Contaminated site management - step 2: preliminary investigation

The need for monitoring and remediation are clarified

The preliminary investigation underlies the evaluation of a polluted site

Data necessary for determining whether a need for monitoring or remediation exists is collected during a preliminary investigation. In the first step, the so-called historical investigation, a search is undertaken for possible causes for the site's pollution. The, as a rule, necessary technical investigation then divulges the potential impact of the site on water, soil or air.

The second step takes up the investigation of a site to determine the pollution level with an eye to its potential environmental hazard: Does the site, although polluted, nevertheless present no long-term environmental danger and can it be retained in the register without any further clarification? Is it polluted and in need of monitoring and appropriate classification in the register? Or is it polluted to such an extent that it is a hazard to the environment and must be remediated as a contaminated site?

Based on the results of the preliminary investigation of a site the authority decides on:

  • pollution without hazard for the environment
  • the need for monitoring
  • the need to remediate = contaminated site

In certain cases the preliminary investigation also reveals that, contrary to expectations, the site is not polluted with wastes at all and can thus be omitted from the register.

Groundwater and surface water:

Groundwater is the most frequently affected protected natural resource. A need to monitor exists in this case where the pollutants leached from the polluting material (leachate, seepage water) at the site exceeds specified concentrations, or where pollutants from the site are already detected in the groundwater. The same holds true for surface waters. In principle, no need for action exists where the current and future seepage water from a polluted site can be safely used as drinking water. Correspondingly, the concentrations listed in the Contaminated Sites Ordinance are oriented as much as possible toward the Swiss food legislation.

In the case of groundwater and surface water, a certain degree of pollution is even tolerated as regards the need to remediate a site. For example, with regard to groundwater, the need to clean up a site starts only where measurements immediately adjacent to the site show that the maximum concentrations listed in the Contaminated Sites Ordinance have been exceeded: The vulnerability of the natural resource to be protected is to be taken into account (is this groundwater useable or not).

Regulations are strict for the protection of groundwater wells in the public interest: No pollutants whatsoever are tolerated here; the need for remediation exists as soon as pollutants from the site can be detected in groundwater wells. After all, who wants to drink solvents!

Additionally, sites are to be remediated if they are in need of monitoring and if they present a substantial danger of contaminating groundwater or surface water. A substantial danger exists where no effective barrier or degradation process prevents the protected natural resource being affected by intolerable pollutant levels.


By no means does all polluted soil (defined as the unsealed top layer of earth in which plants can grow) fall within the scope of the contaminated sites regulations. Only polluted soil within a restricted area and whose pollution comes from clearly defined source is meant.

This may be the case for a heavily polluted soil around a factory smokestack, or where substances that are hazardous to water could seep into the soil due to their storage or transfer. On the other hand, soil with broadly diffuse sources does not fall within the scope of the Ordinance (e.g., along motorways), nor does soil where the substance is put to a specific purpose over a wide area (e.g., copper sulfate used in wine grape cultivation).


It is also possible that toxic or undesirable gases from polluted sites end up in the air where people regularly gather, such as in apartments, basement rooms or construction excavation. In such cases the need to remediate exists if the pollutant concentrates of the interstitial air at the site exceed specific threshold values. The principle holds true, that what applies to the working places also applies to the basement hobby room. The air values of the Contaminated Sites Ordinance are based on the Swiss health and safety regulations (SUVA).

Seriously undesirable odours or particle emissions can emerge from polluted sites as well. In such cases the Contaminated Sites Ordinance refers to the stipulations of the Clean Air Ordinance (CAO) and the requisite implementing ordinance.

Further information


Aids to enforcement


TransSim 2.1 (ZIP, 32 MB, 18.06.2021)Mathematisches Simulationsmodell zur Abschätzung des Schadstofftransportes in der ungesättigten Zone bis zum Eintritt in das Grundwasser. Hilfsmittel für Altlastenfachleute.

PlumBumRisk 1.0 (ZIP, 736 kB, 30.03.2012)Excel-Tool zur Gefährdungsabschätzung bei Schiessanlagen.

Last modification 10.02.2021

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