Analysing how water flows to reach rivers and streams can help to reveal what proportion of agricultural contaminants come from past or more recent land use practices, a new paper shows.

And in good news for authorities grappling with water quality issues, the analysis method uses already existing techniques and data without having to invest in complex and costly new research.

As part of the five-year Critical Pathways programme funded by the Ministry of Business Innovation and Employment (MBIE) Lincoln Agritech’s Hamilton-based scientists studied water and contaminant flows through catchments.

In the Piako River headwater and Waitapu Stream catchments they carried out in-depth investigations, using several innovative techniques, such as airborne geophysical surveys and high-frequency nitrate measurements.

They also looked at 47 other catchments with widely varying environmental and land-use characteristics in Taranaki, Waikato, and Hawkes Bay. Here, they used a modelling method called BACH (Bayesian chemistry-assisted hydrographic separation and load partitioning).

“We developed this method a few years ago specifically to extract some useful information from data-sparse catchments, where more sophisticated models cannot be applied,” said Roland Stenger, Lincoln Agritech’s Principal Scientist, Environmental.

“The only input parameters needed are a long-term flow record and time series for two water constituents that can be used as tracers: in many catchments, that can be the routinely monitored total nitrogen and total phosphorus.”

A tracer is a substance that can be easily tracked as it travels with the water through catchments.

Using one of two tracer combinations (total nitrogen plus electrical conductivity, or total nitrogen plus total phosphorus) the team could calculate how much of a water body’s water came from deep groundwater, shallow groundwater, or near-surface water.

That’s important because near-surface water reaches rivers, streams, and lakes within minutes to days, shallow groundwater takes from a couple of months to two or three years, while deep groundwater can take decades to reach waterways.

By understanding where the water came from, water regulators can understand whether measured agricultural contaminants, such as nitrates, are the result of recent practices, or whether they reflect what happened in the past.

In most catchments studied, shallow groundwater was the most important contributor to local streams and rivers, meaning current practices have the greatest impact on the water in these catchments. Deeper groundwater, associated with long lag times, was the dominant pathway only on the North Island’s volcanic plateau.

Last month Dr Stenger presented the findings to Waikato Regional Council’s Environmental Performance Committee. Its freshwater policy team is now exploring how to best use this new knowledge.

The same methodology could be used to show which pathways are dominant in other catchments around the country.

The paper “Routine stream monitoring data enables the unravelling of hydrological pathways and transfers of agricultural contaminants through catchments” has been published in Science of the Total Environment.

This story has been edited to clarify the results apply to the particular catchments studied.

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