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The City of Portland, Oregon

Portland Water Bureau

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Bacterial Monitoring

The Portland Water Bureau collects and tests over 240 water samples per month to monitor for bacterial contamination and distribution system water quality parameters in Portland’s drinking water. Water samples are generally collected and tested four days a week, Monday through Thursday.

The following are the step-by-step procedures followed by the bureau’s sampling and lab staff to collect and test the water samples. 

Morning Day 1: Sampling

The Water Bureau has trained staff dedicated to collecting water samples, including samples for bacterial monitoring, and water quality parameters in the field. The water quality technicians also maintain all the sampling equipment, including daily calibration of measurement devices, such as pH probes. 

Water Quality Sampling Stations

water quality sampling stations

The Portland Water Bureau has over 170 water quality sampling stations around the city. The sampling station faucets are locked in green metal boxes to prevent tampering and possible contamination. 

Preparing the Water Quality Station for Sampling

 sterilize water quality sampling station

Upon arrival at a sample station, the water quality technician visually inspects the site to ensure that the sample station has not been tampered with and there are no sources of contamination. After unlocking the green box, the technician uses a propane torch to sterilize the sampling area and kill any bacteria that might be on the external surfaces of the sample line faucet. The faucet is then turned on to flush water through the sample line.

Testing Water Temperature and pH

 testing temperature and pH

The first water quality tests the technician preforms are water temperature, pH, and chlorine, all of which do not require a sterile environment. Water temperature and pH are measured using probes and recorded on a field data sheet. 

Testing Chlorine

testing chlorine

The chlorine concentration in the water is measured using a colorimeter. A water sample is collected in a small bottle and a reagent is added to the water. The reagent causes the water sample to change color, which can range from clear when there is a low concentration of chlorine, to dark pink when there is a high concentration of chlorine. The colorimeter measures the concentration of chlorine in the water based on the color of the sample, and the data from the colorimeter are recorded on the field data sheet. 

Preparing to Collect a Water Sample for Bacterial Testing

preparing to collect a water sample

Lastly, the technician collects a water sample that gets tested for bacteria in the lab. Collecting this water sample requires sterile procedures. The technician follows all proper sampling techniques, which includes the industry-standard method of wearing gloves or using hand sanitizer to prevent contamination from their hands. In this case, the technician uses hand sanitizer to sanitize his hands right before sampling. Additional sterile procedures include using a sterile, sealed bottle to collect the sample and, when conditions require, covering the faucet with a shield to protect from environmental contamination such as rain or blowing debris.

Collecting the Water Sample for Bacterial Testing

collecting a water sample

To collect the water sample, the technician breaks the seal of a pre-labelled bottle and collects between 100 and 110 milliliters of water. The water sample is then stored in a cooler with other water samples and ice packs. Keeping the water cool until the technician delivers the samples to the lab is an industry-standard that limits bacterial growth during transit.

Afternoon Day 1: Testing at the Lab

All bacterial monitoring samples are tested by the Portland Water Bureau Water Quality Laboratory.

 colilert bottles and trays

The Portland Water Bureau uses two tests to monitor for bacteria: Colilert-18 (bottles on the left); and Quanti-Tray (trays on right).

Routine bacterial samples from the distribution system are tested using the EPA-approved Colilert-18 test by IDEXX. Colilert-18 simultaneously detects for both total coliform bacteria and E. coli in water after an 18-hour incubation. The version of Colilert-18 used by the Water Bureau to routinely test the drinking water is a presence/absence test.

Routine bacterial samples from the Bull Run Reservoirs and repeat samples from the distribution system are tested using the EPA-approved Quanti-Tray test by IDEXX. Quanti-Tray is a version of the Colilert-18 test that provides information to calculate the number of coliform or E. coli colonies.

The following photos illustrate the lab procedures for all bacterial monitoring samples using the Colilert-18 test. 

Preparing the Water Sample for Bacterial Testing

preparing sample for bacterial testing

When a sample is received at the lab, lab staff first check to make sure the sample bottle is filled to the correct volume and that it is cold. Using sterile procedures that includes wearing gloves and a mask, lab staff adds nutrient-indicators to each sample and then mixes them by shaking.  

Incubating the Water Sample

incubating the water sample

Once shaken, the drinking water samples are added to a warm water bath along with three types of control samples: a sterile water control sample; negative control sample that has non-coliform organisms; and a positive control sample of E. coli. The samples sit in a 95° Fahrenheit (35° Celsius) incubator overnight for at least 18 hours.

Morning Day 2: Test Results at the Lab

Bacterial Testing Results

bacterial testing results

The next morning the samples are retrieved from the incubator. In the photo on the left, the order of the samples are:

(A) E. coli positive control;

(B) sterile water control;

(C) comparator provided by IDEXX;

(D) sample collected from a water quality sampling station;

(E) non-coliform negative control.

Any sample that develops a yellow color equal or greater to the comparator (C) after the 18-hour incubation is positive for total coliform bacteria. E. coli is a type of total coliform bacteria, so as expected, the E. coli control (A) is a deep yellow, indicating the presence of total coliform bacteria.

Any sample that tests positive for total coliforms is then put under a UV light. A sample that contains only total coliform bacteria will not fluoresce, or glow, but a sample that contains E. coli glows bright blue as seen in the photo on the right.

Reporting and Record Keeping

OHA database

All results are reviewed and validated by lab staff. All bacterial testing results are reported to the Oregon Health Authority on a monthly basis. If any sample tests positive for either total coliform or E. coli bacteria, those results are immediately reported to the Oregon Health Authority. All bacterial testing results are publically available on the Oregon Health Authority’s website: The Portland Water Bureau also maintains all test results in a database system.