How do we study subsurface microbial activities?   The study of microbial processes in the subsurface is an important part of understanding the health of the Nation's groundwater supply. About 40% of the water we use is derived from groundwater sources. There have been many studies illustrating how contaminants in water supplies are very difficult to remediate.  We perform injection and recovery experiments (figure at left) to assess how microbes move in the subsurface. The figure on the left is an illustration of the types of experiments done in the field to determine the extent of subsurface microbial movement (Forced Gradient and Natural Gradient) and assess the spread of disease-causing microorganisms in groundwater systems.

Why is groundwater so important? Over the last 10 years, incidence of water-borne illnesses attributed to groundwater has increased by 50%, which is more than 7 million reported illnesses per year. Also, groundwater can be difficult and expensive to clean up. A quart of oil can contaminate about 500 thousand gallons of groundwater.  Viruses, bacteria, and protozoa are differently-sized, have different survival mechanisms, and have different transport characteristics.  For these reasons, study of subsurface microbial transport, protection of groundwater resources, and assessments of pathogen risk are complex and often multidisciplinary.

These figures represent how we do field work to look at the interactions of microbes and contaminants.
We perform injection and recovery experiments (figure at left) to assess how microbes move in the subsurface. The figure on the left is an illustration of the  types of experiments done in the field. A Forced Gradient test is where we try to simulate what happens when water is withdrawn or recharged to an aquifer at high rates and how this affects microbial movement.  A Natural Gradient test is where we want to see what would happen if there are no influences upon flow. In each case, some materials (microbes or chemicals known as tracers) are added to a volume of water which is introduced to the groundwater. This tracer is then monitored as it passes or moves to a sampling spot. We can then assess how and what conditions are necessary for different degrees of microbial transport. In all cases, the microbes that are introduced to the ground are harmless. Nevertheless, they do often contain analogous characteristics specific to pathogens.  Finally, this type of work requires close collaboration and consultation with groundwater geohydrologists/modelers, geochemists, and biologists.

We take samples and look at them with the microscope (figure at right). Click on the microscope to get an idea of what we see.
The yellow fluorescing things are bacteria which have been stained with a dye to make them glow when ultraviolet light is focused on them.


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      Factors Affecting Microbial Movement in the Subsurface

             We all need clean water. Groundwater is a vulnerable resource and should be protected. Sometimes microbial pathogens (disease causing bacteria, viruses, or protozoa) can get into water supplies because they can move or are moved through aquifers. There are many factors which contribute to the movement of microbes through the subsurface. This diagram illustrates some of the major factors. Through coupled laboratory and field experiments, we can estimate the magnitude of each of these effects, the extent to which they contribute to microbial transport and whether pathogens are being mobilized in particular aquifer materials.  Organzations which use our data are the EPA, state & local governments, and environmental consultants. They can then make recommendations to water agencies as to where water supply wells should be placed so that water supplies won't be impacted by municipal sewage plants and industrial facilities.

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