ABSTRACT
The catchment of the River Thames, the principal river system in southern England, provides the main water supply for London but is highly vulnerable to changes in climate, land use and population. The river is eutrophic with significant algal blooms with phosphorus assumed to be the primary chemical indicator of ecosystem health. In the Thames Basin, phosphorus is available from point sources such as wastewater treatment plants and from diffuse sources such as agriculture. In order to predict vulnerability to future change, the integrated catchments model for phosphorus (INCA-P) has been applied to the river basin and used to assess the cost-effectiveness of a range of mitigation and adaptation strategies. It is shown that scenarios of future climate and land-use change will exacerbate the water quality problems, but a range of mitigation measures can improve the situation. A cost-effectiveness study has been undertaken to compare the economic benefits of each mitigation measure and to assess the phosphorus reductions achieved. The most effective strategy is to reduce fertilizer use by 20% together with the treatment of effluent to a high standard. Such measures will reduce the instream phosphorus concentrations to close to the EU Water Framework Directive target for the Thames.
ABSTRACT
The Water Framework Directive (WFD) aims to deliver good ecological status (GES) for Europe's waters. It prescribes the use of economic principles, such as derogation from GES on grounds of disproportionate costs of mitigation. This paper proposes an application of the proportionality principle to mitigation of phosphorus (P) pollution of 544 Scottish lochs at national and local water body scales. P loading estimates were derived from a national diffuse pollution screening tool. For 293 of these lochs (31% of the loch area), GES already occurred. Mitigation cost-effectiveness was assessed using combined mitigation cost curves for managed grassland, rough grazing, arable land, sewage and septic tank sources. These provided sufficient mitigation (92% of national P load) for GES to be achieved on another 31% of loch area at annualised cost of £2.09 m/y. Mitigation of the residual P loading preventing other lochs achieving GES was considered by using a "mop-up" cost of £200/kg P (assumed cost effectiveness of removal of P directly from lochs), leading to a total cost of £189 m/y. Lochs were ranked by mitigation costs per loch area to give a national scale marginal mitigation cost curve. A published choice experiment valuation of WFD targets for Scottish lochs was used to estimate marginal benefits at national scale and combined with the marginal cost curve. This gave proportionate costs of £5.7 m/y leading to GES in 72% of loch area. Using national mean marginal benefits with a scheme to estimate changes in individual loch value with P loading gave proportionate costs of £25.6 m/y leading to GES in 77% of loch area (491 lochs).
