Source identification of Pb Zn contamination in the Allen Basin, Cornwall, S.W. England

It is proposed that geochemical maps expressed in the form of drainage basin segments can identify areas containing contaminated land, and distringuish whether such enrichment is natural or anthropogenic. This technique has been applied to an area of 112 km 2 in Cornwall using maps prepared from a reconnaissance study, apportioning contamination to historical mine wastes and other enrichments to naturally occurring Pb Zn mineralisation. A high density stream sampling programme and soil survey in the Allen Basin for the reconnaissance survey highlighted the presence of a discrete and previously described impersistant galena (and subordinate spalaerite) vein towards the south of the valley acting as a sourceof enrichment. Elevated concentrations of Pb and Zn in stream sediments and water reflected the presence of an anthropogenic source towards the N of the basin; this was a mine-waste tip. The natural enrichment was also detected in the stream sediment and water chemistry. Plots for the downstream dispersion of Pb and Zn from the anthropogenic source were compared with the idealized dispersion model proposed by Hawkes. Only a limited correlation was found between the model and the field measurements, being closest when metal concentrations approached background. Refinement of the model using GIS may improve its reliability and, therefore, the accuracy of the contamination source-separation and identification technique.