GROUNDWATER MODEL ECOSYSTEM LONG-TERM EXPERIMENT

Several hundred thousands of (former) industrial sites and waste deposits have been identified throughout Europe where contaminants are continuously leaching into the soil and groundwater. Studies investigating the natural attenuation of these contaminants at the field scale often lack the spatial resolution and hydrologic controls (boundary conditions) required to fully understand the mass transport and biodegradation kinetics. Furthermore, aquifers cannot be compared before and after contamination to understand how the groundwater ecosystems react chemically and biologically to the contaminant impact. To fill this knowledge gap an indoor aquifer model system (mesocosm, 5 x 0.8 x 0.7m) was constructed. Steady-state conditions were achieved via the constant injection of local groundwater at approximately 1.5 m/day flow rate. Subsequently, the monoaromatic, organic contaminant toluene (BTEX compound), was continuously injected as a point source in dissolved phase at saturation (500 ppm) and the downstream transport and biodegradation was measured via monitoring of physical-chemical key-parameters complemented by compound specific stable isotope analysis (CSIA). The metabolic response of the natural microbial community was followed with total cell counts, ATP analyisis, and bacterial carbon production measurements. At the very beginning, toluene was transported throughout the aquifer with little sign of degradation, but already after 2 weeks 99.9% of the toluene was being biodegraded in the first 15 cm of the flume corresponding to a pore water residence time of approximately 5 hours. Oxygen measurements indicated that the toluene degradation was accompanied by the establishment of anoxic conditions in the center of the contaminant plume. The spatial distributions of total cells and ATP were highly correlated with toluene concentration in the first section of the mesocosm whereas oxygen was inversely correlated. Towards the end of the aquifer, microbial biomass and activity was concentrated at the plume fringes. CSIA confirmed biodegradation was occurring in the first meter of the flume through the observed ¹³C enrichment in toluene molecules. Investigations into the responsible bacterial consortia, presence of functional genes and activity of degradation pathways of toluene are ongoing.