COMBINING GEOCHEMICAL AND NUMERICAL MODELING FOR CHLORINATED SOLVENTS GROUNDWATER CONTAMINATION
Chloroethenes represent a serious risk to aquatic ecosystems and human health on a worldwide scale. Chloroethenes contamination is difficult to solve due to the different optimal redox conditions needed to obtain efficient degradation of each chloroethene. The more chlorinated ones such as tetrachloroethene (PCE) and trichloroethene (TCE) are better degraded via reductive dechlorination (RD) under anaerobic conditions, while the less chlorinated compounds such as dichloroethene (DCE) and vinyl chloride (VC) are better degraded via oxidation (OX) under aerobic conditions. Thus, sequential bioremediation systems (SBSs) able to stimulate anaerobic and aerobic biodegradation are preferred over traditional bioremediation systems targeting single degradation pathways. Assessing the efficiency of SBSs is a challenging task that requires a multidisciplinary approach, spanning from hydrogeology, organic geochemistry and microbiology. While expert knowledge and laboratory analyses provide initial inputs for the design of SBS, field monitoring data are needed to evaluate if and how bioremediation is carried out. Reactive transport models (RTMs) are suitable tools to assist decision makers when designing and monitoring bioremediation efficiency. To date, no applications to SBSs have been reported, possibly owing to the relative novelty of SBS and to the additional modeling challenges (e.g., more complex parameterization) that sequential systems pose over traditional bioremediation systems. RTMs have the capability to integrate and harmonize multiple data and, when properly tailored, to make useful predictions about a system’s behavior when different remediation set-ups are tested in order to optimize the cleanup operations. The general objective of this thesis was to develop methodologies for the evaluation of the efficiency of SBSs applied to chloroethenes aquifer contamination. As case study, we investigated one the largest SBS currently implemented in a polluted alluvial aquifer in Italy. The SBS is about 800 m-long and was created to remediate organic pollutants such as chloroethenes and petroleum hydrocarbons (PHCs). The SBS is made up of a hydraulically upgradient anaerobic (AN) biobarrier, where a reducing substrate is injected to stimulate RD of higher chloroethenes (PCE, TCE), and downgradient aerobic (AE) biobarrier, where OX of lower chloroethenes (DCE, VC) is stimulated through nutrients and oxygen injection. In addition, P&T wells are active downgradient of the two biobarriers in order to intercept the remaining contaminants exfiltrating from the SBS. The first part of the thesis evaluates the initial characterization activities propaedeutic to the installation of the operational (full) scale SBS: Microcosm experiments and in situ tests were carried out to assess the biodegradation potential of the autochthonous microbial communities in the laboratory and in the site under both natural and biostimulated conditions. Both investigations showed a satisfactory efficiency of aerobic and anaerobic degradation only after biostimulation, proving the feasibility as well as the necessity of the full-scale SBS for the cleanup of the site. The analysis of these data constitutes an improvement of the current state of the art in the design and implementation of large-scale SBS. The second part of the thesis is dedicated to the analysis of time series of chloroethenes concentrations and environmental parameters observed during periodic monitoring of groundwater in the site piezometers of the full-scale SBS. Carbon compound-specific isotopic analysis (C-CSIA) were carried out on chloroethenes PCE, TCE, cis-DCE and VC in samples acquired along a flow path crossing both biobarriers. Target chloroethenes concentrations and isotopic compositions were interpreted through a one-dimensional (1D) geochemical model in order to assess the processes controlling the sequential bioremediation and to obtain a first-cut evaluation of the effectiveness of the SBS. The active SBS proved to be efficient in reducing chloroethenes concentrations along the investigated flow path (reduction of concentrations up to 100%). The AN barrier proved to be able to significantly enhance RD of PCE and TCE mainly, increasing natural anaerobic degradation up to 30 times. The AE barrier, instead, proved to have a fundamental role in enhancing OX of cis-DCE and VC. The third part of the thesis focuses on the development of a multidimensional model with enhanced predictive capabilities compared to the 1D model. Different degrees of spatial heterogeneity of degradation efficiency were taken into consideration by setting up two models with different parametrization of degradation constants (i.e. homogeneous and heterogeneous distribution). Moreover, it was also possible to model the P&T system. and simulate different scenarios by deactivating the P&T to determine the most efficient setup of the system (i.e. with or without P&T). Contaminant loadings exfiltrating from the site were quantified and compared between the different scenarios and Damköhler numbers (Da) were calculated to evaluate the spatial heterogeneity of degradation efficiency. The 2D model showed the importance of choosing a spatially heterogeneous configuration rather than a homogeneous one in order to obtain an accurate reproduction of contaminant loadings. Da permitted to prove the heterogeneity of degradation efficiency and pinpoint critical areas in the site where bioremediation is less efficient. Another important result was the demonstration of the P&T importance in abating contaminant loadings and providing a better containment of the plume of contaminants, specifically the more toxic VC whose loadings resulted to be the highest in the site. Overall, the multiscale experimental activities were fundamental for the assessment of the site suitability to sequential bioremediation and for the subsequent implementation of the SBS. The multidisciplinary approach adopted permitted to evaluate the efficiency of the remediation system.
chloroethenes, enhanced bioremediation, sequential bioremediation system, anaerobic organohalide respiration, aerobic oxidative biodegradation, landfill, compound-specific stable isotope analysi, reactive transport model, PHREEQC, RT3D., Settore GEO/05 - Geologia Applicata
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