CONTAMINATED LAND PROJECT FLOW-DIAGRAM
Below is a flow-diagram of the different contaminated land investigations GCS have undertaken. It shows how one phase feeds into another. With each phase a generalised scope of work and the general purpose of the specific phase of work is provided.
At GCS we use a phased approach to Human Health Risk Assessments and Ecological Risk Assessments. Important to note that most sites only require assessments up to a Phase 1.5. This is a generalised project flow-diagram and only serves to educate
The table below provides the benefits of undertaking the various phases of work.
Phase I – Desk Study and Site Walkover
Generally, all contaminated land investigations will begin with a Phase I assessment. During the Phase I assessment a desktop study is completed, and high-level information is requested from the site manager/owner during the site interview regarding:
- history of the site,
- chemical storage areas,
- chemicals stored at the site,
- spill/leak/loss history of the site,
- if previous investigations have been undertaken at the site, and
- if any maps and groundwater chemistry databases are available from previous investigations.
A site walkover is also undertaken while accompanied with a site representative. During the walkover areas of concern are highlighted where product losses could have occurred which may have impacted the environment. These areas are noted, and photographs taken for future reference. These areas are then indicated on a map of the site together with inferred groundwater flow direction, major infrastructure and used to select future soil sampling locations (if needed).
Phase I.5 – Shallow Intrusive Investigation
During the Phase I.5 investigation, the areas highlighted during the Phase I investigation are targeted for soil sample collection. Soil samples are generally collected by first scanning the subsurface with a cable avoidance tool (CAT) to avoid intersecting electrical cables and/or water pipes. Areas free of underground services are then marked-out.
After locations have been selected and cleared of services, ground cover (bricks, concrete, asphalt etc.) is removed with the use of a jackhammer, crowbar (a.k.a gwala) or concrete corer. Once removed, a Johnson handheld auger (with extensions) is then used to collect soils at pre-defined intervals. If volatile organic carbon (VOC) contamination is suspected to be presented the soil will be screened with a photoionizing detector and inspected for visual and olfactory signs of contamination.
Soil intervals with signs of contamination are then collected with laboratory supplied sample containers and sent to the laboratory to be analysed for potential chemicals of concern (decided on in advance based on current and previous suspected contamination).
Also, during the Phase I.5 assessment, a hydrocensus survey is undertaken in search of private borehole/groundwater users. If identified, information such as the water quality complaints, use and groundwater level are collected along with in-field water quality data.
Phase II – Extensive Intrusive Investigation
Based on the findings of the Phase I.5 investigation, a Phase II investigation will be recommended if likely risk to identified receptors was uncovered.
During the Phase II investigation additional soil samples via Johson soil auger may be collected, however in most cases long lasting groundwater monitoring wells will be installed in areas of concern, usually down-gradient of suspected pollution source areas to deeper depths than could be reached via hand auger.
The monitoring wells are usually installed via percussion drilling techniques in areas clear of services. After drilling is complete, factory slotted PVC casing is installed at the desired depth along with silica gravel to ensure long term stability of the well. The wells are then fitted with traffic rated well covers to protect the casing and allow for systematic groundwater monitoring.
Once installed, the groundwater in the wells are purged (well is developed) to allow “fresh” groundwater to enter the well. The well is then left to stabilise for one week after which groundwater monitoring and groundwater sample collection takes place. During groundwater monitoring the water level and depth of well are measured. In-field readings are collected such as the pH, temperature etc. This data is used during the groundwater chemistry data interpretation.
Water samples are collected using either dedicated bailers or low-flow sampling techniques with dedicated tubing. Duplicate water samples are collected along with the rest of the samples to determine representativeness of the laboratory data and to ensure that the laboratory data can be used for interpretation purposes. The water samples are analysed for targeted compounds.
The groundwater laboratory data is then used to determine the:
· Main Contaminants of Concern (COCS).
· Extent of the contamination.
· The migration direction of the contaminants.
· The biodegradation capacity of the groundwater system.
· The likely source(s) of the contamination.
· Theoretical risk posed to identified receptors by screening the results against adopted local and international groundwater standard related to drinking water, indoor vapour inhalation and aquatic water quality (as applicable to the specific site).
The results of the investigation are used to update the conceptual site model for the site and determine to potential risk for the identified source-pathway-receptor (S-P-R) linkages based on the findings of the investigation undertaken at the site.
Additional Phase II
If an increased risks to identified receptors was determined during the initial Phase II assessment, additional Phase II assessment(s) may be required with the main aim of determining the risk to identified receptors and further delineating the impact.
If remediation is likely to be required it is important to know the extent of the impact so it can be mitigated, that is also one of the main reasons for undertaking additional Phase IIs.
During the additional investigations more comprehensive investigations may be undertaken to:
- Further delineate the contamination on and off-site (if needed).
- Determining the interconnection of different aquifer systems.
- Determine if indoor inhalation risks exist.
- Determine the age of the impact.
- Determine the mobility of the contamination and the recoverability potential.
- The aquifer characteristics which govern the migration of the contaminant plume etc.
After the Phase II or additional Phase II assessment(s) it may be determined that even though contamination is present at the site, it does not pose an immediate risk to identified receptors, in which case groundwater monitoring would likely be recommended to determine:
- Changes in the concentrations of the COCs over time – trend analysis.
- Migration characteristics and changes in groundwater flow direction.
- The biodegradation capacity of the groundwater system to naturally attenuate the contamination.
- If likely risk to the identified receptors change or remain the same over time.
If high potential risk to identified receptors is identified with a high possibility to negatively impact the identified receptors a Detailed Qualitative Risk Assessment (DQRA) could be undertaken. During the DQRA site specific parameters are used to establish site-specific target levels (SSTLs), or site-specific clean up levels, which, if in compliance, a theoretical risk would not be present for the modelled pathway(s).
To establish the SSTLs, data collected from the site are inserted into analytical models (RBCA or RISC) which are then used to establish the SSTLs for different modelled scenarios, such as ingestion of groundwater, leaching of contaminants from impacted soils to site groundwater, direct dermal exposure to contaminated soil and vapour inhalation.
ROA and RAP
If it is found that the impact poses a very high risk to the identified receptors (or has already impacted the receptor) after completing the additional Phase II investigation and/or DQRA a remedial options appraisal (ROA) will be undertaken during which all the different remedial technologies are weighed up against one another to decide on a remedial technology which:
- is economically feasible,
- will be able to improve the identified impact, and
- practicality of the technology considering site constraints.
Some remedial technologies include:
- Excavation – Contaminated is removed with a TLB.
- EMNA – Nutrients are added to the subsurface to enhance natural breakdown of contaminants
- VER – A vacuum is applied to the subsurface to remove the impact through different phases (i.e dissolved phase, residual phase, saturated phase and vapour).
- ISCO – An oxidising agent is added to the subsurface to enhance the breakdown of hydrocarbons.
- Pump and treat – Impacted groundwater are pumped from the subsurface and treated on-site.
- Surfactant injection – Emulsifying agent is pumped into the subsurface to encapsulate the contamination and is then extracted via pumping.
After the different technologies pros and cons are weighed up against one another and a remedial technology is recommended a remedial action plan (RAP) will be completed which will lay out a plan to undertake the chosen remedial technology.
After all the options have been considered, a remedial technology chosen and a plan laid out to effectively undertake the chosen technology, remediation can begin.
After remediation has been undertaken at the site, the site soil and groundwater system will be monitored to determine how effective the remedial technology was in decreasing/improving the contamination concentration. If it was effective, additional groundwater monitoring would likely be recommended to ensure that concentrations remain below the SSTLs or other adopted screening criteria. If, however the remedial technology was not effective the remedial strategy would be revised to determine if more events using the same technology would be required, or if an entirely new approach needs to be applied.
A site close out can occur following any one of the above-mentioned phases, if it can be proved/justified that the site does not pose a risk to human health or the environment a site can be closed out and not further environmental investigations need to be undertaken