WATER CHEMISTRY IN SMALL MIXED-LITHOLOGY CATCHMENTS IN THE VALLEY AND RIDGE PROVINCE

The relative importance of physical and chemical weathering is a longstanding discussion in the geosciences. To address the chemical-weathering question, nearly 100 surface waters in the Appalachian Valley and Ridge Province were sampled between August and December 2023. The data from the small watersheds (< 50 km²) were combined with comparable USGS data and existing datasets collected by the authors. The small watersheds were used to evaluate if surface water geochemistry is a strong reflection of the amount or relative amount of carbonate rocks mapped in the watershed.

The field and chemical data included discharge, field geochemical parameters, and major ions. For each sample location, the catchment area was calculated using WhiteBox Workflows for Python, and the percentage of each watershed underlain by carbonate rocks was calculated with Geopandas based on the USGS State Geologic Map Compilation geodatabase. These sites have discharges from less than 1 to greater than 1000 m³/hr. The underlying geology varied but catchments with both 0 to 100 % underlying carbonate rocks were included and provide end-members for the analysis. There was no significant correlation between the percentage or amount of carbonate rock in the watershed with the carbonate-dissolution analytes (Ca and Mg). However, the sites with >85% underlying carbonate were consistent in containing higher concentrations of carbonate constituents. The sites with no contributing mapped carbonates contained lower concentrations of carbonate constituents. The results indicate that watersheds consisting of nearly one rock type may be treated as chemical end-members for the analysis, but that water chemistry does not vary monotonically with proportion of carbonate rock in the watershed.

Although a surface-drainage definition of catchment areas is regularly used in watersheds, it is unlikely to reflect the conditions in karst settings where subsurface flow is important. Furthermore, the calculation of “carbonate contribution” in each watershed is limited by the available geologic data and typically does not consider factors such as deeper rock units and the presence of carbonate minerals in non-carbonate formations. However, even with these limitations there were clear trends between the catchments in this study.