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 - 6.1 Specific Conductance 6. 1.1 Downstream Changes Specific Conductance of Streamflow Increases Downstream Downstream increases in specific conductance result from variations in precipitation, land and water use, and geology. Specific conductance of streamflow increases by at least a factor of 10 from the headwaters to the point where the stream leaves the area. Some of the increase is due to natural processes such as decreasing precipitation downstream and changes in bedrock geology. Most of the increase is related to man's use of the water for agricultural and, to a lesser extent, municipal purposes. Specific conductance is a measure of the ability of water to conduct an electrical current and is expressed in micromhos per centimeter at 25° Celsi- us. Specifid conductance is a useful indicator of water quality because it is simple and inexpensive to measure and is directly related to the concentration of dissolved constituents. Average specific conductance at stream sites is shown in figure 6.1.1 -1. The average specific conduc- tance was determined using all available data at each site. Specific conductance of streamflow is least in the mountainous and mesa areas and increases sig- nificantly in the plains areas because of changes in geologic, climatic, and land and water -use condi- tions. For example, at the headwaters of the Pur- gatoire River, bedrock consists of igneous and metamorphic rocks resistant to chemical weathering and less resistant sedimentary rocks (fig. 6.1.1 -2). Chemical weathering results in dissolution of rock materials and release of dissolved constituents to the water. The headwaters area is primarily forest land and receives the greatest precipitation in the basin. This precipitation and resulting runoff accompanied 48 by slow rates of chemical weathering results in water with an average specific conductance of 295 micro - mhos per centimeter at 25° Celsius at Stonewall, Colo. The foothills area between Stonewall and Trinidad is used primarily for grazing and receives less precipitation. Bedrock geology in this area is dominated by the Raton and Poison Canyon Forma- tions, which are more susceptible to chemical weath- ering. As a result, tributaries in this reach generally contribute water with greater specific conductance than the Purgatoire River. Some small mine drain- ages and seeps in this reach also contribute water with greater specific conductance. Because there is less precipitation in the foothills area, the contribu- tions of water and dissolved constituents are small, and only a small increase in specific conductance occurs. Downstream from Trinidad the valley opens onto the plains formed on more easily erodible and chemically weathered Cretaceous rocks. The flat terrain allows for irrigated agriculture. Much of the irrigation water is consumed by plants- resulting in a concentration of dissolved salts. The unused water, called irrigation return -flow, dissolves salts from the soil and underlying geologic materials as it moves to the river. The concentration of water through evapo- ration and dissolution of chemical constituents result in a large increase in specific conductance between Trinidad and near Hoehne. Similar increases occur on other larger streams in Area 61.