UKWIR – GUIDANCE FOR THE SELECTION OF WATER SUPPLY PIPES TO BE USED IN BROWNFIELD SITES

Article Contaminated Land Laboratories by

Peter Boyd, AECOM Limited and Neil Parry, Geotechnical Engineering Limited

The UK Water Industry Research (UKWIR) has produced this document to replace the heavily criticised 2002 WRAS guidance on water supply pipe materials for contaminated land. Unfortunately there remain several similar problems with the new document. It has been produced by UKWIR with the footnote “Promoting Collaborative Research” but there does not appear to have been any collaboration or consultation with practitioners in the industry. As a result the document fails to reflect commonly accepted industry practice and terminology.

The basis of the document is highly conservative, for example making reference to identifying whether “any chemical may have ever been on a site” and that “samples should be collected at a frequency and depth that will identify any contamination”.  Although it rightly advocates a risk based approach to sampling and assessment many of the recommendations ignore this premise and current guidance, with the end result likely to be overdesign and significantly increased costs for end users.

The document was recently revised, where some of the initial errors were corrected, unfortunately several errors remain and the opportunity to gain widespread acceptance was missed.

The responsibility for the selection of supply pipes is confused between the Developer, Self Lay Organisation (SLO) and Designer.  The responsibility for the production of the crucial Site Assessment Report (SAR) is not clear between the “Developer” or the “Designer” without considering whether they have the necessary contaminated land expertise.  Guidance such as the new Eurocodes define competence roles and from an industry perspective it’s yet another missed opportunity to give some further recognition to the SiLC qualification. It seems to be aimed at the layman but also advocates a very wide ranging and unusual range of laboratory implying the user has a detailed knowledge of soil sampling, preservation and laboratory testing. It is also unfortunate that 30 years after the first edition of BS5930 the document blurs the distinction between site investigation and ground investigation.

Desk Study
The document provides some good general guidance on establishing previous site use and the potential for contamination but insufficient information or reference is provided for a general user to adequately complete this.  It suggests that the findings should be summarised on a map to show current and historic land use but show a level of detail at a scale which would not be possible in most practical instances. implying that the authors have not actually performed this exercise with real data or thought about the preparation of a robust conceptual model, which is the basis for most contaminated land assessment.

The document suggests that the Local Authority may request an SAR as part of the planning process.  There is no recognition of the fact that such an assessment could and perhaps should be incorporated into the routine pre-development desk study, intrusive ground investigation and interpretative reporting process.

Investigation
When looking at investigation the application of photoionization detector (PID) screening is meaningless without further guidance. The “suitable survey pattern” is not defined and ignores the shortcomings of PID readings. The extent of suggested PID testing could also be onerous in most circumstance.

The soil sampling section refers to an unspecified “suitable survey pattern” which is easily confused with the PID screening.  Although it recommends the use of BS10175 for more detailed information on the design of a sampling plan no specific information on sampling for proposed services is included. It makes reference to taking “a spadeful” and the use of a “plastic bag” for samples which may be inappropriate and ignore the complexities of sampling.  Investigation and sampling are assumed to be undertaken via trial pits (likely to be machine dug to achieve the recommended depths) which would be difficult in an urban situation where numerous existing services are present and may not present the best method for obtaining the best samples, particularly for groundwater.  It suggests that if groundwater is present within 1m (or 2m in summer) of the base of the intended trench then a water sample should be taken from “a suitably completed narrow borehole” but establishing groundwater depth may be difficult.

Chemical Analysis
One of the most onerous parts of the recommendations is the imposition of a mandatory analytical suite to be undertaken on all samples.  Despite the fact that a desk study and ground investigation has been undertaken, including PID screening for VOCs, it appears that there is a limited choice for the user of the document in respect of what testing is required. The suite is far from routine with several determinands not generally carried out by any of the commercial laboratories in the UK on soils. To cover the lists as provided would probably cost in excess of £300 per sample.  Notably only organic contaminants are considered with the absence of inorganics such as arsenic.

Confusion extends to the proposed testing suites. The extended VOC suite (by GCMS) contains many non VOCs such as Benzo(a)pyrene and propylene glycol, explosives such as nitroglycerine and nitrotoluene (which are analysed by HPLC), Nitrohydrochloric acid (Aqua Regia a mixture of HCL and nitric acid which again cannot be analysed as a VOC) and Naphtha which is petroleum terminology for an ill defined distillate. There are misspelled chemicals such as “Mesityl” oxide and duplicated compounds such as methyl chloroform (which is 1,1,1 TCE) and Monochlorobenzene which is Chlorobenzene.

Other suites contain similar errors. Petroleum ether is incorrectly listed under ethers. Under mineral oil the document contains a turpenoid, a plasticiser, a flavour, a fatty acid and fuming sulphuric acid with no mention of mineral oil C5-C10, C11-C20 and C21-C40 listed in the “mandatory analytical suite”. A random list of chemicals is listed under Conductivity, Redox and pH including a vitamin, food preservatives and a range of compounds that would either not be found or could not be determined by routine analyses.

The simplistic approach to the determination of redox and resistivity in disturbed samples also causes some concern. This should at least reference BS 1377 Part 3:1990 and mention the benefits of in-situ measurements. Other soil conditions, not necessarily associated with a brownfield site, may also need to be examined for classification, for example “Wetness Class” which, although are not directly related to contamination, are used in the examination of sites for existing or proposed ductile iron pipes.

In relation to chemical testing reference is made to detection limits – but no discussion on how these limits were arrived at is included. These are set at “at least 10 times lower that the screening values identified” which appears to be arbitrary.

Specification of Water Supply Pipes
The final part of the document, as expected, relates to the process of specification of pipes.  It provides a comprehensive list of standards and guidance for each of the options including ductile iron, steel, polyethylene (PE), PE Barrier, PVC and copper.  Further undefined terms which will have a major effect on the specification are included such as “light chemical contamination”

Unfortunately some of the chemistry in this part is also misleading. It gives a conversion from EC to resistivity, which is not applicable to soils as it does not take into account natural moisture content, compaction, voids or the benefit of in-situ measurements.  Redox is used as a criteria without proper reference to BS1377 or acknowledging the problems likely to be encountered with disturbed samples.

Once all of the results of the extended testing have been received, individual chemicals are summed in groups, which appears to be highly questionable considering the differences between each of them.  Further mistakes are noted on the Pipe Selection Table 3.1, below which is provided to make a final selection, notably with disagreements between these figures and those in F.4 (Derivation of ‘data-supported threshold values’ for PE and PVC).  In this table there would be no requirement for any analysis if Barrier Pipe (PE-Al-PE) is used as it passes on all counts.  It is felt that the selection of barrier pipe for all sites will be a frequently exercised option as this is suitable for all conditions, it would also negate the need for any of the desk study, analysis, site assessment and pipe selection process covered in the rest of the document.  A statement that “barrier pipes should be used for all brownfield sites” would make the whole of this document redundant.

Pipe material

All threshold concentrations are in mg/kg

Parameter group

PE

PVC

Barrier pipe
(PE-Al-PE)

Wrapped Steel

Wrapped Ductile Iron

Copper

 

1

Extended VOC suite by purge
and trap or head space
and GC-MS with TIC

0.5

0.125

Pass

Pass

Pass

Pass

1a + BTEX + MTBE

0.1

0.03

Pass

Pass

Pass

Pass

 

2

SVOCs TIC by purge and trap or
head space
and GC-MS with TIC (aliphatic
and aromatic C5 – C10)

2

1.4

Pass

Pass

Pass

Pass

2e + Phenols

2

0.4

Pass

Pass

Pass

Pass

2f + Cresols and chlorinated
   phenols

2

0.04

Pass

Pass

Pass

Pass

3 Mineral oil C11-C20

10

Pass

Pass

Pass

Pass

Pass

4 Mineral oil C21-C40

500

Pass

Pass

Pass

Pass

Pass

 

5

Corrosive (Conductivity, Redox
and pH)

Pass

Pass

Pass

Corrosive if pH < 7
and conductivity
> 400μS/cm

Corrosive if pH < 5 , Eh
not neutral and
conductivity > 400μS/cm

Corrosive if
pH < 5 or > 8
and Eh
positive

Specific suite identified as relevant following Site Investigation
2a Ethers

0.5

1

Pass

Pass

Pass

Pass

2b Nitrobenzene

0.5

0.4

Pass

Pass

Pass

Pass

2c Ketones

0.5

0.02

Pass

Pass

Pass

Pass

2d Aldehydes

0.5

0.02

Pass

Pass

Pass

Pass

6 Amines

Fail

Pass

Pass

Pass

Pass

Pass

Table 3.1: Pipe selection table

Conclusions

Although the document recommends a staged process of desk study, investigation, assessment and specification there are several areas where it is far from satisfactory.  Lack of suitable detail, ignorance of current guidance and an unwieldy and expensive approach to chemical analysis has made the process of selecting suitable pipe materials almost impossible.

Given the potential complexity and cost of the investigation and analysis to fulfil the requirements of the document it is likely that developers and specifiers will often take the simpler approach of always using barrier pipes in brownfield sites when there is any possibility of contamination.  This will be the case in most existing domestic plots and extensions (where the presence of a garage or garden shed would lead to the onerous investigation procedure) and may in turn lead to barrier pipes being unnecessarily specified.  It is also possible that the replacement of lead water pipes will be prevented by the higher costs caused by following this guidance.

We would recommend that the document goes through a further period of consultation including commercial laboratories, consultants and industry groups (such as EIC, NHBC and AGS).  The limitation of desk studies and PID screening should be added and more guidance and reference on the investigation, preparation of a conceptual model and provision of competent personnel given.  A more flexible approach to analysis should be taken, relating the testing to the previous site usage.  The selection process should also be made simpler, making the choice of other pipe materials more likely.