Nitrate Pathways, Processes, and Timing in an Agricultural Karst System: Development and Application of a Numerical Model

Abstract

Nitrogen (N) contamination within agricultural‐karst landscapes and aquifers is widely reported; however, the complex hydrological pathways of karst make N fate difficult to ascertain. We developed a hydrologic and N numerical model for agricultural‐karst, including simulation of soil, epikarst, phreatic, and quick flow pathways as well as biochemical processes such as nitrification, mineralization, and denitrification. We tested the model on four years of nitrate (NO3−) data collected from a phreatic conduit and an overlying surface channel in the Cane Run watershed, Kentucky, USA. Model results indicate that slow to moderate flow pathways (phreatic and epikarst) dominate the N load and account for nearly 90% of downstream NO3− delivery. Further, quick flow pathways dilute NO3− concentrations relative to background aquifer levels. Net denitrification distributed across soil, epikarst, and phreatic water removes approximately 36% of the N inputs to the system at rates comparable to nonkarst systems. Evidence is provided by numerical modeling that NO3− accumulation via evapotranspiration in the soil followed by leaching through the epikarst acts as a control on spring NO3− concentration and loading. Compared to a fluvial‐dominated immature karst system, mature‐karst systems behave as natural detention basins for NO3−, temporarily delaying NO3− delivery to downstream waters and maintaining elevated NO3− concentrations for days to weeks after hydrologic activity ends. This study shows the efficacy of numerical modeling to elucidate complex pathways, processes, and timing of N in karst systems.

Long‐term assessment of nutrient flow pathway dynamics and in‐stream fate in a temperate karst agroecosystem watershed

“…For context, we compare with a mature karst system 21 km from our study watershed (Royal Spring basin). For Royal Spring, three distinguishable hydrologic reservoirs were identified in the master recession curve, with coefficients of 0.50, 0.15, and 0.05 day −1 reflecting the quick, intermediate, and slow flow paths, respectively (Husic, Fox, Ford, et al, ). Regarding slow flow, our results found a master recession coefficient of 0.07 day −1 suggesting 75% of flow in the watershed on an annual basis is governed by recharge through low permeability matrix pores and small fissures.…”

“…Regarding upland dynamics, recent advancements in techniques to quantify karst hydrologic pathways has led to a conceptualization of karst drainage as a series of reservoirs characterized by quick, intermediate, and slow flow pathways (Husic, Fox, Adams, et al, ; Husic, Fox, Ford, et al, ; Rimmer & Hartmann, ). Quickflow in karst watersheds reflects short residence time water originating from sinkholes, swallets, estavelles, and conduits (Pronk, Goldscheider, Zopfi, & Zwahlen, ; White, ).…”

“…Improved hydrologic characterization of karst watersheds has led to improved characterization of nutrient transport pathways. Nitrate concentration and subsequent leaching from the soil and epikarst zone have been found to control spring concentrations and loadings, whereas quickflow following storm events has been found to dilute concentrations relative to groundwater aquifer sources (Baran, Lepiller, & Mouvet, ; Husic, Fox, Ford, et al, and references within). Conversely, dissolved reactive phosphorus (DRP) has been found to be more concentrated in quickflow than in intermediate and slow flow pathways, stemming from agricultural management practices in which high soil P levels at the soil surface become connected to the stream network through macropores and overland flow during quickflow (Baker, Johnson, & Confesor, ; Ford, Williams, Young, King, & Fischer, ; Jarvie et al, ; King, Williams, & Fausey, ).…”

“…These findings would tend to suggest that the prevalence of assimilatory and dissimilatory processes may be mediated by the presence of algal biomass in scoured bedrock channels. Assessing these perceptions in karst agroecosystems is critical given that karst drainages provide thermal buffering, have low dissolved oxygen at low‐flows, and deliver high nutrient loads that stimulate in‐stream biochemical reactivity (Dirnböck, Kobler, Kraus, Grote, & Kiese, ; Husic, ; Husic, Fox, Adams, et al, ; Husic, Fox, Ford, et al, ; Jiang, Hu, & Schirmer, ; Knierim, Pollock, Covington, Hays, & Brye, ; Mahler & Bourgeais, ).…”

Disaggregating Landscape‐Scale Nitrogen Attenuation Along Hydrological Flow Paths

“…While further attenuation is possible in the vadose zone as septic effluent leaves the drainfield (De & Toor, 2015) and reductions in N concentrations of 40% as effluent moves below the drainfield have been observed , we did not consider additional biogeochemical losses of septic leachate in the vadose zone as further attenuation is highly variable (CV = 118%; Katz et al, 2010) and losses are not always observed (Morales et al, 2016). A numerical model in an agricultural karst landscape also found much larger denitrification in the soil compared to epikarst (e.g., vadose zone) (Husic et al, 2019).…”