Dissolved organic matter tracers reveal contrasting characteristics across high arsenic aquifers in Cambodia: a fluorescence spectroscopy study

Organic matter in the environment is involved in many biogeochemical processes, including the mobilization of geogenic trace elements, such as arsenic, into groundwater. In this paper we present the use of fluorescence spectroscopy to characterize the dissolved organic matter (DOM) pool in heavily arsenic-affected groundwaters in Kandal Province, Cambodia. The fluorescence DOM (fDOM) characteristics between contrasting field areas of differing dominant lithologies were compared and linked to other hydrogeochemical parameters, including arsenic and dissolved methane as well as selected sedimentary characteristics. Absorbance-corrected fluorescence indices were used to characterize depth profiles and compare field areas. Groundwater fDOM was generally dominated by terrestrial humic and fulvic-like components, with relatively small contributions from microbially-derived, tryptophan-like components. Groundwater fDOM from sand-dominated sequences typically contained lower tryptophan-like, lower fulvic-like and lower humic-like components, was less bioavailable, and had higher humification index than clay-dominated sequences. Methane concentrations were strongly correlated with fDOM bioavailability as well as with tryptophan-like components, suggesting that groundwater methane in these arsenic-prone aquifers is likely of biogenic origin. A comparison of fDOM tracers with sedimentary OM tracers is consistent with the hypothesis that external, surface-derived contributions to the aqueous DOM pool are an important control on groundwater hydrogeochemistry.