The first stage of any cost effective site investigation is a Tier 1 study, which generally incorporates a historical desk study and site walkover survey. A preliminary site investigation may also be included in order to develop a Conceptual Site Model (CSM) for the likely areas of contaminated material (sources), potential pathways and potential receptors (humans, controlled waters, buildings or ecology).
Elevated concentrations of certain elements/compounds are then compared with suitable, non-site specific screening criteria such as Drinking Water Standards for groundwater or Environmental Quality Standards if a nearby surface watercourse is identified. If recorded concentrations do not exceed the applicable screening levels, the site or individual areas of the site can be assumed to pose no unacceptable risk, if exceedances do occur, further, Tier II investigation is required.
A Tier II investigation usually involves intrusive investigation work based on amended pollutant linkages identified during Tier 1.
Clancy Consulting use a number of models and methodologies to carry out Tier II chronic human health investigations. These include CLEA UK, SNIFFER and RBCA (based upon the ASTM standard E-1739) produced by Groundwater Services Inc. which models both human health exposure and groundwater migration pathways.
For controlled waters, the Environment Agency P20 methodology and CONSIM model are routinely used.
The new CLEA UK model (at the time of writing at beta evaluation stage) which was recently implemented as a tool for evaluating risks to human health was released by DEFRA. The model allows for a probabilistic assessment for the long term human health risks associated with specific elements and compounds which have been toxicologically assessed. To date little progress has been made with respect to the provision of background information to support the modelling and many practitioners have resorted to the use of non UK based toxicological information. Much debate has also been reported with respect to the existing guidance particularly the resulting conservative remedial targets.
Information required for a quantitative Tier II human health risk assessment includes site specific geological information, physical and chemical properties of the unsaturated and saturated zones, site specific human health exposure parameters ingestion rates or breathing rates. The estimated quantity of contaminant inhaled, adsorbed or ingested is compared against acceptable daily intake rates.
For controlled waters the assessment again requires site specific soil and groundwater parameters for inclusion in simple groundwater fate and transport algorithms based on the Domenico equation. The algorithms model the movement of contaminants in the saturated and unsaturated zones to identified receptors such as aquifers, abstraction wells or surface water bodies. Concentrations predicted at the various receptors are compared against target values such as Drinking Water Standards or Environmental Quality Standards.
Groundwater modelling may include conservative biodegradation half-lives in order to account for biodegradation, degradation and natural attenuation processes occurring in the aquifer.
If the contaminant level exceeds the acceptable intake value for any identified receptor it is assumed that there is a potential for an unacceptable level of risk to exist and risk based site-specific target levels (SSTLs) are calculated for the site. The site can then either be remediated on the basis of the SSTLs or an additional investigation instigated in order to facilitate a Tier III risk assessment. The extra cost of data collection within the Tier III investigation is most likely to be offset by the reduced conservatism and uncertainty in the resulting remediation targets.