Hydrology of Area 61, Northern Great Plains and Rocky Mountain Coal Provinces, Colorado and New Mexico

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6.0 SURFACE -WATER QUALITY -- Continued 6.1 Specific Conductance -- Continued 6.1.3 Relationship to Major Dissolved Constituents Concentrations of Major Dissolved Constituents Are Directly Related to Specific Conductance Specific conductance can be used to estimate concentrations of major dissolved constituents. Direct relationships between specific conduc- tance and the concentrations of major dissolved constituents have been developed for water at six streamflow- gaging stations in Area 61 (fig. 6.1.3 -1). The relationships can be used to estimate the concen- tration of major dissolved constituents if specific conductance is known. The relationships for the Purgatoire River at Madrid, Colo. (station 12 in fig. 6.1.3 -1) are illustrated in figure 6.1.3 -2; similar rela- tionships exist for the other five stations. The rela- tionships for all six stations are given mathematically in table 6.1.3 -1. To estimate the expected concentration of any constituent shown in table 6.1.3 -1, multiply the specific conductance, measured in micromhos per centimeter at 25° Celsius, by the value in the "Slope" column for that constituent and add the value in the "Intercept" column. The result is the estimated concentration of the constituent, in milligrams per liter. For example, suppose a specific conductance of 340 micromhos per centimeter at 25° Celsius was measured at the Purgatoire River at Madrid, Colo., and an estimate of sulfate concentration was desired. To make this estimate, multiply 340 by 0.157 (the value in the "Slope" column of table 6.1.3 -1 for sulfate at the station) and add -13.6 (the value in the "Intercept" column). The estimated sulfate concen- tration is 40 mg /L (milligrams per liter). The differ- ence between the estimated and the actual sulfate values is a function of the standard error shown in table 6.1.3 -1- -the larger the standard error, the great- 52 er the possible difference between the estimated and the actual value. Conversely, if the standard error of estimate is small, the actual sulfate concentration is likely to be quite similar to the estimated value. Similar calculations based on the measured specific conductance of 340 micromhos per centimeter at 25° Celsius would result in the following estimated con- centrations of other water - quality constituents: Sodium, 14 mg /L; calcium, 43 mg /L; magnesium, 10 mg /L; bicarbonate, 158 mg /L; chloride, 2 mg /L; and dissolved solids, 197 mg /L. Similar calculations also can be made for other specific conductances at the same station or for other stations. A way to compare the relative abundance of major ions at different stations is by using the pie charts in figure 6.1.3 -1. These charts show the aver- age relative abundance of major ions at each station. Some variations in relative abundance of major ions occur that primarily are related to geology. In the headwaters, where bedrock primarily is igneous and metamorphic rocks and rocks of Pennsylvanian and Permian age (stations 9 and 35), calcium and bicar- bonate are the dominant ions present. As streams flow eastward across the clastic deposits of the Raton and Poison Canyon Formations, the relative concen- trations of sodium and, to a lesser extent, sulfate increase (stations 8, 12, and 30). As streams flow onto the plains formed from Cretaceous rocks, sodi- um and sulfate concentrations increase and become the dominant cation and anion (station 23). r