COMPARISONS OF BACTERIAL COMMUNITIES AND ANALYSIS OF GEOCHEMICAL CONDITIONS IN CHLORIDE IMPACTED FRACTURED BEDROCK WELLS

This study utilizes the characterization of bacterial populations, geochemical trends, and spatial analysis to elucidate source areas for fractures feeding residential wells. The study area is in the town of Sherman, Connecticut, where several residential and public supply wells have had persistent issues with elevated chloride and sodium concentrations since 2015. The town has been monitoring the situation with regular sampling of major ions in 24 wells and surface water in the town center but has not seen a consistent decrease in chloride concentrations since monitoring began. In the context of the local geology and topography, the geochemical analysis indicates the elevated chloride concentrations are limited to wells installed into thin till and are downgradient of large paved areas. Conversely the wells with low concentrations are installed through much thicker and coarser grained alluvium deposits in the local drainage. In addition to conducting geochemical analysis of the data supplied by the town, 24 groundwater and two surface water samples were collected during the March 2018 sampling event for bacterial characterization. By filtering groundwater samples in a 0.22-micron filter, amplifying a portion of the ribosomal RNA through polymerase-chain reaction, and generating sequence reads through a MiSeq sequencer, a dataset was created to represent the thousands of bacterial ‘species” found in that water. Comparisons were made to quantify how similar the communities are between samples to see clear distinctions between samples collected from surface water and well water. Of the groundwater samples there are significant distinctions between the wells installed into thin till and exhibit high concentrations of chloride compared to wells with low concentrations installed through the alluvium in the local drainage. The differences in bacterial populations add context to this hydrologic study and the apparent risk of well contamination from surficial sources.