Proposed Cemetery Thornhill Road Tier One Hydrogeological Risk Assessment Peter Mitchell Associates January 2015
Executive Summary This report uses a desk-based risk assessment technique published by the Environment Agency (EA) and classifies the groundwater vulnerability rating of the site as low. However, this report considers that the proposed development of this site as a cemetery would constitute a high risk to groundwater. The site vulnerability ranking given in this assessment is High mainly as a result of: The high number of expected burials 175 per year The groundwater levels recorded in 1972 in 2 boreholes on the southern and western boundaries of the site The potential for groundwater to collect in sand or gravel lenses or layers within the glacial Till Recommendations It is recommended that this report and the accompanying BGS report be sent to the EA and dialogue should be established with the EA, to ascertain its requirements for further assessment of this site s suitability for development as a new cemetery. Subject to the outcome of this dialogue, if detailed site investigations were required, it is proposed that these should consist of: 1. A topographic survey to provide a basis for designing the cemetery and any necessary drainage infrastructure. 2. An electro-magnetic induction (EMI) survey to provide a basis for establishing the most appropriate locations for excavating test pits down to a maximum depth of 3.5 m and installing a minimum of three dip wells (up to 10 m deep) to monitor groundwater levels. 3. An assessment of the soil profile pits, and to window sample material removed during the boring of the dip wells, in terms of the type, condition and physical properties of the soil exposed. The results will be used to determine factors that may influence the appropriateness of the site for burial purposes and the vulnerability of the environment to contamination from the proposed development. 4. Monitor the groundwater levels in the dip wells over a winter period, i.e. during the period of highest rainfall. Peter Mitchell Associates. January 2015. Page 2 of 24
Peter Mitchell Associates. January 2015. Page 3 of 24
Introduction Cardiff City Council commissioned this initial desk-based assessment of the suitability of a selected site for use as a cemetery. This report follows a method suggested by the Environment Agency (EA) to assess the potential risk of the development to groundwater. Various figures and extracts from a Detailed Geological Assessment report from the British Geological Survey (BGS) have been incorporated into the text below in italic font. Whilst definitive data regarding the pollution from cemeteries is scarce, any planning application for a new cemetery or cemetery extension will be assessed by the local Environment Agency (EA) team in the light of a number of documents published by the EA. These include: GP3, Groundwater protection: policy and practice, published in July 2008 Research and Development Technical Report P223 published in 1999 entitled Pollution Potential of Cemeteries Draft Guidance, Assessing the groundwater pollution potential of cemetery developments published in April 2004 Potential groundwater pollutants from cemeteries published in December 2004. The approach to risk assessment adopted by the P233 report can be summarized by the following excerpt: in order to be able to provide guidance which will enable Environment Agency staff to adopt a consistent approach when assessing the risks associated with the development of human or animal burial grounds. The guidance is directed principally at the potential threats to groundwater resources, but account is taken also of possible risks to surface waters, soils and the atmosphere. The report provides a framework for assessing the risks associated with cemeteries. The first stage is a Tier One preliminary site assessment that provides an initial review of the potential pathways for contamination and receptors in proximity to the site. Peter Mitchell Associates. January 2015. Page 4 of 24
The P233 report sets out the likely types and quantities of pollutants released by the burial of human bodies. The key to whether a site would be considered suitable is the rate at which such pollutants would be transported through the ground to enter water supplies: Pathways which pose the greatest threat to groundwaters from dissolved and particulate contaminants are those where hydrogeological factors allow rapid movement of pollutants from the source to the groundwater Consequently, coarse granular or heavily fractured sub-soils, fissured aquifer materials, or those of restricted mineralogy, are unlikely to offer significant opportunities for attenuation by many of the processes By contrast, aquifers composed of sediments or rocks of mixed mineralogy and in which groundwater flows are irregular, provide more effective protection of groundwater from surface derived pollution. The EA s Technical Report P223 identifies that the number of burials in a proposed cemetery will affect the overall assessment of the environmental risk. Thus a site considered low risk in terms of groundwater vulnerability, automatically becomes a high risk proposal if more than 100 burials are anticipated each year. This relationship between vulnerability class, burial rates and level of risk is shown schematically and featured later in this report. In any case, the EA recommends that all cemetery developments should conform to the following minimum standards: There should be no burials Within 250 metres of any spring, well or borehole used as a source of drinking water; Within 30 metres of any watercourse or spring; Within 10 metres of any field drain. In addition, There should be no standing water in the bottom of the grave when first dug; There should be at least one metre of subsoil below the bottom of the grave; Peter Mitchell Associates. January 2015. Page 5 of 24
The first step in considering this site for use as a cemetery is to assess and score a number of factors against a groundwater vulnerability ranking chart, illustrated below: Groundwater Vulnerability Ranking Chart (Table 5.1 in P223) Ranking Very Low Low Moderate High Very High Drift type Clay Silt Silty sand Sand / gravel Absent Drift thickness >5m >3 5m 3m 0 3m Absent Depth to water >25m 11 25m 10m 5 9m < 5m table Flow Intergranular Fissured mechanism Aquifer Non-aquifer Minor aquifer Major aquifer Abstraction and Source Protection Zone Watercourses Outside Zone 111 Within Zone 111 Close to boundary of Zones 11 & 111 Within Zone 11 Within Zone 1 or <250m from private source >100m >70 <100m >50 <70m >30m <50m <30m and springs Drains >100m >40 <100m 30 40m >10 <30m <10m Figure 1 A scoring scheme is used to provide a comparison mechanism: Figure 2 Vulnerability Element score Total score (Range) Very low 1 2 8 16 Low 3 4 24 32 Moderate 5 6 40 48 High 7 8 56 64 Very high 9 10 72 80 Using this system, a total score (range) for vulnerability ranking can be obtained for each site: Figure 3 Low vulnerability 8 32 Moderate 32 56 vulnerability High Vulnerability 56 80 Peter Mitchell Associates. January 2015. Page 6 of 24
The vulnerability ranking is then considered in the light of burial rates and an overall level of risk projected. Site Location The site under consideration is adjacent to the A469 Thornhill Road, immediately to the north of where it crosses over the M4 motorway, and within a mile from the existing Thornhill Cemetery. The aerial view below shows the approximate location of the proposed new cemetery outlined in Red: Figure 4 Peter Mitchell Associates. January 2015. Page 7 of 24
Site Description The land measures approximately 3.8 hectares (9.4 acres) and is currently used for grazing. A standard adult grave space size in a cemetery usually measures 2.74m x 1.22m (9ft x 4ft). This space permits graves to be excavated to receive typical coffins measuring 1.96m x 0.6m (6ft 5in x 2ft) and still leave sufficient virgin ground between each excavated grave. Making an allowance of 30% of the area of the extension to be used for paths and landscaping, the site has the capacity to provide approximately 8,000 adult grave spaces. At the current rate of demand of 175 new graves per year in the existing Thornhill Cemetery, this new site potentially offers capacity for new adult graves for approximately 45 years. Topography and surface drainage: The site is sloping to the south-west. The highest elevation is in the north-east and is approximately 135 m, sloping down to approximately 115 m in the south-west corner, closest to the M4 motorway. The O.S. map extract below shows a spring and the Llanishen Brook running southwards approximately 235m east of the proposed cemetery: Peter Mitchell Associates. January 2015. Page 8 of 24
Figure 5 Peter Mitchell Associates. January 2015. Page 9 of 24
Site Geology - Superficial Deposits (Drift) Superficial deposits, particularly if they have low permeability, are helpful for cemetery developments in slowing the downward migration of any contaminants that may be released from the decomposition of burials into the underlying bedrock. The map extract below shows the presence of superficial deposits on the site: Figure 6 The superficial deposit at surface is till, a glacial deposit of late Quaternary (Devensian) age. Two key boreholes within the area of interest, on its southern margin (ST18SE211 and ST18SE20) indicate that the superficial deposits are likely to comprise clay and sandy clay and silt with gravel and cobbles and a few boulders of limestone and sandstone. Within the till there may be thin beds of coarse-grained sand and gravel, and thin beds of laminated clay. Beds of sand within the till may be water-bearing, possibly giving rise to running sand conditions in boreholes or excavations. Peter Mitchell Associates. January 2015. Page 10 of 24
Bedrock (Solid) Geology The diagram below shows the bedrock geology in the area: Figure 7 Rockhead Depth: The depth to rockhead (base of the superficial deposits) is estimated to be up to 14 m. This is based on two boreholes in the area of interest (ST18SE211 and ST18SE20). The boreholes are both on the southern margin of the site, close to the M4 motorway. Graves would excavated to between 2m and 3m maximum depth, so would be within the Till superficial deposits. Peter Mitchell Associates. January 2015. Page 11 of 24
Bedrock Geology: The site is underlain by the Llanishen Conglomerate, which is Lower Devonian in age. It comprises about 150 m of red sandstones, siltstones and mudstones with beds of conglomerate; calcrete beds and nodules are common throughout. The strata dip at approximately 50 towards the north and north-north-west. The geology underlying the site is shown in the BGS schematic diagram below: Figure 8 Peter Mitchell Associates. January 2015. Page 12 of 24
Evidence from borehole drilling A number of boreholes have been drilled in this vicinity in the past and provide useful evidence of actual ground conditions. The map extract shows the location of the boreholes Figure 9 Data from some of these borehole records is tabulated below and includes descriptions of ground conditions to 3.5m depth below ground level, i.e. the ground to a greater depth than graves would be excavated: Ref. Date Depth Water level Ground conditions to 3.5m bgl 18 27/09/1972 7m Dry Clayey, silty fine sand and gravelly clay 20 05/11/1972 13.2m 6.5m Silty sandy clay with gravel and cobbles 187 10/03/1972 14.2m 10m Clayey silt with cobbles. Sand, pebbles & silt 209 26/07/1984 10.5m Dry Clayey very silty sand and gravel Sandy, gravelly clay with cobbles. Clayey sandy silt with 211 18/07/1984 10.5m Dry gravel Unfortunately, the records for borehole 83 are missing. Peter Mitchell Associates. January 2015. Page 13 of 24
Hydrogeology and groundwater vulnerability: Figure 10 The BGS report suggests that, whilst the clay within Till deposits is of low permeability, there may be some groundwater in sand and gravel layers within the clay, which could be exposed in excavations. Peter Mitchell Associates. January 2015. Page 14 of 24
Groundwater Source Protection Zones (SPZs) The Environment Agency (EA) has defined Source Protection Zones (SPZs) for 2,000 groundwater sources such as wells, boreholes and springs used for public drinking water supply. These zones show the risk of contamination from any activities that might cause pollution in the area. The closer the activity, the greater the risk. Zone 1 (Inner protection zone) Any pollution that can travel to the borehole within 50 days from any point within the zone is classified as being inside zone 1. This applies at and below the water table. This zone also has a minimum 50 metre protection radius around the borehole. These criteria are designed to protect against the transmission of toxic chemicals and water-borne disease. Zone 2 (Outer protection zone) The outer zone covers pollution that takes up to 400 days to travel to the borehole, or 25% of the total catchment area whichever area is the biggest. This travel time is the minimum amount of time that we think pollutants need to be diluted, reduced in strength or delayed by the time they reach the borehole. Zone 3 (Total catchment) The total catchment is the total area needed to support removal of water from the borehole, and to support any discharge from the borehole. The map overleaf, taken from the web site of the Environment Agency, illustrates the GPZs in the area around Cardiff. Peter Mitchell Associates. January 2015. Page 15 of 24
Figure 11 Inner Zone 1 Outer Zone 2 Total Catchment Zone 3 Special Interest The proposed site lies outside of any Source Protection Zone. Peter Mitchell Associates. January 2015. Page 16 of 24
Aquifers a) Superficial deposits designation Figure 12 b) Bedrock designation Figure 13 Peter Mitchell Associates. January 2015. Page 17 of 24
Principal Aquifers These are layers of rock or drift deposits that have high intergranular and/or fracture permeability - meaning they usually provide a high level of water storage. They may support water supply and/or river base flow on a strategic scale. In most cases, principal aquifers are aquifers previously designated as major aquifer. Secondary Aquifers These include a wide range of rock layers or drift deposits with an equally wide range of water permeability and storage. Secondary aquifers are subdivided into two types: Secondary A - permeable layers 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. These are generally aquifers formerly classified as minor aquifers; Secondary B - predominantly lower permeability layers which may store and yield limited amounts of groundwater due to localised features such as fissures, thin permeable horizons and weathering. These are generally the water-bearing parts of the former nonaquifers. Secondary Undifferentiated - has been assigned in cases where it has not been possible to attribute either category 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. The proposed site lies above a minor (Secondary A) aquifer. Peter Mitchell Associates. January 2015. Page 18 of 24
Indicative Flood Plains Figure 14 Flooding from rivers or sea without defences Flood defences Extent of extreme flooding Areas benefiting from flood defences The proposed site lies outside any of indicative flood plain. Note: Data and maps for Source Protection Zones, Aquifers and Flood Risk were obtained from the Environment Agency s web site on 5/1/2015. Peter Mitchell Associates. January 2015. Page 19 of 24
Groundwater Extraction The records of groundwater occurrence in the till at the site are variable. There are several boreholes close to the site that record the deposits as comprising mainly silty sands with gravel horizons rather than clay. The logs of two boreholes, one on the western edge (ST18SE187) and one in the southwestern corner of the site (ST18SE20), record respectively, a water strike at 8.0 m (the actual borehole log records water encountered at 9.6m, not 8.0m) below ground level in sand and silt with a rest water level at 8.2 m down (c.113 m above OD) and a seepage at 7.0 m below ground level near the junction of gravelly clay with boulders above clayey sand with a rest level at 6.5 m down (c.105 m above OD). However, two other boreholes, one at the south-west corner (ST18SE209) penetrating 12.2 m of till, comprising mainly sand and gravel, over bedrock and one mid-way along the southern boundary (ST18SE211) penetrating mainly clay, were recorded as dry. Two shallow, hand-dug domestic wells borehole (ST18SE247 and 248) with/without a borehole in the base, through till into gravel, about 600 m south of the site, report the yield as fair and good, respectively. None of the four boreholes immediately round the site quoted above, to a maximum depth of 21 m, recorded any water strikes in the bedrock. A 41.5 m deep borehole commencing in Llanishen Conglomerate, 1.2 km to the north-east, had a rest water level 22 m below surface (116 m above OD). A 8.2 m deep well into Llanishen Conglomerate about 1.7 km to the east (ST18SE249) recorded that the water level varied between scarcely enough to almost at the top, indicating seasonal fluctuations of the order of 8 m. The water had a total dissolved solids content of 290 mg/l, chloride 23 mg/l, nitrate 0.6 mg/l and was suitable for drinking. The borehole records suggest that groundwater levels may vary across this site, with groundwater only encountered on the west and south-west edges. Peter Mitchell Associates. January 2015. Page 20 of 24
Site Vulnerability Assessment Criteria Drift Type Drift Thickness Depth to Water Table Flow Mechanism Aquifer Abstraction and Source Protection Zone (SPZ) Watercourses and springs Land Drains Comment Glacial Till 14m 8m Within the Till, any sand or gravel lenses or beds would allow groundwater to move readily within them, whereas movement would be impeded within the clay Glaciofluvial deposits considered a minor aquifer. Outside Zone 3. Spring and stream 235m east of the site None known Figure 15 Peter Mitchell Associates. January 2015. Page 21 of 24
Site Vulnerability Assessment Score Sheet Figure 16 Factor Site Characteristics Ranking Ranking Scores Drift type Glacial Till Very Low 1-2 Drift thickness 14m Very Low 1-2 Depth to water 8m High 7-8 table Flow mechanism Variable Moderate 5-6 Aquifer Minor Aquifer Moderate 5-6 Abstraction and Outside Zone 3 Very Low 1-2 Source Protection Zone Watercourses and >235m Very Low 1-2 springs Land Drains None known Very Low 1-2 Total (range) 22-30 Vulnerability Element score Range Site Very low 1 2 8-16 Low 3 4 24-32 22-30 Moderate 5-6 40-48 High 7-8 56-64 Very high 9-10 72-80 Figure 17 Figure 18 Vulnerability Range Actual Low vulnerability 8 32 22 30 Moderate 32 56 vulnerability High Vulnerability 56 80 Vulnerability Ranking Based upon the total score (range) indicated, I have assigned the site a vulnerability ranking of: Figure 19 Low: X Moderate: High: Scale of Development The number of new graves excavated for full earth burials in the nearby Thornhill Cemetery is 175 per year. Peter Mitchell Associates. January 2015. Page 22 of 24
Figure 20 Level of Risk Number of bodies per year 175 Using the relationship between burial rates, vulnerability and the level of risk, as shown in the diagram below, the appropriate level of risk assessment required is: Figure 21 Low: Moderate: High: X Schematic relationship between burial rates, vulnerability class and level of risk 175 Figure 22 Peter Mitchell Associates. January 2015. Page 23 of 24
Discussion This site offers good potential for development as a cemetery: Factor Comment Size Topography Location Access Ecology The site is sufficiently large to provide for the long-term burial needs of the local community. The site is relatively level and the western boundary hedge and trees provide privacy. The site lies in an attractive setting at the edge of the community which it would serve Access by vehicle to the site is excellent. The development of a cemetery, particularly with new landscaping, could provide for greater diversity than the current use of the site. Figure 23 The site vulnerability ranking given in this assessment is High mainly as a result of: The high number of expected burials The groundwater levels recorded in 2 boreholes on the southern and western boundaries of the site The potential for groundwater to collect in sand or gravel lenses or layers within the glacial Till Trial pits and dip well monitoring across the site during the wetter months of the year would establish actual groundwater levels and the presence or otherwise and extent of sand and gravel layers. These results would inform the need for and design of any mitigation measures to limit potential pollution of groundwater from the use of this site as a cemetery. Peter Mitchell Associates. January 2015. Page 24 of 24