How long does it take for nutrient enriched groundwater to travel from farms to Lake Taupo?
The age of groundwater in the Lake Taupo catchment has been estimated for Environment Waikato by Geological and Nuclear Sciences, who measured concentrations of the cosmogenic isotope tritium, and two anthropogenic tracers: chlorofluorocarbons (CFCs) and sulphur hexafluoride (SF6). These analyses tell us when the water was last in contact with the atmosphere, and the proportions of 'young' and 'old' water in a sample. Results indicate that the mean residence time of groundwater samples in the catchment ranges from about 20 to 75 years.
Higher nitrogen concentrations are found in groundwater with a greater proportion of the water recharged in the 35 years since farming developed in the area. The farming influence is not uniform of course, and not all young groundwater has a high nitrogen concentration. However, in subcatchments where young water is present, we can use these data to assess the potential increase in groundwater nitrogen concentration from current farming. Based on these estimates, the nitrogen load entering the lake is expected to increase for decades to come, even if planning controls are introduced to limit the further intensification of land use.
John Hadfield [ [email protected] ] Environment Waikato