Contaminant mass discharge of chlorinated solvents from sources in clay till: Concepts, quantification and risk assessment

Louise Rosenberg

Research output: Book/ReportPh.D. thesis

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Worldwide, contaminated sites pose a threat to water resources, in particular groundwater. Legacy chemicals, like chlorinated ethenes, exceeding the groundwater quality criteria are vastly found in groundwater. To limit the risk posed to ecosystem health, including humans, clean-ups are needed. However, funds are not unlimited and prioritization of the contaminated sites posing a risk to a given receptor is needed. To better perform a prioritization of contaminated sites, the concept of Contaminant Mass Discharge (CMD) has been suggested as a prevailing metric. A variety of methods to quantify CMD are needed in order to perform risk assessments at different knowledge levels.

In the Northern Hemisphere, many sites are located in clay till settings. Understanding the transport of contaminants in the low-permeability clay till is complex, but vital, in order to protect the underlying aquifer and drinking water abstraction wells. High-permeability features, such as sand lenses and fractures can act as preferential transport pathways. Via diffusion, and later on back-diffusion, the clay till matrix can act as a secondary source increasing the duration of the contaminant emanating to the aquifer. Setting up efficient approaches for investigation of contaminant sources in clay till settings to inform the conceptual site model is key for a reliable risk assessment.

The aim of this thesis is to improve the risk assessment of chlorinated ethene sources in clay till posing a risk to groundwater resources. This was done by: exploring governing and relevant transport processes of chlorinated ethenes in clay till/aquifer systems; assessing field investigation approaches for evaluation of the hydrogeology and contamination at a clay till site; evaluating current methods for quantification of CMD in aquifers and suggesting incorporation of the methods in the Danish risk assessment scheme for contaminated sites. A chlorinated ethane contaminated site in Vassingerød, Denmark, was used as field case for this study.

A comprehensive study of transport and fate processes of chlorinated ethenes in clay till and underlying aquifers was conducted. The study showed a time-depending of dominant processes governing transport of chlorinated ethenes in clay till. Diffusion (enhanced by sorption) and back-diffusion in clay till play a crucial role in distributing and releasing the contaminant. In many cases, contamination with chlorinated ethenes occurred several decades ago, resulting in a more evenly distributed mass of contamination within the clay till. The uncertainty in estimating the stored mass of the contaminant in the source zone of clay till is partly attributed to the sorption coefficient of the soil. In the aquifer, steep concentration gradients in both the horizontal and vertical directions are often observed, indicating low dispersivity values and minimal dilution at local scale.

Through comprehensive field and laboratory studies the vertical hydraulic gradient, sorption coefficient of chlorinated ethenes, and high-resolution, depth-discrete water samples were investigated. Furthermore, a solute transport model was set up to investigate the quantification and governing parameters for CMD at the site and evaluate the effect of fracture aperture and sorption in the clay till. The studies showed that the vertical hydraulic gradient provide insights into the presence of fractures and enhances the accuracy of estimating the infiltration rate at the site. An advancement of using the direct-push method Membrane Interface Probe Hydraulic Profiling Tool (MiHPT) proved valuable for estimating the vertical hydraulic gradient. High-resolution, depth-specific concentration profiles can be considered a footprint of the flow field and provide valuable information about the source zone at the site. Sorption plays a significant role for the mass distribution of contaminants, leaching time and the break-through of contamination into the aquifer. For large fracture apertures, the model showed that the transport primarily happened in the fractures, whereas for smaller fracture apertures, the model showed that the advective and diffusive transport in the clay till was the governing processes.

A need for a CMD quantification method for initial site investigations at clay till sites was identified and led to the development of the ProfileFlux method. This method allows for a time-efficient estimate of CMD that enables the prioritization of contaminated sites based on the associated receptors at a catchment scale. A suggestion for incorporating different CMD methods for different stages of risk assessment frameworks has been provided. CMD has been used for several years by the scientific community, however, it is a new way of thought for many practitioners. Thus, the implementation of CMD in risk assessment is still in its early stage in management of groundwater resources by authorities. Further knowledge sharing and development of protocols could help broaden the use of CMD as a metric for risk assessment of contaminated sites.

In conclusion, this PhD thesis has identified the governing transport and fate processes for legacy spills of chlorinated ethenes in clay till. The work introduced the possibility of using the MiHPT to achieve estimates of the vertical hydraulic gradient in the clay till, and presented the new ProfileFlux method useful for a cost-effective determination of CMD. This can improve the risk assessment of contaminated sites with chlorinated ethene sources located in clay till settings. Prioritizing clean-ups, and thereby using the resources and money properly, will ultimately support the protection of the groundwater.

Finally, for future research topics it is recommended to keep focus on the determination of vertical transport in clay till, further testing of the ProfileFlux method, and develop methods for uncertainty estimates regarding CMD estimates. It was also suggested to develop a protocol for the use of CMD for risk assessment purposes in management of contaminated sites.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages172
Publication statusPublished - 2023


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