RDE613 Audit of hydrodynamic nutrient models for estuaries under OFWAT PR24

Under the water industry national environment programme (WINEP) 2024 price review (PR24), water companies are required to carry out investigations in estuarine and coastal protected areas (Sites of Special Scientific Interest (SSSIs), Special Protection Areas (SPAs), Special Areas of Conservation (SACs), Marine Conservation Zones (MCZs) and Ramsar sites), where there is evidence that conditions are unfavourable for nutrients and water company assets may be significant contributors to nutrient loading. As part of these investigations, 3D hydrodynamic models will be built to model dispersion of contaminants (in particular nutrients) from Wastewater Treatment Works (WwTWs) and other sources and will predict concentrations of contaminants in space and time from point discharges. These investigations have been put forward by Natural England (NE) under Habitats Directive and SSSI drivers.

NE have requested 11 models in estuaries and costal protected areas which are considered unfavourable for nutrients. This is alongside six sites put forward by the Environment Agency (EA) to model chemicals. This contract will be focused on nutrient investigations led by NE, although in some instances models will jointly include chemicals and nutrient investigation so some elements may be combined.

Once produced, the models will aid in improving NE/EA's understanding of the concentration of contaminants in receiving waters and protected areas, to enhance our capability to assess the potential risks and impacts on protected features. These models will be essential to inform decision making when establishing appropriate permit limits for the WwTW needed to restore each estuarine and coastal protected areas to favourable nutrient conditions.

Reporting on progress will be required through a seven step phased process:

Phase 1: Review of behaviour of pollutants of concern in UK estuaries, including sediment

attachment/detachment. (Complete Sept 2023)

Phase 2: Review of existing models.

Phase 3: Data review and gap analysis.

Phase 4: Data collection.

Phase 5: Model development and calibration

Phase 6: Model validation

Phase 7: Scenario testing, with a view to determining permit limits

Models will be produced in line with the model scope produced jointly by the EA and NE, which sets out the specification of requirements for models to be delivered (see Appendix A).