IMPORTANCE OF LITHOLOGIC COMPOSITIONAL DATA IN PREDICTIVE MODELING OF WATERSHED-SCALE SURFACE WATER CHEMISTRY

Models to predict surface water chemistry require large datasets that represent (net) mineral weathering and/or precipitation reactions. Compositional, mineralogical, and alteration data from each lithologic type are needed to determine how the interaction of bedrock and water lead to a particular surface water composition within a watershed. Our group has compiled a large database to determine how different bedrock and alteration types might account for differences in surface water chemistry. Two examples using pH, alkalinity, and conductivity (SpC) data are noted here; other examples will be presented.

The upper Animas River watershed in the San Juan Mountains, Colorado, contains unaltered to highly-altered volcanic bedrock. Data from five different alteration types indicates median pH values increase in going from unaltered but pyritized rocks (pH=3.39) to propylitically-altered rocks (pH=6.56), with other alteration types producing intermediate pH values in surface water. Median alkalinity values increase in a similar manner, from 0.10 (unaltered) to 8.7 mg/L CaCO3 (propylitic). In contrast, median conductivity generally decreases (from 608 to 166 uS/cm) as pH increases toward more alkaline values. These changes probably result from the presence of minerals (e.g., carbonates) in each alteration type that are capable of neutralizing acidity, whereas unaltered rocks contain acid-producing pyrite but have few or no acid-neutralizing minerals.

Numerous watersheds in central Colorado underlain by unaltered felsic bedrock show a median pH of 7.03, median alkalinity of 23.8 mg/L CaCO3, and median conductivity of 47.0 uS/cm. Several watersheds underlain by weakly-altered felsic rocks show (median values) pH of 7.10, alkalinity of 31.1, and SpC of 73.9. Thus, there is little chemical difference between waters derived from these two felsic rock types. These initial results suggest that without a high degree of alteration or mineralization, the bedrock composition may be most important in determining surface water chemistry on the watershed scale.