Environmental chemistry and contamination

The seabed is the recipient of contaminants, and the sediments are a significant pollutant store. The sediments can therefore function for a long time as a source for pollution dispersal in fjords and offshore basins, and for pollutant uptake by organisms. Such pollutants may be heavy metals, organic pollutants like PAH or PCB, or radioactive substances. However, many of these compounds are also found naturally in the sea, occasionally in high concentrations. It is therefore essential to know the background levels before contamination can be proved, and monitoring the natural levels of pollutants in non-contaminated areas is an important aspect of environmental chemistry investigations.

Heavy metals

The term heavy metal covers all metallic chemical elements that have a relatively high specific gravity. Heavy metals are natural components of the Earth’s crust and occur in varying concentrations in most natural materials. Many are toxic, even in low concentrations. Heavy metals have a tendency to bio-accumulate, that is, their concentration in an organism can rise in relation to their concentration in the surroundings. Abnormally high contents of heavy metals in the marine environment can thus have a damaging effect on marine biota and human consumers of seafood.

Human-produced sources of heavy metals

There are many human-produced sources of heavy metals, including present-day and former mines, smelters, sewage plants and more diffuse sources like motor vehicles. Relatively volatile heavy metals and those which easily adhere to airborne particles can disperse over very large areas. Heavy metals can be discharged into river systems through weathering aggravated by acid precipitation. They can be transported with the help of running water, either in solution or as sediments, and ultimately reach the sea where they are deposited. The map showing the distribution of heavy metals enables us to determine the source of pollutants and their dispersal directions.

Lead, mercury and barium in the Skagerrak

The distribution map for these three selected metals demonstrates different sources and transport mechanisms.
The highest lead concentrations are found in the deepest parts of the Skagerrak where fine-grained sediments with a relatively high content of organic material accumulate. Most of these fine-grained deposits derive from southern and central parts of the North Sea.

Mercury-enriched sediments are found in a narrow belt near the coast and show that mercury derives from various sources on land with a relatively limited distribution.

Barium concentrations prove to be highest along the southern and south-western margin of the Norwegian Trench. Significant quantities of barite (BaSO4) are discharged in connection with oil drilling, where barite is a component of drilling mud. Ocean currents transport this barite towards the Skagerrak where it is enriched in silty sediments on the southern flank of the Norwegian Trench.

Pollution levels of heavy metals in the Skagerrak

The absolute concentrations of heavy metals in the Skagerrak are in general low although there is some enrichment in the uppermost layers of sediment. According to the environmental status classes which the Norwegian Climate and Pollution Agency has compiled for heavy metals, sediments in the Skagerrak have mercury and lead contents in levels classified as "insignificant/minor pollution" (class I) and locally in some areas "moderate pollution" (class II).

Pollution levels of heavy metals in Lofoten and the Barents Sea

Concentrations of heavy metals are in general low in Lofoten and the Barents Sea. Some elevated values of lead and mercury have been recorded in the uppermost centimetres of sediment cores from both areas. This influx of heavy metals has been taking place in the past 50 – 100 years.

The Management Plan for the Barents Sea puts priority on three heavy metals in sediment, lead (Pb), cadmium (Cd) and mercury (Hg), and also presents maps for the heavy metals, copper (Cu), chromium (Cr), nickel (Ni) and zinc (Zn), and the elements, arsenic (As) and barium (Ba).

Mercury and lead in the seabed have probably been long-transported by either ocean or air currents. The concentrations of heavy metals are for the most part insignificant (corresponding to class 1). Moderate contamination (class 2) of lead in surface samples has been recorded at a single station on Tromsøflaket.