Groundwater an long term problem for Rotorua lakes

Lake Rotorua’s water quality is likely to get worse before it gets better because of large stores of nitrogen-rich underground water.

A new study commissioned by Environment Bay of Plenty has revealed that most of the groundwater currently entering the lake is too old to reflect the land development and intensification of agriculture in the Rotorua catchment after World War Two.

“Unfortunately, that means the poorer quality water is still underground and will not reach the lake for several decades. We’ve actually seen only the beginning of the problem,” says Paul Dell, Environment Bay of Plenty’s group manager regulation and resource management.

The results have major implications for future lake quality, he says. “It means we can expect quite dramatic increases in nutrients, particularly nitrogen, in the lakes over the coming years – unless ways are found to manage the increasing nutrient loads.”

The study by the Institute of Geological and Nuclear Sciences confirms what regional council scientists suspected – a time delay between land use and its effect on lake water quality.

Agriculture produces excess nutrients (nitrogen and phosphorus), some of which will wash over the top of the land and fairly quickly into nearby waterways. However a lot seeps down through the soil and mixes with water stored in large underground aquifers. Eventually, this nutrient-loaded groundwater will emerge through springs, which flow into the streams feeding the lake, or flow directly to the lake.

For the study, the institute measured the age and proportion of young and old water discharging from these springs and streams, mostly of which are located on the western side of Lake Rotorua.

Initial results show the mean age of the water ranging from 30 years to 170 years. The mean age of water in Hamurana Spring, for example, is 145 years. Only 18% of the flow is young enough to show the effects of agriculture. At the moment, Hamurana Spring contributes about 64 tonnes of nitrogen annually to Lake Rotorua. Scientists expect this to double in the next 50 years as nutrient-loaded water starts to flow through.

At Taniwha Springs, the mean age of water is 64 years, just under half of it being “young” impacted groundwater.

This study is the first part of wider investigations to understand the interactions, time delays and water chemistry of the groundwater system in the Lake Rotorua catchment. Scientists are also investigating groundwater in Lake Okareka.

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