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21  November  2005

Predicting bacterial transport in the vicinity of drinking-water wells: what column- and field-scale tracer tests can tell us and what they cannot

R.W. Harvey

U.S. Geological Survey, 3215 Marine Street, Boulder, Colorado 80303 USA. ( E-mail: rwharvey@usgs.gov )
Accurate prediction of subsurface bacterial transport is important for well head protection, but generally necessitates site-specific information obtained from transport studies that involve media from the area of the aquifer around the well. Because of the difficulties of running such experiments in the field, studies involving bacterial transport through geologic media usually are conducted on the column scale. However, there is a growing body of evidence suggesting that bacterial transport observed in small-scale injection and recovery tests performed in granular, drinking-water aquifers is considerably greater than that observed for a similar distance through columns of repacked sediments in which the pore structure has been altered. However, even in situ injection and recovery tests using labeled bacteria may not yield results that allow accurate prediction of the transport of bacterial pathogens from a contaminant source to a drinking-water well. In part, this is because such tests are not designed to assess the effects of ecological factors, such as the effects of grazing by indigenous protozoa, in-situ growth, and environmental carrying capacity, upon the abundance of bacteria being advected through an aquifer towards a well. Accounting for these ecosystem-level parameters can necessitate additional types of in situ tests.


This paper was presented at the International Workshop "Microorganisms - Water and Aquifers", organized by the Zuckerberg Institute For Water Research, J. Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, held on September 20-21, 2005 in the Sede Boqer Campus, Israel.

Introduction

Most studies designed to delineate the transport behavior of bacteria in drinking-water aquifers are performed in the laboratory. Flow-through column studies provide a greater degree of control and, consequently, are useful in providing detailed, mechanistic information about specific processes affecting the subsurface movement of bacteria. Also, columns can be designed to more nearly meet initial boundary conditions for bacterial transport models and avoid the release of potentially harmful bacteria into drinking water aquifers. However, many processes that govern transport of bacteria in drinking-water aquifers are operative on spatial- and temporal-scales not conducive to laboratory studies. In-situ injection and recovery tests involving bacterial surrogates can provide additional information about the potential for substantive advective migration of pathogenic bacteria in the vicinity of wells. Because of the time, expense, and technical difficulties involved in conducting natural-gradient bacterial transport studies in the field, in situ bacterial tests involving granular aquifers usually are conducted over relatively short travel distances, e.g., <30 m (e.g., harvey and garabedian, 1991; johnson et al., 2001; fuller et al., 2004).

It is often beneficial to combine laboratory experiments that allow better delineation of individual processes and model calibration with field studies that provide a framework in which the applicability of laboratory-derived results can be evaluated under natural conditions. In general, considerably more information can be derived from an iterative approach that combines both column- and small-scale field investigations. However, even a coupled, iterative approach using both column- and small-scale field studies may not account for the combined effects of the various controls that govern microbial transport on an environmentally relevant field scale. There are many factors in addition to the hydrological characteristics of the aquifer itself, which control the movement of bacteria in the vicinity of drinking-water wells. These biotic and abiotic factors include growth, predation by protists, possible parasitism by bacteriophages, motility, lyses under unfavorable conditions, changes in cell size and propensity for attachment to solid surfaces in response to alterations in nutrient conditions, spore formation in the case of some gram-positive species, reversible and irreversible attachment to solid surfaces, detachment from surfaces, and straining (Fig. 1). Many of these processes are inter-related through other factors, poorly understood, and (or) difficult to describe mathematically.

factors in bacterial transport

Figure 1. Factors that affect the subsurface transport of bacteria in the vicinity of drinking-water wells.

Results and Discusion

Column-scale studies

Laboratory-scale studies have resulted in useful and detailed information on a number of processes governing bacterial transport behavior in granular media (Harvey and Harms, 2001). For example, it has been demonstrated that bacterial attachment/transport studies involving core material from the aquifer can allow differentiation between physical and chemical controls (Dong et al., 2002). However, in general, even when subsurface core material is employed results of flow-through, bench-scale studies are generally difficult to extrapolate to the field-scale. There are several reasons for this. The unavoidable alteration of the secondary pore structure caused by repacking, handling, and (or) shipping of the core material typically diminishes bacterial transport (Smith et al., 1985; Harvey et al., 1993). Although generally negligible at the column-scale (Unice and Logan, 2000), hydrodynamic dispersion is quite large (e.g., meters) in field-scale transport studies (Leblanc et al., 1991). Finally, the elevated flow velocities typically used in flow-through columns relative to most granular aquifers can affect the degree of bacterial immobilization (Hendry et al., 1997; Camesano and Logan, 1998).

Field studies

Small-scale injection and recovery studies in which labeled, non-pathogenic bacterial populations are added to and tracked downgradient in an aquifer have yielded valuable site-specific information about the role of local-scale physical heterogeneities (Mailloux et al., 2003), aquifer removal characteristics for pathogens (Schijven et al., 2000), and in-situ bacterial growth (Mailloux and Fuller, 2003). However, small-scale injection and recovery tests have not elucidated the effect of groundwater protists, which can be abundant and very active in drinking water aquifers (Kinner et al., 1998, 2002), upon the fate of bacterial pathogens. Many drinking water aquifers are limited with respect to readily-degraded organic carbon and very little information is available regarding the environmental carrying capacity of these systems. Evidence collected from large-scale surveys (Harvey and Barber, 1992) and from recent down-well incubations in a sole-source aquifer in Cape Cod, Massachusetts suggests that the downgradient abundances of bacteria intentionally or unintentionally introduced to certain groundwater environments may, at least for bacteria capable of in situ growth, be determined more by a site-specific “carrying capacity” than by predation and (or) sorptive losses (Harvey et al., in preparation). The abundance of bacteria at substantive distances downgradient in such systems would be difficult to predict a priori from any existing transport models. This has important implications for transport of certain pathogens in drinking water aquifers and for the practices of aquifer storage and recover and riverbank filtration.

References

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  2. Dong H., Onstott T.C., Deflaun M.F., Fuller M.E., Scheibe T.D., Streger S.H., Rothmel R.K. and Mailloux B.J. (2002). Relative dominance of physical versus chemical effects on the transport of adhesion-deficient bacteria in intact cores from South Oyster, Virginia. Environ. Sci. Technol., 36(5), 891-900.
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