This edition of the blog discusses the different approaches that can be taken to dispose of pumped water from construction dewatering and mine dewatering systems.
Construction dewatering and mine dewatering invariably involve pumping of water and this water must be disposed of in some manner, and disposal must take into account the relevant regulations and the risk of environmental impacts. The disposal of water is sometimes also known as discharge of water.
Dewatering is more correctly described as groundwater control, and is the process of temporarily controlling groundwater for construction excavations or mining projects. As described in a previous blog there are two principal groups of groundwater control technologies: exclusion methods and pumping methods:
Pumping and exclusion methods may be used in combination. One of the drivers for exclusion solutions is often to reduce the pumped flow rate, but it is important to recognise that even groundwater control systems based on exclusion methods will still need to dispose of some pumped water, such as that arising from precipitation, basal seepage and leakage through the cut-off walls.
THE NEED FOR WATER DISPOSAL FROM DEWATERING SYSTEMS
It should be very obvious that any dewatering system needs a reliable water disposal route of adequate capacity. But it is surprising how often this requirement is overlooked until relatively late in the development of a project. On rare occasions it is overlooked completely until pumping is about to begin – in which case progress on the project can be completely stalled until a solution can be developed.
A key challenge for water disposal is that the groundwater element of the water discharge is normally produced 24 hours per day, 7 days per week – the flow never stops. This means that, even for low flow rate systems, it is difficult to ‘store’ water on site, even for a short period, and so most disposal options involve carrying or transporting the water away from site in some manner. On mining projects or large construction projects the dewatering pumping could last several years, and the water disposal route needs to be robust enough to meet the project’s needs over that period. Potential options for disposal of pumped water are outlined later in this blog.
Dewatering systems can also generate water other than the main dewatering flow. At the early stage of a project there may be well pumping tests as part of the site investigation. The water from test pumping must also be disposed of, and while the flow rate will be less than for the main dewatering, it may still need careful planning to get rid of this water at a time before the main project infrastructure is in place. Similarly, water from well development will need disposal in the period before dewatering begins.
Additionally in certain climates, some dewatering systems, such as those for open pit mines or large construction excavations, will sometimes pump a significant proportion of surface water flow. Examples include rainfall during tropical storms, or snowmelt during the spring, and this may place additional stress on water disposal infrastructure.
FACTORS CONTROLLING WATER DISPOSAL FROM DEWATERING SYSTEMS
The requirements for water disposal from dewatering systems are influenced by several factors:
Pumped flow rate: The peak flow rate is a key factor in assessing the suitability of a water disposal route. The groundwater element of pumping will tend to be relatively constant in the short term, although flow rates may increase as excavations are deepened. Conversely, when the excavation is at a constant depth the flow rates may slowly decrease in long term pumping as groundwater storage is removed. However, in some climates the surface water element of the pumped flow will vary during the year, and the peak flow rate to be handled will be groundwater plus surface water. The water disposal route will likely have a maximum flow rate that it can handle. For ‘engineered’ disposal routes such as discharge to sewerage networks this flow rate limit may be known explicitly. Conversely, for natural disposal routes, such as discharge to a river, the upper flow rate limit may not be known. If excessively high dewatering flow rates are discharged to a river or watercourse there is a risk that the capacity of the river channel will be exceeded, creating a risk of the channel overtopping downstream and causing flooding.
Pumped water quality: Most water disposal routes will have water quality limits, beyond which it is not acceptable to discharge, either because it causes adverse environmental impacts in the receiving water body or because it contravenes the applicable regulations. The most obvious issue is where groundwater is contaminated, for example due to previous industrial use of the site or due to leaching from mine wastes. Another example is where the pumped water has a high suspended solids load (as can occur with sump pumping or in-pit pumping). Where water quality from the dewatering discharge is not acceptable for discharge there may be a requirement for an on-site groundwater treatment plant to improve water quality prior to discharge. In any event it is normal practice to pass the dewatering water through a settlement tank or settlement pond to try and reduce suspended solid loading before water is discharged off site.
Duration of pumping: On major projects dewatering pumping may continue for long periods, up to several years in some cases. In such cases the longevity of the water disposal infrastructure needs to be considered, in case the infrastructure will be affected by the project. An example might be where the later stages of an open pit mine requires the diversion of the river into which the dewatering flow is being discharged.
Climate: The dewatering challenges in temperate climates will generally focus on the groundwater element of pumped flow, with episodic surface water flows typically being of modest magnitude only. But in climates subject to rainy seasons, monsoons and tropical storms, or where there are very cold winters followed by spring thaws, the surface water element may be very large during some parts of the year, and the water disposal option must take this into account. One of the challenges can be that the peak pumped flow rate (for example from a tropical storm) must be disposed of at the time when the water disposal infrastructure is already carrying high flow rates due to the climatic conditions.
Location: The terrain and local environment will play a large part in determining what water disposal options are practicable and acceptable in relation to potential environmental impacts. The most appropriate disposal options are likely to be different in a sparsely inhabited arid region, compared to an urban city-centre site, or a site surrounded by ecologically sensitive wetlands.
POSSIBLE WATER DISPOSAL METHODS
A range of methods can be used to dispose of water from dewatering systems. The most common methods are:
Some less common methods include:
Each method is described briefly below.
Disposal to sewerage systems
This option is possible in developed countries, where there is an existing, reliable, sewerage network. If the sewer network has adequate hydraulic capacity, and can accept the pumped water quality, the dewatering water can potentially be discharged to sewer. The sewer network will then convey the water to an ultimate disposal point – typically a river or the sea. In most cases the sewerage system will be owned and maintained by a ‘water company’ or other authority (which is often part of local or national governments). Permission will be needed from these bodies before water can be discharged to sewer, and there will often be fees to pay. In some cases the fee is a ‘volumetric’ fee, where there is a small set cost per cubic metre discharged, and a requirement to accurately monitor the volumes that are discharged. For long duration discharges these small volumetric costs multiplied by the total discharge volume can result in very large costs for water disposal.
Disposal to surface water (rivers, lakes, sea)
In this case the dewatering water is disposed of to an existing body of surface water. If the disposal point is relatively close to the site the discharge may be direct from the dewatering system. If the chosen surface water body is further away there may be a need to construct a connecting water channel or pipeline between the dewatering system and the disposal point. Where an Environmental Regulator exists (such as the Environment Agency in England or SEPA in Scotland) it may be necessary to obtain permission to discharge to surface waters.
Disposal to groundwater (via recharge wells and trenches)
This involves the pumped water being discharged back into the ground either via recharge trenches or infiltration ponds (which can dispose of water at shallow depths) or via recharge wells that can re-inject water at greater depths. Disposal to groundwater in this way is also known as ‘artificial recharge’. The requirement to dispose of dewatering water to groundwater can result from several factors: the desire to reduce external drawdowns (therefore reducing environmental impacts); the desire to reduce net abstraction from a given aquifer (for example to protect drinking water resources); or simply to dispose of water when other possible discharge options are problematical. From a hydrogeological point of view, it is often not a good idea to re-inject water back into an aquifer close to an excavation or mine, because of the risk of the injected water being drawn back to the pumping system. For this reason the recharge wells or trenches often have to be constructed a considerable distance from the excavation. The pumped water is typically conveyed along a pipeline between the dewatering system and the re-injection point. Where an Environmental Regulator exists (such as the Environment Agency in England or SEPA in Scotland) it may be necessary to obtain permission to discharge to groundwater. There are often significant engineering and hydrogeological challenges associated with artificial recharge of groundwater, and it is not an approach to be undertaken without careful investigation, design and monitoring.
Disposal via tanker to off-site facility
For projects where the cumulative volume of dewatering water is small (for example low flow rate pumping tests) and where there is no viable alternative (such as when the pumped water is too contaminated for other disposal routes), then the water can be pumped to mobile road tankers and transported to an off site disposal facility that can handle contaminated water. This option is rarely used due to the high cost and the large number of tankers required.
On-site storage in ponds and reservoirs
On rare occasions large artificial surface reservoirs have been constructed to hold the water pumped from a dewatering system. A possible example might be for an open pit mine or construction project in an isolated area where there are limited watercourses available to which water could be discharged, but where there are large areas of flat lying land available on which an embankment type reservoir can be built. If this approach is taken, a significant design and safety exercise is needed to ensure that the reservoir has adequate capacity to safely contain the predicted volumes of water. Consideration also needs to be given to the impact of time overruns in the project duration, which may cause the cumulative dewatering volumes to exceed the volumes anticipated at design stage.
Evaporation ponds and enhanced evaporation
In arid climates with high rates of evaporation, it may be possible to create shallow ponds, into which the dewatering water is pumped. Evaporation will occur and some water will be lost to the atmosphere. If the ponds are unlined, there may also be an additional loss of water by seepage from the base of the ponds as water infiltrates into the ground. Banks of mechanical evaporators can also be used where small water droplets are added to the airflow created by large fans, generating a stream of moisture blown into the air that will preferentially evaporate. In reality, it will be difficult to evaporate the entire flow rate from a dewatering system of any significant size, but evaporation ponds are sometimes used to reduce the net quantity of water that must be disposed of. It should also be noted that the water remaining behind in the ponds after evaporation will effectively become concentrated and will have increased levels of dissolved salts and other chemical constituents and may be difficult to dispose of for water quality reasons.
Beneficial use of water
If the water quality from the dewatering system is ‘good’, i.e. uncontaminated and with the potential to be used for other purposes, then it may be possible to pipe the water from the site and provide it to nearby users. An obvious example is to provide irrigation water to farmland around a mine site, or in developing countries to provide a source of drinking water to nearby communities. Especially if the water is provided for drinking water purposes there may be regulatory requirements to satisfy, and a regular programme of water quality monitoring should be in place to ensure the pumped water is safe and a fit for purpose.