Freshwater Lake and Pond Water Quality

RIDEM is responsible for assessing the water quality conditions in Rhode Island's lakes and ponds. Fortunately, due to in large part the long-running volunteer monitoring program for lakes coordinated by URI-Watershed Watch, there is a sizable amount of data generated on many of the 237 freshwater lakes and ponds DEM is tracking. Coupled with other data sources including plant surveys, DEM has been able to at least partially assess conditions in 78% of the tracked lakes, ponds and reservoirs.

The following information, derived from DEM's 2010 Integrated Water Quality Monitoring and Assessment Report, summarizes the assessment results for lakes and ponds.

Aquatic Invasive Species

The largest cause of impairment in lakes and ponds is due to the presence of aquatic invasive species (AIS), primarily plants. As of April 2008, DEM reported 36 lakes or ponds had aquatic invasive species in sufficient quantities to impair one or more beneficial uses of the waterbody. DEM field survey work in 2007-2008 documented AIS to be widespread in RI freshwaters - with invasives, found in 73% of the locations inspected. During the same period, the first two infestations of water chestnut in ponds were reported in RI. Surveys for AIS are continuing.

In many RI lakes and ponds, aquatic invasive plants are well established and therefore require a long-term plant management strategy that DEM recommends be part of a comprehensive lake management plan. See the side navigation bars on the left of this page for more information.

Water Pollution

Water pollution problems in freshwater lakes and ponds include excessive nutrients, which can accelerate the eutrophication process, mercury in fish and to a lesser extent metals and pathogens. As of April 2008, fifty-six (56) lakes or ponds were identified as having one or more pollutant related water quality impairment; including 19 lakes known to have fish contaminated with mercury. To restore water quality conditions, DEM develops restoration plans, known as TMDLs, which identify necessary pollution abatement actions. Such plans have been completed for Stafford Pond, Yawgoo and Barber Ponds, Mashpaug Pond and nine eutrophic lakes and ponds. A regional mercury TMDL that applies to 19 lakes has also been completed. Twenty-eight (28) lakes are the scheduled for future TMDL development (2010-2016).

Close to half (48%) of the lake acres for which data were available exhibited impaired aquatic life conditions. Major causes of impairments were excess nutrients and dissolved oxygen. When lakes or ponds receive too many nutrients, the response typically is excess algal or plant growth that upon decay may deplete dissolved oxygen levels. Thirty-three (33) lakes or ponds exhibit elevated levels of the nutrient phosphorus.

Fish consumption advisories have been issued for 20 lakes - all but one involving the contaminant mercury. The regional mercury TMDL cited atmospheric deposition as a primary source of the mercury in RI fish. A full characterization of fish tissue contamination in RI is not possible as the needed data is not currently available for most lakes and ponds. In 2007 DEM initiated a limited fish tissue sampling program to begin to address this data gap.

Pathogen (bacteria) problems were present in a small percentage (4%) of the lake acres for which data were available.

PRIMARY RESOURCES

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