Several studies have demonstrated that the age of groundwater can be included as an essential component of evaluation of the impact of agricultural N mitigation measures on groundwater nitrate concentrations (e. g. Hansen et al, 2017). Its inclusion may help to link changes in land use and agricultural management practices to changes in groundwater quality. Groundwater age determination allows concentrations in grounwater to be related to the time of recharge instead of the time of sampling, which, in turn, makes comparison between groundwater quality and pollutant losses from agriculture possible.
Various measurement techniques and hydrological models are available to estimate the groundwater recharging rates and water ages. The recharging rates can be estimated by direct measurements of water infiltration rates, estimations from various tracers, calculation of water mass balance, and analysis of a hydrograph (Table 7.2). The recharging rates also can be calculated using various types of hydrological models.
Table 7.2: Indicators and methods for lag time estimation
Water age can also be modelled but is determined mainly by age tracers such as radionuclides (e.g. 37Ar, 35S,14C) and stable and transient tracers (e. g. CFCs, SF6, 18O, 3H, 40Ar; Bethke and Johnson, 2008; Busenberg and Plummer, 1992; Laier, 2005).
Time and spatial scales of these methods vary widely and different uncertainties are associated with each method. Therefore, it is recommended to use multiple methods to estimate the recharging rates and water ages (e. g., Bethke and Johnson, 2008; Scanlon et al., 2002).