Shallow permeable aquifers have the potential to disrupt and delay infrastructure projects if not properly accounted for in the initial project planning stages. The presence of shallow aquifers and dewatering demands are an often overlooked component in the infrastructure planning phase, whether the project involves a mine pit or an underground structure such as a basement or parking garage. Dewatering is the process of lowering the water table below the lowest excavation depth by means of a pumping well system to allow for a safe and effective excavation procedure by increasing wall stability and eliminating potential groundwater infiltration into the excavation pit.
Costs associated with dewatering can spiral a project budget out of control if proper consideration is not given to subsurface assessment and can diminish the economic feasibility of an entire project. Aquifer dewatering is a major concern when the project is proximal to a surface water body such as a river or lake, which generally results in a very shallow water table. Seasonal variations of the water table level can add an extra complexity to project timing, depending on the magnitude of the variations. If your project is close to a surface water body or a sensitive ecological environment, it is advantageous to consult with an experienced hydrogeologist during the project planning or proposal stage to avoid unforeseen and costly oversights.
Taking Sediments into Consideration
In Alberta, shallow aquifers generally comprise unconsolidated sediments derived from peri-glacial outwash processes resulting in fluvial and lacustrine depositional environments; the unconsolidated sediments can range from a clay till to clean fluvial sand/gravel. In terms of dewatering, both fine and coarser sediments can be problematic. Fine and coarse sediments can be heavily saturated and difficult to dewater due to varying permeability or hydraulic conductivity. Generally speaking, finer sediments cannot sustain larger pumping rates required in dewatering projects. Coarse sediments require greater pumping rates and more demanding groundwater discharge management methods to adequately dewater higher hydraulic conductivity sediments. An understanding of the aquifer sediment size and sorting is crucial because increased groundwater velocities from aggressive pumping can mobilize finer sediments from within the aquifer and create piping (internal erosion) conditions that can lead to ground subsistence and/or pumping equipment destruction.
Creating a Conceptual Model
A site conceptual model is integral to a successful dewatering plan because without a conceptual model, the appropriate dewatering system cannot be confidently selected. The conceptual model development requires, at minimum, the installation of numerous piezometers around the site. Piezometers provide lithological data (during installation), water table elevation data, and groundwater chemistry data. Although often overlooked, these data can be extracted during a well-planned geotechnical program when completed in consultation with a professional hydrogeologist, and result in considerable cost savings.
The lithology of the site generally determines the dewatering method. The two most common shallow dewatering methods are large diameter vertical pumping wells or a wellpoint system (image below). Large diameter pumping wells are appropriate for coarser sediment conditions, where the small proportion of fine sediments can be developed out with a submersible pump, resulting in low turbidity discharge and robust well deliverability. Wellpoint systems are appropriate for finer sediment conditions, where persistent fine sediment production may be an issue. A wellpoint system consists of a series of small diameter wells that are connected by a header pipe to a wellpoint pump and can also incorporate a separator tank into the system for settling fine sediments (see figure below). The pump creates a vacuum in the header pipe drawing water up from the ground, however the dewatering depth cannot exceed six meters below ground (depending on site elevation above sea level) due to inherent vacuum pump limitations. Therefore, vertical wells must be utilized for dewatering to depths greater than six meters below ground. Cost must be considered as part of the dewatering system decision as the design and intricacies of the wellpoint system can be very expensive compared to vertical well installation.
A proper understanding of fundamental aquifer properties such as hydraulic conductivity, transmissivity, storativity (usually simplified to specific yield in unconfined aquifers), heterogeneity, and anisotropy are critical to the successful design of a dewatering system. Various methods are available to estimate aquifer properties such as slug or bailer tests, pumping tests or even grain size analysis by sieving. The conceptual model should be used to guide the planning process for the selection of a dewatering method (i.e., a slug/bailer test is generally not appropriate for gravel sediments). A slug or bailer test, often conducted as part of the initial geotechnical program, is a simple and cost effective way to estimate hydraulic conductivity but is representative of only a small volume of geological material surrounding the well and may not be a representative elementary volume (REV) of site as a whole. Pumping tests are the preferred method to holistically characterize the aquifer under the site and to provide best estimates of transmissivity and storativity (if observation wells are installed). Transmissivity and storativity estimates from the pumping test can be used to calibrate a predictive forward analytical model to guide dewatering decisions such as dewatering system type (i.e., vertical wells or wellpoint), well quantity, well spacing/distribution, pumping rates, pumping duration and expected volumes. Empirical estimates of heterogeneity and anisotropy are difficult to calculate but a qualified hydrogeologist can estimate these parameters based on lithology and parameterization of analytical models.
Adhering to Regulations
The dewatering process can require continuous pumping for weeks to months which, depending on the site size, can produce tens to hundreds of thousands of cubic metres of groundwater discharge. Pumping must continue throughout the pre-construction and construction phase. Dealing with such large volumes in tanks or water trucks can be cumbersome and expensive so the most practicable option is to discharge to the surface. However, the dewatering contractor or operator must be cognizant of local regulations and the ecological sensitivity of the area (i.e., national parks, wetlands, fish-bearing streams, etc.) prior to discharging groundwater to the environment. Discharged water cannot be discharged directly to a surface water body without prior regulatory approval or be close enough to a surface water body in which foreign debris or sediment are introduced via erosion. For shallow dewatering sites, it is the utmost importance to discharge groundwater sufficiently distant from the site during dewatering to avoid artificial recharge of the aquifer being dewatered.
Special consideration must be given to groundwater quality (chemistry) while dewatering, especially in ecologically sensitive areas. In Banff National Park, for example, limited discharge options are available for dewatering within the Banff town site; the municipal stormwater system is the only viable solution to move such large volumes. However, the stormwater system is routed directly to the Bow River, therefore stringent water quality guidelines to protect aquatic life must be met, such as the Canadian Council of Ministers of the Environment (CCME) Freshwater Aquatic Life and Environmental Quality Guidelines for Alberta Surface Waters. Consequently, discharge water must be held on site or temporarily trucked off site until three consecutive days of water sampling confirm the compliance and reproducibility of the water quality results with the aforementioned guidelines. A contingency plan should be arranged in the event that the water quality does not meet the relevant guidelines. Similar challenges to meeting discharge criteria occur throughout Alberta and are based on municipal, provincial, and federal regulations.
Successful Dewatering
Once the aquifer has been sufficiently dewatered and the infrastructure foundation or underground structure has been constructed, permanent dewatering systems may be necessary (where permitted). These permanent dewatering systems can include, but are not limited to, weeping tile and/or a sump.
In summary, a successful dewatering program should include:
Early engagement with an experienced hydrogeologist, prior to the geotechnical program, to avoid unforeseen and costly oversights and to advise on applicable environmental guidelines to the site;
Development of a conceptual site model through a preliminary exploration program including drilling and installing of piezometers – this program can be combined with the geotechnical assessment resulting in considerable cost-savings;
Using the conceptual model to select the most appropriate site-specific testing method to estimate aquifer properties pertinent to dewatering;
Creation of a calibrated predictive forward analytical model to guide critical dewatering decisions such as dewatering system type, well spacing/distribution, etc.;
Having a primary and contingency plan to manage large amounts of fresh or potentially contaminated discharged groundwater;
Placement of a permanent dewatering system (if necessary) once the foundation or underground structure has been constructed.
Waterline Resources Inc. is a water resource, environmental and information services consulting firm based out of Calgary, AB and Nanaimo, BC, with satellite offices throughout Western Canada. Waterline’s staff includes scientists, engineers, computer scientists and data management technicians that specialize in water well design, water quality analysis and aquifer management. We pride ourselves on exercising scientific principles while working closely with our clients to establish strong relationships for project success. If you have questions about dewatering or require any additional groundwater services, please emails us at info@waterlineresources.com or give us a call at 403-243-5611.