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

Abstract

Abstract. Nitrate (NO −3 ) 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 onfarm 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.

Technical Note: Field experiences using UV/VIS sensors for high-resolution monitoring of nitrate in groundwater

“…The DWS was installed in a flowing spring emergence (spring A) in a karst spring in an agriculture-dominated area in southwest Ireland and the MWS in a flowing spring emergence (spring B) in an urbanised catchment in north-west Jordan. The study sites are described in more detail in a previous study of Huebsch et al (2014) and Grimmeisen et al (2014), respectively. Both springs discharge karst aquifers; however, spring A is located in an agricultural catchment and spring B in an urban catchment.…”

“…a double wavelength spectrophotometer (DWS) and a multiple wavelength spectrophotometer (MWS) used at field sites in Ireland and Jordan, respectively, which were originally used for two different scientific studies (Grimmeisen et al, 2014;Huebsch et al, 2014). The following issues are addressed in the present study: hardware options, ease of calibration, accuracy, influence of additional substances, positive and negative aspects of the two sensors, troubleshooting and trade-offs.…”

“…Built into these regulations is a fundamental need to monitor the quality of drinking water supplies. However, especially in karst and/or fractured aquifers, water quality can change rapidly in a time frame from hours to days (Huebsch et al, 2014;Mahler et al, 2008;Pronk et al, 2009). Nitrate (NO − 3 ) is particularly noted as being a risk to human health when in high concentrations in source drinking water (L'hirondel, 2002) and also contributes significantly to eutrophication of water (Stark and Richards, 2008).…”

“…High resolution flow and nitrate-nitrogen (NO 3 -N) concentration data from short residence time aquifers enable an improved understanding of the mobilisation/dilution dynamics in karst aquifers (Huebsch et al, 2014) and to prevent negative consequences from NO 3 -N concentrations exceeding the maximum allowable concentration (MAC) for drinking water. In the EU, for example, the MAC is 11.3 mg NO 3 -N L −1 , to prevent health concerns (Knobeloch et al, 2000), abortion to cattle or toxicity in livestock (Di and Cameron, 2002).…”

How runoff components affect the export of DOC and nitrate: a long-term and high-frequency analysis

“…Besides high-frequency studies on DOC, several studies were targeting nitrate as well. Huebsch et al (2015) evaluated the performance of UV-VIS 25 spectrometry regarding nitrate and Huebsch et al (2014) investigated the nitrate response of karst springs in Ireland to heavy rainfall events. Instead of investigating how nitrate is related to rainfall-runoff behavior, Aubert and Breuer (2016) studied diel nitrate cycles in streamflow and its driving factors.…”