The importance of groundwater: Making the invisible visible
Why hydrogeology must play a bigger role in civil engineering
Sweco authors: Dr Harriet Carlyle & Caroline Ball
Groundwater is out of sight but should certainly not be out of mind, so we’re doing what we can to raise the profile of this vital resource in the civil engineering industry. Here’s a guide to what groundwater is, why we need to protect it, and the key things planners and engineers need to consider during land, building or environmental development projects.
What is groundwater?
Groundwater is, in short, water found underground in aquifers – rocks, sands and gravels that hold large quantities of water in pores, fractures and fissures. Groundwater supplies springs, rivers, lakes and wetlands, and seeps into our oceans, as well as being abstracted for water supply. Read our full guide here.
Why is groundwater important?
Almost all the liquid freshwater in the world is groundwater, which supplies much of the water we use for drinking and sanitation as well as for agriculture and industry – life would not be possible without it. In fact, in the driest parts of the world, groundwater may be the only water people have. Closer to home, it provides about one third of public water supplies in England and Wales, 6% in Northern Ireland and 3% in Scotland.
Though aquifers lie hidden beneath our feet, groundwater’s impact is visible everywhere, and it is the lifeblood for nationally and internationally important wetland and river ecosystems. Groundwater keeps our rivers flowing throughout the year, even in times with no rainfall, through baseflow, springs and seepage. England’s chalk streams, which support internationally renowned fisheries are well-known examples, as are rivers that disappear into the ground and reappear far away, having travelled underground through cave systems.
As Prof David Kreamer, President of the International Association of Hydrogeologists, asserts, “If earth has a hidden underground pulse, that pulse is groundwater.”
Why does groundwater need protecting?
Groundwater is under threat everywhere from over-abstraction and pollution. For centuries, it has been seen as an infinite resource to be exploited, with over-abstraction leading to land instability and subsidence, seawater intrusion in coastal regions and rivers, lakes and wetlands drying up. It has also been a sink for any number of chemicals applied to the ground such as fertilisers and pesticides, or those deliberately or accidentally discarded as waste such as solvents and fuels.
Climate change presents an additional threat, with wetter winters increasing the risk of groundwater flooding and drier summers increasing the pressure on groundwater resources.
How to protect groundwater…and your project
It’s crucial that developers and planners consider risks to groundwater – and risks posed by groundwater – throughout the project lifecycle. Excavations and foundations below the water table may alter groundwater flow paths and change groundwater levels, potentially reducing flow to rivers, wetlands, springs or abstraction boreholes and wells. Conversely, foundations may act as groundwater dams, potentially causing flooding if groundwater levels reach surface.
Without appropriate mitigation, pollution incidents associated with construction activities present a particular threat to groundwater, for example from refueling, application of chemicals, cleaners and oils, and accidental spillages. Soils and low permeability ground that would normally provide some protection to the underlying groundwater may have been removed through excavation, while piling and foundations may create a direct pathway for potentially contaminated surface water or shallow groundwater to reach deeper aquifer layers. Once groundwater is contaminated, it is very difficult and very expensive to clean up!
Remember that turbidity arising from the generation of fines during excavation and piling can also contaminate groundwater. It presents a particular risk to nearby abstraction boreholes as it may interfere with the water treatment process.
Development projects can benefit groundwater, they can also benefit from it. Incorporating Sustainable Drainage Systems (SuDS) principles into the drainage design helps to control surface water run-off from a development and reduce pollution. Infiltration to ground via soakaways and drainage fields is at the top of the Hierarchy of Drainage. As well as reducing the risk of surface water flooding, infiltration helps to replenish aquifers and the generally slow movement of water through the soil and unsaturated aquifer layers above the water table helps to attenuate pollutants.
Groundwater can also be a source of heating and cooling. A ground source heat pump (GSHP) transfers heat to and from the ground or groundwater via a heat exchanger and is much more efficient in terms of electricity use compared to other forms of heating or cooling.
Making the invisible visible – involving hydrogeologists from conception to completion
Input to options appraisal and preliminary design allows major groundwater risks to be identified early and potentially designed out. Groundwater should be considered in the drainage design, whether this is because of opportunities presented by groundwater SuDS or whether the drainage needs to accommodate groundwater seepage, for example into a road cutting. Groundwater receptors such as rivers, wetlands, springs and abstractions can be identified and mitigation of potential effects on these considered. Regulators appreciate early engagement and the opportunity to work with the project team to remove remove or mitigate risks.
Construction phases present particular risks to groundwater receptors. Moreover, relatively recent legislation means that excavation dewatering now requires an abstraction (transfer) licence for all but very small abstraction volumes, and possibly a discharge environmental permit as well. The abstraction licence process can be lengthy, taking upwards of one year and potentially requiring surveys, pumping tests, monitoring and hydrogeological impact assessment. The level of investigation depends on likely dewatering volumes and the sensitivity of groundwater receptors, and therefore early engagement with regulators is essential. Construction projects have been delayed whilst waiting for the appropriate permits and licences to be granted (and in some case redesign has been required to reduce dewatering requirements), so it is strongly recommended that the process starts well before the construction stage.
Related to this, standard ground investigation techniques often do not provide adequate information on groundwater conditions in terms of groundwater levels, aquifer parameters and groundwater quality. Hydrogeological input to the design and implementation of ground investigations reaps benefits through improved monitoring and testing data to inform groundwater assessments including dewatering appraisals.
Baseline groundwater level and quality monitoring that commences early enough and lasts long enough to capture seasonal changes is also essential. Not only does a good baseline dataset help to inform the permanent and temporary works design and permitting, it can be compared to monitoring data collected during the construction phase. This protects the project by either providing an early warning of potential groundwater level or quality impacts or by demonstrating that there are no impacts, even when anecdotal reports indicate otherwise.
Although groundwater SuDS can be very beneficial, there are also restrictions on discharging to shallow groundwater as attenuation within the unsaturated zone above the water table is effectively an important form of treatment. Moreover, discharges may require and environmental permit. Sufficient ground investigation and monitoring information is required to determine these risks, and drainage designers must be mindful of all infiltration to ground within the drainage system, and not just the final discharge point.
In short, groundwater should be a key consideration in any land, building or resources development project and not only an afterthought, or a problem to solve during works as a result of overlooking it initially. At Sweco, our team of hydrogeologists can offer detailed advice, surveys and assessments in addition to engineering services – contact us below for more information.