Interpreting Drinking Water Test Results (UW

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  • Interpreting Drinking Water Test Resultsby Chris Mechenich and Elaine Andrews

    Musty odors, red-brown stains on plumbing fixtures, bathtubringsall these are unpleasant signs of water qualityproblems, but usually not of harmful contaminants in thewater. Contaminants that may threaten our health are usually notdiscernible by the senses. Drinking water can contain nitrate,bacteria, and pesticides at levels which cannot be tasted or smelled,but which can be hazardous to health.

    If your drinking water comes from a private well, you should testyour water once a year even if you do not observe any smells, stainsor changes in water quality. Only analysis by a certified laboratorycan determine if the water is free of harmful contaminants. Thispublication provides information about how to interpret thelaboratory results for a basic set of tests conducted as part of theUniversity of WisconsinExtension water testing program and thatare recommended for all private wells.

    The tests described in this publication are also conducted on publicwater supplies. If you are using water from a public water utility,consider having the water tested if your home plumbing systemcontains lead or copper pipes or lead solder, if you are installing awater treatment device, or if you have concerns about the water. SeeEvaluating the Condition of Your Public Water Supply (G3558-3), formore information.

    The Initial Set of Water TestsAnyone buying a home with a private well, installing a new well orsimply having their well water tested for the first time should runthe basic set of tests described here. These tests give a good overallpicture of current water quality, indicate possible problems, andprovide a baseline for comparing future test results. Each test isdescribed in more detail in this publication.

    Always have a certified laboratory conduct the tests. Labs certified bythe Department of Natural Resources or Department of Agriculture,Trade and Consumer Protection must meet standards for accuracy. Alist of certified labs is available from county Extension offices andDepartment of Natural Resources (DNR) regional offices.

    This fact sheet is part of a seriesdesigned to help you determinethe quality of your home drinkingwater and to show you techniquesavailable for improving it. To makethe best use of these publications,include them in a household filecontaining well information andwater test results.

    Other fact sheets in the series are:

    Keeping Your Home WaterSupply Safe (G3558-1)

    Evaluating the Condition ofYour Private Water Supply(G3558-2)

    Evaluating the Condition ofYour Public Water Supply(G3558-3)

    Choosing a WaterTreatment Device (G3558-5)

    The Extension bulletin MaintainingYour Home Well Water System(G3399), can be used with thesepublications.

    This series was developed by theUniversity of WisconsinCooperative Extension incooperation with the WisconsinDepartment of Natural Resourcesand the Wisconsin Department ofCommerce.



  • Tests to Identify Contaminants that Harm Health

    BACTERIA. Bacteria, viruses and parasites in water can cause disease.The coliform bacteria test indicates the possible presence of disease-causing bacteria from human or animal waste.

    NITRATE. A form of nitrogen that can dangerously reduce theamount of oxygen in the blood of infants under six months old andmay also harm the unborn. Nitrate is a common contaminant fromfertilizers, septic systems and animal wastes. It often indicates thepresence of other contaminants.

    LEAD AND COPPER. Lead and copper can be leached into water frompipes or solder and can represent a significant health threat.

    Tests to Determine Overall Water Quality

    ALKALINITY. Measurement needed to determine corrosivity.

    CHLORIDE. High concentrations often indicate contamination froma septic system, fertilizer, landfill or road salt.

    CONDUCTIVITY. Measures the ability of water to conduct anelectrical current; can be used to signal the presence ofcontaminants.

    CORROSIVITY INDEX. A combination of several tests that indicatesthe tendency for water to corrode plumbing, or for lime deposits toform in pipes.

    HARDNESS. Helps determine the need for water softening; alsoinfluences corrosivity.

    pH. Indicates water's acidity and helps determine if water willcorrode plumbing.

    After running the initial set of tests, well users should continue totest for bacteria once a year. Its also a good idea to test for nitrateannually for several years. If nitrate levels are consistently low,nitrate tests are not necessary every year. However, a nitrate testshould always be conducted if an infant or pregnant woman isdrinking the water.

    A Note on Drinking Water Standards

    Public water supplies must meet numerical water quality standardsset by the United States Environmental Protection Agency andenforced by the Wisconsin DNR. Routine testing is not required forprivate wells. However, users of private well water should at least beaware of the broad range of contaminants that may be found in wellwater and that concern public health officials.

    Primary standards provide health limits for 82 contaminants as of1995.The list includes 8 inorganic compounds, such as arsenic,copper and lead; pesticides, such as aldicarb and chlordane; volatileorganic chemicals such as benzene and trichloroethylene; PCBs;


    WHAT IS A PARTPER MILLION?Laboratory equipment canmeasure contaminants in water atextremely low levels, such asparts per million (ppm) or evenparts per billion (ppb). Forexample, one part per millioncan be imagined as one redmarble mixed in with 999,999blue marbles, one inch in sixteenmiles, or a minute in the space oftwo years. One part per billion isthe same as two croutons in afive hundred pound salad, or eightdrops of water in an Olympic-sized swimming pool.1 Althoughsuch small numbers may seemquite insignificant, even one partper billion or less of certainchemicals has been found tocause adverse health effects.1These creative part-per-million and part-per-billion examples are from theWaterTest Corporation, New London, NH.

  • microbial pathogens; and radioactive elements. Secondary standardsprovide aesthetic limits for 13 contaminants, such as iron, zinc, colorand odor. The sources and maximum contaminant levels (MCL) forthese substances are described in the publication Private DrinkingWater Supplies: Quality, Testing and Options for Problem Waters listedin the resource section of this fact sheet.

    Testing private well water supplies for all these contaminants wouldbe expensive and is not recommended unless your well is close to aknown or suspected source of contamination.

    The Initial Water Tests:What the Results MeanThe initial set of water tests can provide a good overview of yourwell water quality if you know how to interpret the results. Theinformation below provides a starting point for evaluating yourwater quality.

    Note that water test results are usually presented in milligrams perliter (mg/L) or micrograms per liter (g/L). For example, a water testmight indicate that the water contains 6 mg/L nitrate, meaning thata liter of water contains an average concentration of 6 milligrams ofnitrate. Note also that one mg/L is equivalent to one part permillion (ppm). One g/L equals one part per billion (ppb).

    Coliform Bacteria

    Coliform bacteria are microorganisms found in surface water, soiland in the feces of humans and animals. They do not usually causedisease. However, their presence indicates that fecal wastes may becontaminating the water and means that pathogenic (disease-causing) organisms could be present. If human or animal wastes arecontaminating the water, gastrointestinal diseases, hepatitis or otherdiseases may result.

    Many labs can also test for a specific fecal coliform bacteria, E. coli.The presence of E. coli in a water sample represents an even greaterhealth risk than the presence of total coliform bacteria.

    ACCEPTABLE RESULTS: 0 coliform/100 milliliters (ml) of water. If youhave coliform bacteria present, you should resample. If a second testshows coliform, take corrective action.


    Not present

    Safe Unsafe


  • CORRECTIVE ACTIONS: Coliform bacteria in groundwater indicatethat contaminated surface water is entering groundwater withoutthe filtering effect that soils usually provide. In areas where thebedrock is fractured and close to the surface, or in areas with coarsesand and gravel soils, contaminated surface water can naturally findits way into the groundwater. More often, detection of coliformbacteria in well water is an indication that contaminated surfacewater is entering a well because of defects in well construction ormaintenance.

    If coliform bacteria are detected in the water sample, have anothersample tested. Carefully follow the sampling steps suggested by thelaboratory to ensure that your sampling procedure itself is notcontaminating the water. If the second test shows bacterialcontamination, check the well for defects. Some defects are easilyviewed; others might require excavating around the well. Followthis checklist as you look for obvious defects:

    Is the cap or seal on tightly? Is the well vented? The well capshould fit tightly to keep out surface water and vermin. On adrilled well, the screened vent that allows air to enter the wellmust be securely connected to the cap or seal.

    Is all wiring in conduit (tubing that connects the well with theelectrical box)?

    Is the casing at least 12 inches above the ground? (The casing isthe steel or plastic pipe installed in the bore hole duringconstruction.) Also, if there are visible holes or cracks in thecasing, or if you can move it, there might be a problem.

    Is the well in a pit or basement? If so, i