Risk assessment of chlorinated ethene plumes impacting streams: contaminant mass discharge, field methods and attenuation

Abstract

Contaminated sites across the world are a major threat to human health and the environment. Contaminants from contaminated sites can migrate through aquifers and discharge to streams, leading to decreased surface water quality and ecosystem health. The vast number of sites makes clean-up of all economically infeasible; hence, risk assessment is used to prioritise the sites that pose a risk. When assessing the risk posed to streams, contaminant mass discharge (CMD) is a key parameter. CMD is defined as the contaminant mass passing through a hypothetical control plane per time. Near streams, the CMD approach has several applications due to the possibility to obtain both subsurface and in-stream estimates. However, the advantage of the CMD approach near streams has not yet been fully explored. It is not trivial to measure the CMD near streams due to large spatial variations in contaminant concentrations and hydraulic conductivities, as well as the temporally varying multi-directional groundwater flow. Field tools that can overcome the challenges related to such variabilities are required. The point velocity probe (PVP) and the novel sediment bed passive flux meter (SBPFM) provide direct measurements of seepage velocity and contaminant flux, respectively, and are promising tools for CMD quantification near streams. Chlorinated ethenes are a key contaminant group. After release to the environment, chlorinated ethenes may undergo natural attenuation. This can reduce contaminant mass, but can also lead to the formation of more harmful substances, such as vinyl chloride. Conditions beneath streams can be more favourable for natural attenuation than conditions in aquifers. Nevertheless, risk assessment is challenged by the variation in attenuation observed near streams; some literature studies observe extensive attenuation, while others observe inconsistent attenuation. The aim of the thesis is to improve risk assessment of contaminated sites and plumes impacting streams. This was done by exploring the advantage of the CMD approach near streams, evaluating different field methods for CMD quantification, and quantifying and conceptualising near-stream attenuation of chlorinated ethenes. The Grindsted Stream site was used as a field study site. Detailed field investigations was conducted to quantify subsurface and instream CMDs of chlorinated ethenes. These estimates were shown to have several applications. In-stream CMD discharges along a stretch of Grindsted Stream were used to identify unknown and/or main pathways and sources. Subsurface CMDs can inversely be used to predict stream concentrations, which can support the identification and prioritisation of known contaminated sites. Contaminant mass balance analyses were conducted at the reach scale by comparing total CMDs, as well as CMD-based molar ratios of individual chlorinated ethenes at several subsurface and in-stream transects. Such analyses showed to be capable of improving the conceptual understanding of the system with respect to temporal characteristics of the sources, seasonal trends in stream concentrations, key pathways, and dominant attenuation processes. This level of conceptual understanding is crucial for the design of field campaigns for conservative risk assessment, identification of receptors at risk, and selecting remedial methods. Finally, mass balance analyses can be used to quantify the combined and sometimes even the distinct attenuation processes by assessing mass loss/gain or shift in molar ratios. Several field methods for CMD quantification exist. Darcy’s law-based methods are commonly applied; however, the multi-directional groundwater flow field near streams may decrease the quality of results based on Darcy’s law. PVP-based and SBPFMs-based methods were applied at the Grindsted Stream site. The PVPs were capable of measuring the magnitudes and directions of both horizontal and vertical groundwater flow at the stream bank that could be used to estimate CMD. SBPFMs deployed in a transect in the streambed yielded averaged flux concentrations and specific discharges comparable to results from water samples and various methods for groundwater flow quantification, respectively. DCIP geophysical measurements were additionally applied. DCIP data could improve the description of the concentration distribution – especially for low sampling densities, that are common at large sites – and thus decreased the uncertainty of CMD estimates. Based on a comparison of selected field methods, it was concluded that no single method for CMD quantification near streams is always more advantageous than other methods. The choice of method should rather be based on the purpose and scope of the investigation as well as specific site characteristics. Attenuation is commonly assessed by applying the multiple line of evidence approach. This approach integrates multidisciplinary methods to increase confidence in the interpretation of results and observations at field sites. The majority of the field studies assessing near-stream attenuation of chlorinated ethenes have advanced process understanding. Such knowledge is important for risk assessment, however it cannot stand alone. From a risk assessment perspective, quantifying the effect of the attenuation by applying the CMD approach, as well as developing holistic conceptual models are additionally required. By applying the CMD approach at the Grindsted Stream site, it was found that limited near-stream attenuation of chlorinated ethenes takes place. Volatilisation was identified as the only important attenuation processes removing chlorinated ethenes from the stream, however, the rate of removal is slow. Consequently, the vinyl chloride concentration exceeds the environmental quality criteria for several kilometres, hence Grindsted Stream is at risk. Based on experiences from the Grindsted Stream site and literature case studies, a set of holistic conceptual models were developed. These models show the near-stream attenuation behaviour for discharging chlorinated ethene plumes for various scenarios. In essence, they illustrate that the attenuation – and thus the influence on the surface water quality – depends on the hydrogeology, biogeochemistry and stream characteristics. If favourable conditions for attenuation are only present in parts of the plume discharge zone, a patchy attenuation behaviour will be expected. Moreover, the models identify some key parameters, which – in addition to the CMD – can support reliable risk assessment. These include: geological heterogeneity, seepage velocity, redox conditions, key microorganisms and content of organic carbon in sediments. In conclusion, this PhD thesis has demonstrated the advantages of the CMD approach near streams, compared a selection of field methods for CMD quantification, as well as quantified and conceptualised near-stream attenuation of chlorinated ethenes. This can improve risk assessment and thus strengthen the foundation on which prioritisation of remedial efforts is based. Ultimately, this can support efforts to ensure good surface water quality.