Understanding contaminant loads in farm runoff
NIWA and AgResearch are working together to develop and test practical approaches for farmers to meet environmental targets.
NIWA has designed and will install wetlands on the Telford research farm, a division of Lincoln University. The wetlands are being tested as a practical means to improve the run off from dairy pastures. Initially we need to test the nutrient load and removal occurring in the farm gully then we will construct (plant) a wetland and compare its removal performance.
To evaluate the performance of wetlands we need to measure the quantity and quality of water passing through them. Monitoring stations will measure the load of particulate and dissolved nutrients generated in agricultural runoff and compare this with that leaving, initially, the gully and then the wetland.
Choosing the sites
We are focussing on critical source areas in the landscape, the 20 per cent of the land that produces 80 per cent of the contaminants. Gullies tend to capture runoff from a large area funnelling it into a smaller capture zone, therefore a convenient place to trial our wetlands.
Sampling the water and determining its contaminant load
The wetlands receive a mix of surface run-off with potentially high sediment loads and shallow drainage with dissolved nutrients. We use flow rates and turbidity sensors to trigger sampling during storm events as well as measuring base-flows.
To do this we take a water sample when the water pooling at the bottom of the gully flows through a specially constructed weir. A data logger measures the water level and applies a calibration factor to derive flow rate.
When the water-level rises to a certain height above the bottom of the v-notch, it triggers a water sample and one of the 24 sample bottles fills with water. Later we send the water samples to the NIWA testing laboratory in Hamilton to identify the chemical species and microbes present and to measure their concentrations.
Then we calculate the contaminant load (grams per hour) from the concentration and flow rate. This way we hope to measure the loads of sediments, nutrients and fecal microbes mobilized from pastures, and the interacting effects of grazing episodes and rainfall events.
The instrumentation comprises: a Compact Stilling Well (CSW) developed by NIWA, a water sampling module, a turbidity sensor and a time-lapse camera. The instruments are powered with batteries charged by solar panels.
The CSW is self-contained and houses a water-level measuring instrument and a NEON data logger with a wireless cellular network connection. The data logger records water-level, derives flow rates, initiates the water sampler and sends data to the NEON server.
The water sampling module contains a motorised carousel of sample bottles. The logger initiates a sample when either the measured water-level or the amount of sediment in the water reaches the thresholds programmed into it.
The time-lapse camera provides a visual indication of water-level against a reference marker plate on the weir. This works as a check on the logged values and in events such as weir blockage or overtopping.
NEON server graphics show what's happening at the sites in real time
The NEON metering module at each station records and processes the data then sends it to the NEON server for storage.
Anyone with an Internet browser, and the authority to access the site information from the server, can monitor the sites in real time. The NEON graphics display water sampling progress, water level and derived flow rate in real time.
Historical data is only a click away. Clicking on the parameter you are interested in brings up a table or graph of data over any period of time you choose.
Where to next?
This research will provide information on the abilities of wetlands to capture pollutants in farm run-off before they reach natural waterways, and how best to manage them.
The information will be written into practical guidelines to assist farmers to use wetlands, where appropriate. This will complement good cropping and grazing practices and nutrient management on farms. Providing farmers with a wider range of tools to manage diffuse pollution and help meet environmental targets.
NIWA has previously tested artificial wetlands planted with raupo and other indigenous plant species to 'clean' tile drainage by converting nitrates, dissolved in the water, into nitrogen gas.
Organizations involved in the project
AgResearch, NIWA, Telford Research Farm (Lincoln University) and DairyNZ .
The instrumentation was designed and supplied by the NIWA Instrument Systems group in Christchurch and installed by the NIWA Dunedin Field team.