Trace metals in sediments from the humber estuary: A statistical analysis of spatial uniformity

Concentrations of seven heavy metals (Cu, Zn, Ni, Cr, Nb, Pb, As) in sediments of the Humber estuary and its tributary rivers have been examined using a variety of statistical methods. Concentrations of metals are rather uniform once the effect of grain size is adjusted for. This uniformity extends for some distance beyond the turbidity maximum, and in fact well beyond the normal limit of salt penetration. Detailed statistical analysis of the data do however reveal some trend of concentrations with position in the estuary, with concentrations of Zn, Ni, and Pb being slightly lower towards the mouth of the estuary. Nb concentrations are higher near the mouth of the Humber, but this reflects the location of two large discharges of Nb here. Once these anomalous samples are removed, Nb concentrations are also shown to be spatially homogeneous. The spatial trends in Zn, Ni and Pb may be due to inputs of very contaminated particulate matter to the upper estuary, to inward transport of uncontaminated clay particles from the North Sea to the lower estuary or to the exchange of metals between particulate matter and water. There are however also spatial trends in sediment particle diameter, so some caution needs to be exercised in drawing conclusions.