Understanding Bacteria Sampling
Utilizing a contracted environmental firm and certified laboratory, DC Water initiated daily sampling for E. Coli, an indicator bacterium, at six (6) locations beginning on Jan 29, 2026 with four more locations later added, to assess water quality conditions, sampling locations including the overflow area as well as upstream and downstream sites.
It's important to understand that rain, snow, heat, contaminated runoff, and natural sources of E. coli such as wildlife can cause higher bacteria levels in urban rivers such as the Potomac. After heavy rain experts recommend avoiding the water for at least 48-72 hours. This is reflected in the data, where significant spikes are seen that coincide with rain and weather events but then return to normal including sites upstream of the incident. With no overflows to the river for more than a month prior to completion of emergency repairs, fluctuations in measurements are more likely due to these known factors.
A more detailed explanation is posted on the District Department of Energy and Environment (DOEE) website at doee.dc.gov/release/potomac-interceptor-update-and-faqs
DC Water Locations - Water Quality Sampling
View DOEE's Potomac Interceptor Monitoring page to view comprehensive sampling results from regional agencies and organizations in DC, Maryland, and Virginia.
*Lab results are unavailable on weekends and will be provided the following Monday (excluding holidays).
Sampling Results
July 13 - present (Past results available here)
| E. Coli (MPN/100 ml) | ||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Sample Date | Old Anglers Inn (upstream of collapse) |
Near Drainage Channel @ Overflow (Swainson Island) | Between Minnie's Island and north shore of Potomac River | South Side of Minnie's Island | Sycamore Island | Lock 6 | Fletcher's Boathouse | Georgetown @ Wisconsin Ave | Anacostia @ S. Cap St | National Harbor | ||||||
| 7/13/2026 | 55 | 39 | 57 | 45 | 87 | 93 | 74 | 19 | 34 | 20 | ||||||
| 7/14/2026 | 5 | 18 | 33 | 16 | 73 | 32 | 31 | 25 | 44 | 24 | ||||||
| 7/15/2026 | 10 | 13 | 43 | 9 | 68 | 15 | 55 | 13 | 172 | 10 | ||||||
| 7/16/2026 | 4 | 8 | 4 | 3 | 10 | 8 | 6 | 40 | 86 | 10 | ||||||
| 7/17/2026 | 8 | 24 | 24 | 7 | 25 | 13 | 345 | 12 | 206 | 11 | ||||||
| 7/18/2026 | 4 | 12 | 4 | 3 | 11 | 10 | 29 | 44 | 41 | 6 | ||||||
| 7/19/2026 | 231 | 123 | 101 | 67 | 139 | 161 | 14970 | 248 | 236 | 15 | ||||||
| 7/20/2026 | 29 | 26 | 10 | 11 | 20 | 19 | 179 | 147 | 129 | 32 | ||||||
| 7/21/2026 | 14 | 52 | 19 | 4 | 20 | 13 | 35 | 28 | 19 | 32 | ||||||
| 7/22/2026 | 219 | 461 | 89 | 78 | 4410 | 687 | 1414 | 387 | 1080 | 525 | ||||||
| 7/23/2026 | 34 | 105 | 47 | 38 | 162 | 520 | 99 | 1120 | 520 | 96 | ||||||
| 7/24/2026 | 190 | 19 | 7 | 4 | 141 | 27 | 64 | 410 | 206 | 14 | ||||||
| 7/25/2026 | 21 | 12 | 10 | 9 | 35 | 14 | 178 | 62 | 162 | 1203 | ||||||
| 7/26/2026 | 32 | 12 | 4 | 19 | 21 | 14 | 4 | 108 | 78 | 228 | ||||||
| 7/27/2026 | 12 | 13 | 9 | 3 | 15 | 13 | 9 | 457 | 11 | 15 | ||||||
| 7/28/2026 | 4 | 19 | 12 | 4 | 15 | 3 | 12 | 33 | 48 | 20 | ||||||
| 7/29/2026 | 214 | 361 | 410 | 304 | 3050 | 830 | 1203 | 4800 | 770 | 921 | ||||||
Consistent with public health and U.S. Environmental Protection Agency standards, swimming is not recommended when E. coli levels exceed 410 MPN/100 mL.
For the Potomac River, historical water quality data shows E. coli levels may vary from a range as low as 10 MPN/100mL to as much as 5,000 MPN/100mL on a given day.
Enhanced Water Quality Sampling
On Saturday, April 11, DC Water began enhanced water quality sampling at popular recreation areas along the Potomac River, testing every two hours for three consecutive weeks. This proactive measure is intended to complement the daily water quality sampling already underway. It comes in response to community feedback and provides additional information to residents and recreational users about current river conditions.
| E. Coli (MPN/100 ml) | ||||||
| Sample Date | Sample Time | Between Minnie's Island and north shore of Potomac River | Sycamore Island | Lock 6 @ Feeder Canal | Fletcher's Boathouse | Three Sisters |
| 4/11/2026 | 8-10am | 15 | 10 | 30 | 65 | Not sampled |
| 10am-12pm | 46 | 22 | 68 | 25 | Not sampled | |
| 12pm-2pm | 32 | 45 | 42 | 12 | 2 | |
| 2pm-4pm | 8 | 55 | 33 | 8 | Not sampled | |
| 4pm-6pm | 36 | 18 | 16 | 45 | Not sampled | |
| 4/18/2026 | 8-10am | 37 | 33 | 10 | 15 | Not sampled |
| 10am-12pm | 32 | 28 | 11 | 20 | Not sampled | |
| 12pm-2pm | 14 | 44 | 5 | 20 | Not sampled | |
| 2pm-4pm | 16 | 36 | 9 | 50 | 12 | |
| 4pm-6pm | 20 | 38 | 6 | 64 | Not sampled | |
| 4/25/2026 | 8-10am | 30 | 36 | 6 | 185 | Not sampled |
| 10am-12pm | 22 | 9 | 5 | 32 | 1 | |
| 12pm-2pm | 10 | 6 | 1 | 15 | Not sampled | |
| 2pm-4pm | 5 | 5 | 1 | 26 | Not sampled | |
| 4pm-6pm | 8 | 5 | 1 | 5 | Not sampled | |
Understanding the Data and How it Compares
*Data Provided by DC Department of Energy and the Environment
