LAND USE IMPACTS ON THE GEOCHEMISTRY AND MICROBIOLOGY OF CAVE STREAMS WITHIN THE KNOXVILLE METROPOLITAN AREA

Karst groundwater systems are ubiquitous, reliable aquifers that have higher risks of contamination compared to other aquifer systems due to their connectivity to the surface, such as from sinking streams and sinkholes. Changes to land use / land cover (LULC) above karst systems can impact and alter environmental conditions in the subsurface and affect groundwater biota. Previous studies have focused on understanding how LULC changes affect the geochemistry and microbial communities in surface aquatic systems, but few studies have focused on the impacts of LULC change on water quality and microbiology of cave and karst areas. This study focused on cave systems in the Knoxville Metropolitan Statistical Area, where changes in LULC over a 5-year period (2019–2024), near to and within cave system watersheds, were calculated. Cave stream and sediment geochemistry and microbiology were evaluated and compared to LULC changes. This study also assessed impacts to the caves from a flooding event caused by Hurricane Helene. Overall, watershed boundaries did not predict water quality conditions, but local LULC cover surrounding cave entrances, within a 1-km radius buffer, corresponded to the degree of impact, with the highest nutrient measurements coming from cave streams near high intensity development and with higher human populations. Caves with inflowing streams were the most affected by LULC changes compared to caves with outflowing streams that were more indicative of groundwater conditions buffered from LULC impacts. Stream water and sediment geochemistry generally suggested stream impairment based on elevated concentrations of chloride, nitrate, and trace metals that were indicative of anthropogenic input. Some trace metals were only measured from cave sediments, and concentrations were higher from streams experiencing the most LULC change over time. Moreover, the microbial loads of coliforms and E. coli were higher than recommended by federal agencies, and the presence of other putative pathogens suggested chronic contamination of the cave streams. Dominant bacterial phyla from streams and sediments included Pseudomonadota, Acidobacteria, Bacteridota, Planctomycetota, Verrucomicrobiota, and Chloroflexota. Because there have been limited studies connecting LULC drivers to karst microbial community compositional changes through time, this study provides key information for future research.