This work by our partner Marlborough District Council is not funded by Te Hoiere Project, but has been accelerated and prioritised by Council due to the volume of work going on in the catchment. With the Project so far supporting over 50km of fencing and over 100,000 native plants to improve freshwater, Council is seeking to better understand the flow of nutrients and any potential leaching of nutrients into the ground away from waterways.
A new study by Council’s environmental scientists is looking at groundwater quality and how water moves through parts of Te Hoiere/Pelorus catchment, where hydrology is little understood. Hydrology is the study of how water flows on and below the surface, and how this impacts water quality and quantity alongside land uses.
This one-year study to November 2024 will see several samples taken each season from over a dozen groundwater wells, as well as surface water locations that flow into the Rai River. From Council’s environmental sciences team, Pete Davidson will study the groundwater and Charlotte Tomlinson will lead the science on surface water.
“Rivers and streams are certainly where most of the water quality monitoring takes place. But are the nutrients found in groundwater and wells more interconnected to our rivers and streams than we’ve previously thought? This study will provide some insight,” Pete says.
Pete will drill four new wells over summer, with landowners granting access to 11 existing wells, to measure the water table and water quality. Meanwhile, Charlotte is looking at the levels and flow of surface water that flow into the Rai River. Where the two scientists will overlap is looking at the level of the surface water and how it flows to groundwater (in bores or wells) to better understand how the whole system is working.
Understanding the interaction between groundwater and surface water is not well understood in Te Hoiere/Pelorus, and the flow of nutrient runoff throughout the catchment is even less understood, Pete says. Council only samples one well in Te Hoiere every three months for water quality (not levels) for its State of the Environment reporting.
“Currently, we know that nitrogen is elevated in the groundwater of just one well because that’s the one we regularly monitor for our reporting. But we don’t know if that’s indicative of the whole catchment,” Pete says.
In terms of surface water, Council only has one permanent gauge at Rai Falls to measure water levels. The historic data on levels and flows of the catchment’s waterways have been ad hoc throughout the years. In November, Council began to take measurements at 15 surface water sites around Rai. This information will be coupled with water level measurements in new and existing wells to shed light on how water is moving through the catchment.
“We’ll begin to understand how water moves through gravels in the catchment, both above and below ground,” Charlotte says. “For example, landowners have seen the river go dry in the Opouri River in some reaches, but then water resurfaces downstream. We are trying to understand why,” Charlotte says.
Help from the local community has been key to the study, with some landowners granting access to their wells and others sharing their rainfall data and local knowledge with Council’s environmental science team.
This study is expected to improve how water quality is measured in the catchment. Currently, water quality measurements only take flow into account where this data is available—and there isn’t consistent data in Te Hoiere/Pelorus compared to other areas in Marlborough.
Photos: At the bore closest to the coast, BECA geologist Paul Wopereis inspects drill cuttings and looks for marine sediments.
Council’s Michael Rogers gauges the Rai River using a Surfbee Flow Finder, typically used on moderately sized rivers to measure how fast water is moving through the whole water column. This technology, Acoustic Doppler Current Profiler, is relatively new and uses a gauging boat that is mostly autonomous to take flow measurements.
Charlotte Tomlinson measures river flow of the Tunakino River with a FlowTracker, which uses Acoustic Doppler Velocimetry to determine river flow in smaller, wadable streams.