PAH, BTEX, Mineral oil
Location: Ghent, Belgium
The Interreg’s RESANAT project promotes nature-based remediation techniques for residual soil contamination. The project concerns a full scale field test of a reactive mat (natural catch) in an urban canal threatened by groundwater contamination. The contamination originates from former industrial activities on the bank. The contamination migrates with the groundwater into the surface water, posing spreading and ecological risks. Measurement of groundwater and mass flux allows to identify flow patterns and determine mass discharge to the surface water. Based on the results the most appropriate remediation technique and materials are selected. The results helped our partner to dimension a barrier, both the needed hydraulic conductivity and its capacity to filter contaminant influx.
In 2019, restoration of the drainage capacity of the canal was needed, and the sediment was dredged. By dredging the canal the drainage capacity increased and adsorption of contamination decreased. It led to an increase of contamination in the surface water. The horizontal flux of groundwater and contamination was measured before and after dredging. Horizontal flux samplers were installed on 2 depths, in existing wells in the bank of the canal. Additional vertical flux, upwards and downwards were measured after dredging in the sediment of the canal. In a first stage digital flux measurements of groundwater were performed in 30 locations. The results are available in real time. Based on these measurements 10 locations were selected to install samplers to measure vertical flux over a longer period. The flux measurements provided useful information about the distribution of the inflow over the length of the canal and the effect of dilution and degradation downstream and allowed to refine the conceptual site model.
"Flux measurements created new insights of the distribution inflow of the contamination to the canal, the presence of hot spots, the degradation and dilution occuring downstream." - Tobias Praamstra (TAUW)
How to measure the inflow of contamination to the canal and nearby urban area:
Groundwater flow increased due to dredging of the canal. By traditional sampling only concentration in the groundwater is measured.
The groundwater and mass flux before dredging was rather low. Flux measurements showed an significant increase of the groundwater flux after dredging. The BTEXN flux measured in the wells in the banks increased till 15 times in comparison with the flux before dredging. The PAH flux increased till 3times. After dredging an additional flux of mineral oil is measured. The mineral oil compounds were mobilised after the dredging.
In the sediment of the canal the flux was dominantly upward, from groundwater to surface water. The flux varied strongly because of the heterogeneity of the sediment and characteristics of the canal. The use of passive mass flux sampling in combination with real time continuous flow velocity and direction measurement with digital sensors allowed to characterize exchanges at the interface between groundwater and surface water and to create illuminating insights in the dynamics influencing the water quality. The data led to an effective remediation concept.
Based on the groundwater and mass flux data it became clear where targeted action was needed. With the data it was possible to divide the trajectory into three segments, with each its own dimensions for developing a remediation concept based on the natural characteristics, i.e. drainage capacity, a green adsorbent and biological activity to cope with the inflow of contamination and improve water quality.
In addition, a monitoring plan has been developed including assessments of the effectiveness and progress with flux measurements.