|Loveless, S, Lewis, M A, Bloomfield, J P, Terrington, R, Stuart, M E, and Ward, R S. 2018. 3D groundwater vulnerability. British Geological Survey Internal Report, OR/18/012.|
Groundwater and designated groundwater bodies are potential contamination receptors and understanding their designation and current practices related to their protection is essential in developing an effective 3D vulnerability methodology. The EU’s Water Framework Directive (WFD) and the associated Groundwater Directive (GD) are cornerstones of groundwater governance in England. As transposed by the Environmental Permitting Regulations, these Directives establish a series of environmental objectives for groundwater that include preventing or limiting the inputs of pollutants to groundwater and ensuring that groundwater bodies achieve (and maintain) good chemical and quantitative status In England, responsibility for regulation of groundwater and its management and protection from any potentially polluting activity is the remit of the EA. The Agency’s approach to this is outlined in the document ‘The Environment Agency’s approach to groundwater protection’ (EA, 2017b).
Groundwater bodies are groundwater management units. They exist within aquifers, and are defined within aquifers or contiguous aquifers within which the groundwater resides. Delineation of groundwater bodies takes into account geological boundaries, groundwater flow divides, pollution/abstraction pressures natural chemical variations. UK guidance is that groundwater bodies would not extend below depths greater than 400 m, except where management and protection is required at greater depths, i.e. WFD status and trends objectives apply, (UKTAG, 2011). However, all groundwater, regardless of depth or quality, requires protection from inputs of pollutants, unless the groundwater can be shown to be permanently unsuitable. This is because, for example, groundwater at depth could be part of a pathway for pollutants to travel to designated groundwater bodies, for which particular protective measures might be required. In addition, groundwater at greater than 400 m depth could be considered for protection because of its value as mineral waters, cultural value, or potential for future use.
Considerations of groundwater quality may also influence decisions regarding groundwater protection and the development of a 3D vulnerability methodology. Groundwater quality (total dissolved solids or TDS) data to a depth of nearly 2500 m for England show a lower TDS bound which decreases in quality (increases in TDS) with depth. However, at most depths, TDS concentrations ranging up to three orders of magnitude, and data show that potable water (TDS <1000 mg/l) exists at >400 m bgl in places (see Deep groundwater quality in England). This limit also does not take into account potential future uses for groundwater with a range of qualities and technological developments. It is also well known that deep groundwater flow systems connect to the surface and feed strategically important springs used for recreational purposes (e.g. Bath Spring) and bottled waters. Therefore, a practical framework for protection of deep (>400 m bgl) and brackish waters has been found to be lacking.
This chapter reviews the policies for the protection of groundwater in England, including the associated practical and regulatory guidance, and assesses the suitability of current definitions for vertical and lateral extent of aquifers in England. These definitions are considered in the context of deep groundwater quality data for England and the need to protect deep groundwater systems now and in the future. International best practices are subsequently discussed and details presented in Appendix 3. The resulting classification of receptors based on their importance is then presented.
The following is a summary of aspects of two key (though not the only) EU Directives and guidance on their application to groundwater that have a bearing on the definition of the 3D extent of groundwater systems. Further information is included in Appendix 3 – Defining groundwater.
Overview of EU Directives and guidance
The water framework directive and groundwater directive
Directive 2000/60/EC (EC, 2000), adopted in October 2000, and referred to as the EU Water Framework Directive or simply the WFD, established a framework for community action in the field of water policy, including policy related to groundwater. Directive 2006/118/EC (EC, 2006), known as the Groundwater Directive, was developed in response to requirements of Article 17 of the WFD and sets groundwater quality standards and introduces measures to prevent or limit pollutants entering groundwater.
For groundwater, the key environmental objectives of the WFD, as described in Articles 4.1.b.i. and 4.1.b.ii., are for Member States to:
- “implement the measures necessary to prevent or limit the input of pollutants into groundwater and to prevent the deterioration of the status of all bodies of groundwater” and to “protect, enhance and restore all bodies of groundwater, ensure balance between abstraction and recharge of groundwater, with the aim of achieving good groundwater status at the latest 15 years after the date of entry into force of this Directive [the WFD]”.
The WFD sets out steps and a timeframe for achieving good quantitative and chemical status (outlined in Appendix 3 – Defining groundwater) of European waters, including groundwater. As part of this process, the WFD requires Member States to define and identify groundwater bodies within River Basin Districts and to report to the European Commission (EC) on the status of these bodies. The following groundwater-related definitions are set out in the WFD.
Groundwater is defined in the WFD in Article 2.2 as:
- “all water which is below the surface of the ground in the saturation zone and in direct contact with the ground or subsoil”.
In Article 2.11 an aquifer is defined as:
- “a subsurface layer or layers of rock or other geological strata of sufficient porosity and permeability to allow either a significant flow of groundwater or the abstraction of significant quantities of groundwater”;
In Article 2.12 a body of groundwater or groundwater body is defined as a “distinct volume of groundwater within an aquifer or aquifers”;
As a pre-cursor to establishing the status of a GWB, the WFD requires member states to undertake an initial characterisation (risk assessment) of all groundwater bodies:
- “to assess their uses and the degree to which they are at risk of failing to meet the objectives for each groundwater body under Article 4”.
It requires member states to identify the location and boundaries of groundwater bodies, the pressures to which they are liable, the general character of overlying strata from which the bodies receive recharge and groundwater bodies for which there are directly dependent surface water ecosystems. It also notes that Member States may group groundwater bodies together for the purposes of this initial characterisation.
Annex 2, section 2.2 of the WFD sets out the requirements of further characterisation of groundwater bodies, or groups of bodies, which have been identified as being at risk based on the initial characterisation (Appendix 3 – Defining groundwater).
In addition, Annex 2, section 2.4 of the WFD requires member states to review the impact of changes in groun<syntaxhighlight></syntaxhighlight>dwater levels and to:
- “identify those bodies of groundwater for which lower objectives are to be specified under Article 4 including as a result of consideration of the effects of the status of the body on: (i) surface water and associated terrestrial ecosystems; (ii) water regulation, flood protection and land drainage; and, (iii) human development”.
Similarly, Annex 2, section 2.5 requires member states to review the impact of pollution on groundwater quality and to:
- “identify those bodies of groundwater for which lower objectives are to be specified under Article 4(5) where, as a result of the impact of human activity, as determined in accordance with Article 5(1), the body of groundwater is so polluted that achieving good groundwater chemical status is infeasible or disproportionately expensive”.
After the WFD was adopted, a Common Implementation Strategy (CIS) (EC, 2001) was developed and agreed in May 2001. This sets out a common understanding of approaches to, and implementation of, the WFD, and provided a series of examples of best practice. This is detailed in Appendix 3 – Defining groundwater.
Implementation in England
Allen et al. (2002) describe the interpretation of the WFD and outline procedures used by the EA to undertake the initial delineation and characterisation of the groundwater bodies to meet the requirements of the WFD. The principles set out in Allen et al. (2002) included the following key observations, that:
- “the delineation and characterisation of groundwater bodies [should be] … iterative. Thus, for example, only simple conceptual models are required at first in order to delineate the groundwater bodies, becoming, where required, more sophisticated (and expensive) as the characterisation process proceeds. Iteration also allows for the refining of boundaries or the subdivision or aggregation of groundwater bodies”; that: “Groundwater systems in aquifers should be subdivided or aggregated to form groundwater bodies of a suitable size for management (generally at least tens of square kilometres in area), which will reflect the pressures and impacts on groundwater”; and that: “Groundwater body boundaries should generally be chosen initially on the basis of geology, using WFD aquifer boundaries. If necessary, subsequent subdivision is performed using groundwater divides and finally using flowlines. The groundwater body as delineated will remain constant during a River Basin Management Plan, but may be subdivided or amalgamated with adjacent bodies in subsequent RBMP cycles, dependent on management needs”.
The report concluded with two final principles, that:
- “given that the definition of an aquifer in WFD terms is essentially based on abstraction and flow criteria, and that the lower abstraction limit is small, most geological materials in the UK are likely to be classified as aquifers in WFD terms. The main guiding principle for the delineation of groundwater bodies is that flowlines in an aquifer should not cross from one groundwater body to another. This is to enable groundwater bodies to be treated as coherent hydraulic systems (to aid determination of quantitative status) and to be managed as such.”
Allen et al. (2002) also noted that:
- “there may be geological materials which have sufficient porosity and permeability to support either abstraction or flow (and therefore are potential aquifers in WFD terms) but which do neither when saturated. This could be, for example, because such potential aquifer material lies at depth and therefore is not exploited and does not support surface flow. This material is classified as a potential aquifer on the basis of its aquifer properties, but need not be formally identified as a WFD aquifer”
Note that no explicit guidance was given by Allen et al. (2002) on the delineation of base of aquifers or groundwater bodies.
UK Technical Advisory Group (UKTAG) guidance on implementation of the WFD
UKTAG, the advisory group on implementation of the WFD and Groundwater Directive in the UK, published a paper setting out guidance on the delineation and characterisation of groundwater bodies in the UK in response the requirements of the WFD (UKTAG, 2011). The report refines the definitions of groundwater, aquifer and groundwater bodies, sets out the principles of how groundwater bodies should be delineated, provides guidance on groundwater body depth and the definition of groundwater body horizons and reporting to the EC. The following is a summary of UKTAG definitions and guidance relevant to groundwater body delineation.
In addition to the definitions in the WFD, UKTAG (2011) introduces two new concepts of pore water, as:
- “pore waters in low permeability subsoils (e.g. clays) do not represent groundwater as a receptor, because they do not provide a useful water resource and pollutants going to surface water receptors travel at velocities that are measured on a millimetre-scale per year. Therefore, water in these deposits should not be subject to the same management objectives as, for example, aquifers or groundwater bodies”,
and of groundwater at extreme depth, as:
- “groundwater that exists at extreme depth and is permanently unsuitable for use as a resource, e.g. due to high salinity, should not be considered as a groundwater body”.
These are then related to interpretations of the WFD definitions of groundwater based on their respective roles in environmental management (see Table 3.1 below).
|Water in unsaturated zone||Pore water||Pore water above the water table. Protect as a vertical pathway to groundwater|
|Water in saturation zone||Pore water in low permeability deposits. The concept of the zone of saturation is not relevant in these deposits as it is usually not feasible to define a water table where lateral percolation is impeded. The main role of these strata is as a protecting layer for groundwater|
|Groundwater in strata overlying or underlying groundwater bodies||Groundwater has a value as a lateral or vertical pathway to other receptors. May be usable but only for local supplies <10 m3/day|
|Groundwater in a groundwater body||Groundwater is part of an aquifer and is a receptor as a long term resource that can be exploited for human activities or support surface flows & ecosystems|
|Groundwater that is permanently unsuitable for use||Groundwater which has neither pathway nor resources value. For example, where salinity is greater than seawater.|
Groundwater body depth
- “the main driver for delineating groundwater bodies in three dimensions is groundwater body management”, that “the drivers for groundwater body management relate to its use as a water supply or its contribution to surface water systems. The latter focuses on the unconfined aquifers and, to a lesser extent, discharge from confined aquifers ... Therefore, management of groundwater at greater depths mainly relates to its use for water supply”.
UKTAG (2011) states that:
- “At some depth, depending on the nature of the aquifer, groundwater loses its value as a resource that can be either exploited for human activities or support surface flows and ecosystems”
and goes on to define default depth values for the base of groundwater bodies in the UK, noting that these values:
- “should be amended using local information if available. This information should comprise hydrogeological and hydrochemical information to identify the resource boundaries, preferably through the use of water table information and structural or stratigraphic features that represent aquitards”.
UKTAG (2011) states that the default maximum thickness of groundwater bodies in the UK should be 400 m, with the exception of porous superficial aquifers, such as sand and gravel aquifers, and low transmissivity bedrock, such as the Dalradian, which should have an assumed maximum thickness of 40 m and 100 m, respectively (UKTAG, 2011). Measurement of the thickness should be from the upper extent of the groundwater body downward, where:
- “the upper extent of the groundwater body is the water table. Where information on the level of the water table is not available across the groundwater body as a whole, the upper extent can be considered to lie at ground level”.
It is not explicit in the UKTAG report how this applies to confined groundwater bodies. However, if it is assumed that for most confined aquifers the upper extent of the water table (piezometric surface) is not available, then one possible interpretation of the guidance would be that for confined aquifers the upper extent of the aquifer should be considered to be ground level. It could also be taken as the top of the aquifer unit.
Groundwater protection in England
The EA published a revised approach to groundwater protection in November 2017 (EA, 2017b). The principles and definitions set out in that report and associated documentation are consistent with the previous, more detailed Groundwater protection: principles and practice (GP3) report (EA, 2013).
The EA currently defines principal aquifers, secondary aquifers (secondary A, B and undifferentiated), and unproductive strata (Table 3.2) based on their geological characteristics, the quantity and ease with which groundwater can be obtained from the aquifers, and the extent to which they support flow in rivers and habitats.
|Principal aquifer||Rocks that provide significant quantities of water for people and may also sustain rivers, lakes and wetlands. Formerly referred to as ‘major aquifers’.|
|Secondary aquifers||Rocks that provide modest amounts of water, but the nature of the rock or the aquifer’s structure limits their use. They remain important for rivers, wetlands and lakes and private water supplies in rural areas. Formerly referred to as ‘minor aquifers’.|
|Secondary A||Permeable rocks capable of supporting water supplies at a local rather than strategic scale, and in some cases forming an important source of base flow to rivers.|
|Secondary B||Predominantly lower permeability rocks that may store and yield limited amounts of groundwater due to localised features such as fissures, thin permeable horizons and weathering.|
|Secondary undifferentiated||Designation assigned in cases where it is not been possible to attribute either category Secondary A or B to a rock type. In most cases, this means that the layer in question has previously been designated as both ‘minor’ and ‘non-aquifer’ in different locations due to the variable characteristics of the rock type.|
|Unproductive strata||These are rocks that are generally unable to provide usable water supplies and are unlikely to have surface water and wetland ecosystems dependent upon them. Formerly referred to as ‘non-aquifers’.|
The EA approaches groundwater protection in the context of a risk-based framework, where the technical framework for groundwater risk assessment includes:
- a source–pathway–receptor (S–P–R) approach;
- a conceptual model;
- a tiered approach from qualitative risk screening to detailed quantitative risk assessment (Tier 1–3);
- identification of sources or potential hazards, examining consequences and evaluating the significance of any risk;
- dealing with uncertainties and sensitivity analysis; and
- risk management.
and where this is employed in conjunction with the use of the ‘precautionary principle’. The EA (2017b) provides position statements that apply to developments and activities in SPZ1 for a range of activities, including: Underground coal gasification, coal bed methane and shale gas extraction (C6) and oil and conventional hydrocarbon exploration and extraction (C7). Position statement C6 states:
- “The Environment Agency will, where appropriate, work in partnerships on initiatives to facilitate development of sustainable sources of energy. However, it will normally object to UCG, CBM or shale gas extraction infrastructure or activity within a SPZ1. This includes subsurface SPZ1 areas which are confined by impermeable strata at the surface.
- Outside SPZ1, the Environment Agency will also normally object when the activity would have an unacceptable effect on groundwater. Where development does proceed and where any associated drilling or operation of the boreholes/shafts passes through a groundwater resource, the Environment Agency expects best available techniques (BAT) and pollution prevention measures to be applied to protect groundwater.
- The Environment Agency will expect a detailed hydrogeological risk assessment to be produced for any onshore oil or gas site activity. The assessment must include potential impacts to all groundwater which could be affected, such as any groundwater bearing strata even at depth. Mitigation measures to protect all groundwater will be expected to reflect the sensitivity of that groundwater and any associated receptors. The receptors may include drinking water sources, surface waters and wetlands; as well as the potential uses of deeper groundwater (for example, artificial storage and recovery or geothermal uses).”
and for oil and conventional gas exploration and extraction, C7 states:
- “The Environment Agency will normally object to such hydrocarbon exploration, extraction infrastructure or activity within SPZ1, which will also include any subsurface SPZ1 areas which are confined by impermeable strata at the surface.
Outside SPZ1, the Environment Agency will also normally object when the activity would have an unacceptable effect on groundwater. Where development does proceed, the Environment Agency expects BAT and pollution prevention to protect groundwater to be applied where any associated drilling or operation of the boreholes passes through a groundwater resource.
The Environment Agency will expect a detailed hydrogeological risk assessment to be produced for any onshore oil or gas activity. The assessment must include potential impacts to all groundwater which could be affected, such as any groundwater bearing strata even at depth. Mitigation measures to protect all groundwater will be expected to reflect the sensitivity of that groundwater and any associated receptors. The receptors may include drinking water sources, surface waters and wetlands as well as the potential uses of deeper groundwater (for example, artificial storage and recovery, or geothermal uses).
Where oil and gas activities already exist, the Environment Agency will work with operators to assess and if necessary mitigate the risks. It will normally object to any redevelopment scheme involving retention of oil exploration, extraction infrastructure or activity within SPZ1 unless there are substantial mitigating factors.
GP3 (EA, 2013) provided more information on the concept of “groundwater that exists at extreme depth and is permanently unsuitable for use as a resource” that was used by UKTAG (2011). In GP3 it was noted that:
- “[the WFD] require us to take all necessary measures to prevent the input of hazardous substances into groundwater and to limit the input of non-hazardous pollutants so as to avoid the pollution of groundwater. However, provided it does not compromise the objectives set out in Article 4 of the Water Framework Directive, we may grant a permit for the injection of water containing hazardous substances from hydrocarbon or mining activities or the injection for storage of natural gas or liquefied petroleum gas — but only where the strata have been determined as permanently unsuitable. The geological formation must be examined before being deemed permanently unsuitable. EPR 2010 states that the geological formation must for natural reasons be permanently unsuitable for other purposes. Contamination of the formation as a result of human activity would not be cause for its determination as permanently unsuitable.”
Identifying 3D aquifer extent in England
The 2D extent of aquifers and unproductive strata was mapped for the EA’s aquifer designation maps using geological maps of the ground surface or rockhead at a scale of 1:50 000 (see www.bgs.ac.uk/products/hydrogeology/aquiferDesignation.html; and www.apps.environment-agency.gov.uk/wiyby/117020.aspx). These maps show aquifer designations for superficial deposits (Quaternary age) (575 units), and bedrock formations (3700 units). These designations are used for groundwater vulnerability mapping (Carey and Thursten, 2014) to help manage potentially contaminating activities at or near the ground surface.
There is currently no equivalent designation at depth, in part due to a lack of knowledge of the vertical and lateral distribution of aquifers at depth. The first systematic study to characterise the 3D distribution of principal aquifers in England and Wales was undertaken as part of a recent BGS/EA co-funded project (iHydrogeology, www.bgs.ac.uk/research/groundwater/shaleGas/aquifersAndShales/maps/home.html; Loveless et al., 2018) to map the vertical separation between the top of selected shale formations and base of overlying principal aquifers across England and Wales. This entailed modelling, at a scale of 1:625 000, of the top surface of six major shale and clay units that are potentially oil/gas bearing (the Kimmeridge Clay Formation, Oxford Clay Formation, the Lias Group, Marros Group, the Bowland Shale Formation and the Upper Cambrian shales) and the depth to base of 11 geological formations corresponding to bedrock principal aquifers (Crag Group, Chalk Group, Lower Greensand Group, Spilsby Sandstone Formation, Corallian Group (limestone), Great and Inferior Oolite groups, Triassic sandstones, Zechstein Group, Permian sandstone, Carboniferous limestone, and Border Group (Fell Sandstone)). Surfaces were created using BGS’s National Geological Model (UK3D) of the UK (Mathers et al., 2014). The 3DGWV LFV project extends this work and accounts for the vertical and lateral extent of both principal and secondary aquifers in England using UK3D2015 (Waters et al., 2016), which includes additional geological cross sections and named bedrock units since the iHydrogeology project, improving the spatial and stratigraphic resolution, respectively. Due to the very local and sometimes discontinuous nature of many of these units it is not justifiable to produce interpolated surfaces and thus full subcrop maps for all of the potential receptors across England. Instead, potential receptors on the geological sections in the 3DGWV LFV project have been attributed with EA aquifer designations. Designations, lateral and vertical extents can be viewed within the LFV software.
Water quality of deep groundwater systems in England
Water quality standards, including specific electrical conductance (SEC) or total dissolved solids (TDS), are used to regulate supply and use of groundwater in England and elsewhere. The Council of the European Union (1998) specified that the maximum SEC at 20°C should be 2500 µS/cm (about 1625 mg/l TDS), and the water should not be aggressive. This limit has been embodied in the water supply regulations for England that specify the maximum admissible concentrations and values for parameters in drinking water for both public supply (The Water Supply (Water Quality) Regulations (2016)) and private water supplies for human consumption (Private Water Supplies (England) Regulations (2016)). The World Health Organisation (WHO) describes water with a TDS of <600 mg/l as good quality and that with a TDS of >1000 mg/l as increasingly unpalatable (WHO, 2011). For comparison, the US EPA (2017), states a guideline maximum TDS value of 500 mg/l in the Secondary Drinking Water Standards but considers an underground drinking water source to have a TDS of <10 000 mg/l.
Understanding of groundwater quality at depth is integral to 3D groundwater vulnerability and risk assessments and, arguably, the related policy development and management decisions. Water quality tends to deteriorate with increasing depth as lower hydraulic gradients and slower groundwater movement result in longer residence times during which the water can interact with the host rock and result in increased mineralisation. There are some exceptions to this, for example, in the East Midlands better quality Sherwood Sandstone groundwater occurs below more mineralised shallower waters associated with the overlying gypsiferous Mercia Mudstone and groundwater subjected to recent near surface pollution from agriculture or coal mining.
Deep groundwater quality in England
There are 13 public water supply sources with depths over 400 m in the BGS Wellmaster database (a comprehensive database of water borehole logs in the UK) in England, though none are over 500 m deep.
Table 3.3 indicates that these boreholes all terminate in sandstone aquifers (either the Lower Greensand or the Sherwood Sandstone). Water in silicate aquifers is generally less mineralised than that in carbonate ones. However, it is not clear whether the lack of boreholes >400 m deep in carbonate aquifers is due to a decrease in dissolution and secondary fractures affecting yields or poor groundwater quality at depth within these aquifers.
|Aquifer||Number of supplies||Depth (m)|
|Palaeogene, Chalk and Folkestone Formation (Lower Greensand Group)||1||422|
|Folkestone Formation (Lower Greensand Group)||6||400, 414, 442, 457, 468, 489|
|Folkestone Formation (Lower Greensand Group) and Hastings Beds (Purbeck Group)||1||433|
|Sherwood Sandstone Group||5||400, 414, 430, 431, 500|
The Geothermal Data Catalogues provide the most complete data on groundwater quality at depth, including information on locations, depths, sample types and aquifers. The first comprehensive catalogue of underground temperature, heat flow and hydrogeochemical data was published in 1978 by the Department of Energy (Burley and Edmunds, 1978). This was updated by the British Geological Survey’s ‘Investigation of the geothermal potential of the UK’ project in the 1980s and published in three revisions (Burley and Gale, 1982; Burley et al, 1984; Rollin, 1987). The majority of the data were derived from drill stem tests (Table 3.4).
Data from the Geothermal Data Catalogues have been digitised and anomalous values removed. Site locations given to the nearest 1 km (in some cases 10 km) were cross-referenced by location names and depth and identified to at least the nearest 100 m. Mine drainage data were also removed, since these analyses may not be representative of natural groundwater conditions and the depth from which the water drains is ambiguous. The remaining 500 analyses range from springs (surface) to a maximum borehole depth of 2385 m, although the number of observations decrease significantly with depth (Table 3.5). Where the source rock was not recorded but a depth provided, borehole logs were used to identify the formation from which the water sample was most likely derived. Where only a borehole depth (not sample depth) was available, the sample was assumed to be from the formation at the final borehole depth.
Analyses were from a range of formations, but primarily the Chalk, Sherwood Sandstone, Zechstein Group, Coal Measures, Millstone Grit and Carboniferous Limestone (Table 3.6). Where no TDS was recorded in the original data, it was assumed to be the sum of all the ions quoted (major ions plus silica), although in many cases some ionic concentrations (mainly potassium, bicarbonate, sulphate and silica) were not recorded and hence the calculated TDS content is a minimum estimate. The dataset is a collection of all data available at the time of compilation, rather than being a comprehensive review of water quality from different formations at specific depths. For example, many more data exist for aquifers at shallow depths which are not included, and these data indicate that the lowest TDS range for any unit in Table 3.6 relates to the London Clay at depths of >150 m, not a formation generally considered to form a significant aquifer, is anomalous and an artefact of the way data was originally selected for inclusion in the catalogues.
|Data source||Number of sites|
|Spring (including thermal springs)||10|
|Drill stem test||309|
|Depth (m)||Number of sites|
|Period||Formation||Number of sites||Depth range (m)||TDS range (mg/l)|
|Kellaways and Oxford Clay formations||4||105–833||10812–47625|
|Bridport Sand Formation||14||0–1180||321–143470|
|Bowland Shale||2||0 (springs)||637–1195|
|Lower Limestone Shale||2||1684–1834||87875–101610|
|Devonian||Old Red Sandstone||3||104–1919||225–136744|
Figure 3.1 shows TDS as a function of depth for all of the Geothermal Data Catalogue data. Figure A3.2 to Figure A3.7 in Appendix 3 – Defining groundwater show TDS as a function of depth highlighting data for the Chalk, Sherwood Sandstone, Zechstein Group, Coal Measures, Millstone Grit and Carboniferous Limestone. Figure 3.1 shows that there is significant variation in TDS at any given depth. For example, at ~400 m bgl, measured TDS may vary by over three orders of magnitude from ~100 to >100 000 mg/l. The corollary of this is that a given TDS may be found over a wide range of depth intervals. For example, TDS values of 10 000 mg/l have been reported from the near surface down to depths of >1 km.
However, groundwater at greater depths will generally be older, allowing more time for water-rock interaction and hence more mineralised. Hence there is a broadly linear lower bound to the distribution of TDS (Figure 3.1). This means that for a given depth interval an equivalent minimum TDS can be approximately identified. The lower TDS bound for a given depth indicates a maximum depth of ~900 m for potable groundwater in England (maximum TDS ~1625 mg/l based on the current statutory SEC limit for potable groundwater of 2500 µS/cm) (Figure 3.2). Groundwater below ~1,750 m is likely to be more saline than seawater (35 000 mg/l TDS) (Figure 3.2). However, estimation of depth intervals associated with specific TDS thresholds will depend on the precise location and shape of the lower bound to the TDS-depth trend and these figures should only be taken as approximate values. A similar lower bound to groundwater quality-depth data has also been described for data from California (Kang and Jackson, 2016). In California, however, the lower bound is lower than for England, reflecting lower TDS at greater depths. This difference could result from a range of factors, including the length of time that groundwater has been in contact with the host rocks which in turn is a function of the hydrogeological setting, rock hydraulic conductivity, and rock solubility.
Groundwater protection summary
According to the groundwater governing frameworks in England (WFD and GD), all groundwater should be protected from the input of pollutants. There is a general characteristic of increasing mineralisation of groundwater with depth (Figure 3.2). Consequently, in practice, the use and hence protection of groundwater <400 m bgl has been prioritised. However, with an increasing focus on the use of deeper geological environments for potential hydrocarbon development, there is a need to consider the application of protection for deeper, more highly mineralised groundwater. Although such groundwater is currently not considered as a groundwater body and so is not subject to the same management objectives as a groundwater body (UK TAG, 2011), recognition of the importance of deep groundwater as a pathway, as well as its potential for future uses, means that it should still be afforded certain defined protections.
Potential receptor classification
POTENTIAL RECEPTOR CLASSIFICATION
| In the 3DGWV methodology, potential receptor units are used to assess groundwater within different strata, in accordance with possible differences in the groundwater condition at different depths, laterally, and within different geological units. Each geological unit identified in the geological sequence within the AOI should be classified as a potential receptor on the basis of their geological and hydrogeological properties (primarily identified through their EA aquifer designation) and their shallowest depth in the AOI.
The 400 m default maximum depth for Groundwater Bodies in the UK (UKTAG, 2011) is central to receptor classifications in the absence of groundwater quality data (Table 3.7). However, where there are data for the TDS content of the groundwater within the unit (Table 3.7), this should be the determining factor in receptor classification. The groundwater quality boundaries are defined according to WHO (2011) with potable water having TDS <1000 mg/l, slightly brackish water, which can be used for potable mineral water supply and agriculture, parks and gardens, from 1000–3,500 mg/l (EPA Victoria, 1997) and brackish water up to 35 000 mg/l. UKTAG (2011) suggests that groundwater with no resource value may be ‘permanently unsuitable’ for use, for example, where its salinity is greater than that of seawater, i.e. the TDS exceeds 35 000 mg/l.
The 3DGWV LFV model can be used to identify the EA aquifer designation attributed to a particular geological formation. Where the aquifer designation is variable, local information should be used to identify the nature of the unit, for example from the EA website:
It should be noted that aquifer designations are only shown at outcrop. Where units are confined, aquifer designation should be obtained from nearby outcrops of the potential receptor with the same lithology.
The shallowest depth of the unit should be used for aquifer depth, so if the top of the second principal aquifer in (Figure 2.2) was at 300 m bgl, and the base was at 500 m bgl, the unit should still be classified as potential receptor class A. Where there is evidence (for example chemical) that groundwater bodies in the same aquifer unit may be separated by a barrier such as a fault, the receptor classification of the groundwater bodies can be assessed separately. TDS may also be estimated by summing all of the cations and anions to provide a minimum value, or by conversion of an SEC value, if available.
Where new information as to the groundwater quality in a particular potential receptor becomes available, this should be used to update the potential receptor classification.
- ENVIRONMENT AGENCY. 2017b. The Environment Agency’s approach to groundwater protection. November 2017 Version 1.1 [online]. Available from https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/658135/LIT_7660.pdf [cited 16 January 2018].
- UKTAG. 2011. Defining and reporting on groundwater bodies, UK Technical Advisory Group on the Water Framework Directive, working paper V6.22/Mar/2011 [online]. Available from http://www.wfduk.org/resources%20/defining-and-reporting-groundwater-bodies. [cited 12 September 2014].
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