Choosing and testing your ground (heavy metals)

What to understand before starting

• An internet connection to consult the public databases.
• An hour of free time.
• The exact address of the plot to study.
• Ideally, the known history of the plot (former uses).

Detailed steps

1. Step 1

   Consult Géorisques

   Go to georisques.gouv.fr. Enter the address of your plot. The national platform gathers on one map the natural risks, the known polluted sites (BASOL), the former potentially polluting activities (BASIAS) and the cadastral history. Everything is free and public. Note what you find.

2. Step 2

   Check BASIAS specifically

   BASIAS lists hundreds of thousands of former industrial or service activities in France since 1850: petrol stations, mechanic workshops, printers, tanneries, gasworks, oil depots. Many are not in local memory but have left traces in the soil. An activity less than 100 m away can concern your plot.

3. Step 3

   Observe the neighbouring buildings

   Houses built before 1949: a risk of lead in flaking paint that may have reached the garden soil. Old metal sheds, garages: possible traces of hydrocarbons, copper, zinc. Visible asbestos (fibre-cement roofs from before 1997): a further investigation for that distinct risk.

4. Step 4

   Measure the distance to roads

   Lead from petrol contaminated soils in the immediate vicinity of major roads for 60 years. A plot less than 30 m from a busy major road deserves a soil analysis for lead. Beyond 100 m, the risk drops sharply.

5. Step 5

   Decide whether a soil analysis is needed

   If Géorisques flags a nearby BASOL/BASIAS, if the plot is less than 30 m from a major road, if the buildings are old with flaking paint, or if you plan to grow for children under 6: an analysis is recommended. Otherwise, judge according to your own level of caution.

6. Step 6

   Choose an analysis laboratory

   In France, several accredited laboratories offer soil analysis for heavy metals. SADEF (Aspach-le-Bas, Alsace), Auréa AgroSciences (several sites), INOVALYS, LAMS are references. Expect 60 to 200 € depending on the number of elements tested. Ask at minimum for: lead (Pb), cadmium (Cd), arsenic (As). Add mercury and chromium if an industrial area is suspected.

7. Step 7

   Take the sample correctly

   Make a composite sample: 5 to 10 sub-samples from different spots on the plot, 20 cm deep (the vegetable rooting zone), mixed in a clean bucket, from which you take 500 g for the laboratory. Clean plastic or stainless tools, no galvanised tools (zinc) or lead-handled ones.

8. Step 8

   Read the results without panicking

   The laboratories give values in mg/kg of dry soil. There is no specific regulatory threshold for amateur gardening in France. Useful benchmarks: French background level Pb 10-30 mg/kg (BRGM); the order of 8 January 1998 on sewage-sludge spreading sets, for the receiving soil, Pb ≤ 100 mg/kg and Cd ≤ 2 mg/kg (this text sets no threshold for arsenic, there is no regulatory 'soil' value for As in France). Above these values, precautions are needed; beyond double them, reconsider the project or switch to raised beds.

Concrete examples

• A community garden in Paris's 20th arrondissement: BASIAS reveals a former gasworks 50 m away → analysis → 280 mg/kg Pb → conversion to raised beds on geotextile.
• A 19th-century country house in the Loire: flaking paint on the gable → analysis at the base of the façade → 450 mg/kg Pb → vegetable garden moved 10 m from the wall.
• A rural plot with no history, 300 m from a secondary road: no analysis deemed necessary, a classic start.

Géorisques flags nothing, am I safe?

Not entirely. BASIAS only covers declared and inventoried activities. Many old lead paints and leaded petrol leave no trace in the public databases. If your plot is in an urban or peri-urban area, the history remains a factor to consider.

My analyses exceed the 100 mg/kg lead threshold, is it serious?

Not catastrophic, but it is a signal: you must adapt. See the guide 'Growing on doubtful soil'. Solutions exist: raised beds, choice of low-accumulating crops, moderate liming, rigorous washing of the vegetables. For young children (under 6), heightened caution.

Is a simple mulch enough to protect from metals?

No. Mulch protects from contaminated-dust splash, which is useful, but it does not reduce the availability of the metals to the roots. The fundamental solutions are amendment (organic matter, liming if acid soil) and above all physical separation with healthy soil (beds on geotextile).