Appendix A9.1: Private Water Supply Supplementary Environmental Information

Transcription

1 Appendix A9.1: Private Water Supply Supplementary Environmental Information 1.1 Introduction SEPA objected to the planning application for the Project in their consultation response dated 19 October 2013 on the ground of lack of information with respect to Private Water Supplies (PWS) and note that PWS1 falls within the 100m buffer zone of the access track. SEPA recommend that: The precise location and nature of the PWS should be confirmed. Further assessments must demonstrate that the risk to the groundwater abstraction is not significant by establishing the size of the Zone of Contribution feeding groundwater to the water supply and identifying the proportion of flow that will be reduced as a consequence of any construction. Where an impact on a water supply is established the applicant should agree with the owner of the abstraction to provide an alternative supply. This appendix provides a hydrogeological risk assessment of the potential impacts of the revised Project on PWS1 (location illustrated on Figure A9.2) and proposed mitigation to ensure that the risk of disruption to this abstraction is not significant. 1.2 Nature of the PWS The abstraction is located within the landholding of Knockgardner Farm and supplies the residential property at Threethorns, which is situated approximately 1.3km to the north of the abstraction location. Further details regarding the nature of PWS1 have been obtained via a site visit on 25 June 2014 and discussions with the landowner of Knockgardner Farm and the property owners at Threethorns. During the site visit the conditions were dry and overcast with rainfall in the preceding 24 hours, although weather conditions had been dry prior to this. Water quality parameters were recorded at the abstraction location using a field testing meter. The PWS details, including setup, history and water quality, are summarised in Table 1, Table 2 and Figure 1. The abstraction point is believed to be served by both surface water and groundwater inflows. A small surface water inflow enters the first brick chamber (see Table 1) from a small, vegetated gully upstream. It is estimated that the catchment area of this surface water supply is only ~ 0.5ha. The open water upslope is covered by blocks, presumably to reduce silt input to the supply. There is also a pipe inflow which is thought to provide a groundwater inflow, however the details regarding the installation of the private water supply are unknown. The supply is piped between the abstraction and Threethorns via gravity with no pumping. The exact route of the pipeline is unknown but is likely to be situated close to the ground surface; the pipeline was disrupted when the land was ploughed for forestry. 14/7048/001/GLA/O/R/001 Revision B1 Page 1 of 8

2 Table 1: PWS1 Summary Key Parameters Description Location of abstraction Knockgardner Farm landholding (NGR , ) at ~ 208mAOD Properties served by supply Source of supply Abstraction description Treatment of supply History Threethorns residential property with two occupants. Surface water and groundwater inflows. A piped inflow and surface water inflow enter a small brick chamber (60cm x 70cm x 50cm deep) covered by wooden boards. This water then flows into the main brick chamber (120cm x 185cm x 70cm deep) with a wooden cover in which the abstraction point is located. The main chamber appeared to have a solid base and was covered by several centimetres of silt. The water depth in this chamber was 50cm on day of visit. UV filter with regular maintenance. PWS could not provide for all the water requirements during drier summer months when operating as a dairy farm ~30 years ago and as such a mains supply was installed. Current occupants formerly ran a B&B at Threethorns with all water requirements sourced from the PWS. No lack of pressure or drying up of PWS has been experienced during past 14 years. Alternative supply available? Yes mains supply is available (currently not connected). Table 2: PWS1 Water Quality Sampling (25/06/14) Sample Description Temp ( o C) ph EC (µs/ cm) DO (mg/l) Pipe inflow No colour, no particulates Surface water inflow Very slight colour, some very small particulates /7048/001/GLA/O/R/001 Revision B1 Page 2 of 8

3 Main chamber (covered) Main chamber (uncovered) View of inflow between first and main chamber First chamber showing pipe and surface inflow Small gully upslope of PWS View north towards Threethorns from PWS Figure 1: PWS1 Photographic Summary 14/7048/001/GLA/O/R/001 Revision B1 Page 3 of 8

4 1.3 Hydrogeological Risk Assessment A hydrogeological conceptual model of the potential impact of the access track on the abstraction is provided in Figure 2 and a summary of the hydrogeological conditions is provided below: Thin mineral soils are present at the abstraction location, consisting of poorly draining non-calcareous gleys derived from the Blair association 1. There are no drift deposits recorded in this location. Devensian till is recorded to the east. It is likely that a zone of weathered bedrock is situated at the interface between the soils and bedrock. The underlying solid geology consists of Swanshaw Sandstone Formation (sandstone) at the abstraction location. Information provided by SEPA classifies the bedrock aquifer associated with this sandstone formation as being moderately productive with groundwater movement via intergranular or fracture flow. The Hydrogeological Map of Scotland classifies this bedrock aquifer as being locally important with groundwater flow dominantly in fissures and other discontinuities. A band of conglomerate is situated approximately 25m downslope of the abstraction. SEPA s bedrock aquifer mapping indicates that this formation may be of lower productivity and dominated by fracture flow. The proposed access track is situated 60m downslope of the abstraction within this conglomerate band at ~ 197mAOD. The precise nature of the boundary between the sandstone and conglomerate bands is unknown, however the BGS mapping suggests that bedrock in this region dips towards the south-east at an angle of The groundwater catchment/zone of contribution to the abstraction location, based on contours is estimated as being ~ 2ha. This would mainly involve flows in the weathered zone and is larger than the surface water catchment as it is not affected by the influence of artificial drainage on the surface water flows, which leads to the diversion of surface runoff further up-gradient. 1 Soil Survey of Scotland Ayr Sheet 14 and part of 13. Published by The James Hutton Institute. 14/7048/001/GLA/O/R/001 Revision B1 Page 4 of 8

5 It is unknown whether there is a contribution from fracture flow at the abstraction location. The groundwater and surface water catchments, based on contour data are small, and so given the reliability of the supply it is considered possible that there is a contribution from fracture flow. The zone of contribution associated with any fracture flow is unknown. No fault lines or similar features are recorded in the BGS map. An extensive intrusive investigation would be required to establish the presence and location of fractures and discontinuities within the sandstone and conglomerate bands, which would inform the zone of contribution to the abstraction and the potential connectivity between the abstraction and proposed access track. A qualitative conceptual model has therefore been developed (see Figure 2) rather than undertaking a quantitative assessment. The likelihood of a fracture or discontinuity providing connectivity between the access track and abstraction is considered to be low, however the conceptual model assumes that such a feature could be present as this information is currently unknown. The key construction activity that could affect the abstraction has been identified as the lateral excavation of bedrock to provide a cutting into the hillslope for construction of the access track across this relatively steep slope. If a fracture connecting the access track and abstraction locations exists, the excavation could disrupt the groundwater flow and affect the groundwater levels at the abstraction location. This impact is assessed as being possible and of Medium magnitude if no mitigation measures are implemented. There is a risk that the PWS pipeline could be damaged during construction as it is believed to lie close to the ground surface. This impact is assessed as being likely and of Medium magnitude if no mitigation measures are implemented. Measures to manage the risk of pipeline damage are outlined in section 1.4. The PWS was assessed in the ES as being of Medium sensitivity and as such the significance of the impact of the Project on the abstraction and supply is assessed as being Medium and potentially significant if no mitigation measures are implemented. Mitigation measures are therefore proposed in section /7048/001/GLA/O/R/001 Revision B1 Page 5 of 8

6 Figure 2: Hydrogeological Conceptual Model Potential Impacts 14/7048/001/GLA/O/R/001 Revision B1 Page 6 of 8

7 1.4 Mitigation and Residual Impact The key construction activity that has the potential to affect the abstraction is cutting into the hillslope for construction of the access track. It is proposed that excavation into bedrock will not occur for a length of at least 100m of access track centred on this location. A level surface and foundation for the access track will be constructed at this location by building-up on the downslope side of the track location rather than through excavation of a cutting into the hillslope, as illustrated in Figure 3. Avoiding excavation of bedrock at this location will prevent the disruption of groundwater flows and levels and reduces the associated impact magnitude of constructing the access track on the abstraction to Negligible. The route of the water supply pipe will be established prior to works commencing via the site investigation. Suitable protection of pipe will be employed where is passes across the proposed access track and 10m up-gradient and down-gradient of the track. If the water supply pipe needs to be disrupted during the works, the occupants of Threethorns will be notified immediately to allow the mains supply to be reconnected whilst the water supply pipe is repaired. Baseline, construction and post-construction monitoring of the PWS water quality and yield will be undertaken. The potential impact of the revised Project on this PWS abstraction will be not significant if these mitigation measures are employed. 14/7048/001/GLA/O/R/001 Revision B1 Page 7 of 8

8 Figure 3: Hydrogeological Conceptual Model - Mitigation 14/7048/001/GLA/O/R/001 Revision B1 Page 8 of 8