Transport of industrial lead in snow through soil to stream water and groundwater

Abstract

Determination of isotopic compositions and concentrations of Pb in stream water, groundwater, soil and bed-rock in a subalpine watershed, combined with concentrations of Ca, Na, K, Mg, Fe and Mn and mineralogical data, show that ∼ 90% of the industrial Pb in snow is trapped in a subunit of the upper 2 cm of soil. This is composed of humus plus Fe-hydroxides and Mn-oxides, where industrial Pb mixes isotopically with Pb already accumulated there from previous inputs of industrial Pb and natural rock-Pb. The ^(206)Pb/^(207)Pb ratio of Pb in the upper 2-cm soil accumulation reservoir is different from that of industrial Pb added from snow each year, because it is a mixture of natural rock-Pb derived from weathering (∼ 20%) which has a ^(206)Pb/^(207)Pb ratio different from industrial Pb, and because the ^(206)Pb/^(207)Pb ratio of industrial Pb accumulated from atmospheric inputs during the past century (∼ 80%) has changed considerably during that time. During spring melt, when streams are fed mostly by surface runoff, Pb in the surface soil accumulation reservoir is released back into the runoff and into streams. At this time the proportion of industrial Pb in mountain stream waters is that in the surface soil accumulation reservoir, ∼ 80%. Concentrations and isotopic composition of Pb in soil profiles, and the relation between Pb and organic matter concentration in soil indicate little or no industrial Pb accumulates in the reservoir below 2-cm depths. These data, combined with measured proportions of Ca and Pb and ^(206)Pb/^(207)Pb ratios in groundwater, indicate that most of the Pb in aquifers at depth below 30 cm originates by weathering from natural rock-Pb. Although aquifers are charged by snow-melt, industrial Pb is removed from the water as it percolates downward through the soil accumulation reservoir. Therefore, during times when streams are fed mainly from these aquifers, stream waters contain mainly natural rock-Pb and some industrial Pb in colloids originally contained in snow-melt. Concentrations of Pb in stream waters tend to remain constant both throughout different seasons (when rates of stream discharge change 20-fold), and over the last decade (when Pb inputs from snow decreased 6-fold) because Pb in those waters originates from a fixed reservoir in the soil.

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