ThisEffect is of the the Title Paradox for Fact Valley Sheet Unit on the Dissolved-Solids Load of the Dolores River near Bedrock, Colorado, 1988–2001 —By Daniel T. Chafin Prepared in cooperation with the Bureau of Reclamation Abstract concentrations of dissolved sodium and chloride account for most of the increase in salinity. According to water- Discharge of brine with an average dissolved- quality data (Andrew Nicholas, Bureau of Reclamation, solids concentration of about 256,000 milligrams per written commun., 2002), the brine has an average liter from alluvium in Paradox Valley, a collapsed salt dissolved-solids concentration of about 256,000 milligrams anticline, substantially increases the dissolved-solids per liter (mg/L) in alluvium in the vicinity of the Dolores load of the Dolores River. In 1996, the Bureau of River. The Colorado River Basin Salinity Control Act of Reclamation began operation of the Paradox Valley 1974 (Public Law 93–320, amended in 1984 as Public Law Unit, a series of brine-withdrawal wells completed in 98–569) authorized construction of the Paradox Valley Unit alluvium along the Dolores River and a deep-injection by the Bureau of Reclamation (BOR) as one of the projects well for the brine, to decrease flow of brine into the implemented to control salinity in the Colorado River river. This report presents the findings of a study to Basin. determine the effectiveness of the Paradox Valley Unit from 1988 through September 2001. Colorado Differences in dissolved-solids load of the River Basin Dolores River between two gaging stations, one Dolores upstream and one downstream from the Paradox River Basin DENVER 109° Valley Unit, indicate that an average dissolved-solids Grand 39° 108° Junction load of about 313 tons per day (an annual average of C O L O R A D O about 115,000 tons) was contributed by brine inflow O to the Dolores River before operation of the Paradox Dolores L O R A D Valley Unit began in July 1996. By September 30, U T A H 2001, the dissolved-solids load contributed by brine C O San Juan had declined to an average of about 29 tons per day— Bedrock River Basin a decrease of about 90 percent. This decrease might River have been facilitated by a decrease in precipitation and Paradox Valley streamflow into the Paradox Valley during the last few 38° years of the assessed period. McPhee Reservoir 0 10 20 MILES INTRODUCTION 0 10 20 KILOMETERS Discharge of saline ground water (brine) to the Dolores River, as it crosses the Paradox Valley (fig. 1), Figure 1. Location of the study area (from Watts, 2000). substantially increases the dissolved-solids load of this river, a tributary of the Colorado River. Increases in U.S. Department of the Interior WRIR 02–4275 U.S. Geological Survey January 2003 The Paradox Valley Unit, as described by Watts the brine that would have otherwise flowed into the (2000), consists of 12 shallow (less than 80 feet deep) Dolores River. Watts (2000) estimated that operation production wells, a pipeline connecting the wells to a of the Paradox Valley Unit reduced the dissolved- treatment facility, and a deep injection well (fig. 2). solids load to the Dolores River by about 32 percent, The well field is located along the Dolores River and is the dissolved-sodium load by about 36 percent, and designed to intercept brine before it flows to the river. the dissolved-chloride load by about 37 percent. The brine is injected into rocks of Precambrian and Since 1998, BOR modified operation of the Paleozoic age, primarily the Leadville Limestone of Paradox Valley Unit to optimize interception of brine Mississippian age. The injection zone is at depths of 14,068 to 15,857 feet below land surface. Test opera- flowing into the Dolores River. Early in 2002, BOR tion of the Paradox Valley Unit began in 1980, and requested that the USGS reevaluate the effect of the production operation began in 1996. Paradox Valley Unit on removal of dissolved solids During 1999, the U.S. Geological Survey from the Dolores River. (USGS), in cooperation with the BOR, studied the effect of the Paradox Valley Unit on dissolved solids, sodium, and chloride in the Dolores River. The study Purpose and Scope evaluated changes in water quality from October 1987 This report reevaluates the effectiveness of the through September 1998. The resulting report (Watts, 2000) estimated that, during nonpumping periods, Paradox Valley Unit in decreasing dissolved-solids about 76 percent of the dissolved solids, about 86 load to the Dolores River by evaluating the entire percent of the dissolved sodium, and about 90 percent period of the water-quality record through September of the dissolved chloride in the Dolores River near 30, 2001 (at the time, the latest date for quality- Bedrock (station 09171100; fig. 2) probably was assured, continuous data in USGS databases). This derived from ground-water flow of brine in the reevaluation is based on estimates of dissolved-solids Paradox Valley. Watts (2000) also concluded that loads at water-quality stations upstream and down- decreases in median concentrations of dissolved stream from the Paradox Valley and on brine-with- solids (from 1,570 to 1,115 mg/L), sodium (480 to drawal data for the Paradox Valley Unit. 294 mg/L), and chloride (760 to 470 mg/L) at that station between nonpumping and pumping periods indicate that the Paradox Valley Unit captured part of Geohydrologic Setting The following description of the geohydrologic 1,030,000 1,040,000 setting is modified from Watts (2000). Paradox Valley, 09171100 EXPLANATION in southwestern Colorado (fig. 1), is a collapsed Paradox Valley 630,000 diapiric salt anticline, trending northwest-southeast. 09171100 Active water-quality monitoring site and The Paradox Valley is about 24 miles long and 3 to W number Pa x C rado r 09171100 5 miles wide. The Dolores River crosses the valley Streamflow-gaging station and number about midway across the long axis of the valley, Dolores R WELLS 620,000 BRINE PRODUCTION entering and leaving through deep and narrow canyons E 0 1 MILE that were eroded several hundred feet through the P a 09169500 ra dox 0 1 KILOMETER sandstone and shale that form the valley walls. Uncon- C r Injection Well solidated alluvium overlies the salt- and gypsum- Base from U.S. Geological Survey digital line graphs, 1:100,000 Colorado state plane coordinate system, southern zone. bearing rocks along the flood plain of the Dolores North American datum 1927 River and, locally, the valley floor. Maximum reported Figure 2. Locations of U.S. Geological Survey gaging thickness of the alluvium is 129 feet. The alluvium is a stations and Paradox Valley Unit brine-withdrawal wells and injection well (modified from Watts, 2000). source of ground water for irrigation wells in the western end of the valley. Approach Water-quality records include 315 discrete water-quality analyses (159 for station 09169500 and Dissolved-solids loads contributed to the 156 for station 09171100), which were collected Dolores River by seepage of brine were estimated by approximately monthly between January 4, 1978, and water-quality records for two USGS gaging stations February 27, 2002. Because these analyses did not (fig. 2): (1) Dolores River at Bedrock (station include concentration of alkalinity, alkalinity (as 09169500), upstream from Paradox Valley; and 2 – CO3 ion) was calculated as the mass required to (2) Dolores River near Bedrock (station 09171100), balance the sum of charges of major ions in the downstream from Paradox Valley. Loads at these sites sample. Masses of major ions, including alkalinity, were estimated only for days in the record for which were then summed to provide an estimate of data were available for both sites (that is, matched-pair dissolved-solids concentration for each sample. Linear observations). regression was used to determine the relation between Dissolved-solids loads at these two gaging specific conductance and dissolved-solids concentra- stations were estimated with the following approach: tion for all samples at both sites (fig. 3). This strong (1) dissolved-solids concentrations for discrete water- relation (R2=0.995, indicating the fraction of the varia- quality samples were estimated as the sum of the tion in dissolved-solids concentration that is explained concentrations of major ions; (2) a linear equation was by variation in specific conductance) was then used to developed to predict dissolved-solids concentrations estimate dissolved-solids concentration for days when from specific-conductance measurements; (3) daily dissolved-solids loads were estimated as the product of only specific conductance was measured for salinity mean-daily discharge and estimated mean-daily (as described in the next paragraph). dissolved-solids concentration; and (4) dissolved- Most water-quality data for stations 09169500 solids concentrations from discrete water-quality and 09171100 consist of mean-daily streamflow samples were used to estimate mean-daily dissolved- and mean-daily specific conductance, as determined solids concentrations for some days without specific- by continuous measurements (generally every conductance data. 15 minutes) from water-quality monitors. The usable 14,000 12,000 10,000 8,000 6,000 DS = 0.5653 x SC –13 N = 315 4,000 R2 = 0.995 2,000 DISSOLVED-SOLIDS (DS), AS SUM OF CONSTITUENTS, IN MILLIGRAMS PER LITER 0 0 5,000 10,000 15,000 20,000 25,000 SPECIFIC CONDUCTANCE (SC), IN MICROSIEMENS PER CENTIMETER AT 25° CELSIUS Figure 3. Relation between specific conductance and dissolved-solids concentration at gaging stations 09169500 and 09171100, January 1988–February 2002 record began on August 1, 1991, and consists of brine-withdrawal volumes (Andrew Nicholas, Bureau of 3,714 days for which dissolved-solids loads could be esti- Reclamation, written commun., 2000) and the average mated at both stations.