How to Map Hydrocarbon Contamination of Groundwater Without Analysing for Organics
The Geological Surveys of Latvia and Norway and the Norwegian Defence Research Establishment have undertaken a risk-based investigation and assessment of a former Soviet military oil depot at Viestura Prospekts, Riga, using the firms Dames & Moore (U.K.) and Geo-Konsultants as subcontractors.
The site lies on homogeneous fine-medium grained Baltic Ice Lake sands containing a shallow unconfined water table. Georadar, calibrated against monitoring boreholes, has been used to map the north-westward gradient of the water table.
16 monitoring boreholes have been employed to assess groundwater contamination. Free phase oil has been proven in boreholes near fuel storage bunkers and at the oiltransfer railhead.
Vertical electric sounding profiling (VES), a geophysical technique measuring the apparent resistivity of the sediments, has been employed at the site. VES showed two plumes of low resistivity leading away from the bunker area and the railhead in a NW direction. These may represent plumes of groundwater with high ionic content.
Oil degradation releases CO2, which enhances weathering of carbonate and silicate mineral phases in the aquifer, releasing alkalinity and several cations and elevating the water’s electrical conductivity (EC), explaining the VES map. Additionally, degradation consumes O2 and other electron acceptors, promoting reducing conditions and releasing dissolved iron. Thus, a combination of high Fe, alkalinity, ion content, EC and low oxygen, sulphate and nitrate, may be used as low-cost inorganic indicators of hydrocarbon contamination.
KeywordsVertical Electric Sound Gravel Aquifer Fuel Bunker Shallow Sand Monitoring Borehole
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