Demonstrating trend reversal of groundwater quality in relation to time of recharge determined by 3H/3He.

Recent EU legislation is directed to reverse the upward trends in the concentrations of agricultural pollutants in groundwater. However, uncertainty of the groundwater travel time towards the screens of the groundwater quality monitoring networks complicates the demonstration of trend reversal. We investigated whether trend reversal can be demonstrated by relating concentrations of pollutants in groundwater to the time of recharge, instead of the time of sampling. To do so, we used the travel time to monitoring screens in sandy agricultural areas in the Netherlands, determined by (3)H/(3)He groundwater dating. We observed that concentrations of conservative pollutants increased in groundwater recharged before 1985 and decreased after 1990. Thereby, we demonstrated trend reversal of groundwater quality. From this research we concluded that (3)H/(3)He dating can be used to facilitate (re)interpretation of existing groundwater quality data. The presented approach is widely applicable in areas with unconsolidated granular aquifers and large agricultural pressures on groundwater resources.