TIPOS DE EVALUACIONES

The WFD is a secondary driver for the Thames Tunnel project. This requires surface waters to achieve and maintain ‘good status’. Under Article 13(1) of the WFD, EU Member States are required to ensure that a river basin management plan is produced for each river basin lying entirely in their territory.

In the UK, the Environment Agency has responsibility for the production of river basin management plans. The River Basin Management Plan (RBMP) for the Thames River Basin District was

published in December 2009 after a period of consultation from December 2008 to June 2009. Section 4 of the RBMP, ‘The State of the Water Environment Now’, identifies, in table 3, the main reasons for not achieving good ecological status or potential in rivers. Section 6, ‘The state of the water environment in 2015’ notes that one of the objectives of the Water Framework Directive is to aim to achieve good status in water bodies by 2015. However, for 75% of surface water bodies this target cannot be met by this date. The Plan sets out where good status cannot be achieved by 2015. In these cases an alternative objective of good status or potential by 2021 or 2027 is set. Section 7 explains the three river basin management cycles, 2009-2015, 2015-2121 and 2021- 2027. Achieving good status in all water bodies by 2027 is a significant challenge.

The Thames River Basin Catchments are discussed individually in Section 8 of the RBMP. The Tidal Thames is included within the Estuaries and Coastal Waters Catchment and on page 79 it is stated that:

“The water quality concerns for the Thames estuary centre around the impacts of storm discharges from the five major sewage works which serve London and from the combined sewer network. These discharge to the estuary frequently, resulting in drops in dissolved oxygen, and increases in aesthetic pollution, risk to health and fish kills.

Improvements to the sewage treatment works along the tidal River Thames and the construction of the London Tideway Tunnels are planned to be delivered by Thames Water over the next two river basin cycles. These major projects represent the primary measures to address point source pollution from the sewer system and are fundamental to the achievement of good status in this catchment”

Key actions for the catchment are itemised on page 80. These include:

“Improvements to the London sewerage network to reduce the impact of storm sewage on water quality in the Thames Tideway - Thames and Lee Tunnels.”

Annex B of the RBMP is a large compendium of ‘objectives for our waters’. Section B23 ‘Estuaries’ sets out various ‘proposed actions for estuaries’ and details the ‘ecological potential’ and ‘chemical status’ of estuaries within the Thames water basin. Schedules setting out ‘ecological potential’ and ‘chemical status’ of Thames Lower, Thames Middle and Thames Upper estuary make it clear that the status of those stretches of the river are currently moderate and that they will not achieve ‘good’ status by 2015. They do, however, indicate “Good potential by 2027”.

4.7 Water quality impacts

4.7.1 Introduction

Discharges from CSOs contain a mixture of foul sewage, groundwater infiltration and storm water runoff, and contain pollutants such as suspended and colloidal solids and volatile organic matter that contribute significantly to the organic and chemical oxygen demand in the river, which, in turn, result in oxygen depletion and nutrient enrichment of the river through London. These conditions are most prevalent during summer months when temperatures are higher and oxygen in the river is more subject to depletion.

Water quality is influenced by physical, chemical and biological conditions and is commonly compared to a set of standards against which compliance is assessed. In England and Wales, the Environment Agency is the competent authority for quality monitoring. The following parameters are among the most useful and important in assessing quality:

 Biochemical oxygen demand (BOD) - a measure of the amount of biodegradable organic

matter in water. It is expressed as the number of milligrams of oxygen required by

micro-organisms to oxidise biodegradable organic matter in one litre of water. As the BOD of a water body increases, so does the demand for oxygen, which causes a subsequent depletion in the dissolved oxygen (DO) concentration to the detriment of water quality.

 Total ammonia – a compound of nitrogen and hydrogen which, at high concentrations, is

toxic to freshwater organisms. It is also oxidised by certain bacteria in water causing a depletion of dissolved oxygen. The measurement of ammonia and other inorganic nitrogen compounds is usually referenced to the nitrogen and hence ammonia is often referred to as ammoniacal nitrogen.

 Un-ionised ammonia - a form of ammonia that arises under conditions of high temperature

and/or pH (alkaline conditions). This form of ammonia is more toxic to freshwater organisms than other forms of ammonia.

 Dissolved oxygen (DO) – the amount of oxygen dissolved in water expressed as milligrams

of oxygen dissolved in one litre of water. It is influenced by temperature, atmospheric pressure, re-aeration, photosynthesis and the decay of pollutants (as indicated inter alia by ammonia and BOD levels). DO concentration decreases with rising temperature and low DO concentrations can result in fish kills.

 Nitrogen and phosphorus – these are commonly referred to as nutrients. High

concentrations can give rise to eutrophication, the condition where nutrient enrichment supports dense plant populations the decomposition of which kills aquatic life by reducing DO concentrations.

In England and Wales, there are water quality standards that should be met in order to maintain water quality. Whilst those for freshwaters are well-established, and were in many instances statutory, those for estuaries were historically less well-developed. For this reason the TTSS developed a suite of water quality standards applicable to the Thames Estuary. Water quality is influenced by a number of sources, which are outlined in further detail below:

 Continuous discharges – typically continuous discharges come from treated effluent from

sewage treatment works, and the quality of the final effluent depends on the nature of the sewage treatment works. Specifically the BOD, ammonia and phosphate in final effluent are considered to assess the impact on water quality in the receiving watercourse.

 Intermittent discharges – this is the discharge of untreated sewage into a watercourse during

times of high flow in the sewer system, typically from CSOs or storm tanks at STWs. As the discharges are untreated (although perhaps screened), they may discharge high levels of pollutants into a watercourse which can cause detrimental impacts on water quality, in particular a depletion of DO.

 Diffuse pollution – pollution from diffuse sources can include, for example, agricultural runoff,

highway runoff, or runoff from contaminated land. Frequently diffuse runoff enters a watercourse without being treated and can add significant pollutant to a watercourse.