Pollutant swapping in constructed agricultural wetlands

Freer, Adam and Quinton, John and Surridge, Ben and McNamara, Niall P. (2016) Pollutant swapping in constructed agricultural wetlands. PhD thesis, Lancaster University.

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Diffuse agricultural pollution presents a major challenge to global water quality management, requiring the adoption of new land management practices such as constructed agricultural wetlands. These wetlands, promoted in agri-environment schemes, may effectively intercept rainfall-mobilised phosphorus (P), nitrogen (N) and carbon (C). However, wetlands may potentially facilitate ‘pollutant swapping’: the transfer of one form or pathway of pollution for another, as a result of mitigation efforts. Retained pollutants may be remobilised through solubilisation or as the greenhouse gases (GHGs): methane (CH4), carbon dioxide (CO2) and nitrous oxide (N2O). Therefore this research examines the potential for agricultural wetlands to ‘swap’ local improvements in water quality, for (1) increased pollution to groundwaters and (2) to the atmosphere. GHG exchanges from an agricultural wetland (area 0.032 ha) in Cumbria, UK were monitored over an 18 month period, using floating gas chambers, ebullition traps and diffusive gas exchange models. While the wetland was a net sink of particulate C and N, mean net releases of CO2 (2249 – 5724 mg m-2 d -1 ), N2O (0.93 – 2.04 mg m-2 d -1 ) and CH4 (169 - 456 mg m -2 d -1 ) were significantly greater than those from adjacent riparian land. Wetland releases of CH4 were most significant in terms of potential atmospheric impact compared to other wetland GHG releases. Shallow groundwater samples extracted from a piezometer network surrounding the study site, illustrated that retained sediments acted as a source of NH4-N and DOC to surface and local groundwaters but mitigated leaching and outward transport of NO3-N to surface and groundwaters. Field and laboratory microcosm experiments demonstrated that pollutant swapping of GHGs and nutrients may be increased during periods of reduced water oxygen content associated with eutrophic conditions. In wetland designs with water depths >0.5 m, anoxic conditions may perpetuate in lower water column zones and facilitate increased CH4 and NH4-N production and storage. Additionally, microcosm studies identified that disturbance of bottom sediments by stormflow may elicit heightened GHG and nutrient releases. Therefore the net impact of wetland construction in catchments may need reconsiderations, with respect to the potentially detrimental effects on water and the atmosphere. However upscaling of observations suggests that wetland implementation in the UK is unlikely to significantly increase GHG budgets. Use of shallower wetlands with vegetation or inlet baffles may reduce CH4 emissions by encouraging oxidation and protecting sediments from storm flows.

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Thesis (PhD)
?? greenhouse gasgroundwaterdiffuse pollutionmethane ??
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12 Sep 2016 13:26
Last Modified:
16 Jul 2024 05:36