally, salinity from six additional sites in basins that are not affected by irrigated agriculture or urbanization was monitored from WY 2008 to 2010 to quantify what portion of salinity may be from nonagricultural or natural sources. In the Upper Gunnison area, which refers to Gunnison River Basin above the site located on the Gunnison River below the Gunnison Tunnel, estimated mean annual salinity load was 110,000 tons during WY 1989-2004. Analysis of both study periods (WY 1989-2004 and WY 1989-2007) showed an initial decrease in salinity load with a minimum in 1997. The net change over either study period was only significant during WY 1989-2007. Salinity load significantly decreased at the Gunnison River near Delta by 179,000 tons during WY 1989-2004. Just downstream, the Uncompahgre River enters the Gunnison River where there also was a highly significant decrease in salinity load of 55,500 tons. The site that is located at the mouth of the study area is the Gunnison River near Grand Junction where the decrease was the largest. Salinity loads decreased by 247,000 tons during WY 1989-2004 at this site though the decrease attenuated by 2007 and the net change was a decrease of 207,000 tons. The trend results presented in this study indicate that the effect of urbanization on salinity loads is difficult to discern from the effects of irrigated agriculture and that natural sources contribute a fraction of the total salinity load for the entire basin. Based on the calculated yields and geology, 23-63 percent of the estimated annual salinity load was from natural sources at the Gunnison River near Grand Junction during WY 1989-2007. The largest changes in salinity load occurred at the Gunnison River near Grand Junction as well as the two sites located in Delta: the Gunnison River at Delta and the Uncompahgre River at Delta. Those three sites, especially the two sites at Delta, were the most affected by irrigated agriculture, which was observed in the estimated mean annual loads. Irrigated acreage, especially acreage underlain by Mancos Shale, is the target of salinity-control projects intended to decrease salinity loads. The NRCS and the USBR have done the majority of salinity control work in the Lower Gunnison area of the Gunnison River Basin, and the focus has been in the Uncompahgre River Basin and in portions of the Lower Gunnison River Basin (downstream from the Gunnison Tunnel). According to the estimates from the USBR and NRCS, salinity-control projects may be responsible for a reduction of 117,300 tons of salinity as of 2004 and 142,000 tons as of 2007 at the Gunnison River near Grand Junction, Colo. (streamflow-gaging station 09152500). USBR and NRCS estimates account for all but 130,000 tons in 2004 and 65,000 tons in 2007 of salinity load reduction. The additional reduction could be a reduction in natural salt loading to the streams because of land-cover changes during the study period. It is possible also that the USBR and NRCS have underestimated changes in salinity loads as a result of the implementation of salinity-control projects.

Colorado↗