Environ. Sci. Technol. 1994, 28,662-669
Lead Migration in Forest Soils: Response to Changing Atmospheric Inputs Eric K. Miller' and Andrew J. Friedland Environmental Studies Program and Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755 Decreased use of leaded gasoline has caused a rapid reduction in atmospheric P b deposition to terrestrial and aquatic ecosystems of the northeastern United States. In forest soils, the response to decreased P b deposition has been more rapid than was anticipated based on previous studies of Pb cycling in forested ecosystems. To better understand the observedecosystem response, we estimated both short- (25 y) and long-term (190 y) time series of the regional average atmospheric Pb concentrations in precipitation, cloudwater, and air. We combined these time series with modeled wet and dry deposition velocities to calculate the time-dependent Pb flux at different elevations in montane forests of the northeastern United States. The total integrated flux of anthropogenic P b to northeastern forests since industrialization is estimated to range from -17 kg ha-1 at low elevations to >30 kg ha-l at elevations above 1200m. We used the Pb deposition time series and measurements of organicsoil horizon P b content made in 1966,1980,and 1990to compute dynamic solutions for the responsetime of the atmosphere-organic soil system for P b storage in soils of several forest types. The response time for Pb storage in organicsoil increased with increasing elevation from 17 y in the northern hardwood forest zone to 77 y in the subalpine spruce-fir forest zone. These response times calculated by analysis of the dynamic system are significantly lower than previous estimates made using steady-state assumptions (150-500 y). A linear-rate model of Pb transport predicts the observed pattern of P b accumulation and release by the forest floor of these two forest zones in response to the fluctuating atmospheric inputs. Using this model, we estimate that P b concentrations will stabilize at -18 and -4 pglg in the organic soil horizons of the subalpine spruce-fir and northern hardwood forests, respectively, if atmospheric P b concentrations stabilize at current values. Maximum Pb concentrations in organic soil occurred around 1980 and were -200 pglg in the spruce-fir forest and -85 pg/g in the northern hardwood forest. The time required to reach steady-state concentrations is estimated to be -400 y for the spruce-fir forest and -90 y for the northern hardwood forest. The large pool of anthropogenic P b which was deposited on northeastern forests during the 1950sto the 1980s is being redistributed in the soil profile rather than being retained in the organichorizon for several centuries as was previously thought. Analysis of Pb transit times in mineral soil horizons indicates that the large pulse of anthropogenic lead deposited after 1960 may begin to be released to upland streams beginning sometime in the middle of the next century. ~
Introduction Lead concentrations in the environment have been increasing for several centuries due to human activities including mining and smelting of ores, the combustion of fossil fuels, and the dissemination of lead through industrial processes ( I , 2). The most significant release of P b into the global environment has been the use of P b 662
Environ. Sci. Technoi., Vol. 28, No. 4, 1994
additives in gasoline (3). This recent release of P b into the environment has caused the atmospheric concentration, long-range transport, and atmospheric deposition of P b to increase by several orders of magnitude during the past 70y (4). While much of the P b emitted by automobiles is deposited in narrow corridors along roads and highways (51,a significant portion is released as volatile compounds or sorbed on fine aerosols. Lead in these forms can be circulated into the upper troposphere and transported thousands of kilometers as the residence time of P b in the atmosphere ranges between 7 and 14 days (6). Several studies have documented increased rates of Pb deposition in Greenland and polar snow since the use of leaded fuels began ( 4 ) . Studies of lake sediments, peats, and the organic horizon of forest soils have all indicated that atmospherically transported P b as accumulated in the northeastern United States over the past century (7,B). In the eastern United States, atmospheric deposition of P b occurs primarily by rainfall at low elevations, with cloudwater interception playing a greater role at high elevations (9)and dry deposition contributing up to 20% of the total Pb flux at some locations (10). Both the amount of precipitation and the amount of cloudwaterinterception increase with increasing elevation ( I I ) , and therefore, Pb deposition is greater at higher elevations. Johnson et al. (12) and Friedland et al. (7) demonstrated that both the concentration and the amount of P b in the organichorizon of forest soils increased with increasing elevation in the northeastern United States. Both studies attributed this pattern to greater atmospheric deposition rates at higher elevations compared to lower elevations. Lead consumption as a gasoline additive peaked during 1978 in the United States and declined rapidly to pre1943levels by 1988(13). Several studies have documented a rapid atmospheric and environmental response to the decline in the use of leaded gasoline (14,151. Boutron et al. (16)reported that P b deposition rates decreased rapidly in Greenland snows during this period, probably as a result of reduced Pb emissions from North America (17). Lead concentrations reported for precipitation in the northeastern United States during the late 1960sand early 1970s were greater than 30 pg L-' ( 9 , I B ) but had declined to 17 pg L-1 by 1982 (19) and 1pm) of the soil solution suspension,which may be important for OM-Pb complexes (47-49). For example, Beier et al. (49) have reported that significant differences exist between lysimeter types for the collection of large organic molecules in the size range 1-10pm. We hypothesize that the portion of the apparent P b transport not explained by dissolved P b transport may be due to a mobile solid phase of colloidal or slightly larger dimensions which is not efficiently sampled by lysimeters. It has long been observed that clay particles (especially colloidal sizes
