Using groundwater age and hydrochemistry to understand sources and dynamics of nutrient contamination through the catchment into Lake Rotorua, New Zealand

摘要

The water quality of Lake Rotorua has steadily declined over the past50 years despite mitigation efforts over recent decades. Delayed response ofthe groundwater discharges to historic land-use intensification 50 years agowas the reason suggested by early tritium measurements, which indicatedlarge transit times through the groundwater system. We use the isotopic andchemistry signature of the groundwater for detailed understanding of theorigin, fate, flow pathways, lag times and future loads of contaminants. Aunique set of high-quality tritium data over more than four decades,encompassing the time when the tritium spike from nuclear weapons testingmoved through the groundwater system, allows us to determine detailed agedistribution parameters of the water discharging into Lake Rotorua.The Rotorua volcanic groundwater system is complicated due to the highlycomplex geology that has evolved through volcanic activity. Vertical andsteeply inclined geological contacts preclude a simple flow model. Theextent of the Lake Rotorua groundwater catchment is difficult to establishdue to the deep water table in large areas, combined with inhomogeneousgroundwater flow patterns.Hierarchical cluster analysis of the water chemistry parameters providedevidence of the recharge source of the large springs near the lake shore,with discharge from the Mamaku ignimbrite through lake sediment layers.Groundwater chemistry and age data show clearly the source of nutrients thatcause lake eutrophication, nitrate from agricultural activities andphosphate from geologic sources. With a naturally high phosphate loadreaching the lake continuously via all streams, the only effective way tolimit algae blooms and improve lake water quality in such environments is bylimiting the nitrate load.The groundwater in the Rotorua catchment, once it has passed through thesoil zone, shows no further decrease in dissolved oxygen, indicating an absenceof bioavailable electron donors along flow paths that could facilitatemicrobial denitrification reactions. Nitrate from land-use activities thatleaches out of the root zone of agricultural land into the deeper part ofthe groundwater system must be expected to travel with the groundwater to the lake.The old age and the highly mixed nature of the water discharges imply a veryslow and lagged response of the streams and the lake to anthropogeniccontaminants in the catchment, such as nitrate. Using the age distributionas deduced from tritium time series data measured in the stream dischargesinto the lake allows prediction of future nutrient loads from historicland-use activities 50 years ago. For Hamurana Stream, the largest stream toLake Rotorua, it takes more than a hundred years for thegroundwater-dominated stream discharge to adjust to changes in land-useactivities. About half of the currently discharging water is still pristineold water, and after this old water is completely displaced by wateraffected by land use, the nitrogen load of Hamurana Stream willapproximately double. These timescales apply to activities that causecontamination, but also to remediation action.