Field and Laboratory Studies Linking Hydrologic, Geochemical, and Microbiological Processes and Enhanced Denitrification during Infiltration for Managed Recharge

Abstract

We present linked field and laboratory studies investigating controls on enhanced nitrate processing during infiltration for managed aquifer recharge. We examine how carbon-rich permeable reactive barriers (PRBs) made of woodchips or biochar, placed in the path of infiltrating water, stimulate microbial denitrification. In field studies with infiltration of 0.2–0.3 m/day and initial nitrate concentrations of [NO3-N] = 20–28 mg/L, we observed that woodchips promoted 37 ± 6.6% nitrate removal (primarily via denitrification), and biochar promoted 33 ± 12% nitrate removal (likely via denitrification and physical absorption effects). In contrast, unamended soil at the same site generated <5% denitrification. We find that the presence of a carbon-rich PRB has a modest effect on the underlying soil microbial community structure in these experiments, indicating that existing consortia have the capability to carry out denitrification given favorable conditions. In laboratory studies using intact cores from the same site, we extend the results to quantify how infiltration rate influences denitrification, with and without a carbon-rich PRB. We find that the influence of both PRB materials is diminished at higher infiltration rates (>0.7 m/day) but can still result in denitrification. These results demonstrate a quantitative relationship between infiltration rate and denitrification that depends on the presence and nature of a PRB. Combined results from these field and laboratory experiments, with complementary studies of denitrification during infiltration through other soils, suggest a framework for understanding linked hydrologic and chemical controls on microbial denitrification (and potentially other redox-sensitive processes) that could improve water quality during managed recharge.

Mobilization of Arsenic and Other Naturally Occurring Contaminants during Managed Aquifer Recharge: A Critical Review

“…Similarly, in situ amendments for infiltration basins have been proposed, including addition of carbon sources (e.g., wood chips) to create a horizontal permeable reactive barrier and promote denitrifying conditions. − These studies focus on nitrate removal from recharge water during infiltration and associated shifts in microbial community structure. While nitrate is a primary contaminant, preventing nitrate from reaching deeper groundwater supplies can also minimize the potential for oxidation of reduced immobile geogenic contaminant phases (e.g., arsenian pyrite, uraninite) limiting mobilization potential along the flow path.…”

Influence of agricultural managed aquifer recharge on nitrate transport: The role of soil texture and flooding frequency

“…Nitrate leaching and N transformation processes have been studied in more detail in traditional MAR systems such as storm water or treated wastewater infiltration basins (Abiye et al, 2009;Ben Moshe et al, 2020;Goren et al, 2014;Gorski et al, 2019;Schmidt et al, 2011). Depending on the infiltration rate of the native soil, Schmidt et al (2011) observed a 30-60% removal of NO 3 − in agricultural storm runoff recharged in a 3-ha infiltration basin.…”

“…Although traditional MAR systems have similar end goals to Ag‐MAR, major hydrologic differences exist between these systems. In contrast with MAR infiltration basins, which often maintain a constant head of several meters for several weeks, creating a thick saturated layer beneath the basin surface (Gorski et al., 2019; Schmidt et al., 2011), previous Ag‐MAR projects often had smaller heads (10–50 cm). In addition, on agricultural fields planted with perennial crops, the water was applied in short‐lived pulses on high‐infiltration‐capacity soils to avoid waterlogged conditions that could potentially harm the crops.…”

Natural and forced soil aeration during agricultural managed aquifer recharge

“…Monitoring soil physical-biogeochemical processes during MAR has been extensively studied (Danfoura & Gurdak, 2016;Ganot et al, 2018;Gorski et al, 2019;Greskowiak et al, 2005;McNab et al, 2009;Rodríguez-Escales et al, 2020;Schmidt et al, 2011;Vandenbohede et al, 2013); however, since Ag-MAR is a relatively new technique in the MAR toolbox, to date only a few studies have monitored these processes in actual agricultural fields during Ag-MAR (Bachand et al, 2014(Bachand et al, , 2016(Bachand et al, , 2019Dahlke et al, 2018;Dokoozlian et al, 1987). Most Ag-MAR studies have focused on developing soil suitability guidelines (O'Geen et al, 2015), regionalscale aquifer storage estimations (Scanlon et al, 2016), water availability analysis (Kocis & Dahlke, 2017), hydro-economic analysis (Gailey et al, 2019), and benefits evaluation using numerical modeling (Kourakos et al, 2019;Niswonger et al, 2017).…”