Groundwater time delay in the Lake Rotorua catchment
Environment Bay of Plenty is undertaking a study with the Institute of Geological and Nuclear Sciences (GNS) to understand the groundwater delays within the catchment of Lake Rotorua. This work is vitally important for understanding land-use effects on Lake Rotorua. The focus of the study is the western catchments of Lake Rotorua, the area of greatest known flux of groundwater to the lake and where most of the streams are spring fed.
GNS is using tritium isotope and two anthropogenic tracers, chlorofluorocarbons (CFCs) and sulphur hexafluoride (SF6), to measure the age and proportions of young and old water discharging from the groundwater fed springs and streams that flow into Lake Rotorua. Initial results indicate that the mean residence time of samples in the catchment ranges from 30 to 170 years. The ‘percentage young’ fraction (with ‘young’ defined as being less than 40 years old) ranges from 1% to 84%. This means that most of the groundwater that is now discharging to the lake is too old to reflect the post-war land intensification in the western catchment. High nitrate concentrations are related to the young waters. Based on these age determinations, the nitrogen load entering the lake is expected to increase into the future, even if land-use intensity was reduced now.
Dougall Gordon, Environment Bay of Plenty [[email protected]] Uwe Morgenstern, Geological and Nuclear Sciences [[email protected]]