EXTRACT. MONITOR. CONTROL.

Transcription

1 GUIDANCE OF ABSTRACTION AND DISCHARGE PERMITS FOR OPERATIONS When considering any potential dewatering requirement, it is necessary to consider the following questions: Are we allowed to abstract groundwater, and will the pumping impact third parties? Where can we dispose of the water, and what needs to be done to allow discharge? What can be done to minimise water disposal costs? Does the water need to be treated prior to discharge? EXTRACT. MONITOR. CONTROL.

2 Abstraction for Dewatering Abstraction for Dewatering Prior to the enactment of the Water Act (00), construction dewatering was exempt from requiring an abstraction licence. This is now not the case, but the type of licence and level of effort required to make an application is dependent upon the nature of the individual project and the perceived level of risk. For a small minority of short-term schemes which operate for less than 8-days, a Temporary Licence can be obtained. In the majority of cases, a Transfer Licence will be appropriate unless the water is used for an intervening use, such as Dust Suppression. If the For guidance, the Environment Agency considers a scoring system, based on aquifer characteristics, locality of water-dependent conservation sites, water-resource availability and dewatering abstraction flows: Tier (Basic) Effort Required Groundwater conceptual model developed using easily available data Suggested Design Tools Analytical equations, as set out in CIRIA C55 Weighted Score 0 to 0 discharged water is used for a secondary process and not returned back to the environment, a Full Licence will be required. An application for a licence will require a hydrogeological impact assessment (HIA), which is then summarised in an Environmental Statement to support the application. The amount of work required for the HIA is assessed using a tiered approach, and can also have an impact on the tools used to design a dewatering system, with groundwater modelling required in more complex cases. Tier (Intermediate) Conceptual model tested using more detailed data from limited field investigations Tier (Detailed) Conceptual model tested using numerical modelling techniques Simple D steadystate models D modelling, such as MODFLOW, with time dependent analysis to 0 to 0

3 Abstraction for Dewatering Abstraction for Dewatering Criteria and Classes Score Weight Aquifer Characteristics Karst Principal (Major) Aquifer Secondary (Minor) Aquifer Unproductive Strata Water-Dependent Conservation Sites Habitats Directive (Natura 000) Sites Sites of Special Scientific Interest Other Designations (Including National Parks and AONB) None Water Resource Availability Status Over-Abstracted Over-Licensed No Water Available Water Available Dewatering Quantity Very Large (> 5,000 m /d) Large (,500 to 5,000 m /d) Medium (,000 to,500 m /d) Low (<,000 m /d) Working through three common dewatering scenarios we might encounter, we would expect the following: Case (A Small Wellpoint Scheme): A contractor requires dewatering for a simple trench excavation to install 00m of drainage pipes and manholes up to.5m deep, in superficial silty sand deposits. There are no known conservation areas locally. The expected duration for the drainage works is weeks. A wellpoint system is proposed with flows of 5 l/sec (m /day). In this instance, a Temporary Licence should be sufficient as the dewatering works are less than 8 days duration. Should there be a concern over potential programme delays, and we used the proposed scoring system, a Tier study should be sufficient. Case (Cofferdam Depressurisation): A contractor plans to excavate a 0 x 0m sheet-pile cofferdam for the installation of an underground tank at a wastewater treatment works. The excavation is founded in clay, but below the clay is a layer of highly permeable sandy gravels (e.g. River Terrace Deposits, classed as a minor aquifer) with a sufficient pressure to cause a risk of basal uplift. The expected duration for the works is 0 weeks. In order to depressurise the underlying sandy gravels, a deepwell scheme is proposed with a total flowrate of 0 l/sec (50m /day). There is an adjacent wetland conservation area, but not in direct hydraulic continuity with the confined aquifer. Based on the scoring system, a Tier assessment is sufficient, despite the high expected flowrates involved.

4 Abstraction for Dewatering Abstraction for Dewatering Case (Deep Shaft Dewatering): A deep shaft is being constructed for a tunnel with an The process for the hydrogeological impact assessment should follow the flow chart below: invert level below the top of the confined chalk aquifer in an urban area with other nearby licenced abstractors. Working on a range of bulk permeability estimates for the Chalk, the overall flowrate for a proposed deep well system could range from 0 to 00 l/s (70 to 8600m /day). As the aquifer is confined, there is expected to be no direct impact on water-dependent conservation sites. The works involves a major aquifer with potentially high flows, so a Tier assessment is required. Detailed sensitivity analysis may be required to Step : Establish the regional water source status Step : Develop a conceptual model for the dewatering operation and surrounding area Step : Based on the conceptual model, identify all potential water features which are susceptible to flow impacts establish the range of impacts due to the uncertainty on the flowrates. Aqufier Characteristics Water-Dependent Conservation Sites Water Resource Availability Dewatering Quantity Case Case Case Score Weight Total Score Weight Total Score Weight Total 9 9 TOTAL 5 9 Step 6: Assess the significance of the net flow impacts Step 7: Define the search area for drawdown impacts Step 5: For the relevant water features, allow for the mitigating effects of any discharges associated with the abstraction to arrive at net flow impacts Step 8: Identify all features within the search area which could potentially be impacted by drawdown Step : Apportion the likely flow impacts to the water features, again based on the conceptual model Step 9: For all these features, predict the likely drawdown impacts

5 Abstraction for Dewatering Abstraction for Dewatering Step 0: For the relevant water features, allow for the effects of any measures being taken to mitigate the drawdown impacts Step : Assess the significance of the net drawdown impacts Step : Assess the water quality impacts As designers and specialist sub-contractors, the Project Dewatering Limited team have the capabilities to carry out the hydrogeological impact assessment associated with our designs. Importantly, any groundwater modelling work carried out is only as good as the input parameters and understanding of the aquifer conditions; inevitably, a high quality study would require information from pumping tests to calibrate the model, and further validation of the model, using monitoring data during the actual dewatering operations. Step : Develop a monitoring strategy, focussing on the features likely to experience flow or drawdown impacts Step : If necessary, redesign the mitigation measures to minimise the flow and drawdown impacts Top tip: Always engage with the Environment Agency as early on as possible to ensure the correct application is made and sufficient time is available to complete the process. Hydrogeological impacts assessments from dewatering should consider:. Detrimental impacts on surface water features sustained by groundwater. Derogation of existing abstractors. Contamination of groundwater and surface water

6 Discharge of Groundwater Discharge of Groundwater Disposal of clean, uncontaminated groundwater is usually carried out to either a surface water body, such as an adjacent river, or to sewer. Sewer discharge is often the least preferable option in terms of the expense accumulated through volumetric discharge charges, remembering that dewatering is a -hour, 7 day per week operation. Discharge rates can be as high as 0.70 per m for some areas of the country. Both the local sewer connections and wider network will have a limit on the flowrate that can be accepted; in many urban areas there is simply not sufficient capacity for the required dewatering discharge rate. An assessment will be made by the water company responsible for the network as part of the application process, and will take into account both the quality and quantity of water. In order to provide details on the expected quality, groundwater sample analysis results are essential. A discharge consent will place limits of the water quality parameters and contaminant concentrations that can be accepted, but are less stringent than for a discharge licence to surface water body. Top Tip: Where dewatering is expected, always include groundwater sampling as part of any pumping test or site investigation works to assist with discharge applications Discharge to rivers or estuaries can be a convenient and relatively cost effective way to dispose of water where the site is located nearby. Even where the discharge location isn t directly adjacent to the site boundary, specialist temporary pipe infrastructure can often be used, although there are planning issues to consider when running pipe work outside of the site boundary. The costs of installing pipework are still likely to be less than the costs of sewer discharge for longer duration projects with high flowrates. Discharge pipe arrangement in a busy city centre for large-scale dewatering. Project Dewatering Limited can design and install complex discharge pipelines to cross roads and structures.

7 Discharge of Groundwater Discharge of Groundwater A Regulatory Position Statement has recently been published by the Environment Agency on Recharge back to the ground via reinjection wells is an option that can be considered where February 06 entitled Dewatering Building Sites and Other Excavations: Environmental ground conditions are suitable. This is ideal where there is a physical cut-off wall between the Permits. This allows temporary discharge of clean dewatering water to a surface water body dewatered area and the location of the recharge wells, where there are water-bearing for up to months without a permit where the discharge does not: strata separated by a lower permeability layer, or a combination of both. Commonly, Pollute surface water or adversely affect aquatic life, or designated sites or species recharging of the water will lead to some increase in overall rate of abstraction required and Result in the spread of non-native invasive species, parasites or disease on larger schemes a groundwater modelling exercise is recommended to fully understand Cause flooding from surface water Cause erosion of the banks or bed of surface water the flow balances. In all cases, the Environment Agency should be consulted if a recharge scheme is considered. Enter the surface water feature less than 500m upstream of a designated conservation site (including SSSI, SAC, SPA, Ramsar, Nature reserves, wildlife sites) Where there is legacy contamination in the groundwater, treatment of the abstracted groundwater is likely to be required prior to disposal. In this event, a Mobile Plant Permit is For longer periods of discharge, or where there is known contamination, a bespoke required to allow operation of a temporary treatment plant, and a Deployment Notice needs environmental permit will always be required. to be submitted to the Environment Agency prior to commencement.

8 For more information about how we can design, implement and manage your dewatering project, please give us a call on or enquiries@project-dewatering.co.uk Published: March 06 Correct at the time of going to print References: Environment Agency (007), Hydrogeological impact appraisal for dewatering abstractions, Science Report SC0000/SR Preene et al. (000), CIRIA Report C55: Groundwater Control Design and Practice, Construction Industry Research and Information Association Published Feb 06