Thickness Design of Cover Systems for Contaminated Land: BRE/AGS/NHBC: 2004

Starting in 2017, the AGS Contaminated Land Working Group (AGS CLWG) undertook a review of the 2004 BRE/AGS cover systems document to determine:

a) if it was still fit for purpose given current regulatory practice and guidance,
b) what awareness of it there was within the industry and with regulators,
c) how much it was being used and accepted by regulators as evidence,
d) what was current general practice,
e) if the document and software required updating,
f) if it might be applicable or adaptable for sites with trace levels of asbestos.

As part of this review, several questionnaires were forwarded to regulators and private industry practitioners and awareness was raised at several industry events as well as within the various AGS working groups. In conclusion the results of this review can be summarised as follows:

The document is still considered technically fit for purpose, though the fact that it specifically excludes considerations of the emplacement of break layers and water table movements somewhat limits its applicability under current practice at present. That it has not been updated or reviewed since it was published in 2004, and whilst still available is quite expensive, also probably somewhat limits its use by regulators or practitioners.

The document has suffered from a general change in practice since it was first published and perhaps more importantly a change in the way waste is currently defined and perceived in practice during developments. This is (in part) due to regulatory waste changes and interpretations as well as changes in CDM. This is seen as being two-fold in practice, firstly in that the placement of materials from off-site as cover can now potentially be considered a de-facto waste action. Secondly, the technical mixing of potentially contaminated materials in situ with uncontaminated material is contraindicated by current guidance, being seen as potentially “diluting” the contaminants in the underlying materials. This paradox, since the underlying material is probably not technically a waste per se, has certainly in part limited the uptake of this methodology by both practitioners and regulators.

In addition, the responses clearly indicated that many practitioners and regulators were unaware of this document and its potential uses, possibly due to institutional loss of knowledge and experience over time since publication, as well as changes in general contaminated land and waste practice. In addition, it would appear that not many practitioners were presenting such evidence or systems on behalf of clients or developers, probably for essentially the same reasons.

It was noted in the responses that many regulators now default to generic requirements for cover systems. The usual default being 600mm or “Two Spades depth” and usually overlying a break layer. This later element was seen by most regulators and some practitioners as being more effective, acceptable and risk averse in that a break layer provides added security, i.e. by providing an obvious pathway linkage break. Such actions also avoid having to consider potential mixing issues and therefore complies more justifiably with current contaminated land exposure models, remediation practice and guidance by “removing” a pathway rather than just mitigating it.

Unfortunately, this trend may not be truly more sustainable or cost effective as the associated costs and materials used in providing break layers are not always going to be as sustainable as not doing so or accepting lesser depths of cover. Practitioners may therefore logically be taking this lead from the regulators and therefore not considering using the document and methodology on behalf of their clients, even if they are aware of it. This is because the associated costs of justification and arguing the case in a scientific and robust manner may be more than simply accepting emplacement of a break layer and provision of the full cover depth required by the regulator.

On the basis of the above findings, it is considered that current policy and lack of awareness make this a rarely used methodology in current practice. The document and software could be updated relatively easily to make them more user friendly as the general underlying science is still considered current and relevant. However, the general lack of enthusiasm noted for its use in light of the issues highlighted above (especially the general regulatory trend towards the use of generic depths and break layers) probably do not justify the associated costs of doing so at this time.

In conclusion, the basic science is still considered very sound, and the document would benefit from a wider awareness in the community and amongst regulators were it not for the tendency towards the more risk averse inclusion of break layers and standard depths. This has perhaps been a quite reasonable response to changes in regulatory frameworks since the document and software were first produced in 2004 and also probably reflect a general loss of institutional knowledge regarding the methodology over time.

There remains a potential for the methodology to be resurrected and updated for application in solid contaminants such as trace asbestos in soils, though sadly the supporting science is not currently available to justify such works in other than a speculative way.

On this basis the position of the AGS is that updating of this document is probably not sustainable or cost effective at present, but may be so in the future if:
1. Regulatory attitudes change,
2. Practitioners feel it still has utility in certain circumstances,
3. It has greater awareness within the community,
4. It can be adapted to meet additional and scientifically justifiable uses.

Article contributed by AGS Contaminated Land Working Group