A review of processes important in the floodplain setting

This report reviews the physical and geochemical processes reported in the literature and likely to be operating in the floodplain setting. The review supports a study of the Port Meadow, located within the floodplain of the River Thames to the northwest of the city of Oxford, an area affected by urban pollution. It focuses on floodplains but includes both material for the hyporheic zone and also generally for riparian zones. It describes the processes, generically covers case studies where these processes have been studied and then assesses the impact on nutrient and other pollutant attenuation. Floodplains are periodically-flooded areas along rivers and lakes and we can envisage the floodplain setting as a heterogeneous and constantly changing environment where nutrients and other contaminants are attenuated or removed from groundwater discharging to surface water. These changes may be controlled predominantly by water levels. The surface water/groundwater interface is a crucial control point for lateral nutrient fluxes between uplands and aquatic ecosystems and for longitudinal processes along rivers and wetlands. The concept of redox zones constitutes an important framework for understanding the behaviour of nutrients and contaminants in groundwater. The addition of low molecular weight carbon from the river to shallow groundwater during seasonal inundation of flood plains provides the potential to further stimulate the redox environment. To understand water movement and hence nutrient movement in floodplains we need to understand residence times, the character of multiple flow paths, and if and when hydraulic gradient reversal occurs. Nutrient migration is therefore controlled by water levels, the distribution of subsurface permeability, plant uptake and redox zone patterns. In contrast, point sources, such as landfills, unlined animal waste stores and leaking sewers, appear to lead to a relatively stable system with a carbon and nutrient-rich source term which generates an established redox sequence in a limited volume of groundwater down gradient. The shape and size of such plumes may be affected by drought or heavy rainfall. On-site sanitation systems demonstrate the same sequence of redox zones but the highly reducing conditions are found within the tank and such schemes may rely on oxidation of reduced species in the effluent during transport through the unsaturated zone. Nitrate, the predominant oxidised form of nitrogen, is readily transported in water and is stable under a range of concentrations. However, anaerobic carbon-rich sediments, such as those in floodplains, have the potential to support large populations of denitrifying bacteria. Denitrification is rapid compared to groundwater flow particularly under low flow conditions and nitrate is lost from the groundwater system under these conditions. In contrast, phosphorus removal by soil or sediment retention and biota uptake results in accumulation within the system. Sediments can be both a source and a sink of dissolved phosphorus in surface water and shallow groundwater. Opposite conditions are favourable for nitrogen and phosphorus management with anaerobic conditions favouring denitrification but enhanced desorption and release of phosphorus. The important processes affecting other contaminants, such as those present in landfill leachate, dissolved organic matter, xenobiotic organic compounds, inorganic macrocomponents, and heavy metals are dilution, sorption, ion exchange, precipitation, redox reactions and degradation. For many contaminants these processes provide significant natural remediation, limiting the effects of the leachate on groundwater. In peri-urban environments, such as the Oxford Port Meadow, there will be many different sources of nutrients and contaminants and the distribution of zones of low redox may be both spatially and temporally variable. Other indicators will need to be applied, such as cocontaminants, to help to understand fate and transport of nutrients and contaminants.