Over 50 percent of the United States population utilizes groundwater as its drinking water source. Approximately 96 percent of the groundwater users live in small rural areas often utilizing small individual wells, where the resources for treatment and monitoring are limited. Consequently, groundwater contamination caused by pathogens or chemicals often remains undetected. This failure to detect groundwater contamination is due to the existence of a large number of wells and high cost of sample analysis. In the state of Illinois alone, there are 3400 public supply wells and about 500,000 private wells. The average cost of an organics scan of the sample in 1981 was approximately 1000 dollars per sample.
In 1984, Craun stated that "contaminated water causes almost half of the outbreaks of water-born diseases each year in the United States" (Craun, 1979, 1984). The organisms most responsible for water born diseases include: bacteria such as Salmonella and Vibrio , viruses such as hepatitis virus and poliovirus, and protozoa such as Cryptosporidium and Giardia (Teutsch, 1991, Brock, 1994). Defective and improperly designed or installed septic tanks, sewage sludge land applications, undetected sewage system leakage, and improperly maintained and enclosed bioremediation sites are the major sources for pathogens in groundwater (Gallagher, 1996, Teutsch, 1991, Abu-Ashour, 1994).
Two factors determine whether or not the groundwater will become contaminated with pathogens are survival and transport rate of pathogens within the system. The survival rate is influenced by the environmental conditions, the physical and chemical properties of water and soil in the system, as well as the identity and physiological state of the organisms. The transport of pathogens is affected by the physical and chemical properties of water and soil, hydrology of the system, and microbial characteristics (Teutsch, 1991, Abu-Ashour, 1994).
The properties and conditions of the surrounding environment control the identity and physiological state of the microorganisms which influences the survival and transport capabilities of the organisms. One important characteristic of the organisms is their ability to compete for substrate and nutrients. In general, an organism will have a shorter survival time in the presence of its predators or in the presence of competing organisms, where the resources (substrate, nutrients) become limited. Floc forming organisms will have an increased chance for survival because of the protection and opportunity to share resources within the floc. The ability to survive and transport will be affected by the size of the individual organism or its floc. Transport will be influenced by the size of other members of the population by limiting the pore spaces the organism is able to move though without encountering blockage (Teutsch, 1991, Abu-Ashour, 1994).
The physical and chemical characteristics of water and soil have a great influence on the survival and transport of the microorganisms, as indicated above. Pathogens generally survive longer in waters and soils where organic matter is readily available; organic matter can provide substrate and nutrients to the organisms. However, toxic organic matter such as antibiotics will decrease the survival rate of the microorganisms. Microorganisms thrive in moisture rich soils, because water is the means of substrate and nutrient transport as well as organism transport. The pH of the environment is an important variable; pH affects the solubility of compounds and the charge distribution in the system, which affects the adsorption of organisms onto the soil. The advantages of soil adsorption, similar to the floc formation advantages, include protection and nutrient and substrate sharing by the organisms. In addition to pH, adsorption is also affected by the soil type, texture, particle size distribution, and pore size distribution (Teutsch, 1991, Abu-Ashour, 1994). All of the above are related to the surface charges and the surface area available for adsorption as illustrated in the figures.
The environmental characteristics that affect the microorganisms survival and transport include: temperature, water content, hydrology of the area, and application methods of organisms to the system. Temperature affects the die-off rates of the organisms, especially in viruses. Higher temperature have increased die-off rate of the organisms while lower temperatures have lower die-off rates. Temperature also affects the density and viscosity of water altering the water velocity (Teutsch, 1991). An increase in temperature will also cause increased molecular diffusion. The water content and the hydrology affect the water velocities and direction of flow in the ground. The route and frequency of organisms introduction to the soil determines the number of organisms in the groundwater system (Abu-Ashour, 1994). For example, a one time contamination from a defective septic tank will result in a lower number of microorganisms in the soil than frequent contamination of soil by land application of sludge.
The typical assumptions in these models include (Abu-Ashour, 1994):
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Student Authors: K. Perri and A. Fallon, kperri@vt.edu,
afallon@vt.edu
Faculty Advisor: Daniel Gallagher, dang@vt.edu
Copyright © 1998 Daniel Gallagher
Last Modified: June 7, 1998
