ICES C M 1996 Theme session R Results of baseline studies of contaminants in Baltic sediments Quality assurance of sediment sampling and the distribution of trace elements in the Baltic Sea sediments
The Sediment Baseline Study was planned and organized by a special group within the ICES Working Group on Baltic Marine Environment as an outcome of the attempt to review the multitude of pollution studies of the Baltic Sea Sediments. In this contribution, we report the general conduct of the study, review the experiences gained with respect to quality assurance, give a tentative list of procedures necessary for selecting sedimentation sites suitable for monitoring work, together with a preliminary characterization of the trace element distribution in the surface sediments.The most important elements which must be considered and/or included and also well documented in a comprehensive quality assurance plan for sediment sampling to ensure that the data generated has the appropriate quality include the description of the project including the objectives and the data quality needed, site selection criteria, sampling period frequency, sampling procedures for each major measurement including subsampling, storage procedures and sample treatment, and a plan for laboratory quality control checks. Each step of the procedures should be duly documented in order to provide a possibility for tracing the results both in time and in quality. The ICES/HELCOM Baltic Sea Sediment Baseline Study sewed as an example where the quality aspects of sampling and analysis were tested and elaborated. Introduction Marine sediments provide a possibility to assess in a consistent manner the changes in the environment. Not only the present day distribution of contaminants can be exposed, but also at the same time the history of the sampling site, providing certain hydrochemical and biolo@cal conditions are met. The use of biota as a pollution indicator in the Baltic Sea suffers from the drawback that only very few, if any, species are represented in all the subareas in sufficiently large quantities. In order to assess the use of sediments as environmental indicator medium, the pollutant studies in the Baltic Sea sediments were reviewed recently (Perttila and Briigmann, 1992) One of the conclusions was that while a large amount of work has been invested in the sediment studies, both sampling and analysis methods. as well as the positions of the sampling sites. have been selected using varying criteria. For these reasons the reported concentrations are seldom comparable. A suggestion was made to carry out a baseline study covering all the important net sedimentation areas of the Baltic Sea (Fig. 1)The Sediment Baseline Study was planned and organized within the ICES Working Group on Baltic Marine Environment. In this contribution. we report the general conduct of the study, give a list of the selected sedimentation sites, together with a preliminary characterization of the sites and approximate sedimentation rates. and trace element distribution in the surface sediments to the extent they are available at present. During the process of the Sediment Baseline Study. several new contributions have been published in other connections on the distribution of pollutant concentrations in the Baltic Sea sediments. not listed in the aforementioned Review. These include the Gulf of Bothnia (Leivuori and Niemisto, 1994) the Gdansk Bay together with the south eastern Baltic Proper (Szczepanska and Uscinowicz, 1994) and preliminary reports on the Riga Bay project of the Nordic Council of Ministers. Quality aspects of the sediment sampling have been recently reviewed by Perttila and Pedersen (1996) Their recommendations are largely based on the experiences of the Sediment Baseline Study. Aspects of quality assurance in sediment work Quality assurance is the total system of activities required to guarantee the appropriate quality of the product and is therefore an integrated part of all sampling programs. The sample to be analyzed has to be been taken adequately, otherwise all efforts in the laboratory for good quality control are futile, since the obtained analytical result, however accurate, does not represent the concentration at the sampling site. A relevant quality assurance procedure is highly dependent on the future use of the data. In consequence. the objectives of the project should be well defined and clearly stated. In principle the user of the data also should define the necessary data quality parameters like precision, comparability. and accuracy. Monitoring of the geographical distribution and time trends of element/compound concentrations in the sediments is often the aim of the study. The sediments provide an excellent archive in which to study the past and present of environmental pollution. However, especially in this case the sampling site selection is of crucial importance. The site should be representative of the sea area and it should have peaceful sedimentation conditions. Because of internal currents and other varying hydrographic effects, the net sedimentation rate may change significantly from site to site. For monitoring purposes it is essential to repeat the sampling at pre defined intervals at exactly the same sedimentation sites. The first selection of a position for pollution monitoring sampling should thus be carried out with care. The following aspects should be considered: Hydrography (currents, vertical structure of water mass) Structure of the sedimentation basin Benthic communities (bioturbation) Rate of sedimentation Sediment structure (overall topography and the fine structure of the sediment surface In highly polluted areas (eg. in regions where dumping of military wastes is known to have occurred) special precautions must be taken for clean up procedures in cases pf accidental lifting of hazardous materials on deck. Chemicals are carried to the sediments mainly via sedimentation, that is, via the sedimenting particles. which "scavenge" the water column of pollutants, as well as feed pellets and inorganic detrital particles. The main part of the sedimenting particles are composed of aggregates of dead plankton, in or on which the pollutants have accumulated or adsorbed. Evidently, in the absence of strong currents, the strongest pollution signals are found in the vicinity of the pollution source. and the signal weakens with an increasing distance from the source. However, very seldom the primary sedimentation site is the final sink for materials. More often, macroscale current system lifts (resuspends) the material from the sediment surface and transports it gradually to the final sink. E.g. in the Baltic Sea. where the bottom topography is very irregular. there are nevertheless only a very few final sedimentation basins in each sea area. Also the general hydrography should be considered. The structuring of the water column may prevent the vertical cycling of the water mass and the penetration of atmospheric oxygen to the bottom and anoxic and oxic conditions may vary. A continuous monitoring in such areas will usually not be meaningful as. precipitation/dissolution properties vary with the redox potential for certain compounds (eg. iron and phosphate) the bentluc communities may change as a consequence of the oxygen availability which in turn will affect the bioturbation of the sediment surface and disturb the analytical signal the diagenetic processes within the sediment vary with the redox potential e.g. through the varying solubility These aspects should be studied and considered before a decision is taken on the site selection for e.g. long term environmental monitoring. The bottom topography is very seldom monotonous in coastal and estuarine areas where sediments may have their most significant pollution signals. Rather, the overall basin is often divided into smaller sedimentation "holes" separated by sharp edges. The whole area should therefore be scanned by echo sounding before the sampling position is selected. The navigation system of many ships usually allows the coupling of navigation data with the echogram data, so that the bottom topography can be described visually in a three dimensional picture (Fig. 3) The most promising areas can be selected from such an echogram data. However, the bottom sediment bed cannot be deduced only from the topography. The scanning of the area should also be carried out using a low frequency <lOkHz) sounding, which, while giving a signal of the sediment surface, also penetrates the surface and gives signal of other. deeper lying reflector surfaces. Thus the depth of the soft sediment layer can be measured and used for the selection of the most suitable position. To a certain extent, also some other qualities of the sediment bed can be deduced from a low frequency echogram; e.g. the occurrence of gas, which probably has mixed up the sediment layer or will mix the sample when taken it on board. In shallow areas <200m) the possibility of mechanical disturbance of the bottom should also be checked before sampling, e.g. from the bottom trawling for fishery purposes. The most powerful method for the detection of the fine structure of the sediment surface is the side scan sonar (SSS) An experienced SSS user can easily detect e.g. the signs of bottom trawling in the echograms. Again. in order to avoid mixed up areas it is essential to be able to track down the positions of the SSS pictures by means of coupling to the ship's navigation data. All three above mentioned methods should be applied when selecting a suitable position for a representative sediment sampling. The two first sources of information can nowadays be obtained commercially in a single computerized instrument, but, as described above, they can relatively easily be obtained combining the echo sounding data with the ship's navigation data. Using e.g. any surface contouring program. a three dimensional picture of the bottom can be constructed. In spite of all mapping techniques, the final test for selection of a position is provided by the visual inspection of a test core. The sediment core description shou