Bacteria Health & Safety

OARS continues to address this critical question through our E. coli bacteria monitoring program. Bacteria and other pathogens can enter the rivers through leaking sewers or septic systems, stormwater runoff, wildlife waste, or pet waste. High levels of E. coli, an indicator bacteria, have been linked by the EPA to an increased risk of gastrointestinal illness in swimming waters.

Our bacteria monitoring efforts help keep the Sudbury, Assabet, and Concord rivers safe for recreation and wildlife. This dashboard provides up-to-date results from our regular water sampling, showing bacteria levels across key locations in the watershed. Use this interactive tool to check water quality conditions before swimming, fishing, or paddling, and stay informed about the health of your rivers.

VIEW DASHBOARD ON SEPERATE WEBPAGE

OARS volunteers collect water samples biweekly from May through September at seven core locations in the river system, plus an additional site in River Meadow Brook. Samples are analyzed in a lab, with E. coli counts recorded as the Most Probable Number (MPN) of colony-forming units (cfu). The EPA swimming safety thresholds are:

Single-sample limit: 235 cfu/100mL
Seasonal geometric mean limit: 126 cfu/100mL

We use these results to assess which river sections are safe for swimming, identify pollution sources, and work with local authorities to mitigate contamination.

2024 Bacteria Monitoring Results

Our 2024 monitoring revealed trends similar to previous years. The Maynard, Ashland, and Lowell sites had seasonal geometric means exceeding the swimming threshold of 126 CFU/100 mL. These locations have consistently shown elevated bacteria levels.

The Carter Avenue site (Concord River, Billerica) and Danforth Street (Sudbury River, Saxonville) continued to show low bacteria levels, remaining swimmable throughout the season. This year, a new site at West Concord (Route 62, Assabet River) was added, with mostly swimmable conditions recorded.

The River Meadow Brook site (Lowell) remains a major concern, consistently returning high bacteria levels. In response, we continue sharing our findings with Lowell Wastewater Utility to encourage remediation efforts.

In 2024, we also conducted a special bacteria study in Ashland (SUD-236). DNA analysis confirmed human fecal contamination and a survey of stormwater pipes identified several potential sources of sanitary sewage entering the river. Further investigation is ongoing.

Site#	Description	River	Samples	Exceed-ances	% Exceeded	2024 Geo-Mean	2023 Geo-Mean	2022 Geo-Mean	2021 Geo-Mean	2020 Geo-Mean	2019 Geo-Mean
ABT-007	Waltham Street, Maynard	Assabet	9	7	78%	294	195	193	164	289	121
ABT-176	Wheeler Road, Hudson	Assabet	9	3	33%	228
ABT-033	Route 62, Concord	Assabet	9	1	11%	117
CND-009	Rogers Street, Lowell	Concord	9	3	33%	185	191	203	153	216	147
CND-093	Carter Avenue, Billerica	Concord	9	0	0%	26	51	9
RVM-001	Lawrence Street, Lowell	River Meadow	9	7	78%	396	502	504	412	427
SUD-144	Danforth Street,Saxonville	Sudbury	9	1	11%	58
SUD-236	Chestnut Street, Ashland	Sudbury	9	3	33%	174	186	201	169	348	151

* Orange shding designates > 245 exeedance of BAV threshold or swimming geo-mean exceeddance.

2024

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2023

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2022

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Older Summaries

In addition to routine monitoring, OARS conducts special studies to track bacteria pollution sources.

2020–2022: A focused study in Lowell identified ongoing sewage contamination in River Meadow Brook. We shared our findings with Lowell Wastewater Utility in 2023, and we continue to push for remediation. Read Study Report here.
2023: A special study in Maynard (ABT-007) pinpointed bacterial sources, with results provided to Maynard DPW in April 2024. Read study report here.
2024: In Ashland (SUD-236), we used qPCR DNA analysis to determine whether bacteria came from human or animal sources. The results confirmed human DNA in the water, and additional investigations identified potential sewage leaks. Read study report here.