Angus B. MacKenzie

Stable lead isotopic characterisation of the historical record of environmental lead contamination in dated freshwater lake sediment cores from northern and central Scotland.

Sediment cores from three Scottish freshwater lakes, Loch Ness in the remote north and Loch Lomond and the Lake of Menteith, much closer to the heavily populated and industrialised central belt were analysed for 210Pb, 137Cs, Pb and stable Pb isotopic composition (206Pb/207Pb). The radionuclide data were used to establish chronologies for the Loch Ness and Loch Lomond cores, but a chronology could not be developed for the Lake of Menteith core, in which the surface sediment had been subject to intense mixing. Although Pb concentrations generally started increasing during the mid-17th Century, a small peak occurred for Loch Ness in the early 16th Century, perhaps attributable to the influence of medieval mining and smelting in mainland Europe. Temporal trends in the pattern of Pb accumulation were similar for Loch Ness and both sites in Loch Lomond, with 40-50% of the anthropogenic Pb deposited prior to the 20th Century. Fluxes of anthropogenic Pb to the lake sediments peaked during the 1950s at all locations where chronologies could be established. The 5-fold increase in anthropogenic Pb inventory for the southern basin of Loch Lomond relative to Loch Ness reflected geographical proximity to the main polluting sources. The 206Pb/207Pb data for anthropogenic Pb in the sediments from Loch Ness and Loch Lomond exhibited largely similar trends related to five different time periods. Pre-1820, the 206Pb/207Pb ratio was close to that for coal (1.181). From 1820 to 1900, a fairly constant 206Pb/207Pb ratio of approximately 1.17 probably resulted from a combination of emissions from the smelting of indigenous Pb ore (1.170) and coal burning (1.181) in Scotland, and industrial activity to the south in England, where Australian Pb of characteristically low 206Pb/207Pb ratio (1.04) was already in use. From 1901 to 1930, the 206Pb/207Pb ratio declined by <0.01, due to the increasing influence of Australian Pb. From 1931 to 1975/1985, the 206Pb/207Pb ratio of anthropogenic Pb declined by a further 0.03 to 0.04, to minimum values from approximately 1975 to 1985, primarily a consequence of car-exhaust emissions of Pb arising from the introduction of alkyl Pb petrol additives (206Pb/207Pb approximately 1.06-1.09). From 1975/1985 to the mid-1990s, the 206Pb/207Pb ratio of anthropogenic Pb increased by up to 0.015, a consequence of a reduction in car-exhaust emissions of Pb, resulting from reductions in the maximum permitted concentration of Pb in petrol, and the introduction and increasing uptake of unleaded petrol. Source apportionment calculations, on the basis of 206Pb/207Pb values in surface sediment, suggested that the contribution of Pb emissions from the use of leaded petrol was 27-40% of the atmospheric burden by the mid-1990s, in line with estimates from rainwater 206Pb/207Pb data.

Isotopic evidence of the relative retention and mobility of lead and radiocaesium in Scottish ombrotrophic peats

Abstract The reconstruction of historical trends in atmospheric deposition of contaminant metals by use of their concentration profiles in accumulating, ombrotrophic peat deposits, in conjunction with radionuclide-derived chronologies, is subject to uncertainty because of potential post-depositional mobility both of the radionuclides and of the contaminant metals. Results are presented here for a study of the distribution of 210Pb, Pb, stable Pb isotopes and radiocaesium in the unsaturated, surface zone of ombrotrophic peat deposits in three contrasting locations in Scotland. 210Pb chronologies are derived and the implied historical variations in atmospheric depositional fluxes and atom ratios of contaminant Pb are considered, along with total inventories, relative to known historical trends in input and proximity to heavily populated, industrial areas. Similarly, radiocaesium concentration profiles, activity ratios and inventories are considered in the context of the 210Pb chronologies and known inputs. It is concluded that the historical trends in the depositional flux and stable Pb atom ratio of contaminant Pb derived on the basis of the 210Pb chronologies are in good agreement with known temporal variations in these parameters, consistent with post-depositional immobility of Pb under the conditions investigated here. The study confirms that the high degree of mobility of radiocaesium in organic soils renders it of little value in providing chronological information, but suggests that the presence of small amounts of mineral matter may restrict this mobility.

Distributions, inventories and isotopic composition of lead in 210Pb-dated peat cores from contrasting biogeochemical environments: Implications for lead mobility

Abstract The use of concentration profiles of lead and other contaminant metals in 210 Pb-dated ombrotrophic peat cores for reconstruction of historical trends in atmospheric deposition has become relatively well established. However, uncertainty remains over the validity of the assumption of post-depositional immobility of lead in peats. In particular, a number of studies have suggested that in saturated peat systems, lead is subject to diagenetic remobilisation and redistribution, with the result that 210 Pb and lead profiles do not provide a historical record of deposition. Results are presented here for lead concentrations, inventories and stable isotope ratios and for 210 Pb activities in two peat cores from locations close to the Glasgow industrial area in west-central Scotland. Contrasting biogeochemical conditions prevailed at the two sampling sites, with one being unsaturated and ombrotrophic while the other was saturated and minerotrophic. The results for the ombrotrophic peat core were compatible with information from other peat and lake sediment core studies, data for archived herbage samples and with known historical trends both in industrial activity and in the use of lead as an additive to petrol, giving a high degree of confidence in the 210 Pb chronology and implied record of deposition. This provides strong support for the suggestion that lead is immobile in such systems. In contrast, the lead concentration profile and stable isotope data for the minerotrophic peat provided apparent temporal variations that were inconsistent with other studies and known historical trends, confirming that the saturated peat core did not provide a record of atmospheric deposition. This observation is consistent with the suggestion that lead is subject to diagenetic remobilisation and redistribution in saturated peats.

Climate variability over the last two millennia in the North American monsoon region, recorded in laminated lake sediments from Laguna de Juanacatlán, Mexico

High-resolution titanium (Ti) data obtained using an ITRAX XRF core scanner from a laminated sediment core from the Laguna de Juanacatlán, western central Mexico yield a unique high-resolution record of runoff (precipitation) change for the last 2000 years. In the absence of reliable, long-term meteorological records, comparison of the Ti data with information from the rich Spanish colonial period archives and the post-Independence period, confirms that Ti is a proxy for runoff. This interpretation is supported by comparison with other high-resolution archives from the surrounding region, primarily tree rings and other lake sediment sequences. The Juanacatlán Ti record is therefore a proxy for summer, monsoonal rainfall. The record provides new evidence from the Pacific margin of tropical North America of the occurrence of dry conditions through much of the Classic period (c. AD 300—900), and wetter conditions during the later Medieval period (c. AD 1200—1350). The period commonly known as the ‘Little Ice Age’ (LIA) shows considerable variability, with dry conditions in the early part (c. AD 1400—1600) and wetter conditions, punctuated by multiyear droughts through the eighteenth century. A notable feature of the record is the apparent decoupling of lacustrine sedimentation from the climate since the mid-twentieth century, possibly resulting from anthropogenic disturbance. Preliminary interpretations of the Ti record indicate that patterns are consistent with changes in monsoon strength associated with ENSO and solar forcing over the last two millennia.

Investigation of contaminant metal dispersal from a disused mine site at Tyndrum, Scotland, using concentration gradients and stable Pb isotope ratios

Results are presented for a study of Pb and Zn concentrations and stable Pb isotope ratios for mining wastes, river sediments and 210Pb-dated peat cores from the vicinity of a derelict Pb/Zn mine at Tyndrum in central Scotland. Mining was carried out at Tyndrum between 1741 and 1862 and the original waste dumps were reworked between 1916 and 1925. Little remains of the original buildings and workings and the contemporary distribution and dispersion pathways of the residual waste are poorly defined. The mine and ore processing waste dumps were readily identified as highly polluted, barren areas, with concentrations of up to 21 and 3.4% Pb and Zn, respectively. The Pb in the waste was characterised by a 206Pb/207Pb atom ratio of 1.146±0.004 and a 208Pb/207Pb ratio of 2.432±0.007. Significant quantities of waste were also found along riverbanks and in river channel sediments in the vicinity of the mine, with concentrations of up to 5.22% Pb and 2.35% Zn. Concentrations of Pb and Zn in river channel sediments decreased markedly with increasing distance downriver from the mining area. However, the decrease in Pb concentrations in the <53 μm fraction was less pronounced, with a relatively high concentration of 0.076% being observed at a distance of 6.5 km from the mine, suggesting significant fluvial transport of this size fraction of waste. The stable Pb isotopic characteristics of the river sediments were consistent with the mine waste being the dominant source, along with minor inputs from local bedrock and vehicle exhaust emissions. The peat core data revealed high levels of Pb deposition throughout the period of the mining operation and very high levels of input in the early 20th century, almost certainly as a result of the reworking of the former waste dumps. As with the river sediments, the isotopic characteristics of the Pb in the peat cores were consistent with a dominant input from the mine waste and minor contributions from bedrock and vehicle exhaust emissions. The results suggest that waste from the mining operation has been a significant source of contaminant heavy metals for several hundred years and continues to be so. The apparent fluvial transport of Pb from the mine site is consistent with previous work suggesting that Tyndrum mine waste is the probable origin of anomalously high levels of Pb deposited in the sediments of Loch Tay, some 25 km to the east. The study highlights the utility of stable Pb isotope analyses in the investigation of sources and environmental dispersion of contaminant Pb.

A comparison of antimony and lead profiles over the past 2500 years in Flanders Moss ombrotrophic peat bog, Scotland

Two cores collected in 2001 and 2004 from Flanders Moss ombrotrophic peat bog in central Scotland were dated (14C, 210Pb) and analysed (ICP-OES, ICP-MS) to derive and compare the historical atmospheric deposition records of Sb and Pb over the past 2500 years. After correction, via Sc, for contributions from soil dust, depositional fluxes of Sb and Pb peaked from ca. 1920-1960 A.D., with >95% of the anthropogenic inventories deposited post-1800 A.D. Over the past two centuries, trends in Sb and Pb deposition have been broadly similar, with fluctuations in the anthropogenic Sb/Pb ratio reflecting temporal variations in the relative input from emission sources such as the mining and smelting of Pb ores (in which Sb is commonly present, as at Leadhills/Wanlockhead in southern Scotland), combustion of coal (for which the Sb/Pb ratio is approximately an order of magnitude greater than in Pb ores) and exhaust emissions (Pb from leaded petrol) and abrasion products from the brake linings (Sb from heat-resistant Sb compounds) of automobiles. The influence of leaded petrol has been most noticeable in recent decades, firstly through the resultant minima in Sb/Pb and 206Pb/207Pb ratios (the latter arising from the use of less radiogenic Australian Pb in alkylPb additives) and then, during its phasing out and the adoption of unleaded petrol, complete by 2000 A.D., the subsequent increase in both Sb/Pb and 206Pb/207Pb ratios. The extent of the 20th century maximum anthropogenic enrichment of Sb and Pb, relative to the natural Sc-normalised levels of the Upper Continental Crust, was similar at approximately 50- to 100-fold. Prior to 1800 A.D., the influence of metallurgical activities on Sb and Pb concentrations in the peat cores during both the Mediaeval and Roman/pre-Roman periods was discernible, small Sb and Pb peaks during the latter appearing attributable, on the basis of Pb isotopic composition, to the mining/smelting of Pb ores indigenous to Britain.

Remobilization of Sellafield-derived radionuclides and transport from the north-east Irish Sea

Plutonium-239,240 activities in seawater from the north-east Irish Sea, collected in January 1992, were in the range of 0.17–4.30 mBq l−1 in solution and 7.2–62.3 Bq kg−1 in suspended particulates. The 238Pu:239,240Pu activity ratios for the soluble fraction were consistent with those of the underlying sediments but inconsistent with contemporary discharges from Sellafield, indicating that most of the plutonium in solution was derived from re-dissolution from the sediment. The activity ratios for the particle associated material were more variable, but again, on average, typical of the sediments. The annual loss of 239,240Pu from the north-east Irish Sea in solution was estimated to be 1.19 TBq and that associated with suspended particulates to be 0.11 TBq. From this, a halving time of 350 years for the loss of 239,240Pu activity from the sediment by dissolution was determined. A similar treatment of 137Cs data yielded an annual loss of 86 TBq in solution and a halving time of 23 years. Calculations using a numerical model of the Irish Sea yielded similar estimates of current loss rates and permitted the estimation of total cumulative losses from the Irish Sea by advection through the North Channel of 7.9 TBq of 238Pu, 48 TBq of 239,240Pu 2.6 × 104 TBq of 137Cs.

Environmental radioactivity: experience from the 20th century — trends and issues for the 21st century

Environmental radioactivity has been an important area of research throughout the 20th century, with recent work having been stimulated mainly by: (i) the remarkable power of radionuclides as tracers of the rates and mechanisms of environmental processes and (ii) the potential health implications of contaminant radionuclides in the environment. A review is presented of salient aspects of environmental radioactivity, including consideration of tracer applications, sources and environmental impact of anthropogenic radionuclides, radioactive waste disposal and future exploitation of nuclear energy.

A historical record of atmospheric depositional fluxes of contaminants in west-central Scotland derived from an ombrotrophic peat core

An ombrotrophic peat core from west-central Scotland was subjected to multi-element analysis and 210Pb dating in order to estimate historical trends in atmospheric deposition of contaminants. The inferred depositional flux of ash (total inorganic material) exhibited small, transient increases from as early as the mid 18th century, followed by a larger increase from the mid 19th century to a pronounced maximum approx. 1960. A significant decrease was observed from the peak ash deposition of 29.2 g m−2 year−1 to the contemporary level of 11 g m−2 year−1, although this is still an order of magnitude greater than the pre-industrial value of 1 g m−2 year−1. Depositional fluxes of a wide range of elements (scandium, copper, arsenic, antimony, samarium, cobalt, bromine, lanthanum and lead) also exhibited major increases from the mid 19th century and peak values in the mid 20th century (approx. 1940–1950). Aluminium, cerium and bromine also showed evidence of transiently increased deposition as early as the mid 18th century. Normalisation of the concentrations of individual elements to that of aluminium confirmed that the peak depositional fluxes in the mid 20th century were not due to increased re-deposition of soil minerals from the atmosphere, but were attributable to other sources of contamination including heavy industry, coal burning and vehicle exhaust emission. Nevertheless, re-deposited soil minerals were estimated to constitute an important fraction of the ash, with peak input at the start of the 20th century and subsequent deposition at a relatively constant level, about half that of the maximum flux. Excellent agreement was observed between the inferred trends in contaminant deposition and the known history of industrial development of the area.

Historical accumulation rates of mercury in four Scottish ombrotrophic peat bogs over the past 2000 years

The historical accumulation rates of mercury resulting from atmospheric deposition to four Scottish ombrotrophic peat bogs, Turclossie Moss (northeast Scotland), Flanders Moss (west-central), Red Moss of Balerno (east-central) and Carsegowan Moss (southwest), were determined via analysis of (210)Pb- and (14)C-dated cores up to 2000 years old. Average pre-industrial rates of mercury accumulation of 4.5 and 3.7 microg m(-2) y(-1) were obtained for Flanders Moss (A.D. 1-1800) and Red Moss of Balerno (A.D. 800-1800), respectively. Thereafter, mercury accumulation rates increased to typical maximum values of 51, 61, 77 and 85 microg m(-2) y(-1), recorded at different times possibly reflecting local/regional influences during the first 70 years of the 20th century, at the four sites (TM, FM, RM, CM), before declining to a mean value of 27+/-15 microg m(-2) y(-1) during the late 1990s/early 2000s. Comparison of such trends for mercury with those for lead and arsenic in the cores and also with direct data for the declining UK emissions of these three elements since 1970 suggested that a substantial proportion of the mercury deposited at these sites over the past few decades originated from outwith the UK, with contributions to wet and dry deposition arising from long-range transport of mercury released by sources such as combustion of coal. Confidence in the chronological reliability of these core-derived trends in absolute and relative accumulation of mercury, at least since the 19th century, was provided by the excellent agreement between the corresponding detailed and characteristic temporal trends in the (206)Pb/(207)Pb isotopic ratio of lead in the (210)Pb-dated Turclossie Moss core and those in archival Scottish Sphagnum moss samples of known date of collection. The possibility of some longer-term loss of volatile mercury released from diagenetically altered older peat cannot, however, be excluded by the findings of this study.