STABLE LEAD ISOTOPIC CHARACTERISATION OF THE HISTORICAL RECORD OF ENVIRONMENTAL LEAD CONTAMINATION IN DATED FRESHWATER LAKE SEDIMENT CORES FROM CENTRAL AND NORTHERN SCOTLAND

John G. Farmer (J.G.Farmer@ed.ac.uk), Lorna J. Eades (Department of Chemistry, University of Edinburgh, Edinburgh EH9 3JJ, Scotland, UK), Angus B. MacKenzie (SURRC, East Kilbride, Glasgow G75 0QF, Scotland, UK), Anthony E. Bailey-Watts and Alexander Kirika (NERC Centre for Ecology and Hydrology, Penicuik EH26 0QB, Scotland, UK)

ABSTRACT

The ²⁰⁶Pb/²⁰⁷Pb data for anthropogenic lead in sediment cores from Loch Ness, Loch Lomond and Lake of Menteith in Scotland were found to exhibit largely similar trends related to five different time periods (pre-1820, 1820-1900, 1900-1930, 1930-1980 and 1980 to the present) during the past few hundred years. The trends can be interpreted in terms of temporal variations in the input of anthropogenic lead of differing isotopic signature from the smelting of indigenous lead ores, coal combustion, the smelting of Australian lead and car-exhaust emissions of lead.

INTRODUCTION

Historical records of the input of lead to the environment are increasingly based not only on the lead concentration of archival material (e.g. grass, moss) and of dated sections of peat bog, lake sediment and ice cores but also on the associated stable lead isotopic composition, which may be indicative of sources such as lead ore smelting, coal combustion and car-exhaust emissions (Sugden et al, 1993; Farmer et al, 1996, 1997). In this study, lead and ²⁰⁶Pb/²⁰⁷Pb data are presented for radiometrically dated sediment cores from three freshwater lakes in Scotland: Loch Ness, situated about 160 km to the north of Glasgow and remote from industry; Loch Lomond, 30 km to the north-west of Glasgow and much closer to the densely populated, formerly heavily industrialised central belt (Farmer, 1994); and Lake of Menteith, 20 km to the east of Loch Lomond and near to the ombrotrophic Flanders Moss peat bog (Farmer et al, 1997).

METHODS

The mini-Mackereth (MM) and Jenkin (J) coring techniques were used to collect sediment cores from Loch Ness (19 November 1990, Urquhart Bay, water depth 170 m, MM core length 128 cm), Loch Lomond North (16 July 1995, Ardlui Basin, water depth 60 m, MM core length 90 cm), Loch Lomond South (26 November 1991, Fault Basin, water depth 18 m, MM core length 80 cm, J core length 14.4 cm) and Lake of Menteith (13 November 1996, water depth 13 m, MM core length 56 cm, J core length 22 cm).

Lead concentrations were determined in MM cores by flame AAS after digestion of dried sediment sections (0.2-5 cm thick) in hot 8 M HNO₃/11.6 M HCl and stable lead isotopic ratios by ICP-MS after digestion in hot 8 M HNO₃ (Farmer et al, 1996). Radionuclide determinations were carried out on MM cores from Loch Ness and Loch Lomond North and on J cores from Loch Lomond South and Lake of Menteith, employing g-spectrometry for both naturally occurring ²¹⁰Pb and anthropogenic ¹³⁷Cs (Eades et al, 1998) with the exception of ²¹⁰Pb by a-spectrometry (MacKenzie et al, 1979), via grand-daughter ²¹⁰Po, for Loch Lomond North and Loch Lomond South.

RESULTS AND DISCUSSION

For Loch Ness, Loch Lomond North, Loch Lomond South and Lake of Menteith, concentrations of lead in the sediment cores ranged from baseline values of 12, 24, 15 and 28 mg/kg to maximum values of 83 (8-10 cm), 170 (14-15 cm), 158 (6.3-6.6 cm) and 250 (12-12.5 cm) mg/kg, respectively. Measured ²⁰⁶Pb/²⁰⁷Pb ratios ranged from baseline values of 1.216±0.008, 1.160±0.001, 1.174±0.002 and1.171±0.002 to minimum values of 1.146 (2-4 cm), 1.136 (5-6 cm), 1.127 (3.6-3.9 cm) and 1.142 (3.2-3.4 cm), respectively. Sedimentation rates (based on ²¹⁰Pb and/or ¹³⁷Cs) of 15.1±1.7 mg/cm²/y for Loch Ness, 118.6±2.6 mg/cm²/y (to a depth of 1.127 g/cm²) and 75.5±9.2 mg/cm²/y (below 1.127 g/cm²) for Loch Lomond North, 30.9±1.3 mg/cm²/y for Loch Lomond South and 49.3±10 mg/cm²/y (to a depth of 1.38 g/cm² and 47.6±19 mg/cm²/y (below 1.38 g/cm²) for Lake of Menteith were obtained.

After correction of measured lead concentrations for baseline lead values, fluxes of anthropogenic lead deposition to the sediments were calculated using the measured sedimentation rates and plotted, along with corresponding calculated ²⁰⁶Pb/²⁰⁷Pb ratios of anthropogenic lead, against calendar date for each of the four cores (Fig. 1).

The fluxes of anthropogenic lead increased during the 19^th Century and peaked ca. 1950-1960 for all lakes. The flux was about one order of magnitude lower for Loch Ness, a consequence of its more remote location. Differences in the magnitude of fluxes for Loch Lomond North and Loch Lomond South probably reflect site-specific variations in sedimentation rate, sediment focusing etc. For Loch Ness, Loch Lomond North and Loch Lomond South, 40-50% of the anthropogenic lead inventory was deposited prior to 1900.

Pre-1820, a mean ²⁰⁶Pb/²⁰⁷Pb ratio of 1.175-1.180 for anthropogenic lead in Loch Lomond sediments is close to that of ~1.18 for Scottish coal (Farmer et al, 1999), indicating the increasing influence of coal burning as a source of energy during the Industrial Revolution, while values sometimes in excess of 1.20 for the much less contaminated Loch Ness indicate a small geogenic input, perhaps from volcanic emissions (Monna et al, 1999) in Iceland to the north-west.

From 1820-1900, a consistent ²⁰⁶Pb/²⁰⁷Pb ratio of ~1.17 for anthropogenic lead in Central Scotland is similar to that for lead from Leadhills in SW Scotland (Sugden et al, 1993), the main source of Scottish lead ore, but more probably reflects a combination of that from the smelting of this lead, coal burning and the smelting of ²⁰⁶Pb-depleted Australian lead (²⁰⁶Pb/²⁰⁷Pb ~1.04) to the south in England, where supplies of indigenous lead had largely expired by the mid-19^th Century (Farmer et al, 1999). Using the constant 1820-1900 ²⁰⁶Pb/²⁰⁷Pb value appropriate to each of the lake sediment cores (i.e. 1.177±0.004 (from 1840) for Loch Ness, 1.172±0.003 for Loch Lomond North, 1.170±0.002 for Loch Lomond South and 1.172±0.005 for Lake of Menteith) the 20^th Century values for the ²⁰⁶Pb/²⁰⁷Pb ratios of anthropogenic lead were calculated and plotted versus calendar date in Fig. 2.

From 1900-1930, a slight decline (<0.01) in the ²⁰⁶Pb/²⁰⁷Pb ratio of anthropogenic lead, much less than the 0.025 observed in archival herbage from southern England (Bacon et al, 1996), is indicative of the growing influence of ²⁰⁶Pb-depleted Australian lead. From 1930-1980, a significant decline of ~0.035-0.05 in the ²⁰⁶Pb/²⁰⁷Pb ratio of anthropogenic lead is attributable to the then growing influence of car-exhaust emissions of lead arising from the use of ²⁰⁶Pb-depleted additives in petrol (²⁰⁶Pb/²⁰⁷Pb ~1.06-1.09) (Sugden et al, 1993). From 1980 to the present, an increase of ~0.005-0.015 in the ²⁰⁶Pb/²⁰⁷Pb ratio of anthropogenic lead reflects the reduction of the maximum permitted lead concentration of leaded petrol and the introduction and increasing uptake of unleaded petrol in the UK (Farmer et al, 2000).

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Figure 1: Fluxes (o) and associated ²⁰⁶Pb/²⁰⁷Pb ratios (·) of anthropogenic lead at sediment core sites in Loch Ness, Loch Lomond North, Loch Lomond South and Lake of Menteith versus calendar year.

Figure 2: Change (D) in the ²⁰⁶Pb/²⁰⁷Pb ratio of anthropogenic Pb relative to the appropriate 1820-1900 mean for anthropogenic lead in sediment cores from Loch Ness, Loch Lomond North, Loch Lomond South and Lake of Menteith. The best-fit polynomial curves through the data are shown.