Understanding the age distribution of water at key locations in a catchment is crucial to understanding water-quality dynamics. Current trends in water quality may be a combination of both historical land use change and lag times to a point of interest in the catchment. To assess the temporospatial relationship between Nitrate-N (N) concentrations in fresh waters and changes in intensive agriculture in the Upper Waikato catchment (New Zealand), an integrated surface water-groundwater MODFLOW-MODPATH model was developed. The model provided the estimation of water travel times from different locations in the catchment to its end point (surface water) using particle tracking. The model simulates steady-state saturated flow and particle transport, and includes the Waikato River and all the major tributaries in the catchment (using MODFLOW’s STR package). This integrated surface water-groundwater model was calibrated against measured groundwater levels, river gains and losses, and water quality observations of N and tritium. To address the insufficient tritium data availability in some areas, a relationship between SiO2 concentrations and mean resident time (MRT) using measured tritium was used
A particle was released into the water-table at the centre of each (uniform) model cell, and the travel time for each particle to reach its end-point was calculated. The mean age and age distribution of water at different locations was estimated from the collection of particle travel times contributing to the end point. The MRT for water contributing to the average flow at tributaries was estimated between 5 to 101 years, with an average of 51 years. The model results showed that MRT alone can be misleading, and it is important to also identify the age distribution of water discharged from sub-catchments. Approximately two thirds of the flow from most sub-catchments had ages that were less than the MRT. This is because MRT distribution is skewed by a few contributions from very long travel times. The model also suggests that travel times from areas that are subject to more recent land-use intensification can differ to travel times for the catchment as a whole. Therefore, it is important to consider both the historical and spatial distribution of land use change. This work demonstrates how groundwater modelling can be used as an effective tool for unravelling the effects of variable lags in the contaminant loads from historical land use changes when investigating current trends in water quality.