Contaminants travelling through the coastal zone do so via three media:

• rivers
• atmosphere
• groundwaters and sediments

Traditionally, river inputs to coastal waters were seen as the principal route of contaminant transport, and for a number of substances passage through estuary and river systems are indeed obligatory. Flows are one-way.

Within ELOISE, the topic of riverine contaminant supply to coastal waters focused largely on heavy metal inputs from historical mining regions - Rio Tinto and Rio Odiel discharging to the Spanish Mediterranean, the Danube exiting to the Black Sea, the Humber and Rhine flowing to the North Sea, the Vistula in the Baltic basin and the Idrijca entering the Adriatic. These issues are local in scale.

The long term legacy of these elevated metal discharges may be restricted. Contaminants are removed from the river en route by biological or physical processes, this removal continuing after the river reaches the sea. There are also options for active management to limit initial transfer to river water via attention to surface runoff characteristics within catchments.

For specific other contaminants supply to the sea from the atmosphere dominates. This pathway is also common for general contaminants arriving at marine locations which are distant from land. Atmospheric transport is not uni-directional, and flows can occur:

• from the land via the atmosphere to the sea
• from the sea to the atmosphere, potentially subsequently to the land

  Lindane A white crystalline powder used as an agricultural insecticide that causes adverse health effects in domestic water supplies and also is toxic to freshwater and marine aquatic life. See Glossary for a complete list of all terms.

As much as 85% of lindane reaching the Baltic, and 50% of metals reaching the Mediterranean, may arrive via the atmosphere. Chemical forms less prone to washout in precipitation may be transported far from their point of release, whilst those in or converted to more easily scavenged forms display specific patterns of input to the sea.

For atmospheric contaminants deposited to coastal waters, mercury and persistant-organic-pollutants (POPs) have received attention within ELOISE and IMPACTS. Both of these are semi-volatile with potentially reversible air/sea transfer, and both are bioaccumulative, adding complexity to biogeochemical cycling. These features create a regional/global issue. Description of deposition, exchange, partitioning, sedimentation, uptake etc. has been improved within ELOISE and IMPACTS.

An opposing atmospheric transport, from the sea to the land, occurs with sulphur. The coincidence of determining factors can render coastal seas a sufficiently important atmospheric source that in certain areas these emissions can have dominate local terrestrial acidification. ELOISE observations have found higher coastal water concentrations of sulphurous compounds than previously recognised.

Relevant to global scale environmental change, river plumes transport the greenhouse gases methane, nitrous oxide (N₂O), carbon dioxide etc. Estuaries/coastal zones also represent a coincidence of factors arising from their land use, drainage, population, industry, etc. which favour the biological production of greenhouse gases.

The interplay of transport routes can be complex, e.g. with the riverine supply of contaminants to coastal waters which results in gas release to the atmosphere and subsequent transport back to land.

Contaminants moving from land to sea generally concerns substances of anthropogenic origin. Releases from the sea to the atmosphere conversely often involves naturally occurring contaminants, although their production may be subject to anthropogenic influence.

Movement in groundwaters of terrestrial origin with subsequent seabed release has received much less attention, but has illustrated the potentially long time scale of contaminant release – decades.