Scale aspects of microbial transport and removal in groundwater

Abstract

Groundwater is an important source of drinking water around the world; therefore, its microbiological quality is of the highest importance to human health. The focus of this doctoral thesis is on the transport and fate of microorganisms in groundwater meant for drinking water production.One of the greatest challenges involved in the process of providing safe drinking water is the assessment of the presence and concentrations of waterborne pathogens in the source water, i.e. groundwater. Measuring this is laborious and expensive and thus, cannot be done continuously. Often, pathogen concentrations are too low to detect, but can still be at unacceptable levels for public health. Because of this, it is important to use groundwater that has been sufficiently filtered through the porous media of the subsurface to reduce concentrations of waterborne pathogens sufficiently. However, the distance that groundwater should travel before it can be considered safe depends on the type of pathogen in addition to the geochemical composition of the aquifer. Because of these variations in aquifer composition, ideally, aquifers should be tested separately for their capacity to remove microorganisms from groundwater. A good way of performing such testing is to use tracer tests with microbial surrogates. Unfortunately, this is not always possible because of the practical difficulties involved. Furthermore, microbial surrogates do not necessarily accurately model the target pathogen in every material, and since pathogens commonly cannot be used in the field, column tests are often used to compare the surrogate to the pathogen. For these reasons, laboratory column tests are often done instead of field tests. However, these tests are performed on a much smaller scale; therefore, methods need to be developed to upscale the results based on the actual conditions of the field site. After obtaining the correct removal capacity of the aquifer, the minimum groundwater protection zone around a drinking water pumping well needs to be calculated. One method for performing this calculation is to define a health-based target of an acceptable risk (infections/person/year), and to perform a quantitative microbial risk assessment (QMRA) using different amounts of microbial removal, which can then be translated to microbial transport distances to select a suitable groundwater protection zone. Another option is to look at unconventional methods for measuring microbiological groundwater quality, which could help us in certain cases, for example at remote locations where a microbiological laboratory is not readily accessible. The goal of this doctoral thesis is to explore the transport and fate of microorganisms in groundwater at different scales, by answering the following questions: (1) What are the most important parameters and processes that influence upscaling of microbial transport in porous media? (2) How does the size of well protection zones change when moving from one calculation method to another, and what influence does pumping rate play? (3) How could alternative methods for the detection of potentially adverse bacteria be used to help with providing safe drinking water?Following the Introduction, Chapter 2 describes the use of field and column tracer tests to investigate the processes involved in subsurface microbial transport. It was found that subsurface heterogeneity and how much it conduces preferential flow were the most important factors affecting upscaling of microbial transport, and were more important than the size or type of microbial tracer. Chapter 3 describes the calculation of safe setback distances in a well field near Vienna, Austria for two pathogens, namely Cryptosporidium and Campylobacter. According to our best estimates, specifically for this study site and only for these two pathogens, it was found that the groundwater protection zone did not significantly change in size when switching from the 60-day travel time method to the QMRA method (using an acceptable risk level of 10-4 infections/person/year). However, QMRA needs many input parameters that are difficult to measure accurately, and relying on literature values instead of accurate measurements can lead to larger setback distances. Chapter 4 deals with a semi-quantitative, easy-to-use microbiological test called the Laboratory Biological Activity Reaction Test (LAB-BART) that tests for metabolic activity of potentially adverse bacteria in groundwater, which was applied at two riverbank filtration sites in Austria in conjunction with conventional methods used to assess the chemical and microbiological groundwater quality. Although these tests are not as accurate as other methods, such as polymerase chain reaction (PCR), it was found that LAB-BART can serve an important purpose in terms of pre-screening waters of interest in remote locations, or as indicators of potential changes in groundwater quality due to any changes in the flow field, especially when problems with well clogging or biofouling arise.In this doctoral thesis, further insight was gained into the processes and parameters influencing microbial transport and how this knowledge can be used to define safe groundwater protection zones for drinking water production. Results based on our work imply that there is a large difference between microbial transport at different scales, even at the same location, and a thorough understanding of different processes (including preferential flow) in the hydrological system is necessary to ensure microbially safe drinking water from groundwater.