Currently in the U.S., 283.7 million people obtain their water supply from public water systems, while 42 million have private drinking water supplies. Typically private drinking water supplies are drawn from the ground water through wells, but some households also use water from streams or cisterns. Private systems, while not federally regulated, are vulnerable to biological contamination from sewage, improper well construction and poor-quality water sources. The U.S. Environmental Protection Agency (EPA) encourages households to take special precautions to ensure the protection and maintenance of their private drinking water supplies.
Measures and Testing
In households, water used for drinking and cooking should be free
of disease causing microorganisms that cause illnesses such as gastroenteritis, typhoid fever, cholera and dysentery. To detect contamination problems early, the EPA recommends that private water supplies be tested annually for nitrate and coliform bacteria. Testing must be done if a problem is suspected from other potential contaminants, such as radon and pesticides. Testing is also recommended when repairs or improvements are made to a private well.
The biological quality of drinking water is determined by tests for coliform group bacteria. The presence of coliform typically indicates disease causing microorganism contamination. The standard for coliform bacteria in drinking water is less than 1 coliform colony per 100 milliliters of sample (<1/100ml).
If a contaminant is detected, test results from laboratories include the concentration of the contaminant and an indication if the concentration exceeds a drinking water quality standard. The initial presence of coliform requires re-testing of the water supply. The re-tested sample should be analyzed for fecal coliform organisms; if positive, prompt action is required.
Conditions Causing Contamination
Coliform bacteria may contaminate well water from the following conditions:
- Loose or worn seals on the drilled and driven well;
- Defective, too short or inadequate ly sealed casings in the drilled and driven well;
- Cracked or loose-fitting cover
on a dug well;
- Defective wall lining or cracked
concrete apron of a dug well;
- Repair to well structure of sub- merged pump;
- Flooding of the well from hurricanes, floods, heavy rainfall or other natur- al disasters;
- Breakdown and repair of a septic tank system;
- Wells drilled into fractured rock formations; and
- Wells located in areas where ground waters are subject to continuous contamination from outside sources.
Purification of drinking water containing microbiological contamination requires some form of disinfection treatment to kill or render microbiological organisms harmless. Of the available disinfection treatment methods for private water systems, chlorination is most commonly used.
The addition of chlorine to a water supply readily combines with chemicals dissolved in water, microorganisms, plant material, odors and colors. Chlorine that is “used up” by these components comprise the chlorine demand of a treatment system. Sufficient amounts of chlorine must be added to a water supply to meet the chlorine demand and provide residual disinfection. Free or residual chlorine is the amount of the disinfectant that does not combine with components in the water, and is available for continuous disinfection.
Prior to installing a permanent means of chlorine disinfection onto a well, it must be ensured that contamination originates from the ground water itself and that it is not a temporary condition. When the “one time only” addition of chlorine to a water supply known as shock chlorination does not eliminate bacteriological problems, a permanent means of continuous chlorination is required.
Shock Chlorination Measures for Wells
Permanent and continuous chlorination of a private water supply can be done through a chlorine pump, solid feed unit, aspirator or a suction device. Unlike public water systems that utilize chlorine in the gaseous form, a private water system uses liquid chlorine or dry chlorine depending on the chosen method of chlorination.
In 1979 the EPA adopted a trihalomethane (THM) regulation limiting the allowable level of this carcinogenic disinfection by-product in drinking water. The maximum contaminant level for total THMs in drinking water is 0.10mg/L. THMs are chemicals that are formed, primarily in surface water, when naturally occurring organic materials combine with free chlorine. Because groundwater rarely has high levels of organic materials such as humic and fulvic acids, chlorinated private wells contain much lower levels of THMs.
Chlorination is generally an inexpensive treatment method and proven to be effective against a broad spectrum of pathogens. Although is has shown itself to be effective against waterborne bacteria and viruses, it only provides some degree of protection against protozoan agents. Notwithstanding, a private water supply should utilize a treatment system that kills or neutralizes all pathogens in the water through an automatic, simply maintained and safe process. Chlorination remains the most popular choice of treatment for private water supplies by homeowners.