Construction phase surface water pollution control

Silt is the pollutant that gets contractors prosecuted. A single storm on an open earthworks site can push enough suspended solids into a watercourse to smother spawning gravels for a kilometre downstream, and unlike an oil spill it happens without anyone doing anything wrong in particular. Controlling construction phase surface water pollution is therefore about designing the temporary drainage before the topsoil comes off, not about reacting when the ditch turns brown.

Why silt dominates the risk

Stripped ground generates suspended solids at rates orders of magnitude above the same ground under grass. Fine clay particles stay in suspension for hours, pass straight through a straw bale and reach the receiving water looking like milky tea.

The legal exposure is wide. Causing or knowingly permitting a water discharge activity without an environmental permit is an offence under the Environmental Permitting (England and Wales) Regulations 2016, and causing poisonous, noxious or polluting matter to enter controlled waters is an offence under section 85 of the Water Resources Act 1991. Neither requires intent. The person who opened the valve and the company that employed them can both be liable.

Councils increasingly condition this alongside the permanent drainage design, and the lead local flood authority will look for it, as our note on what the lead local flood authority checks sets out.

Temporary drainage and settlement

The temporary system is a design, not an improvisation. It needs a route for every part of the site at every stage of the earthworks, and it has to work when the permanent system does not yet exist.

The components are familiar. Cut-off ditches at the head of the site divert clean upslope water around the works rather than letting it become dirty. Graded falls and temporary channels take site water to a collection point. Settlement lagoons or tanks hold it long enough for solids to drop out, sized on the contributing area and the design rainfall rather than on what fits in the corner. Where retention time alone will not achieve the discharge quality, flocculant dosing or a proprietary treatment unit follows the lagoon.

Two design points are routinely missed. A lagoon needs de-silting access for a machine, and nobody plans for where the arisings go. And the discharge from the lagoon needs somewhere to go at a rate the receiving ditch or sewer can take, which is the same greenfield rate argument as the permanent Drainage Strategy, applied to a temporary condition.

Wheel washing, haul routes and stockpiles

Mud on the road is the visible failure, and it usually signals that the site drainage is failing too.

Wheel washing works when the wash water is recirculated and settled rather than discharged to the nearest gully, and when the exit run after the wash is long enough and hard enough that tyres stay clean. A brush and a hose at the gate is not wheel washing.

Haul routes should be stone-surfaced, drained to the site system, and kept away from the site boundary where run-off can escape onto the highway or into a roadside ditch. Stockpiles are the other reservoir of silt: site them away from watercourses and drains, on a graded base draining back into the site, and seed or cover any pile that will stand for more than a few weeks.

Concrete washout and fuel storage

Concrete and cement wash water is highly alkaline, often above pH 12, and lethal to fish at dilutions that seem trivial. Provide a lined, bunded washout area with a defined capacity, use it for every chute wash and every pump clean, and take the hardened arisings off site. Never wash out onto the ground or into a drain.

Oil storage on a construction site is regulated. Above the threshold in the oil storage regulations, tanks need secondary containment with capacity of at least 110 per cent of the largest container, sited away from drains and watercourses, with fill points and valves inside the bund. Mobile bowsers, generators and static plant all count. Keep spill kits at each storage location and at the site entrance, and make sure the people on nights know where they are.

Site welfare produces foul water too. Chemical toilets and canteen waste need tankering or a connection, and a temporary connection to the public foul sewer needs the sewerage undertaker's agreement. Site the welfare compound where the tanker can reach it without crossing the working area, because the alternative is a vehicle movement through wet ground twice a week.

Permits and consents for temporary discharges

Route the question early, because consenting takes weeks.

1. Discharge of treated site water to a surface watercourse usually needs an environmental permit, or falls under an exemption or a standard rules permit depending on the volume and quality.
2. Discharge to a public sewer needs a trade effluent consent from the sewerage undertaker, and undertakers frequently refuse silty water outright.
3. Dewatering an excavation may need an abstraction licence as well as a discharge permit, depending on volume and duration.
4. Works in, over or under a main river need a flood risk activity permit, covered in our piece on flood risk activity permits.
5. Works affecting an ordinary watercourse need land drainage consent from the lead local flood authority.

Nothing about this sequence is fast, and the application usually asks for the discharge quality and volume you intend to achieve, which means the treatment design has to exist first.

What goes in the construction management plan

Most of this is secured through a condition requiring a Construction Management Plan to be approved before work starts. The version that gets approved first time is specific.

It names the receiving water and the drainage route at each phase. It shows the temporary drainage on a drawing, phase by phase, with the lagoon sized and located. It sets out the monitoring: who checks the discharge, how often, against what trigger, and what happens when the trigger is exceeded. It names the permits applied for and their status. It includes the spill response procedure and the training regime. And it identifies a responsible person by role, with the authority to stop work.

Generic method statements copied between sites are the reason these conditions get refused. An officer who has read twenty of them recognises the twenty-first.

Keep the plan alive after approval. The phase drawings only work if someone updates them when the earthworks sequence changes, and the sequence always changes. A short monthly review, minuted, is enough to show that the approved plan is being followed rather than filed.

Enforcement and the cost of getting it wrong

Enforcement for water pollution sits with the Environment Agency rather than the council, and the sanctions run from advice and guidance through enforcement undertakings to prosecution, with fines for corporate offenders set by reference to turnover and culpability. The clean-up cost, the programme delay and the loss of the discharge consent usually exceed the fine.

The realistic protection is boring: an adequately sized lagoon, someone whose job it is to look at the outfall every morning, and a written record showing that they did. This is general guidance, not legal advice; check the position with the Environment Agency, your sewerage undertaker and your local planning authority before relying on it.

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