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Hot-spots of Phosphorus and Nitrogen delivery in Time and Space in agricultural catchments

Periodic Reporting for period 1 - HotPaNTS (Hot-spots of Phosphorus and Nitrogen delivery in Time and Space in agricultural catchments)

Reporting period: 2016-01-01 to 2017-12-31

Human activities, mainly diffuse pollution from agriculture, have led to a gradual deterioration of water quality in streams, rivers and seas and their eutrophication. Combating eutrophication is proving difficult and expensive and emphasizes gaps in our current understanding of sources and fate of agricultural pollution. One potential reason for this is poor targeting of diffuse agricultural pollution both in space and time by existing monitoring networks. To address this scientific challenge, we tested a hypothesis that organic matter measurements obtained with low-cost and portable optical sensors could provide vital information on hot-spots of diffuse pollution in space and time. To test this hypothesis, the project evaluated spatial and temporal relationships between phosphorus and nitrogen, two main nutrients responsible for eutrophication, and organic matter measurements in agricultural streams. These relationships were tested over two years for 10 agricultural catchments and 9 observation fields in Sweden, covering a gradient in climatic, hydrological, biogeochemical and land use conditions. Preliminary results show that: (1) relationships between nutrients and organic matter are site-specific and dependent on dominant geology, soil properties and dominant flow pathways in the catchment and (2) turbidity and organic matter fluorescence measurements obtained with optical sensors can characterise sources and dynamics of diffuse pollution. These findings show that we can use optical sensor technology to improve targeting of eutrophication pressures both in space and time. In the long-term, this can help us to mitigate eutrophication by better control of sources and their delivery pathways.
The work focused on better understanding of spatial and temporal relationships between phosphorus and nitrogen concentrations and organic matter measurements in agricultural catchments. This evaluation was carried out based on collection of both low- and high-temporal resolution water quality data:
1. Low-temporal resolution data have been collected every two weeks for 10 agricultural streams and 9 drained observation agricultural fields, for 2 years. This sampling allowed to build a large database of phosphorus, nitrogen and organic matter responses to varying weather, flow and land use conditions.
2. High-frequency resolution data including turbidity, organic matter fluorescence, water temperature and flow discharge have been every 15 minutes for year and a half in one selected agricultural stream, with high eutrophication pressures.

Preliminary results against the original project objectives

Task 1 Building of knowledge base
• Building a database of water quality, nutrient concentrations and optical measurements for the SMPA samples collected throughout the project – The database was created in Matlab and includes 50 weeks of data on optical measurements (absorbance and fluorescence spectroscopy), nutrients, total and dissolved organic carbon and turbidity (Figure 1).
• Testing the relationship between in situ optical measurements and nutrient concentrations to account for variation in catchments’ characteristics and flow conditions – The relationships between optical measurements and nutrient concentrations are catchment-specific and their strength vary on a seasonal basis.
• Evaluation of the quenching effects from turbidity, temperature and inner-filter effect, their spatial and temporal variability and calibration and validation of the compensation equations – Quenching effects are much higher for clay catchments but in all catchments undergo significant seasonal variation driven by flow dynamics. Those effects are correctable through compensation equation for the turbidity and temperature range observed in the catchments.

Task 2 Hot-spots of nutrient delivery
• Deployment of the optical sensors for real-time measurements at sampling points in the 2 pilot catchments and collection of water samples to target storm events and agricultural practices – Initial analysis indicated that sand catchments show low quenching effects, thus the optical sensor was deployed in a clay catchment to understand how turbidity and optical measurements vary on a seasonal basis (Figure 2).
• Testing the relationship between in situ optical measurements and nutrient concentrations to account for a variation in flow, water quality characteristics, differences in catchments’ characteristics and agricultural land use – Turbidity measurements can be used as proxy for sediment and particulate phosphorus concentrations. Although CDOM and tryptophan-like fluorescence do not provide universal correlations with nutrient concentrations (since they are site- and season-specific), they provide proxy for understanding solute dynamics.

Task 3 Evaluation of the method’s potential and limitations
• Evaluation of the efficacy of the optical measurements as a proxy for nutrient concentrations based on the database built in Tasks 1 and 2 – as above.
• Determination of the method’s potential, limitations and practical requirements to be used in agricultural catchments to provide quantitative evidence of hot-spots of nutrient delivery – Optical sensors are the future of water quality monitoring. They are portable and accurate and therefore enable targeting hot-spots of nutrient delivery both in space (for different catchments and streams) and time (throughout the hydrological year). However, evaluation of the quenching effects is needed for each sampling site which will result in development of the compensation equations for turbidity and temperature. Another limitation highlighted by this project is biofouling as seen in Figure 2 for uncorrected CDOM measurements in summer 2017. Biofouling results in a build-up of microorganisms on the windows of optical sensors resulting in a gradual decrease of signal between the sensor maintenance visits. This effect is particularly observed during the summer months in clay catchment streams which show high degree of eutrophication, as can be seen in the graphical abstract. Regular maintenance, calibration and cleaning of the optical windows is mandatory.
Understanding distribution of eutrophication pressures in space and time is one of the key questions in hydrochemistry and an important management challenge. Current methodologies to target eutrophication pressures rely on both water quality data from stream networks and spatially-distributed models linking water quality with the catchment properties. Optical sensor technology and organic matter fluorescence can provide additional source of information on eutrophication pressures. When installed in streams over long time, optical sensors can provide information on hot-moments of pollution dynamics in relation to weather and flow conditions. When installed in streams at several locations, optical sensors can provide information on both temporal and spatial distribution of eutrophication pressures. Potential applications can include improved targeting of hot-spots of eutrophication in time and space, for better monitoring (increasing our understanding of pollution dynamics) and mitigation (reducing losses of phosphorus and nitrogen from agriculture and thus reducing the costs for farmers, environment and society).