In order to determine the natural background values of heavy metals in Dutch sediments, we took approximately 1000 samples from 14 borings to a depth of 5 to 113 meters in the south of the Netherlands, and analyzed them for major-and trace elements. Grain-size distribution and organic carbon content was determined for a part of the samples. The sediments studied comprise Upper Tertiary and Lower Pleistocene marine, estuarine and fluviatile deposits. Provenance varies between Rhine, Marine and the former Campine river system. We found that the natural contents of heavy metals can be described as a function of sediment provenance, grain-size and diagenetic processes.
Shifts in sediment provenance between Rhine and a former Campine river system result in alternations between sediments with high and low contents of muscovite. Apart from that, glauconite-rich Tertiary deposits occur. Variations in muscovite, clay and glauconite have major effects on the heavy metal contents of the sediment:
In glauconite-poor sediments, the concentrations of the heavy metals Co, Cr, Cu, Ni, Pb and Zn are determined by the contents of clay minerals and micas. This results in a positive linear correlation between the concentrations of these heavy metals and the Al-content and a logarithmic correlation with the medain of the grain-size. Ba shows a better correlation with K than with Al, as both Ba and K have higher contents in muscovite than in clays as a result of which their ratio to Al is grain-size related. This means that the ratio Ba/Al is dependant of the (provenance-related) mica content of the sediment.
In glauconite-rich sediments, the concentrations of As, Co, Cr, Cu, Ni, Pb and Zn are linear correlated with K, as they all occur in glauconite in constant ratios. Here however, Ba and Al are not correlated with K because of the low concentrations of Ba and Al in glauconite. As no significant amounts of clay minerals are present in these coarse sandy sediments, all Ba and Al are present in micas, resulting in a linear relation between the two. Because of the coarse-grained nature of the glauconite grains, the concentrations of As, Co, Cr, Cu, Ni, Pb and Zn are higher in coarser sediments.
Because in glauconite-poor deposits diagenetically formed pyrite is the major source for As, the concentrations of As are positively correlated with S contents. High As values (max. 98 ppm) can be found especially in organic rich layers, as syn- or postsedimentary formed pyrite is concentrated there. Apart from As, also Ni shows accumulations in organic rich layers (up to 400 ppm), but there is no clear relation with the S contents. Ni is probably present in pyrite in variable quantities, or it forms a separate sulfide phase. Diagenetic accumulations of Co, Cr and Zn occur only locally, associated with high Ni or As concentrations in organic rich layers.
Heavy metal concentrations that are determined by sediment provenance and grainsize effects as described above can be regarded as natural background values. Especially because of the spatial character of provenance effects, these background values show regional variations. Diagenetic accumulations can result in higher values for some elements. For legislative purposes the relation between concentrations of Al, K and S on the one hand and As, Ba, Co, Cr, Cu, Ni, Pb and Zn on the other can be used to describe or define the natural background of heavy metals. We found these correlations are significantly better and easier to use than correlations with grainsize parameters and organic matter contents. Dutch legislation however, is based on the comparison between measured heavy metal contents and standardized background values in which only clay and organic matter contents are considered to be significant. We argue that legislation can be improved considerably by using above mentioned correlation between heavy metals and Al, K and S, and by introducing regional parameters based on sediment provenance and probability of diagenetic accumulations. More research is needed however determine the nature and behaviour of diagenetic heavy metal accumulations.