Importance of soil for groundwater vulnerability assessment Importance du sol pour l'estimation de la vulnérabilité des nappes

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1 Scientific registration nº : 1861 Symposium nº : 35 Presentation : poster Importance of soil for groundwater vulnerability assessment Importance du sol pour l'estimation de la vulnérabilité des nappes LEONAVICIUTE Neda State Land Survey Institute, Sapiegos 12, 3000 Kaunas, Lithuania Introduction In Lithuania almost all drinking water is derived from groundwater sources. Some of these sources, particularly shallow Quaternary aquifers, are already known to be contaminated beyond the strict limits imposed on drinking water by Europe Union. The recent economic changes in Lithuanian agriculture have led to an increase in the demand for less expensive, shallow boreholes to serve individual farms. It is important therefore to set up a national system whereby the likely availability of clean, cheap, potable shallow groundwater for the rural inhabitants can be assessed and protected. In the search for tools to deal with contamination of groundwater, the concept of groundwater vulnerability was introduced in the late 1960s. The concept of groundwater vulnerability is based on the assumption that the physical environment may provide some degree of protection to groundwater against the human impact. The earth materials may act as natural filters to screen out some contaminants (IDA, 1995). Although there are many published methodologies for assessing groundwater vulnerability (IAH, 1994), their adopting to Lithuania is limited whether by the lack of representative data or specific purpose of the studies. In 1996 a joint British Lithuanian project funded by the UK Environmental Know How Fund (EKHF) was carried out in order to investigate the feasibility of using British groundwater vulnerability mapping techniques in Lithuania. The major advantages of British methodology are a relatively low cost technology for producing the maps at scale 1:50,000, because it uses existing archive soil and geological databases, and a clearly defined classification system which can be readily used and understood by non-specialists. Furthermore, it should be emphasised that this methodology considers equally the importance of soil and geological deposits in groundwater vulnerability assessment. The processes causing elimination and/or attenuation of pollutants in the subsurface occur at much higher rates in the biologicallyactive soil zone, as a result of its higher clay mineral and organic matter content and bacterial populations. The influence of the soil is of great importance in connection with diffuse agricultural pollution and soil should be regarded as one of the principal factors in the assessment of groundwater vulnerability in the Lithuanian environment. 1

2 This paper demonstrates the significance of soil zone for groundwater vulnerability assessment trough the presentation of British methodology altered to Lithuanian conditions, and extends the application of soil information stored in Lithuanian State Land Survey Institute for other than agricultural purposes such as protection of the environment. Organisation and methodology Groundwater vulnerability Groundwater vulnerability depends on the natural characteristics of a site and relates to the ease of vertical movement of pollutants. It is assessed on the physical, chemical and biological properties of the soils and geological deposits. The factors that together define the vulnerability of groundwater resources to a given pollutant or activity are: presence and nature of overlying soil; presence and nature of Quaternary deposits; depth to groundwater. It is convenient to subdivide geological deposits according to their permeability as the higher the permeability, the less the capacity for attenuating contaminated recharge entering at their surface. A soil classification based upon the physical and chemical properties of undisturbed soil have been developed, to assess the potential for pollutants to move down through the soil. This soil leaching potential classification can be applied to all soil types (Palmer et al., 1997). Geological classification The initial starting point for the geological classification was that used in the UK (Palmer et al., 1995) but this was found to be inappropriate for Lithuania. Although the Pre- Quaternary deposits are important as sources of good quality groundwater, Quaternary deposits are generally very thick and overlie solid rock only at considerable depth and, therefore, in terms of assessing groundwater vulnerability, it is relevant to consider the physical properties of the Quaternary deposits. In addition, it is from the shallow Quaternary deposits that demand for groundwater is increasing at present. As a consequence, a 4-category classification based on the relative permeability of the uppermost Quaternary deposit (estimated from the lithological information shown on drift maps prepared by Lithuanian Geological Survey) has been devised (Table 1). 2

3 Table 1. Generalised aquifer classification of Quaternary deposits Aquifer Class Permeability Deposits A Very High Gravel, sand and gravel, Holocene aeolian sand and Pleistocene alluvial sand; B High Sand (except that identified above); C Moderate Silt, peat and till (except oldest Baltic Till); D Low Oldest Baltic Till and glaciolacustrine clay. Unfortunately, there was no data available on fracturing in tills. The presence of fracturing in clay-rich deposits has a major influence on the rate at which pollutants can pass through them into underlying aquifers (Adams and Foster, 1992). Hence, the assessment of the areas covered by till and glaciolacustrine clays should be approached with caution until more detailed information becomes available. Soil Soil leaching potential classification The national series of 1:50,000 scale soil maps have been used as a basis for the work. These maps represent a simplification of the complex pattern of soils shown on individual collective farm maps at 1:10,000 scale. Each soil map unit shown on the relevant 1:50,000 scale map have been allocated a soil leaching potential. An indication of the effect soil layers might have on pollutant fate and behaviour is provided by the concept of Soil Leaching Potential. This is a measure of the ease with which pollutants might move through and out at the base of a soil profile taking into account those soil properties which may attenuate the pollutant or bind it onto soil particles (Palmer et al., 1995). A very simple classification of soil leaching potential is used which involves three classes and six subclasses. The High leaching potential soils will have least capacity to attenuate pollutants and Low leaching potential soils will have the greatest potential. Leaching potential relates to a wide range of pollutants that are soluble in water and have the capacity to adsorb onto clay and/or organic matter (Table 2). 3

4 Table 2. Soil Leaching Potential classes (Palmer et. al., 1997) Soil Leaching Potential High Intermediate Category H1 H2 H3 HU I1 I2 Pollutant transmission characteristics Readily transmit liquid discharges. Readily transmit a wide range of pollutants. Readily transmit non-adsorbed pollutants and liquid discharges but have some ability to attenuate adsorbed pollutants. Assumed to readily transmit a wide range of pollutants. Possibly transmit a wide range of pollutants. Possibly transmit non-adsorbed pollutants and liquid discharges but are unlikely to transmit adsorbed pollutants. Low L Unlikely to transmit any pollutants. Characteristics of soil which affect pollutant degradation The degree of degradation of any contaminant is dependant upon the residence time within the soil zone which in turn is determined by the speed of pollutant movement through the soil and by how much of the pollutant remains in the non-mobile phase. These are determined by: depth, duration and type of waterlogging; soil texture; slowly permeable horizon; organic matter content. Depth duration and type of waterlogging In many low-lying sites, seasonal soil waterlogging is caused by fluctuating groundwater within the soil profile. In such circumstances, even relatively short-lived pollutants, such as biodegradable pesticides and herbicides, which enter the soil may reach groundwater. Such soils are rated as having the highest risk and are classified as High (H1) leaching potential. Soil texture The volume of water retained by sandy soil is relatively small and most is in the mobile phase, therefore, the pollutants are likely to leach quickly out of the soil profile due to the naturally high permeability. A subclass (H2) is therefore established in the High Leaching potential class to cater for sandy soils. Conversely, soils with a high silt and/or clay content are able to retain relatively large amounts of water, most of which is nonmobile. Water and pollutants will leach through the soil slowly, giving more time for degradation to occur. These soils are grouped as Intermediate (I1) Leaching potential. Slowly permeable horizon Subsoil which are clay-rich, dense and coarsely structured act as significant barriers to vertical water movement and cause seasonal waterlogging in the soil layers above. Where slowly permeable subsoil extends below 1m depth the downward movement of pollutants is strictly limited. Such soils are classified as Low Leaching potential (L). Locally, 4

5 especially on sloping ground, excess water may be readily shed laterally downslope or the slowly permeable layer may not be laterally continuous and hence allow some downward percolation. In these situations the soils are placed in the Intermediate (I1) Leaching potential class. Organic matter content The ability of a soil to adsorb pollutants depends upon its cation exchange capacity which, under temperate soil forming conditions, depends mainly on organic matter and clay contents. Thus, the lower the soil organic matter and clay contents, the lower the ability of the soil profile to attenuate potential pollutants. With regard to this characteristic the different soils are grouped into relevant Soil Leaching Potential subclasses. The complete description of the Lithuanian soil classification is beyond the scope of this paper but Table 3 gives the general characteristic of the soils within each of the soil leaching potential sub-classes. Table 3. General characteristics of soils within each soil leaching potential subclass. Soil Leaching Potential class High (H) Intermediate (I) Soil Leaching Potential sub-class H1 H2 H3 U I1 I2 Soil characteristics Gleyic and gley soils with slowly permeable subsoil; Well-drained gravely soils; Undrained peat soils with permanently wet topsoils. Well-drained, sandy or sandy over gravel soils with a low topsoil organic matter content. Well-drained, loamy over gravel soils. Soils in urban areas, designated development or resort land and areas of past or present mineral workings. Deep, well drained, loamy and clayey soils with a topsoil of low or moderate organic matter content. Peat or peaty soils which have been drained for agricultural use. Low (L) L Gleyic and gley soils with slowly permeable subsoil. Map compilation The soil leaching potential map is laid directly over the relevant geological permeability map and groundwater vulnerability zones are drawn. These zones identify all the different soil leaching potential classes occurring within each individual area of Quaternary deposits Types A, B and C. It is assumed that Type D deposits provide protection to any underlying groundwater because of their naturally low permeability. Information on the type of soil overlying these deposits is therefore not relevant to groundwater vulnerability studies. The Groundwater Vulnerability zones are colour coded in such a way that the stronger the intensity of colour the higher the leaching 5

6 potential. In order not to overload groundwater vulnerability maps with information, maps such as land use, depth to shallow groundwater, actual mineralisation of groundwater have been prepared additionally. Results and discussion of the maps The main results of British-Lithuanian project were three 1:50,000 scale groundwater vulnerability maps representing contrasting landscape regions in Lithuania. Each map covers a land area of about 16 km x 18.5 km. One of the pilot maps has been digitised and presented in a computer drawn form. One of the major limitations in vulnerability assessments is the absence of judging their validity (Chapel, 1993). Therefore, attempts have been made to validate the Lithuanian groundwater vulnerability maps, firstly, by assessing the quality of the National Soil Map and, secondly, by comparing the groundwater vulnerability classifications at locations where analyses of shallow groundwater exist. The estimated level of certainty portrayed in 1:50,000 scale soil maps, from which soil leaching potential maps have been derived, was equal to 84%. Therefore, use of the 1:50,000 scale National Soil Map to assess soil leaching potential did not significantly compromise the quality of the final groundwater vulnerability maps. The validation possible from available shallow groundwater analyses suggests that the methodology satisfactorily predicts the intrinsic groundwater vulnerability, although further hydrochemical data is required to fully validate the methodology. Differences in the frequency distribution of nitrate levels are consistent with the soil leaching potential classification. For example, a large proportion (85%) of wells with nitrate levels above 45 mg/l are located in areas of High leaching potential; a lower proportion (47%) of wells with nitrate levels below 4.5 mg/l are located in areas of Intermediate leaching potential. Although the data in all areas is sparse, this available information prove the significance of soil zone for groundwater quality, since it exercises a major control over the leaching of agricultural pollutants. The preparation of groundwater vulnerability maps in Lithuania faces certain problems connected with the lack of geological and soil data and the differences between the Lithuanian and the UK soil classifications. For instance, the Lithuanian soil classification does not distinguish the cause of waterlogging that is of great importance for groundwater vulnerability studies. Also the cut-offs between the soil wetness categories in these soil classifications are drawn in different places, i.e. Lithuanian well drained soils suffers from waterlogging more frequently than the UK. Therefore, subjective interpretation has been used to overcome these fundamental differences in soil mapping. Conclusions This work represents the first collaborative work between Lithuanian geologists and soil scientists. It has resulted in the development of a nationally applicable groundwater vulnerability classification methodology which recognises the importance of both geological deposits and soil types in attenuating and retarding potential pollutants and which, based upon its low cost approach using archive data, is relevant to the current needs of Lithuania. 6

7 The validation possible from available shallow groundwater analyses suggests that the methodology satisfactorily predicts the intrinsic groundwater vulnerability, although further hydrochemical data is required to test fully the methodology, and demonstrates the significance of soil zone for groundwater vulnerability assessment The maps at scale 1:50,000 therefore can be used as a basis for predicting those areas of land where underlying groundwater resources are vulnerable to surface-derived pollutants. However, the maps based on this methodology should not be used for local studies which will involve detailed soil and accurate hydrogeological investigations. The groundwater vulnerability concept is designed to summarise complex earth science data in a form which can be readily used and understood by non-specialists. Relevant data are presented in simple and clearly defined classification systems which do not assume specialist knowledge. The maps are therefore suitable for use by a wide range of organisations and individuals who have a responsibility to manage land in an environmentally aware way. Bibliography Adams, B., and S.S.D. Foster (1992). Land-surface zoning for groundwater protection. Journal of Institution of Water and Environmental Management 6: Addiscot, T.M., Whitmore, A.P., and D.S. Powlson (1991). Farming, Fertilizers and the Nitrate Problem, Melksham, UK: Redwood Press Ltd. Alley, W.M. (ed) (1993). Regional Groundwater Quality, New York: Van Nostrand Reinhold. Chapel H. (1993). Pesticides in Drinking Water, New York: Van Nostrand Reinhold. Giorgini, A., and F. Zingales (1986). Agricultural non-point source pollution: model selection and application. Developments in Environmental Modelling 10: Hiscock, K. M., Lovelt, A.A., Brainard, J.S., and J.P. Parfitt (1995). Groundwater vulnerability assessment: two case studies using GIS methodology, Quarterly Journal of Engineering Geology 28: Illinois Department of Agriculture (1995). Identification of Areas Vulnerable to Groundwater Contamination in Illinois, and Recommended Management Practices. Illinois: Illinois Department of Agriculture Bureau of Soil and Water Conservation. International Association of Hydrogeologists (1994). Guidebook on Mapping Groundwater Vulnerability, eds. Vrba J. and A. Zaporozec, Volume 16, Hanover: Heise. Palmer R.C., Holman I.P., and N. Leonaviciute (1997). Introducing Multidisciplinary Groundwater Vulnerability Mapping to Lithuania, Bedford: Cranfield University, Soil Survey and Land Research Centre. Palmer, R.C., Holman, I.P., and M.A. Lewis (1995). Guide to Groundwater Vulnerability Mapping in England and Wales, London: HMSO. Key words: groundwater vulnerability, diffuse pollution, soil leaching potential, mapping Mots clés : vulnérabilité des nappes, pollution diffuse, lessivage potentiel du sol, cartographie 7