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 on 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 the high-resolution monitoring of karst aquifers to capture the high variability of hydrochemical processes, which occur at time intervals of hours to days.
Highlights
The consequences of groundwater contamination by reactive nitrogen (Nr, e.g. nitrate NO−3 ), derived from agricultural sources, is of major concern in many countries (Galloway and Cowling, 2002; Spalding and Exner, 1993; L’hirondel, 2002)
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 onfarm borehole groundwater fluctuation data
By looking at different nitrate characteristics during storm events, we aim to answer the following questions: what are the key factors controlling increased or decreased nitrate concentrations in karst springs as response to storm events? Does it depend on the karst system alone, the hydrological situation or land use and/or of a combination of all these components together? the objectives of the present study are to formulate a conceptual model of possible scenarios of nitrate responses during storm events, and to verify this hypothesis using other examples from the literature together with data from our study site
Summary
The consequences of groundwater contamination by reactive nitrogen (Nr, e.g. nitrate NO−3 ), derived from agricultural sources, is of major concern in many countries (Galloway and Cowling, 2002; Spalding and Exner, 1993; L’hirondel, 2002). As groundwater response times affect the physical and economic viability of different mitigation measures, there is a realisation that such responses must be incorporated into environmental policy Such processes are poorly understood (Sophocleous, 2012), where nitrate discharges unabated from high N input agricultural systems underlain by thin free draining soils and karst aquifers (Huebsch et al, 2013). In karst regions, characterising nitrate dynamics in aquifers can help to predict when concentrations are likely to breach this MAC or not No such standard exists for surface water but instead, in countries such as the Republic of Ireland, a much lower MAC of 11.5 mg NO−3 L−1 exists for estuaries (Statutory Instruments S.I. No 272 of 2009). Improving our conceptual model of nitrate mobilisation and/or dilution in karst systems will allow us to better manage agricultural systems in the future
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