A REPORT on PFAS contamination at Gingin satellite airfield has recommended the installation of up to three new offsite shallow groundwater monitoring wells.
The report by Jacobs also recommends an ongoing monitoring plan should be developed for the site and implemented so as to monitor PFAS concentration trends over time within the management area.
The Jacobs assessment of hydrogeological conditions at the site and the surrounding area included consideration of per- and polyfluoroalkyl substances (PFAS) dispersion, specifically Perfluorooctane sulfonate (PFOS) and Perfluorohexane sulfonate (PFHxS) using – what the report said was a highly conservative analytical approach.
According to the report published on the Department of Defence website the monitoring wells should be installed in the superficial aquifer downgradient of the site, between the identified on-site source areas and the existing off-site sentinel monitoring well (GC19).
“Jacobs recommends that these wells are installed at a distance where groundwater concentrations are below either laboratory reporting limits for PFAS or drinking water guidelines (0.07 µg/L),’’ the report said.
“Based on existing information this would equate to about 500m from the boundary of the site, but would be nominated following further consideration of groundwater travel times and observed reduction in PFAS groundwater concentrations away from the identified sources.
The report said the monitoring plan should include the sampling and analysis of existing groundwater monitoring wells at the site as well as the three new proposed wells.
It also said sampling of sentinel well GC19 should also be included in the monitoring plan.
The assessment was carried out due to a concentration of PFOS and PFHxS being detected at the southern boundary of the site.
The Water Corporation extracts groundwater from the superficial, Leederville, Mirrabooka and Yaragadee aquifers, via several bores, for Perth’s water supply.
The closest water supply bores to the site is the Pinjar production bore P140 located 7.5km south-southwest of the site boundary.
“This bore is inferred to be screened in the superficial aquifer and · Pinjar production bore P145 located 7.5km south-southwest of the site boundary.
“This bore is screened from 111 – 183 mbgl, which is inferred to be in the Leederville aquifer.
“These bores are located in the Gnangara underground water pollution control area (Gnagara UWPCA) and are part of a network that supplies drinking water to Perth, the Peel region and towns in the Goldfields as part of a wider integrated supply system.
“Based on the hydrogeological conceptual model and the outcomes of the analysis performed, it is acknowledged that while the potential for PFAS impacts to the Gnangara UWPCA cannot be ruled out, Jacobs considers that the risk to the current identified offsite receptors – the Pinjar production bores – has not occurred and future risk of this occurring is acceptably low.
“This determination is based on consideration of the following: · PFAS was not detected in either of the two recent rounds of groundwater sampling and analysis performed by WA Water Corporation at the Pinjar P140 and P145 abstraction bores. ·
“PFAS was not detected in monitoring well GC19 located half way between the boundary of the site and the two abstraction bores above.
“The location of this well is such that is can be used to provide a reliable early indication as to whether PFAS might be detected in the Pinjar abstraction bores at some point in the future (i.e. it is highly unlikely that PFAS would be detected in the Pinjar abstraction bores without first being detected in the GC19 monitoring well).’’
The report said a highly conservative analysis of PFOS and PFHxS dispersion away from the identified source areas had been performed.
“The results indicate that the time period before which PFOS and PFHxS (or other PFAS compounds) would theoretically be detected at concentrations in exceedance of the drinking water criteria at the GC19 sentinel monitoring well location would be substantial – ranging from 50 years to 150 years.
“In reality, it is highly unlikely that PFAS would be detected in either sentinel monitoring well GC19 or the Pinjar production bores in the future given these dispersion time periods (which would likely be significant), or the fact that a continuous source of PFAS is no longer present at the site.
“As a result of the calculated travel time, a more strategic approach to monitoring and managing risk can be undertaken.’’
The human health risk assessment (HHRA) report said based on the assessment’s findings the risks to typical base personnel at Gingin satellite airfield from chronic exposure to non-potable water impacted with (PFAS) was determined to be low and acceptable.
“This is on the basis that whilst the tap water at the site does contain levels of PFAS and the water is used for various non-drinking purposes, incidental exposure from the PFAS is low and acceptable and thus the potential consequences to health are insignificant,’’ the HHRA said.