Leachability testing

Few people ever pass a development site and think: ‘I wonder what type of leachability test was carried out here?’ But then, few people outside the construction and infrastructure industries have the slightest idea what leachability tests are.

Leachability tests are a key area of environmental work. Such tests are carried out on proposed building sites, many of which are brownfield sites that have been built on previously. An initial site investigation and assessment is made of the scope and extent of contamination. Then, referring to the site conditions, proposed use and geology, the risks of possible mobility of contaminants and the potential impact on groundwater/controlled waters are identified. This is when the leaching tests become important. When a house developer plans to build on a plot of land where contamination is suspected, he will generally have the plot investigated to ensure that the land is suitable for its intended use and to provide information to support his planning application. A key aspect is to examine the risk of harm to relevant receptors, among which is the potential impact on groundwater, which could in turn represent a health hazard to users of the water.

Contamination reports

It is normal planning law that such contamination reports are required. Similarly, where remediation has taken place on the site, compliance with appropriate quality standards is also checked during development and certainly prior to the commencement of building works. When a developer purchases land for building, a clear timeline is followed. With building permission granted, the site is investigated by environmental consultants and a report is prepared on the land. The consultants examine the land and its geology, while considering contamination issues as well as the site’s environmental setting and its potential relevance to controlled waters including groundwater. They create a mathematical model which looks at how, for instance, water may be affected by contamination and what route the water may take off the site (called a Source Pathway Receptor model). A specialist company, meanwhile, is asked to give an analytical breakdown of the soil to see if it contains such potential hazards as metals, petroleum hydrocarbons and various chemicals including poly-aromatic hydrocarbons and cyanides. Each site’s potential contamination is assessed based upon its prior use, and appropriate targeted analysis of potential pollutants is then performed.

Crucial analysis

That analysis, coupled with a leachability test, is crucial in deciding whether the site needs to be cleaned up or ‘remediated’. If necessary, a remediation contractor is brought in to design and carry out the cleaning operation. On the face of it leachability tests are a tiny cog in the entire process, but they could not be more important. There is significant pressure in the United Kingdom to build new homes, many of which are erected on brownfield sites due to previous targets to bring such land back into beneficial use and the new National Planning Policy Framework which encourages the re-use of land. Developers who fail to meet safety requirements in preparing their sites, which could previously have been the homes of petrol stations, dry-cleaning businesses or chemical factories – could be faced with massive financial liabilities. Many environmental consultants and developers are unaware of the variety of leachability tests available, and should inform themselves better before making such crucial decisions. A leachability test essentially means bringing soil samples in contact with water and measuring what contaminants are released or ‘leach out’ into the water.

Standard batch test

The standard batch test most commonly used is covered by the Environmental Agency’s British Standard BS EN 12457. It is quick to do and widely available. It specifies that granular soil, with particles no bigger than 4mm, is put into a type of bottle also containing water at a 2:1 liquid-soil ratio. The receptacle is then turned over and over for several hours. At the end, the liquid, or leachate, is drawn off and analysed for contaminant levels. The batch test can then enter a second stage, where the liquid-soil ratio is 8:1, or you can do a simple one-stage test with a 10:1 ratio for up to 24 hours. Clearly, the soil particles rub against each other quite a lot during testing. In this sense, it is aggressive in nature and the results are likely to be an overestimate of what would leach out of the same sample of soil if it were left lying in the ground without such abrasion. Why does this matter? Because the European Parliament and Council’s Environmental Quality Standards Directive (EQS), published in 2008 and which lays down the standards for priority substances and certain other pollutants in order to achieve good surface water chemical status, is stringent in its requirements.

Concern and difficulty

Of course, if the batch test reveals no more than a minimal contamination risk then there is no problem, but what we frequently find is that BS EN 12457 produces results which, by being overstated, cause concern because they fail to meet the forecasts of the site’s conceptual model. It could mean a project being badly delayed – and time is money – or even, in rare cases, being shelved. It could also mean developers feeling obliged to undertake expensive remediation measures to clean up the site when it is not in fact necessary. Some consultants, however, realise the limitations of this test – which was initially developed for waste classification and not for environmental uses – and are increasingly aware of the alternatives, especially when dealing with sites with significant or complex contamination issues. Alternatives include tank testing and up-flow percolation testing, but many laboratories do not offer these tests. The tank test is quite a long test and can last from anything between four and 64 days. A relatively new test, it is more realistic and generally provides a lower figure than the BS EN 12457 test. When testing, a soil sample is taken that is essentially ‘monolithic’, meaning it has a fairly solid, clay-type consistency, rather than being broken up into small particles. It is then immersed into a tank, where it is left without being agitated or turned.

Hard to argue against

Only the surface of the material interacts with the solvent that leads to the leaching. This better simulates what would happen on-site –and is more accurate in analysing how much remediation will be required. It’s very hard to argue against, from a scientific point of view. It makes sense. The up-flow percolation test, meanwhile, is used for situations where the soil is looser and made up of smaller particles like sand or pebbles, as can often be the case on industrial sites. Here, the soil, granular enough to allow liquid to pass through it, is confined in a column-shaped receptacle. Water is slowly pumped through at a rate of a few centimetres a day, thus simulating groundwater or rainwater gradually working its way through the material. This process can last around 30 days before the leachate is analysed for contaminants. Despite the greater sophistication, accuracy and usefulness of these latter two tests, there is very little official guidance available on the issue.

New technologies, new science

Even less laboratories offer the up-flow test than the tank test – it’s a question of development. This is a new area so there is less demand from the market at the moment, but we have always been committed to developing new technologies and new scientific methods – it’s our mantra. We firmly believe there should be more official guidance to clarify the difference between leachability tests. While the batch test is covered by BS EN 12457 there is no other British Standard or guidance document produced by the Health and Safety Executive or the Environmental Agency that really shows how you relate the tank test to work on site. BS EN 15863 does describe the tank test, but that document was only published in 2015 and relates more to waste than soil. There is some further guidance on the tests available, by the document BS EN ISO 18772, which discusses the difference between characterisation and compliance testing, but we have rarely come across clients who know about it. DEFRA (the Department for Environment, Food and Rural Affairs) has made things easier for people to interchange information and knowledge by helping to develop the idea of conceptual models so that everybody is on the same hymn sheet, but now, without further official documentation, we are in an area where there is no sheet to sing off. We think greater awareness and understanding should be promoted within the industry.

Most economic solution

The need for more sophisticated leachability testing won’t apply in every situation, of course – each case is different. We do think, however, that developers and consultants should be asking themselves one critical question: Are you doing the right leachability test to help you ensure that you produce the most suitable – and economic – solution to the site you are trying to remediate? The testing and remediation industry is an exciting place to be. Things have changed constantly over the years and continue to do so, not only within the field of testing processes but also in remediation, in environmental matters and in the wider industry’s health and safety considerations. Where once contamination parameters and thresholds were applied by rule of thumb and across the board, now each individual case is considered and investigated on its own merits. Remediation techniques have continued to develop, as has the associated legislation. In the past, if a contaminated area was identified on site, contractors would simply ‘dig and dump’ – dig out the contaminated soil and dump it in landfill. Today that approach is regarded as unsustainable, environmentally unfriendly and, with the introduction of new tax regimes, uneconomic.

New ways of working

That has led to new, novel ways of working, such as soil washing, vapour purging and bioremediation, where hydrocarbons are destroyed on contaminated sites by the encouragement of biological activity in the soil through the addition of air, heat and various fertiliser, organic matter and enzymes. Bioremediation can take a long time, so another new idea called ‘encapsulation’ is becoming more and more popular. Put in simple terms, the soil is stabilised by the addition of a cementatious-type substance(s) as well as other ingredients that turn loose, granular waste into something like a stiff clay, which helps to seal off the contaminants and limit leaching to acceptable levels. Published: 11th May 2016 in AWE International