Radiogenic Isotopes - Application of Geochemical Tracers to Fluvial Sediment - page 99

Fig. 4.6 Downstream changes in total Pb concentration ( red circles )and 208 Pb/ 206 Pb ratios
( orange circles ) along the River Wear, Northern England. Sample sites 1 through 27 are shown
on ordinal axial and cover a distance of 86.5km. Error bar refers to isotopic measurements (from
Shepherd et al. 2009 )

An excellent example of the use of Pb isotopes to determine Pb provenance was
provided by Shepherd et al. ( 2009 ) for the River Wear in northeast England. They
analyzed a variety of geological materials within the catchment including the channel
bed sediments along an 87km reach of the river for selected Pb isotopes (Fig. 4.6 ). As
is common, Pb concentration data were also collected to provide additional details
on the levels of contamination associated with the various Pb sources (Fig. 4.6 ). At
Site 1 the 208 Pb/ 206 Pb ratio is comparable to geological materials found in the outer
mineralized zone of an orefield. The high Pb concentrations found in the channel bed
sediments are therefore likely to reflect the influx of Pb-contaminated sediment to the
river from historic mining operations, including abandoned mines, waste piles and an
old processing plant that produced about 60 000 t of Pb concentrate (Dunham 1990 ).

The rapid downstream declines in Pb concentration from Sites 1-5 are thought to
reflect dilution processes and the deposition of dense, finely milled Pb ore within the
channel bed. Downstream of Site 5, Pb concentrations semi-systematically increase
until reaching Site 9, whereas 208 Pb/ 206 Pb ratios continue to decline. The divergent
changes in Pb concentration and 208 Pb/ 206 Pb ratios apparently reflect the influx of Pb-
enrichedmine waste from two tributary valleys where the Pb ore was derived from the
inner zone of the orefield. Rocks within this inner zone exhibited a lower 208 Pb/ 206 Pb
ratio than those found within the outer zone of the orefield, thereby lowering the
ratio within the channel bed sediment. Between Sites 10 and 11 the 208 Pb/ 206 Pb
ratio increases abruptly, reflecting the input of Pb from a tributary basin (Bollihope
Beck) that contains another cluster of mines. Pb concentrations do not significantly
increase indicating that the tributary provides only a limited quantity of the Pb to the
axial channel of the River Wear (about 18% of the total). There are no further direct
inputs of Pb from mining operations downstream of Site 11. Without the isotopic

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