Mobilisation or dilution? Nitrate response of karst springs to high rainfall events

Abstract

Abstract. Nitrate (NO3-) contamination of groundwater associated with agronomic activity is of major concern in many countries. Where agriculture, thin free draining soils and karst aquifers coincide, groundwater is highly vulnerable to nitrate contamination. As residence times and denitrification potential in such systems are typically low, nitrate can discharge to surface waters unabated. However, such systems also react quickest to agricultural management changes that aim to improve water quality. In response to storm events, nitrate concentrations can alter significantly, i.e., rapidly decreasing or increasing concentrations. The current study examines the response of a specific karst spring situated on a grassland farm in south Ireland to rainfall events utilising high-resolution nitrate and discharge data together with on-farm borehole groundwater fluctuation data. Specifically, the objectives of the study are to formulate a scientific hypothesis of possible scenarios relating to nitrate responses during storm events, and to verify this hypothesis using additional case studies from the literature. This elucidates the controlling key factors that lead to mobilisation and/or dilution of nitrate concentrations during storm events. These were land use, hydrological condition and karstification, which in combination can lead to differential responses of mobilised and/or diluted nitrate concentrations. Furthermore, the results indicate that nitrate response in karst is strongly dependent on nutrient source, whether mobilisation and/or dilution occur and the pathway taken. This will have consequences for the delivery of nitrate to a surface water receptor. The current study improves our understanding of nitrate responses in karst systems and therefore can guide environmental modellers, policy makers and drinking water managers with respect to the regulations of the European Union (EU) Water Framework Directive (WFD). In future, more research should focus on high resolution monitoring of karst aquifers to capture the high variability of hydrochemical processes, which occur at time intervals of hours to days.

Nitrogen loss from karst area in China in recent 50 years: An in‐situ simulated rainfall experiment's assessment

“…To find out the correlation between rainfall and [DN], we train three fitting models according formula . First‐order exponential decay best describes the correlation between [DN] and rainfall (mm) (Figure ), which is likely to be associated with the dilution affect (Huebsch et al., ). The equations best describing surface and subsurface runoffs at CQ are as follows: where Equation and belong to surface runoff and subsurface runoff, respectively.…”

“…To find out the correlation between rainfall and [DN], we train three fitting models according formula (5). First-order exponential decay best describes the correlation between [DN] and rainfall (mm) (Figure 8), which is likely to be associated with the dilution affect (Huebsch et al, 2014). The equations best describing surface and subsurface runoffs at CQ are as follows:…”

Insights into agricultural influences and weathering processes from major ion patterns

“…With respect to the common dual‐flow concept for groundwater flow in karst aquifers (Atkinson, ; Ford & Williams, , ; Goldscheider & Drew, ; Kiraly, ; White, ), the observed pattern at the Wiesent source reflects the diffusive flow component through fractures and pores. In contrast to the faster reacting conduit flow system that transports water along pipe‐like conduits and cave stream channel, the diffusive flow reflects average values and long‐term trends (Huebsch et al, ). Subsequently, likely has accumulated over decades in the deep karst aquifer and seasonal variations became negligible.…”

“…The Cl − excess could in part be balanced by excess Ca 2+ and Mg 2+ that does not account for carbonate weathering. When assuming that this cation excess stems from fertilizer use in the upstream part of the catchment, it supports the hypothesis that fertilizers are more pronounced in upstream river sections and may have entered the river via diffusive flow in the present karst system (Huebsch et al, ).…”

Hydrologic and Geochemical Dynamics of Vadose Zone Recharge in a Mantled Karst Aquifer: Results of Monitoring Drip Waters in Mystery Cave, Minnesota

“…Fertilizer and pesticide pollutants such as nitrate and atrazine are commonly detected in shallow carbonate aquifers of southeastern Minnesota due to agricultural practices on the surface. These chemicals migrate down through the vadose zone, and may accumulate in the epikarst and/or saturated zone (Kamas et al, 2015;Huebsch et al, 2014). For example, Alexander et al (1999) reported that an anthropogenic Ca+Mg / Cl+NO 3 chemical component of up to 6 meq/kg is detectable in some at Garden of the Gods indicates that this area of the cave is a significant focal point for discharge of vadose zone waters held in storage above the cave.…”