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 6.1.2 Relationship to Stream Discharge Specific Conductance of Streamflow Is Inversely Related to Discharge Relationships between specific conductance and streamflow are affected bygeology. Relationships between specific conductance and streamflow have been developed for the 10 stations shown in figure 6.1.2 -1. The linear relationships between the logarithms of discharge and specific conductance were developed using the method of least squares at stations where visual inspection of the plotted data indicated a linear trend. The rela- tionships (fig. 6.1.2 -2) are inverse; when discharge increases, specific conductance decreases. The posi- tion and slope of the lines in figure 6.1.2 -2 are significantly affected by local bedrock geology. The specific conductance of streamflow is largely determined by the specific conductances of its two major components -- direct runoff and base flow. Direct runoff to streams occurs as a result of snow - melt or rainfall runoff. Direct runoff usually has a low specific conductance because the relatively pure water from snowmelt or rainfall has minimal oppor- tunity to dissolve salts enroute to the stream. The highest discharges of streams in Area 61 generally occur as a result of direct runoff, causing the curves in figure 6.1.2 -2 to slope down at high discharges. When direct runoff is not occurring, streamflow may be maintained at base flow by ground -water inflows. Because ground water is in contact with geologic materials for a much longer period than direct ru'n- 50 off, ground water generally has a higher specific conductance because of dissolution of rocks and minerals. The result is a greater specific conductance of streamflow during base flow. The specific conductance of base_ flow depends on the geologic formation or formations from which base flow is contributed (fig. 6.1.2 -2) at each site. The highest specific conductances in base flow occur at sites where the base flow originates in Cretaceous rocks and the lowest at sites where base flow origi- nates from igneous, metamorphic, Pennsylvanian, or Permian rocks. Intermediate specific conductance occurs at sites where base flow is contributed from the Raton and Poison Canyon Formations. These trends in the specific conductance of base flow are similar to what is observed in ground -water samples from wells completed in these formations. The inverse relationship of specific conductance to discharge and associated seasonal variations is further exemplified at the streamflow- gaging station on the Purgatoire River at Madrid, Colo. (fig. 6.1.2 -3). Similar seasonal variations can be expected at other sites shown in figure 6.1.2 -1.