Tunnelling, Chalk and turbidity: conceptual model of risk to groundwater public water supplies

Abstract

Tunnelling for infrastructure developments within sections of Chalk below the water table creates suspended sediment, which may give rise to turbidity risks where tunnel alignments pass close to abstraction boreholes used for public water supply. In high-transmissivity zones in Chalk it is predicted that groundwater velocities within fractures are high enough to maintain continual suspension and transport of sediment. The assessment of risk from turbidity has been conceptually modelled in a manner akin to a source–pathway–receptor contaminant transport model. Turbidity characteristics in Chalk are described as well as turbidity effects upon public water supplies. Data from a case study of a proposed tunnelling project have been used in this study. Data acquired from the site investigation stage indicated that turbidity levels of over 4000 Nephelometric Turbidity Units (NTU) were created from drilling and abrading the Chalk, which may be a proxy for what occurs at tunnelling cutting faces. The sediment generated had a d80 particle passing size value of 10.5 µm and groundwater velocities in fractures were calculated to be c. 0.5 mm s−1; literature values suggest that higher velocities are attainable and therefore these values are considered sufficient for suspending the sediment. The conceptualization used information from hydrogeological, geotechnical, engineering and water-resource assessments. The risk assessment part of the model focuses on how turbidity could be released from tunnelling machinery in specific circumstances (the source term), how the continued suspension of sediment and movement within the aquifer (the pathway) could occur and assessment of consequences at the receptor. The conceptualization concludes that there is a plausible risk of turbidity pollution in the Chalk aquifer from tunnelling, with a discussion on further data acquisition and approaches to quantitative analysis and modelling.Thematic collection: This article is part of the Ground models in engineering geology and hydrogeology collection available at: https://www.lyellcollection.org/cc/Ground-models-in-engineering-geology-and-hydrogeology