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Chapter 7 Soil contamination

There are a wide range of potential pollutants that can contribute to soil contamination. These pollutants enter the soil system by either atmospheric deposition or land-based human activities. This chapter is divided in three sections, describing soil contamination by:

7a Soil Contamination by Atmospheric Pollutants

7a.1 Summary

• There is a strong link between acidification of soils and emissions of sulphur and nitrogen.
• Many Scottish soils are particularly sensitive to acid inputs of sulphur and nitrogen from the atmosphere. This is due to many upland soils being derived from slow weathering, acidic soil parent materials and comprising organic rich soils.
• The sensitivity of soils to acidification has been assessed through the use of a critical load approach applied to the 1:250 000 soil map units of Scotland.
• Emissions and deposition of sulphur have been reduced through international abatement strategies whereas nitrogen emissions and deposition have declined by a very small amount.
• Exceedance of Critical Loads for soil acidification in Scotland has been estimated for the period 1995 to 1997 to be 85%. This will decline to 35% by 2010 due to reduced emissions and deposition.
• Exceedance of Critical Loads for eutrophication in Scotland has been estimated to range from 2 % for acid grassland to 76 % for coniferous woodland and will decline by only approximately 15 % by 2010.
• Recovery of soils to previous levels of base saturation, is likely to take decades, in contrast to the signs of recovery in some water bodies, and their fish populations, which have recovered their acid neutralizing capacity over a few years.
• Accidental emissions such as the Chernobyl accident, and their location and impacts are impossible to predict.

7a.2 Introduction and Description of Threat

The burning of fossil fuels and industrial emissions to the atmosphere of pollutants such as sulphur, nitrogen and heavy metals give rise to the deposition of these pollutants, often thousands of km away from their source. The impact of these pollutants on soils and their related ecosystems is variable in time and space and is determined by a number of factors such as: deposition history, pollutant mobility and accumulation in the soil and ecosystem sensitivity. Within Europe the approach selected to scientifically assess and to evaluate potential policies to abate the impact of these pollutants has been the development of the critical load approach (see under NEGTAP below).

7a2.1 Acidification

Acidification occurs when gaseous emissions of SO₂, NO and NO₂ are converted to sulphuric or nitric acids which fall as 'acid rain'. The acid inputs provide both a source of protons to exchange for base cations (Na ⁺, K ⁺, Ca ²⁺, Mg ²⁺) and mobile anions ( SO₄ and NO₃) to remove displaced cations from the soil through leaching. A wide range of soils in Scotland are susceptible to acidification. Some guidance on which soil types are the most sensitive is provided in Langan and Wilson (1993). This study partitioned the soils of Scotland into the five critical load classes (after Nilsson and Grennfelt, 1988). The most susceptible soils are developed on quartzites and granites. The next most sensitive soils have low base saturation, and are developed in metamorphic schists, gneisses and granulites of pre-Cambrian age covering large areas of upland Scotland. Soils in these two classes have inherently low base saturation, which is further reduced by acid inputs. The low weathering rates in these soils imply that recovery of base saturation levels will take decades during the reduced emission levels currently in existence. This is in marked contrast to water courses and lochs, which have already exhibited clear signs of recovery. There are also natural processes of acidification in soils involving respiration to produce carbon dioxide, organic acid produced by decomposition of organic matter and base cation uptake by plants. A recent estimate for the 1990 decade (Fowler et al., 2002) suggested that acid inputs exceeded the critical loads for more than half the 1 km grid squares .The largest areas of exceedance were in central and southern Scotland. Peatlands and acid grasslands were the most affected habitats. The main findings of a review of the impact of nitrogen deposition in forest ecosystems (Emmett, 2002) were that soil is the main sink for increased N deposition. There are limits to how much N can be stored in soil in the long term. Nitrate leaching will eventually occur in N-saturated soils. The review states that ammonium uptake by vegetation and soil microbes and increased nitrification can lead to acidification of some sensitive soils and a reduction in the availability of other nutrients.

Langan et al. (2004) report that a significant proportion of different woodland habitats in Scotland and more widely across Great Britain have been recipient of pollutant deposition in excess of their critical load compared with the 1995-97 data. In comparison to future modelled deposition loading for 2010 there is a significant decrease in the area in which the critical load is exceeded. This data is reproduced in the Table 7.1 below. The geographical distribution of these areas of exceedance can be obtained by reference to the original paper.

Nitrogen can also play a role in the acidification of soils depending on the nitrogen transformations in the soil. However, currently the emphasis of the impact of nitrogen deposition on natural and semi-natural ecosystems is its role in eutrophication.

Table 7a.1 Critical load exceedance by country and woodland habitat type calculated using deposition data for 1995-97 and predicted modelled data for 2010.

7a.2.2 Eutrophication

Nitrogen is a major nutrient in soils and eutrophication by reduced ( NH₃ and NH₄⁺) and oxidized (mostly NO₃^-) forms of N is becoming more important as sulphur emissions reduce. The extent of exceedance of critical loads of eutrophying inputs during the 1990s varied according to habitat. According to Fowler et al. (2002) the largest exceedances of critical loads for nutrient nitrogen for woodland ground flora occur across extensive areas of the lowlands in south, central and north-east Scotland. Peaty soils are also sensitive to eutrophying inputs, but do not occur extensively in areas where nitrogen deposition is high.

7a.2.3 Radiocaesium contamination

Soil contamination by radiocaesium has occurred as fallout from nuclear weapons testing and as a result of the accidental release from Chernobyl in 1986. Weapons testing has ceased. Radiocaesium from the Chernobyl accident affected areas of the uplands in Scotland when deposited during rain events. Caesium remains available for uptake by vegetation in peaty soils. Uptake by animals is highest in summer when vegetation growth is highest and grazing animals are on higher ground.

7a.3 General Policy Issues

The UK approach to gaseous emission reductions is set out by the Department of the Environment, Transport and the Regions (The Air Quality Strategy for England, Scotland, Wales and Northern Ireland, 2000). In relation to acidification and eutrophication, this strategy document uses air pollution limits in ambient air for oxides of nitrogen and sulphur dioxide taken from various European Air Quality Daughter Directives, which control emissions of sulphur and nitrogen by setting:

­ Maximum sulphur content of heavy fuel oil and gas oil

­ Emission standards for motor vehicles.

7a.4 Evidence

7a.4.1 Current Status

There is a lack of long term soil monitoring data in Scotland and the UK. However there are a small number of site manipulation studies and modelling approaches that have tried to identify the extent of the impact, particularly of atmospheric deposition of nitrogen and sulphur.

The effect of acid deposition on ecosystems in Scotland

The pH of peat across Scotland has been related to the level of acid deposition (Skiba et al., 1989). Dystrophic peats were found to have pH below 3.0 where deposition exceeded 0.8 kg H ⁺ ha ^-1 year ^-1. Additional evidence for change with time, between 1956 and 1997, in Grampian region (Miller et al., 2001) at an upland site managed as a heather dominated rough pasture receiving no fertilizer showed an increase in exchangeable hydrogen throughout all soil horizons in the order of 100-400%. The same study also reported large decreases in available magnesium and calcium in the humic layer. White and Cresser (1998) reported that there was a strong positive relationship between soil pH and rainfall pH and soil pH and deposition inputs of strong acid anions in soils developed on granitic and quartz rich parent materials across a pollution gradient in eastern Scotland. In a study of soil profiles in forests of NE Scotland, Billett et al. (1988) demonstrated that between 37 years between two sampling dates (1949 and 1986) the surface organic horizons had been acidified between 0.07 and 1.28 pH units in 80% of the sites and that at 70% of the sites the mineral horizon had been acidified below 40 cm. The authors account for this acidification as due to both base cation uptake within the forest biomass and that in the deeper mineral horizons soil acidification as a result of acid deposition inputs was a contributor.

Eutrophication in ecosystems in Scotland

Data from two Countryside Surveys (Barr et al., 1993; Haines-Young et al., 2000) was used (McGowan et al. 2001) to examine the evidence for eutrophication in Scotland. In the second survey, a greater number of species suited to fertile conditions occurred within lowland raised bogs, blanket bogs and dwarf shrub-heath across Scotland, while the number of species declined by 6-7 % in dwarf shrub-heath. Upland soils show an increased nutrient status, which has been used to explain a shift from heaths to grasslands in the uplands of Scotland. In the arctic-alpine heaths, Racomitrium lanuginosum has declined by 75 to 95 % over the last century (Thompson and Baddely, 1991). However, it is important to note that other factors such as increased competition between plants and different land management impacts such as fire? and grazing will have significant influence on the occurrence and growth of many dwarf-shrub species.

7a.4.2 Data Availability, suitability and quality:

There are a number of reviews and primary sources of material and references available that detail spatial and temporal impacts of atmospheric pollution on ecosystems. A selection of the most relevant ones is:

• The UK Air Pollution Information System ( APIS) is located at www.apis.ac.uk: APIS is a support tool for staff in the UK conservation and regulatory agencies, industry and local authorities for assessing the potential effects of air pollutants on habitats and species. As such, it aims to enable a consistent approach to air pollution assessment across the UK. The web pages consist of a database which can be queried by the user. The site does not currently allow the user to search the database by country and in that sense is not Scottish specific. Recently amended by CEH with new model for ammonia emission impacts for Scotland ( SE report unpublished 2006). However, there are four ways in which you can search the database, one of which is by location. Using this facility, together with either a known locality, National Grid Reference ( NGR), or the inbuilt NGR location map, it is possible to search the available data for further information on that site. The other methods of searching the database are by:- Habitat of which there are 32 categories ( e.g. raised bog , montane heath) which can also be matched against Biodiversity Action Plans or the Habitats Directive, Annex 1 species; Pollutant type, a choice of querying by acid deposition, ammonia, N deposition, nitrogen oxides, ozone and sulphur dioxide is available; Issue in which the user can choose from acidification, eutrophication, photochemical oxidants, accumulation of toxic substances, direct toxicity of atmospheric pollution and particulate matter. The site also contains an overview section in which the user can search by pollutant impact by issue, impact by ecosystem or legislation. Finally the site contains a glossary, unit conversion and an extensive reference list from where the original findings have been published.
• The air quality archive www.airquality.co.uk/archive/reports/reports.php?action=category&section_id=10 This site setup on behalf of the Department of the Environment, Rural Affairs Department and the Devolved Administrations contains a digest of background information on air pollution, available data from UK networks, local air quality monitoring, research information and reports.
• The National Expert Group on Transboundary Air Pollution ( NEGTAP) report www.nbu.ac.uk/negtap/docs/finalrep_web/NEGTAP_C5Soils.pdf. This work provides a comprehensive review and details the evidence and uncertainties for sulphur, nitrogen and ground level ozone and their impacts on ecosystems, including soils in the UK. The report is structured around principal chapters dealing with emissions; deposition of sulphur, nitrogen, ozone and acidity; modelling concentrations and depositions; effects on soils; effects on freshwaters; effects on vegetation; the European perspective and recovery. The chapter dealing with soils starts with a description of the process of soil acidification and the role of acid deposition. The chapter moves on to discuss soil indicators of change and the evidence base from which this has been taken. The evidence base is provided from a number of studies that includes sites and studies from Scotland. The chapter concludes with a description of the approach used to predict the impact of acid deposition on UK soils based on the principles of the United Nations- Economic Commission on Europe ( UN- ECE) Convention on Long Range Transport of Air Pollution. Under this convention it was agreed to adopt an effects based approach to international negotiations on reductions of air pollutants. The favoured approach to this requirement was to use a targeted approach based on critical loads. A critical load is defined as the highest deposition of acidifying compounds that will not cause chemical changes leading to long-term harmful effects on ecosystem structure and function. The approach and methods of calculations are presented in the references given below and included in the report.
• Hydrology and Earth System Sciences, special issue on the sustainability of UK upland forestry (2004, Volume 8, Number 3). This journal contains a number of papers (Hall et al. (2001); Hodson and Langan (1999); Hornung et al. (1995)) relating to the impacts of atmospheric pollution on forests in Scotland and the UK. These include the results of a manipulation experiment characterizing the impacts of acidity and nitrogen loading on Sitka Spruce (Sheppard et al., 2004) and the development of the acidity critical load approach to assess woodland habitats in Great Britain (Langan et al., 2004)

7a.4.3 Future trends

• For Scotland and more generally for the UK and Europe, emissions and therefore the potential impacts on soils and ecosystems from air pollution are lessening. The main references are: ( www.nbu.ac.uk/negtap/docs/finalrep_web/NEGTAP_C5Soils.pdf) and Birnie et al., (2001).
• There is little unequivocal evidence of recovery from soil acidification with most data suggesting continuing acidification into the 1990s. However there is some evidence of pH increases in upland soils in 1998/99 (compared with 1978) and on arable and grassland soils over the last 10 to 20 years, see: ( www.nbu.ac.uk/negtap/docs/finalrep_web/NEGTAP_C5Soils.pdf). The partial revisiting of the NSI in Scotland funded by SEERAD will shed further light on any trends. In addition, CS2007 should produce trend data that will inform this debate.

7a.4.4 Impactson soil functions

Biomass production

Arable crops and improved grassland require soil pH to be at certain levels for optimal growth - pH 6.0-6.2 for arable crops and pH 5.6-5.8 for improved grassland. Clearly soil acidification will affect these values, but management intervention through liming can control pH. Most cultivated Scottish soils are maintained at artificially high pH values and liming is a standard part of farm practice. However financial pressures on farming may lead to lack of maintenance in soil pH values on cultivated land.

Most woodland species and semi-natural plant species are tolerant of acid conditions and will not be affected by soil acidification. Only some broadleaved species such as ash that favour more base-rich conditions may be affected.

Environmental interactions

Soil acidification contributes to surface water acidification so in this respect there is a negative interaction; this is probably the biggest impact on any soil function. However there are encouraging indications that recovery is taking place (McCartney et al., 2003). Soil acidification can also have a deleterious influence on the ability of soils to buffer other pollutants such as heavy metals.

Biodiversity

There is limited evidence that soil acidification or eutrophication has had an effect on the range of semi-natural habitats that Scottish soils support; indeed increased acidity will if anything help any initiative to reinstate habitats that favour acid conditions. The biodiversity within the soil may be changed subtly by changes in soil pH, but that is not to say that a change is necessarily negative or positive. Changes in soil microbial biomass and microbial activity were reported in a simulated nitrogen input experiment (Johnson et al., 1998). The results of this study showed significant increases in microbial biomass and activity in nitrogen-limited heathland. The soil under acidic grassland showed a decrease in microbial biomass and there was no change at the calcareous grassland site.

Soil acidification generally has little impact on the provision of a platform, provision of raw materials and protection of cultural heritage functions.

7a.4.5 Related threats

Soil acidification interacts with the threat of contamination by increasing the potential for pollutants to be mobile in more acid soils. There are potentially strong linkages between future emissions and impacts that atmospheric pollution may have under varying scenarios of climate change.

7a.5 Conclusions

Scotland contains large areas of soils derived from acid parent materials. These soils have inherently low base status and have been affected by acid deposition. The effects include removal of base cations and their replacement on the exchange complex with hydrogen or aluminium. Low weathering rates and the potential for desorption of sulphates mean that these affected soils will take of the order of decades to recover to pre-industrial revolution levels, if they ever will. These soils will remain in this acid sensitive state for a long period even under the current régime of reduced emissions. Evidence for increased eutrophication appears to suggest that a wider range of species are colonizing affected peat soils. The amounts of nitrogen deposition have not been reduced by the same proportion as those of sulphur, so nitrogen deposition remains a threat to upland soil quality.

7a.6 References

Barr, C.J., Bunce, R.G.H., Clarke, R.T., Fuller, R.M., Firse, M.T., Gillespie, M.K., Groom, G.B., Hallam, C.J., Hornung, M., Howard, D.C. and Ness, M.J. (1993) Countryside /survey 1990 Main Report. Department of the Environment, London.

Billett M.F., FitzPatrick E.A. and Cresser M.S. (1988) Long term changes in the acidity of forest soils in north east Scotland. Soil Use and Management 4, 102-107.

Birnie R et al (2002) The Land Resources of Scotland: trends and prospects for the environment and natural heritage. The State of Scotland's Environment and Natural Heritage (eds. Usher, Emmett, B.A. (2002)

The Impact of Nitrogen Deposition in Forest Ecosystems: A Review. CENTRE FOR ECOLOGY AND HYDROLOGY (Natural Environment Research Council) CEH Project No: C00311 DEFRA Terrestrial Umbrella Phase II

Fowler, D., Dragosits, U., Pitcairn, C., Sutton, M., Hall, J., Roy, D. and Weidemann, A. (2002) Deposition of acidity and nitrogen and exposure of terrestrial surfaces to ozone in Scotland; mapping critical loads, critical levels and exceedances.. Scottish Natural Heritage Research, Survey and Monitoring Report No. 169.

Haines-Young, R.H., Barr, C.J., Black, H.I.J., Briggs, D.J. Bunce, R.G.H., Clarke, R.T., Cooper, A., Dawson, F.H., Firbank, L.G., Fuller, R.M., Furse, M.T., Gillespie,

M.K., Hill, R., Hornung, M., Howard, D.C., McCann, T., Morecroft, M.D., Petit, S., Sier, A.R.J., Smart, S.M., Smith, G.M., Stott, A.P. Stuart, R.C. and Watkins, J.W. (2000) Accounting for Nature: Assessing Habitats in the UK Countryside. Department of the Environment, Transport and the Regions, London

Hall, J., Hornung, M., Kennedy, F., Langan, S.J, Reynolds, B. and Aherne, J. (2001) Investigating the uncertainties in the Simple Mass Balance equation for acidity critical loads for terrestrial ecosystems. Water, Air and Soil Pollution: Focus 1: 43-56.

Hodson, M.E. and Langan, S.J. (1999) Consideration of uncertainty in setting critical loads: the role of weathering rate determination. Environmental Pollution, 106, 73-81

Hornung, M., Bull, K., Cresser, M., Hall, J., Langan, S.J., Loveland, P. and Smith, C. 1995. An empirical map of critical loads for soils in Great Britain, Environmental Pollution, 90, 301-310

Johnson, D., Leake, J.R., Lee, J.A. and Campbell, C.D. (1998) Changes in soil microbial biomass and microbial activities in response to 7 years simulated nitrogen pollutand nitrogen deposition on a heathland and two grasslands. Environmental Pollution 103, 239-250.

Langan, S.J., Hall, J., Reynolds, B., Broadmeadow, M., Hornung, M. and Cresser, M.S. (2004) The development of an approach to assess critical loads of acidity for woodland habitats in Great Britain.

McGowan, G.M., Palmer, S.C.F., French, D.D., Barr, C.J., Howard, D.C. and Smart, S.M. (2001) Trends in broad habitats: Scotland 1990-1998. Unpublished report.

Langan, S. and Wilson, M.J. 1994. Critical loads of acid deposition on Scottish soils. Water, Air and Soil Pollution, 75 177-191.

Miller, J.D., Duff, E.I., Hirst, D., Anderson, H.A. and Bell, J.S. (2001) Temporal change in soil proerties at an upland Scottish site between 1956 and 1997. The Science of the Total Environment 265, 15-26.

Nilsson, J. and Grennfelt, P. (1988) Critical Loads for Sulphur and Nitrogen. Report from a Workshop held at Skokloster, Sweden in March 1988, published by Nordic Council of Ministers, Copenhagen.

Skiba U., Cresser M.S., Derwent R.G. and Futty D.W. (1989) Peat acidification in Scotland. Nature 337, 68-69.

Thompson, D.B.A. and Baddely, J.A. (1991). Some effects of acidic deposition on montane Racomitrium lanuginosum heaths. In The Effects of Acid Deposition on Nature Conservation in Britain, ed. By S.J. Woodin and A.M. Farmer. Nature Conservancy Council, Peterborough. pp. 17-28.

White C. C., Smart R. P. and Cresser M. S. (1998) Effects of atmospheric sea-salt deposition on soils and freshwaters in northeast Scotland. Water Air and Soil Pollution 105, 83-94.

7a.7 Further Reading

Ashmore MR, Bell JNB, Reily CL. (1978) A survey of ozone levels in the British Isles using indicator plants. Nature, 276: 813-815.

Mackey and Curran) pp 41-81. (2002) The Stationery Office Edinburgh.

Billett M.F., Parker-Jervis F., FitzPatrick E.A. and Cresser M.S. (1990) Forest soil chemical changes between 1949/50 and 1987. Journal of Soil Science 41, 133-135.

Black K.E., Lowe J.A.H., Billett M.F. and Cresser M.S. (1993) Observations on the changes in nitrate concentrations in seven upland moorland catchments in north eastern Scotland. Water Research 27, 1195-1199.

Carreira J.A., Harrison A.F., Sheppard L.J. and Woods C. (1997) Reduced soil P availability in a Sitka spruce ( Picea sitchensis (Bong) Carr) plantation induced by applied acid-mist: Significance in forest decline. Forest Ecology and Management 92, pp53-166.

Leith ID, Sheppard LJ, Pitcairn CER, Cape JN, Hill PW, Kennedy VH, Tang YS, Smith RI, Fowler D (2001) Comparison of the effects of wet N deposition (NH ⁴Cl) and dry N deposition (NH ³) on UK moorland species. Water Air Soil Pollution 130 (1-4): 1043-1048.

Ling K.A., Power S.A. and Ashmore M.R. (1993) A survey of the health of Fagus sylvatica in southern Britain. Journal of Applied Ecology 30, 295-306.

McCartney, A.G., Harriman, R., Watt, A.W. Moore, D.W., Taylor, E.M., Collen, P. and Keay, E.J. (2003) Long-term trends in pH, aluminium and dissolved organic carbon in Scottish fresh waters; implications for brown trout ( Salmo trutta) survival. The Science of the Total Environment 310, (1-3): 33-141.

Pitcairn, C. E. R., Fowler, D., Leith, I. D., Sheppard, L. J., Sutton, M. A., Kennedy, V. and Okello, E. (in press) Bioindicators of enhanced nitrogen deposition. Environmental Pollution.

Power S.A., Ashmore M.R., Cousins D.A. and Sheppard L.J. (1998b) Effects of nitrogen addition on the stress sensitivity of Calluna vulgaris. New Phytologist 138, 663-673.

Uren S.C., Ainsworth N., Power S.A., Cousins D.A., Huxendurp L.M. and Ashmore M.R. (1997) Long term effects of ammonium sulphate on Calluna vulgaris. Journal of Applied Ecology 34, 208-216.

Sanger L.J., Billett M.F. and Cresser M.S. (1993) Assessment by laboratory simulation of approaches to amelioration of peat acidification. Environmental Pollution 81, 21-29

Sheppard L.J., Crossley A., Cape J.N., Harvey F., Parrington J. and White C. (1998) Early effects of acid mist on Sitka spruce planted on acid peat. Phyton (Horn Austria), 39: 1-25.

Sheppard, S. C. (1992) Summary of phytotoxic levels of soil arsenic. Water, Air, and Soil Pollution 64, 539-550.

Skiba U., Sheppard L.J., MacDonald J. and Fowler D.(1998a) Some key environmental variables controling nitrous oxide emissions from agricultural and semi-natural soils in Scotland. Atmospheric Environment 32, 3311 - 3320.

Skiba U., Sheppard L.J., Pitcairn C.E.R., Leith I., Crossley A., van Dijk S., Kennedy V.H. and Fowler D. (1998b) Soil nitrous oxide and nitric oxide emissions as indicators of the exceedance of critical loads of atmospheric N deposition in seminatural ecosystems. Environmental Pollution 102, 457 - 461.

Smith C. M. S., Cresser M. S. and Mitchell R. D. J. (1993) Sensitivity to Acid Deposition of Dystrophic Peat in Great Britain. Ambio 22, 22-26.

White C. C., Cresser M. S. and Langan S. J. (1996b): The importance of marine-derived base cations and sulphur in estimating critical loads in Scotland. Science of the Total Environment 177, 225-236.

White C.C., Dawod A.M. and Cresser M.S. (1996) Nitrogen accumulation in surface horizons of moorland podzols: evidence from a Scottish survey. Science of the Total Environment 184, 229-237.

White C.C., Dawod A.M., Cruickshank K., Gammack S. and Cresser M.S. (1995) Evidence for acidification of sensitive Scottish soils by atmospheric deposition. Water, Air and Soil Pollution 85, 1203-1208.

Yesmin L., Gammack S.M. and Cresser M.S. (1996) Medium term response of peat drainage water to changes in nitrogen deposition from the atmosphere. Water Resources 9, 2171-2177.

Yesmin L., Gammack S.M. and Cresser M.S. (1996b) Effects of atmospheric nitrogen deposition on ericoid mycorrhizal infection of Calluna vulgaris growing in peat. Applied Soil Ecology 4, 49-60.

7 b Soil contamination by pesticides

Use of pesticides in agriculture and forestry and accidental spillages can contaminate soils and water courses. This section presents and assesses the evidence of the threat from pesticides to soil function.

7b.1 Summary

• Pesticides are controlled by a strict regulatory framework within both the EU and UK.
• UK Ministerial approval for use is under scientific advice from The Advisory Committee for Pesticides.
• Although a limited number of pesticides in rivers and groundwater are monitored, data sets are not complete and pesticide concentrations or exceedances of quality standards are not regularly published.
• Evidence from England and Wales in an experiment with five pesticides shows no adverse effects on crop productivity or on microbial biomass or its activity, as assessed by C or N mineralization.
• Evidence on the effect of an organophosphate pesticide on soil microbial biomass from four sites throughout Scotland was inconclusive.
• Pesticide usage surveys in Scotland have been used in the development of a screening tool for pesticides. There are several separate surveys undertaken but it would be of considerable benefit if they were to be combined to give a common dataset.

7b.2 Introduction

Pesticides are widely used within agriculture to control or eliminate unwanted insects, fungi and weeds, thereby improving crop yield, quality and reliability. Pesticides are biocides and therefore highly toxic substances. They are used within a strict regulatory framework which should ensure that contamination of soils and water from pesticide usage does not occur. Nonetheless there are cases of misuse and spillages which do result in pollution incidents prosecuted by SEPA.

Pesticides have a range of potential impacts on soil functions, but the most likely threats are to:

Environmental interaction

Pesticides reaching surface or ground waters through leaching, drainflow, surface runoff or spray drift may impact on the quality of surface waters, but pesticide usage should have no significant impact on the filtering function of the soil itself, unless the absorption capacity of a soil for a particular substance is exceeded. This latter situation is unlikely to occur as it would only result from long periods of application at rates considerably in excess of approved usage rates. Residues in the soil could potentially impact on microbial decomposition processes in the soil and thus have some impact on soil carbon storage.

Support for ecological habitats and diversity

Pesticide usage is aimed at eliminating target plant, insect or fungal species, thereby deliberately reducing above ground diversity. There are also potential threats to non-target organisms.

Biomass production

Use of pesticides is aimed at increasing biomass production directly. The only potential threat to biomass production would be from residues building up in the soil to toxic concentrations.

Pesticides are unlikely to have any impact on the soil's ability to provide raw materials, act as a platform for built infrastructure or protect cultural heritage.

7b.3 Policy

The regulatory framework for the use of pesticides is enshrined in various pieces of Scots and UK legislation (The Water Environment and Water Services (Scotland) Act 2003; The Food and Environmental Protection Act 1985; The Control of Pesticides Regulations 1986; The Plant Protection Products Regulations 1995; The Plant Protection Products (Basic Conditions) Regulations 1997; The Pesticides (Maximum Levels in Crops, Food and Feedingstuffs) (Scotland) Regulations 2000) and, in the case of sheep dips, the Veterinary Medicines Regulations 2005. The major driver for policy is, however, now EU directive 91/414/EEC, the 'Authorisations Directive' which provides the framework within which pesticides are authorised for use across the European Union.

Approval of pesticides for use in the UK is granted by UK Government ministers. Ministers are guided by advice from an independent scientific advisory committee (The Advisory Committee for Pesticides) which bases its advice on an assessment by scientists from DEFRA's Pesticides Safety Directorate of a data package provide by the approval holder (normally the manufacturer. The assessment includes calculation of likely concentrations of the pesticide and its metabolites in the soil and in surface and ground water. Calculations are based on field and laboratory determinations of persistence and mobility and computer models and field trials of loss from the soil and movement to surface and ground waters. Predicted concentrations in soil and water are compared with toxicity thresholds for various organisms and approval is only granted where there is an assurance of no significant environmental damage.

The Directive and Scots and UK legislation provide the framework under which pesticides are approved and used and they are reinforced by a wide variety of guidelines, covering both agricultural uses of pesticides and other uses. These guidelines include the Prevention of Environmental Pollution From Agricultural Activities Code Of Good Practice ( PEPFAA Code) (Scottish Executive, 2005), The Use of Herbicides in the Forest (Forestry Commission, 1995) , The Safe Use of Pesticides for Non-agricultural Purposes (Health and Safety Commission, 1991) and The Groundwater Protection Code: Use and disposal of sheep dip compounds (Defra, 2001)

7b.4 Evidence

Current status

EU Directives, and associated UK and Scottish legislation should ensure that approved pesticides do not cause environmental and ecological harm. Soil plays an important role in mediating the impacts of pesticides on groundwater through adsorption and immobilisation of active substances; however adsorption within the soil may lead to the build-up of pesticide residues detrimental to soil quality. In one long-term experiment in England, five pesticides were applied in various combinations over a 17 year period and no adverse effects on crop productivity were found (Bromilow et al., 1996). The continuous use of the same pesticides, either singly or in combination, also had no measurable long-term harmful effects on the soil microbial biomass or its activity, as assessed by C or N mineralization (Hart and Brookes, 1996). In Scotland, synthetic pyrethroid was applied to plots at four sites and organophosphate was applied at two of these sites in the spring of 2002 (Aitken, et al., 2002). The soils were sampled between 30 and 62 days later. All the soils had similar loamy textured topsoils (sandy clay loam, sandy loam or sandy silt loam) and only one was described as having a high organic matter content. Analyses of the soils showed that the pesticides had degraded and that there had been little adverse effect on the soil microbial biomass, mineralisable nitrogen or soil respiration rate though there may have been a weak indication of an adverse effect by the organophosphate on the soil as measured by the Biolog. However, greater differences were attributed to soil variability than to the effects of the applied pesticides. Thus, there is little evidence of pesticides having a detrimental effect on the biomass production or microbial biodiversity function of the soil.

Approved pesticides should also not contaminate surface and ground waters, although there appear to be few independent data to confirm this. The Environment Agency of England and Wales has a programme of routine monitoring of pesticides in surface and ground waters ( http://www.environment-agency.gov.uk/yourenv/eff/1190084/business_industry/agri/pests/).Under this programme, concentrations of a wide range of pesticides are monitored normally 4 times per year at more than 1,500 freshwater sites. The annual reports based on monitoring does reveal some exceedance of environmental quality standards but the pesticides implicated are often either no longer used or about to be withdrawn. This monitoring raises no serious concerns over the use of pesticides arising from agricultural practices in England and Wales. In Scotland, SEPA monitors some 12 pesticides in groundwater at 248 sites across Scotland and a smaller number at the 58 harmonised monitoring sites on Scottish rivers. However, data sets are often incomplete and there are no published reports of exceedance of environmental quality standards or of trends in concentrations for Scotland.

Anthony et al. (2006) modelled the risk of applied pesticides reaching water courses in sufficient quantities to impact on aquatic life. One of the intermediate stages in this modelling work was to identify the annual load of pesticides applied to land in Scotland based on estimates of crop type taken from SEERAD's Agricultural and Horticultural census data and pesticide usage surveys. The data were summarised to a resolution of 1 km ² grid cells. The modelled out put was divided into 2 groups: Priority substances that are toxic and persistent in the environments including Atrazine, Chlorpyrifos, Endosulfan, Isoproturon, Simazine and Trifluralin and, secondly, the remaining pesticides in common use, grouped according to the action and leaching potential of the active ingredient.

7b.4.1 Data availability

Pesticide usage in Scotland is monitored by the Scottish Agricultural Science Agency (Table 1) with arable surveys reported biennially and all others reported every four years since 1990. As the timing of surveys varies between land use type it was not possible to gather all data for a single reference year for use within the diffuse pollution screening tool (Anthony et al., 2006). Data were available separately on the application of pesticides to arable crops (Kerr and Snowden, 2001), grassland and fodder crops (McCreath, 1998), sheep dip (Thomas, 1998) and aerial spraying for bracken control (Wardman and Thomas, 1998). These reports summarise the quantities of active ingredient applied by crop type and the percentage of crop receiving each pesticide. These usage surveys are designed to provide an estimate of national pesticide usage and there are inherent statistical errors when these data are disaggregated using agricultural census information and presented at a 1 km ² resolution. Anthony et al. (2006) also caution that the pesticide usage data is dated and there may have been changes in their since the survey was completed.

Table 7b.1 Year of pesticide usage surveys in Scotland (from Anthony et al., 2006).

Loadings for each 1 km ² grid cell were calculated based on the surveys of pesticide usage, the recommended application rate per treatment and the percentage of the cropping area that received one or more treatments (Table 2 and figure 1). The map clearly shows that the majority of pesticide usage is concentrated in the arable areas of eastern Scotland with more limited usage in the south west which is dominated by improved grassland.

Figure 7b.1 The distribution of total pesticide loading in Scotland based on land cover and agricultural census data (from Anthony et al., 2006)

Table 7b.2 Modelled total annual loads of Priority Substances and selected pesticides applied in Scotland (from Anthony et al., 2006).

7b.4.2 Gaps in data or evidence

Pesticide concentrations in surface and ground waters in Scotland are monitored but the results are not systematically published. However, in the light of the data published for England and Wales, it is unlikely that these data will indicate any significant threat to waters in Scotland.

The evidence of minimal reduction in soil microbial communities (Hart and Brookes, 1996) could be strengthened with further work.

7b.5 Future trends

Pesticide usage is unlikely to increase in the short to medium term and is likely to become more stringently regulated as food safety becomes an increasingly important issue.

7b.6 References

Aitken, M., Sym, D.G., Douglas, J.T., Campbell, C.D and Burgess, S.D.J. (2002) Impact of industrial wastes and sheep dip chemicals applied to agricultural land on soil quality. Final report to SEPA.

Anthony, S., Betson, M., Lord, E., Tagg, A., Panzeri, M., Abbott, C., Struve, J., Lilly, A., Dunn, S., DeGroote, J., Towers, W. and Lewis, D. (2006) Provision of a screening tool to identify and characterise diffuse pollution pressures: Phase II. Final Report WFD19 (230/8050. Sniffer .

Kerr, J. and J. P. Snowden (2001) Arable Crops 2000. Scottish Agricultural Science Agency, East Craigs, Edinburgh, pp. 84.

McCreath, M. (1998) Grassland and Fodder Crops (1997) Scottish Agricultural Science Agency, East Craigs, Edinburgh, pp. 63.

Thomas, L. A. (1998) Pesticide usage in Scotland: Sheep (1996) SASA, East Craigs, Edinburgh.

Wardman, O. L. and M. R. Thomas (1998) Aerial Applications, UK, (1997) Pesticide Usage Survey Report No. 149, CSL, York.

The Control of Pollution Act 1974

The Food and Environmental Protection Act 1985

The Control of Pesticides Regulations 1986

The Plant Protection Products Regulations 1995

The Plant Protection Products (Basic Conditions) Regulations 1997

The Pesticides (Maximum Levels in Crops, Food and Feedingstuffs) (Scotland) Regulations 2000

Prevention of Environmental Pollution From Agricultural Activitiy Code ( PEPFAA). SEERAD, 2005

EU directive 91/414/EEC, the 'Authorisations Directive'

The Use of Herbicides in the Forest (Forestry Commission, 1995)

The Safe Use of Pesticides for Non-agricultural Purposes (Health and Safety Commission, 1991).

The Veterinary Medicines Regulations 2005 SI 2745

The Groundwater Protection Code: Use and disposal of sheep dip compounds (Defra, 2001)

7c Anthropogenic Pollution by Heavy Metals

This section describes current data sources of heavy metal levels in Scottish soils and the impacts associated with heavy metal addition to soil through atmospheric deposition or organic waste recycling.

7c.1 Summary

• Heavy metals occur naturally in the environment and some fulfil a number of key functions in relation to plant growth and health.
• Some soils have naturally high levels of metals, for example ultrabasic rocks and some support plants and communities of high conservation value.
• Atmospheric deposition of heavy metals is generally low over Scotland and is predicted to decline.
• Organic wastes including sewage sludge are one of the prime anthropogenic sources of heavy metal inputs into soils and their application is highly regulated. The recycling of other wastes with elevated metal levels either follow voluntary codes (farmyard manure and slurry application on the farm at which the waste was produced) or under exemptions within the framework of the Waste Management Licensing Regulations. Under the second scenario there is a requirement that the waste brings either agricultural or ecological improvement. Exceedence of the metal limits set out in the sewage sludge regulations can be used as evidence by the regulatory authorities for the lack of improvement.
• Sewage sludge, and other organic waste recycling to land, is projected to continue and a watching brief on the results from the sewage sludge network is highly recommended. There is emerging evidence that relatively low metal concentrations, of Cu but particularly of zinc, at levels below the current regulatory threshold, are having a significant impact on biological fertility.

7c.2 Introduction and Description of Threat

Heavy metals occur naturally in soils as a lithogenic signature inherited from the geological parent material from which the soil has formed. Where these natural background levels are enhanced by additions, damage to the soil can occur. There are two main ways that added metals can enter the soil. The first of these is by atmospheric deposition whilst the second is by the application of fertilizers and certain organic wastes such as pig slurry, sewage sludge (Nicholson et al., 1993) and wastes generated by specific industries e.g. distilling. Of these wastes, only sewage sludge is regulated in terms of heavy metal additions to soil and only then for addition to soil for agricultural purposes. Some other wastes have much higher concentrations of specific metals (Towers and Campbell 1998). In the case of other wastes, however, SEPA does in practice have good grounds for refusing to register exemptions under Waste Management Licensing if the limits for metal concentrations in soil stated in the Sludge Regulations were likely to be exceeded. SEPA could argue that the waste would damage soil quality, which would breach the requirement under Waste Management Licensing Regulations for the party applying the waste material to demonstrate the waste brings 'agricultural improvement' or 'ecological improvement'.

Metal addition to land by waste recycling is much more confined than that resulting from atmospheric deposition and nowadays is primarily targeted onto cultivated agricultural land. In the past, wastes were applied in some areas to semi-natural vegetation to improve the sward quality for grazing, but this practice has ceased. More recently, sewage sludge has been applied to forestry and in land restoration projects but there have been some concerns about application rates and techniques. The most obvious damage has occurred to water courses as a result of over application and although no damage to soil has been reported, any effects may be occur on the longer term.

Sewage sludge to land is very well regulated and documented in terms of volume and quality of the sludge and the quality and location of the receiving soil. Much less is known about the other wastes with enhanced levels of heavy metals although the SEERAD June census data may provide some indication of the likely location and volumes of application of specific wastes such as pig slurry. In addition some activities involving use of waste materials for treatment of soil for agricultural benefit or ecological improvement qualify for a registered exemption from licensing if they meet the requirements detailed in Regulations 7, 9 and 19 and Schedule 3 of the Waste Management Licensing Regulations 1994 (as amended). As a result of amendments to the Regulations in 2003 and 2004, some exempt activities, including all those relating to recovery of waste on land, are now required to keep records, for a period of at least two years, of the quantity, nature, origin, destination and method of recovery or disposal of all waste used in connection with an exempt activity. These records have either to be submitted to SEPA or made available on request. SEPA therefore has records which includes soil and waste analysis of all such exempt activity in Scotland.

Records relating to exempt waste applications to land held by SEPA often contain heavy metal concentration data for wastes and soils. As applications for waste management licensing exemptions must demonstrate that 'agricultural improvement' or 'ecological improvement' will occur as a result of the disposal of the waste to land, the limits for metals in soils quoted in the Sludge (Use in Agriculture) Regulations (1989) are often used by SEPA as guideline values to assess whether the waste application is likely to have negative effects on soil quality.

The first real indication that heavy metals may be impairing soil function came in the late 1980s and early 1990s when there was some evidence that heavy metals from sewage sludge were causing damage to the soil microbial population. While regulations exist for the amount of heavy metals that can be introduced into agricultural soils through the application of sewage sludge, these are based on known effects on plant growth and productivity. In the late 1980s and early 1990s, there was some evidence that heavy metals from sewage sludge were causing damage to the soil microbial population and its functioning. This culminated in the production of the 'Bradshaw Report' (1993) which recommended the establishment of a series of experimental sites where different sludges would be applied under standard protocols and a set of standard analysis undertaken on the receiving soils. Previous studies in England showed that following long-term application of sewage sludge, accumulated heavy metals had a marked effect on the microbial population and diversity of soil function. In particular, the survival of effective rhizobia which trap atmospheric nitrogen in soils was reduced and the strains which adapted to the increased heavy metal concentrations were ineffective. This was considered to have a long term consequence on the fertility of the soil.

Other potential consequences of elevated soil heavy metal levels are perturbation of the C and N cycle, soil respiration and reduction in functional diversity. Heavy metals are not known to have any impact on the protection of cultural heritage but depending on soil metal concentrations, there is a small chance that there may be a negative interaction on the provision for building and raw material functions.

Erosion of soil with enhanced levels of metals and subsequent transport to water courses will pose greater risks than uncontaminated soil. The same general statement also applies to soils that are subject to landslides or at risk from flooding (see Chapter 6)

7c.3 General Policy Issues

Sewage sludge is considered a waste. It may, however, be applied to agricultural land under the Sludge (Use in Agriculture) Regulations (1989) and the Sludge (Use in Agriculture) (Amendment) Regulations 1990, which enforce the provision of EC Directive 86/278/EEC. These set maximum annual applications for metals contained in sludge and maximum permitted metal concentrations in agricultural soil treated with sludge. Guidelines for the use of sewage sludge on agricultural land are given in the Code of Practice for the Agricultural Use of Sewage Sludge ( HMSO, 1996). In Scotland these are summarised in the PEPFAA code of good practice, where reference is made to the recommendation of the RCEP (1996) that untreated sludge is not used on agricultural land. Sewage sludge may also be applied to non-agricultural land under a paragraph 8 Waste Management Licensing Exemption, but the operator must demonstrate the sludge will bring 'ecological improvement' in order to comply with the Regulations. The Manual of Good Practice for the Use of Sewage Sludge in Forestry (Forestry Commission, 1992) (Forestry Commission Bulletin 107) also endorses the applicability of these regulations in the forestry industry. This document has been recently updated by Moffat (2006) and has been extended to include the use of sludge in land reclamation.

7c.4 Evidence

7c.4.1 Evidence of current status

Unlike a number of the other pollutants, there is relatively good information on the background levels of heavy metals in Scottish soils. The best source of information is the SEPA publication on background levels of pollutants in Scottish soils (Paterson et al., 2002, ( http://www.sepa.org.uk/pdf/publications/reports4sepa/contaminants_scottish_soils.pdf).

Data exist for 19 metals, from the National Soils Inventory for Scotland, and summaries for cadmium, chromium, lead, nickel, copper and zinc appear in Paterson et al. (2002). The values for all the metals are lower than those in the equivalent datasets for England, Wales and Northern Ireland.

Data on heavy metal concentrations in soils are also available from Countryside Survey 2000 (CS2000). Comparison of median values for selected metals from the NSIS and CS2000 datasets are remarkably similar considering the different structure of the sampling grid and the details of how the soils were actually sampled (Table 7c.1).

Table 7c.1 Heavy metal concentrations (mg kg ^-1) in soils of Scotland from NSIS and CS2000 data (median values).

Soil pollution transects (Fig. 7c.1 http:/www.macaulay.ac.uk/tipss) are a third source of data. Although fewer in number, transects provide a useful indication of metal concentrations in soils relative to built up areas; namely, upland uncultivated land where inputs are more likely to be from atmospheric deposition. Cadmium and lead have higher values in the two southern transects than in the other two transects whereas the other elements have by far the highest concentrations (Table 7c.2) in the Central Lowlands transect . This is a clear indication that intense industrial and other activities in the Central Belt are the prime source for the diffuse anthropogenic background for these contaminants. The data for Fe confirm this conclusion and identify iron and steel making as a probable major contributor to the anthropogenic inputs of Cr, Cu, Ni and Zn.

Figure 7c.1 Location of sampling points on Scottish soil transects relative to built-up areas ( www.macaulay.ac.uk/tips s).

There is little information on the background concentrations of Hg in Scottish soils, mainly as result of inadequate analytical methods for the low background concentrations found in soils. Recent improvements in analytical methodologies make it feasible to measure Hg concentrations in soils and a systematic survey would be beneficial in establishing baseline levels for any evaluation of Hg accumulation in soils. Although major emissions of Hg occur in the tropics, the metal migrates to cooler latitudes (a "distillation" effect) and is now present throughout the globe. Scottish soils could therefore in theory be vulnerable to deposition of Hg but there is little evidence if any for diffuse contamination of Scotland's soils. The chemistry of Hg is complex and its toxicity highly dependent on chemical form. Mercury deposited from the atmosphere is generally inorganic but microbial processes in soils can convert Hg into highly toxic methylmercury. Similarly, there are no systematic data on arsenic levels which is potentially toxic at low concentrations in soil and water. In the UK arsenic contamination is associated primarily with tin mining which is not widespread in Scotland. However, elevated levels have been found in small localised areas due to the long term use of seaweed as a soil amendment (Castlehouse et al., 2003).

Table 7c.2. Mean values and ranges (mg kg ^-1) for a range of heavy metals in the four transects across Scotland.

7c 4.2 Availability of data for monitoring trends

There are a number of datasets that might be used for this purpose and include:

• NSIS (n = 721). This has already been done in England and Wales where there is an apparent decline in most metals. However it is considered that a major factor in this is that the changes are linked to changes in analytical techniques and may not reflect real change.
• CS2000. Around 400 soil samples from approximately 100 separate 1 km squares in Scotland (H. Black, pers. comm.) were analysed for seven heavy metals as part of CS2000.
• The TIPPS transects (n = 29).
• The sewage sludge network sites (n = 2). As already described, these have devised to specifically monitor metal build-up in soils and the microbiological response to that.
• The Soil and Herbage study (n = c.30). This dataset is due to be available soon and will contain information on heavy metal levels in soil and herbage.
• The Environmental Change Network ( ECN) sites (n = 3).

7c.5 Evidence of threat and damage

Modelling

Estimates of atmospheric inputs have been modelled by MacDonald et al. (2001) and there is a clear differentiation between the relatively high deposition rates in the Central Belt and the lower rates over much of the Highlands. Deposition rates in the Southern Uplands fall between these two extremes indicating 'import' of metals from other parts of Europe, including England. It is fortuitous that the soils more sensitive to heavy metal addition are broadly co-incident with lower levels of deposition (Anthony et al., 2006). These findings have been supported to a large degree by Ashmole et al. (2001) who have developed a critical load approach to heavy metals. This suggests that critical loads have not been exceeded for cadmium anywhere or for lead at the upper critical limit of 8 µg l ^-1. However, the critical load and critical limit have been exceeded in some areas at the lower critical limit for lead (2 µg l ^-1). It should be stressed that these results are based on modelling. As such they cannot be viewed as evidence of damage but are indicative of where the greatest likelihood of damage may occur based on current understanding of processes.

Deposition of metals from the atmosphere is in decline (MacDonald et al., 2001) and it is likely that this trend will continue.

Experimental studies

In the Leadhills area of Southern Scotland, near the site of former smelting and mining (Schön and Paterson, in preparation), the metal contents of the soils are considerably enhanced compared with those obtained for similar soils outwith the immediate area of mining and smelting. The levels of contamination with lead, zinc and copper are extremely high in the alluvial soils in the floodplain of the Glengonar Water (up to 22,000 mg kg ^-1) but high levels, particularly of lead, are also observed in soils from higher ground around a local smelter. These high levels of lead in the environment have been linked to a number of biological impacts including elevated levels of lead in the blood of the local population, particularly children, signs of chronic lead poisoning in fish stock from the headwaters of the Glengonnar Water and the suggestion that lead in the local environment may lead to a skeletal disorder of lambs grazing contaminated pastures. Whilst there is no direct evidence of damage to the soil per se, there are significant and serious biological impacts.

A major study was started in 1993 (referred to in Section 7c.2) on the effect of sewage sludge on soil fertility. A consortium of funders ( DOE, MAFF, UKWIR, The Welsh Office and SOAEFD) commissioned a group of organisations ( ADAS, WRc, IACR Rothamsted, MLURI and SAC) to set up nine experimental sites in England, Scotland and Wales. The same sewage sludges, naturally high in copper, zinc or cadmium were applied at specific rates so that a range of soil metal concentrations, relative to the current regulatory limit were achieved. The two sites in Scotland (Hartwood and Auchincruive) represent typical grassland. The sites were chosen to cover a range of soil types from well-drained sandy soils to the poorly-drained and fine textured soil at Hartwood.

The effects of the metal additions on soil microbial activity (biomass carbon, soil respiration and rhizobia) were key to this study. The general trend across all sites was for no effect of metals on the soil respiration rate suggesting there was no metal-induced microbial stress. On the other hand a significant decrease in microbial biomass-carbon was observed at Hartwood for the Cu treatments. This effect was also seen at some, but not all, of the other sites. This decrease can be interpreted as copper toxicity to the microbial population. The same effect has been observed at some sites for the Zn treatments but these did not include the Scottish sites at Hartwood or Auchincruive.

The most significant finding of the study has been the elimination of rhizobia in the high zinc treatment plots at Hartwood and signficant reductions in other Zn treatment plots (Fig. 7c.2). This reduction in rhizobia numbers, since the first additions of the Zn-containing sludges, appears now to be progressive with time. The y-axis of Fig 7c.2 has a logarithmic scale so the reductions in rhizobia numbers were by several orders of magnitude. The rhizobia numbers in the Zn 3 treatment have been reduced below detection limits by 2003. In addition, clover plants grown in the affected soils were unable to nodulate and fix nitrogen. Decreasing rhizobia numbers have also been observed in the long term build up Zn treatments (LT Zn) which has Zn concentrations of 86 mg kg ^-1. If this trend were to continue then complete loss of rhizobia might occur on all the soils with Zn elevated by sludge addition

Figure 7c.2 Trend in Rhizobia numbers ( MPN) with time in control plots and plots amended with sludge with Zn at different elevated concentrations in soil.

The reason for the drastic elimination of rhizobia in some of the plots cannot be stated with certainty but the body of evidence is that the presence of Zn added with sludge was the main factor. All plots at Hartwood were successfully maintained at pH 5.8 so the relatively low soil pH could not be an explanation for zinc availability per se. Although most of the treatments received large doses of organic matter at the start of the experiment, this was not the case for the LT Zn treatment which received only a small addition of sludge each year. Furthermore, the Cd-rich sludge treatment (Cd4), with highest coincident elevated Zn levels, also showed signs of reducing Rhizobium numbers (data not shown) and these sludges had a different quality of organic matter. Consequently, the only common factors between the Zn 1, Cd 4 and LT Zn treatments seem to be the inherent soil characteristics and the common Zn concentration. Although the reduction in rhizobia numbers has been most marked at Hartwood, similar trends are now being/have been observed at some of the other eight sites ( www.ukwir.org/news/figures/03sl014%20exec%20summ.doc) so the inherent soil characteristics are probably not a factor in the rhizobia toxicity but in the relative rates at which rhizobia numbers drop. The added Zn is therefore the most likely agent of rhizobia toxicity.

This project provides sound scientific evidence that Zn accumulation can be detrimental to biological fertility where sewage sludge and other wastes are applied to land and that certain soils in Scotland may be particularly sensitive. The soils at Hartwood are relatively extensive in the Central Belt where much of the sludge is likely to be spread and research is continuing to identify why this soil should be so sensitive. The significance of these data is that the loss of rhizobia is now being seen at Zn concentrations below the current maximum limits and with sludge additions that are permissible under current legislation. Although the clover rhizobia used in this trial are relevant primarily to grassland, the effects seen could be indicative of further effects on soil microbes and on biological fertility in general.

Metal levels in sewage sludge have declined over the last 20 years, on average by around 50%, with the reduction in cadmium being 76% (Smith 2002). Data on trends on other organic wastes are unavailable. These trends are encouraging and in their recent draft sewage sludge strategy, Scottish Water and the PFI operators intend to recycle approximately half of the projected volumes of sewage sludge (currently 151 000, rising to 175 000 tonnes dry solids in 2025) to agricultural land. Recycling strategies for sewage sludge are usually quite precautionary and monitor build-up of metals at regular intervals to maintain a safety margin between actual and maximum permissible levels. However, given the results that are emerging from the UKSS network, some additional caution should be exercised and perhaps there is a greater need to identify Zn-rich sludges and target applications to less sensitive soil types until this phenomenon is more fully understood.

Other sources and future scenarios

Phosphate fertilisers are known to contain small concentrations of cadmium and this can vary depending on the product in question. Perhaps surprisingly, little is known about the Cd content of fertilisers used in Britain and therefore, the amount of Cd entering the soil through this source cannot be determined (Chaudri et al., 1995). More recently Adams et al. (2002) have suggested that the extent of cadmium contamination in commonly used British agricultural fertilizers is a possible area for future research. Given the length of time that fertilizers have been applied to agricultural land, there is no evidence that levels of cadmium in soil have been elevated above that where food or environmental safety may be compromised nor indeed are starting to approach it.

Concern has been expressed that other metals derived from industrial processes are being released into the environment and are accumulating in soils. Interest in some elements has grown in part because new analytical methods for measuring the very low levels found in the environment have been developed in recent years. Toxic elements such as Sb and Tl occur at low levels in the environment but their industrial use is increasing. The need for improved waste water treatment to protect the environment from Tl produced in industrial processes has been highlighted (Peter and Viraraghavan, 2005) but evidence for widespread accumulation of Tl is lacking. Most evidence for increased levels in the environment has come from contaminated soils close to industrial plants but the analysis of peat cores by Shotyk and colleagues (Frank et al., 2003, Krachler et al., 2003, Krachler and Shotyk, 2004, Krachler et al., 2005, Roos-Barraclough et al., 2006, Shotyk et al., 2005a, Shotyk and Krachler, 2004, Shotyk et al., 2005b, Shotyk et al., 2004) have demonstrated the increased deposition of some of these metals since the start of the industrial era. An indication of the background levels of almost all elements in Scottish soils was given by the analysis of the major parent materials (Ure et al, 1979) but there have been no studies aimed at demonstrating accumulations of these elements. The linkage between accumulations and threats to the environment or human health is still probably speculative.

The removal of leaded petrol from the marketplace in most developed countries has had a marked effect on the diffuse deposition of Pb to soil. For example, the rates of deposition to the upland catchment at Glensaugh have been reduced many fold over the last few decades (Farmer et al., 2005). The reduction of lead in traffic emissions has, however, led to increases in other metals. There is some concern that the now widespread use of platinum group elements ( PGE) in the catalytic converters used in cars will lead to a steady and diffuse accumulation in soils. The levels in soils are very low and there have been no long-term studies on accumulations so the long-term consequences of the release of PGE to the atmosphere and the impacts on soils are unknown. The removal of lead from petrol has resulted in other organic compounds being used to increase the octane number of petrol. One of these is methyl cyclopentadienol manganese tricarbonyl (18 mg l ^-1). In those countries where use of this compound is allowed Mn has effectively replaced Pb as an emission in car exhausts. This has raised concerns because Mn is not only an essential element but is also toxic at high concentrations. A thorough review of the literature came to the conclusion that the very weak cause-and-effect relationships do not justify concern about environmental exposure to manganese (Finley, 2004). The effects of increased levels of Mn in soil are unknown but the accumulations will be small compared with the background levels - Mn has been used as a fertilizer for many years with no adverse impact - so the effects are also likely to be small.

7c.6 Conclusions

• Heavy metals occur naturally in the environment and some for example, copper, if deficient, can adversely affect plant growth. At high levels, most of these metals are phytotoxic or zootoxic and are considered as pollutants.
• Total metal concentrations in soils are relatively well characterized but there are important omissions e.g. arsenic and mercury. We recommend consideration be given to collecting systematic wide area data on these elements to determine background levels and spatial trends.
• Atmospheric deposition of heavy metals is generally low over Scotland and is predicted to decline.
• Organic wastes including sewage sludge are one of the prime anthropogenic sources of heavy metals into soils and application is highly regulated. The application of sewage sludge to land is highly regulated whereas the application of most other organic wastes is subject to exemption issued by SEPA under the Waste Management Licensing Regulations. We recommend that consideration be given to how other organic wastes could be considered under a framework similar to that for sewage sludge.
• Sewage sludge, and other organic waste recycling to land, is projected to continue and a watching brief on the results from the sewage sludge network is highly recommended. There is emerging evidence that relatively high metal concentrations, particularly of zinc but at levels below the current regulatory threshold, in sewage-sludge amended soil is having a serious impact on biological fertility.

7c.7 References

Adams M L, Zhao F J, McGrath S P, Nicholson, F A, Chalmers A, Chambers B J, and Sinclair, A. H. 2002 Evaluation of factors affecting the cadmium and lead content of British wheat and barley. Report to the UK Home-Grown Cereals Authority ( HGCA)

Anthony, S., Betson, M., Lord, E., Tagg, A., Panzeri, M., Abbott, C., Struve, J., Lilly, A., Dunn, S., DeGroote, J., Towers, W. and Lewis, D. 2006. Provision of a screening tool to identify and characterise diffuse pollution pressures: Phase II. Final Report WFD19 (230/8050. Sniffer .

Ashmore M et al 2001 Development of a Critical load Methodology for Toxic Metals in Soils and Surface Waters: Stage I and II. Contract Report by the University of Bradford to DEFRA

Bacon, J. R. , Campbell, C. D. , Coull, M. C. , Chambers, B. J. , Gibbs, P. A. , Chaudri, A. M. , McGrath, S. P., Carlton-Smith, C. H. and Aitken, M. N. (2001) A long-term study of the effects of heavy metals in sewage sludge on soil fertility and soil microbial activity. In: Recycling and reuse of sewage sludge, edited by R. K. Dhir, M. C. Limbachiya, and M. J McCarthy, London:Thomas Telford, 2001, p. 87-96.

CEC (1986) Directive on the protection of the environment and in particular of the soil when sewage sludge is used in agriculture. Official Journal L181/6, 4 July 1986. CEC, Brussels

Chaudri A M, Zhao, F J, McGrath S P and Crosland A R (1995) The Cadmium content of British Wheat Grain. Journal Environmental Quality 24, 850-855 (1995)

Department of the Environment (1996) Code of Practice For Agricultural Use of Sewage Sludge. HMSO.

Farmer, J. G., Graham, M. C., Bacon, J. R., Dunn, S. M., Vinogradoff, S. I., MacKenzie, A. B., 2005. Isotopic characterization of the historical lead deposition record at Glensaugh, an organic-rich, upland catchment in rural N.E. Scotland. Science of the Total Environment 346, 121-137.

Finley, J. W., 2004. Does environmental exposure to manganese pose a health risk to healthy adults? Nutrition Reviews 62, 148-153.

Forestry Commission (1992) A Manual of Good Practice for the Use of Sewage Sludge in Forestry, Forestry Commission Bulletin 107. Forestry Commission , Edinburgh.

Frank, J., Krachler, M., Shotyk, W., 2003. Atmospheric deposition of arsenic and selenium in ombrotrophic peat bogs. Journal De Physique IV 107, 1427-1427.

Gibbs, P. A., Chambers, B. J., Chaudri, A. M., McGrath, S. P., Carlton-Smith, C. H., Bacon, J. R., Campbell, C. D. ,and Aitken M. N. (2006) Initial results from a long-term, multi-site field study of the effects on soil fertility and microbial activity of sludge cakes containing heavy metals. Soil Use and Management 22 (1):11-21.

HM Government (1989) The Sludge (Use in Agriculture) Regulations 1989. Statutory Instrument No. 1263. London, HMSO.

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www.ukwir.org/news/figures/03sl014%20exec%20summ.doc (updated with more recent information from Phase 3)

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