Hydrodynamic characterisation of a groundwater - surface water system and evaluation of BTEX, PAHs decay and heavy metals fate
Most contaminated sites related to past industrial activities in Europe are adjacent to rivers and urbanized areas. These sites pose a major problem for water authorities and policy makers in terms of contaminants mobility, natural attenuation (or degradation) and risk assessment for environment and humans. Natural attenuation in groundwater is only effective if hydrogeochemical conditions of the system are favourable and contaminant degrading microorganisms are present. To evaluate this effectiveness, many site specific factors have to be considered, among which the dynamics of groundwater fluxes and groundwater – surface water interactions and biogeochemical processes. The site of concern in this study is a brownfield located in the north bank of the Meuse River, upstream of the city of Liège, Belgium. The soil, the unsaturated zone and the gravel aquifer are heavily contaminated by organic pollutants (BTEX and PAHs) and metals due to past industrial activities of coke production in the XXth Century. Various point sources of contaminants were delineated in the site. Benzene and Toluene concentrations in groundwater were found up to 50.000 μg l-1 and 23.000 μg l-1 respectively in source areas, while pollution due to metals was less important, with presence of Fe (> 6400 μg l-1), Zn (40 μg l-1), Co (18 μg l-1), Cu (>9 μg l-1), Pb (>100 μg l-1) and Cr (2 μg l-1) mainly with higher concentrations in some hot spots. The Meuse River level is established at around 59.5 meters a.s.l. by dams. However, river water levels fluctuate continuously with amplitude varying between a few centimetres up to 2 meters during winter and spring seasons. The main objectives of the research investigations were 1) to evaluate whether an interaction exists, at the level of the mentioned brownfield, between groundwater and the neighbouring river; 2) to assess the dynamics of such interactions and to quantify groundwater fluxes as the main potential vector of mobility of contaminants offsite; 3) to determine the potential of bacterial degradation of organic compounds and fate of metals contaminants in the specific environment of the site; and 4) to integrate groundwater – surface water dynamics with potential degradation of contaminants in order to evaluate further evolutions of contaminants in the aquifer. A detailed monitoring of groundwater and surface water levels, together with a series of field tests like pumping and tracer tests, contributed to a good knowledge in hydrodynamics of the contaminated aquifer. Groundwater and surface water monitoring datasets were analyzed in order to characterize hydraulic dynamics of the groundwater – surface water system. Groundwater heads observed were strongly influenced by Meuse River stages, and groundwater flow in the transition zone (seepage) was oriented in the direction going from the aquifer to the river under normal conditions. The use of an analytical model, however, pointed out that small changes on water river level were enough for a groundwater flow inversion, so seepage going from the Meuse River into the aquifer. Organic compounds (mainly benzene and low weight PAHs) have been studied with respect to their intrinsic bacterial degradation potential. Two independent stable isotope-based approaches (laboratory and field) were used to determine in situ biodegradation rates for benzene, and for the two- and three-ring aromatic hydrocarbons naphthalene and acenaphthene. In the laboratory, microcosms were set up with 13C-labeled substrates as well as with groundwater and with sediments from the site. The increase in 13C-CO2 over time was monitored by GC-IRMS analysis and used to calculate biodegradation rates. Benzene, naphthalene and acenaphthene were found to be biodegradable by the intrinsic microbial community under in situ-like conditions. The respective biodegradation rates decreased with increasing number of aromatic rings and were significantly lower under anoxic conditions. Apart from the microcosm study, in situ-biodegradation rates could be retrieved in a field study from 13C/12C signatures of residual groundwater contaminants using first-order kinetics. Biodegradation rates of lab- and field-based approaches were found to be in good agreement. Batch tests revealed that the heavy metals could be precipitated under sulphate (present at the site) reducing conditions. This in situ bioprecipitation process can be induced by the presence of electron donors and plays an important role in the natural attenuation process. Other conditions as aerobic or nitrate reducing conditions, did not lead to heavy metal immobilization. This gives specific indications about future site management and land use. It was also proved that the present heavy metal concentration did not influence the PAH biodegradation. However, PAH biodegradation under aerobic conditions is very slow. Further investigations will be necessary to evaluate the effect of aerobic biodegradation (addition of air) on the mobilization of the heavy metals, bound to the aquifer. The continuous changes of the groundwater flow direction observed in the studied site, lead to surface water flowing into the aquifer. This is likely to be an additional source of oxygen for the aquifer. This influx of oxygen-saturate water could enhance the degradation of BTEX and PAHs in the regions of the aquifer which are affected. Further investigations are needed to come to a qualitative evaluation to what extent oxygen from river water contributes to the removal of BTEX and PAHs at the site.