Effect on Groundwater Quality

Groundwater quality (i.e. the chemical composition of the water) varies naturally from place to place and from aquifer to aquifer (see Section 3.7). In some cases, groundwater is almost pure enough to be potable with only minimal treatment in the form of chlorination to destroy any harmful bacteria. In other locations, the groundwater may contain considerable impurities, which could be naturally occurring or man-made. It is important to realize that pumping from groundwater lowering systems will change natural groundwater flow in aquifers and may cause existing contamination plumes or zones to migrate. Two of the most important cases to be considered are contaminated groundwater, often left over from industrial land use, and intrusion of saline water in coastal areas.

Movement of Contaminated Groundwater

The study of groundwater contamination is a major field in itself, and the reader is commended to texts such as the one by Fetter et al. (2018) to obtain the full background on the subject. Contaminants interacting with groundwater flow exist in one of three forms (or phases):

  • 1. Dissolved (or aqueous) phase. A wide range of substances are soluble in water and so become part of the water itself, travelling with it.
  • 2. Non-aqueous phase. This describes liquids that are immiscible with water. They may have densities lower than that of water (light non-aqueous phase liquids or LNAPLs) and will float on top of the water table (examples are petrol and diesel compounds). Dense non-aqueous phase liquids (DNAPLs) also exist, which are denser than water and tend to sink below the water table until they meet a low-permeability layer (examples include chlorinated hydrocarbons such as trichloroethylene). The non-aqueous phase is sometimes described as ‘free product’, meaning that it is a form of contamination existing separately (in a different phase) from the water.
  • 3. Vapour phase. Volatile compounds in the contaminant can move in gaseous form in the unsaturated zone above the water table.

Some contaminants (e.g. hydrocarbons such as petroleum products) may create all three phases when they reach groundwater.

Contamination may be caused by a variety of land uses: [1]

The absence of active use of a site does not provide assurance that the site is uncontaminated. A legacy of contamination may exist in the ground and groundwater for years or decades after the pollution is stopped. A guide to the types of pollution that can be expected from former industrial sites can be found in CIRIA reports on contaminated land (Harris etal., 1995). A desk study (see Section 11.6) should investigate former uses of a site to determine the risk of contaminants being present at problematic levels.

As described in Chapter 3, groundwater is constantly in motion, and where contamination exists, that will tend to move too, gradually forming a plume stretching away from the original source of contamination. The rate and direction of movement of contamination depend on many factors, including hydraulic gradients, the geological structure of the aquifer, the nature of the contaminant and any chemical changes in the contaminant with time. Detailed consideration of these factors is beyond the scope of this book, and the references cited earlier are recommended for further study. However, it is vital that anyone designing or carrying out a groundwater lowering understands that pumping may change considerably the existing groundwater gradients and velocities, affecting both the magnitude (generally increasing flow velocities) and the direction. This means that groundwater lowering can cause the extent of a contamination plume to change, perhaps much more rapidly than previously. If movement of contamination is of real concern, a thorough site investigation followed by development of a groundwater flow and contaminant transport model is essential, and specialist advice should be obtained at an early stage.

When planning groundwater lowering on or near a contaminated site, there are two important issues to be addressed in addition to the dewatering design itself:

  • 1. How can the influence of groundwater lowering on the contamination plume be minimized or controlled? The use of physical cut-off barriers (Chapter 18) to hydraulically separate the groundwater lowering system from adjacent contaminated sites is a method often used (see Section 17.14).
  • 2. How can the discharge be disposed of? The water pumped from the wells may contain problematic levels of contamination, preventing direct discharge to watercourses or sewers. On occasion, it has been necessary to establish a temporary water treatment plant on site to clean up the discharge water quality (see Section 17.14).

Methods based on groundwater lowering technology can be used to help clean up sites. Pumping of groundwater and treatment of discharge prior to disposal, with the aim of reducing contamination levels, is known as the ‘pump and treat’ method. This is a specialist method, and its effectiveness should be compared with other competing clean-up techniques (see Holden et al., 1998). Further details are given in Section 17.14. A project example is discussed in Case History 27.11.

Saline Intrusion

Saline intrusion describes the way more mineralized water is drawn into freshwater aquifers under the influence of groundwater pumping. This is a particular problem where large volumes of groundwater are abstracted for potable supply, because if saline water reaches the well, it may have to be abandoned. Saline intrusion principally affects coastal aquifers, but saline water can sometimes be found in inland aquifers where the water has become highly mineralized at depth.

Saline intrusion is a complex process affected by aquifer permeability, rate of recharge, natural groundwater gradients and the effect of any existing pumping wells. Any significant groundwater lowering operations will affect the boundary between fresh and saline water. If saline water is drawn to the groundwater lowering system, that may not be a problem in itself (provided that the water can be disposed of), but any saline water drawn towards nearby supply wells is of much greater concern. The risk of saline intrusion may need to be investigated using numerical modelling to assess the effect on local and regional water resources; the reader is referred to hydrogeological texts such as Younger (2007) for further details.

Although not strictly a case of saline intrusion, in arid countries, there is a risk of affecting the salinity of wells used by local communities. In some hydrogeological settings, fresh or brackish water lenses may exist above a generally saline water table and are often exploited by shallow wells to provide water for irrigation and livestock. Preene and Fisher (2015) highlight that in those circumstances, even small changes in groundwater levels caused by dewatering pumping could cause significant changes in water quality in the shallow wells. It is possible that the salinity of the water in the well may increase dramatically, rendering the well unusable.

Effect on Groundwater Borehole or Spring Supplies

This book mainly deals with groundwater as a problem needing to be controlled to allow construction excavations to proceed, but, as was highlighted at the start of this chapter, groundwater is also a resource used by many. Groundwater is obtained from wells and springs as part of public potable water supplies and for private supplies for domestic dwellings and industrial users such as breweries, paper mills, etc. and by farmers for irrigation and watering of livestock. If temporary works groundwater lowering is carried out in the vicinity of existing well or spring abstractions, there is a risk that the abstractions will be ‘derogated’ - in other words, it will be harder for the user to abstract water, and in extreme cases, the source may even dry up completely (Figure 21.9). The interaction between ground- water supplies and civil engineering works is discussed further by Brassington (1986).

Occasionally, water quality from a spring or well source may deteriorate as a result of changes in the groundwater flow direction. This can occur by various mechanisms:

  • • Changing the mix of water pumped from the water source (well or spring) This can occur when the water source taps into multiple aquifer zones - e.g. a shallow and a deeper water bearing zone - but where the dewatering works affect only one zone. This will affect the chemical make-up of the water from the source. As noted earlier, in arid countries where wells tap into both fresh and brackish water zones, there is a risk that nearby dewatering activities may reduce the freshwater flow, increasing the salinity and rendering the source unusable.
  • • Drawing in of different water quality towards the water source so that it eventually yields water of inferior chemistry. Examples include by drawing in contaminated groundwater (Section 21.4.10), saline water from coastal waters (Section 21.4.11) or poorer-quality water from abandoned mine workings (Neymeyer et al., 2007).

The effects are often temporary, and may cease soon after the end of groundwater control pumping, but can cause considerable inconvenience and cost to groundwater users. The legal issues must also be considered, since in England and Wales, licensed groundwater abstractors have a legal right to continue to obtain water (see Section 25.4).

When groundwater lowering is carried out for a construction project, the primary effect on nearby abstractions is likely to be a general lowering of water levels, which will affect

Derogation of groundwater sources,

Figure 21.9 Derogation of groundwater sources, (a) Impact on borehole. Dewatering system lowers groundwater level at water supply borehole, (b) Impact on spring. Dewatering system reduces flow from spring. (After Preene, M and Brassington, F C, Water and Environmental Management Journal, 17, 59-64, 2003.)

operating water levels in existing wells, with a corresponding reduction in output. The magnitude of the reduction in output will depend on factors including:

  • 1. Aquifer characteristics, including permeability and storage coefficient
  • 2. Distance between groundwater lowering wells and supply wells, and their location in relation to any existing hydraulic gradients in the aquifer
  • 3. The dewatering pumping rate and period of pumping (low-flow-rate and short-duration pumping systems will have less of an effect on supply wells)
  • 4. The depth, design and condition of the supply wells and associated pumping system

Any rational assessment of the effect of groundwater lowering on supply wells will require some form of conceptual groundwater model to be developed. This could then be used as the basis for a numerical model, or an initial assessment could be made using the methods of Chapter 13, treating the groundwater lowering system as an equivalent well. The assessment of potential impacts is discussed in Section 21.10.

Permanent dewatering systems (see Chapter 20) can also have an impact on nearby groundwater sources. Impacts may be caused not only by systems that are actively pumped by wells but also by projects where linear engineered features (such as road or rail cuttings) are drained by gravity (Figure 21.10).

If the estimated effects on the supply wells are small, this may be deemed acceptable with no further mitigation measures. However, if the effects are more severe, Powers (1985) suggests the following mitigation measures:

  • 1. If only a few low-volume users are affected, and the dewatering period is short, the lost supply might be replaced by a temporary tanker supply.
  • 2. If the supply well is deep but with the pump set at a fairly high level, it may be possible to install higher-head pumps at greater depth in the supply well. This would allow abstraction to continue even with the additional drawdown generated by groundwater lowering.
  • 3. If the supply wells are shallow, it may be necessary to deepen the wells, perhaps into another aquifer. Alternatively, for small-diameter shallow wells, it may be more economical to simply drill a new, deeper well.
  • 4. A portion of the dewatering discharge may be piped to the affected user by temporary pipeline. Depending on the water quality, point of use treatment may need to be provided to ensure that the water is suitable for use.
  • 5. Public water mains may be extended into the affected area, giving a permanent benefit for the money spent.
Groundwater abstraction from linear construction projects,

Figure 21.10 Groundwater abstraction from linear construction projects, (a) Flow to springs prior to construction (b) Reduced flow to springs following construction. (Redrawn from Preene, M and Brassington, F C, Water and Environmental Management Journal, 17, 59-64, 2003.)

Some of these measures have huge cost, time and public relations implications and clearly need to be compared with the alternative of constructing the project without groundwater lowering or even relocating the project away from the supply wells.

A desk study (see Section 11.6) will be valuable in identifying the presence of any vulnerable water abstractions (well or spring sources) that may be impacted by a proposed groundwater control scheme. National and regional governmental bodies and environmental regulators often hold records of the location of water sources. Larger sources may have SPZs delineated around them, within which engineering works require special permission from the regulators.

If the effects on nearby groundwater abstractions are of real concern, it is essential that they are addressed early in the planning of a project, because it is unrealistic to expect the contractor to bear all the costs and risk of some of these measures. The project client will have to face up to the potential need for some of these measures and perhaps allow for them when negotiating with landowners for wayleaves, etc. The mitigation measures might be included at the very start of site works as part of the enabling works. Alternatively, the supply wells may be monitored during the works, with a contingency in place that the mitigation measures will be applied if the well is affected beyond a certain pre-defined level.

Other Effects

Occasionally, other, less common side effects may be of concern.

21.4.13.1 Damage to Timber Piles

It is widely recognized that timber piles supporting older structures may be detrimentally affected by drawdown of water levels. This is a particular issue in Scandinavia, where buildings founded on timber piles are commonplace (Peek and Willeitner, 1981). In cities such as Copenhagen, this is such an important issue that it has led to the introduction of local regulations that prohibit significant lowering of groundwater levels in specified areas. This has influenced the groundwater control techniques used on several major infrastructure projects in Copenhagen, leading to widespread use of artificial recharge systems (Bock and Markussen, 2007).

Powers (1985) states that the damage to timber piles and foundations may result from fungi present in the timber thriving in an aerobic environment created if groundwater levels are drawn down, exposing the tops of the piles to air. However, Powers also states that the most severe cases of aerobic attack have been when piles were exposed in excavations, and that observed decay due to drawdown has been less severe. This is probably because the oxygen supply to the timber surface is not increased substantially when the piles are in dense or fine-grained soils, even when groundwater levels are lowered.

Nevertheless, a sensible approach is to proceed cautiously when working in areas when older structures are founded on timber piles. Even if aerobic attack does not compromise pile stiffness, soil consolidation and pile downdrag due to negative skin friction should be considered.

21.4.13.2 Vegetation

It is rare for groundwater lowering systems to have a noticeable effect on vegetation. This is mainly due to the short-term nature of pumping and the fact that plants generally draw their water from immediately below the surface, above the water table. This zone is much more likely to be affected by changes in precipitation and infiltration than by deeper pumping. For longer-term pumping (for certain types of quarries or open pit mines), this issue may need to be considered further, and the services of an experienced ecologist can be very useful in this regard.

21.4.13.3 Impact on Archaeological Remains

The continued in situ preservation of archaeological remains may also be dependent on stable groundwater levels, and there have been cases of degradation associated with large- scale dewatering works (French and Taylor, 1985). Numerical modelling has been used to assess groundwater lowering beneath areas of archaeological interest (Garrick et al., 2010).

  • [1] Industrial processes - mainly from spillages or leakages from stored materials. Sites ofconcern include not only manufacturing and chemical production sites but also vehiclestorage areas, airports and other locations where fuel and detergents are used. • Landfilling and waste disposal - from both official and unofficial disposals. • Agricultural practices - such as fertilizer or pesticide use. • Urban use - including leaking sewers, fuel spills, etc.
 
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