Influence of Agricultural Managed Aquifer Recharge and Stratigraphic
Heterogeneities on Nitrate Reduction in the Deep Subsurface
Abstract
Agricultural managed aquifer recharge (AgMAR) is a proposed management
strategy whereby surface water flows are used to intentionally flood
croplands with the purpose of recharging underlying aquifers. However,
legacy nitrate (NO3-) contamination in
agriculturally-intensive regions poses a threat to groundwater resources
under AgMAR. To address these concerns, we use a reactive transport
modeling framework to better understand the effects of AgMAR management
strategies (i.e., by varying the frequency, duration between flooding
events, and amount of water) on N leaching to groundwater under
different stratigraphic configurations and antecedent moisture
conditions. In particular, we examine the potential of denitrification
and nitrogen retention in deep vadose zone sediments using variable
AgMAR application rates on two-dimensional representations of
differently textured soils, soils with discontinuous bands/channels, and
soils with preferential flow paths characteristic of typical
agricultural field sites. Our results indicate that finer textured
sediments, such as silt loams, alone or embedded within high flow
regions, are important reducing zones providing conditions needed for
denitrification. Simulation results further suggest that applying
recharge water all-at-once, rather than in increments, increases
denitrification within the vadose zone, but transports higher
concentrations of NO3- deeper into the
profile. This transport into deeper depths can be aggravated by wetter
antecedent soil moisture conditions. We conclude that ideal AgMAR
management strategies can be designed to enhance denitrification in the
subsurface and reduce N leaching to groundwater, while specifically
accounting for lithologic heterogeneity, antecedent soil moisture
conditions, and depth to the water table.