THE ROLE OF CATCHMENT ON LAKE POLLUTION BY Pb

AT LOCHNAGAR, SCOTLAND

Handong Yang* (e-mail: hyang@geog.ucl.ac.uk), Rick Battarbee, Neil Rose (Environmental Change Research Centre, University College London, 26 Bedford, London, WC1H 0AP, UK) and John Boyle (Geography Department, Liverpool University, Liverpool, L69 3BX, UK)

Abstract

Lochnagar is a remote Scottish mountain lake. The catchment soils of Lochnagar are very organic, and hence have a high affinity for heavy metals resulting in enrichment in the soils. With soil erosion in the catchment, these metals are washed into the lake. Pb budget shows that present amount of Pb washed from the catchment into the lake is 12.2 time those deposited directly from the atmosphere

Pb deposition density has been reduced in the recent years. As the catchment is the major suppler of Pb to the lake, Lochnagar sediments have not recorded this reduction. This implies that since the catchment has become a pool for anthropogenic Pb, with Pb deposition reduced, the catchment will become an increasingly more important source of Pb to the lake. The influence of this source on the lake will exist for a considerable time, and hence restoration of the lake ecosystem from present level of Pb pollution could be delayed by the influence of the catchment.

Introduction

Anthropogenic emissions of heavy metals greatly exceed biogenic input (Nriagu and Pacyna, 1988). The surplus of these elements eventually reach the two endpoints of the metal cycles: soils, and marine and freshwater sediments (Birch et al., 1996), via a variety of pathways e.g. industrial outlets, atmospheric deposition and leaching.

Lake sediments store important information about past conditions of a lake and its catchment. Records of the distribution and abundance of natural and anthropogenic chemicals can be preserved in sequentially deposited, undisturbed sediment profiles. Geochronological data can be used to document historical changes in levels and compositional complexity of chemicals found in sediment profiles (Catallo et al., 1995). Lake sediments also provide an excellent archive for historic changes in industrial discharge of potentially toxic metals such as Pb (Rippey et al., 1982; Engstrom et al., 1994; Renberg, 1986). The historical analysis of Pb from sediments can therefore provide an insight into trends in Pb inputs to ecological and human systems.

Lochnagar is a remote mountain lake in the Cairngorms region of Scotland and is a key site in national and international monitoring networks. It has been contanimated by heavy metals (Battarbee et al., 1995; Jones et al., 1993). No distinguishable inflow feeds Lochmagar, anthropogenic heavy metals at the site are derived solely from the atmospheric deposition. The loch drains to the north-east through a series of small pools. This paper reported Pb inventories in the lake sediments and the catchment soils, and showed the importance of the catchment to the lake on Pb pollution through the Pb budget.

Methods

Seventeen lake sediment cores along five transects radiating from a central point of the lake were taken using Glew corer in 28th June and 2nd July. These sediment cores were subsamled at 0.5 cm intervals from the surface to 7 cm depth, and then at 1 cm intervals to the bottom of the cores. Ten soil cores representing different areas of the catchment were taken in July 1997. The soil cores were silced at 1 cm intervals from the surface to 10 cmdepth, and then at 2 cm intervals to the bottom. All the samples were freeze dried. Pb has been analysed in all the cores using XRF (Boyle,1999). All the sediment and soil cores were dated and stratigraphically correlated by using spheroidal carbonaceous particle (SCP) method (Rose et al., 1995; Yang 2000) and ²¹⁰Pb. Dates for sediments were calculated according to the constant sedimentation rate model (Oldfield and Appleby, 1984; Hermanson, 1991; 1993; Flower et al, 1994; Boyle et al., 1998). Water loss via the outflow was estimated through a stageboard calibrated by dilution gauging at a range of flows. Bulk deposition and lake water samples were collected monthly. Samples were acidified, and measured by ICP-MS.

Results and Disscussion

Lochnagar catchment soils are very organic, the loss on ignition value is up to 99%, as organic soils have high affinity for heavy metals (Livett et al., 1979; Sigg, 1994), this results in enrichment of Pb in the soils. With the dates dated by the SCP profiles and 210Pb (Yang et al., 2000; Yang 2000), the inventories of total and anthropogenic Pb in the lake sediments and the catchment soils were calculated and shown in Table 1.

Table 1 Totla and anthropogenic Pb inventories (kg) in the lake sediments

and the catchment soils since 1860

	Total	Anthropogenic
Sediments	150	93
Soils	936	839

Figure 1 Chronological inventories of Pb in Lochnagar sediments

The chronological inventories of Pb base on the whole lake basin was set up (Figure 1), which shows a slow, steady increase over the period between the 1860s and the 1880s followed by a rapid increase in Pb loading to the 1940s. Since then, Pb loading to the sediments has fluctuated to the present. The increase in rate of Pb loading between the 1910s and the 1940s is about 2 times of the rate between the 1880s and the 1910s.

Input of Pb in Lochnagar is only from deposition, The output of Pb is through the outflow. As Pb loading to the sediments was relatively stable in the last twenty years, it is reasonably assumed that the inventories of Pb in the lake water and the catchment vegetation were stable during this period. The amount of atmospheric Pb deposition (1810+193 g) calculated from the measured Pb load for 1998 is less than the amount of Pb lost via the outflow and stored in the lake sediments (998+1169 g) (Figure 2). This implies that trace metals washed in from the catchment (maybe including erosion of the catchment soils) have made a great influence on the metal budgets in the lake, the yearly storage of trace metals in the lake sediments may not correspond well to the load of the lake. This means that even if the Pb load is reduced, the yearly Pb storage in recent years in the sediments is not reduced to the same level.

Figure 2 Yearly total Pb budget in Lochnagar for 1998. Figures in ( ) show dates.

998 g

Loss via outflow

Vegetation

Lake water

					1169 g
				Sediments
	Old (before 1998)
				Lake system

The Pb budget for 1998 shows that the amount of Pb washed into the lake is 12 times the amount deposited directly from the atmosphere. Erosion in the catchment reduces Pb storage in the soils, and eroded soils carry high amounts of Pb into the lake. In 1998, if all the “new” Pb deposited in the catchment was washed into the lake, the total storage of Pb in the catchment would still be reduced by 550 g. Pb deposited in the catchment from anthropogenic sources in the last 140 years is more than 839,000 g, which is 464 times the yearly atmospheric deposition of Pb in 1998. This suggests that Pb deposition has been greatly reduced.

The yearly Pb budget for 1998 also implies that catchment erosion has changed the balance of supply for trace metals from deposition and the catchment. The distribution proportion of Pb in the compartments within the catchment ecosystem has changed and this results in errors in establishing historical load by matching the present load to the sediment record. Although Pb load has been reduced, as Pb has been washed (including eroded) into the lake, this has obscured or concealed the decline of Pb load in the sediment record. Therefore, historical load cannot be simply obtained by matching present deposition data to the sediment record. As Pb deposition varies over a large range, long-term Pb deposition monitoring is needed. If the mass-balance in this period still shows that the amount of Pb washed from the catchment into the lake is larger than the amount deposited on the catchment, i.e. the net yearly storage in the terrestrial catchment is negative, then this means that the balance of distribution of Pb has changed, and historical atmospheric load cannot be obtained by matching deposition data to the sediment record.

Assuming the yearly storage of Pb in the lake sediments is constant in the 1990s, the Pb budget for 1998 shows that the amount of Pb washed from the catchment into the lake is 2,360 g, which is 12 times higher than the amount of Pb deposited directly onto the lake. If the rate of Pb washed from the catchment into the lake is 2,360 g/year, the stored anthropogenic fraction in the catchment could maintain this rate of supply for a further 356 years. This implies that the catchment could be a major source of Pb to the lake for a long time.

References

Battarbee, R.W., Jones, V.J., Flower, R.J., Appleby, P.G., Rose, N.L. and Rippey, B., 1995,

Bot. J. Scotl., 48(1): 79-87.

Birch, L., Hanselmann, K.W. and Bachofen, R., 1996, Wat. Res., 30(3): 679-687.

Boyle, J.F., Machay, A.W., Rose, R.L., Flower, R.J. and Appleby, P.G., 1998, J. of Palaeolimnology, 20: 135-150.

Boyle, J.F., 1999, X-Ray Spectrom, 28: 000-000.

Catallo, W.J., Schlenker, M., Gambrell, R.P. and Shane, B.S., 1995, Environ. Sci. Technol., 29(6): 1436-1445.

Engstrom, D.R., Swain, E.B., Henning, T.A., Brigham, M.E. and Brezonik, P.L., 1994, In:

Environmental Chemistry of Lakes and Reseroirs. Edited by Baker, L.A., 33-66.

Flower, R.J., Rippey, B., Rose, N.L., Appleby, P.G. and Battarbee, R.W., 1994, J. of Ecology, 82: 581-592.

Hermanson, M.H., 1991, Environmental Science & Technology, 25: 2059-2064.

Hermanson, M.H., 1993, Water Science and Technology, 28:33-41.

Jones, V.J., Flower, R.J., Appleby, P.G., Natkanski, J., Richardson, N., Rippey, B. Stevenson, A.C. and Battarbee, R.W., 1993, J. of Ecology, 81: 3-34.

Livett, E.A., Lee, J.A. and Tallis, J.H., 1979, J. of Ecology, 67: 865-891.

Nriagu, J.O. & Pacyna, J.M. 1988, Nature, 333:134-139.

Oldfield, F. and Appleby, P.G., 1984, In: Lake Sediments and Environmental History. Edited by Haworth, E.Y. and Lund, J.W.G., University of Minnesota Press, Minneapolis, 93-124.

Renberg, I., 1986, Hydrobiologia, 143: 379-385.

Rippey, B., Murphy, R.J. and Kyle, S.W., 1982, Environ. Sci. Technol., 16: 23-30.

Rose, N.L., Harlock, S., Appleby, P.G. and Battarbee, R.W., 1995, The Holocene, 5(3): 328-335.

Sigg, L., 1994, In: Chemical and Biological Regulation of Aquatic System. Edited by Buffle, J., De Vitre, R.R., Lewis Publishers. 175-196.

Yang, H., Rose, N.L. Battarbee, R.W., 2000, The Holocene.

Yang, H., 2000, Trace Metal Storage in Lake Systems and its Relationship with Atmospheric Deposition with Particular Reference to Lochnagar, Scotland. Unpublished PhD thesis. University College London.