Abstract
Groundwater contamination and transport of viruses and bacteria in aquifers are a major concern worldwide. To ascertain the ability of these aquifers to remove pathogens, tracer tests with microbial surrogates are carried out. These tests are laborious and may require special permits, and therefore, column tests are often done instead. Unfortunately, results from column tests tend to grossly overestimate removal rates when compared to the field scale, which can lead to an underestimation of groundwater contamination risks. Scale is an important consideration when examining pathogen transport through porous media, as pathogen removal is rarely a linear process. In this study, field tests were carried out with endospores of Bacillus subtilis and coliphage phiX174 over a distance of 25 m in an alluvial gravel aquifer near Vienna, Austria. The sandy gravel material from the field site was also used in column tests with the same tracers. Both attachment-detachment and colloid filtration theory were used to model these tests, as well as log-removal rates per meter. The results show that the spatial removal rate (log/m) is approximately 2 orders of magnitude higher on the column scale, when compared to the field. A comparison with the literature showed a correlation between the heterogeneity of the porous media and the difference in removal rates between the column and field scale.
Highlights
Groundwater is an important source of drinking water for many people around the world
The zeta potentials for both microbes were measured in 4 different sterile filtered matrices: groundwater sampled from P24 at the field site, Vienna tap water used for the column tests and a standard 10 mM NaCl solution buffered to pH 7.3 and 8.0 using NaHCO3, for literature comparison
In double-distilled water at pH 7, the zeta potential of phiX174 was −31.78 mV, and it became less negative as the pH increased after reaching a minimum around a pH of 5.5.47 This may explain why phage phiX174 was less negative in Vienna tap water, which had a higher pH than the Lobau groundwater, whereas the zeta potentials of spores of B. subtilis were similar in Vienna tap water and Lobau groundwater
Summary
Groundwater is an important source of drinking water for many people around the world. Removal of pathogens in the subsurface varies greatly depending on the type of microorganism and its interaction with site-specific aquifer material, so it is often impossible to transfer the results from one site to another.[11] Soil characteristics, such as chemical attributes, rock fractures, lenses of higher permeability, and physical heterogeneity, can negatively influence the removal of pathogens during transport.[12−14] Though it has been shown that preferential transport pathways are responsible for decreased pathogen removal rates in porous media, more research is needed to adequately describe these processes.[15]. Observed removal processes in columns might not be representative of the field scale, and this method often grossly overestimates microbial removal rates and parameters controlling attachment.[17,18] it is essential to understand how scale affects colloidal transport in porous media and to identify the dominant factors that influence the upscaling of transport processes. We hypothesize that one reason average removal rates per meter are higher at the column scale when compared to the field scale is because of soil heterogeneity and the fact that preferential flow elements, such as cracks or lenses of coarser material, are not captured at the column scale
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