Helen Culshaw, Senior Advisor from the Environment Agency’s (EA) Permitting Geoscience Operations Team (GOT) explains how they audit permit applications for the deposit of inert waste that could impact the subsurface environment and answers commonly raised queries from operators for these deposits.
When we pick up an application the first element looked at is the Conceptual Site Model (CSM) proposed. This will underpin a Hydrogeological Risk Assessment (HRA). The CSM describes the site and its environment, both natural and man-made in four dimensions, including time.
“Conceptual models present the hypothesised relationships between the source (S) of a hazard, the pathways (P) by which exposure might occur, and the receptors (R) – those features of the environment that we value and that could be harmed” (Green Leaves III, Defra 2011). These are known as the Source – Pathway – Receptor linkages and inform the risk assessment required.
Where a CSM is flawed, through uncertainty in any of the linkages or from insufficient site investigation, it can have a significant impact on any risk assessment undertaken and therefore the time it takes to audit and determine an application. GOT are screening application documents early as they arrive with the EA to identify where it would be in the applicant’s interest to withdraw their application and undertake more work to reduce the CSM uncertainties.
The CIWM ‘Landfill: Understanding the Conceptual Site Model’ virtual course provides detailed step-by-step guidance on developing a Conceptual Site Model (CSM) as well as equipping learners with comprehensive guidance on how to maintain a systematic and sustainable approach to environmental management of landfills.
The Source
The source for these permits is the inert waste proposed to be deposited, either in an inert landfill or recovered under a deposit for recovery operation.
“My site won’t contain or create leachate because it is inert”
Unless you have a completely impermeable surface above your waste deposit for all time this is unfortunately not true.
Leachate is “water that has percolated through a solid and leached out (or removed) some of the constituents” (Google Oxford Languages), and the definition of inert is “waste that does not undergo any significant physical, chemical or biological transformations…. The total leachability and pollutant content of the waste and ecotoxicity of the leachate must be insignificant, and in particular not endanger the quality of surface water and/or groundwater“ (Landfill Directive Council Decision, 1999).
So, while some water will enter the inert waste deposited, any leachate produced should by definition be of a quality that will not cause any impact on the receiving environment.
“I’m going to just apply the landfill directive inert waste acceptance criteria to my procedures”
This is a common proposal in deposit for recovery applications, where testing of the waste is required. These are the waste acceptance criteria (WAC) in the Landfill Directive that set concentration limits that landfills designed for inert waste can accept (Landfill Directive Council Decision, 2002).
The Landfill Directive inert WAC were developed using a specific conceptual model assuming the presence of a 1 m thick attenuating layer (or geological barrier if you are talking about a landfill) of permeability 1 x 10-7 m/s. This model assumed the site was located above the water table. The criteria were developed before arsenic and lead were reclassified as hazardous substances (more on this later) in 2016.
Therefore, depending on your site setting and design just relying on the inert WAC for landfill as your WAC may not be sufficient to show that your proposal will not have the potential to have an impact on the water environment, including groundwater.
For deposit for recovery permit applications, it is the applicant’s responsibility to show that the WAC they will use will not cause pollution. Therefore, the upper limit proposed must be assessed in the HRA and shown to be appropriate.
“I am not going to accept non-inert material”
At even the best run sites there is always the potential for accidental acceptance of non-inert material. This may also occur if the producer has misrepresented their source to you [Picture 1]. There is an increased risk of acceptance if your site is also treating non-inert waste from mistakes made in the transfer of any resulting wastes for deposit. When a site is in a sensitive area (Box 1 – see: landfill operators: environmental permits: plan the environmental setting of your site.) an application needs to justify that the risk of non-compliance can be remedied before pollution is caused.
From our observation of landfill sites taking inert soils,
we have found that the three most mobile contaminants that could leach from deposited soils are sulphate, ammoniacal nitrogen and nickel (all non-hazardous pollutants). We would expect the input of non-inert waste to be considered in an HRA for these substances, along with at least one of either arsenic or lead to cover the fate of hazardous substances. Further information is provided in our article advice for commonly raised questions on completing Hydrogeological Risk Assessments for inert waste deposit permit applications.
The Pathways
The pathways in a CSM are all routes by which pollution could travel away from the site. These could be natural through the underlying geology and groundwater, or over the ground surface, or, man-made through engineering or other structures that could be present such as culverts.
“I have put three boreholes in”
Developing the CSM and undertaking site investigation to reduce uncertainties is an iterative process. The reference to three boreholes is the absolute minimum you would need to work out the groundwater flow direction and is the minimum prescribed in the Landfill Directive. This is to give one measuring point in the in-flow (up-gradient) region and two in the down-flow (down-gradient) region. There are sites where there is groundwater flow in more than one direction, and this would not be picked up with only three boreholes [Picture 2]. This can mean that an assessment doesn’t pick up the pathway to a more sensitive receptor, such as a surface water body or spring. We have also had sites where the three boreholes are installed in a single line, or no boreholes at all, which will lead to even more uncertainty.
Sufficient site investigation must be undertaken to confirm what the groundwater flow directions are at sensitive sites. If uncertainties remain when developing the CSM these need to be remedied with further phases of investigation before embarking on the risk assessment. Scrimping on site investigation at the beginning of the process of trying to obtain a permit is a false economy. If additional site investigation is needed during the determination of a permit this can prolong the process, and we are likely to return an application.
“Do I really need a year of monitoring data?”
You must be able to show that you have taken account of the seasonal variation at your site, as well as any potential effects on the groundwater throughout the life of the activity. This means that you must have representative monitoring rounds for seasonal high-water levels and seasonal low water levels. Gathering a year of monitoring data is the easiest way of ensuring both these are covered. It is also in your interest to make sure that you have suitable knowledge of the variation in the quality of the water environment at your site when proposing compliance limits. It will be your risk where a full year of data is not provided, and in this case, permits are generally issued with pre-operational conditions to collect more data to ensure that the water environment is protected, and appropriate compliance limits set. We have generally said that four months of data is the minimum we would expect when an application is provided for a new site. For variations we would usually expect presentation of two years of data to back up changes.
We often see a lack of suitable groundwater level contour plots provided within applications. The plots must be developed from representative monitoring rounds for seasonal high-water levels and seasonal low water levels. Often plots are presented using average or maximum water levels for each borehole, which may not be representative of groundwater flow at the site at any given point in time. This can impact the groundwater flow direction identified in the CSM and therefore which receptors could be affected by the deposit activity.
“I’m going to be using enhanced engineering”
The Landfill Directive places specific requirements on the engineering properties of the geological barrier. For deposit for recovery there are no specific legislative requirements for the attenuation layer. The hydraulic conductivity of the engineering needs to be shown to be appropriate through the hydrogeological and any stability risk assessments.
The hydraulic conductivity required by the landfill directive of 1×10-7 m/s for a 1 m thickness, is relatively permeable, and will generally allow all water (leachate) that reaches the base of a landfill or waste deposit to drain through, albeit at a potentially slower timescale than the underneath geology, attenuating contaminants. This should mean that any boreholes installed in the waste mass are generally dry.
However, if a good quality clay or other material is placed such that a lower hydraulic conductivity, such as 1×10-9 m/s or better, is achieved (which may also reduce further once compressed by the waste above) then less leachate will be able to pass through the barrier/attenuation layer leading to the potential for a build-up of leachate within the waste. The water balance of the rate of infiltration of water to the waste, and the permeability of the barrier/attenuation layer will govern the volume of leachate that will build-up and whether it might breakout at the surface.
Sites that propose to use a good quality clay or other material to provide their protection engineering must demonstrate that the impact of a leachate build-up to any sensitive receptors has been assessed in the HRA. This should reference a clearly explained water balance calculation, which can be more complicated if the site design is below the surrounding groundwater table level.
” I’ve been told I can’t dewater my site to place the inert waste in a dry environment”
This isn’t necessarily a showstopper to the inert waste activity, but the application must justify why dewatering cannot take place. The published guidance landfill operators: landfills for inert waste) includes the steps that must be undertaken to ensure that the site (whether a landfill or deposit for recovery) will not cause an unacceptable discharge to the water environment. We find applications are often missing the justification for why dewatering cannot take place and how the increased risks from this method (due to acceptance of non-compliant wastes) will be managed.
If you are wanting to use inert waste material to construct the required geological barrier or attenuation layer this needs to be shown to be of an appropriate chemical quality. This must consider the presence of any hazardous substances within the waste. [Box 2]. Hazardous substances should not be confused with hazardous waste or the hazardous properties used to classify waste. Any input of hazardous substances must not be discernible in the groundwater, so the WAC for these substances must be shown not to exceed the natural background quality or the minimum reporting values.
Even if you are dewatering prior to constructing your site, the impact the waste to be deposited could have on any discharge of water to achieve this must also be detailed and discussed within the CSM and HRA.
The Receptors
Receptors are the humans or environment that could be affected by any pollution from the source that reaches them through the pathways. For a HRA, we are looking at any groundwater beneath the site and any sensitive surface water features.
“My site isn’t in a drinking water groundwater Source Protection Zone”
The Environmental Permitting (England and Wales) Regulations (2016) incorporated the requirements of the Water Framework Directive and the Groundwater Directives into the Schedule 22 Groundwater activities. These require permits to prevent the input of hazardous substances to groundwater and to limit the input of non-hazardous pollutants to groundwater to ensure these do not cause pollution of the groundwater.
While a site might not be in an area designated as a Source Protection Zone (SPZ) for groundwater supply if the site is located on an aquifer or below the water table there is still a requirement to protect the groundwater. This means that there cannot be an input of hazardous substances that is detectable (discernible), or pollution caused from the input of non-hazardous pollutants. There may also be private drinking water supplies in the area that are not covered by a designated SPZ.
“My site sits on a non-aquifer or an already contaminated site, so I don’t have any receptors of concern”
While there may not be an underlying groundwater receptor at your site, there may be a risk of surface flow to any sensitive surface water receptors from any build-up of leachate. This will need to be addressed in the CSM and HRA.
If the groundwater beneath your site is already contaminated this does not mean that it isn’t a receptor of concern. You cannot add more contamination that would prevent the quality improving in the future, and you will still have to prevent the input of hazardous substances to it. In addition, the groundwater may form a pathway to a more sensitive surface water receptor that will need to be assessed.
“But the consultant has assessed the risk to the Principal aquifer that sits underneath a low permeability unit”
While a low permeability unit is likely to provide protection to a Principal aquifer below, there may be lower class aquifer, such as a Secondary A aquifer above the low permeability unit that would be a more sensitive receptor to the waste deposited. The presence of springs in the area around the site can be an indicator of this as they are surface expressions of groundwater. These must be shown to be protected for the same reasons explained above in relation to a site being outside a SPZ. You may even be below the groundwater table in this aquifer. We have had applications that have not made any assessment of these near surface aquifers.
Summary
The CSM is key to any permit application to deposit waste as it informs the assessment that must be undertaken to show that the deposit will not cause pollution.
Where a CSM is flawed it can have a significant impact on the time it takes to audit and determine an application.
Further useful information can be found within the following:
- Landfill operators: environmental permits – Plan the environmental setting of your site
- A McMahon et al, July, 2001. Guide to good practice for the development of conceptual models and the selection and application of mathematical models of contaminant transport processes in the subsurface.
If you have specific technical queries for your application after reading this article and the published guidance you can request pre-application discussions at Get advice before you apply for an environmental permit.
