How effective are reedbeds, ponds, restored and constructed wetlands at retaining nitrogen, phosphorus and suspended sediment from agricultural pollution in England?

A high priority topic within the Department for Environment, Food and Rural Affairs (DEFRA) water quality programme is the mitigation of diffuse rural pollution from agriculture. Wetlands are often cited as being effective at reducing nutrient and sediment loadings to receiving waters. However, the research in this area is inconsistent, and whilst most studies have shown that both natural and constructed wetlands retain nutrients and sediments, others have shown that they have little effect, or even increase nutrient and sediment loads to receiving water bodies. DEFRA has commissioned a systematic review on the use of wetlands to mitigate N, P and SS inputs from agriculture to receiving freshwater in England. The review will encompass a comprehensive literature search on all available material on the subject, both published and unpublished within the British Isles. Specific inclusion criteria will be adhered to and a formal assessment of the quality and reliability of the studies will be undertaken. The data will then be extracted and a data synthesis undertaken. The review will inform an evidence-based policy that can be implemented by stakeholders

Phosphorus release from sediments in a treatment wetland: Contrast between DET and EPC0 methodologies

Wetlands are capable of reducing nutrient loadings to receiving water bodies, and hence many artificial wetlands have been constructed for wastewater nutrient removal. In this study, diffusive equilibrium in thin films (DETs) and equilibrium phosphorus concentration (EPC0) analysis were used to examine the role of sediment as a nutrient source or sink in a constructed treatment wetland in summer. The effect of dredging on sediment-water nutrient exchange was also studied. Soluble reactive phosphorus (SRP), ammonium (NH4+) and sulphate (SO42−) concentration profiles were measured by DET across the sediment-water interface (SWI) in both a settling pond and iris reed bed within the wetland. The SRP concentrations in the sediment pore-waters of the settling pond were extremely high (up to 29,500 μg l−1) near the SWI. This is over an order of magnitude higher than the levels found in the water column, which in turn are over an order of magnitude higher than environmental levels proposed to limit eutrophication in rivers. The profiles demonstrated an average net release of SRP and NH4+ from the settling pond sediment to the overlying water of 58 mg m−2 d−1 (±32 mg m−2 d−1 (1 sd)) and 16 mg m−2 d−1 (±25 mg m−2 d−1 (1 sd)), respectively. The DET SO42− concentration profiles revealed that the sediment was anoxic within 2 cm of the SWI. Dredging of the reed bed made no significant difference to the P release characteristics across the SWI. The EPC0s were much lower than the SRP concentration of the overlying water, indicating that the sediment had the potential to act as a phosphate sink. The apparent contradiction of the DET and EPC0 results is attributed to the fact that DET measurements are made in situ, where as EPC0 measurements are ex situ. These results show that substantial releases of P can occur from wetland sediments, and also highlight the need for caution when interpreting ex situ EPC0 analytical results

Understanding the hydrochemical evolution of a coastal dune system in SW England using a multiple tracer technique

An improved knowledge of the hydrology of coastal dune systems is desirable for successful management of their diverse ecology under a changing climate. As a near-pristine coastal dune spit system, Braunton Burrows (SW England) is an ideal location for the study of the natural processes governing recharge to the dune groundwater system and the evolution of its water quality. Whereas previous investigations have tended to focus on inter-dune slacks, this study has also given attention to infiltration through the high dunes. Cores were taken through dunes and the resulting sand samples processed to provide information on grain size distribution and porewater chemistry. Groundwater samples were obtained from beneath dunes and slacks. A variety of geochemical techniques were applied including hydrochemistry, stable isotopes and residence time indicators. The unsaturated zone profiles indicate the existence of piston flow recharge with an infiltration rate of 0.75–1 m/yr, although faster rates probably also occur locally. Groundwater beneath the high dunes gave ages in the range 13–16 yr, compared to the dune slack groundwater ages of 5–7 yr, and an age of 22 yr for groundwater from the underlying mudstone aquifer. The chemistry of waters in both unsaturated and saturated zones is dominated by Ca and HCO3, supplemented by variable amounts of other ions derived from marine aerosols and limited reaction with sand grains and their coatings. The main chemical evolution of the porewaters occurs rapidly through the mobilisation of surface salt crusts and dissolution of shell carbonate. This situation changes little in the underlying groundwater, though an evolution towards reducing conditions increases the concentrations of redox-sensitive species such as Fe and Mn. The rapid chemical evolution of the infiltrating water means that its composition will respond quickly to changes in the supply of shell material and/or marine salts, which are possible consequences of climate change. However, the residence time measurements suggest the dune aquifer has a relatively long turnover time which will to some extent buffer such changes. The results of the present study should be transferable to natural dune systems in similar coastal situations

Internal loading of phosphorus in a sedimentation pond of a treatment wetland: Effect of a phytoplankton crash

Sedimentation ponds are widely believed to act as a primary removal process for phosphorus (P) in nutrient treatment wetlands. High frequency in-situ P, ammonium (NH4+) and dissolved oxygen measurements, alongside occasional water quality measurements, assessed changes in nutrient concentrations and productivity in the sedimentation pond of a treatment wetland between March and June. Diffusive equilibrium in thin films (DET) probes were used to measure in-situ nutrient and chemistry pore-water profiles. Diffusive fluxes across the sediment–water interface were calculated from the pore-water profiles, and dissolved oxygen was used to calculate rates of primary productivity and respiration. The sedimentation pond was a net sink for total P (TP), soluble reactive P (SRP) and NH4+ in March, but became subject to a net internal loading of TP, SRP and NH4+ in May, with SRP concentrations increasing by up to 41 μM (1300 μl− 1). Reductions in chlorophyll a and dissolved oxygen concentrations also occurred at this time. The sediment changed from a small net sink of SRP in March (average diffusive flux: − 8.2 μmol m− 2 day− 1) to a net source of SRP in June (average diffusive flux: + 1324 μmol m− 2 day− 1). A diurnal pattern in water column P concentrations, with maxima in the early hours of the morning, and minima in the afternoon, occurred during May. The diurnal pattern and release of SRP from the sediment were attributed to microbial degradation of diatom biomass, causing reduction of the dissolved oxygen concentration and leading to redox-dependent release of P from the sediment. In June, 2.7 mol-P day− 1 were removed by photosynthesis and 23 mol-P day− 1 were supplied by respiration in the lake volume. SRP was also released through microbial respiration within the water column, including the decomposition of algal matter. It is imperative that consideration to internal recycling is given when maintaining sedimentation ponds, and before the installation of new ponds designed to treat nutrient waste

A simple method for assessing the vulnerability of wetland ecosystem services

Wetlands can provide many ecosystem services, but throughout the world are exposed to a range of pressures. In some locations these are well documented and are being successfully addressed. However at other locations, lack of data, resources and methods are hindering thorough assessment. This is particularly the case in remote wetlands such as those in the Himalayan region. This paper presents a methodology developed and tested on three wetland sites in Nepal. The method gives a structure to data collection and analysis and leads the user through to producing a vulnerability assessment of wetlands. We found that in each case the method provided a prioritised table of values and threats and this helped to make the development of a site management plan straightforward. The action plan highlights data gaps and identifies priority areas for attention. The results from our assessment of Rupa Lake are also presented in this paper