Author of Response: Thomas Harter, Cooperative Extension Specialist, Groundwater Hydrology, University of California-Davis, thharter@ucdavis.edu

The main difference in the environmental fate of pathogens, when compared to that of other pollutants, is how they get into a stream or into groundwater—or, better said, how they do not get there. While invisible to the human eye, most pathogens are giants of the micro-world. In contrast, chemical pollutants are tiny, and millions of these molecules would have to be lined up to become as long as a single bacterium. A typical bacterial pathogen is a relatively rigid oval or stick-shaped body that is a little less than one micrometer long (about one-hundredth of the thickness of human hair), much too large to fit between the clay or silt particles of many clayey, silty, or loamy soils. Traveling across the soil surface in runoff and traveling into the soil pores during infiltration, most pathogens are therefore quickly strained by soil particles. Only in sandy soils is the pore space indeed large enough to provide ample traveling space for pathogens. Even there, pathogens frequently collide onto grain surfaces where they tend to become permanently attached. Ultimately, most pathogens are strained or filtered out of the water cycle long before reaching groundwater or a stream. Even if pathogens reach an aquifer, the aquifer itself will filter most remaining pathogens over relatively short distances (100 ft to 300 ft), unless the aquifer has very large pores (gravel aquifer, karst aquifer, basalt aquifer).

Apart from being strained by tight pore spaces or filtered and adsorbed onto soil particles, the transport mechanisms for pathogens are identical to those for nitrates or phosphorus, for example: Once in a stream or in a gravelly aquifer, pathogens travel along and mix with the water flow. Like some other pollutants, pathogens have a propensity to temporarily attach to grain surfaces or to surfaces of particles suspended in (muddy) stream water, a process referred to as sorption. Also, like some organic pollutants (e.g., pesticides), pathogens lose their viability and degrade or are consumed by other micro-organisms while traveling in streams or groundwater. Because of this similarity in the transport mechanisms, some chemicals may be used as indicators to either identify the potential for pathogen contamination or, once a pathogen contamination has occurred, to establish potential sources of that contamination.