BORRAS AIRFIELD & THE HOLT ESTATE. Planning Application to Extend Borras Quarry: Hydrological & Hydrogeological Assessment.

Transcription

1 BCL Document Reference: BCL/htl/B110/031.doc BORRAS AIRFIELD & THE HOLT ESTATE Planning Application to Extend Borras Quarry: Hydrological & Hydrogeological Assessment August 2005 Report prepared for: Tarmac Limited Wolverhampton Science Park, Wolverhampton, WV10 9RU Tel: , Fax: web: Company Registration Number: Registered in England & Wales. Registered Office: 33 Wolverhampton Road, Cannock.

2 BCL Document Reference: BCL/htl/B110/031.doc BORRAS AIRFIELD & THE HOLT ESTATE Planning Application to Extend Borras Quarry: Hydrological & Hydrogeological Assessment August 2005 Report Prepared By Henry Lister B.Sc. M.Sc. Senior Hydrogeologist Report Checked By Gavin Chaplin B.Sc. M.Sc. Senior Hydrogeologist Wolverhampton Science Park, Wolverhampton, WV10 9RU Tel: , Fax: web: Company Registration Number: Registered in England & Wales. Registered Office: 33 Wolverhampton Road, Cannock.

3 CONTENTS Page Number 1.0 INTRODUCTION Background Data sources THE BASELINE SURVEY Location and land use Topography Regional Geology Geology of Borras Airfield Geology of the Holt Estate Meteorology Hydrological Setting Watercourses Surface water bodies Regional hydrogeology Site hydrogeology Groundwater levels background data presented in previous reports Data from active piezometers on Borras Airfield Data from active piezometers on the Holt Estate Groundwater head contours Aquifer hydraulic conductivity Geological factors affecting aquifer properties Licensed groundwater and surface water abstractions Private water supplies Conceptual hydrogeology Further investigation Vicarage Moss SSSI... 21

4 3.0 QUARRY DEVELOPMENT AND RESTORATION Programme of working Method of working Restoration IMPACT ASSESSMENT Groundwater levels and flow Groundwater level rainfall analysis Water Environment Protection Scheme Groundwater quality Impact upon the surface water regime Impact upon licensed and unlicensed abstractions Surface water run-off in the Holt Estate SUGGESTED PLANNING CONDITION Recommendations SUMMARY AND CONCLUSIONS Background Baseline survey Development proposals Impact assessment APPENDICES Appendix I Appendix II Appendix III Appendix IV Appendix V Appendix VI Appendix VII Appendix VIII Figures Rainfall data Data from EA s piezometers Data from Tarmac s piezometers Licensed abstractions within a 4 km radius of the Site Private abstractions within a 4 km radius of the Site Correspondence between the EA and WCBC Water Environment Protection Scheme

5 1.0 INTRODUCTION 1.1 Background In May 2003, Tarmac Limited (Tarmac) submitted a Planning Application (supported by an Environmental Statement), seeking permission to extend Borras Quarry. The aim was to consolidate the working and restoration strategies at Borras Airfield and to extend the workings on to the Holt Estate land (which together comprise the Application Site). This would involve continued working on the existing Borras Airfield, in a phased manner and to a selective depth below the already permitted 62 maod (but above the water table), and extending in 6 phases on to the Holt Estate land, to the immediate north of the Airfield Tarmac is now submitting an amendment to the Planning Application. This amendment proposes the working of the Holt Estate land by eliminating Phase H6 of the working programme and providing greater stand-offs from the limit of sand and gravel extraction to residential properties. Phase H6 was intended to be the last phase of working of the Holt Estate land but, by excluding this phase, it is possible to retain a kettle hole and escarpment, which are considered to be visually important landscape features. Additionally, some minor boundary changes have taken place on the southern area of the Holt Estate land. These revisions have been brought about in response to issues raised by statutory and non- statutory consultees and following the publication of the Minerals Technical Advice Note (TAN) produced by the Welsh Assembly Government in This report forms part of the amended Environmental Statement. It details the baseline (pre-existing) hydrological and hydrogeological regime within and surrounding the Application Site. It includes an assessment of the potential impacts upon this regime as a result of the Proposed Development and, where necessary, outlines a schedule for mitigation of such impacts Consultations have been held with the Environment Agency (EA) and the Countryside Council for Wales (CCW) and account has been taken of their comments in this assessment and in the design of the Proposed Development. Page 1

6 1.1.5 Fuller details of the Proposed Development are contained within the Environmental Statement and accompanying Planning Application. 1.2 Data sources A programme of desk and field study was undertaken to define the baseline hydrological and hydrogeological regime at the Application Site. This involved the inspection and correlation of information from the following sources: i. Ordnance Survey (OS) topographic map, Pathfinder sheet number 789, Wrexham (North), at a scale of 1:25,000. ii. iii. iv. British Geological Survey (BGS) map, sheet number 121, Wrexham (drift and solid), at a scale of 1:63,360. Accompanying BGS memoir. Mineral Assessment Report Number 61, Sand and Gravel Resources North of Wrexham, Clwyd, Institute of Geological Sciences. Geological information from exploration drilling programmes undertaken at the Application Site. v. Reports detailing the hydrology and hydrogeology of the area, which include: Application for the Determination of New Conditions, Borras Airfield and Borras Hall Farm, Borras Quarry, Wrexham. Hydrological and Hydrogeological Assessment, prepared by Tarmac Quarry Products Limited in March 2000 (Reference 1). Hydrogeological Assessment of the Holt Estate, Wrexham, prepared by Aspinwall & Company in December 1994 (Reference 2). Hydrogeological Assessment of an Area of Proposed Mineral Extraction at Borras Farm, near Wrexham, Clwyd, prepared by Aspinwall & Company in January 1985 (Reference 3). vi. Groundwater level monitoring data from piezometers operated by Tarmac and the EA. Page 2

7 vii. Surface water level and flow measurements from gauge boards and spot gauging exercises (undertaken by Tarmac and the EA). viii. Rainfall data from the nearest EA gauging station. ix. EA database of licensed ground and surface water abstractions. x. Wrexham County Borough Council (WCBC) database of private (unlicensed) water supplies. xi. xii. Information from the EA and English Nature (EN) detailing local waterdependent ecological resources. A water features survey of the Site and its environs. Page 3

8 2.0 THE BASELINE SURVEY 2.1 Location and land use The Application Site is centred upon land at National Grid Reference (NGR) SJ , which is approximately 3.5 kilometres (km) north east of Wrexham town centre, within the county borough of Wrexham, North Wales The Site location is shown on Drawing B110/130a in the main body of the report. All other figures referred to by this report are included within appendix one Borras Airfield is currently being worked in accordance with a planning permission for sand and gravel extraction, which permits extraction to a depth of 62 metres above Ordnance Datum (maod) The Holt Estate comprises agricultural land, principally used for livestock grazing The boundaries of the Application Site are shown on Drawing B110/130a. 2.2 Topography The Application Site forms part of the Wrexham Delta terrace, an area of glacial sand and gravel deposits. The terrace lies between the high ground of the Hope and Esclusham Mountains to the west and the extensive low-lying Cheshire Plain to the east (figure two) Borras Airfield is relatively flat with an average elevation of 72 maod, reduced from a pre-quarrying level in excess of 75 maod The Holt Estate consists of flat to gently undulating fields, averaging 75 maod in elevation. The highest point is 81 maod, at the northern limit of the Application Site. There are occasional depressions (65-70 maod), which represent kettle holes (glacial outwash features) formed within the sand and gravel deposits. Beyond the northeast boundary of the Site, three of these depressions support pond features (figure two and section 2.9). Page 4

9 2.2.4 To the north and west of the Site, the land is gently undulating and hummocky, typical of glacial outwash deposits, with a series of kettle holes and ponds. The ponds (section 2.9) include Vicarage Moss (a Site of Special Scientific Interest, SSSI), The Flash, Pantyr-ochain, Coed-yr-bychan and Tommy s Wood The deeply incised valley of the River Alyn lies 1.75 km to the north west of the Site The suburbs of Wrexham extend to within 250 m of the south west boundary of the Site. The land is gently undulating but, compared to areas abutting the north and west of the Site, there are fewer ponds (Borras Bog and Acton Lake, section 2.9) Immediately to the east of the Site, a north-south trending escarpment marks the abrupt transition from the Wrexham Delta terrace on which the Site is located (some 75 maod) down to the lower ground of the Cheshire Plain (less than 15 maod). A number of small un-named ponds and spring-fed streams occur along this escarpment at elevations of between maod The River Dee occupies a broad shallow valley on the Cheshire Plain. The valley is aligned south-north and lies approximately 4.0 km to the east of the Site boundary at its closest point To the south of the Site, the escarpment changes to a northeast-southwest alignment. A number of small un-named ponds and spring-fed streams emerge at elevations of between maod The River Clywedog occupies a shallow valley located some km to the south of the Site A restored landfill site, Llan-y-pwll, is located some 250 m to the south of the Site boundary, situated upon the eastern edge of the Wrexham Delta terrace at an elevation of maod. 2.3 Regional Geology The regional geology is detailed on BGS sheet number 121 ( Wrexham ) and in the associated memoir and mineral assessment report. An extract from the BGS map is shown in figure three. Page 5

10 2.3.2 The drift deposits at the Site form part of the Wrexham Delta terrace, a thick accumulation of glacial sand and gravel, which generally overlies boulder clay. The BGS map indicates that the sand and gravel pinches out along a north-south oriented line (less than 0.5 km to the east of the Site), which represents the escarpment at the edge of the Wrexham Delta terrace. To the east of the escarpment, the boulder clay is extensive and laterally persistent Data collected during this assessment suggests that the sand and gravel deposit may, in places, extend into the boulder clay to the east of the escarpment (section 2.20) Across the majority of the Site, the drift material is underlain by the Erbistock Beds, part of the Carboniferous Upper Coal Measures. These outcrop in the valley of the River Alyn at NGR SJ and comprise purple and red sandstones, marls, conglomerates and breccias, with thin impure limestones and coal seams At the eastern extremity of the Site, the Erbistock Beds are unconformably overlain by Triassic Sherwood Sandstone (a soft, reddish brown, cross-bedded sandstone). During drilling of the Borras Quarry abstraction borehole, located at NGR SJ , Erbistock Beds were encountered rather than Sherwood Sandstone, contrary to information shown on the BGS map. It is therefore inferred that the boundary between these two rock types should in fact be at least 500 m to the east of the position indicated on the BGS map. 2.4 Geology of Borras Airfield An accurate model of the geology of Borras Airfield was defined as a result of an extensive drilling programme carried out by Tarmac in The deposit has proved to be highly variable but can be sub-divided into the following broad lithologies: Soils / Made Ground up to 2.0 m thick. Where present, the soils are generally black, sandy, often silty, with gravel. There is no distinct subsoil. However, large parts of Borras Airfield have made ground and concrete at the surface or exhibit no natural soils due to previous extraction activities. Page 6

11 Upper Sands and Gravels up to 20 m thick. This unit is a variable horizon comprising clean to clayey, reddish brown sand and gravel. Typically the material consists of a medium to coarse sand with fine to coarse gravel, which is sub-rounded. A large area of this unit has been wholly or partially worked in the past. Lower Sands up to 30 m thick. Underlying the Upper Sands and Gravels, this unit is variable in nature and distribution. The colour is typically buff brown but varies from orange red to dark brown to whitish. Grain sizes range from fine sand with silty clay to medium sand. Boulder Clay This was encountered or, where drilling became too difficult to progress, alluded to in the majority of the 1996 series of boreholes. In general, it is a stiff reddish grey clay, with occasional fine to medium grained angular gravel fragments. It is sometimes overlain by a softer buff brown silty clay unit. The upper surface of the boulder clay varies between maod. A north northwest south southeast trending buried channel extends across the central part of Borras Airfield The controls exerted upon watertable elevation and aquifer properties by the geological configuration of the area are discussed in section Geology of the Holt Estate Exploration drilling programmes were undertaken in 1979, 1989, 1990, 1994 and 2000 to examine the geological sequence at the Holt Estate Topsoil across the Holt Estate is an average of 0.26 metres in thickness and subsoil varies from 0.2 to 0.8 m in thickness In general, the soil is underlain by the sand and gravel deposit. Similar to Borras Airfield, a sub-division between Upper Sands and Gravels and Lower Sands has been identified during exploration In the northern area of the Holt Estate (and in the base of kettle holes), accumulations of boulder clay are encountered beneath the soils horizon. These range from m in thickness and comprise reddish brown, stiff to firm, silty to sandy clay with occasional gravel. Page 7

12 2.6 Meteorology Long term average monthly rainfall data, recorded in millimetres (mm), for the period are included in appendix two. The data have been obtained from the EA rainfall gauging station at Five Fords (NGR SJ ), which is some 5 km to the south of the Site Potential evapotranspiration rates, measured in mm/month, for the period are presented as average monthly data in appendix two. The data have been obtained from the (former) Ministry of Agriculture, Fisheries and Food (MAFF): Technical Bulletin 34, Climate and Drainage (1976) Collection of the average monthly rainfall data and potential evaporation data has allowed for the estimation of effective rainfall following the method of Grindley ( The calculation of actual evaporation and soil moisture deficit over specified catchment areas, Hydrological Memorandum 38, Meteorological Office, Bracknell, U.K., November 1969). Effective rainfall is the amount of precipitation available for infiltration to the watertable and for surface water run-off, after satisfying any soil moisture deficit and actual evapotranspiration. The latter two parameters are dependent upon vegetation cover. Therefore, effective rainfall rates have been calculated for a range of vegetation types: crops, grass and bare ground. The derived monthly average data are tabulated in appendix two The derived annual average effective rainfall rate at the Site increases from 241 millimetres/annum (mm/a) in areas planted with crops to 248 mm/a for grass and up to 293 mm/a in areas of bare ground (such as ploughed fields). 2.7 Hydrological Setting The catchment area boundaries of three river systems (the River Alyn, the River Clywedog and the River Dee) extend across the Application Site The River Alyn lies to the west and north of the Site. Its valley follows a meandering course, varying from a north-south to west-east alignment. At its closest approach, the river lies some 1.75 km to the north west of the Site, has an elevation of maod and flows in a northeasterly direction. On a regional scale, it flows from west to east across the Wrexham Delta terrace. Page 8

13 2.7.3 At its closest approach, the River Clywedog is located some km to the south of the Site and has an elevation of maod. It flows from west to east across the Wrexham Delta terrace. The orientation of the valley shows little variation but numerous river meanders occur upon the valley floor The River Alyn and River Clywedog descend on to the lower-lying Cheshire Plain and join the River Dee, which flows from south to north. It lies approximately 4.0 km to the east of the Site boundary and has an elevation of 5-10 maod, at its closest point. As with the River Clywedog, the orientation of the valley shows little variation but numerous river meanders occur upon the valley floor. 2.8 Watercourses There are no surface watercourses within the Site boundary (figure two) The largest watercourses in the vicinity of the Site are the Rivers Alyn, Clywedog and Dee (section 2.7) There are a number of un-named spring-fed streams that are within a 1 km radius of the Site (i.e. in closer proximity than the aforementioned rivers). These are identified by reference to the name of the nearest property. They occur at various points along the east-facing escarpment that divides the Wrexham Delta terrace on which the Site is located from the lower ground of the Cheshire Plain. They generally emerge at elevations of between maod Hoseley Bank Farm stream arises some 650 m to the northeast of the proposed extraction area within the Holt Estate, at the maod contour level. A flow gauging exercise was undertaken in August 1997 using an impellor flow meter. Three readings were taken at NGR SJ , with measured discharge varying from litres/second (l/s). A similar gauging exercise was undertaken at this location by the EA in September 1996 and the flow rate measured 0.74 l/s. The stream flow northeastwards to join the River Alyn Springfield stream arises 1 km to the east of the proposed extraction area within the Holt Estate, at an elevation of 35 maod. It flows in a northeasterly direction and is a tributary of the River Dee. During the 1996 flow gauging exercise, a flow rate of 1.40 l/s was recorded at NGR SJ Page 9

14 2.8.6 The source of Common Wood Farm stream is located 660 m east northeast of Borras Airfield, emerging from the escarpment at the maod contour level. The flow rate observed in September 1996 was less than 0.1 l/s (NGR SJ ) Two streams arise in the vicinity of Plas Bostock, some km to the east of Borras Airfield. The springs are at an elevation of maod. In September 1996, the flow rate in Plas Bostock stream (at NGR SJ ) was 0.2 l/s. A lower flow rate (less than 0.1 l/s) was recorded in Plas Bostock II (at NGR SJ ) The Common Wood Farm and Plas Bostock streams converge and flow in a northeasterly direction to join the River Dee Holt Lodge Farm stream arises 350 m to the southeast of Borras Airfield, at an elevation of some 45 maod. It flows towards the south southeast and joins the River Clywedog. In August 1997, the flow rate was not sufficient for gauging purposes. 2.9 Surface water bodies Over the majority of the Site (with the exception of the lagoon system on Borras Airfield), the unsaturated zone within the sand and gravel deposit is free-draining such that surface water does not accumulate There are three un-named ponds located close to the north and east boundaries of the proposed extraction area within the Holt Estate (figure two). These are henceforth referred to as Woodlands Pond, South Vicarage Pond and Walnut Tree Pond. At their closest approach to the Site, the distance between pond margin and extraction boundary is 25 m, 45 m and 50 m respectively Walnut Tree Pond and Woodlands Pond occupy shallow depressions measuring 25 m by 50 m. Based upon the OS map, ground elevation adjacent to these ponds is at approximately 75 maod. These ponds represent shallow, abandoned clay workings, filled by rainfall run-off. Page 10

15 2.9.4 South Vicarage Pond comprises three interconnected depressions (forming a shape). The depressions measure approximately 30 m by 30 m, 50 m by 80 m and 50 m by 125 m. The water level in the pond was monitored from November 1990 to October 1993 and varied between maod. The relevant data are contained in Reference 2. The pond lies at the south west edge of a woodland block, which together form Vicarage Gorse County Wildlife Site (CWS 314) A kettle hole complex, known as Vicarage Moss, is located adjacent to, but outside, the northern boundary of the Holt Estate. The complex, which comprises one very large kettle hole and several smaller subsidiary kettle holes, is a Site of Special Scientific Interest (SSSI). The largest kettle hole is 200 m in diameter and forms a deep basin occupied by mire vegetation. This same vegetation also occupies one of the smaller kettle holes. An area of carr (wet woodland) occurs at the northern edge of the mire. The SSSI boundary extends to within 75 m of the proposed extraction area at the Holt Estate, at its closest approach. Water level in the largest mire was monitored from November 1990 to July 1991 and varied between maod. The relevant data are contained in Reference There are a number of ponds located to the west of the Holt Estate (figure two). The closest is Tommy s Wood, which is located some 45 m from the western boundary of the Site. The water level in the pond was monitored from August 1984 to January 1985 and varied between maod. The relevant data are contained in Reference Coed-yr-bychan Pond is approximately 255 m to the west of the Holt Estate. Reference 3 records a water level of maod for the same monitoring period Pant-yr-ochain (CWS 312) comprises a number of small ponds surrounding a large lake. One of the satellite ponds is some 100 m from the Site boundary; however, the lake itself is located 365 m to the west of the Site, at its closest approach. Reference 3 records the water level in the lake varying from maod for the period August 1984 to January The Flash is located 625 m to the west of the Site boundary. Its water level is monitored by the EA and ranged from maod for the period 1984 to December 2002 (appendix three). Page 11

16 Borras Bog (CWS 315) lies some 160 m to the west of Borras Airfield. Reference 3 records the water level in the bog varying from maod for the period August 1984 to January There is a small, un-named pond within the grounds of Wrexham Golf Club. It is situated at NGR SJ , approximately 170 m to the south of the Borras Airfield extraction boundary. A gauge board was installed within this pond in March 2004; thereafter, surface water level has been recorded on a monthly basis. During this period, water level has varied from maod. It is noteworthy that the pond dried out completely in June There are numerous small, un-named ponds located to the east of the Holt Estate and Borras Airfield (figure two). These occur along the escarpment between the Wrexham Delta terrace (on which the Site is located) and the lower ground of the Cheshire Plain. The ponds are generally encountered at or below the 50 maod contour level. The closest to the Site boundary is some 350 m east of Borras Airfield Regional hydrogeology The Triassic Sherwood Sandstone is classified as a Major Aquifer by the EA s Policy and Practice for the Protection of Groundwater (Regional Appendix, Welsh Region). The sandstone strata (soft, reddish brown, fine to medium grained, cross-bedded) exhibit high permeabilities, both intergranular and fissure. The overlying drift deposits restrict recharge but provide protection from contamination by surface activities. High abstraction rates have resulted in the localised depression of groundwater levels and in salinity problems at some sources The Erbistock Beds (part of the Carboniferous Upper Coal Measures) comprise purple and red sandstones, marls, conglomerates and breccias, with thin impure limestones and coal seams. The sandstone strata yield large but (temporally and spatially) variable quantities of water, thus they have Minor Aquifer status. Flow is predominantly through joints and fissures as well as tension zones caused by mining activities. Page 12

17 The Wrexham Delta terrace is a thick accumulation of glacial sand and gravel (of Quaternary age), which generally overlies boulder clay. The sand and gravel deposits are relatively thin and variable but with good intergranular permeability. They are of limited importance as aquifers, providing local supplies only (Minor Aquifer). The watertable is near-surface and therefore vulnerable to pollution; however, the silt and clay content of the deposit provides some attenuation of contaminants Site hydrogeology Based upon the model of the Site geology detailed in sections 2.4 and 2.5, it is evident that the drift deposits comprise sand and gravel underlain by a laterally persistent layer of boulder clay. Although many of the 1994 and 1996 boreholes proved a substantial thickness of boulder clay (up to 8 m), the only documented location at the Site where the drift has been fully penetrated is in the 180 m deep Borras Quarry abstraction borehole (NGR SJ ). This recorded a total drift thickness of 60 m, which included approximately 20 m of boulder clay In light of the collected data, it is considered that the sand and gravel drift deposits at the Site are not in hydraulic continuity with the underlying solid geology i.e. they form two separate aquifers. This is due to the intervening layer of boulder clay. The base of the sand and gravel aquifer is identified as the upper surface of the boulder clay Groundwater levels background data presented in previous reports Reference 1, which was prepared in March 2000, presents data collected from groundwater level monitoring boreholes (piezometers) operated by Tarmac and the EA in the vicinity of the Site. This includes data for Borras Airfield as well as nearby active and restored sand and gravel workings (Borras Farm, Borras Hall Farm and Caia Farm). In addition, Reference 1 contains groundwater level data obtained from WCBC for piezometers operated at Llan-y-pwll landfill site (some 250 m to the south of the Site boundary) These groundwater level data have been collated and analysed in Reference 1. Page 13

18 The piezometer data presented in Reference 1 have been updated within this report. A number of piezometers have been lost since the submission of Reference 1. Current groundwater levels at these locations have therefore been estimated by reference to neighbouring, active piezometers e.g. water level at BH has been estimated using current data from BH (active). This has been achieved by examining the historic relationship between the hydrographs for BH and BH Based upon this historic relationship, the current water level difference at the two locations has been estimated Data collected by the EA are presented as a series of hydrographs in appendix three Data obtained from Tarmac s piezometers up to February 2005 are examined in sections 2.13 and Data from active piezometers on Borras Airfield There are four piezometers on Borras Airfield, one of which was installed in July 1996 (BH 96/46); the remainder were constructed in November 1999 (BH 99/02, BH 99/03 and BH 99/04). The locations of these piezometers are shown in figure four Groundwater level data obtained from these piezometers for the period January 2000 to February 2005 are tabulated in appendix four. The data were collected on a quarterly basis from January to September 2000, thereafter on a monthly basis. They are presented as a series of hydrographs in figure five In terms of spatial distribution, the highest groundwater levels on Site are recorded in BH 99/46 ( maod) and BH99/02 ( maod), which are towards the southern and western boundaries of Borras Airfield. These two piezometers show similar temporal variations. Minimum water levels were observed at the start of the monitoring period, steadily rising to peak levels in November-December 2001, after which they remained relatively constant until mid Subsequently, they have declined back towards the minimum elevations recorded during the monitoring period. Page 14

19 The lowest groundwater elevations (in terms of spatial distribution) occur in BH 99/03 ( maod) and BH 99/04 ( maod), towards the centre and eastern parts of Borras Airfield. These two piezometers have exhibited similar variations in water level during the monitoring period. Water levels rose from January 2000 onwards, attaining maximum levels in September 2002 through to June 2003, and then exhibiting a slight but steady decline to current levels (which are some 0.3 metres below peak levels) Data from active piezometers on the Holt Estate There are thirteen active piezometers operated by Tarmac on the Holt Estate, four of which were installed in November 1990 (BH 11, 12, 13 and 14), the remainder were constructed between May and October 1994 (BH 15, 16, 17, 18, 19, 20, 21 and 22). The locations of these piezometers are shown in figure four Groundwater level data obtained from these piezometers for the period January 2000 to February 2005 are tabulated in appendix four. The data have been collected on a monthly basis, with the exception of the period March to June 2001, when no data were retrieved due to access restrictions during the Foot and Mouth Crisis. They are presented as a series of hydrographs in figure six The spatial distribution of groundwater levels on the Holt Estate has been examined. The highest water levels are recorded in BH 16 ( maod), which is located at the south west corner of the Holt Estate. The most depressed water levels are observed in BH 14 ( maod), towards the north east boundary of the Site It is considered that BH 17 measures a perched water level. Its neighbouring piezometers, BH 23 (100 m to the west) and BH 11 (60 m to the east), record significantly lower groundwater levels (6 m and 8 m lower levels respectively). Therefore, data from BH 17 has not been used when determining groundwater head contours for the Site (section 2.15) Temporal variations in groundwater level are dependent upon spatial distribution. The piezometers located in the southern and western parts of the Holt Estate (BH 11, 16, 17, 18, 19, 22 and 23) all exhibit a consistent temporal trend in water level: Page 15

20 Minimum water levels are recorded at the commencement of monitoring (January- October 2000) and are recurring at present (February 2005). Peak water levels are attained in July 2001, although the trend in the preceding months cannot be analysed due to missing data during the Foot and Mouth Crisis. There is generally a decline in water level from July 2001 to date, although a temporary recovery occurs each winter-spring. BH 19 shows the least variation in water level during the monitoring period (0.4 m), compared with a maximum of 1.6 m variation in BH The piezometers in the northern and eastern parts of the Holt Estate (BH 12, 13, 14, 15, 20 and 21) can be grouped together based upon their similarity in hydrograph trend: Minimum water levels are recorded in January Allowing for minor seasonal variations, groundwater elevations show a steady increase from January 2000 to January Peak water levels are generally maintained between February 2002 and October 2003, after which there is a gentle decline for the remainder of the monitoring period. BH 20 shows the least variation in water level during the monitoring period (2.0 m), compared with a maximum of some 3.0 m variation in BH Groundwater head contours Data from Reference 1 and from the active piezometers on the Holt Estate and Borras Airfield have been merged to produce a watertable contour plan for the Site, showing peak groundwater levels to date (figure seven) The groundwater flow direction indicated by the contour plan is predominantly from west to east across the Site. The hydraulic gradient is extremely variable, ranging from a Site maximum of 0.08 to sub-horizontal. Page 16

21 The watertable contours shown in figure seven do not extend across the eastern edge of Borras Airfield. The saturated section of the sand and gravel aquifer is limited in its lateral extent by a mounding in the topography of the underlying boulder clay (Reference 1) This mounding represents the eastern flank of a buried channel encountered at the centre of Borras Airfield (section 2.4.1). One branch of this channel leads to the north then north east and another branch goes to the south east, thus groundwater flows in these two directions around the mounded boulder clay Aquifer hydraulic conductivity Reference 1 provides hydraulic conductivity data for the sand and gravel deposits at Borras Airfield, Caia Farm and Borras Farm At Borras Airfield, estimates of hydraulic conductivity vary from 1-20 metres/day (m/d), based upon grain size grading analyses Values for hydraulic conductivity at Caia Farm and Borras Farm have been derived from piezometer (falling head) tests. They range from m/d It is considered that these variations in hydraulic conductivity are related to spatial differences in the silt content of the sand and gravel deposit i.e. higher silt percentages cause reduced hydraulic conductivity Geological factors affecting aquifer properties The variability in watertable elevation observed across the Site is not uncommon in the sand and gravel drift deposits of this area. It is considered to result from the influence of several controlling factors: Variations in the silt content of the sand and gravel deposits, with reduced aquifer hydraulic conductivity in areas of high silt. Undulations in the topography of the boulder clay, which are imposed upon the watertable in the overlying sand and gravel deposits. Perching on silt and clay layers. Page 17

22 Based upon this information, it is considered that the steepest hydraulic gradients occur where the sand and gravel has a relatively high silt content and is underlain by a comparatively steeply-dipping section of the boulder clay topography. Conversely, shallow hydraulic gradients prevail where the sand and gravel has a low silt content and the underlying boulder clay is relatively flat Licensed groundwater and surface water abstractions Details were obtained from the EA of licensed groundwater and surface water abstractions within a 4 km radius of the Site (appendix five). A total of 22 abstractions were identified, the locations of which are illustrated in figure eight There is one licensed abstraction within the Site boundary. This is the Borras Quarry abstraction borehole, operated by Tarmac and used for sand and gravel washing. The licensed abstraction rate is in the region of 0.5 megalitres/day (Ml/d), derived from the Carboniferous Upper Coal Measures; the actual usage is some 0.15 Ml/d With the exception of Tarmac s borehole, the closest licensed abstraction is at Hoseley Bank Farm (NGR SJ , some 800 m to the north east of the proposed extraction area within the Holt Estate). The licensed rate is Ml/d, sourced from the sand and gravel drift deposits and utilised for agricultural purposes All other licensed groundwater abstractions lying within the radius of search utilise less than 0.01 Ml/d, for agricultural purposes. The majority are from the sand and gravel drift deposit. However, three in the Commonwood area and two at Cooksbridge Farm abstract from the Triassic Sherwood Sandstone There are two licensed surface water abstractions in the vicinity of the Site: one is from the River Alyn at Singret Quarry (approximately 2 km to the north west of the Site), using in the region of 0.55 Ml/d for sand and gravel washing; the other is from a tributary of the River Clywedog, on Wrexham Industrial Estate (Caparo Wire Ltd., 1.5 km to the south east of the Site), utilising some 0.03 Ml/d for industrial purposes. Page 18

23 The EA has been consulted with regard to Source Protection Zones (SPZs) for groundwater abstractions in the vicinity of Borras Quarry. SPZ definitions are given in the EA s Policy and Practice for the Protection of Groundwater. The Triassic Sherwood Sandstone underlying the eastern margin of the Site has Zone III status (Source Catchment) The westerly extent of the SPZ coincides with the geological boundary between the Sherwood Sandstone and the Erbistock Beds (as defined on the BGS map, an extract of which is shown in figure three) During drilling of the Borras Quarry abstraction borehole, located at NGR SJ , the Erbistock Beds were encountered rather than the Sherwood Sandstone, contrary to information shown on the BGS map. It is therefore inferred that the boundary between these two rock types should in fact be at least 500 m to the east of the position indicated on the BGS map Private water supplies The private water supplies register held by WCBC s Environmental Health Department has been consulted. A total of 10 abstractions were identified within a 4 km radius of the Site (appendix six); their locations are illustrated in figure eight In closest proximity to the Site is the private borehole supply at Pant-yr-ochain Farm (NGR SJ , some 550 m to the west of the Holt Estate). This is a domestic supply for a single dwelling, abstracting less than 5 m 3 /day (i.e. Category 1, Class F). It is assumed that the water is sourced from a shallow supply (i.e. the sand and gravel drift deposits) and WCBC notes that it has a history of high nitrate levels The other private supplies are to the north east of the Holt Estate, over 1.75 km from the Site boundary. It is assumed that they are shallow supplies from the sand and gravel drift deposits. Two of the abstractions are Category 2, Class 5 (less than 2m 3 /day); the remainder are Category 1, Class F Conceptual hydrogeology The information detailed in sections 2.2 to 2.19 informs an assessment of the interaction between groundwater and surface water bodies in the vicinity of the Site. Page 19

24 In general, groundwater movement is from west to east. Hydraulic gradients are highly variable, controlled mainly by the silt content of the sand and gravel, the topography of the underlying boulder clay and the extent of interburden (silt/clay) layers A number of exploration boreholes drilled towards the eastern margin of Borras Airfield have proved a mounding in the topography of the boulder clay (i.e. at the base of the sand and gravel deposit). Water strikes recorded in these boreholes coincide with the upper surface of the boulder clay. Consequently, it is concluded that there is a negligible thickness of saturated sand and gravel. The only groundwater occurring on this mounded area is that derived from rainfall recharge i.e. a limited amount draining across the surface of the boulder clay The mounded area forms an easterly limit to the saturated section of the sand and gravel aquifer. It is considered to represent the eastern flank of a buried channel. One branch of this channel leads to the north then northeast, the other branch heads to the southeast. Thus groundwater flow is split in these two directions The BGS map indicates that the sand and gravel deposit pinches out some 0.5 km to the east of the Site, along the north-south trending escarpment that separates the Wrexham Delta terrace from the low-lying Cheshire Plain. Groundwater surfaces at this escarpment as a series of minor springs and seepages, forming small streams and ponds. Flow rates in these streams are limited, in the order of 1 l/s or less The low flow rates observed in these streams suggest that some groundwater from the sand and gravel aquifer remains sub-surface. It is therefore postulated that the sand and gravel deposit may, in places, extend into the boulder clay beneath the Cheshire Plain (contrary to information shown on the BGS map) Groundwater elevations in the monitoring piezometers have been compared with surface water levels in adjacent ponds The Flash, Pant-yr-ochain Lake and its satellite ponds, Tommy s Wood and Coed-yrbychan are all in hydraulic continuity with the groundwater in the sand and gravel drift deposits. This is indicated by the concordance of surface water levels and interpolated groundwater contours (figure seven). Page 20

25 Vicarage Moss SSSI and the surrounding ponds (Walnut Tree Pond, Woodlands Pond and South Vicarage Pond) exhibit water levels that are up to 20 m above groundwater elevation, indicating a perched system Borras Bog is also interpreted as being a perched system, although here the level difference between the surface water and groundwater systems is only m The pond at Wrexham Golf Club exhibits a water level of some 62 maod. The extrapolated groundwater level at this location equates to circa 58 maod, based upon readings obtained from the nearest piezometers. This would suggest that the pond is a perched feature which, given its topographic setting (within a minor valley feature), is supported by rainfall run-off. This interpretation is endorsed by anecdotal evidence from management at the club, who recall that an abandoned borehole, drilled adjacent to the pond, encountered a clay lens beneath the soils Further investigation Vicarage Moss SSSI A walk-over survey has been undertaken in the vicinity of Vicarage Moss SSSI and South Vicarage Pond in order to define the rainfall catchment area for the surface water features. The extent of the catchment area is illustrated upon figure nine, as discussed at a meeting with the EA on 18 th February The geological logs for piezometers and exploration boreholes drilled on the Holt Estate have been inspected Two piezometers (BH 13 and 20), a Mineral Assessment Borehole (MAB 1) and three exploration boreholes (89/7, 89/8 and 89/9) are located within or adjacent to the boundary of the rainfall catchment area (figure nine). The geological logs show that the soil is immediately underlain by a layer of clay (described as soft to stiff, red to brown, silty to sandy boulder clay), ranging in thickness from 2.50 m up to 9.10 m The remaining Holt Estate piezometers are located some m from the boundary of the rainfall catchment area. The geological logs for these piezometers show that the soil is directly underlain by the sand and gravel deposit, with no intervening layer of clay. The only exception is BH 23, which is located within a dry kettle hole towards the south west corner of the Holt Estate. Page 21

26 These data lend support to the interpretation that Vicarage Moss SSSI and South Vicarage Pond are not supported by the watertable in the sand and gravel aquifer. They are isolated from the sand and gravel by an intervening layer of relatively impermeable clay. This clay layer prevents rainfall run-off from draining into the sand and gravel. Instead, the rainfall run-off feeds into the SSSI and ponds i.e. it is a perched system As stated in section , surface water level in the SSSI and ponds is some 20 m above groundwater elevation in adjacent piezometers. Page 22

27 3.0 QUARRY DEVELOPMENT AND RESTORATION 3.1 Programme of working The Site, comprising Borras Airfield and the Holt Estate, covers an area of 173 hectares. Extraction of sand and gravel has been taking place on Borras Airfield since the 1960s and large areas have already been substantially disturbed Quarry development proposals involve the extraction of Upper Sands & Gravels and Lower Sands at the Holt Estate together with continued working of Lower Sands at Borras Airfield. Drawing Nos. B110/134a and B110/135a within the Supplementary Statement show the proposed depth of extraction. The Site has been subdivided into 9 phase areas (4 on Borras Airfield, 5 on the Holt Estate). A number of phase areas will be worked concurrently to enable the blending of Upper Sands & Gravels and Lower Sands. 3.2 Method of working Comparison of the peak watertable elevation recorded at the Site (figure seven) with the extraction profile (Drawing Nos. B110/134a and B110/135a) informs an assessment of the requirement for sub-watertable working or dewatering There is a minimum stand-off of 1 m between the base of extraction and the underlying watertable (based upon peak groundwater levels recorded up to February 2005). There is no requirement for sub-watertable working or dewatering The water management procedures currently employed on Site will be maintained. The same water supply will be utilised to facilitate the washing of sand and gravel within the processing plant Following processing, water from the plant will be directed through the lagoon system. This will allow the silt within the water to settle out so that it can be re-circulated from the clean water lagoon to the processing plant Stockpile dewatering techniques are already in place on site, to minimise the loss of water by infiltration into the ground beneath the processing plant. Page 23

28 3.2.6 This water re-circulatory system minimises the loss of water from sand and gravel processing. A small amount of water will be lost as a result of evaporation from the lagoons and water lost within the sold product. 3.3 Restoration The restoration proposals for Borras Airfield involve creating an area of nature conservation and amenity grassland, interspersed with woodland blocks. There will be wild flower meadows and habitat provision for sand martins, bees and wasps. A small wetland area with shallow water and reedbeds will be retained at the centre of the Airfield It is proposed to restore the Holt Estate primarily back to best and most versatile agricultural land. This will be achieved through progressive restoration. The preexisting field boundary pattern will be reinstated. A woodland buffer will be planted adjacent to Vicarage Moss SSSI and alongside the southeast boundary Up to 1 hectare (ha) of wetland a soakaway outfall for agricultural underdrainage is included in the restoration proposals for the Holt Estate. This will be upon the lowest ground towards the eastern extreme of the restored agricultural area in Phase H5 of the Holt Estate. Page 24

29 4.0 IMPACT ASSESSMENT 4.1 Groundwater levels and flow The proposed development will not involve sub-watertable working or dewatering. Therefore, there will be no lowering of the watertable as a result of the quarrying activities and no drawdown-related impact upon groundwater levels and flow Localised effects upon groundwater levels and flow can occur as a result of a change in effective rainfall. As detailed in section 2.6.4, recharge by effective rainfall depends upon land use at the Site Effective rainfall in extraction areas: Borras Airfield is mostly stripped of vegetation cover. Therefore, on-going sand and gravel extraction will involve no change in land use and consequently no change in effective rainfall. The Holt Estate is currently used for rearing livestock, with grass vegetation cover. In such areas, effective rainfall averages 248 millimetres/annum (mm/a). This increases to 293 mm/a in areas of bare ground, such as sand and gravel workings (i.e. an additional 45 mm/a). Therefore, an assessment has been undertaken of the potential quarry-related impact upon groundwater levels and flow resulting from an increase in effective rainfall. The Holt Estate will be worked in 5 phase areas. Restoration will be progressive in order to minimise the area being worked at any one time. During sand and gravel extraction in the Holt Estate, the maximum area of bare ground subject to increased effective rainfall will be in the region of 35 hectares. Within this 35 ha area, it is estimated that there will be an additional 45 mm of effective rainfall recharge during the year. This equates to an increase of less than 0.15 mm/day, which will be confined to a localised section (35 ha) of the regional catchment (several km 2 ). It is thus considered that the effect upon groundwater levels and flow will be negligible. No mitigation is considered necessary in respect of this potential impact. Page 25