NRA Water Resources 28

Ig UIDE TO GROUNDWATER VULNERABILITY MAPPING IN ENGLAND AND WALES National Rivers Authority

GUIDE TO GROUNDWATER VULNERABILITY MAPPING IN ENGLAND AND WALES R C Palmer, I P Holman (Soil Survey and Land Research Centre, Cranfield University)

N S Robins, M A Lewis (British Geological Survey)

The Information Centre National Rivers Authority Waterside Drive © National Rivers Authority, 1995.

Applications for reproduction should be to HMSO Copyright Unit, St Crispins, Norwich NR3 1PD

First edition 1995

ISBN 0 11 310103 1 ISSN 1358-328X

This Guide was produced for the NRA by the Soil Survey and Land Research Centre (SSLRC) and The British Geological Survey (BGS) under a contract constituting part of the NRA’s Research and Development programme.

Designed by HMSO Graphic Design

Cover and text printed on recycled materials (HMSO Recycled Register — Score 60.0) FOREWORD

In 1992 the National Rivers Authority published its Policy and Practice for the Protection of Groundwater. The implementation of the policy depends upon the definition of groundwater source protection zones and on the preparation of vulnerability maps. This guide is one of two volumes which provide the background to the production and use of these two policy tools and complement the original policy document.

The companion volume to this Guide is the Guide to Groundwater Protection Zones in England and Wales. GUIDE TO GROUNDWATER VULNERABILITY MAPPING IH ENGLAND AND WALES

CONTENTS

1 INTRODUCTION 1 1.1 Aims of the guide 1 1.2 Background 1 1.3 Groundwater vulnerability 3

2 PRINCIPLES USED IN GROUNDWATER VULNERABILITY MAPPING 6 2.1 Water cycle 6 2.2 Hydrogeology 6 2.3 Soil 9 2.4 Factors affecting pollutant movement in soil 12

3 MAP PRODUCTION METHODOLOGY AND DATA SOURCES 15 3.1 Overview 15 3.2 Geology 15 3.3 Soil 18 3.4 Low permeability drift deposits 22

4 USE AND INTERPRETATION OF THE MAPS 23 4.1 Overview of information provided by the maps 23 4.2 Example interpretations 25 4.3 Regional differences in low permeability drift interpretation 27

5 LIMITATIONS OF THE GROUNDWATER VULNERABILITY MAPS 29 5.1 Introduction 29 5.2 Geological data limitations 29 5.3 Soil data limitations 31 5.4 Drift data limitations 32 5.5 Site specific limitations 34

6 REFERENCES 37

APPENDICES 1 Stratigraphic column and strata in aquifer classes 38 2 Commonly occurring soil series within soil leaching potential classes 44 3 Key sheet for understanding the NRA Groundwater Vulnerability Maps 45 4 Glossary of Terms 46

IV 1 INTRODUCTION

1.1 AIMS OF THE GUIDE This guide is designed to be used with the series of 1:100 000 scale maps which form part of the National Rivers Authority (NRA) Policy and Practice for the Protection of Groundwater (abbreviated in this document to Groundwater Protection Policy - GPP), published in 1992. The maps depict groundwater vulnerability, a concept based on interpretations of geology and soil, and the series is being progressively published over a 4'/2 year period with full coverage of England and Wales expected by 1998. The programme is being carried out by the Soil Survey and Land Research Centre (SSLRC) and the British Geological Survey (BGS) to take advantage of the existing soil and geological databases of the two organizations.

The aims of this guide are to: different ways by the various users of the Groundwater Vulnerability • give the background to the Maps. groundwater vulnerability Those who only require sufficient mapping programme guidance to use and interpret the • describe the groundwater, maps are advised to consult: geology, soils and contaminant transport principles used in the • Chapter 1 - Introduction classifications forming the basis • Chapter 3 - Map production of the maps methodology and data sources • describe the methodology and • Chapter 4 - Use and data sources used in the interpretation of the maps. production of the maps Those users requiring specialist • provide an overview of how to technical information on the use the maps through example principles used in the compilation of interpretations the maps and on the conceptual • discuss some limitations of limitations of the maps should also groundwater vulnerability consult: mapping in general and at 1:100 000 scale in particular. • Chapter 2 - Principles used in groundwater vulnerability This guide, together with the Source mapping Protection Zone Guide, provides a • Chapter 5 - Limitations of the complement and update to the GPP Groundwater Vulnerability and is directed towards the Maps following readership:

• NRA officers, and particularly 1.2 BACKGROUND non-specialists in groundwater L2.1 The importance of groundwater protection Groundwater is contained naturally • developers and their agents within underground water-bearing • planning, central government strata (aquifers') of various types and other statutory authorities across the country. It is present • interest groups within all rocks but some contain • consultants. more than others and are therefore Because of the likely different levels more important for water resources. of technical knowledge between the The volume of water stored in the many map user-groups this guide is pores and fractures of aquifers vastly aimed at the informed non-specialist exceeds the volumes of fresh water lay person. However, as the guide in lakes and rivers in England and provides both general and detailed background information, it is 1 Words shown in italics are defined in the envisaged that it will be used in glossary GUIDE TO GROUNDWATER VULNERABILITY MAPPING IN ENGLAND AND WALES

Wales. Many of the most productive to the inaccessibility of the aquifers, such as the Chalk or strata, the slow rates of Triassic sandstones, occur in the groundwater movement and the densely populated relatively dry low levels of microbiological eastern and central parts of England activity. Processes which take and consequently, groundwater has a place in days or weeks in surface strategic significance in providing water systems are likely to take high quality water for public supply. decades in groundwater systems. It provides approximately 35 per cent of present national demand but It is better to prevent or reduce the in some southern and eastern risk of groundwater contamination regions more than 80 per cent. than to deal with its consequences. Groundwater is also an important source for industry and agriculture 1.2.2 Groundwater vulnerability within as well as sustaining the baseflow of the legislative framework rivers. Rivers can dry up without In 1980 the European Commission sufficient volumes of groundwater introduced a Groundwater Directive inflow as occurred in Chalk streams (80/68/EEC) aimed largely at the during the drought of the early control of discharges of a wide 1990’s. Baseflow often provides the range of potentially toxic bulk of a river’s flow in summer so substances. These are grouped by that groundwater quality is crucial the Directive into List I and List II in maintaining surface water quality substances (NRA 1992). The to the benefit of aquatic ecology, Directive did not address either wetland habitats and recreational diffuse pollution arising from the uses. use of agricultural fertilizers or the Groundwater is increasingly essential links to the management of under threat. Pollution can arise abstraction. from many activities carried out on, At a Seminar on Groundwater or near, the land surface. This may held in the Hague in November be at a particular place (a point 1991, EC Environment Ministers source) or be widespread over large recognized the limitations of areas (a diffuse source). Examples of existing community-wide point source pollution include regulations and adopted an action spillages of chemicals from programme for the future industrial sites which give rise to protection of groundwater. intense but localised effects. In Groundwater is recognized as a contrast, nitrate pollution from finite, natural resource of great value agriculture is a diffuse source which which should be managed and can build up over many years and protected on a sustainable basis. The affect significantly larger volumes of subsequent declaration stressed that groundwater. The protection of the objective of sustainability should groundwater quality is of critical be implemented through an importance for the following integrated approach, which means reasons: that surface water and groundwater should be managed as a whole, • The degradation of a widespread paying equal attention to both high quality water resource, quality and quantity aspects and through contamination of an taking into account all interactions aquifer, might necessitate the with the soil and atmosphere. development of more expensive The NRA (and the Environment water resources, if available, or Agency after April 1996) has a duty expensive water treatment to monitor, maintain and protect before use. groundwater resources from • Once polluted, groundwater is pollution with the aid of various costly and difficult, if not legislative provisions. These impossible, to rehabilitate due provisions implement EC Directives

2 including the 80/68/EEC Directive contamination from surface or near­ on the Protection of Groundwater surface derived pollutants, Against Pollution Caused by recognizes that the risk of pollution Certain Dangerous Substances, the from a given activity is greater in Control of Pollution Act 1974, the certain soil and hydrogeological Environment Protection Act 1990 situations than in others. and the Water Resources Act 1991. Groundwater vulnerability Together they control the storage depends on the natural and discharge of dangerous characteristics of a site and relates chemicals to groundwater, control to the ease of vertical movement of the disposal of wastes, and control pollutants. It is assessed on the the handling of potential pollutants physical, chemical and biological used in the agricultural industry. properties of the soils and rocks At its time of creation in 1989, beneath the site as these control the the NRA inherited various ease with which a pollutant can Groundwater Protection Policies migrate to the water table. The from the former Water Authorities. factors which together define the In order to consolidate and vulnerability of groundwater standardise the existing policies and resources to a given pollutant or to take account of new duties activity are: imposed on the NRA, including the • presence and nature of overlying need to support the EC objective on soil sustainability of groundwater quality • presence and nature of drift and quantity, the NRA has adopted deposits a new policy framework for • nature of solid geological strata protecting groundwater that is . in the unsaturated zone outlined in the Groundwater • depth to groundwater. Protection Policy Groundwater Vulnerability Maps, and Source It has also to be recognized that Protection Zones (which are being these intrinsic properties can be produced separately), are integral modified by man-made structures or parts of the Groundwater excavations. Protection Policy. The maps deal The key to groundwater with the areal zonation of land vulnerability classification lies in the above aquifers according to the unsaturated zone - that volume of vulnerability to pollution of the the soil, and unsaturated aquifer or groundwater contained in the confining beds situated above the underlying aquifer, whereas Source water table or potentiometric surface., Protection Zones define areas respectively. In the absence of immediately surrounding a fissures in the unsaturated zone, groundwater source or group of water movement is essentially slow sources according to the travel times and takes place within the for groundwater flow to the source. interconnected pore spaces; the chemical environment is aerobic and generally alkaline. This provides the 1.3 GROUNDWATER VULNERABILITY potential for: 1.3.1 Factors which affect groundwater vulnerability ^ • interception, adsorption and Many human activities present a elimination of bacteria and potential hazard to groundwater viruses. quality and in most cases a • attenuation of heavy metals, and significant element of the total other inorganic compounds. hazard to quality depends on natural • adsorption and biodegradation soil and geological conditions. The of organic compounds. concept of groundwater The presence of vertical fissure vulnerability, that is the systems, however, may enable the susceptibility of groundwater to rapid flow of pollutants from the GUIDE TO GROUNDWATER VULNERABILITY MAPPING IN ENGLAND AND WALES

land surface directly to the water the NRA to use its existing table and the potential therefore for statutory powers in a consistent and the above processes to take place uniform manner and they will guide will be greatly reduced. it in its response to the various There are many published statutory and non-statutory methodologies for assessing consultations it has with other groundwater vulnerability (IAH, organisations whose decisions can 1994). A review of the available affect groundwater. techniques (Adams and Foster, 1991) enabled the NRA (later followed by the Department of the 1.3.3 Purpose of the Groundwater Environment for Northern Ireland and also the Scottish Office Vulnerability Maps Environment Department) to adopt The Groundwater Vulnerability the easily visualized approach Maps are an aid both to developers described in this guide. planning new activities, and to The NRA 1:100 000 scale Planners assessing new proposals or Groundwater Vulnerability Maps drawing up strategic planning have been prepared to reflect the documents (Local Plans/Structure different degrees of groundwater Plans). They will allow better vulnerability according to a range of informed judgements to be made on soil properties and geological the location of new developments. criteria (Foster and Skinner, 1995). By consulting the maps, in The main purpose of the maps is to conjunction with the policy communicate such scientifically statements and matrices in the derived land subdivisions to those Groundwater Protection Policy, land whose knowledge of the earth users and developers will know to sciences may be limited but who avoid proposing potentially need to make land use and land polluting activities in highly management decisions based on the vulnerable areas, while favouring data portrayed in the maps. sites on Non-Aquifers or sites with soils of low leaching potential where a more permissive view is likely to 1.3.2 Groundwater Vulnerability Maps be taken by the NRA. It is within the Groundwater Protection Policy important to recognize this role as a A full vulnerability assessment of ‘first pass’ screening tool in site the risks posed to groundwater by assessment. Site specific studies will potentially polluting activities on always be needed for detailed the land surface can only be proposals. achieved by local studies which, in The maps also have a use in the many cases, will involve detailed soil consideration of existing activities and hydrogeological investigations. which may be giving rise to However, in the context of strategic (presently undetected) pollution. land use planning the NRA They will allow owners of multiple (Environment Agency) is producing sites to prioritise their investigative the series of Groundwater and subsequent remedial actions Vulnerability Maps to allow where historical practice may have planners, developers and regulatory given rise to land and groundwater bodies to make informed contamination. judgements on the location of new This is the first time this type of developments. information has been published in The policy statements in the England and Wales and it will prove Groundwater Protection Policy and an invaluable source of initial the information provided on the strategic information to local Groundwater Vulnerability Maps do planning authorities, other not, of themselves, have a statutory regulatory bodies, developers, status. However, they will enable consultants and the general public.

4 L Introduction

1.3.4 Groundwater vulnerability mapping soils, but only those soils overlying All groundwaters are controlled Major and Minor Aquifers have waters and are afforded protection, been classified on the maps. regardless of whether, or how, they Wherever geological are currently used. However, for investigations identify low strategic land use planning it is permeability surface drift deposits convenient to subdivide permeable overlying Major and Minor Aquifers strata into the categories of Major their presence is indicated on the Aquifers and Minor Aquifers. Due maps by a stipple ornament. A to relatively high permeability, Major stipple is used to highlight the Aquifers generally have less capacity inherent variability in permeability for attenuating contaminated of many drift deposits, both laterally recharge entering at their surface and vertically, to emphasise the need than Minor Aquifers. This division for detailed site specific information is to a certain extent also coincident to assess individual situations. with their water resource potential. The Groundwater Vulnerability Low permeability strata are Maps therefore show the degree of classified as Non-Aquifers although risk of a pollutant, which originates groundwater will be present and at the surface, moving down into an may be an important part of surface underlying aquifer by assessing water baseflows. (Figure 1.1) the: A soil classification, based upon • soil type the physical and chemical properties • likely presence/absence of low of undisturbed soil, has been permeability surface drift developed by SSLRC to assess the deposits likelihood of pollutants moving • permeability of aquifer material. down through the soil column. This classification can be applied to all

FIGURE 1.1 Overview of information layers used to compile a Groundwater Vulnerability Map

Groundwater Vulnerability Map GUIDE TO GROUNDWATER VULNERABILITY MAPPING IN ENGLAND AND WALES

2 PRINCIPLES USED IN GROUNDWATER VULNERABILITY MAPPING

2.1 WATER CYCLE The water cycle, depicted in Figure 2.1, has no beginning or end but it is convenient to describe it starting with the process of evaporation whereby water vapour enters the atmosphere. Evaporation is not restricted to open water bodies, such as oceans, lakes, streams and reservoirs. Precipitation (rain, hail, snow or mist) intercepted by leaves and other vegetative surfaces can also evaporate, as can soil moisture in the upper layers of the soil.

Water vapour is held in the their type (sandstone, granites, atmosphere and transported by limestones etc.). An aquifer contains global weather systems until useful quantities of groundwater conditions cause the vapour to within its interconnected pore condense to form droplets which spaces and cracks which can be fall as precipitation. Precipitation abstracted. Classification of rocks which reaches the ground, without according to their physical being intercepted, can follow a properties relating to the occurrence number of different pathways of groundwater is, therefore, most within the water cycle. Precipitation pertinent to any assessment of the which drains across the ground vulnerability of groundwater to surface is termed runoff, and can pollution. directly enter stream channels. A Groundwater is the water proportion, however, infiltrates into contained within a rock system both the ground, and drains downwards within the unsaturated zone as well through the unsaturated zone by as below the water table in the gravity at a rate dependant on the saturated zone (Fig. 2.1). soil permeability, a function of the size of the pores and their degree of continuity. At some depth, the soil 2.2.2 Importance of the unsaturated or rock is saturated with water. As zone the water moves through the Above the water table, unconfined unsaturated and saturated zones, its aquifers have an unsaturated zone, chemistry is changed by interactions which may include unconsolidated with the surrounding soil and rock drift deposits and, above these, the material. biologically active soil zone. The Groundwater, although it often nature of any drift cover is flows at a very slow rate, may not important with regard to the stay underground for ever. It can downward percolation of pollutants discharge as a spring or as slow through the unsaturated zone. A seepage into stream, lake or ocean cover of uniform clay grade beds. Water flowing in a stream can material, such as clay till, will therefore originate either from provide a hydraulic barrier, whereas runoff or from groundwater that has sand and gravel as in glacial outwash seeped into the stream bed. The deposits will readily allow the groundwater contribution is termed downward passage of pollutants. baseflow. In the case of mobile conservative pollutants (e.g. road salt) the unsaturated zone merely introduces 2.2 HYDROGEOLOGY a time lag before the arrival of the 2.2.1 Introduction pollutant at the water table without The rocks that underlie England and significant attenuation. Wales can be classified according to In other cases (e.g. degradable their age (see Appendix 1), organic compounds) the degree of according to their mode of attenuation is highly dependent formation or merely according to upon the unsaturated zone but is

6 Impermeable Layer FIGURE 2.1 Diagrammatic representation of the water L->s— cyde showing groundwater and surface water Oil Storage Tanks relationships and groundwater pollution risks

Saturated Zone Principles Used Groundwater in Mapping Vulnerability GUIDE TO GROUNDWATER VULNERABILITY MAPPING IN ENGLAND AND WALES

difficult to predict. It will depend where the water table is at its lowest on the: elevation, again by definition the • physical and chemical areas where groundwater discharge (springs or baseflow to rivers) is at characteristics of the pollutant its greatest. • geochemical environment Rocks which are less able to • time taken to travel from surface to water table (Robins et transmit groundwater may form a al., 1994), which depends upon hydraulic barrier, or a confining layer to deeper, more permeable the intensity of application of strata. Such is the case in the the pollutant, the prevailing London Basin, where London Clay rainfall, the presence or absence confines groundwater in the of by-pass fissures or low underlying Palaeogene sands and the permeability horizons (e.g. marl Chalk. The latter forms a confined bands) and the thickness and aquifer beneath parts of London, prim ary p erm eability of the but are unconfined where they unsaturated zone. outcrop beyond the London Clay For diffuse source pollutants, transit cover to form the Chalk downlands time in the unsaturated zone can be (Figure 2.2), A confined aquifer assessed from average annual whose recharge area may be distant, infiltration rates and the moisture is better protected from direct retention properties of the soil. In percolation of pollutants. In the practice, saturated flows only occur London Basin, the Chalk remains for short periods of time under field susceptible to surface pollution at conditions, but in most strata (other outcrop beneath the Chalk than fine-grained unconsolidated downlands north and south of the sediments) maximum pollution risk London Clay cover, but not in the occurs wherever the unsaturated confined area beneath the London zone is thinnest. The lithology of the Clay. unsaturated zone, its degree of The Groundwater Vulnerability consolidation and the presence of Maps only classify aquifers which fracturing, are the key factors outcrop at surface or beneath drift beneath the soil zone in assessing cover; these tend to be unconfined the vulnerability of groundwater to aquifers. Aquifers that are confined pollution. by bedrock, which are concealed and protected from pollution by their 2.2.4 Role of the saturated zone confining cover, are not shown. Groundwater flow is induced from Groundwater in many aquifers areas where the water table is such as the Chalk or the Permian highest, which are, therefore the and Triassic sandstones is contained main areas of recharge, to areas both in intergranular pore spaces

NORTH DOWNS FIGURE 2.2 A diagrammatic geological cross section of the London Basin Principles Used in Groundwater Vulnerability Mapping

and in the many cracks which 2.3 SOIL characterize these rocks. Although 2.3.1 Introduction to soils storage of water in the intergranular Soil is the upper layer of the earth’s pore spaces is usually more crust and is the product of complex significant than that in the cracks, it interactions, which are most intense is the cracks which are likely to be near the surface, between climate, the main route for groundwater living organisms, parent material flow. These are termed fractured and relief. Soils develop through the aquifers. However, the dimensions, accumulation of unconsolidated and importance of fractures for mineral grains from the physical and groundwater flow vary greatly from chemical weathering of rock aquifer to aquifer. fragments and the addition of There are three key physical organic material (humus) from properties which all rocks, both vegetation. For the purposes of aquifers and non-aquifers possess. assessing pollutant movement, soil These are: is here considered to be the upper weathered parts of the earth which • permeability (which is based on are affected by living organisms, and the size and connectivity of the which undergo seasonal changes in pores and other voids within a moisture content, temperature and given rock unit) which controls gaseous composition. In the UK, as the ease with which in most other temperate countries, groundwater can flow the soil zone extends to about 2 m • porosity, or volume of void depth. space (water and air filled) A vertical section through soil, as relative to the unit volume of seen in the face of a pit or aquifer excavation is known as a soil profile • amount of groundwater that (Fig. 2.3). A soil profile usually may be released from the aquifer contains a number of distinct (storage coefficient). horizons, or layers, which are the Clearly aquifers with high product of various soil forming permeabilities and large storage processes taking place within it. coefficients are likely to be most There is usually a complex assembly susceptible to pollution from the of horizons within a soil profile but surface simply because water can a simple three-fold division is move relatively rapidly through their discussed below. From the surface interconnected pore spaces, cracks downwards these are: and fractures in the unsaturated and saturated zones. • Topsoil - This is usually dark There is no consideration of the brown in colour due to the processes taking place in the incorporation of decomposed saturated zone within this plant residues. It is relatively assessment of groundwater organic - and nutrient-rich and vulnerability because once a is the zone of maximum pollutant has arrived at the water biological activity. It is also table the groundwater is assumed to subject to the greatest changes be contaminated. However, the in temperature and moisture. prevailing groundwater flow Most roots, plant and animal direction (hydraulic gradient) and life, from moles to earthworms, physical properties of the aquifer to microscopic bacteria and will dictate the direction, speed of fungi are found here. movement, diffusion and possible further attenuation of the pollutant • Subsoil - This is the altered within the groundwater body. parent material where soil forming processes are active; minerals are actively broken down and altered, plant GUIDE TO GROUNDWATER VULNERABILITY MAPPING IN ENGLAND AND WALES

FIGURE 2.3 1. ARABLE FARMLAND 2. WOODLAND Diagrammatic representation of a soil General properties profile showing the three Depth cm main horizons (or layers). 0 Slightly stony light T o p s o il or medium loam

I A b f r v j ?

Stony medium v^rti-rrH loam '~V i ‘ I : j ! ) y _ F- ! ^ -vL J S u b s o il i & j >Al ~<-L r\ & ^ i « j- i C2? cm J l —r- -v- ° ^ &\Q . 4& £ e T « ^ ® gp.‘§ P ^ !s f S ^ ^ S

Very stony flint gravel Parent Material

100 cm

nutrients are released and 2.3.2 Soil classification in England and considerable reorganization of Wales soil material occurs, involving The characteristics of the soil at any the binding of soil particles to particular location depend on five form aggregates. Organic main groups of factors. These are: material is restricted to living and dead plant roots and is • the physical and chemical much less abundant than in the constitution of the parent topsoil. material • past and present climate • Parent material - This is the • relief and hydrology relatively unaltered material at • length of time during which soil the base of the soil profile in forming processes have been which the soil is developed. This acting zone is least affected by soil • ecosystem , including the forming processes and the extensive modifying effects of original rock structure can man’s activities. usually be seen. The depth to parent material within the soil In order to provide a consistent and depends on its resistance to systematic basis for differentiating weathering and the length of the characteristics, properties and time the weathering processes relationships of soils in England and have been active. Wales, a hierarchical soil

10 Principles Used in Groundwater Vulnerability Mopping

classification has been developed This identifies where aquifers (Avery, 1980). This classification, are very close to the land adopted for use throughout England surface. and Wales, groups soils that behave • organic matter content. This is in a similar and therefore predictable related to the ability to adsorb way. pollutants. The lowest category in the system For example using the above (Clayden and Hollis, 1984), and the criteria, the Aberford series is unit shown on detailed soil maps, is defined as a well drained, fine loamy the soil series, of which there are textured soil developed in hard or over 700 established. Soil series are shattered limestone bedrock within named after the place where they 80 cm depth. were first described or are extensive. The spatial distribution of soil For example, the Aberford series series within the landscape is very was first described near the village complex and, locally, can be difficult of Aberford in North Yorkshire to show on a map even at 1:10 000 during the early stages of the soil scale. For maps at a scale of survey of the Leeds district 1:100 000 or smaller, amalgamations (Crompton and Matthews, 1970). of soil series into soil complexes or Each soil series has a limited and soil associations are usually made. defined range of diagnostic Soil complexes contain two or more properties that distinguish it and soil series which are intricately allow its consistent national mixed in the landscape such that recognition, so that soils belonging they cannot be mapped separately. A to the Aberford series are soil association is a grouping of soil recognised throughout the areas of series that occur together, often in Jurassic and Magnesian Limestone predictable patterns, on the same in eastern and southern England. A geological parent material and which useful definition is given by differ in characteristics related to Robinson (1943) - “a group of soils local variations in texture, relief and similar in character and arrangement hydrologic conditions. They are of horizons of the profile, and generally characterised and named developed under similar conditions after the most frequently occurring from one type of parent material”. component soil series (termed the The diagnostic properties used to lead series), although the other soil differentiate soil series are the series may have different soil relatively permanent properties leaching potentials to the lead series. which determine soil behaviour, but For example in a typical area which are not easily changed by land mapped as Aberford association, it management practices. Several is predicted that approximately important diagnostic properties 55 % of the soils consist of used to define soil series also Aberford series (H3 soil leaching influence the movement of potential), 30 % Elmton series (HI pollutants in soil: soil leaching potential - a very • depth and duration of shallow soil over limestone), 10 % waterlogging. This is associated Dullingham series (II soil leaching with either high groundwater potential - a deep soil found in levels or precipitation. A high narrow valley bottoms and formed soil water table may indicate a in accumulated weathered limestone limited unsaturated zone. debris) and 5 % other undefined soil • texture. This is based on the series. For explanation of soil relative proportions of sand, silt leaching potential classes see Inset and clay-sized particles in a soil on page 21. . (particle size distribution) and is closely related to permeability. • parent material type and depth. GUIDE TO GROUNDWATER VULNERABILITY MAPPING IN ENGLAND AND WALES

2.4 FACTORS AFFECTING POLLUTANT produce saturated conditions at or MOVEMENT IN SOIL near the surface for at least a short 2.4.1 Introduction period. The rate of pollutant Factors which influence how likely a movement through the soil will then pollutant applied at the soil surface be dependant upon the vertical or is to move through the soil profile sub-vertical saturated soil include: permeability. Potential pollutants may therefore move rapidly (several • characteristics of the potential meters per day) out of the upper contaminants soil layers and hence bypass the • physical and chemical zone of maximum microbiological characteristics of the soil activity, and consequent including degradation. - characteristics which encourage lateral pollutant Diffuse pollutants movement Diffuse source pollutants, compared - characteristics which affect the with liquid discharges, are speed of pollutant movement characterized by much lower rates - influence of soil texture on of application and therefore lower pollutant mobility concentrations of pollutant are - characteristics affecting the present in the soil water. degradation or attenuation of Unsaturated soil conditions usually potential contaminants. predominate in which soil permeability is generally several 2.4.2 Characteristics of the potential orders of magnitude lower than contaminants when the soil is saturated. Potential The Groundwater Vulnerability diffuse source pollutants therefore Maps consider the ability of the soil remain in the upper soil layers and unsaturated zones to attenuate significantly longer than most three types of pollutant: pollutants derived from a liquid discharge. Where pollutants have • liquids (e.g. from slurries, physico-chemical characteristics manures, waste water irrigation which allow attenuation by and large spillages) degradation and adsorption, these • diffuse pollutants which under processes lead to a reduction in the certain circumstances can be volume and concentration of adsorbed (or retained) in the pollutant leaching from the soil. For soil layers (e.g. pesticides, diffuse source pollutants, the certain other organic leaching potential of many soils to compounds and some metal- non - or weakly adsorbed based inorganic compounds) compounds will be greater than • diffuse pollutants which are their leaching potential to more soluble and can readily pass strongly adsorbed ones. through the soil layers without being adsorbed (e.g. nitrate). 2.4.3 Physical and chemical Liquids characteristics of soil The rate of pollutant movement The physical and chemical through the soil resulting from characteristics of soil which liquid discharges is much faster than influence the soil leaching potential the movement associated with classification are summarized on diffuse pollutants derived from page 21. either agrochemicals or the redevelopment of contaminated Characteristics which encourage lateral pollutant land. movement In the case of liquid discharges, Subsoil horizons which are dense, the large volumes often involved coarsely structured, clay-rich and act

12 as significant barriers to water such as biodegradable pesticides and movement are termed slowly herbicides, which enter the soil are permeable. Soil layers above a slowly likely to reach groundwater. Such permeable horizon suffer seasonal soils are classified as High (HI) waterlogging and water movement Leaching potential. in these layers is lateral where slopes Raw lowland peat soils that are allow. saturated for most of the year are Where slowly permeable subsoils also classified as High (H i) extend below lm depth the Leaching potential. They occur in downward movement of pollutants undrained sites under semi-natural is strictly limited and it is unlikely bog or fen vegetation, usually where that any pollutant applied at or near peat is still accumulating. the surface will penetrate below 2 m Where hard bedrock, rock-rubble depth. Such soils are classified as or gravel occurs immediately Low Leaching potential. beneath the topsoil layer or where Locally, especially on sloping only a thin subsoil is present, ground, soil characteristics indicate potential contaminants at the soil the occurrence of only slight surface are likely to reach the seasonal waterlogging above the underlying geological material very slowly permeable layer. Here excess quickly. Because the ability of water may be readily shed laterally unweathered geological material to downslope or the slowly permeable attenuate potential pollutants is far layer may not be laterally less than that of soil material, little continuous and hence allow some additional attenuation of the downward percolation. In these contaminant is likely to occur. Such situations the soils are placed in the soils are classified as High (HI) Intermediate (II) Leaching potential Leaching potential. class. The thicker the subsoil overlying In upland sites, a combination of rock, rock-rubble or gravel the high rainfall and low permeability greater is the potential to attenuate bedrock or drift can lead to the diffuse source pollutants due to the development of permanently wet increased time the pollutant is peaty topsoils. These blanket peats subject to degradation and readily shed excess rainfall laterally adsorption in the soil zone. and are classified as Low Leaching Therefore soils with a greater potential. thickness of subsoil above the parent material, while still considered to be High Leaching Characteristics which affect the speed of potential are given a lower H3 pollutant movement subclass. Pollutants can pass rapidly to groundwater or through the soil zone either where the soils suffer Influence of soil texture on pollutant mobility from a seasonally high water table When a pollutant enters the soil it caused by fluctuating groundwater partitions, or separates, into three or where the soils are shallow over phases. Some of the pollutant will bedrock, rock-rubble or gravel. adsorb onto clay particles and In many lowland soils, seasonal organic matter within the soil waterlogging is caused by matrix, some will volatilize, or fluctuating groundwater within the evaporate, into the gaseous phase soil profile. Even when and the remainder will remain as a underdrainage is installed it is likely liquid. After partitioning, the that during the wettest parts of the concentration of pollutant in the year, there will be groundwater at soil is determined by the relative relatively shallow depth within the volume of pollutant and water soil. In such circumstances, even retained within the soil. Only a relatively short-lived pollutants, proportion of this retained liquid, GUIDE TO GROUNDWATER VULNERABILITY MAPPING IN ENGLAND AND WALES

that is the mobile fraction, which Intermediate Leaching potential and varies between soil types, is likely to a subclass 12 established for peat move downwards through the soil, soils. taking pollutants with it. Any potential pollutant within the Soil characteristics affecting degradation or remaining non-mobile liquid attenuation of potential pollutants fraction, which resides in very small In broad terms, the ability of a soil pores, will leach only very slowly by to adsorb pollutants depends upon the process of diffusion into the its cation exchange capacity which, mobile water. under temperate soil forming For a conservative pollutant such conditions, depends mainly on as road salt, that does not degrade organic matter and clay content, and cannot be metabolized by plants although/)// can be an important or micro-organisms, the entire modifier. Thus, the lower the soil volume of pollutant held within the organic matter and clay contents, non-mobile phase will eventually the lower the ability of the soil diffuse out and leave the soil profile. profile to attenuate potential However, for non-conservative pollutants. Based on a statistical pollutants, this period of diffusion analysis of 6 500 soil samples by allows the pollutant to be degraded SSLRC, it has been possible to by microbial activity such that less group soils into 4 broad classes of of the pollutant originally retained adsorption potential according to in the non-mobile phase may be their overall clay and average topsoil able to pollute groundwater. organic matter contents under a In sandy soils, the volume of range of land uses. water retained by the soil is Soils with a low adsorption relatively small and most of the potential comprise well drained retained water is in the mobile sandy or near-sandy soils with low phase. In these soils, therefore, the clay and high sand contents concentrations of any dissolved throughout and a low average pollutant can be relatively high and topsoil organic matter content. a large proportion is likely to leach These soils are classified as High quickly out of the soil profile. Sandy (H2) Leaching potential. soils thus have significantly less Soils with a moderate adsorption potential than other soils to potential comprise well drained attenuate potential pollutants sandy or near-sandy soils with low through the processes of dilution, clay and high sand contents retardation and degradation. A throughout and a moderate average subclass (H2) is therefore topsoil organic matter content. established in the High Leaching These soils are classified as High potential class to cater for sandy (H3) Leaching potential. soils. Deep, moderately permeable Conversely, peat soils and soils loamy and clayey soils with a low or with a high silt and/or clay content moderate adsorption potential and are all able to retain significantly which do not suffer marked seasonal larger amounts of water, most of waterlogging are classified as which is held tightly in the non- Intermediate (II) Leaching mobile phase. In such soils, potential. therefore, the concentrations of any Soils with a high adsorption dissolved pollutant are likely to be potential comprise soils with lower than in sandy soils and, organic mineral or peaty textures in because a large proportion will be some part. Due to their high organic retained in the non-mobile phase, matter content these soils are most will leach only slowly, giving classified as Intermediate (12) more time for degradation to occur. Leaching potential. These soils are grouped as

14 3 MAP PRODUCTION METHODOLOGY AND DATA SOURCES

3.1 OVERVIEW The production of each Groundwater Vulnerability Map (Fig. 3.1) involves five main stages, with a consultation period for Quality Assurance checks after each of the first four stages. These ensure that potential errors are found and corrected before they are compounded in subsequent stages. Discrepancies and local NRA requirements are discussed as early as possible in the process. The five stages are as follows and the numbers correspond to those in Figure 3.1. 1. Preparation of an aquifer classification which places all geological units present in a map sheet area into the categories of Major Aquifer, Minor Aquifer, Non-Aquifer (Section 3.2.2). Low permeability drift deposits (Section 3.4) are also identified at this stage. 2. Preparation of Ordnance Survey (OS) film bases showing the extent of the various aquifer classes and of the low permeability drift deposits 3. Overlaying soil leaching potential classcs (Scction 3.3.2) onto areas of Major and Minor Aquifers 4. Production of a four colour ‘cromalin’ final proof map 5. Production of final artwork from which the printers make the printing plates.

3.2 GEOLOGY geological classification which 3.2.1 Sources of Geological Data begins with the following statement: The Groundwater Vulnerability Geological strata which contain Maps are compiled from the most groundwater in exploitable recent BGS 1:50 000 or 1:63 360 quantities are termed aquifers, scale geological maps. Generally, whereas rocks which are largely modern geological mapping is impermeable and do not readily published at 1:50 000 scale but transmit water are termed non­ coverage is not yet complete (Fig. aquifers. Aquifers vary in their 3.2). Mostly these are published general and hydraulic maps, but in a few cases properties and can be classified compilations of recent, as yet as fractured, fracture- unpublished, mapping are made intergranular and intergranular available. Where there is no map and it is these properties, coverage at these scales suitable particularly in the upper 1:25 000, 1:10 000 or 1:10 560 scale unsaturated zones, which form maps have been used. The few the basis of the groundwater remaining gaps have been filled with vulnerability assessment. Old Series 1:63 360 scale maps and locally 1:250 000 and 1:625 000 scale The three basic geological classes are maps have been used for the solid Major Aquifers (Highly permeable), and drift deposits respectively where Minor Aquifers (Variably no other mapping is available. The permeable) and Non-Aquifers source material has been (Negligibly permeable) (Table 3.1). photographically adjusted to It is important not to confuse the 1:100 000 scale in order that the use of these classification on the pertinent boundaries can be Groundwater Vulnerability Maps extracted. with groundwater resource potential, because the descriptors relate specifically to the vulnerability 3.2.2 Geological Classification of the unsaturated zone and not The legend to each vulnerability necessarily to the saturated zone map includes a description of the resource potential. Information on GUIDE TO GROUNDWATER VULNERABILITY MAPPING IN ENGLAND AND WALES

FIGURE 3.1 Stages in producing Groundwater Vulnerability Mops □

□

□

□

16 Map Production Methodology and Data Sources

FIGURE 3.2 Geological map coverage at 1:50 000 and 1:63 360 scale (excluding Old Series Maps) used in the compilation of the Groundwater Vulnerability Maps

resource potential is available on Minor Aquifers (Variably hydrogeological maps (Fig. 3.3). permeable): Fractured or potentially The three classes are broadly fractured rocks, which do not have a described as follows: high primary permeability (e.g. Major Aquifer (Highly permeable): Millstone Grit), or other formations Highly permeable formations of variable permeability (e.g. usually with a known or probable alluvium) including unconsolidated presence of significant fracturing. deposits, such as the Crag, or This class includes the outcrop areas permeable drift deposits (e.g. glacial of the three regionally most sands and gravels). Full lists of these significant aquifers: the Chalk, the aquifers are given in Appendix 1. Permian and Triassic sandstones and Non-Aquifers (Negligibly the Jurassic limestones (the latter permeable): These are formations largely a confined aquifer). Full lists which are generally regarded as of these aquifers are given in containing insignificant quantities of Appendix 1. groundwater, although some may GUIDE TO GROUNDWATER VULNERABILITY MAPPING IN ENGLAND AND WALES

TABLE 3.1 Major Aquifers (highly permeable) Minor Aquifers (variably permeable) Generalized Classification of Non«Aquifeis (negligibly permeable) Types of Strata (refining Chalk blown sand Palaeogene clays Appendix 3 in NRA, 1992) Upper Greensand alluvium Gault and other Lower Cretaceous clays Lower Greensand dry valley deposits Jurassic clays Spilsby Sandstone river terrace deposits Penarth Group Jurassic limestones glaciofluvial sand and gravel deposits Mercia Mudstone Group Upper Lias Sands Crag Permian marls Dolomitic Conglomerate Pliocene gravels Silurian (except limestones) Sherwood Sandstone Group Palaeogene sands Ordovician Permian sandstones Lower Cretaceous sands (except Cambrian greensands and Spilsby Sandstone) Magnesian limestones Precambrian Jurassic limestones, sandstones and Carboniferous Limestone ironstones (except where Major intrusive igneous rocks Aquifers) Permian breccias and conglomerates (SW England) Coal Measures Millstone Grit Lower Carboniferous limestones and shales Old Red Sandstone Silurian limestones volcanic rocks See Appendix 1 for detailed classification and regional variations.

yield water in sufficient quantities and Wales. Both published and for domestic use. Full lists of these unpublished information, at a aquifers are given in Appendix 1. variety of scales ranging from All permeable drift deposits are 1:25 000 to 1:250 000, have been mapped as Minor Aquifers; although used. Approximately one-third of wherever they overlie a Major England and Wales is mapped at Aquifer this takes precedence. All detailed scales of 1:63 360 or greater permeable drift deposits are mapped (Figure 3.4) while the whole of regardless of whether they contain England and Wales is covered by the groundwater (potentially an aquifer) National Soil Map at a scale of or are dry (the base of the deposit is 1:250 000 (Soil Survey Staff, 1983). situated above the water table) as In general, individual soil types this may vary from season to (known as soil series) are shown on season. Drift deposits can vary the more detailed soil maps, while greatly in composition vertically and soil associations (or groupings of horizontally, so that their hydraulic soil series of similar character) are properties which are used in the mapped on the National Soil Map. aquifer classification may change Soil series and soil associations have over very short distances. been discussed in greater detail in Section 2.3.2. 3.3 SOIL 3.3.1 SSLRC soil maps 3.3.2 Soil leaching potential The soil information used in the classification Groundwater Vulnerability Maps is A methodology for classifying of based upon maps and reports soils into three leaching potential produced since 1939 by the Soil classes has been devised by SSLRC Survey and Land Research Centre, for use in the Groundwater formerly the Soil Survey of England Vulnerability Maps. This

18 Map Production Methodology and Data Sources

FIGURE 3J Availability of hydrogeology maps for England and Wales

classification embraces all soil series be adsorbed in the soil layers in past and current use on detailed • diffuse source pollutants which soil maps; and all the soil are soluble and can readily pass associations of the National Soil through the soil layers without Map. With regard to the soil being adsorbed. associations, the individual soil However, due to the widely series are classified according to differing chemical properties of their leaching potential and the these types of pollutants, the soil leaching class found to be the most leaching potential classification widespread is then used to define devised by SSLRC is of necessity the association. generalized. In order to produce a As described in Section 2.4, the single map that illustrates the maps aim to illustrate the ability of vulnerability of groundwater to the soil to attenuate three types of these differing pollutants, the pollutants: properties of a ‘representative’ • liquids pollutant was used. This ■p • diffuse source pollutants which ‘representative’ pollutant is: under certain circumstances can

GUIDE TO GROUNDWATER VULNERABILITY MAPPING ENGLAND IN AND WALES

FIGURE 3.4 Areas af detailed published soil maps produced by SSLRC at (a) 1:25 000 scale and (b) 1:50 000 or 1:63 360 scale used in the production of the maps.

(b)

/ Mop Production Methodology

• soluble in water, so that it potential classes (High, moves through the soil column Intermediate and Low) and six in solution subclasses (H i, H2, H3, II, 12 and • able to adsorb (or stick) onto HU-Urban). See inset and clay particles and organic Appendix 2. The classification is matter. applied to soils overlying Major and Minor Aquifers but not to those It is recognized that some overlying Non-Aquifers. pollutants which pose a threat to groundwater quality do not have these properties. For these 3.3.4 Urban or Disturbed Soils pollutants, the leaching potential Soil information relating to urban may be higher or lower than that areas is often limited and soil maps shown on the maps. The use of the are locally less reliable than for rural maps in assessing groundwater areas. This is often also the case for vulnerability to some of these disturbed sites such as restored pollutants is discussed in Sections mineral workings where it may be 4.1.2 and 5.6.3. impossible to give reliable soil The classification groups all soils leaching potential classes without into one of three soil leaching detailed site investigation. For

SOIL LEACHING POTENTIAL CLASSES The classification defines 3 main categories of leaching potential ranging from High to Low.

High Soil Leaching Potential The High category is subdivided into 4 subclasses with soils in the HI subclass having a greater soil leaching potential than H2 and H2 soils a greater potential than H3. HI Soils with groundwater at shallow depth. Soils with rock, rock-rubble or gravel at shallow depth. Undrained lowland peat soils with permanently wet topsoils. H2 Sandy soils with a low topsoil organic matter content. H3 Sandy soils with a moderate topsoil organic matter content. Soils with rock, rock-rubble or gravel at relatively shallow depth within the soil profile. HU Soils in urban areas and areas of restored mineral workings.

Intermediate Soil Leaching Potential The Intermediate category is subdivided into 2 subclasses; mineral soils are placed in II and peat soils in 12. 11 Deep loamy and clayey soils unaffected by marked seasonal waterlogging, with a topsoil of low or moderate organic matter content. 12 Lowland peat soils which have been drained for agricultural use.

Low Soil Leaching Potential There is no subdivision of the Low category of soil leaching-potential. L Soils with a dense subsoil which restricts downward water movement. Upland soils with a permanently wet peaty topsoil. GUIDE TO GROUNDWATER VULNERABILITY MAPPING IN ENGLAND AND WALES

example, imported topsoil and Wherever there is no site specific subsoil materials used in restoration detailed information on drift schemes can differ markedly in deposits there is uncertainty about permeability and thickness to the their composition and thickness. original natural soil. Extensive urban They are therefore treated areas and large restored mineral everywhere as a special case in this workings are therefore assumed to groundwater vulnerability have High leaching potential, until assessment. proved otherwise, but are designated Generally, wherever low uniquely by the HU subclass. permeability drift deposits occur at Locally, because of the small the surface and overlie (directly or 1:100 000 scale of the Groundwater indirectly) Major Aquifers or Minor Vulnerability Maps, the age of some Aquifers they are interpreted of the OS base maps and the lack of separately from the aquifer site specific information, a High classification and depicted on the (HU) leaching potential designation map by a black stipple ornament. may not be shown in some relatively The presence of the stipple small built up areas or in areas of ornament indicates that a drift active or recently restored mineral deposit of generally low workings. However, it should not be permeability (such as till or peat) is assumed that the soil is everywhere known to be present, but that the undisturbed in these areas but it degree of protection afforded to the should be borne in mind that the underlying aquifer is uncertain. leaching potential is always based on However, the aquifer is probably an undisturbed soil profile. better protected than if low permeability drift was absent. In these areas, local site specific 3.4 LOW PERMEABILITY DRIFT DEPOSITS enquiries will be required and, if Variable, but generally low need be, exploratory boreholes permeability drift deposits are drilled in order to establish the difficult to classify in terms of the nature, thickness and integrity of likelihood of pollutant movement the low permeability drift cover. through them. Clearly where there Locally, where the low is 10 m or more thickness of clay- permeability drift deposits are rich till (boulder clay) overlying an considered to be sufficiently thick aquifer, that aquifer is likely to be (greater than 5 m) to afford well protected from surface complete protection to underlying pollutants. On the other hand a 3 m Major or Minor Aquifers from thickness of silty till containing surface derived pollutants those sand and gravel lenses and vertical aquifers are not shown. The special fractures may offer little, if any, cases where this has been carried protection to an underlying aquifer. out are discussed in Section 4.3

2 2 4 USE AND INTERPRETATION OF THE MAPS

4,1 OVERVIEW OF INFORMATION PROVIDED BY THE MAPS The maps incorporate three layers of information, which from the surface downwards are: • soil leaching potential classes (High/Intermediate/Low) and subclasses • presence, where applicable, of low permeability drift deposits at the surface above aquifers by stipple ornament • permeability of the geological deposits (Major/Minor/Non-Aquifers). Together these layers of information produce 27 different vulnerability combinations which are shown on the maps. In order to use the maps to their full potential it is necessary: • to appreciate the potential complexity of the possible combinations of aquifer types and drift deposits which are represented by the restricted number of simple classes shown on the maps (fig. 4.1) • to understand the relevance of the soil leaching potential classification to individual pollutant groups • to interpret the relationship between soil leaching potential classes and presence or absence of low permeability drift deposits at the surface.

4.1.1 Combinations of aquifer types and unlikely to be vulnerable to surface- drift deposits derived contamination in this Any site in England and Wales will position. Figure 4.1 shows be underlain by one or more of the schematically the range of strata following types of geological unit; that have been encompassed by each permeable drift deposits; low of the five combinations. permeability drift deposits; Major Aquifers; Minor Aquifers; or Non- 4.1.2 Treatment of different pollutant Aquifers. However, for any vertical types sequence of such deposits the It is recognized that many types of Groundwater Vulnerability Maps pollutant with differing chemical reduce the natural complexity to and physical properties pose a threat show only 5 combinations based to groundwater quality. Table 4.1 upon permeability characteristics. below indicates the relative These are: vulnerability of groundwater to • Major Aquifer pollution from 3 broad groups of • Major Aquifer with drift stipple pollutants applied to soils with • Minor Aquifer leaching potential classes given in • Minor Aquifer with drift stipple the key to the maps. Non-adsorbed • Non-Aquifer. diffuse pollutants are those which are applied across wide areas and The simplification of the many which do not bind onto the soil geological sequences to arrive at one (e.g. nitrates from agricultural use) of these 5 combinations is achieved while adsorbed diffuse pollutants by only classifying strata down to can include chemicals such as the first solid deposit and by pesticides and herbicides. restricting the classification of drift to the surface deposit. Geological units occurring beneath the uppermost solid strata are never considered in the aquifer classification, even if they may potentially be Major or Minor Aquifers. It is considered they are GUIDE TO GROUNDWATER VULNERABILITY MAPPING IN ENGLAND AND WALES

FIGURE 4.1 The range of geological sequences encompassed in the combinations used on the maps

Major Aquifer without stipple

Key

Low permeability drift

□ Permeable drift (Minor Aquifer)

Major Aquifer with stipple 1 1 Solid Minor Aquifer

□ Major Aquifer

I Non-Aquifer

N.B. Vertical scale exaggerated

Minor Aquifer without stipple

Low permeability drift > 5 m thick*

Minor Aquifer with stipple

Non-Aquifer

Low permeability drift > 5 m thick**

The treatment of thick deposits of low permeability drift in parts of Northumbria and Yorkshire and North West NRA Regions. See Section 4.3.2 for further detail. * * The treatment of thick deposits of low permeability drift in parts of Northumbria and Yorkshire NRA Region. However, in North West NRA Region the Major Aquifer is shown as Major Aquifer with stipple ornament. See Section 4.3.2 for further detail.

24 TABLE 4.1 Soil teaching Pollutant type A provisional assessment of potential class liquid Konoadsorbed diffuse Adsorbed diffuse the relative vulnerability of discharges pollutants pollutants groundwater to pollution HI ✓ ✓ ✓ ✓ ✓ ✓ from certain pollutant types H 2 ✓ ✓ ✓ ✓ ✓ ✓ applied to soils with H 3 ✓ ✓ ✓ ✓ ✓ different teaching potential 11 ✓ ✓ ✓ 12 ✓ ✓ X

L X X X ✓ The larger the number, the greater the risk of pollutant moving towards groundwater. X Negligible risk of pollutant moving towards groundwater.

4.1.3 Combinations of soil leaching 4.2 EXAMPLE INTERPRETATIONS potential classes and the presence or 4.2.1 Application for disposal of liquid absence of drift stipple waste within the soil zone All groundwaters are controlled Waste may be disposed of beneath waters, and are therefore protected the topsoil, but within the soil zone, by both European and national through a number of activities, such legislation against pollution so that as: for most types of land use and many • near-surface soakaways activities which present a threat to • septic tank discharges groundwater quality, the most • injection of farm or sewage relevant information provided by sludges. the maps will be the assessment of the protection afforded to In addition, in areas where the groundwater by the soil and topsoil has been removed but the unsaturated zones. remainder of the soil profile is In order to fully utilise the undisturbed, the application of information on the maps it is potential contaminants and the necessary to understand and spillage of chemicals will be interpret the relationship between equivalent to disposal beneath the the various combinations of the topsoil. presence or absence of low At first glance, the Groundwater permeability drift deposits (stipple Vulnerability Maps may not appear ornament) and the soil leaching to be directly suited to provide an potential classes given in Table 4.2. interpretation on such issues, as the A schematic ranking is given to each point of entry of the pollutants into combination according to the the soil-rock column is beneath the vulnerability of groundwater in surface. However, by re-interpreting underlying aquifers ranging from 11 the leaching potential classes (Table (lowest) to 1 (highest). In those 4.3) it is possible for the maps to combinations which suggest there is provide some guidance in such a discrepancy between the soil and cases. drift geology classification, the soil However, there are a number of classification has, for convenience, cautionary points which need to be been assumed to be correct. borne in mind when considering However, there will be some such activities: localities where the geological • soil types which make excellent mapping is more detailed and/or soakaway material, such as more recent than the soil maps and sandy soils, have high soil here the geological mapping may leaching potentials (H2 and H3) take precedence. • soils with Low leaching potential which provide significant protection to GUIDE TO GROUNOWATER VULNERABILITY MAPPING IN ENGLAND AND WALES

TABLE 4.2 Soil Leaching Presence/absence of Relative vulnerability Provisional interpretation A schematic ranking of the Potential dass drift stipple of groundwater in relative vulnerability of underlying aquifers* groundwater in underlying Low (L) Present 11 High degree of protection is afforded to any aquifers represented by the underlying groundwater. Low (L) Absent 10 Soil data suggesting high degree of protection range of combinations of present. soil leaching potential dosses and presence or Intermediate (12) Present 9 Groundwater quality within peat is at risk but a absence of low permeability significant degree of protection is likely to be surface drift deposits afforded to groundwater within underlying solid formations due to low permeability of peat. (stipple ornament). Intermediate (12) Absent 9 Groundwater quality within peat is at risk but a significant degree of protection is likely to be afforded to groundwater within underlying solid formations due to low permeability of peat.

Intermediate (11) Present 8 If drift deposits are of low permeability, excess water is likely to be shed laterally within the soil zone so that there is a moderately high degree of protection afforded to any underlying groundwater. Intermediate (11) Absent 7 Moderate degree of protection is likely to be afforded to groundwater by deep loamy and/or clayey soils.

High (H3) Present 6 The low permeability drift deposit will be thin but will afford some limited protection to groundwater. High (H3) Absent 5 Very limited protection is likely to be afforded to groundwater due to the shallow depth or very permeable nature of the soils overlying aquifer material.

High (H2) Present 4/3 Mismatch between datasets suggested - soils are deep and sandy (permeable), geology indicates low permeability drift at surface. High (H2) Absent 3 O nly limited protection is likely to be afforded to groundwater due to the highly permeable nature of these soils.

High (H I) Present 2 The low permeability drift will afford only very limited protection to groundwater due to the very shallow depth to aquifer material or groundwater. High (H i) Absent 1 Only extremely limited protection is likely to be afforded to groundwater due to the shallow depth to aquifer material or groundwater. '* 11 Lowest vulnerability 1 Highest vulnerability

groundwater generally make • In soils which suffer seasonal poor soakaway material waterlogging, either associated • depending upon local with high groundwater levels topography and the proximity (some HI soils) or poor of surface water features, lateral infiltration of precipitation (L downslope movement of and some II soils), pollutants within the relatively underdrainage may be installed permeable upper horizons of to alleviate soil wetness. The soils classified as Low leaching drains then provide the potential may pose a threat to potential for the rapid surface water quality. In movement of applied liquids, addition, the downslope such as slurry, out of the soil movement within the soil may profile and into nearby surface lead to the contaminated water water courses. recharging nearby aquifers by lateral movement into and Using the information provided through more permeable soil in Table 4.3 it may be types. recommended, in conjunction with

26 Use and Interpretation of the Maps

TABLE 4.3 SSLRC Soil Presence/ Provisional revised soil leaching potential doss leaching absence of A provisional modification potential dass drift stipple of soil leaching potential L Present High degree of protection likely. dosses for the disposal of Absent* potential pollutants within 12 Present If peat is deeply drained, may stay as 12. Otherwise should be treated as H i due to the the soil zone, taking into proximity of the water table though some shallow peats may be best treated as 11. account the nature of the Absent These probably include areas of peat soils too shallow to be identified by geological maps. Generally these soils should be treated as HI or, more rarely as 11 or H2 surface drift deposits. depending on the depth to groundwater or presence of sandy subsoil textures respectively.

11 Present Where the soils contain slowly permeable horizons, they remain as 11 or, depending upon the degree of waterlogging become L. Absent Where the soils are moderately permeable, they remain as 11, but may become HI or H3 (if aquifer material or a water table is present at relatively shallow depth).

H3 Present* Absent Sandy soils become H2 or locally 11 where the sands overlie loamy or clayey material. Shallow soils over aquifer material and soils containing groundwater at relatively shallow depth may become H i.

H2 Present* Absent Most soils remain as H2 but if groundwater is at relatively shallow depth these soils may become HI. Where aquifer material is at relatively shallow depth these soils become H3 and where the sands overlie loamy or clayey material they become 11.

HI Present* Absent All soils remain as HI, but the risk of groundwater contamination is increased. ”■ The occurrence of these combinations require further investigation before modifications can be proposed.

the Acceptability Matrices in the 4.3.2 Treatment of thick low permeability GPP, that soil zone disposal does drift cover not take place in areas classified on In and around the Vale of York and the maps as High (HI, H2 & H3) on Holderness in Northumbria and leaching potential or in areas of Yorkshire Region; wherever thick shallow peat soils of Intermediate clayey drift deposits (5 m thick) (12) leaching potential. If an aquifer overlie potential solid Major and is known to be present within less Minor Aquifers, those potential than about 2 m below ground aquifers are considered to be surface, then care should be taken in protected from surface-derived disposing of any liquids in areas pollutants. Where spatial data is classified as Intermediate (II) available delineating areas with this leaching potential. This information type of thick clayey drift cover the should be considered in addition to aquifer classification has been used the determination of the type of to zone the surface drift deposits aquifer present (Major or Minor (permeable deposits are shown as Aquifer). Minor Aquifers and low permeability deposits as Non- Aquifers). Potential Major and Minor Aquifers have so far been 4.3 REGIONAL DIFFERENCES IN LOW taken out of the classification PERMEABILITY DRIFT INTERPRETATION system in this manner on Maps 8, 9, 4.3.1 Introduction 12, 13. The range in permeability of In the North West Region deposits classified as generally of (Vulnerability Map 16) along the low permeability has necessitated River Mersey, the Sherwood the use of regional special cases. The Sandstone Group is known to be cases made so far are described locally overlain by more than 5 m of below. clay within the drift sequence. In GUIDE TO GROUNDWATER VULNERABILITY MAPPING IN ENGLAND AND WALES

these areas where permeable drift classified as low permeability drift deposits are present at the surface deposits similar to the treatment of the Sherwood Sandstone is shown as upland peats. a Minor Aquifer but where low permeability drift deposits are at the surface, the sandstone is portrayed 4.3.4 Brickearth as a Major Aquifer with low Brickearth varies considerably in permeability drift stipple ornament. composition. In Sussex, on Sheets 45 and 46 where it is relatively 4.3.3 Peat permeable, it is shown as a Minor On a few of the early vulnerability Aquifer but further east and west maps (Sheets 18, 24 and 25, mainly along the south coast (Sheets 44, 47 in East Anglia) drained lowland and 52) it is classified as a low peats were considered to offer only permeability drift and where partial protection to underlying applicable indicated by a stipple aquifers and were therefore not ornament. On Sheet 33, the older classified as low permeability drift. (glacial) brickearths are shown as However, subsequently all peat low permeability drift, while the deposits shown on geological maps post-glacial brickearths are classified whether drained or undrained, are as Minor Aquifers.

28 5 LIMITATIONS OF THE GROUNDWATER VULNERABILITY MAPS

5.1 INTRODUCTION Although the methodology of producing the Groundwater Vulnerability Maps incorporates various Quality Assurance checks, there are a number of unavoidable limitations and necessary compromises associated with the data and methodology that need to be borne in mind when using the maps. These include: • attempting to portray the 3-dimensional Continua of the soil and geological columns in 2-dimensions • mismatches resulting from the overlaying of information of differing scales • attempting to sub-divide a continuum of soil, geological and contaminant properties into a number of discrete classes • limitations resulting from overlaying data from different sources with different base maps.

Limitations manifest in the geological map relates to a geological, soil and drift data, lithological and strati graphical together with limitations in a site- nomenclature, not to a given rock specific context are discussed in the type. For example, the Coal following sections. However, it Measures comprise sandstone, shale, should be remembered that this seat-earth and coal, each with very series of maps are intended as a different hydrogeological properties first-pass assessment of the likely but which are shown collectively as vulnerability of groundwater in one unit, except for significant England and Wales. Many of the sandstones and coals which may be limitations highlighted in this mapped separately. On the chapter are intrinsic to any Groundwater Vulnerability Maps the groundwater vulnerability Coal Measures are classified assessment not dealing with site collectively as a Minor Aquifer, even specific information. though the shales, seat-earth and coal strata have the properties of 5.2 GEOLOGICAL DATA LIMITATIONS Non-Aquifers. 5.2.1 Introduction The aquifer classification depends General limitations imposed by the on mapped geological units quality of geological mapping may corresponding to aquifer units. This is generally satisfactory, as both are be due to: based on lithology. However, locally • lack of data on the variability of important aquifer units may be too mapped units thin to map and may be • inadequate description of drift undifferentiated from Non- deposits Aquifers. • presence of bypass features In addition litho-stratigraphical o difficulties of portraying multi- units may change in lithology and and sequential aquifer systems hydrogeological character from as a single aquifer. place to place, for example the Upper Greensand. Locally, 5.2.2 Lack of data on the variability of permeability characteristics change gradually but sufficiently for the mapped units unit to change aquifer types, in Generalizations may have to be these circumstances, wherever made in compiling the geological classes because of insufficient data possible the boundary has been on the variability of mapped units. drawn at the margin of a map sheet to ease compilation. A single mapped unit on a GUIDE TO GROUNDWATER VULNERABILITY MAPPING IN ENGLAND AND WALES

5.2.3 Inadequate description of drift zone to be ineffective. They include deposits cracks, joints and other features Drift deposits are widespread in offering secondary permeability such England and Wales yet drift geology as in limestones, sandstones and the maps are not as universally available Chalk, and topographic depressions as those for solid bedrock geology. in which recharge potential is For some areas drift mapping is enhanced. They also include sink incomplete (showing only river holes in karst limestone rocks and terrace deposits and alluvium) and man-made features such as some older maps do not show even soakaways and other excavations the most rudimentary subdivisions which allow the whole of the of the glacial deposits such as sand unsaturated zone to be by-passed. and gravel from till. It is likely These typical bypass features may therefore that, in some areas of turn an apparently secure non- England and Wales, permeable drift vulnerable aquifer into one at risk. deposits qualifying as Minor Aquifers will be more extensive than 5.2.5 Portrayal of multi- and sequential shown on the Groundwater aquifer systems Vulnerability Maps. It is a convention, except for some Poor understanding of lithology, areas of North West and thickness and variability of drift Northumbria and Yorkshire Regions deposits has resulted in the use of a discussed in Section 4.3, that where stipple ornament on this map series the uppermost solid geological unit to represent low permeability drift is a Major or Minor Aquifer, the at the surface. This is because it is extent of that unit is shown on the often not possible to characterize map regardless of the properties of drift sufficiently accurately to any overlying drift deposits. As a incorporate them into a consistently result, the aquifers delineated on the defensible aquifer classification Groundwater Vulnerability Maps do system. However, where the not necessarily correspond to the deposits are known to contain permeable horizons, they are geological parent material of the overlying soil. generally classified as Minor Aquifers, rather than Non-Aquifers Ground-water Gley Soils is one of the 10 Major Soil Groups (e.g. some brickearths, morainic recognised by SSLRC in the drift). In this way the maps national soil classification system. therefore represent a worst case This group of soils is developed scenario in relation to drift deposits. within or over permeable materials The identification of geological and suffers likely periodic seasonal boundaries beneath significant waterlogging associated with thicknesses of superficial drift fluctuating groundwater levels. Due deposits is inevitably less reliable to the shallow depth of groundwater than for boundaries occurring at the during a considerable part of the surface. year, they are classified as High (H i) soil leaching potential. 5.2.4 Presence of bypass features However, the soil water table may More specific limitations arise from not be in hydraulic continuity with the uncertainty of geology itself. the groundwater body in the Regardless of bedrock geology and underlying mapped aquifer. It is drift deposit cover, bypass features therefore necessary to take account may be present that allow the rapid of local topography and geology in passage of percolating rainwater order to ascertain whether the high from the land surface to the water groundwater levels in the soil are table. These features may enable the related to the aquifer beneath or are moderating effects of the in fact perched some distance above unsaturated zone below the soil it.

30 Limitations of the Groundwater Vulnerability Mcps

FIGURE 5.1 Alluvial Alluvial soil (H I) Dingrammotic cross section groundwater table Marine alluvium of the possible misrepresentation of the groundwater vulnerability Chaik Impermeable of some confined aquifers. potentiometric glacial drift surface

Chalk Aquifer (Major Aquifer)

For example, on Groundwater 5.3 SOIL DATA LIMITATIONS Vulnerability Sheet 13 (Humber 5.3.1 Variability within soil mapping Estuary), a large part of the units southern shore of the Humber General purpose soil maps produced estuary is mapped as Major Aquifer by SSLRC show the distribution of with an HI leaching potential (Fig. soils by the use of three types of 5.1). The high groundwater levels map unit, according to the scale of which give the soils their high the final map. These are: leaching potential occur within the more permeable layers of the marine • Soil series alluvium. The Chalk, which • Soil complexes constitutes the Major Aquifer, is • Soil associations. confined beneath a thick sequence The way in which the soil leaching of impermeable glacial drift deposits potential classification is assigned which in turn underlie the marine differs according to the type of map alluvium. The Chalk is therefore unit. likely to be protected from any pollutants that may be within the Soil series alluvial groundwater body, and the In general, if a block of land is HI leaching potential classification shown on a detailed soil map to be is overly pessimistic in relation to uniformly one soil series, it contains Chalk but highly pertinent to at least 70 % of that named series. groundwater bodies in the marine The remaining 30 % may include alluvium. several other, often unnamed, series. It is therefore locally important Although these are usually similar that the Groundwater Vulnerability to the named series, due to the Maps are interpreted in conjunction natural heterogeneity of soil, with the soil and geological occasional contrasting soils may be information on which they are found. based. There are many cases where The soil leaching potential class drift Minor Aquifers support soils given to this block of land is that of whose leaching potential is classified the named series. There are, on the maps, but which overlie older therefore, small patches within the Major or solid Minor Aquifers mapped area of the soil series which are the strata classified on the (maybe up to 30 %) which may be map. If low permeability drift of lower or higher soil leaching deposits occur between the zoned potential. Minor or Major Aquifer and the permeable drift deposit at the Soil complexes surface, there is the potential for When two or more soil series are . . overstating the vulnerability of the • intricately mixed such that they zoned aquifer. cannot be mapped separately, even on detailed soil maps, map units called soil complexes are defined. GUIDE TO GROUNDWATER VULNERABILITY MAPPING IN ENGLAND AND WALES

When assigning a leaching potential (Table 5.1), which in turn relates class to these units it is based on the back to the mapping quality of the soil series included within the original source soil map (Figure complex with the highest leaching 3.4). potential class, unless this series is described to be of very limited extent. It is likely therefore that 5.4 DRIFT DATA LIMITATIONS significant proportions of soil 5.4.1 Mismatch with soil information complexes may be of lower leaching There are certain combinations of potential than that assigned, soil leaching potential classes and although discrete areas will be of low permeability drift stipple limited lateral extent. ornament which may occur when there is a difference in interpretation between soil and drift Soil associations geological mapping. These are: Soil series and soil complexes are used on detailed soil maps which are • Low leaching potential without available for about one-third of drift stipple England and Wales. Elsewhere, the • High (H2 and sandy H3) leaching potential classes are based leaching potential with stipple on the soil associations of the • Intermediate (12) leaching National Soil Map. In all but 39 of potential without drift stipple these, the series providing the name • High (HI and shallow H3) of the soil association (the lead leaching potential with stipple. series) forms an extensive and often These differences can result from: dominant component of most delineations. Therefore the soil • the availability of more detailed leaching potential class given to the soil mapping than geological association is that of the lead series, mapping, or vice versa although there will inevitably be • inadequate mapping of drift areas within the association deposits on many early (generally 30—60 %) where soils geological maps may have either a higher or lower • disagreement in the definition leaching potential. of a drift aquifer The remaining 39 soil associations • a lack of understanding or are more variable in composition, information on the texture of having a number of inextensive and some drift aquifers. dissimilar soils and are named after The individual cases outlined above the soil series which characterises are discussed in the following them best. Each of these sections. associations is given the leaching potential class which occurs most widely among its constituent soil Low leaching potential without drift stipple series. For example, the Holidays Soils are routinely examined to a Hill association is assigned an II depth of 1—1.2 m during a soil leaching potential class as it contains survey and the leaching potential 30 % Holidays Hill series (II), 20 class is assigned based upon the % Shirrel Heath series (H3), 20 % properties of the soil to 1 m depth. Kings Newton series (L), 10 % For a soil to be classified as Low Isleham series (12), 10 % Rapley leaching potential it must have series (II) and 10 % unspecified soil physical properties preventing the series. movement of water (and pollutants) The reliability of the leaching down through the soil profile to potential estimates given on the 1 m depth. It is implicit in this soil vulnerability maps therefore classification that a permeable depends on the scale of soil map deposit must extend below 1 m and the type of soil map units used depth to qualify as an aquifer.

32 limitations of the Groundwater Vulnerability Maps

TABLE 5.1 Map scale Mapping Unit Basis of leaching Reliability of leaching potential estimate potential estimation The basis and reliability of soil leaching potential Soil Series Named soil series Very good but may include small areas of different 1:10 000 leaching potential estimates for each typo of 1:25 000 soil map unit. 1:50 000 Soil complex Soil series with Good but possibly includes large areas of lower 1:63 360 highest leaching leaching potential, although individual patches of potential limited extent tSoil Association Lead soil series Regionally good but likely to include restricted areas of lower & higher leaching potential 1:100 000 1:250 000 ^Variable Soil Most extensive Regionally satisfactory but may include substantial Association leaching potential areas of lower & higher leaching potential class * Name of Soil Association given in capitals on the legend for the 1:250 000 scale Soil Map of England and Wales ’•'Name of Soil Association given in mixed case type on the legend for the 1:250 000 scale Soil Map of England and Wales

However, it is only in the last two (in texture), much is silty or sandy decades that field geologists have clay and there may also be sand and closely examined surface drift gravel horizons within it. Therefore, deposits (to 1—1.5 m depth) with the presence of a low permeability hand augers. In older geological drift stipple ornament on sandy surveys, therefore, the soils, whilst appearing to suggest a characterization of drift deposits data mismatch, may be providing was less comprehensive and both additional information on the local low permeability and permeable lithology of the till. drift deposits may have been omitted from geological maps which Intermediate (12) leaching potential without would now be shown. In other areas stipple the permeable topsoil and shallow Soils with Intermediate (12) leaching subsoil of some Low leaching potential have peaty topsoils and potential soils (with dense slowly peat can extend to depths of permeable subsoils) were mapped as 100+ cm. Peat develops at the permeable drift deposits and have surface of these soils in response to been shown on the Groundwater high groundwater levels within the Vulnerability Maps as Minor soil profile. While all groundwaters Aquifers. In these cases, the Low are protected according to the leaching potential class refers to the Groundwater Protection Policy peat whole soil profile to 1 m depth is insufficiently permeable to yield which includes the aquifer material groundwater in exploitable as defined by BGS and the quantities and has therefore been underlying slowly permeable layer. classified by the BGS as a low If these very shallow permeable permeability drift deposit. Many layers are the object of protection geology maps only indicate the policy decisions at a local level, the presence of peat if it is greater than vulnerability of groundwater will be 1 m in thickness. higher than that shown on the map. Soil maps occasionally identify surface peat deposits up to 1 metre High (H2 and sandy H3) leaching potential with thick which are not shown on stipple geology maps. In these situations Soils within the High (H2) or those Intermediate (12) leaching potential sandy soils classed as High (H3) soils without stipple indicate where leaching potential are sandy to permeable drift deposits or solid below 80 cm depth by definition. Minor Aquifers or Major Aquifers The occurrence of a sandy soil underlie the peat. Groundwater in developed in till may reflect the these deposits is likely to be nature of the till. Not all till is clay afforded significant protection by GUIDE TO GROUNDWATER VULNERABILITY MAPPING IN ENGLAND AND WALES

soil horizons of high organic carbon potential soils representing the soils content. developed in clay-with-flints are However, as discussed in Section broadly similar to, but do not 4.3.3, peat is treated in a different exactly match the drift stipple so manner on Sheets 18, 24 and 25 that the stipple occasionally overlaps around the Wash. Here, peat is onto the shallow High (HI and H3) considered permeable where soil leaching potential soils developed in survey evidence shows the soils to the Chalk. be deeply drained and under arable Alluvium can vary in texture from cultivation. Underdrainage lowers sandy to clayey, yet geological maps the groundwater levels, resulting in seldom give any indication of this. oxidation and shrinkage of the peat Locally, where such geological which increases its permeability. information is available, areas of Therefore, on these 3 maps peat is clayey alluvium are classified by classified as a permeable drift BGS as low permeability drift and deposit but is not depicted as an so are given a stipple ornament. aquifer. However, soil information often In coastal areas soils developed in indicates that these deposits are marine alluvium with thin peat sufficiently permeable, at least in topsoils over clayey mineral subsoils their upper parts, for the principal are classified as Intermediate (12) source of waterlogging within the leaching potential. Often these areas soil profile to be due to receive a stipple ornament because groundwater. An alluvial soil of of the low permeability of the High (HI) leaching potential shown marine alluvium. In this case the on the map with a low permeability classification of an Intermediate (12) stipple ornament therefore indicates leaching potential class with stipple a difference of interpretation of . should not be interpreted as permeability between the soil indicating the presence of thick scientist and geologist. peat.

5.5 SITE SPECIFIC LIMITATIONS High (HI and shallow H3) leaching potential 5.5.1 The Ordnance Survey National Grid with stipple Soils classified as High (HI and The vulnerability maps are produced shallow H3) leaching potential by overlaying information taken because aquifer material is found at from soil and geology maps, at a relatively shallow depth within the range of scales from 1:10 000 to soil profile are unlikely to contain a 1:250 000, onto an Ordnance Survey sufficient thickness of clay to be 1:100 000 scale national grid base recognised by geologists as a low map. Compromises in accuracy permeability drift deposit. In those result from this approach for the small areas where drift stipple is following reasons: shown on such soils it is likely to • Prior to overlaying, most soil result from a mismatch between soil and geology maps have to be and geological mapping. In certain either photo-enlarged or cases, the pattern of the drift stipple reduced to exactly 1:100 000 ornament on the vulnerability maps scale. When photo-enlarging and a lower soil leaching class is small scale maps to 1:100 000 broadly similar but not coincident. scale, the National Grid lines These slight discrepancies may be rarely coincide with those on caused by the different scales and the OS published film bases, objectives of the original surveys. even though all maps are An example relates to the clay-with- precisely 1:100 000 in scale. flints deposits over the Chalk in Overlaying has routinely been Kent (Sheet 47), where the extent of carried out using fixed features the Intermediate (II) leaching such as rivers and railways,

34 Limitations of the Groundwater Vulnerability Maps

rather than the National Grid. This equates to areas on the maps of Whilst this ensures an accurate 3 mm by 5 mm or 1 mm by 15 mm. fit adjacent to water courses, it In the Jurassic rocks of the cannot be guaranteed that the Cotswolds, for example, very thin overlays are perfect fits. It must bands of sands and clays (Minor be borne in mind that a Aquifers) have been separated from misalignment in any of the maps the Major Aquifer limestones on the of only 1 mm is equivalent to dip slopes. Where the sands and 100 m on the ground. In most clays outcrop on scarp slopes, they areas of the maps, the are much too narrow to show at misalignment of soil and 1:100 000 scale. Inevitably, geological boundaries are likely amalgamation and simplification of to be less than 1-1.5 mm. It is some units has therefore been possible that, in some areas of carried out. Outcrops which are 1:250 000 scale mapping with continuous are therefore few fixed features, misalignment occasionally omitted where they are of soil boundaries of up to 2-3 thinnest, and a potential ribbon-like mm may occur. However, most pattern is broken up. The result is boundaries between soil series an exaggeration of Minor Aquifers or soil associations in the field at the expense of Non-Aquifers and are gradational, rather than of Major Aquifers at the expense of abrupt, so that any Minor Aquifers and/or Non- misalignment is likely to be less Aquifers. serious than might first appear. The vulnerability maps incorporate soil information collated • Some of the older geology maps from maps ranging in scale from published by the BGS date back 1:25 000 to 1:250 000 while to the turn of the century, ( geological information is derived before the National Grid was from 1:10 000 to 1:250 000 scale routinely placed on all base maps. Features mapped at a scale of maps produced by the OS. 1:25 000 are sixteen times smaller in Fixed features have to be used area when reduced to a scale of in aligning the overlay of these 1:100 000, while features at 1:10 000 publications. Due to the errors are 100 times smaller in area when inherent in these old maps and reduced to 1:100 000 scale. As a their use of uncertain result, a great deal of simplification cartographic projections, there has had to be carried out to ensure may be a misalignment of an acceptable compromise between boundaries of up to 2-3 mm in the representation of natural some areas. complexity and the ease of use of the maps. 5.5.2 The incompatibility between These unavoidable compromises natural complexity and map scale place strict limitations on the The natural variability of soil and resolution and precision of map geological properties is extremely information. Detailed 1:25 000 scale complex. Even the most detailed soil and geology maps often have maps (1:10 000 scale) are map separates of between 1 and 5 approximations to the actual/ hectares in extent. These are clearly distribution of soil types and too small to show at 1:100 000 scale. geological units. An enlargement or magnification of Further to these limitations in the published 1:100 000 scale source maps, some geological vulnerability map will, in these simplification has, of necessity, been instances, not necessarily provide carried out in parts of recently the best available information for a published geological maps. specific-'site. Outcrops of less than 15 hectares in In .^e of this map series, the area have generally been omitted. /', ./ of soils, geological strata and GUIDE TO GROUNDWATER VULNERABILITY MAPPING IN ENGLAND AND WALES

potential contaminants that are may not adsorb onto soil particles. covered is wide, and the The sub-classes of the High and classifications used are, of necessity, Intermediate classes explain generalized. As a result, individual something of the likely speed of sites and circumstances will always movement of pollutants through the require further and more detailed soil and take into account the assessments to determine the adsorptive capacity of the soil. specific impact of potentially The movement of insoluble polluting activities on groundwater pollutants depends on their resources. individual properties such as density and viscosity. The leaching potential 5.5.3 Range of pollutants classification cannot be used directly The simple 3-fold leaching potential for these pollutants and requires classification (High, Intermediate refining by amalgamating certain and Low) has been devised for sub-classes and perhaps setting up soluble pollutants and liquid new sub-classes before these specific discharges, either of which may or pollutants could be catered for.

36 /* / / / ✓ r ' 6 REFERENCES

ADAMS, B. AND FOSTER, S.S.D. (1991). INTERNATIONAL ASSOCIATION OF National groundwater protection HYDROGEOLOGISTS (1994). policy: Hydrogeological criteria for Guidebook on mapping division of the land surface area. groundwater vulnerability, Editors NRA R&D Note 6. J Vrba and A Zaporozec. International Contributions to AVERY, B.\E (1980). Hydrogeology Vol. 16. Soil classification for England and Wales. (Higher categories). Soil NATIONAL RIVERS AUTHORITY (1992). Survey Technical Monograph Policy and practice in the protection No. 14. of groundwater. NRA, Bristol.

CLAYDEN, B. AND HOLLIS, J.M. (1984). ROBINS, N.S., ADAMS, B„ FOSTER, S.S.D. Criteria for differentiating soil AND PALMER, R.C. (1994). series. Soil Survey Technical Groundwater vulnerability mapping: Monograph No. 17. the British perspective. Hydrogeologie, 3, 35-42. CROMPTON, A. AND MATTHEWS, B. (1970). Soils of the Leeds District. Memoir ROBINSON, G.W (1943). of the Soil Survey of Great Britain. Mapping the soil of Britain. Discovery, 4, 118-121. DUFF, RMcLD. AND SMITH, A.J. (ED) (1992). Geology of England and Wales. SOIL SURVEY STAFF (1983). Geological Society, London. The National Soil Map of England and Wales, 1:250,000 scale (in six FOSTER, S.S.D. AND SKINNER, A.C. (1995). sheets). Soil Survey of England and Groundwater Protection: the Wales, Harpenden. science and practice of land surface zoning. In Proceedings of the International Association of Hydrological Sciences conference (Groundwater Quality; Remediation and Protection, Prague 1995). "\ \ GUIDE TO GROUNDWATER VULNERABILITY MAPPING IN ENGLAND AND WALES

\ APPENDIX 1 STRATIGRAPHIC COLUMN AND STRATA IN AQUIFER CLASSES

TABLE A l.l Recent aeolian, organic, fluvial, lacustrine and marine deposits Stratigraphic Column (based on Pleistocene glacial deposits Duff and Smith, 1992). No time East Anglia London Basin Hampshire Basin and IoW Dorset S.W England correlations are implied. Norwich Crag v Red Crag Netley Heath Beds Coralline Crag St Erth Beds Lenham Beds Bovey Hamstead Beds Formation Bembridge Beds Osborne Beds Headon Beds Barton Group Hengistbury Beds Boscombe Sand Selsey Sind f Branksome Marsh Farm Fm I Bracklesham \Sand EarnleySand J Group Wittering Fm J f Poole Bagshot Formation I Formation London Clay Formation Claygate Member Haldon Blackheath and Gravels Oldhaven Beds/ Harwich Formation -Woolwich Formation and Reading Formation / Lambeth Group- Thanet Sand Fm I I

Yorkshire Lincolnshire North Norfolk Bedfordshire Weald IoW

Q \< 'wiicimn_ 'vJiuuu / Upper Greensand T Hunstanton Formation /\ Gault Clay Formation \/ e Speeton Carstone Formation Carstone Clay Woburn Folkestone Beds "1 L Sandrock I Lower t Formation Sutterby Marl Sands Sandgate Beds > G Ferruginous (Greensand Formation Hythe Beds J S Sands [LGS] a Skegness Clay Atherfield Clay c Roach Formation Weald ' Vectis Tealby Clay Fm Dersingham Clay Formation e Formation Formation Wessex Claxby Formation Formation 1 Wealden o Sandringham Tunbridge Wells Sands u V/ Spilsby Formation Formation Wadhurst Clay Ashdown Beds >' i s Purbeck

38 Srratigraphic Column and Strata in Aquifer Classes

TABLE AM Dorset Cotswolds East Midlands Yorkshire Srratigraphic Column Purbeck Group (based on Duff and Smith, Portland Group 1992). No time correlations Kimmeridge Clay Formation Ampthill Clay Formation ore implied. Cantd. Corallian Group West Walton Formation I Brantingham Fm Oxford Clay Formation------Kellaways Formation ------Cornbrash Formation Forest Marble Formation Blisworth Clay I Great Oolite White Lst Fm Blisworth Limestone. Fuller’s Earth Hampen Fm Taynton Lst Fm Rutland Formation Sharp’s Hill Fm Chipping Norton Ravenscar Group Lst Fm Lincolnshire Limestone Formation Inferior Grantham Formation -Oolite - Group Northampton Sand Formation Dogger Formation L Bridport Sand Fm. Cotteswold Sand 1 Downcliff Clay U pper Lias Whitby Mudstone Fm A Junction Bed S Thorncombe Sand Marlstone Rock Formation Cleveland Ironstone Fm

G Eype clay Middle Lias Staithes Sandstone Fm R Green Ammonite Beds O Belemnite Marls Lower Lias clays Redcar Mudstone Fm U Black Ven Marls I Lower Lias P Shales with Beef Blue Lias Hydraulic Ists

Penarth Group Triasstc Mercia Mudstone Group Sherwood Sandstone Group Aylesbeare Mudstone Group (S.W England)

Cumbria Durham Yorkshire S.W England

St Bees Sandstone P Upper Permian Marl/Roxby Formation e r Eden Upper Magnesian Lst/Seaham Formation Upper Magnesian Lst/ m Shales/St Brotherton Formation i Bees Shales Middle Permian Marl/Edlington Formation a and Evaporites Middle Magnesian Lst/Ford Formation n Lower Magnesian Lst/ Cadeby Fm. Lower Magnesian Lst/Raisby Formation Lower Permian Marl Penrith Sandstone Basal Permian Sands and Breccias Luccombe Breccia Exeter Group Vexford Breccia Wiveliscombe Sandstones

Coal Measures Carboniferous Millstone Grit Carboniferous Limestone

Devonian Old Red Sandstone Silurian shalesI with subordinate limestones Ordovician predominantly shales Cambrian predominantly shales Precambrian GUIDE TO GROUNDWATER VULNERABILITY MAPPING IN ENGLAND AND WALES

TABLE AI.2 river gravels (Middle Thames valley) Major Aquifers (highly Chalk permeable) Red Chalk/Hunstanton Formation Upper Greensand (except east of Hog’s Back) Carstone (Norfolk) Lower Greensand (undifferentiated and Folkestone and Hythe Beds) Dersingham Formation (sands) Spilsby Sandstone Sandringham Formation Portland Stone Corallian/Brantingham Formation (Yorkshire) and Osmingcon Oolite (Dorset) Cornbrash Formation (where in hydraulic continuity with underlying limestones) Forest Marble and Great Oolite limestones (south of Oxford) Inferior Oolite/Lincolnshire Limestone Upper Lias (Coueswold, Midford, Yeovil) sands Junction Bed (Dorset) Dolomitic Conglomerate Sherwood Sandstone Group Permian sandstones (e.g. Dawlish Sandstone Formation, Collyhurst Sandstone, Bridgnorth Sandstone, St Bees Sandstone, Penrith Sandstone Upper Magnesian Limestone/Brotherton and Seaham Formations Middle Magnesian Limestone/Ford Formation Lower Magnesian Limestone/Cadeby and Raisby Formations Carboniferous Limestone (limestones except in northern England)

TABLE A1.3 made ground Minor Aquifers (variably landslip permeable) head (sandy) coombe deposits blown sand brickearth (sandy) calcareous tufa alluvium dry valley deposits alluvial fan deposits river terrace deposits (except Middle Thames) storm beach deposits raised beach deposits glaciofluvial sand and gravel deposits undifferentiated glacial drift (morainic drift, buried channel deposits) sand and gravel of unknown age/origin disturbed Blackheath Beds Crag (Norwich, Red and Coralline Crags) Pliocene gravels (e.g. Netley Heath Beds, Lenham Beds and St Erth Beds) Hamstead Beds Bembridge Limestone Osborn Beds Headon Beds Bovey Formation Barton Group (sands)

40 Stratigraphy Column end Strata in Aquifer Gasses

TABLE A1.3 Bracklesham Group (sands) Minor Aquifers (variably Bagshot Formation permeoble) Contd. London Clay Formation (sands) Claygate Member Blackheath and Oldhaven Beds/Harwich Formation Woolwich Formation and Reading Formation/Lambeth Group Thanet Sand Formation Haldon Gravels Upper Greensand (east of Hog’s Back) Carstone (except Norfolk) Whitchurch Sands Formation Sandgate Beds Roach Formation Weald Clay (sandstones and limestones) Tealby Limestone Claxby Formation Tunbridge Wells Sands Spilsby Formation Wealden Beds (sands) Wadhurst Clay Formation (sands) Ashdown Beds (except clay) Purbeck (sands) Portland Sands Corallian Group (except Yorkshire and Osmington Oolite) West Walton Formation (limestone) Kellaways Sand/Osgodby Formation Cornbrash Formation Ravenscar Group Blisworth Limestone Glentham Formation (limestones) Fuller’s Earth Rock Northampton Sand Upper Lias (Yeovil/Bridport) sands Junction Bed (not Dorset) Marlstone Rock Formation Pennard Sands Dyrham Siltstone Formation Lower Lias (limestones) Blue Lias White Lias/Langport Member Sandstones in Mercia Mudstone Group (e.g. Sneinton and Woodthorpe Formations) Permian breccias and conglomerates (S.W England) Basal Permian Sands Coal Measures (including “Barren Measures”) Millstone Grit Culm Carboniferous Limestone (limestones in northern England, Yoredales, Limestone Shales) Old Red Sandstone/Devonian Silurian limestones volcanic rocks GUIDE TO GROUNDWATER VULNERABILITY MAPPING IN ENGLAND AND WALES

TABLE A1.4 Norwich Crag (clays) Non-Aquifers (negligibly Bembridge Marls permeable) Barton Group (clays) Bracldesham Group (clays) London Clay Formation Gault Clay Formation Speeton Clay Formation Sutterby Marl Skegness Clay Atherfield Clay Weald Clay Formation (clays) Snettisham Clay Tealby Clay Formation Grinstead Clay Formation Wealden Beds (clays) Wadhurst Clay Hundleby Clay Ashdown Beds (clays) Purbeck (clays) Kimmeridge Clay Formation Ampthili Clay Formation West Walton Formation (clays) Ancholme Clay Group Oxford Clay Formation Kellaways Clay Member Blisworth Clay Glentham Formation (clays) Fuller’s Earth clays Grantham Formation Upper Lias (clays)/Whitby Mudstone Formation Downcliff Clay Middle Lias (clays) Eype clays Green Ammonite Beds Lower Lias (clays)/Rcdcar Mudstone Formation Belemnite Marls Black Ven Marls Shales with Beef Penarth Group (except Langport Member) Mercia Mudstone Group (except Dolomitic Conglomerate and sandstones) Manchester Marl Upper Permian Marl/Roxby Formation Middle Permian Marl/Edlington Formation Wetherby Member Lower Permian Marl St Bees Evaporites and shales/Eden Shales Silurian (except limestones) Ordovician Cambrian Precambrian intrusive igneous rocks

42 Strctigraphic Column and Strata in Aquifer Gooes

TABLE A l i head (clayey) Low permeability drift clay-with-flints deposits loess brickearth (except where sandy) peat (except on maps 18, 24 and 25) shell marl lacustrine deposits tidal flat deposits saltmarsh deposits marine and estuarine alluvium glaciolacustrine deposits till GUIDE TO GROUNDWATER VULNERABILITY MAPPING IN ENGLAND AND WALES

APPENDIX 2 COMMONLY OCCURRING SOIL SERIES WITHIN SOIL LEACHING POTENTIAL CLASSES

TABLE A2.1 HI H2 H3 Soils with High leaching Andover Beam ed Aberford potential Blackwood Bridgnorth Anglezarke Elmton Bromsgrove Badsey Fladbury Cuckney Eardiston Longmoss Newport Munslow Newchurch Wick Neath Sherborne Panholes Wallasea Rivington Wisbech Shirrell Heath Sollom Sonning Swaffham Prior Waltham

TABLE A2.2 11 12 Soils with Intermediate Batcombe Adventurers’ leaching potential Blacktoft Altcar Bromyard Burlingham Carstens Coombe C unisden Denbigh East Keswick Evesham Fyfield Halstow Hanslope Ludford Manod Milford Whimple Worcester

TABLE A2.3 Beccles Soils with Low leaching Brickfield potentiol Cegin Clifton Denchworth Dunkeswick Foggathorpe Hafren Ragdale Salop Wickham Wilcocks W indsor Winter Hill

44 APPENDIX 3 KEY SHEET FOR UNDERSTANDING THE NRA GROUNDWATER VULNERABILITY MAPS

The degree to which groundwater is vulnerable to contamination from pollutants applied or discharged on the land surface is shown by the various shades and colours on the map. Three types of information are used in compiling the maps: • Soil - the ease with which pollutants move down through soil is indicated by the intensity of purple and brown colours — the denser the colour, the faster the movement. • Low permeability surface drift deposits - indicated, where present, by a stipple ornament. • Aquifer types - purples (Major Aquifer); browns (Minor Aquifer) and green (Non Aquifer).

Soils Soils overlying Major and Minor Aquifers (see below) are ranked High (dark purple or dark brown), Intermediate (mid purple or mid brown) or Low (pale purple or pale brown) according to their Soil Leaching Potential, a measure of the relative speed of pollutant migration through the soil zone. High (HI, H2, H3) Soil Leaching Potential: these soils have very little ability to prevent pollutants entering groundwater due to either: • Groundwater present at very shallow depth within the soil zone • Very shallow soils over aquifer forming rocks • Physical properties that allow pollutants to move quickly through the soil. Soils in built-up areas and areas of past or present mineral extraction are uniformly classified as High (HU) Soil Leaching Potential until demonstrated otherwise by more detailed desk assessment or site - specific investigation. Intermediate (11, 12) Soil Leaching Potential: these soils have some ability to prevent pollutants entering groundwater due to either: • Physical properties that enable pollutants to bind onto soil particles • Physical properties that limit the speed of pollutant movement through the soil • Considerable thickness of soil. Low (L) Soil Leaching Potential: these soils prevent pollutants entering groundwater due to either: • Physical properties that prevent pollutants moving downwards through the soil • Impermeable strata beneath the soil.

Low permeability drift deposits A stipple ornament is used to indicate where geological maps suggest low permeabilty drift deposits to be present at the surface which may help to impede the downward movement of pollutants. The thickness and vertical consistency of the drift deposits (which is often very variable) and whether they are underlain by permeable drift are not considered.

Aquifer types Geological deposits are divided into Major, Minor and Non-Aquifers (potentially water bearing rocks). Major Aquifers are used extensively for public water supply (and hence the protection of groundwater quality is of major concern) while Minor Aquifers are largely used for private supply but may support some public abstractions. Although Non-Aquifers have little importance for groundwater supplies they may, due to their impermeable nature, promote lateral movement of pollutants within the soil zone towards Major or Minor Aquifers. APPENDIX 4 GLOSSARY OF TERMS

Adsorption Fractures/Fissures Process by which a thin layer of a substance Natural cracks in rocks that enhance rapid accumulates on the surface of a solid water movement. substance. Inorganic Aerobic Chemicals which are not carbon-based, such In the presence of the atmosphere or free as salt, nitrate fertilizers etc. oxygen. Leaching Aquifer Removal of soluble substances by action of Permeable strata that can transmit and store water percolating through soil, waste or rock. water in significant quantities. Lithology Attenuation The general characteristics of a sedimentary Breakdown or dilution of a contaminant in rock including mineral composition and water. texture. Baseflow Organic That part of the flow in a watercourse made Chemicals which are carbon-based, such as up of groundwater and discharges. It sustains pesticides, dry cleaning solvents etc. the watercourse in dry weather. Outcrop Biodegradation Where strata are at the surface, even though Microbial breakdown of a compound. they may be covered by soil cover. Bypass flow Permeability Liquid flow through preferential routes in measure of a soil or rock’s capacity to cracks, fractures etc. whereby flow is more transmit water. rapid than within the soil or rock matrix. PH Cation the degree of acidity (or alkalinity) of a Positively charged ion (see Cation Exchange solution or soil and expressed in terms of the and Cation Exchange Capacity) pH scale. Cation exchange Potentiometric surface The interchange between cations in solution A surface that represents the level to which and cations on the surface of clay or organic water will rise in tightly cased wells. colloids. Primary Permeability Cation exchange capacity Permeability related to flow between grains The sum total of exchangeable cations that a within the aquifer. soil can adsorb. Recharge Confined aquifer Water which percolates downwards from the Where permeable strata are covered by a surface into groundwater. substantial depth of impermeable strata such that the cover prevents infiltration. Secondary Permeability Permeability related to groundwater flow Confining beds within fissures rather than between grains Impermeable strata which prevent infiltration (see Primary Permeability) to underlying permeable strata. Septic tank Conservative pollutant Small tank receiving and treating sewage by Pollutant which can move readily through the bacteria where effluent overflows. aquifer with little reaction with the rock matrix and which are unaffected by Slowly permeable (soil) biodegradation (eg chloride). Subsurface layer which acts as a significant barrier to water movement when the soil is Diffuse source pollutants saturated. They are generally dense and clay- Pollution from widespread activities with no rich. one discrete source Soakaway Diffusion System for allowing water or effluent to soak The process by which ions and molecules into the ground, commonly used in dissolved in water move from areas of higher conjunction with septic tanks. concentration to areas of lower concentration. Temperate Moderate, without extremes. Drift deposits Term used to include all unconsolidated Unconfined aquifer superficial deposits (eg. ftuvioglacial, alluvium An aquifer in which the water surface is etc.) overlying solid rocks. formed by the water table which is free to fluctuate under atmospheric pressure and can Ecosystem thus reflect changes in storage in response to A system involving the interactions between abstraction and recharge. a community and its non-living environment. Unsaturated zone Water table Zone of aquifer between soil and watertable Top surface of the saturated zone within the which is partly saturated (ie that part of the aquifer. aquifer above the water table.

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The NRA is committed to the principles o f stewardship and sustainability. In addition to pursuing its statutory responsibilities as Guardians o f the Water Environment, the NRA will aim to establish and demonstrate wise environmental practice throughout all its functions. GUIDE TO GROUNDWATER VULNERABILITY MAPPING IN ENGLAND AND WALES

In 1992 the National Rivers Authority published its Policy and Practice for the Protection of Groundwater. The implementation of the policy depends upon the definition of groundwater source protection zones and the preparation of vulnerability maps. This guide is one of two volumes which provide the background to the production and use of these two policy tools and complement the original policy document.

The companion volume to this guide is the Guide to Groundwater Protection Zones in England and Wales.

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