Adopt the mentality that every positive coliform result has a source. Thoroughly investigate to determine potential sources. Do not flush and disinfect the entire system following a positive routine result.
Fixing Positive Coliform Results Understand that repeat samples are intended to determine whether coliform contamination is widespread or isolated. Take repeat samples at the correct locations. Review past monitoring results to see if there are any trends.
Fixing Positive Coliform Results If the system uses sodium hypochlorite for disinfection, review: –procedures for replacing or freshening-up the hypochlorite feed solution –chlorine residual monitoring frequency in the distribution system –locations for chlorine residual monitoring –expiration date of test kit chemicals –water usage records, particularly peak usage vs. chlorine feed and contact time
Investigative Sampling If after thoroughly inspecting the system for potential pathways for the coliform contamination you still have not identified a source, take investigative samples of the: –water source –storage tank –each section of the distribution system Check the box on the the bacteriological sampling form that the samples are “special” or “investigative”.
If Potential Coliform Source Cannot Be Identified Shutdown the system if a source cannot be found. Flush and disinfect the storage tank and distribution system completely if a source cannot be identified. If the source is determined to be contaminated, disinfect it obtaining a free chlorine residual of 100 mg/L.
If Potential Coliform Source Cannot Be Identified Premix your chlorine solution in a container. Pour the solution into the well through a hose which the hose is being raised and lowered through the water. Hold the chlorine in the source for at least 24-hours. Pump the well or flush out the spring box until the chlorine residual is 1 mg/l or less.
Disinfection of the Distribution System To disinfect the distribution system chlorinate the source and pump or feed the water into the system. Let the water run out of each tap until a chlorine odor is detected. Hold the chlorinated water in the distribution system for at least 24-hours. Keep the chlorinated water out of nearby surface water.
Can surface water runoff infiltrate around the well casing?
Seal any cracks in the concrete pad around the casing. Pad should extend above grade and slope away from the casing. Hand pump pad should have a drain that discharges to grade at least 100 feet away.
Stagnant water due to excessive storage volume may cause grown of heterotrophic (noncoliform) bacteria.
Corrosion inside very old tanks will affect the quality of the water
Provide a separate water line with backflow prevention on it to meet fire demands
Cross Connections Has the water system pressure dropped below 20 psi shortly before the positive coliform result due to: –water line break –high water demand (e.g., opening of a hydrant to fill fire engines) Are backflow preventors installed on all potential cross connections?
A reduced pressure backflow preventor should be installed on lawn irrigation lines Backflow preventors should be tested annually to ensure they are functioning properly
Are vacuum breakers installed on all hose bibs?
Install vacuum breakers with manual drains to prevent the tap from freezing
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