School of Earth & Environmental Sciences
Davies, Bethany L., Vapour intrusions from ground water contamination in the Sutherland Shire, New South Wales: The relationship between subsurface sediment composition and risk to human health, BEnviSci Hons, School of Earth & Environmental Sciences, University of Wollongong, 2015.
Groundwater contamination, in particular petroleum hydrocarbons, can result from leaking underground storage tanks, numerous industrial processes or accidents occurring during the transportation of petroleum products. Vapours can emanate from groundwater contamination, and have the potential to significantly decrease the quality of indoor air and negatively impact human health. However, vapours are not likely to travel more than 1.5 m from the source to the receptor, and attenuate significantly in the presence of oxygen.
Currently national guidelines suggests that 1.5 m and 10 m of clean soil overlying groundwater contamination is required to attenuate dissolved-phase hydrocarbons and Light Non-Aqueous Phase Liquids (LNAPL) respectively to non-detectable levels. However, due to the nature of the sedimentary rock successions in the Sutherland Shire, these minimum soil depths are rarely met. Therefore, this study aimed to assess the relevance of the recommended guidelines to the Sutherland Shire. Additionally, it aimed to make an assessment on whether the geological successions that typify the Sutherland Shire form a suitable barrier to vapour intrusions.
In this thesis six sites were examined; three underlain by Hawkesbury Sandstone and three underlain by Botany Sand. Available groundwater monitoring results and soil vapour results were obtained from the Council and additional soil vapour monitoring results were acquired when necessary. Additionally, a petrological study was undertaken to reveal the compositional and structural characteristics of the sedimentary rock successions and unconsolidated soils and sands of the Hawkesbury Sandstone and Botany Sand substrates.
The results from groundwater monitoring and soil vapour testing at sites underlain by Hawkesbury Sandstone showed no risk to human health via vapour intrusions. There was however, an exceedance of the site specific screening levels for benzene, toluene and total xylene (470,000 µm/m3, 1,100,000 µm/m3 and 140,000 µm/m3 respectively) at one monitoring well. This is likely, however, to represent a preferential pathway within the subsurface. As a result of the general lack of vapour exceedances at sites underlain by Hawkesbury Sandstone, it is suggested that the low porosity and permeability of the sedimentary unit acts as a barrier to vapour intrusions.
Conversely, groundwater monitoring and soil vapour sampling was inconclusive at sites underlain by Botany Sand within the Sutherland Shire, due to the lack of soil vapour data. However, calculations quantifying the rate of natural attenuation suggested it occurs rapidly in the presence of oxygen (average between 30-45% in 80 cm of clean soil overlying LNAPL). Therefore, it was concluded that although sites underlain by Botany Sand may pose a human health risk as a result of vapour intrusions, vapours may not require the minimum soil depths to attenuate to non-detectible levels.
The results of this study suggest the minimum soil depths may not be applicable in all situations. Rather, a site specific assessment may be required, particularly for shallow contaminants in sites underlain by highly porous and permeable sediments.