Best Practice Guide to Dewatering Excavations

For construction projects, highways & utilities maintenance 

Dewatering an excavation

Whether you manage a construction site or a roadside maintenance team, at some point you’ll need to empty an excavation that has filled up with rainwater or ground water. Particularly in city centres where deep pilings and basements mean the water has nowhere else to go.

Long gone are the days when you could simply just pump the water out and discharge it into the environment. Legislation ensures that operators not abiding by pollution prevention guidance are at risk of prosecution and hefty fines.

However, the expense and disruption associated with some dewatering remedies can lead to malpractice and contamination of surface water.

Fortunately, if you are well-prepared you can dewater most small to medium-sized excavations quickly, cost-effectively and compliant with environmental regulations.

This Best Practice Guide to Dewatering Excavations tells you how. We’ve broken it down into 3 steps:

Step 1: Do You Need a Discharge Permit?

There are two main issues here: how clean is the water and where are you going to discharge it?

If the water is clean, you won’t need a permit.

Silt left on road & pavementIf the water is sediment-laden, you can only discharge a minimal quantity to land without a permit. Make sure that dirty water doesn’t overflow onto pavements and roads as this is a pedestrian and traffic hazard not to mention unsightly for residents. This is a common and illegal practice of roadside contractors.

If you filter out the sediment first, to regulator-approved standards, you will be able to discharge to surface water. You might still need a permit. If your dewatering activities are going to continue for more than three consecutive months, you must apply for a permit.

If the water is contaminated with hydrocarbons, you should try to discharge it to the foul sewer. You will need a permit from the public sewerage undertaker which will incur a fee and there will be a process of approval. However, if you are performing emergency works, approval will usually be granted quickly.

If this is not an option, you will need to remove the waste water using vacuum tankers or clean it yourself before dewatering. Vacuum tankers can be prohibitively expensive – in the region of £1,000 per load – and may cause disruption to your project. If the level of contamination is high, you may have no choice.

If you can clean the water first to regulator-approved levels, you could discharge to land or surface water.

You can discharge to land without an environmental permit. If you cause pollution to the land, your dispute will be with the landowner. If discharging to surface water, you will most likely need an environmental permit.

In all cases, contact your local environment agency for advice. It is against the law to be operating without a permit when you need one. Permits can take three or four months to obtain so plan well in advance. Advice can also vary depending on who you talk to so don’t assume the advice you received in one area of the country will be the same in another area. (How to apply for a discharge licence in England, Wales, Scotland & Northern Ireland)

Make yourself familiar with the Temporary Dewatering from Excavations to Surface Water regulatory positioning statement (Updated 30 April 2020) or the equivalent in your jurisdiction. There are other factors, including your proximity to protected areas, which will determine whether or not you need a permit.

Step 2: How to Test for Hydrocarbon Contamination