Marine sediments represent sinks for heavy metal contamination. Creatures such as marine bivalves (oysters and mussels) feed on sediments and so can become intoxicated with heavy metals. Plants may also redistribute metals from sediments which is why they are monitored in ecological risk studies.
Speaking of Sediment...
At first sight, sediment might appear to be a barren and inconsequential medium that is unlikely to have any effect on marine biota or man, since it sits, apparently inertly, at the bottom of the sea, river or lake; but this couldn’t be further from the truth. Sediment provides sources of nutrients (both organic and inorganic) to a range of creatures and flora and consequently, assessing the ecological risk of metals in sediments is an important task in evaluating marine pollution.
Sediment is the name given to the particulate substances that collect on the ocean floor or in a lake or river bed. The nature of these sediments is, by definition, heterogeneous since they are made up from a wide range of materials, including debris from the exoskeletons of marine invertebrates. Sediment may be deposited into a water body by a number of different mechanisms. These include aeolian (wind) deposition of dust (e.g. soil particles) picked up by a swirling wind and then dropped over water when the wind dies; run-off of material from fields into the river system or directly into the ocean and erosion of material that makes up the watercourse itself.
Metal Meets Sediment
Once the sediment particles enter the water body, they will gradually fall through the water column and come to rest at the interface between the watercourse and its bed – or the seafloor in an ocean. The rate at which this process happens (the sedimentation rate) will depend upon the size (most importantly, the surface area) and weight of the descending particle, water currents, the buoyancy of the water body and, to a small extent, the temperature of the water. As particles fall through the water column, they can scavenge chemicals (including metal ions) onto their surface and these will become incorporated in the sediment.
Metals can enter the sediment by becoming absorbed onto sediment particles or by contamination of the sediment before it enters the aquatic system. Equally, the material deposited as a sediment may contain the metals directly (for example, as an ore). While it is possible for a sediment particle to become charged with heavy metals as it falls through the water column, this would be unusual since the concentration of heavy metals in seawater or fresh water are usually very low (except in the aftermath of a specific pollution event).
Essential And Toxic Metals
Some metal elements are essential to living organisms and play a vital biological role (Mn, Cu, Zn, Fe, Se, Na, etc.), but other metals have no known biological function and can be harmful or even fatal to an organism if ingested in sufficient quantities (e.g. Pb, Hg, As and Cd). Contaminated sediments pose a hazard to biota because they can be taken up by marine plants and subsequently ingested by higher organisms or because certain organisms feed by filtering sediment through their gut and so may bioaccumulate toxic metals within their tissues (e.g. mussels and oysters). The concentration of heavy metals in top predators can become orders of magnitude higher than in the water through the process of bioaccumulation (this was partially responsible for the human tragedy in Minimata). For this reason, assessing the ecological risk of metals in sediments is an important part of evaluating the health of the coastal (or terrestrial) environment. Marine sampling campaigns fall in to two types: hot spot monitoring or trend monitoring.
The surface layer of sediment can be disturbed by storms (in shallow waters) or by creatures which feed on the sediment (a process known as bio-turbation), potentially releasing pollution into the marine environment. In order to assess the heavy metal loading of a sediment, samples must be collected for laboratory analysis (usually, this is done by a specialist marine laboratory). Two forms of sampling are usually conducted for the determination of metals in sediment: grab samplers or corers. Corers tend to be used if scientists wish to reconstruct the pollution history of a given site since the deeper segments of a core sample represent historical events as newer sediment deposits bury older ones (often, naturally occurring radiotracers can be used to date the cores).
Sampling And Analysis
Once back at the laboratory, the sediment will be sieved to reject material above a certain particle size (typically, the cut-off is 200 µm) and then it will be destroyed by digestion with strong acids to solubilize the metals contained within. The sediment digests are then diluted appropriately and analyzed using modern analytical equipment such as ICP-MS, ICP-OES or AAS which will identify and quantify the metals present. It is common practice for a marine laboratory to collect sentinel organisms (e.g. mussels, oysters and fish) together with the sediment samples in order to gain a better understanding of the ecological situation with respect to heavy metals once the tissues are analyzed.
Certain metals can be linked to human activities; for instance, high silver concentrations are associated with inputs from sewage. Mercury and arsenic can arise from industrial pollution, or from mining activities. Excessive concentrations of transition metals can be associated with metallurgical contamination (often point source and potentially an artifact of sampling) whereas lead and cadmium point to industrial pollution. Unusually high barium signatures may be linked to oil exploration as the metal is used in artificial muds used as lubricants for drilling activities.
Through these sediment assessments and sampling, scientists will continue to identify the ecological risks caused by these particular metals with the hopes to control them or their ability to harm.