Evaluation of subsurface microbial transport using microbial indicators, surrogates and tracers
The goal of this chapter is to provide the readers with useful information that can help with evaluating the microbial contamination risk of groundwater from faecal sources, determining safe setback distances and selecting suitable sites for wastewater reclamation.
Estimating microbial contamination risk in groundwater from faecal wastewater disposals requires knowledge of faecal pathogen loading, microbial attenuation in subsurface media and their transport pathways, which are the essential information required for Quantitative Microbial Risk Assessment (QMRA).
The chapter will outline the available indicators, surrogates, mathematical models, laboratory and field methods that are used for evaluating microbial subsurface transport, and summarise the current knowledge on the capacity of subsurface media in attenuating microbial contaminants. The usefulness and limitations of the exiting tools will be discussed to guild the end-users to find the right decisions about management, treatment and monitoring.
A number of useful tracers (salts, dyes, DNAs and emerging chemical tracers) for establishing flow connection, identifying and tracking the sources/pathways of faecal contamination in groundwater are described. Microbial tracers, microspheres and novel “micro mimics” (biomolecule-modified particles) to determine microbial attenuation and transport in porous media are discussed. Information on their applicability and representativeness are given, including their enumerable concentration range by detection methods.
Mathematical models are given to describe processes involved in microbial transport in subsurface media, including advection, dispersion, degradation (persistence/inactivation), attachment, detachment, straining and colloidal associated transport.
Laboratory methods are described for characterising the physiochemical properties of (bio)colloids, soil and aquifer media, determining microbial inactivation rates using incubation tests, examining particle attachment/detachment between water-solid interface in batch tests, and studying mechanisms and processes of microbial transport in porous media using column experiments. Relative attenuation of pathogens, indicators and surrogates determined from column studies in various porous media is summarised.
Field studies of microbial transport are discussed in investigating the impacts of preferential flow and heterogeneity of porous medium and (bio)colloid properties on microbial transport. This is followed by a summary of microbial removal rates and transport parameter values derived from field studies using indicators, surrogates and pathogens in soils, vadose zones, and groundwater (with links to other chapters). Based on microbial removal rates derived from field studies, suitable sites for wastewater reclamation are recommended.
One section presents model tools for calculation of protection zones/setback distances and for including river bank filtration (RBF) as part of QMRA. For RBF these tools are QMRAspot and QMRAcatch.
Finally, a selection of field studies covering developing and developed countries are described. These studies encompass relatively homogenous fine-grain aquifers (sandy aquifers) and heterogeneous aquifers (alluvial sandy gravels, karst, fractured rocks), focusing on cases of groundwater contamination by wastewater disposals.