Using historic mapping in a contamination desk study

A Phase 1 contamination desk study stands or falls on the map sequence. Successive Ordnance Survey editions covering a site from the middle of the nineteenth century to the present show what was there, when it arrived and when it went, and that history is what generates the list of potential contamination sources the rest of the assessment tests. Everything else in the study, from the geology to the regulatory searches, is interpreted against it.

Reading the maps properly is a skill, and it is mostly about noticing absences.

What the editions reveal

Work chronologically and record what changes. The first large scale county surveys show a rural or early industrial baseline. Late nineteenth century editions catch the expansion of works, gasworks, tanneries, foundries, railway land and engine sheds. Interwar editions pick up filling stations, garages and small industrial units. Post-war National Grid editions show redevelopment, and often the first clear evidence of infilled ground.

Two patterns matter more than any individual feature.

A use that appears and then disappears within two editions is a warning, because whatever was in the ground did not leave when the building did. A pond, pit or quarry present on one edition and absent on the next is almost always infilled rather than drained, and what it was filled with is the question.

Note the site boundary generously. Contamination migrates, and a works immediately off site upgradient is as relevant as one within the red line.

Identifying tanks, works, pits and infilled ground

The annotation is where the information density is. Tanks are commonly marked, and a tank shown once and never again is a candidate for a buried structure. Works, depots, yards, sidings, sewage works, sand and gravel pits, brickworks and clay pits all appear by name.

Look for the quiet indicators too. Unlabelled rectangles inside an industrial curtilage. A siding running into a building. A weighbridge. Hachures showing a change in level where a pit was worked. A boundary shown in one edition and lost in the next, suggesting ground reshaping.

Mineral extraction followed by residential development is the classic sequence, because the void was filled before anyone recorded what went into it. Where you see it, expect made ground of unknown composition and depth, and expect a ground gas regime to be part of the investigation.

Reading annotation conventions

Abbreviations vary by period and are worth checking against a key rather than guessing. Tanks, pumping stations, sluices, mileposts, wells and springs all have conventional forms. Levels are shown as spot heights, and comparing spot heights between editions is one of the quickest ways to detect ground raising.

Scale matters. A feature legible at 1:2500 may vanish at 1:10560, and a study that relies only on the smaller scale for early periods will miss small structures. Where both scales exist for a period, look at both.

Survey date and publication date are different, sometimes by years. Cite both, because a feature might have existed for several years before the survey that recorded it.

Aerial photography as a companion source

Photographs are dated precisely and they show what maps generalise. Post-war coverage across England is extensive, and it reveals infilling in progress, spoil heaps, tank farms, hardstanding, demolition phases and vegetation patterns that mark buried structures.

For a site where a pit was filled between two map editions eleven years apart, a run of photographs can narrow the date to a year and sometimes show the material arriving. That matters, because the era of filling is a reasonable proxy for what the fill contains.

Use them alongside published geological mapping and archived borehole logs, which is the same discipline used when finding borehole and geology records for a site at the ground investigation stage.

Building the source, pathway and receptor picture

The point of the history is a conceptual site model. For each potential source, identify a plausible pathway and a receptor, and be honest where any of the three is missing.

Sources: fuel and chemical storage, process residues, ash and clinker, made ground of unknown origin, asbestos in demolition fill, gasworks residues.

Pathways: direct contact with soil in gardens, ingestion by children, dust inhalation, vapour and ground gas ingress into buildings, leaching to groundwater, lateral migration along granular services trenches.

Receptors: future residents, particularly in gardens, construction workers, controlled waters, buried services, and ecological receptors on some sites.

No linkage, no risk in the regulatory sense. A hydrocarbon source 4 metres down under a sealed commercial yard with no groundwater pathway is a different proposition from the same source under a garden. National policy takes a suitable for use approach, so the assessment is always against the proposed development rather than against an abstract standard, and the layout can change the answer. Move the gardens away from the former tank position and the linkage weakens before a single sample is taken, which is why the findings belong on the constraints plan before the design starts.

Recording sources so the study is auditable

Cite every map by scale, survey date, publication date and sheet. Reproduce the extracts as figures, dated and labelled, with the site boundary overlaid consistently at the same scale across editions.

Do the same for photographs, regulatory searches and any anecdotal information, and separate observed fact from interpretation in the text. A reviewer should be able to see which conclusion rests on a map annotation and which rests on your judgement.

This discipline matters more than it sounds. Desk studies are read years later by people deciding whether an intrusive investigation was adequately scoped, and an unsourced assertion is worth nothing at that point.

Where the evidence triggers intrusive work

Some findings effectively decide it. A mapped tank, an infilled pit or pond, a former industrial use on the site itself, a gasworks within influencing distance, railway land, or made ground of unknown depth across the development footprint. Any of these leads to a Phase 2 site investigation designed to test the specific linkages the desk study identified.

Design the investigation from the model, not from a standard grid. Boreholes positioned to test a mapped tank location and the infilled corner of a site tell you more than an even spread across a rectangle.

Two practical notes on timing. Commission the desk study before exchange where you can, because a gasworks on the neighbouring plot is a price question. And do not let the desk study sit for two years before the investigation, since regulatory records, the surrounding land uses and the proposed layout will all have moved on and the reviewer will notice the dates.

PlanScope drafts the planning documents around the ground work, and you can enter a postcode on the home page to see which reports a site is likely to need. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. Land contamination reporting must be prepared by suitably qualified and experienced persons in accordance with the relevant British Standard, so treat a draft as a starting point for that professional rather than a substitute. This is general guidance, not legal advice, so check the position with your local planning authority before relying on it.