Evidence for a site inside a groundwater source protection zone

A groundwater source protection zone changes the technical evidence base rather than the principle of development. Two decisions drive everything: whether surface water will be infiltrated to ground, and how the foundations will be installed. Both are assessed against the risk of mobilising contamination or creating a pathway into the aquifer that supplies a public abstraction, and the Environment Agency will comment on both.

The zones are defined by travel time to the abstraction point. The inner zone represents a 50 day travel time, the outer zone 400 days, and the total catchment the whole area contributing to the source. Restrictions tighten sharply towards the inner zone. For the document list that applies to a scheme in one of these zones, the companion note on documents for a scheme in a source protection zone sets out the pack. What follows is about the evidence inside those documents.

What the designation means for development

The zone is not a constraint in the way a flood zone or a conservation area is. There is no sequential test and no presumption against development. What changes is the standard of proof required about what will reach the groundwater.

Three categories of proposal attract most attention. Development on land with a history of contamination, because disturbance can mobilise contaminants that are currently stable. Development proposing infiltration drainage, because that creates a direct route to the water table. And development requiring deep foundations, because piles can penetrate a protective layer.

A fourth, less obvious, is anything involving the storage or handling of fuels, oils or chemicals, including the construction phase. Many conditions on these sites relate to the build rather than the finished development.

Where the site is clean, the drainage discharges to a sewer and the foundations are shallow, the evidence requirement can be modest. Establishing that early is worth doing before any investigation is commissioned.

Confirming the zone and the aquifer beneath you

Do not rely on a consultant's constraints plan alone. Check the designation directly against the Environment Agency's published spatial data, and record the zone, the date of the data and the abstraction it protects.

Then establish what lies beneath. The aquifer designation of the bedrock and any superficial deposits matters as much as the zone, because a principal aquifer directly beneath made ground is a very different proposition from one confined beneath 20 metres of clay. Geological mapping gives the starting point and the site investigation confirms it.

Three further checks are worth making at the same time. Whether any private water supply, well or borehole exists within the area that could be affected. Whether the site falls within a nitrate vulnerable zone or any other water related designation. And whether the authority's own local plan carries a water protection policy with its own requirements, which several do.

An illustrative example of how this changes things: a former vehicle depot in an outer zone, underlain by sand and gravel over a principal aquifer, will need intrusive investigation and a controlled waters risk assessment before the drainage strategy can even be drafted. The same scheme in a total catchment zone over a thick clay sequence may need only a desk study.

The groundwater risk assessment the Environment Agency expects

The structure follows the source, pathway and receptor model, and the consultee reads it in that order.

Start with a conceptual site model from the desk study: the site history, the potential contaminant sources, the geology and hydrogeology, the pathways by which contaminants could travel and the receptors at risk, which here includes the groundwater itself as a controlled water.

Then the intrusive phase. Soil and groundwater sampling positions targeted at the identified sources, monitoring wells installed to allow repeat groundwater sampling rather than a single snapshot, and analysis for the contaminants the history suggests rather than a generic suite. Groundwater levels and flow direction need establishing, because a receptor down gradient of the source is in a different position from one up gradient.

The assessment then compares measured concentrations against assessment criteria derived for controlled waters, using an established methodology, and concludes on whether a risk exists requiring remediation. Where it does, an options appraisal and a remediation strategy follow, with a verification plan setting out how completion will be demonstrated.

Land contamination investigation and reporting is a field where the local planning authority expects work prepared in accordance with BS 10175 by suitably qualified and experienced persons, and a licensed or competent professional is legally required for this kind of assessment. A land contamination assessment drafted by PlanScope is a starting point for that professional rather than a substitute for them.

Infiltration drainage inside a protection zone

The drainage hierarchy puts infiltration first, and inside an inner source protection zone that preference frequently has to give way. The Environment Agency's published position is restrictive about infiltration where it could mobilise contamination or where the unsaturated zone provides insufficient attenuation, and deep bore soakaways are opposed in these zones.

So the Drainage Strategy has to state a position and justify it rather than defaulting to soakaways because the hierarchy says so. The usual alternatives are attenuation with a restricted discharge to a sewer or watercourse, lined attenuation features where ground conditions would otherwise infiltrate, and permeable paving over a lined sub base acting as attenuation rather than infiltration.

Where infiltration is proposed, the case needs four elements. Infiltration testing to an established method with results at the proposed depth. Confirmation that the infiltration point is in clean material and not through made ground. A treatment train sized for the pollution hazard of the contributing surfaces, following the approach in the recognised SuDS guidance. And a stated separation between the base of the infiltration feature and the highest recorded groundwater level.

Car parks, service yards and roads carry higher pollution hazard than roofs, so the treatment requirement differs by surface. A strategy that applies one solution across the whole site usually over provides in one place and under provides in another.

Piling and the risk of creating a pathway

Piling becomes a planning matter inside a protection zone because a pile can create a vertical route through a low permeability layer that is currently protecting the aquifer beneath.

The expected document is a piling risk assessment following the established industry guidance on piling and penetrative ground improvement on land affected by contamination. It should set out the ground model, the contamination status, the piling method proposed, whether the method generates arisings or displaces material, the depth relative to any confining layer, and the mitigation.

Mitigation options in practice are method selection, limiting pile depth so a confining layer is not penetrated, grouting or sealing around piles, using a method that reduces the annulus around the pile, and in some cases redesigning the foundation to avoid deep piles altogether. The assessment should be prepared alongside the Geotechnical Assessment rather than after it, since the foundation solution and the groundwater position are interdependent, and professional review is strongly advised.

Expect a condition requiring a piling method statement to be approved before that work starts. Build the time for it into the programme, because it is one of the conditions most commonly discharged late.

Fuel, chemicals and the construction phase

Oil storage on most non domestic sites is regulated, with requirements for secondary containment sized above the capacity of the tank. Inside a protection zone, authorities and the Environment Agency pay closer attention to how fuel, oils and chemicals are stored during construction as well as in the completed development.

A construction environmental management approach should cover bunded storage positions away from drains and watercourses, refuelling procedures and drip trays, spill kits and a response procedure, wheel washing and silt management, concrete washout containment, and a prohibition on discharging anything to ground.

Dewatering during excavation needs its own thought. Pumping contaminated groundwater to a sewer or a watercourse may require a consent or a permit, and that application takes time.

Where a discharge to ground or to surface water is proposed in the finished development, an environmental permit may be needed separately from planning permission. Check it early, because a permit refused after permission is granted leaves the drainage strategy unimplementable.

The document set and the conditions to expect

For a site in a source protection zone, a typical submission carries the Phase 1 desk study with conceptual site model, the Phase 2 intrusive investigation including controlled waters risk assessment, a remediation options appraisal and strategy where risk is identified, a verification plan, the drainage strategy with the infiltration position stated and justified, a piling risk assessment or a commitment to produce one, and the construction management approach. The full set of document types sits at all 66 report types.

Conditions to expect are a phased contaminated land set covering investigation, remediation scheme, verification report and unexpected contamination, approval of a piling method statement before piling starts, approval of the detailed drainage design, and a restriction on infiltration without prior written approval.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.