THE ROLE OF WEATHERED AND UNWEATHERED BEDROCK CHEMICAL COMPOSITION AND MINERALOGY IN PRODUCING VARYING STREAM WATER CHEMISTRIES IN A HEADWATER CATCHMENT IN WESTERN NORTH CAROLINA

Headwater streams are understudied, especially the causes of different water chemistry at small spatial scales. Previous work in Gribble Gap, a 0.4 km² headwater catchment in the Southern Appalachians, measured differences in the groundwater and surface water chemistry between the northern and southern tributaries in Na, Ca, and Si. Relative homogeneity in factors that have been known to affect water chemistry (precipitation, vegetation, temperature, and land use history) across the watershed constrain potential drivers of water chemistry differences to the soil or bedrock. The purpose of this study was to investigate the role of bedrock in producing different water chemistries within Gribble Gap. Two transects across the northern and southern portions of the catchment were sampled for both weathered and unweathered bedrock to determine if there was spatial variation in bedrock mineralogy and elemental composition. Thin sections were prepared and analyzed with a petrographic microscope. Samples were also pulverized and analyzed for whole-rock mineralogy using Powder XRD, digested and analyzed for major elements using ICP-OES, and underwent a leaching experiment to determine if mobile ions are released more readily from the rock on one side of the catchment compared to the other. The dominant bedrock sampled was biotite gneiss as shown using thin section and XRD analyses, however, one sample of amphibolite was also identified. Thin sections and XRD analyses showed minimal variation in mineralogy and mineral abundances across the biotite gneiss in Gribble Gap. Data from the leaching experiment showed that there was little variation in the weathering products of the bedrock across Gribble Gap, however, the isolated amphibolite sample released ions more readily than biotite gneiss. Based on evidence from thin sections, Powder XRD, and bulk chemical analyses, there is no apparent systematic variation in the bedrock lithology of Gribble Gap. As the bedrock does not show substantial variation across the catchment, it cannot account for the measured differences in water chemistry between the northern and southern streams. Future studies should expand spatial coverage of bedrock samples in Gribble Gap and obtain unweathered bedrock samples, in part to understand the distribution of amphibolite within the watershed.