Tanyard Lane, Part 2A Vapour Intrusion Investigation by Campbell Reith
Shortlisted for Best Research or Application in the Advancement of Science, Technology or Digital Innovation in the Brownfield Sector
Summary
Detailed site investigation and assessment has been carried out for a former dry cleaners to determine whether identified contamination constitutes a Part 2A determination due to vapour risk to neighbouring residents. Due to the site’s complexity, a detailed conceptual site model was set up to identify the key data uncertainties and inform the investigation. The subsequent investigation comprised: installation of multi-level wells to enable groundwater and soil vapour sampling from specific depths and to identify the potential for DNAPL; shallow vapour wells; and in-house sampling over several months using passive diffusion tubes, sub-slab sampling via vapour pins, and measurement of differential pressure through the floor slabs. The scale/complexity of works are thought unprecedented in the UK and provide new data on several aspects of vapour intrusion (VI) assessment that can be used by the industry. The works were successful and confirmed the absence of risk, providing much needed reassurance to the residents. Risk to groundwater also required assessment but is not covered in this presentation.
Introduction
The brownfield site in Ross-on-Wye, Herefordshire, is owned by Persimmon. Historical investigation works by others identified chlorinated hydrocarbons (CHC) contamination of soils and groundwater due to the site’s historical use as a dry cleaners. A groundwater plume of CHC travels roughly southward from the site, therefore migrating beneath the properties of an off-site residential estate to the south – this is shown on the figures 1 and 2. Geology comprises fractured sandstone and fracture flow predominates groundwater movement, with the potential presence of DNAPL also noted.
On-site soil vapour concentrations of perchloroethylene (PCE) and trichloroethene TCE) were greater than 600 and 5000 times the calculated target indoor air concentrations (TIAC - obtained directly from the air quality threshold value used in the derivation of C4SL and therefore representing a low level of toxicological risk) respectively. As such, the Contaminated Land Officer (CLO) at Herefordshire Council contacted Persimmon regarding the site’s potential to be determined as potentially Contaminated Land under Part 2A of the Environment Protection Act and the Contaminated Land Statutory Guidance (2012). Persimmon Homes agreed to carry out a voluntary investigation to assess the risks and, if necessary, undertake voluntary remediation which included the potential need for the residents to be immediately moved from their houses until remediation / risk mitigation could be carried out. CampbellReith were employed to review all information, design the investigation, carry out detailed assessment, and provide technical advice to the communication strategy.
Preliminary Conceptual Site Model (CSM)
The principle vapour intrusion (VI) pollutant linkages, with the key data uncertainties that informed the design of the investigation, are shown on Figure 1. A plan of the site layout which shows the orientation of the CSM section is provided on Figure 2.
Given the complexities associated with the site a multiple lines of evidence approach was required to avoid potential over-reliance on one data source. The presence of fractured bedrock and potential DNAPL meant that modelling of groundwater and soil vapour data to predict in-house concentrations could not be relied on to predict risk and an over conservative estimate could result in evacuation of residents, significant retrospective remediation to the buildings, and potential reductions to house values, all of which would cause significant and unreasonable stress to the residents.
Vapour Intrusion Investigation Design
The investigation comprised shallow vapour wells and multi-level wells to enable groundwater and soil vapour sampling from specific depths at the same location. Because of the data uncertainties, the the sensitivity of the residents, and the implications of both under and over conservatism in the vapour assessment, in-house monitoring was also recommended. Due to the lack of UK guidance regarding detailed design of in-house vapour monitoring and the absence of similar UK projects, the US OSWER guidance was used. This promotes limiting return visits, where possible, as they cause disruption and stress to the building occupants. The qualitative guidance was used to inform the number and locations of properties selected for in-house monitoring and investigation design accounted for the anticipated variability in the in-house vapour concentrations due to potential heterogeneities in the subsurface materials, building construction and age, and occupants’ activities.
Figure 1 - Preliminary Conceptual Site Model
In-House Sampling Methodology
A combination of passive sampling by appropriate thermal desorption tubes (selected following discussions with the laboratory and tube manufacturer to ensure they accommodated the varying uptake rates of the numerous contaminants of concern - CoCs) and active sampling from sub-slab vapour pins (via vacuum canisters) was used. One vapour pin per property was connected to a differential pressure meter which recorded pressure below the slab and inside the property (data logged every 15 minutes, downloaded on each return visit) to assess whether there was a pressure differential that could result in advective air flow into the property from the subsurface. In addition, ‘grab’ samples were obtained via vacuum canister at ground floor of each property on the first visit. Monthly rather than weekly sampling was proposed to limit disruption to the residents as far as practicable but although research is available that confirms that the tubes are reliable over a 7-day sampling period, no evidence was found regarding sampling over 1-month. There was concern that uptake rates would decrease over longer sampling times and could result in under-estimation of actual in-house concentrations. In a novel approach, the methodology was therefore amended to include two tubes per location. For the first month, one tube per location was replaced every week. At the end of the first month the second tube was also removed for analysis and the data compared against those from the weekly samples. For each location a mean concentration was calculated from the weekly results and compared against the concentration for the monthly sample. The concentrations obtained over 1 month were consistently lower than the respective mean weekly concentrations for all CoCs detected – for PCE they were lower by an average of 25% (ratios ranged between 0.68 and 0.80). A conversion factor based on the lowest ratio was calculated and applied to the subsequent monthly results, in the knowledge that any significant exceedances would need to be considered further, given the conservative ratio choice. The authors believe this is the first time this method has been employed and the results and statistical calculations will be written up so they can be used by industry.
Communication Strategy and Walkover Surveys
A detailed communication strategy was agreed between the main stakeholders. In summary this comprised posting of information leaflets to the residents, followed by several public exhibitions by CampbellReith and Persimmon to explain the issues and proposed investigation strategy to the residents. These were followed by home visits by CampbellReith to personally meet the residents and build up trust. This was followed by detailed site walkovers to assess foundation design, type, and condition of the utility entry - specifically the sewers, and identification of potential background vapour sources. An 80% visit success rate was achieved for the property walkovers and internal monitoring was enabled for 14 properties out of the planned 15 (see figure 2). The preparatory work invested in liaising with the residents, explaining the situation and answering all their concerns was key to the success of the whole project and it is proposed that this will form the basis of an information bulletin to inform similar works.
Figure 2 - In-House Monitoring and Walkover Survey Locations
Results and Background Sources
For each property, results from the indoor ‘grab’ samples and the passive samples were compared against the pumped canister samples taken from the sub-slab vapour pins. PCE was the only CHC detected both below slab and inside the properties and all lines of evidence confirmed it was from an on-site source. In-house concentrations were all almost two orders of magnitude lower than the TIAC.
Several other CoCs that were known to exist on-site were detected within some properties and some, such as benzene and TCE, were detected above the respective TIACs. However, these were not detected in the respective sub-slab concentrations which indicated that these were due to in-house sources. Other VOCs that were not identified on-site, such as acrolein and acetone were also detected in-house and some were detected sub-slab but at much lower concentrations suggesting that the concentrations were due to an-house source. Assessment of the pressure differential data indicated the potential for movement of air from in-house to sub-slab which supported the theory of an in-house source. In some properties additional samples for specific VOCs were also taken from first floor locations where sources were suspected (bathroom and bedroom, for example) and these returned much higher concentrations than those from ground floor, further supporting the conclusion that they were from an in-house source/s.
The data set therefore represents a source of background concentrations of compounds that could be used in the derivation of risk assessment screening values. Production of Defra’s C4SLs used values from the US due to the absence of UK derived data and a review of these indicates the majority are higher than those detected in this project. It is proposed that the data will be statistically assessed to produce minimum, maximum and median values that could then be used as a data source by industry.
Vapour wells were installed close to properties where possible. Guidance (e.g. CIRIA C682) suggests the potential for concentrations of CHC in soil vapour to be higher directly beneath the properties and so vapour wells could underestimate the risk if used in isolation. The opposite was found in this project with soil vapour concentrations of PCE recorded up to 10 times the maximum concentrations detected sub-slab. The reason for this is not fully understood but one explanation is heavy (recorded) rainfall causes saturation of surface soils that then significantly impede vertical migration leading to a build-up in vapour concentrations emanating from the impacted underlying groundwater. Increased soil vapour concentrations would then result as groundwater levels drop and soil pore spaces become unsaturated, leading to volatilisation from pore water just above the falling water table.
Conclusions
The in-house data were combined with those collected from the soil vapour wells and groundwater boreholes (not detailed in this presentation due to space restrictions) to finesse the CSM. PCE was the only CHC detected both below slab and inside the properties, with little variation in either data set. The principal intrusion pathway was identified as volatilisation from groundwater, upwards diffusion through the unsaturated zone and migration through the concrete floor slabs. The indoor air concentrations were not impacted by meteorological changes (e.g. drops in barometric pressure) or fluctuations in groundwater level, and the data, supported by J&E modelling, confirmed that migration was limited by diffusion rather than advection.
The data uncertainties were resolved, enabling a definitive decision to be made using a multiple lines of evidence approach. They confirmed that the CSM is suitably strong to allow a definitive risk assessment to be made and conclude the risk to residents is low. The report was approved by the CLO and UKHSA in November 2022 and can be used by the residents to assist any property sales. The works, although costly, have definitively proven the lack of risk and confirm that remedial works, such as retrofitting of vapour protection measures, are not required, both of which have given the residents much needed assurance.
The project represents a valuable resource for future VI work. It is being written up as a scientific paper to enable the methodology and data to be used. Items of interest include the use of thermal desorption tubes over a one-month sampling period; background VOC data for use in the derivation of assessment criteria; and data informing the relationship between sub-slab concentrations and those associated with soil vapour close to the property in question. A separate bulletin will also be produced detailing the communication strategy used that proved so successful. The works were iteratively designed via regular stakeholder meetings which included Herefordshire Council and their specialists (EPG) and UKHSA. Simon Firth provided input to the modelling on behalf of CampbellReith.