Onsite wastewater systems treat and disperse pathogens and chemicals, by definition, in the soils near where people live and recreate. Properly designed, installed and operated on-site systems can be as effective as municipal wastewater treatment plants in reducing the public health risks associated with wastewaters; however, contamination of drinking waters and surface waters due to on-site systems does occur and people do contract gastrointestinal and other illnesses from these wastewaters. We explore the conditions associated with known and potential exposure to wastewater contaminants and review the literature on waterborne disease outbreaks and population-based (epidemiological) studies.

Exposure

Exposure to pathogens, nutrients and other chemicals found in a home s wastewater occurs through a few potential routes:

$ In the home. A septic tank or drainfield that is clogged or "backed up" increases the opportunities for people in the home to be exposed to pathogens, because toilets, sinks, and other drains are not allowed to drain. This would appear to be a relatively small risk to inhabitants, for such conspicuous problems are understood by the inhabitants to be unpleasant and to be avoided. Generally such conditions exist for a period of a few hours to a few days.

$ On the soil surface. A drainfield that is not disposing of wastewater may force wastewater to the soil surface. Young children and pets are most likely to have direct contact with the pathogens in wastewater, spreading bacteria or viruses directly occupants or visitors in the home. Also, insects can act as vectors for pathogens found in wastewater. Residents of the individual home are generally immune to the pathogens, but visitors are at greater risk to contracting disease from failing systems.

$ Well water. For those with private wells or community wells that are not properly disinfected, there are risks to the consumers of well water when onsite systems  fail. Not only are pathogens of concern, but also nitrates are a public health problem, particularly to infants because of the disruption to the oxygen carrying capacity of the blood. Viruses, bacteria or other pathogens can contaminate well water if systems or soils are not effectively treating wastewaters. Saturated soils are a particular problem, because of changes in treatment efficacy and because of changes in pathogen persistence and movement.

$ Surface waters. Runoff from rains or surfacing of wastewaters increases the contamination potential for surface waters. Rivers, lakes, ponds, swamps, estuaries, etc., are commonly contaminated with wastewater contaminants as well as from many other sources. Whether the surface water is used for drinking water or for recreation, there are risks associated with septic wastes that find their way to surface waters.

It is very difficult to measure or estimate the numbers of individual who are exposed to wastewater contaminants from onsite treatment and disposal systems. Equally, it is not easy to estimate the number of bacteria or viruses that individuals are exposed to from either properly or improperly operating onsite wastewater treatment systems.

Health Effects

Summary of Pathogens, Nutrients, and Other Chemicals

There are 4 general types of microorganisms in wastewater and important families or species represented in each group

$ Bacteria - Salmonella, Shigella, E. Coli, Campylobacter
$ Viruses - Norwalk-like (caliciform) viruses, Hepatitis A,
$ Protozoan - Giardia lamblia, Cryptosporidium
$ Worms (Helminthes) - roundworms, tapeworms

Estimates of the number of pathogens in wastewater are variable, particularly those associated with disease. If people are sick and shedding viruses, bacteria or other pathogens, then large numbers can be found in wastewater. Feces are made up of approximately 10% bacteria. Many benign species make up the bulk of the bacteria found in waste.

Health Effects

Several conditions are related to the likelihood that a person will get infected by a pathogen:
(1) the virulence and infectivity of a pathogen, (2) which reservoirs for the pathogen exist, (3) the route of transmission, (4) the entry portal, and (5) how susceptible an individual or population is to a microorganism. Age, hormonal status, physical and mental health, and race play roles in the potential for infection. A summary of minimal infectivity doses is provided in Table 1.

Cases of waterborne disease outbreaks that can be traced directly to onsite wastewater systems are relatively few in number. However, it is known that reports of disease outbreaks under-represents the true number of people getting gastroenteritis and other diseases. There are 23 million private wells in the U.S. (most of them without disinfection) and probably close to ¾ million in North Carolina alone. With over 40 percent of North Carolina households having an onsite wastewater system and a large number of drinking water wells, there is a considerable potential for wastewater- related illnesses. Another indication of the potential for exposure is the reported 15 to 20% of onsite systems that are  failing in any given year. This reported failure rate is not necessarily a measure of those that are contaminating surface soils, groundwater or surface waters, but rather a somewhat unquantified potential for contamination and human exposure.

The following is meant to highlight what we can learn from cases of waterborne disease outbreaks and from other studies about health effects related to onsite wastewater systems.

$ CDC surveillance "probably underestimates true incidence of WB disease outbreaks"
$ Review of CDC outbreak incidence reports for 1992 to present shows about 30 cases in the U.S. potentially involve onsite wastewater.
$ Sources and etiological agents often unknown or undetermined
$ High percentage of drinking water-associated outbreaks had problematic or untreated water systems
$ 1997-98 Giardia and cryptosporidium often from groundwater sources.
$ All bacteria related outbreaks since 1989 from groundwater sources.
$ Only 2 viral outbreaks confirmed since 1991, despite improvements in viral detection. However, among undefined etiology, symptoms are often consistent with viral syndrome.

Table 1. Minimal infective doses for some pathogens

In the U.S. 51% of waterborne disease outbreaks were preceded by precipitation events above 90 percentile.
$ Surface water outbreaks in month of extreme rain and groundwater had 2 month lag periods
$ Cryptosporidium in Milwaukee had heavy rains
$ In Finland "main reasons for fecal contamination of groundwater were floods and surface runoffs" . In largest outbreak, lake water was rapid sand filtered and chlorinated (0.2-0.3 mg Cl₂/L), yet 90% of residents got sick

From the analysis of available onsite wastewater exposure and health effects literature, the following conclusions are drawn.

$ Exposure to wastewater contaminants occurs routinely. Non-routine circumstances of wastewater exposure are those most often causing large and publicized outbreaks of disease. Recreational areas, special events, and summer homes disproportionately represented among WBDOs
$ The relatively small number of reported onsite-related disease outbreaks and numbers of individuals with reported disease might greatly under-represent the true health impacts of onsite systems.
$ Viruses are of particular concern, because under some conditions they can be persistent in the environment and also travel further and faster than other pathogens. Also, nitrogen reduction in onsite systems should receive greater emphasis particularly where private wells are used, in part because of the potential cumulative impacts on groundwater.
$ A systems approach to onsite wastewater treatment should be considered by the public health and planning communities. Wastewater issues are part of a broader public health and environmental picture involving drinking water, groundwater recharge, and land use and disaster/emergency planning.
$ Long-term management of onsite systems is a critically important element in reducing public health impacts and management needs to be more strongly integrated into the regulatory and financial programs involved in onsite systems.

Research should be supported that will help measure health impacts of onsite wastewater treatment and disposal systems. Epidemiological studies and molecular tracking studies could be among the types of studies initiated.

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Please address any questions to Dr. David Lindbo.

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This page created by
Roland O. Coburn, Research Technician I on 1/10/03.

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