Changes in Magnetic Mineralogy Through a Depth Sequence of Hydrocarbon Contaminated Sediments

Sediments, soils and groundwater can act as a natural storage for many types of pollution. This study aims to investigate ferro(i)magnetic phase formation and transformation in the presence of organic contaminants (hydrocarbons) and its relation to bacterial activity, in particular in the zone of fluctuating water levels. The work extends previous studies conducted at the same site. The study area is a former military air base at Hradčany, Czech Republic (50°37'22.71"N, 14°45'2.24"E). Due to leaks in petroleum storage tanks and jet fuelling stations over years of active use the site was heavily contaminated with petroleum hydrocarbons, until the base was closed in 1991. This site is one of the most important sources of high quality groundwater in the Czech Republic. During remediation processes the groundwater level in the sediments fluctuated, driving the hydrocarbon contaminants to lower depth levels along with the groundwater and leading to magnetite formation (Rijal et al., Environ.Pollut., 158, 1756-1762, 2010). In our study we drilled triplicate cores at three locations which were studied earlier. Magnetic susceptibility (MS) profiles combined with other magnetic properties were analyzed to obtain the ferro(i)magnetic concentration distributions along the depth sections. Additionally the sediment properties, hydrocarbon content and bacterial activity were studied. The triplicate cores were used to statistically discriminate outliers and to recognize significant magnetic signatures with depth. The results show that the highest concentration of ferrimagnetic phases (interpreted as newly formed magnetite) exists at the probable top of the groundwater fluctuation (GWF) zone. For example at one of the sites this zone is found between 1.4-1.9 m depth (groundwater table at ~2.3 m depth). High S-ratio and the correlation of ARM with MS values confirm the contribution of magnetite for the ferro(i)magnetic enhancement in the GWF zone. In the previous studies the MS signals revealed small-scale isolated features, but with the use of triplicate cores significant trends of MS could be identified, showing an increase from the lowermost position of the groundwater table upward. Bacterial activity is likely responsible for magnetite formation in this depth range as indicated by most probable number (MPN) results of iron-reducing bacteria.