Concealed by darkness: Combination of NMR and HRMS reveal the molecular nature of dissolved organic matter in fractured-rock groundwater and connected surface waters.

Detailed molecular composition of solid phase extracted dissolved organic matter (SPE) collected from fractured-rock groundwater was compared to connected surface river water at two different watersheds in the unconfined chalk aquifer of Champagne in France using full scan ultrahigh resolution electrospray and photoionization Fourier transform ion cyclotron mass spectrometry (FT-ICR MS), Orbitrap tandem MS (MS/MS) and H magnetic resonance spectroscopy (NMR). H NMR spectroscopy indicated that groundwater SPE carried a higher contribution of aliphatic compounds while surface river waters SPE were enriched in carboxyl-rich alicyclic molecules (CRAM), acetate derivatives and oxygenated units. Furthermore, we show here that use of photoionization (APPI(+)) in aquifer studies is key, ionizing about eight times more compounds than ESI in surface river water samples, specifically targeting the dissolved organic nitrogen pool, accounting for more than 50% of the total molecular space, as well as a non-polar, more aromatic fraction; with little overlap with compounds detected by ESI(-) FT-ICR MS. On the other hand, groundwater SPE samples did not show similar selectivity as less molecular diversity was observed in APPI compared to ESI. Mass-difference transformation networks (MDiNs) applied to ESI(-) and APPI(+) FT-ICR MS datasets provided an overview of the biogeochemical relationships within the aquifer, revealing chemical diversity and microbial/abiotic reactions. Finally, the combination of ESI(-) FT-ICR MS and detailed Orbitrap MS/MS analysis revealed a pool of polar, anthropogenic sulfur-containing surfactants in the groundwaters, likely originating from agricultural runoff. Overall, our study shows that in this aquifer, groundwater SPE contains a significantly reduced pool of organic compounds compared to surface river waters, possibly related to a combination of lack of sunlight and adsorption of high O/C formulas to mineral surfaces.