Benzene dynamics and biodegradation in alluvial aquifers affected by river fluctuations

in Ground Water (2013)

The spatial distribution and temporal dynamics of a benzene plume in an alluvial aquifer strongly affected by river fluctuations was studied. Benzene concentrations, aquifer geochemistry datasets, past ... [more ▼]

The spatial distribution and temporal dynamics of a benzene plume in an alluvial aquifer strongly affected by river fluctuations was studied. Benzene concentrations, aquifer geochemistry datasets, past river morphology and benzene degradation rates estimated in situ using stable carbon isotope enrichment were analysed in concert with aquifer heterogeneity and river fluctuations. Geochemistry data demonstrated that benzene biodegradation was on-going under sulphate reducing conditions. Long-term monitoring of hydraulic heads and characterisation of the alluvial aquifer formed the basis of a detailed modelled image of aquifer heterogeneity. Hydraulic conductivity was found to strongly correlate with benzene degradation, indicating that low hydraulic conductivity areas are capable of sustaining benzene anaerobic biodegradation provided the electron acceptor (SO42-) does not become rate limiting. Modelling results demonstrated that the groundwater flux direction is reversed on annual basis when the river level rises up to two meters, thereby forcing the infiltration of oxygenated surface water into the aquifer. The mobilisation state of metal trace elements such as Zn, Cd and As present in the aquifer predominantly depended on the strong potential gradient within the plume. However, infiltration of oxygenated water was found to trigger a change from strongly reducing to oxic conditions near the river, causing mobilisation of previously immobile metal species and vice versa. Monitored natural attenuation appears to be an appropriate remediation strategy in this type of dynamic environment provided that aquifer characterisation and targeted monitoring of redox conditions is adequate and electron acceptors remain available until concentrations of toxic compounds reduce to acceptable levels. [less ▲]

Benzene dispersion and natural attenuation in an alluvial aquifer with strong interactions with surface water

in Journal of Hydrology (2009)

Field and laboratory investigations have been conducted at a former coke plant, in order to assess pollutant attenuation in a contaminated alluvial aquifer, discharging to an adjacent river. Various ... [more ▼]

Field and laboratory investigations have been conducted at a former coke plant, in order to assess pollutant attenuation in a contaminated alluvial aquifer, discharging to an adjacent river. Various organic (BTEX, PAHs, mineral oils) and inorganic (As, Zn, Cd) compounds were found in the aquifer in concentrations exceeding regulatory values. Due to redox conditions of the aquifer, heavy metals were almost immobile, thus not posing a major risk of dispersion off-site the brownfield. Field and laboratory investigations demonstrated that benzene, among organic pollutants, presented the major worry for off-site dispersion, mainly due to its mobility and high concentration, i.e. up to 750 mg L 1 in the source zone. However, benzene could never be detected near the river which is about 160 m downgradient the main source. Redox conditions together with benzene concentrations determined in the aquifer have suggested that degradation mainly occurred within 100 m distance from the contaminant source under anoxic conditions, and most probably with sulphate as main oxidant. A numerical groundwater flow and transport model, calibrated under transient conditions, was used to simulate benzene attenuation in the alluvial aquifer towards the Meuse River. The mean benzene degradation rate used in the model was quantified in situ along the groundwater flow path using compound-specific carbon isotope analysis (CSIA). The results of the solute transport simulations confirmed that benzene concentrations decreased almost five orders of magnitude 70 m downgradient the source. Simulated concentrations have been found to be below the detection limit in the zone adjacent to the river and consistent with the absence of benzene in downgradient piezometers located close to the river reported in groundwater sampling campaigns. In a transient model scenario including groundwater–surface water dynamics, benzene concentrations were observed to be inversely correlated to the river water levels, leading to the hypothesis that benzene dispersion is mainly controlled by natural attenuation. [less ▲]