| Ctwc - Naugatuck Reg-Terryville System | Clinton, CT | 5,607 | 9 | Clean |
| Berlin Water Control Commission | Berlin, CT | 5,128 | 677 | Detected |
| Ctwc - Shoreline Region-Chester System | Clinton, CT | 5,030 | 156 | Clean |
| Portland Water Department | Portland, CT | 5,010 | 65 | Detected |
| Colchester Sewer & Water Commission | Colchester, CT | 4,020 | 159 | Detected |
| Ctwc - Naugatuck Reg-Thomaston System | Clinton, CT | 3,789 | 84 | Clean |
| Ledyard Wpca - Ledyard Center | Ledyard, CT | 3,294 | 13 | - |
| Aquarion-Litchfield Regional | Shelton, CT | 3,097 | 12 | Clean |
| Ctwc - Shoreline Region-Sound View | Clinton, CT | 3,028 | 134 | - |
| Connecticut Valley Hospital | Middletown, CT | 2,928 | 149 | - |
| Worthington Fire District | Berlin, CT | 2,875 | 57 | - |
| Pratt & Whitney | Middletown, CT | 2,800 | 187 | - |
| Fairfield Hills | Newtown, CT | 2,610 | 64 | - |
| Weston Schools and Municipal Buildings | Weston, CT | 2,600 | 493 | - |
| Ridgefield High and Middle School | Ridgefield, CT | 2,575 | 100 | - |
| Scwa, Montville Division (Mtv) | Gales Ferry, CT | 2,570 | 28 | - |
| Scwa, Tower-Ferry View Division | Gales Ferry, CT | 2,567 | 54 | - |
| Wolcott Water Department | Wolcott, CT | 2,550 | 137 | - |
| Ctwc - Gallup System | Clinton, CT | 2,538 | 46 | - |
| Groton Long Point Association | Groton, CT | 2,400 | 69 | - |
| Connecticut Correctional Institute | Wethersfield, CT | 2,385 | 460 | - |
| Ledyard Wpca, Gales Ferry System | Ledyard, CT | 2,369 | 23 | - |
| Salmon Brook District Water Dept | Granby, CT | 2,151 | 127 | - |
| Noank Fire District | Noank, CT | 1,947 | 59 | - |
| Aquarion-Salisbury | Shelton, CT | 1,932 | 23 | - |
| Ctwc - Hebron Center Division | Clinton, CT | 1,927 | 2 | - |
| Aquarion-New Hartford | Shelton, CT | 1,872 | 132 | Clean |
| Scwa, North Stonington Division (Nst) | Gales Ferry, CT | 1,860 | 33 | - |
| New Fairfield High/Middle School | New Fairfield, CT | 1,791 | 576 | - |
| Ctwc - Plainfield System | Clinton, CT | 1,713 | 23 | - |
| Durham Center Division | Durham, CT | 1,601 | 298 | - |
| Aquarion-North Canaan | Shelton, CT | 1,481 | 15 | - |
| Tariffville Fire District Water Dept | Tariffville, CT | 1,477 | 112 | - |
| Leonard J.tyl Middle School&high School | Oakdale, CT | 1,466 | 328 | - |
| Scwa, Mohegan Division | Gales Ferry, CT | 1,428 | 17 | - |
| Consolidated & Meeting House Hill School | New Fairfield, CT | 1,425 | 294 | - |
| Killingly High School & Agricultural Ctr | Dayville, CT | 1,400 | 128 | - |
| Ctwc - Legend Hill System | Clinton, CT | 1,368 | 118 | - |
| Aquarion-Marlborough | Shelton, CT | 1,348 | 633 | - |
| Candlewood Shores Tax District | Bethel, CT | 1,315 | 382 | - |
| Montville Water Supply | Uncasville, CT | 1,300 | 87 | - |
| Coventry High & Nathan Hale Schools | Coventry, CT | 1,252 | 18 | - |
| Tolland Water Department | Tolland, CT | 1,251 | 19 | - |
| Lone Oak Campground | Quincy, CT | 1,250 | 29 | - |
| Aquarion-Woodbury | Shelton, CT | 1,209 | 231 | - |
| Woodstock Academy | Woodstock, CT | 1,125 | 199 | - |
| Region 18 Schools - Lyme Street | Old Lyme, CT | 1,102 | 130 | - |
| Aquarion-East Derby | Shelton, CT | 1,101 | 32 | - |
| Ctwc - Thompson System | Clinton, CT | 1,070 | 77 | - |
| Ctwc - Shoreline Region-Point O Woods | Clinton, CT | 1,068 | 113 | - |