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Groundwater age dating as a tool for nitrogen pollution risk assessment in Croatia

Groundwater is a valuable natural resource and has to be protected from any chemical pollution. The most common source of groundwater contamination is intensive agricultural activity and, as its consequence, an increased nitrate concentration in groundwater. Agricultural activity is most intensively represented in lowland areas where the most productive aquifers are often located. In these aquifers, groundwater has relatively long travel time of pollutants from the ground surface to the observation borehole. Using groundwater dating, the measured concentrations of pollutants can be directly related to the time of recharge (BROERS, 2004 ; MORGENSTERN et al., 2015 ; VISSER, 2009). In that sense, the question arises can we detect trends and trend reversal in groundwater quality using groundwater age? Studies have shown that this is possible (HANSEN et al., 2012, 2017 ; VISSER, 2009). Groundwater quality in Croatia is generally relatively good. Mean nitrate concentrations in groundwater are mostly below 15 mg/L, while in the southern, karst areas, they rarely exceed 5 mg/L. However, in the western (Istria peninsula) and north-western parts of Croatia, concentrations of nitrates in the groundwater may exceed the threshold values [BIONDIĆ et al., 2016 ; NAKIĆ et al., 2016]. As a result, in the Water Management Plan for 2016 – 2021 (HRVATSKE VODE, 2016) groundwater bodies (GWBs) in these areas are declared as GWBs of poor chemical status and are at risk of failure to achieve the goal of "preventing status deterioration". Dating of groundwater age using environmental tracers in these areas started two years ago and the research is still ongoing. The aim of this research is to develop and improve tools to detect trends of nitrates in groundwater in Croatia. Groundwater samples were collected at 10 locations. Environmental tracers, as CFCs (chlorofluorocarbons), SF6 (sulphur hexafluoride), 3H (tritium) and 3He (helium-3) were measured. Their content in the groundwater was compared with historical data on their content in the air based on which the mean groundwater age was assessed. The estimated mean groundwater age was analysed in relation to the pressures from the agricultural activity. The research points out interesting results that can contribute to a better assessment of the future nitrate trends in the investigated areas.

environmental tracer, mean residence time, groundwater age distribution, nitrates, trend

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