Formatted Title
Unravelling Contaminant Migration Pathways at a Former Pesticide Manufacturing Facility
Background/Objectives
A Detailed Site Investigation (DSI) was conducted to identify, and where necessary remediate, impacted soil and perched and regional groundwater at a former pesticides manufacturing facility. The DSI was required prior to divestment of the property and within the regulatory framework to assess and deem the site suitable for ongoing commercial / industrial use and remove potential ongoing sources of impact to the regional groundwater aquifer. The site is located in an industrial precinct within the Melbourne (Victoria, Australia) metropolitan area.
The site is underlain by basaltic rock to on average approximately 3 feet below surface, with concrete gravel subgrade and residual basaltic clay present above bedrock. Groundwater beneath the site is present at depths of between approximately 25 and 30 feet below ground level in a fractured basalt rock aquifer with limited suitability for extractive use. The fractured basalt is present beneath the site from near-surface (on average approximately 3 feet below ground level). There is potential for impacted groundwater to discharge to surface water bodies down-gradient of the site and impact upon sensitive freshwater water ecosystems. Shallow perched water was identified at some areas within site boundaries, hosted in shallow subgrade gravels below concrete pavement.
Approach/Activities
Initial DSI stages involved compiling and reviewing desktop information in relation to former pesticide manufacturing activities, including the nature and layout of liquid waste storage and handling infrastructure. Soil testing targeting 3 to 4 foot deep blind pits (i.e., pits intended to be directly pumped out, so not connected to any underground pipe network) in manufacturing plant rooms identified a previously unknown network of redundant underground liquid effluent pipes. Further review of historic company files revealed proposed service construction plans (i.e., not ‘as built’ plans) dating back to original construction of the facility. Excavations to expose the redundant pipes found some, but not all, redundant pipes to be present where shown on the old plans. In some cases obvious impact (i.e., stained, odorous or discoloured soils) was observed in trench fill sands and gravels surrounding the redundant pipes suggesting they have acted as preferential migration pathways.
Ground penetrating radar (GPR) and closed circuit television (CCTV) surveys were conducted to confirm the presence and location of redundant effluent pipes shown on the historic plans. In some areas pipes were found at locations not shown on the plans. The GPR and CCTV surveys were followed by a test pitting program to further investigate impacted trench fill and check whether residual liquids may be present in the pipes. The nature of significance of impacted trench fill was confirmed through laboratory analysis which help build the conceptual site model by identifying actual contamination source areas and migration pathways.
Perched water samples collected from monitoring wells and during the removal of pits reported elevated concentrations of pesticides with the potential to pose unacceptable risk to the health of future maintenance or construction workers who may be exposed in the future. A risk assessment was conducted to develop site-specific trigger levels based on likely exposure scenarios.
The DSI also considered the potential for contaminated perched water to impact the stormwater system via cracks in pipes or pits. A CCTV survey identified areas with visible cracks in pipes. These areas were tested through a program of targeted soil sampling. Stormwater sampling was also conducted at selected locations on-site, as well as upstream and downstream of the site.
Monitoring wells screened in the regional fractured basalt aquifer reported elevated concentrations of various pesticides formerly manufactured at the site. Where most elevated, reported concentrations of some pesticides exceeded groundwater trigger levels relevant to environmental values. The DSI therefore needed to determine whether any ongoing contamination sources, e.g., blind pits, redundant effluent pipes, residual soil contamination or perched groundwater, could act as an ongoing source of impact to the regional groundwater aquifer and therefore require remediation to make the site suitable for ongoing commercial / industrial use.
Results/Lessons Learned
Where pesticides were identified at elevated concentrations in soils or perched water, the relative severity of impact in the regional groundwater aquifer was often low or contamination was not detected. In many cases specific pesticides reported at relatively elevated concentrations in soil or perched water were absent in regional groundwater. Information obtained from literature reviews leachability analytical data from impacted areas explains the general absence of correlation between impacts in soil / perched water and regional groundwater impacts, whereby the downward migration of pesticides is limited by the presence of low permeability basaltic clays and their ability to sorb contaminants.
Where pesticides were found to have impacted the regional groundwater aquifer, impact was generally confined to production and waste storage areas. In some cases specific contaminants were absent or reported at relatively low concentrations in soil or perched water, however more significant impact was reported in regional groundwater. These observations support the conclusion that blind pits are the most important migration pathway between former primary sources and regional groundwater, whereby pesticide-impacted waste water would have leaked through cracks in pit bases before entering fractures in basaltic bedrock just below to pit bases.
While evidence of contaminant migration was observed in trench fill around some redundant effluent pipes, our understanding of the significance of impacted soil as a secondary contaminant source suggests this is not an important pathway to impact in the regional aquifer.
There is overall limited correlation between contaminants identified in stormwater and those identified in perched water. This indicates that while there is potential for contaminated perched water to infiltrate the stormwater system, other pathways are the primary causes of impact to stormwater. This conclusion is supported by the results of the CCTV survey which only found a relatively small number of defects, which where present, were small and mostly in areas where perched water does not occur.