SPATIOTEMPORAL DYNAMICS OF MICROBIAL COMMUNITIES IN CONTAMINATED SUBSURFACE

Groundwater serves as drinking water for 50% of United States residents, but a study conducted by the United States Geological Survey (USGS) found hazardous geological or manmade contaminants in approximately 20% of U.S. groundwater sources. Some contaminates were introduced due to industrial activities, such as at the Oak Ridge Reservation (ORR) in Tennessee. It is possible these contaminates could be reduced or immobilized by microbial activity, but the understanding of how spatiotemporal variability of subsurface geochemistry and the drift, dispersal, and selection of associated microbial communities is vital to understanding these potential processes. To understand the spatiotemporal relationship between microbial communities and geochemistry, a 70-day high-resolution time series survey of 27 wells was completed at the ORR. This study design was to obtain diurnal and seasonal fluctuations at three sites with mild, moderate, and high levels of nitrate and heavy metal contamination. One well at each site was selected for additional analyses by sampling for microbial communities in groundwater and sediment-associated environments simulated with sediment traps. Groundwater was filtered through 8 µm [micrometer] and 0.2 µm [micrometer] filters for 16S rRNA [ribosomal ribonucleic acid] gene sequencing and shotgun metagenomic analysis. In each of the wells used for microbial sampling, 18 unamended sediment traps were deployed throughout the sampling period for a time series soil analysis.

The results revealed distinct succession patterns in the abundant phyla and cell size of the microbes affected by the contamination levels and rainfall events. During a drought period captured over the course of the study, we established a baseline of microbial communities and geochemistry present in the groundwater at each site. We determined planktonic Actinobacteria became more abundant in all three contamination levels in the changing geochemistry after heavy rainfall. However, sediment-associated Firmicutes and Bacteroidota consistently increased after rainfall events. The largest geochemical fluctuations after rainfall were seen in the mild contamination level with sharp increases in nickel and aluminum. The drastic subsurface changes over time and space within these extreme environments show that any future bioremediation efforts at contaminated sites like the ORR require dynamic efforts that are adaptable to each site.