Groundwater metabolome responds to recharge in fractured sedimentary strata

Understanding biogeochemical conversions of dissolved organic matter (DOM) in aquifers is paramount for the effective management of groundwater supplies. On its passage through the critical zone, DOM is subject to biogeochemical conversions and therefore carries cross-habitat information useful for monitoring and predicting the stability of groundwater ecosystem services. Groundwater metabolomics assesses this information. However, challenges arise from insufficient knowledge on groundwater metabolite composition and dynamics, and the necessity to maintain analytical conditions for long-term monitoring. We explored fractured sedimentary bedrock by 5-year untargeted metabolomics monitoring for oxic perched and anoxic phreatic sites along a hillslope recharge area, to evaluate DOM as groundwater tracer. Dimension reduction by principal component analysis revealed that metabolome dissimilarities between distant wells coincide with transient cross-stratal flow indicated by groundwater levels and environmental tracers. The metabolome was highly variable lacking seasonal patterns, and did not segregate by geographic location of sampling wells thus ruling out surface vegetation or (agricultura) land use as driving factor. The metabolome time series provide detailed insights into subsurface responses to recharge dynamics. Metabolomics monitoring provides information on groundwater flows, and allows concluding about below ground ecology and water quality evolution, required to understand the impact of interannual wet-dry cycles.