Naturally Elevated Levels of Arsenic within Early-Mid Jurassic Bedrock

Geo-Integrity Ltd. recently undertook a Phase I Desk Study and Phase II Ground Investigation at a proposed residential site near Broadway in the Cotswolds AONB. The desk study identified the site was directly underlain by the Dyrham And Marlstone Rock Formation, which is often encountered as sandy mudstone with interbeds of silt, fine-grained sand and impersistent beds of ferruginous limestone and ironstone.


Naturally elevated amounts of arsenic are often present within the Dyrham and Marlstone Rock Formation. Arsenic is a non-metallic element which is strongly associated with sulphide minerals and its toxic character is well known. Naturally elevated levels occur within some geological units including Early-Mid Jurassic bedrock which contain ironstones that are known to contain elevated arsenic concentrations compared to other soil and rock types. This does not imply that these soils pose a significant risk to human health, but that further assessment should be undertaken to determine whether there is the possibility of significant harm.


As such, as part of this investigation it was recommended and agreed bioaccessibility tests would be undertaken on representative near surface samples in the form of Physiologically-based extraction testing (PBET) to evaluate the bioaccessible fraction of arsenic within the underlying soils. The PBET attempts to simulate what occurs in the human stomach and gut by digesting the soil in a mixture that simulated typical stomach contents, such as acids and enzymes, at body temperature.


An analysis of the encountered arsenic levels was then undertaken using the CLEA analysis tool to determine a site specific assessment criteria (SSAC) level of the arsenic at the site, factoring the exposure to the oral ingestion exposure pathway. The analysis indicated the recorded arsenic levels fall below the SSAC and therefore poses no significant risk to human health.


Without the above PBET testing the recorded arsenic levels at this site were elevated above the Generic Assessment Criteria Values  (GAC) and as a result further remediation methods would have been required costing the client unnecessary additional money and time.



Here at Geo-Integrity we aim to provide professional advice to protect the end-users of the site in a cost-effective manner. If you have a planning condition on one of your sites which we could advise on please contact us on 01280 816409 or drop us an email.

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