Soil and groundwater contamination with gasoline and diesel oil : assessment of subsurface hydrocarbon contamination resulting from a fuel release from an underground storage tank in Vanstad, Skåne, Sweden
Abstract: Spills and leaks from storage tanks constitute the most common groundwater contamination source with petroleum products. Gasoline and diesel pools do not spread extensively along the water table, but dissolved constituents are transported in groundwater on relatively larger distances. Volatile hydrocarbons migrate in the vadose zone dependently on distribution and extent of the dissolved and pure phase. When influx exhausts, fuel trapped in pores at a residual saturation still remains as a potential contamination source. Several thousand among 22 000 contaminated sites in Sweden are result of petrol stations contributing to soil and groundwater pollution. In 2003, investigations carried out by SWECO VIAK AB in the property of a former petrol station in Vanstad in Sjöbo commune in Skåne, revealed subsurface contamination with petroleum hydrocarbons and lead resulting from an underground storage tank fuel leakage. After the tank remediation it was recommended to collect water samples for oil index analysis in 2005. The property is located within a proposed water protection area. The aim of the thesis was to proceed further investigations in the vicinity of the property in order to assess overall spatial contamination and its influence on the water protection area. The investigations were carried out from April to June 2005 and included: coring, groundwater levelling, groundwater and soil sampling, oil index and volatile compounds analyses. On the basis of the field data evaluation, it is inferred that hydrocarbons in a gaseous phase can migrate upwards through all the units comprising the vadose zone, but sand and gravelly sand constitute the preferential pathways. Dissolved compounds can be transmitted northwards of the tank through silty and clayey till as well as through silty and gravelly clayey sand. Assuming the worst case scenario, the solutes are transported due to advection at the rate 5.4x10-8 m/s. It was further calculated that hydrocarbons can pass in groundwater 1.7 m per year, what gives 71 m in 42 years. The “clean line” was subsequently delineated with regard to this distance. Laboratory analyses showed that groundwater is not contaminated with hydrocarbons. The oil index values are below a detection level of an instrument (50 μg/L) applied. None of BTEX is registered in a vapour headspace. N-Heptane and MTBE detection must be due to an error of the instrument applied. There is no negative impact on the proposed water protection area. Total concentrations of volatile compounds in the soil vapour (TVOC) do not exceed 20 ppm. Such low concentrations are of very marginal importance and the soil is considered as not contaminated. It is concluded that long term, subsurface fuel releases do not necessarily result in extensive soil and groundwater contamination. The fact that contamination is not observed in the investigated area, although the distribution calculations imply so, underlines a huge role of natural attenuation as a factor limiting subsurface contamination with petroleum hydrocarbons.
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