Abstract :
A rapid procedure is described to evaluate the significance of inorganic and organic pollutants in fine-grained sediments. The air- (<30°C) or freeze-dried samples are screened within 30 min. in stainless steel sieves (<0.04 mm) prior to chemical analysis. This procedure is also suitable for analysis of such elements as Hg and many non-volatile organic pollutants. The sediment size fraction of <0.04 mm was chosen in order to separate the smallest grain-size obtainable by rapid dry-screening techniques; this is representative of the grain-size of suspended matter transported at low and medium river discharge (0.1–0.7 m/s). The screened fraction (<0.04 mm) of aquatic sediments is more homogeneous in grain-size and distribution, and the fine particles are more capable of adsorbing pollutants than coarser fractions. However, large differences are observed in grain-size distribution and adsorbable phases from one sample to another, even in the smallest size fractions (<0.04, <0.02, <0.002 mm). Clay minerals, Fe- and Mn-(oxy)hydrates, amorphous silica and humic substances all adsorb water at normal humidity. In order to estimate and standardise the adsorption capacity of the surface of these screened sediment fractions, the weight loss after heating at 105°C is used, instead of tedious quantification of all the above mentioned sediment phases. In addition, this procedure allows the rejection of sediment samples, unsuitable for monitoring purposes, which have very low adsorption capacities. In order to correct for the variable adsorption capacity of different sediment samples or size fractions, the weight loss (WL) at 105°C can be inserted in the widely used GeoIndex formula (Igeo105°=log2 (Cmeasured/Cbackground*WL105°C). This improves the evaluation of pollution in aquatic environments.
