Vertical change in dissolved organic carbon and oxygen at the water table region of an aquifer recharged with stormwater: biological uptake or mixing?

Abstract

Decreases in dissolved organic carbon (DOC) and dissolved oxygen (DO) with increasing depth below the groundwater table are often considered as evidence for aerobic respiration; however, they may reflect mixing of infiltrating water and groundwater. We found that groundwater DOC concentration was on average 0.3 mg C l -1 higher and DO concentration 1.5 mg O 2 l -1 lower at recharge sites replenished with stormwater than at reference sites fed by direct infiltration of rain water from the land surface. Groundwater DOC increased and DO decreased with increasing vadose zone thickness (VZT) at both recharge and reference sites. There was no significant interaction between the effects of stormwater infiltration and VZT. Vertical changes in DOC and DO below the groundwater table at recharge sites could account for by simple mixing of infiltrating stormwater and groundwater. Moreover, aquifer sediment respiration (SR) was not significantly higher at recharge sites than at reference sites. However, slow filtration column experiments showed that SR increased significantly with an increasing supply of easily biodegradable DOC. We conclude that the observed reduction in DOC below the groundwater table at recharge sites was essentially due to water mixing rather than biological uptake because of the low biodegradability of the DOC and the short transit time of stormwater in the upper layers of groundwater. Our results highlight the need to distinguish between the effect of hydrological and biological processes on DOC and DO patterns below the groundwater before conclusions are made on the efficiency of groundwater in degrading surface-derived DOC.

The removal kinetics of dissolved organic matter and the optical clarity of groundwater

“…5). Surface-derived recharge then mixes and dilutes with ambient groundwater (Foulquier et al 2010) leading to an initial DOC concentration. DOC is then removed from the aquifer/compartment by the combined effects of biodegradation and sorption.…”

“…Studies in other groundwater environments have also reported an inverse relationship between DOC concentrations and the thickness of the unsaturated zone (Pabich et al 2001;Chapelle et al 2013), and DOC concentration decreases along discrete groundwater flowpaths (Findlay and Sobczak 1996;Baker et al 2000). The decrease in DOC concentrations has been attributed to a combination of biodegradation (Findlay and Sobczak 1996;Baker et al 2000;Rauch and Drewes 2005;Grünheid et al 2005), sorption onto aquifer materials (Neff and Asner 2001;Kalbitz et al 2003), and dilution (Foulquier et al 2010).…”

Origins and bioavailability of dissolved organic matter in groundwater

“…Other alteration processes must be active as surface DOM infiltrates into the groundwater. Hydrologic mixing can influence DOC concentrations in groundwater (Foulquier et al 2010), but mixing does not appear to be the major factor controlling the composition of DOM in groundwater. Sorption alters the composition of DOM by preferentially removing hydrophobic macromolecules, but it has less influence on bioavailable components (e.g., amino acids and carbohydrates) (Jardine et al 1989;Kaiser et al 2004;Inamdar et al 2012).…”

Composition and Dissolution of a Migratory, Weathered Coal Tar Creosote DNAPL

“…On the sink side, the physical accessibility of solid-phase geogenic TEA, predominantly Fe(III), and Mn(IV) species, is dependent on their crystallinity, which is not commonly analyzed except for sites of research interest (Villatoro-Monzón et al, 2003;Lovley et al, 2004;Scherr et al, 2016). Reduced species may be recharged in the upper aquifer portions, as molecular oxygen infiltrated by rain (Foulquier et al, 2010) or on a discontinuous scale from upgradient regions, such as nitrate from agricultural use. Source dissolution processes are characterized to a better extent for electron acceptor DNAPLs, such as single-component halogenated hydrocarbons (Klenk and Grathwohl, 2002;Guilbeault et al, 2005;Johnston et al, 2014).…”