Helen Culshaw, Senior Advisor from the Environment Agency’s Permitting Geoscience Operations Team (GOT) explains how the associated Hydrogeological Risk Assessments (HRA) are audited, and common queries that arise.
This article is part of a series and should be read with the Environment Agency advice on commonly raised questions for preparing water environment Conceptual Site Models for inert waste deposit permit applications.
“What is a risk assessment and when does it need quantitative modelling?”
After looking at the Conceptual Site Model, the next element we audit in a submission for a waste deposit Environmental Permit is the Hydrogeological Risk Assessment (or groundwater risk assessment).
Risk assessment is a staged approach to answering the question “What are the potential environmental consequences of the placement of the waste, and the likelihood of this happening?” This should mean that the greater the risk to the water environment, or the more uncertainty there is about the risk, the more complex the risk assessment is likely to need to be.
We often find that the assessments submitted for inert landfills or deposit for recovery operation applications jump straight into the highest stage of complexity (Detailed Quantitative Risk Assessment), frequently using modelling tools that are inappropriate for the site setting, without having completed the first stages needed to identify the actual risk posed by the waste proposed. Using a tool that is too complicated or the wrong one for the site setting leads to more issues when auditing the HRA. All input values for models must be fully justified and we find that the use of complicated models leads to more questions and a lengthier determination time.
The first stage of a risk assessment should always be a risk screen to identify what constituents of the deposited wastes pose a potential hazard. If the wastes proposed do not require testing (I.e. they are from a single well characterised source and carry no risk of contamination), then quantitative modelling may not be required at all. Where testing is required, then comparing the source concentrations against water quality standards and the natural background groundwater quality (usually defined as the Environmental Assessment Level) should be the first step, as it will identify substances that may pose a risk. We have had quantitative risk assessments use a source term that has a lower concentration than the water environment, which makes provision of a quantitative model pointless as there is no real risk to the environment from the waste to start with.
In choosing an assessment level where the groundwater is already contaminated this must use the water quality standard, as discussed in the article Environment Agency advice on commonly raised questions for preparing water environment Conceptual Site Models for inert waste deposit permit applications. 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 under Schedule 22 of the Environmental Permitting Regulations (2016).
A risk screen can be supported by basic dilution or attenuation calculations; this is described in the Green Leaves document (Defra, 2011) as a Semi-Quantitative risk screen as it involves some calculations. If these conservative calculations, which can be carried out in simple excel spreadsheets, show little likelihood of pollution this may be sufficient to answer the question posed within the risk assessment.
The further levels of risk assessment incorporate a full quantitative assessment, where more complicated calculations are undertaken to work out the pollution risk to groundwater. The Generic Quantitative Risk Assessment (GQRA) uses conservative worst-case data and assumptions. This level of assessment can also sometimes be undertaken in excel spreadsheets without needing to use a more complicated software tool.
The choice of a worst-case value for all inputs to a model can also be complicated, as an individual input may give opposing effects on different results. For example, a high hydraulic conductivity can result in a high dilution within the aquifer giving a lower concentration, but a faster travel time, which could generate a higher concentration at a receptor where less biodegradation occurs. As such the GQRA should look at the sensitivity of the results to different input values where the choice of a worst-case is complicated. Where site specific input values are known these are important to include rather than using unrealistic values from literature sources.
Finally, if a worst-case GQRA assessment isn’t sufficient Detailed Quantitative Risk Assessment (DQRA) is undertaken. This would use ranges of realistic input values to represent the site setting and address the uncertainty in these values. The tools that do this level of assessment usually employ a probabilistic approach, whereby the output gives a certainty percentage to the result value. We look for a 95% certainty to be provided in results. Where DQRA is undertaken we expect the HRA report to demonstrate how the ranges applied to the input values have been derived.
The site setting and the waste type to be deposited will impact what level of risk assessment may be required. We have collated a table (see Box 1) that gives a guide to the level of risk assessment that we expect may be required to provide the necessary confidence to answer the risk assessment question under different situations. The specifics of your site conceptual model may mean that more complex assessment is needed than identified in the table.
“What software tools should be used?”
When a software tool is used to undertake any calculations, the assumptions that these models make must be shown to be appropriate for the site setting and design. For example, both the Environment Agency supported LandSim and ConSim software tools have a set conceptual model that is for waste to be placed above the groundwater table and therefore will not necessarily give reliable results where waste is to be placed below the water table. The hydraulic containment spreadsheet was developed for those landfills that operate by managing their leachate level lower than the external groundwater level. For most inert sites this practice is not undertaken and so the tool is unlikely to be applicable, without a very clear justification for it.
“But I am not going to accept non-inert material”
The assessment will also need to cover the risk of accidental acceptance of non-compliant wastes, as discussed in the article Environment Agency advice on commonly raised questions for preparing water environment Conceptual Site Models for inert waste deposit permit applications.
We have suggested ranges of suitable concentrations that could be considered in an assessment of accidental acceptance of non-inert material (see box 2). These are based on a limited data set from monitoring carried out of leachate quality at these types of inert site. Should you as the operator wish to undertake any programme of leachate monitoring within your site to produce a more suitable concentrations for use in any such assessments, the Environment Agency would support such initiatives. We may require such monitoring in a permit if a site is in a very sensitive area to validate any assessment provided.
What makes a risk assessment report easier to audit?
When we determine an application, we must back up our decisions by referencing an applicant’s report, which can be requested from the EA under freedom of information. Therefore, the presentation of a clear concise report makes it easier for everyone to see how decisions have been arrived at. Reports need to be focussed on the application task in hand referencing relevant data and the appropriate EPR regulations. We need the data and detail that supports the application but not everything that consultants think is interesting! We don’t need every page from desk study information sourced from database search reports, or pages of model result printouts where graphs of the results are sufficient.
It is good practice to list each component of the Conceptual Site Model that is represented in any models produced and explain how these are dealt with. We expect that borehole logs from any site investigation are provided, to give the evidence for the hydrogeological properties of the CSM pathways.
The report needs to document the basis for the decisions made at the various stages of the risk assessment. It will need to explain how the CSM has been translated into any models provided and discuss any simplifications or assumptions that had to be made. All input values used need to be justified, and explained, and the calculations used presented. Where graphs or statistics are provided, we do need the electronic excel versions to allow us to interrogate the data and justifications, a PDF version will not allow a full audit.
The information submitted should allow the work to be audited and, if necessary, reproduced. Models may not be accepted, or further documentation may be requested if the supporting information is inadequate. In many cases this could significantly increase costs and timescales on projects (A McMahon et al, July, 2001).
Summary
The submission of a clear and concise hydrogeological risk assessment that provides justification for the level of assessment applied to answer the question “What are the potential environmental consequences of the placement of the waste, and the likelihood of this happening? should enable a shorter application determination time. Any software tool used should be suitable for the site-specific situation and it is not always necessary to provide a complicated model which can lead to more questions on how input values are justified and applied.
Further useful information can be found within the following:
- Groundwater risk assessment for your environmental permit
- 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.
