Environ. Sci. Technol. 1999, 33, 3362-3367
Natural Lead Concentrations in Pristine Boreal Forest Soils and Past Pollution Trends: A Reference for Critical Load Models RICHARD BINDLER,* M A J A - L E N A B R A¨ N N V A L L , A N D INGEMAR RENBERG Department of Ecology and Environmental Science, Umeå University, S-901 87 Umeå, Sweden OVE EMTERYD AND HARALD GRIP Department of Forest Ecology, Swedish University of Agricultural Sciences, S-901 83 Umeå, Sweden
Knowledge of natural, prepollution concentrations of heavy metals in forest soils and temporal trends of soil pollution are essential for understanding present-day pollution (ecotoxicological assessments) and for establishing realistic goals for reductions of atmospheric pollution deposition (critical loads). Soils not exposed to deposition of atmospheric pollution no longer exist and, for example, present lead (Pb) pollution conditions in northern European soils are a consequence of nearly 4,000 years of atmospheric pollution. We use analyses of Pb concentrations and stable Pb isotopes (206Pb/207Pb ratios) of ombrotrophic peat and forest soils from southern Sweden and a model for Pb cycling in forest soils to derive an estimate for the prepollution concentration of Pb in the mor layer of boreal forest soils and to back-calculate Pb concentrations for the last 5,500 years. While the present-day concentrations of the mor layer are typically 40-100 µg g-1 (0.251.0 g m-2), Pb concentrations of pristine forest mor layers in Sweden were quite low, e0.1 µg g-1 (e1 mg m-2). Large-scale atmospheric pollution from the Greek and Roman cultures (ca. 0 AD) increased Pb concentrations to about 1 µg g-1. Lead (Pb) concentrations increased to about 4 µg g-1 following the increase of metal production and atmospheric pollution in Medieval Europe (ca. 1000 AD).
Introduction There is currently great political interest in Europe to control long-range transport and deposition of heavy metals using the critical loads concept, i.e. the idea of only allowing atmospheric deposition levels that are harmless to the environment (1). The critical loads concept for heavy metals requires knowledge of natural background conditions preceding large-scale atmospheric pollution and historical pollution loads. Natural concentrations of Pb are assumed to be 10-15 µg g-1 in the mor layer (2, 3) of boreal forest podzol soils in Sweden and are at present often 40-100 µg g-1. It is clear that the long-range transport of pollutants has had a long-term effect on heavy metal cycles throughout the * Corresponding author: phone: (+46) 90 7869784; fax: (+46) 90 7866705; e-mail:
[email protected]. 3362
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 19, 1999
northern hemisphere as evidenced by lake sediment and peat cores in Europe (4, 5, 6, 7, 8, 9) and ice cores from Greenland (10, 11), which record atmospheric Pb pollution beginning as early as 1500-2000 BC. Although pristine environments no longer exist, estimates for background Pb concentrations and assessment of temporal trends for Pb pollution in forest soils can be made by coupling data on past atmospheric deposition rates, drawn from paleoarchives, with present-day understanding of Pb biogeochemical cycling in forest soils (12, 13, 14, 15, 16). Podzols (17) are acidic soils characteristic of the circumpolar boreal coniferous forests, which cover large areas of Fennoscandia, Russia, and North America. These podzols are stratified into a surficial organic horizon (O horizon), which is a humus layer, specifically called mor layer, that covers the mineral soil like a blanket, a gray eluvial horizon (E horizon) where Al and Fe are leached, a dark illuvial horizon (Bs horizon) where Al, Fe, and organic complexes have accumulated, and below, a gradation into unaltered parent mineral soil (C horizon). The physical mixing rate between the mor layer and underlying mineral soils is low due to limited digging soil fauna in podzols. The Pb deposited from the atmosphere is initially trapped in the mor layer, and gradually transported down into underlying mineral soil horizons. Transport processes for Pb in soils are largely dependent on the decomposition and downward migration of organic matter. Recent studies of Pb cycling in soils have shown short mean residence times for the Pb accumulated in forest organic horizons, typically from 25 to 75 years (12, 13, 14, 15, 16). We use Pb concentrations and Pb stable isotope ratios (206Pb/207Pb) in soils, peat bogs, and tree rings from southern Sweden to derive an estimate for the natural, prepollution concentration of Pb in the mor layer of boreal forest soils. We apply data from paleoreconstructions of Pb deposition from peat bog records to contemporary models of Pb cycling in forests to develop a 5,500 year back-calculation for Pb concentrations in the biologically active mor layer. Developing a reasonable estimate for the natural, pre-pollution levels of Pb as well as for other heavy metals is important for assessments of their effects on soil biota; ecotoxicological studies lack a natural reference point. With the increasing interest to establish critical loads for heavy metals in the environment, it is important to understand the long-term processes that have contributed to the present status. Accumulation of Pb, and other heavy metals, has occurred over a long-term period, therefore, future environmental goals must be viewed in a long-term perspective.
Methods Peat cores were collected from three ombrotrophic bogs, Store Mosse, Trolls Mosse, and O ¨ nneby Mosse in southern Sweden, using a Wardenaar corer (18) for the top one meter and a Russian peat corer for the layers below. AMS radiocarbon dates (Ångstro¨m laboratory, Uppsala University, Sweden) were calibrated to calendar years (19). The coring site on Store Mosse was located on the southern section of the bog near to coring sites of an earlier study on the spatial and temporal changes in the plant community of the bog (20). The development of Store Mosse from a Carex fen into an ombrotrophic bog, with three major bog phases, has been previously described in detail (20, 21, 22). On the basis of strong similarities in ash content, Pb and Hg concentrations, and peat accumulation rates, the bog development of Trolls Mosse and O ¨ nneby Mosse are assumed to be similar to Store Mosse. 10.1021/es9809307 CCC: $18.00
1999 American Chemical Society Published on Web 08/14/1999
Using past atmospheric Pb deposition rates calculated from the peat bogs and current understanding of Pb behavior in the soils (12, 13, 14, 16), we can model Pb concentration changes in boreal forest mor layers in southern Sweden over the last 5,500 years. Lead concentrations are a function of the mass of the mor layer and the inventory of Pb stored in the mor. The Pb inventory in the mor layer in a given year (MorPb(t)) can be expressed as (14):
MorPb(t) ) MorPb(t-1) + input(t) - output(t)
(2)
where t is a given year, MorPb(t-1) is the pool of Pb stored in the mor layer in the year prior to the year t, input(t) is the deposition of Pb in year t, and output(t) is the loss of Pb from the pool in year t. The output(t) is expressed as a first-order equation: FIGURE 1. Map of Sweden with study sites: ombrotrophic bogs (•), Store Mosse (1), Trolls Mosse (2), and O2 nneby Mosse (3), and soils, Bottnaryd (B), Norra Kvill (K), and Ha1 sthult (H). Soil profiles were taken from three old-growth forest sites, Bottnaryd, Ha¨sthult, and Norra Kvill; these forests have not been actively logged in at least the last approximately 200 years. Three profiles were collected at Ha¨sthult and two profiles each at Bottnaryd and Norra Kvill. To characterize the spatial variability of Pb in the mor layer a series of replicate samples of the mor layer were collected at each site. Samples from the soil profiles were collected using a stainless steel soil corer in hand dug soil pits after the sampling face was first scraped clean with a stainless steel trowel. Cores from 200 to 300 year old Scots pine (Pinus sylvestris) were collected nearby the soil pits at each forest site using a 5 mm incremental borer at breast height; two trees 15 m apart were cored at Ha¨sthult. Locations of the study sites are shown in Figure 1. Lead concentrations and isotope ratios of peat and soil samples were determined using ICP-MS (23, 24) following digestion with strong acid (HNO3 + HClO4, 10:1), with treering samples being digested only in HNO3. All concentrations are reported on a dry mass basis. Analytical quality control includes use of both standard reference material and internal reference material. Analytical error is
