The coastal environment in many parts of the world has been extensively developed. Ports and their environs offer convenient sites for many industries to process, refine or manufacture goods from imported raw materials. Oil is routinely transferred to coastal refineries for conversion to a range of plastics and petrochemical products. This industrialisation is accompanied by a corresponding threat to a sensitive environment from deliberate and fugitive discharges of a wide range of substances; influent rivers, themselves carrying their load of natural and anthropogenic contaminants, add to this pressure. Engineering projects designed for coastal defence, flood control, energy generation and other purposes can also significantly alter natural sediment erosion and deposition patterns.

Estuaries, the mixing zones between fresh and salt water, can be defined several ways. The definition can be as geographical as boundaries, sometimes international, on maps, whilst geomorphologists will likely define an estuary differently by the outlet channel shape. In this article, however, our understanding is served by defining the estuary by water composition criteria, bounded by river water composition at the landward end and seawater at the other. As such, estuarine boundaries vary in time geographically, as fresh water can push the estuary seaward under spate conditions whilst strong tides can push the boundaries substantially landwards. Coastal waters have near full-strength seawater composition, whilst being subject to land derived influences.
Characteristics of estuarine and coastal waters
 

A different approach to pollution risk assessment is required for marine water as compared to that of surface freshwater because of physical (density) characteristics in combination with a rapidly changing chemical composition as freshwater meets saline water. Although specifics will vary, estuarine environments in general act as effective physical and bio-reactor zones which strongly influence the net contamination flux from rivers into the receiving coastal environment.

In typical temperate estuaries inflowing river water, containing perhaps 100-150 mgdm-3 total dissolved solids (TDS) dominated by calcium, magnesium, (hydrogen-)carbonate and silicate ions, meets high salinity (33 000 – 35000 mgdm-3 TDS) water dominated by sodium and chloride ions. Salinity gradients can result in water stratification (described later), but will the increase in ionic strength in the water also destabilise electrostatically separated organic colloids and dissolved organic material carried in river water which coagulate to form fresh settleable particles. Turbidity in the estuaries is increased by the landward movement of sediments carried by deep residual currents.

Estuaries are typically muddy environments with localised areas of extremely high suspended solid concentrations (several hundred mgdm-3 is not uncommon) referred to as a turbidity maximum zone (TMZ). The TMZ in an estuary is not fixed spatially, as it shifts up- and down-stream depending upon factors including freshwater flow, tidal condition (spring/neap bi-weekly tides and high/low-water daily tides) and channel shape, but will normally be found near the low-salinity part (2000-5000 mgdm-3 TDS)of the mixing zone.

The rapidly changing estuarine environment can affect the transport of dissolved and particulate contaminant fluxes. River borne contaminants encounter different dominant counter-ions and high TDS conditions. This enhances the precipitation of colloidal species (e.g. iron, manganese) and induces co-precipitation of dissolved species with high partition coefficients, including many metals, phosphates and trace organic substances. Passage of dissolved constituents through the TMZ allows extensive sorption reactivity transferring reactive dissolved pollutants to the particulate phase.

Another feature of natural estuarine water chemistry is the co-existence of depressed dissolved oxygen concentrations with the TMZ. Here, increased oxygen demand in the water column results from decomposition of the organic component of the suspended load. During spring-tides in warm summertime conditions, this natural oxygen depletion can be severe (<4 mgdm-3) enough to prevent fish migration. Oxygen sags can also influence redox-sensitive water chemistry. Notably iron and manganese rich particulate matter can re-release metals and other co-precipitated contaminants into the water column as chemical reduction solubilises Fe2+ and Mn2+. Downstream of the TMZ, water oxygenation usually rapidly improves, inducing oxidative reactions of pollutants.

Estuarine reactivity is complex, but there is a tendency for dissolved pollutants with a high partition coefficient to become particulate on passage through to the coastal zone. This particulate matter is largely retained within the estuary. Tidal asymmetry, whereby flood-tides with greater water velocity and short duration carry particulate matter landward more effectively than the ensuing, gentler, ebb tide permits seaward transport, meaning particles have difficulty escaping to sea. Estuaries tend to silt-up extensively and require dredging to keep shipping channels open.

Physico-chemical reactions are important to pollutant pathways in coastal zones. However, microbial activity in partially submerged mudflats and during algal bloom events in the water column can also significantly modify contaminant fluxes. For example, studies aimed at quantifying fluxes of land-derived nutrients (N, P) to the North Sea from eastern and southern England (Joint Nutrient Study, JONUS) highlighted the importance of inter-tidal mudflats acting as bioreactor zones for extensive denitrification, resulting in much smaller fluxes of nitrogen to coastal waters than would be anticipated from simply multiplying river water concentrations by freshwater flows.

The net effect of extensive estuarine and coastal zone reactivity on pollutant transport can be difficult to evaluate without detailed modelling. Regulators have adopted an approach to consent setting to brackish and marine waters not only recognising the inherent sensitivity of the environment, but also the likelihood for extensive dispersion of discharges and the potential for complex pollutant reactivity. Predictive assessment is further complicated as estuarine and coastal waters are prone to stratification caused by the presence of water of contrasting salinity (saline water is denser than fresh) and/or temperature (colder water being denser than warm). Complex patterns of water movement derived from vertical stratification (layering of water masses with depth) and horizontal stratification (cross-embayment) can result in a more limited effluent dispersion and can result in distinctive poorly diluted plumes extending significant distances. Stratification patterns for a given coastal area are not necessarily consistent and can vary not only seasonally, as might be anticipated with changing water temperatures, but also with “water energy”, the combined effect of tidal strength, freshwater discharge volume and channel geometry.

In this dynamic environment, regulators, operators and consultants wishing to ensure discharges are abated and emitted to a proportionate degree are best advised to use carefully calibrated models to predict dispersion under a variety of seasonal and tidal conditions, including depth variations. Off-the-shelf software packages usually require additional data to model specific channel geometries. Additional equations may be required to model contaminant reactivity to account for physico-chemical and biologically mediated reactivity.

Limiting coastal contamination

The dispersive potential of large coastal water bodies to dilute contaminating inputs is very high albeit complex. Adverse anthropogenic influences on estuarine and coastal water quality have been shown across the globe. Nutrients, long suspected of contributing to coastal as well as freshwater eutrophication are now viewed as substances requiring abatement in many coastal areas. Inputs of toxic and harmful substances, including a range of metals, pesticides, hydrocarbons, endocrine disrupting substances (EDC) and persistent organic pollutants (POP) are also regulated.

Coastal eutrophication whereby increased nitrogen and phosphorus loads lead to excess micro and macro-algal growth, can increase the frequency of foam events and can lead to suppressed oxygenation as organic matter sinking through the water column or settling on the bed decomposes (for example in the Kattegat between the Baltic and North Seas). The diffuse flush-out of nutrients from agricultural land, from surface flow and via groundwater, is probably an important source of both N and P to the coastal environment. Local direct discharges of wastewater also contribute.

It is unclear as to whether the main limiting nutrient is N or P; in freshwaters P is normally limiting and is thus the key nutrient to control, whilst in open oceans N is usually limiting as P can be more rapidly recycled within the water column. In intermediate coastal waters inputs of land-derived N can be significant although research in several coastal regions (including the North Sea and coastal waters off the NE USA) still suggest N to be limiting, with inputs via atmospheric deposition being influential.

Coastal eutrophication also potentially influences the balance of algal species (e.g. diatom, dinoflagellate, etc.) dominance by improving the competitive edge for species more adept at utilising one or other of the key nutrients (N, P and Si). In open oceans the C:Si:N:P ratio tends towards 106:15:16:1 (molar basis, the Redfield ratio), and species balance and “average biomass composition” reflects this. In the coastal zone, large-scale N and P inputs from agricultural sources are not usually matched by Si inputs, shifting nutrient proportions away from the Redfield ratio.

Heavy metal inputs can have distinctive localised impacts. For example, sacrificial zinc anodes on boat hulls can produce measurable increases in water contamination in the vicinity of harbours. Industrial acidic iron-rich effluents form brightly-coloured effluent plumes as alkaline seawater neutralises the effluent and iron oxidation/precipitation results in localised coating of the bed and intertidal zones unless abated. A wide range POP have both lethal and sub-lethal impacts on biota including shellfish. Tri-butyl tin (TBT) is now much reduced in use after androgenic impacts on molluscs were proven, despite the limited effectiveness of alternatives biocides.

Accidents from tanker activity are, thankfully, relatively rare. However coastal zones where there are oil transport, transfer and refining activities are prone to chronic hydrocarbon contamination from low-level fugitive releases. In such areas, natural bioremediation can become effective as a well-adapted microbial sediment population becomes endemic.

Emissions controls are well developed in the USA, Europe and elsewhere. Discharge consents cover the release of harmful substances under various legislative measures from point sources. In the EU, consents for emissions to coastal waters ensure adequate controls for microbial contamination (Bathing Waters Directive), nutrients, suspended solids and COD (Urban Wastewater Treatment Directive) and for toxic metals and organics (e.g. Shellfish Waters Directive). Approaches to diffuse pollution control include the adoption of River Basin Management Plans (RBMP, under the Water Framework Directive, WFD), whilst the worst excesses of oil spill damage are mitigated by extensive accident risk assessment and spill-containment planning under Control of Major Hazards (COMAH) Regulations.
 

Holistic approaches to coastal protection
 

The controlled emission of harmful substances from point and diffuse sources is one key approach taken to protect the estuarine and coastal environment. Greater understanding of the effects and reactivity in a dynamic water environment gives regulators confidence that a proportionate yet effective solution is now being implemented in Europe, the USA and elsewhere. In recent years it has been recognised that additional protection of the marine environment is required to control a number of other anthropogenic activities which have degraded habitats to the detriment of marine life. In part, this is because such impacts are often trans-boundary, requiring the adoption of international agreements. Several such agreements have been reached.

Examples include the Environmental co-operation in the Danube Act (EU, 2001) aimed at reducing harmful discharges (including nutrients) and oil pollution incidents in the Black Sea and, also in Europe, several regional conventions to improve environmental protection of the Baltic (Helsinki Convention), Mediterranean (Barcelona Convention) and NE Atlantic (OSPAR). Although the emphasis of regional agreements varies, emission control, reduced pollution from shipping and oil transportation, and restrictions on dumping of waste and dredging are included. Agreements are normally reached to co-operate with scientific study and monitoring. Elsewhere, the intention to impose bans on commercial fishing and mining in a 500 000 km2 area of sea and sea-floor surrounding some of its Pacific islands was announced (January 2009) by the USA.

The need for a holistic approach to marine protection is recognised. In the EU, the recently adopted Marine Strategy Framework Directive (MSFD, July 2008) sets objectives that member states achieve good environmental status for their marine waters by 2020. Objectives include preventing deterioration and, where possible, restoring marine ecosystems. Many countries are already introducing legislation to meet many of the requirements of the MSFD; transposition into member state national law is required by 2010. In the UK the Marine and Coastal Access Bill being prepared has wide ranging objectives covering emission controls, the establishment of Marine Conservation Zones (MCZ) to link with existing conserved areas to form Marine Protected Areas (MPA), improved fish and shellfish management, licensing and enforcement, the introduction of detailed locally informed marine planning and improved access to the coast for recreational and leisure purposes.

It is proposed to introduce a new “one-stop” agency, the Marine Management Organisation (MMO), to oversee marine protection issues. Implementation of such legislation will need to be consistent with existing legislation. For example, RBMP (developed within WFD requirements) covers point and diffuse source emission controls and includes estuaries and coastal waters. Here, the Environment Agency (responsible for implementing the WFD) has consulted with other organisations such as Natural England (with a conservation and biodiversity remit) to ensure RBMP will protect and improve marine habitats.

Marine conservation is an important additional strand to be adopted in this holistic approach in the UK. Existing initiatives, including the assignment of a variety of protected areas (e.g. Marine Nature Reserves, Special Areas of Conservation, Special Protected Areas), have been somewhat uncoordinated. The proposed Marine Bill will add the flexibility to ensure effective protection of endemic and threatened or rare marine species, habitats and areas important for species life-cycle stages together with the preservation of areas of special geological or geomorphological interest. Marine planning and extensive sea-bed mapping should ensure a balance between effective conservation and resource extraction and utilisation.

Balancing the needs of socio-economic development and coastal defence with environmental protection provides difficult challenges to governments. In the EU, an approach to manage this, Integrated Coastal Zone Management (ICZM), is recommended and has been adopted in the UK. ICZM takes a long-term view to sustainable uses of the coastal zone (including both estuarine and coastal waters and adjoining land areas of a locally defined extent). With guiding principles that include adaptive management working, where possible, with natural forces, additional Shoreline Management Plans are harmonised with RBMP and locally agreed Regional Plans informing an over-arching Marine Plan that covers sometimes sensitive issues such as managed retreat and decisions on resource use or harvesting in the coastal zone.
 

Conclusions
 

The coastal environment is a heavy utilised zone with fragile habitats that can be strongly influenced by man. The estuarine mixing zone and coastal waters are dynamic making predictive pollutant modelling complex because of the combination of three-dimensional stratification and the potential for extensive physico-chemical and biologically mediated reactivity. Despite a huge dilution potential, anthropogenic impacts on water quality are regularly observed. Nonetheless, a better understanding of pollutant pathways has arisen over the past couple of decades, and regulators are now using informed modelling to ensure point source discharges are tightly controlled and diffuse-source pollutants are managed. More recently, legislative frameworks are being developed to include emissions control in combination with other important planning, resource management, conservation and habitat protection objectives for a holistic approach to coastal water management. The importance of interacting sustainably with the sensitive marine environment is being widely recognised.