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

Response: Pathogens from animal waste (zoonotic pathogens) persist in surface water and groundwater for periods ranging from less than a day to several weeks or even months. For example, after excretion from cow and calves or from wild mammals, the infectious stage of the protozoic Cryptosporidium parvum has been shown to be viable for more than six months while in the natural environment. The chance of survival decreases exponentially with time. Most pathogens die off quickly (within hours or days). Only few manage to survive for more than a week. In manure land application areas, most pathogens generally lose their infectivity within three months. Also, bacterial pathogens of common concern (E. coli H157:O7, Salmonella, Campylobacter, Cyclospora cayetanensis, Shigella, Leptospira, Pseudomonas, Mycobacterium, Vibrio, Francisella tularensis) are unable to multiply in the natural environment. Due to their physiology, viruses and the resting stages of parasites (Hepatitis virus, Norwalk virus, rotaviruses, cysts, oocysts, ova, e.g., of Giardia, Cryptosporidium, Microsporidia) can also only multiply in their host environment, which typically is the digestive tract of humans, mammals, and other animals.

Very few pathogens of concern grow or multiply in water and in soils. Bacteria that do grow and multiply in water are typically nuisance organisms that affect the odor and taste of water or may lead to discoloration but do not cause human illness. But there are exceptions, for example, the pathogen Legionella pneumophila (causing Legionnaires' disease), which grows in warm, stagnant water, independent of an animal source. Other pathogens that may grow in warm, stagnant water independent of animal sources are uncommon in the United States (due to the use of disinfection), but widespread elsewhere. These include Cholera (Vibrio cholerae) and two pathogenic, free-living amoebae, Naegleria fowleri, and Acanthamoeba.

The survival rate of zoonotic pathogens excreted into the environment is highly dependent on numerous environmental factors, especially temperature, moisture, exposure to UV light (sunshine), and predation by other organisms. High temperature, lack of moisture, and exposure to sunlight usually lead to rapid die-off of pathogens. In contrast, cold but not freezing temperature and a moist environment lend themselves to longer survival times, on the order of days to months. Survival of pathogens is not as long in surface water as in groundwater due to the exposure to light and often warmer temperatures. While shorter-lived, pathogens in surface streams may travel large distances within that short period of time, increasing the chance of being ingested by a host through drinking water consumption. Surface water is therefore very vulnerable to microbial contamination.

Pathogens in groundwater have longer survival times (weeks to months), but groundwater is generally not easily accessible to pathogens. Groundwater is much more protected from contamination with pathogens due to the overlying soils, which provide an excellent filter to most pathogens. Pathogens in recharge reach shallow groundwater only if soils are very sandy to gravelly or in the presence of large soil pores, cracks formed by shrinking and swelling soils, pore tubes created by decaying roots, or fractures in surficial bedrock. Commonly, pathogens occurring in groundwater aquifers have been transported to the aquifer through poorly sealed wells, backflow of contaminated water into the well casing, or uncontrolled runoff into abandoned, open well casings and into gravel-filled drainage wells.

In the vicinity of animal farms, their housing area, and near manure storage and application areas, shallow groundwater (at depths not exceeding a few tens of feet) is known to have a much higher risk for the presence of fecal coliform indicator bacteria. This is independent of the survival time of the pathogens due to the frequent loading at the land surface.