A study into how salt contamination impacts groundwater microbiology.

Abstract

Groundwater is a valuable resource currently under threat from salt contamination due to over-extraction of water, sea level rise (SLR), and anthropogenic activities. While these physical processes are well researched, the impact of salt contamination on microbial communities is less so. Microorganisms play an important role in ecosystems as decomposers and nutrient cyclers. Their role in groundwater microbial communities could be of utmost importance to the maintenance of groundwater system health, which humans rely on. Salt is known to have anti-microbial properties and therefore, salt contamination may cause a change in the groundwater microbial community, impacting their function.

A laboratory-based, packed sand column experiment was undertaken. To do so, a groundwater microbial community was established in sand-packed columns with a mutual, recirculated water source collected from a groundwater well in Canterbury, New Zealand. Once established, the packed columns were split into two sets: (1) packed columns continuing to receive fresh groundwater; and (2) packed columns receiving fresh groundwater spiked with sodium chloride. Differences in the saltwater packed column’s microbial community was compared to the freshwater packed column’s microbial community. Intrinsic permeability was used as an indicator for bioclogging and was measured for each packed column throughout the experiment. Destructive sampling was undertaken at four time points to characterize the microbial community, where the aquifer media was used to carry out protein assays, total cell counts, heterotrophic plate counts, and 16S rRNA amplicon sequencing.

Salt contamination was observed to have varying degrees of impact on the microbial community. Salt-induced bioclogging had little influence on the intrinsic permeability. while impacts on heterotrophic plate counts, total cell counts, and live cell counts did not significantly differ, their behaviour indicated a level of salt impact. Total protein indicated the saltwater packed column’s microbial community were stressed by the presence of salt, while 16S rRNA amplicon sequencing provided evidence of microbial community composition divergence over time.

This work will support future research into how salt exposure impacts groundwater microbial communities and its services in a broad sense, while specifically contributing to studies establishing evidence-based salt contamination limits in groundwater, globally.