Protazoa and Virus Monitoring - Drinking Water
Massachusetts Water Resources Authority

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’s Surface Water Treatment Rule sets inactivation requirements for Giardia, while the Long Term 2 Enhanced Surface Water Treatment Rule sets inactivation requirements for Cryptosporidium. MWRA meets all treatment requirements for inactivation of both Giardia and Cryptosporidium through use of ozone and UV disinfection. 

Wachusett Reservoir raw water is sampled weekly at Carroll Water Treatment Plant (CWTP).  The Quabbin Reservoir has biweekly raw water sampling which is conducted at Brutsch Water Treatment Facility. Giardia and Cryptosporidium results are reported as number of oocysts per 100 Litres (L).

Giardia and Cryptosporidium sampling were initiated in 1994.  In mid-January 1999, sampling was increased to weekly at the Cosgrove Intake. 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 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.

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 to Department of Public Health is made, and additional sampling and watershed review actions are undertaken:

For Cryptosporidium: Above 0.1 oocysts/L, or 10 oocysts/100 L
For Giardia: Above 0.3 cysts/L, or 30 cysts/100 L

At or below this level, no special actions are required.

A summary of these results is listed below.

For more information, please contact:

Beverly Anderson, MWRA Public Health Project Manager

Updated June 26, 2024