The release of phosphorus to porewater and surface water from river riparian sediments

Sediments can be both a source and a sink of dissolved phosphorus (P) in surface water and shallow groundwater. Using laboratory mesocosms, we studied the influence of flooding with deionized water and simulated river water on P release to solution using sediment columns taken from a riparian wetland. The mesocosm incubation results showed that rather than retaining nutrients, sediments in the riparian zone may be a significant source of P. Concentrations of dissolved P in porewater reached more than 3 mg L(-1) and in surface water over 0.8 mg L(-1) within a month of sediment inundation. The reductive dissolution of P-bearing iron (Fe) oxides was the likely mechanism responsible for P release. Dissolved P to Fe molar ratios in anaerobic samples were approximately 0.45 when columns were flooded with water that simulated the chemistry of the adjacent river. This suggests there was insufficient Fe in the anaerobic samples to precipitate all P if the solutions were oxygenated or transported to an aerobic environment. If the anaerobic wetland solutions were delivered to oxygenated rivers and streams adjacent to the riparian zone, the equilibrium concentration of P in these systems could rise. The timing of P release was inversely related to the nitrate (NO3-) concentration in floodwater. This indicates that in riparian zones receiving low nitrate loads, or where NO3- loads are being progressively reduced, the risk of dissolved P release may increase. These findings present particular challenges for restoration and management in riparian areas.     

Enhancing soluble phosphorus removal within buffer strips using industrial by-products

Using industrial by-products (IBPs) in conjunction with buffer strips provides a potentially new strategy for enhancing soluble phosphorus (P) removal from agricultural runoff. Here, we investigate the feasibility of this approach by assessing the P sorption properties of IBPs at different solution-IBPs contact time (1–120 min) and solution pH (3, 5.5, 7.5), as well as possible adverse environmental effects including P desorption or heavy metal mobilisation from IBPs. Batch experiments were carried out on two widely available IBPs in the UK that demonstrated high P sorption capacity but different physicochemical characteristics, specifically ochre and Aluminium (Al) based water treatment residuals (Al-WTR). A series of kinetic sorption–desorption experiments alongside kinetic modelling were used to understand the rate and the mechanisms of P removal across a range of reaction times. The results of the kinetic experiments indicated that P was initially sorbed rapidly to both ochre and Al-WTR, followed by a second phase characterised by a slower sorption rate. The excellent fits of kinetic sorption data to a pseudo-second order model for both materials suggested surface chemisorption as the rate-controlling mechanism. Neither ochre nor Al-WTR released substantial quantities of either P or heavy metals into solution, suggesting that they could be applied to buffer strip soils at recommended rates (≤30 g kg^?1 soil) without adverse environmental impact. Although the rate of P sorption by freshly-generated Al-WTR applied to buffer strips reduced following air-drying, this would not limit its practical application to buffer strips in the field if adequate contact time with runoff was provided.     

Bringing diverse knowledge sources together--a meta-model for supporting integrated catchment management

Integrated catchment management (ICM), as promoted by recent legislation such as the European Water Framework Directive, presents difficult challenges to planners and decision-makers. To support decision-making in the face of high complexity and uncertainty, tools are required that can integrate the evidence base required to evaluate alternative management scenarios and promote communication and social learning. In this paper we present a pragmatic approach for developing an integrated decision-support tool, where the available sources of information are very diverse and a tight model coupling is not possible. In the first instance, a loosely coupled model is developed which includes numerical sub-models and knowledge-based sub-models. However, such a model is not easy for decision-makers and stakeholders to operate without modelling skills. Therefore, we derive from it a meta-model based on a Bayesian Network approach which is a decision-support tool tailored to the needs of the decision-makers and is fast and easy to operate. The meta-model can be derived at different levels of detail and complexity according to the requirements of the decision-makers. In our case, the meta-model was designed for high-level decision-makers to explore conflicts and synergies between management actions at the catchment scale. As prediction uncertainties are propagated and explicitly represented in the model outcomes, important knowledge gaps can be identified and an evidence base for robust decision-making is provided. The framework seeks to promote the development of modelling tools that can support ICM both by providing an integrated scientific evidence base and by facilitating communication and learning processes.