Comprehensive risk assessment for the remediated site

To assess whether the environmental risk in the groundwater caused by the chlorinated alkenes can be reduced by treatment with Carbo-Iron, a comprehensive risk assessment was performed.

The environmental risk before and after application of Carbo-Iron was assessed, integrating risk indices for the chemical, ecotoxicological and physico-chemical component.

The desired remedial effect of Carbo-Iron is discernible in the decreasing chemical and ecotoxicological risk indices in all groundwater samples investigated [D.1, Figure 5 and D.2, Tables S9 and S12] during the first 58 d after Carbo-Iron injection. This effect is most pronounced in samples from the contamination zone I (wells GWM1 and RKS13). While the chemical and ecotoxicological risk indices decreased during this first post-treatment period, Carbo-Iron caused increased physico-chemical risk indices due to a reduced redox potential (in zone II (CMT2) on d 190 and in zone III (RKS24) on d 31 and d 58) or an increased conductivity and a lower pH (in zone II (CMT1) on d 93) [D.1, section 3.3]. This effect on redox potential can be assumed to be temporary, caused by the reaction of Carbo-Iron with the pollutants and the oxygen in the groundwater, i.e.

ceasing with loss of reactivity of Carbo-Iron. However, with the available data it is not possible to verify these assumptions.

The risk indices increased again after the first 58 d of Carbo-Iron application. This was expected, since the amount of Carbo-Iron used in the field study was not sufficient for a complete remediation of the groundwater [D.1, section 3.3]. In most contamination zones, the risk indices reached values slightly lower than before the application of Carbo-Iron. However, in zone II (CMT2), the risk index for the ecotoxicological risk increased on d 93 and d 190 above all previous values and the environmental risk, representing the mean of all three risk components, was higher than before the application of Carbo-Iron. On these days, concentrations of ethane, ethene and 1,1-dichloroethene and 1,2-dichloroethene reached their maximum values. Similarly, increased concentrations were observable for all zones investigated in the study. In zone II, the measured concentrations exceeded the relatively low PNEC values for 1,1-dichloroethene and 1,2-dichloroethene. These increased risk ratios represent the main reason for the increased ecotoxicological risk in zone II. The presence of these transformation products and the simultaneous absence of vinyl chloride is typical for PCE degradation by Carbo-Iron and thus indicates proper functioning of the remediation (Mackenzie et al., 2012). Yet, the increased risk is not desirable, especially in the long term. In the present study, effects of Carbo-Iron on the pollutants beyond d 190 were not evaluated and the fate of the transformation products was not examined. However, Vogel et al. (2018) observed in a field study

that 600 days after treatment with Carbo-Iron groundwater microorganisms grew on the active carbon component of Carbo-Iron and that biogenic dehalogenation of the halogenated alkenes occurred. Thus, in the long term and considering the decreasing influence of Carbo-Iron on the physico-chemical risk index, further decreasing values for the environmental risk are expected.

These finding indicate that the use of Carbo-Iron for the treatment of groundwater contaminated with chlorinated alkenes is promising. Still, it should be kept in mind that further remediation methods are available. To choose the most appropriate method for the respective groundwater, the environmental risk of these methods should be assessed and compared to the risk indices obtained in the present study. While a detailed assessment of these methods was not feasible in the present thesis, in the following a brief and tentative evaluation of several techniques available is provided.

Conventional ex situ processes, e.g. soil excavation and pump and treat, have been used for the remediation of PCE and TCE in groundwater (see reviews by Lemming et al., 2010; Pant and Pant, 2010). However, remediation of groundwater polluted with chlorinated hydrocarbons with these techniques is usually progressing relatively slow, the costs are high (mainly because of a high energy demand by the equipment), the structure of the aquifer was often physically destroyed by excavation and the treatment was inefficient (Bankston et al., 2013; Majone et al., 2015). The low efficiency of soil excavation is based on the density of PCE and TCE, which is higher than the density of water, leading to an enrichment of the pollutants below the groundwater (Pant and Pant, 2010).

Water extraction techniques and subsequent ex situ treatment are impeded by the low water solubility and high volatility of the chlorinated alkenes: the pollutants easily adsorb to organic and sand particles (Aggarwal et al., 2006; Lee et al., 2007; Zytner, 1992) and volatize into porous media in the head space of the groundwater (Marrin and Kerfoot, 1988; Pavlostathis and Mathavan, 1992).

Based on their lack of efficiency and intrusion in environmental structures in the aquifer, conventional ex situ methods are not suited for the environmentally safe remediation of chlorinated hydrocarbon pollution in groundwater (Lemming et al., 2010b; Majone et al., 2015).

Thus, in situ treatment methods have been developed and increasingly used for the treatment of chlorinated hydrocarbons in the last two decades (Lemming et al., 2010a). To operate effectively with minimal maintenance over the time necessary for remediation, the in situ concept was further developed to establish permeable reactive barriers, emplaced into the groundwater to intercept the flow path of a contaminant plume (Faisal et al., 2018). With in situ methods, chemical oxidation of chlorinated hydrocarbons with permanganate (Watts and Teel, 2006), hydrogen peroxide (Gates

and Siegrist, 1995) and persulfate (Tsitonaki et al., 2010) were investigated. These oxidative treatments are expected to lead to a full mineralization of e.g. PCE to chloride, water and carbon dioxide (Yan and Schwartz, 1999). However, hydrogen peroxide is highly unstable and persulfate is only moderately stable in groundwater (Watts and Teel, 2006), demanding constant control of the remediation process and high amounts of the respective remedial agent. Additionally, transformation products of oxidative degradation often include reactive transformation products, potentially having a high toxicity. Considering the demand for large amounts of the remedial agent and the toxicity arising from reactive transformation products, the environmental risk of oxidative remedial techniques is likely to be higher than for the reductive-acting Carbo-Iron.

Reductive treatment for removal of chlorinated alkenes is an attractive option, based on high dechlorination rates, even at low temperatures, and the theoretical absence of chlorinated end products (Bruin et al., 1992). Compared to oxidative treatment techniques, the stoichiometric efficiency of reductive chlorinated hydrocarbons degradation is clearly higher. Several remediation agents are available and from those, nFe⁰ and composites with nFe⁰ were emerging in the past decade (Patil et al., 2016; Tosco et al., 2014). Though extensively used in the USA (Mueller et al., 2012) ERA for iron-based remedial agents are not available. However, based on results from ecotoxicity tests with several nano-iron particles (Bhuvaneshwari et al., 2017; Hjorth et al., 2017;

Nguyen et al., 2018; Patil et al., 2016; Schiwy et al., 2016; Semerád et al., 2018; Xue et al., 2018;

Zheng et al., 2008) it seems unlikely that the environmental risk of these remediation agents is higher than that of Carbo-Iron investigated in the present thesis.

Another in situ technique for the removal of chlorinated hydrocarbons from groundwater is bioremediation. This involves the extraction of groundwater, but unlike the pump & treat technology, the extracted groundwater is enriched with nutrients (e.g. soybean and lactate (Lemming et al., 2012)) and injected in a recharge well. The nutrients then activate the bacteria present in the groundwater, which results in the degradation of the contamination (Juwarkar et al., 2010). Three metabolic processes are able to degrade chlorinated ethenes: reductive dechlorination in anaerobic conditions, co-metabolism under anaerobic conditions and oxidation in aerobic or anaerobic conditions (Pant and Pant, 2010). Factors such as low temperature, anaerobic conditions, low levels of nutrients and co-substrates, the presence of contaminants in concentrations that can be toxic for the bacteria, and the physiological potential of microorganisms can limit the efficiency of microbial degradation of contaminants (Megharaj et al., 2011).

Furthermore, in anaerobic conditions and depending on the presence of other organics acting as

electron donors, the possibility exists that the final or long-term transformation products of this remediation are dichloroethenes or vinyl chloride (Bardos et al., 2018; Russell et al., 1992).

Dichloroethenes and vinyl chloride are more mobile than PCE and TCE (Chambon et al., 2010) and vinyl chloride is cancerogenic (see section 3.2). Moreover, the PNEC of 0.025 mg/L and the target value of 0.005 mg/L for vinyl chloride are the lowest of the pollutants investigated in the present thesis [D.1, Table 2 and D.2, Table S10]. Increasing concentrations of vinyl chloride would thus increase the chemical and ecotoxicological risk and the resulting environmental risk. This is, however, strongly depending on the aforementioned conditions in the groundwater.

A further alternative to the treatment of groundwater with Carbo-Iron is a no-action scenario. In this case, concentrations of the pollutants would be diluted by infiltrating ground water (Lemming et al., 2012). While this leads to lower pollutant concentrations, the contamination is spread over a larger area. Depending on the microbial community in the aquifer, natural bioremediation could potentially occur (Hancock et al., 2005). This process would take longer than any of the aforementioned remediation techniques and as mentioned for the induced bioremediation, the degradation of chlorinated alkenes to vinyl chloride is possible. Depending on the transformation products of the biological degradation (e.g. vinyl chloride) and the flow rates of the infiltrating groundwater, the environmental risk in the contaminated area would decrease very slowly:

Lemming et al. (2012) estimated a time frame of 800 years necessary until TCE concentrations in the groundwater meet the quality criteria.

In summary, the in situ instalment of a reactive barrier with the reductive remediation agent Carbo-Iron is highly suitable for the treatment of chlorinated alkenes. Compared to pure nFe⁰, the composites of activated carbon and nFe⁰ have the benefit of higher remediation efficiency based on the adsorptive enrichment of hydrophobic contaminants (e.g. chlorinated alkenes) on the active carbon component and subsequent reductive reaction with the nFe⁰ component (Mackenzie et al., 2012). Additionally, the increased mobility of Carbo-Iron (Busch et al., 2015, 2014) facilitates the in situ installation of a permeable reactive barrier in the aquifer (Georgi et al., 2015).

However, a certain environmental risk of Carbo-Iron exists (i.e. by potentially temporary transformation products and Carbo-Irons impact on physico-chemical parameters in the groundwater, see section 4.4), though it is most likely limited to the area close to injection points in the contaminated site. Considering the comparatively low environmental risk (Lemming et al., 2010b), in situ bioremediation may represent a suitable alternative to the groundwater treatment with Carbo-Iron. During bioremediation, the favoured reductive dechlorination pathway of the

pollutants provided by nFe⁰ is possible, too. However, as mentioned above, degradation might not proceed completely to nontoxic products and dichloroethenes and vinyl chloride can accumulate.

A thorough assessment of the respective contaminated site and the conditions in the groundwater is thus necessary to determine the suitability of bioremediation. In this context it should be considered that a stimulation of bioremediation was observed after the treatment of groundwater with nFe⁰ (Bardos et al., 2018) and Carbo-Iron (Vogel et al., 2018).