Relations of Main-Stem Reservoir Operations and Specific Conductance in the Lower Arkansas River USGS Fact Sheet

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Relations of Main -Stem Reservoir Operations and Specific Conductance in the Lower Arkansas River, Southeastern Colorado INTRODUCTION Two main -stem reservoirs on the lower Arkansas River, Pueblo Reservoir and John Martin Reservoir, are used to reg- ulate and manage streamflow for irrigation and domestic water supplies, flood control, and recreation. Water storage and release operations for these reservoirs can affect streamflow and water quality of the down- stream river. One water - quality property that can be affected is specific conductance. In 1996, the U.S. Geological Survey com- pleted a study of the effects of Pueblo Reservoir and John Martin Reservoir opera- tions on specific conductance at five sites located on the Arkansas River between Pueblo Reservoir and Lamar (fig. 1). This fact sheet presents an overview of the study findings. A detailed discussion of the study results is presented in Lewis and Brendle (1998). Specific Conductance and Dissolved Solids Specific conductance, in microsie- mens per centimeter at 25 degrees Celsius (gS /cm), is a property of water that is attributable to salinity, or dissolved solids, which is a measure of salts such as sodium chloride or calcium bicarbonate. Pure water has a very low specific conductance —a few hundredths of a microsiemen per centime- ter. As the dissolved - solids concentration of water increases, the specific conductance increases (fig. 2). I.eadville 104° 105° Granite �1as° 103° COLORADO DENVER i 1 Study area Colorado Springs o f ndy l` Bue� • . �" Vista P t Salida CC tyn 102° \ Pciala d S'Q.P Avondale n John Martin Reserva' , R� vofr Pueblo �� e a 38 �orles Rl �� Nepesta Lam S, Las Animas La Junta EXPLANATION WATER - QUALITY SAMPLING SITE -- -L -1 -- NEW MEXICO OKLAHOMA 25 50 75 MILES 0 25 50 75 KILOMETERS Rgure 1. Location of study area. U.S. Department of the Interior U.S. Geological Survey I I I Specific Conductance Can Affect Water Suitability High specific conductance indicates high dissolved - solids concentration; dissolved solids can affect the suitability of water for domestic, industrial,,and agri- cultural uses. The secondary maximum contaminant level for dissolved solids in drinking water is 500 milligra�rls per liter (mg/L) (U.S. Environmental lfirotection Agency, 1986). In the lower Arkansas River, 500 mg/L of dissolved solids is equivalent to a specific conductance of about 700 to 800 gS /cm (Cain, 1987). At higher levels, drinking water may have an unpleasant taste or odor or may even cause gastrointestinal distress. Additionally, high dissolved - solids concentration can cause deterioration of plumbing fixtures and appliances. Relatively expensive water - treatment processes, such as reverse osmosis, are needed to remove excessive dissolved solids from water. Agriculture also can be adversely affected by high- specific- conductance water. Depending on the crop, agricultural losses might occur when salinity reaches 700 to 850 mg/L (U.S. Department of the Interior, 1994), which is equivalent to a specific conductance of about 950 to 1,200 gS /cm in the Arkansas River (Cain, 1987). The salinity hazard of irrigation water is a relation developed by the U.S. Salinity Laboratory (Richards, 1954) that describes the qualitative effect of saline water on irrigated crops. The hazard is based on the specific conductance of the water and is divided into four classes of salinity hazard ranging from low (less than 250 gS /cm) to very high (greater than 2,250 g6/cm). As specific conductance increases, special agricultural management practices may be needed, and crops having a substantial salinity tolerance may need to be grown. Generally, crops with a higher salinity tolerance have a lower market value than more salt- sensitive crops (Miles, 1977). USGS Fact Sheet 166 -97 July 1998