Protazoa and Virus Monitoring - Drinking Water
Protozoa Monitoring Program
Giardia and Cryptosporidium are protozoa that are of significant concern in drinking water supplies, because they are the most difficult organisms to inactivate with disinfection. EPA sets inactivation requirements for Giardia, while requirements for Crypto are still being developed.
Giardia and Crypto sampling were initiated in 1994 (monthly samples were taken at Shaft 4 and were later moved to to the Cosgrove Intake). In mid-January 1999, sampling was increased to weekly at the Cosgrove Intake. Bi-weekly sampling is conducted at the Chicopee Valley Aqueduct Intake.
Giardia and Cryptosporidium results are reported as number of oocysts per 100 Litres (L).
Until March 2004, MWRA used the EPA-approved method, ASTM D19 (ICR) with 100 L samples. Under this method, identifications were grouped into 2 categories: presumed (no internal structures identified) and confirmed (1 or more internal structures identified).
From July 1997 to March 2004, no samples confirmed positive for Giardia, and no samples were presumptive or confirmed positive for Cryptosporidium.
In April 2004, MWRA began testing samples using the newly EPA-approved Method 1623 with 50 L samples, which is about twice as sensitive as ICR method – 3 to 4 times the recovery rate with half the volume. Under the new method, identifications are grouped into 3 categories: empty (no internal structures), amorphous structure (structure not consistent with a normal organism), and one or more internal structures. The latest results under this method are listed above.
Neither method listed above is capable of determining if cysts are alive or dead, or if they are capable of causing human infection. Even without knowledge of infectivity, MWRA has developed trigger levels for these organisms above which notification and other actions would be undertaken, as recommended by Professors Rose and Haas, leading researchers in the field of assessing risk of giardiasis and cryptosporidosis in water supplies.
Below 10 cysts per 100 L, no special actions are required. MWRA has never exceeded this level.
MWRA is also participating in a federally funded research study with Tufts Medical School. Weekly composite samples of 1000L are taken and investigated using Method 1623. The running average for both Giardia and Crypto is 0.026 cyst/100 L. A summary of these results is available at www.mwra.com.
Virus monitoring was initiated in July 1997 as part of EPA’s Information Collection Rule (ICR) program. The approved method under this program is the ICR method for total culturable viruses. This method detects the presence of many viruses that may be associated with human infection, however there are also viruses detected by this method that are not of concern to humans. While this test does not prove the presence of viruses capable of causing infection in people, it does measure the presence of a group of enteric viruses commonly found in fecally contaminated waters and EPA believes these are at least somewhat representative of human pathogenic viruses. Under the ICR program, if virus levels exceeded 100 MPN/100 L, additional monitoring would have been required. MWRA has never exceeded this limit. New virus test methods are being researched and QA staff is reviewing these improvements. In most samples, levels of viruses have been below the detection limit. In 11 samples since July 1997, viruses were detected at low levels. Note: The turn around time for virus test results is typically a minimum of 6 weeks from sampling.
For more information, please contact:
Joshua Das, MWRA Public Health Project Manager
Updated - March 23, 2017