Large-scale study on groundwater dissolved organic matter reveals a strong heterogeneity and a complex microbial footprint

Dissolved organic matter (DOM) in terrestrial groundwater is generally low in concentration compared to inland surface waters. However, the overall amount of groundwater DOM is huge, as there is 100 times more fresh groundwater than fresh surface water. To date, research on groundwater DOM has merely focused on specific threats to humans such as e.g. DOM and heavy metal complexations and DOM from hydrocarbon contamination. A comprehensive, large-scale study of groundwater is still missing. Here, we examine DOM properties in a large-scale approach with regards to surface characteristics such as land use and altitude, aquifer characteristics as well as microbial features. We analyzed 1600 water samples from 100 groundwater bodies all over Austria with regards to their DOM quantity, quality and bacterial abun-dance (BA). DOM quality was evaluated with self-organizing maps on fluorescence excitation-emission-matrices (EEMs) combined with Ward clustering and subsequent parallel factor analysis to describe DOM properties of each clus-ter. We evaluated how these clusters differed among each other, based on DOC and nitrate concentrations, BA and se-lected environmental characteristics. Our results show that fluorescence components in groundwater resemble components found in other groundwater studies, in studies from forest streams, the dark ocean, agricultural catchments and wastewater treatment plants. The latter fluorescence components were associated with a cluster that is characterized by agricultural and urban land use, as well as by high nitrate concentrations. Clusters with an increased abundance of high-molecular weight and humic components, commonly associated with vascular plant and soil origin, correlated with a higher bacterial abundance. This observation provides evidence that elevated numbers of suspended bacteria mainly originate from the surface. Our study shows that DOM fluorescence can be a fast monitoring tool to identify aqui-fers under anthropogenic stress and delineate sensitive recharge areas with high surface-groundwater interaction.

Automated summary

This study examined the properties of dissolved organic matter (DOM) in terrestrial groundwater on a large scale, considering surface characteristics, aquifer characteristics, and microbial features. The results showed that DOM fluorescence components in groundwater are similar to those found in other studies, with associations to land use and nitrate concentrations. Clusters with higher bacterial abundance were correlated to high-molecular weight and humic components, indicating a surface origin for suspended bacteria. The study suggests that DOM fluorescence can be used as a fast monitoring tool to identify groundwater aquifers under anthropogenic stress and delineate sensitive recharge areas.