CHANGES IN LEAD BIOGEOCHEMISTRY IN RESPONSE TO DECREASING ATMOSPHERIC INPUTS

In the 1970s, the U.S. Congress passed legislation restricting the sale of gasoline with alkyl-lead additives. In the intervening years, the amount of lead (Pb) consumed in gasoline has declined sharply, resulting in lower rates of atmospheric Pb deposition. At the Hubbard Brook Experimental Forest (HBEF) in New Hampshire, the input of Pb in precipitation has declined by 97% between 1976 and 1989. The purpose of this paper is to evaluate the long-term response of forest and associated aquatic ecosystems to these declining inputs of Pb. Lead continues to accumulate in the forest ecosystem of the HBEF, in spite of lower inputs, due to extremely low losses in drainage water. However, between 1977 and 1987 the Pb content in the forest floor declined by 29%. Lead now appears to be accumulating in the mineral soil. The stratigraphy of Pb in sediment from a nearby lake shows continued Pb deposition, though at a declining rate. Much of our understanding of the long-term patterns of lead accumulation in ecosystems has been shaped by paleoecological studies, in which inputs of lead are assumed to be irreversibly retained. Using a regression model and historical information concerning Pb consumption in gasoline, we estimated Pb inputs to the HBEF ecosystem during the period 1926-1989. Based on this analysis and our field observations, it is clear that Pb mobility at the HBEF is greater than previously reported: approximate to 30% of the total atmospheric Pb input between 1926 and 1987 was not retained in organic soils or pond sediments. The potential for Pb pollution in drainage water may therefore be greater than suspected in regions with continued high rates of atmospheric Pb deposition.