2022 Status of Beneficial Use Impairments

BUI 1:
BUI 2:

BUI 3:

BUI 4:

BUI 5:

BUI 6:

BUI 7:

BUI 8:

BUI 9:
Drinking water

BUI 10:

BUI 11:

BUI 12:

BUI 13:

BUI 14:

What is a Beneficial Use Impairment? (BUI)


The St. Lawrence River offers many ecological, recreational and economic beneficial water uses. When human activity negatively interferes with the use or enjoyment of a water use, it’s called a Beneficial Use Impairment (BUI). This may include changes to the chemical, physical, or biological integrity of the ecosystem. In order for the St. Lawrence River Area of Concern to be “delisted”, or removed from the list of Areas of Concern, there can longer be any BUIs. Recognizing that it is not feasible to restore all BUIs completely due to human influences having permanent impacts on the natural environment, a set of “delisting criteria” have been set for each BUI. These are measurable environmental conditions that need to be achieved before the BUI can be considered restored and the Area of Concern delisted.

Summary of Existing BUIs


The following is a summary of the status of the BUIs which are either impaired or need further assessment in the St. Lawrence (Cornwall) Area of Concern.

Printable Resources:    •BUI Booklet     BUI Poster       BUI Rack Card

BUI 1- Restrictions on Fish and Wildlife Consumption

Status: Impaired

Reasons for impairment:

  • Mercury from historic Cornwall industry

Delisting Criteria:

  1. Contaminant levels in fish in the Area of Concern are the same or less than those in upstream non-Area of Concern areas in the St. Lawrence River.
  2. Restrictions same or fewer/less restrictive than upstream non-Area of Concern areas in the St. Lawrence River.
  3. No restrictions on the consumption of snapping turtle in the Area of Concern.


The results of long-term monitoring (since 1975) of mercury in fish show that the levels have declined significantly over time in Area of Concern fish. For Block 15 (considered the Area of Concern in the Ontario Guide to Eating Fish), recent mercury concentrations are 33-59% lower than historical mercury concentrations for all four species of interest (walleye, yellow perch, smallmouth bass and northern pike).

Consumption advisories have improved within the AOC and other blocks along the St. Lawrence River. Fish consumption advisories from within the Area of Concern are generally more restrictive compared to upstream St. Lawrence River locations.

There are no consumption guidelines for snapping turtles either federally or provincially. Although the 2017-2018 Guide to Eating Ontario Fish does include a reference on snapping turtles, this is advice only. Sampling of turtle eggs in the Raisin River and upstream of the Moses Saunders Dam found the concentrations of both mercury and PCBs have been declining over time and that the concentrations in the Area of Concern are the same as upstream.

All major industrial discharges have been eliminated in the Area of Concern. This has resulted in the cessation of direct industrial wastewater releases containing mercury (and other industrial contaminants). Ongoing sources of mercury to the AOC include possible diffuse emissions from widespread low level contaminated sediments, and natural sources that act as the primary sources of elevated levels in sport fish. A mercury ‘track down’ investigation to detect any remaining fugitive releases of mercury has been completed and did not identify any ongoing fugitive sources.

BUI 2- Tainting of Fish and Wildlife Flavour

Re-designated from impaired to not impaired in 1997.

BUI 3- Degradation of Fish and Wildlife Populations

Status: Impaired

Reasons for impairment:

  • Seaway and dam construction
  • Fishery exploitation
  • Loss of habitat due to development along shorelines and in wetlands
  • Possible contaminant impacts

Delisting criteria:

  1. Maintenance of marsh bird and amphibian populations and diversity at or above Great Lakes non-Area of Concern averages as determined by the Marsh Monitoring Program of Bird Studies Canada.
  2. Presence of successfully reproducing osprey in the Area of Concern for a minimum 5 years.
  3. Maintain no net loss of populations of great blue herons, great egrets, black-crowned night herons, common terns, and herring and great black-backed gulls on Dickerson Island and Strachan Islands or other suitable areas within or immediately adjacent to the Area of Concern.
  4. Temporal trends in contaminant levels in herring gulls, night herons, snapping turtles and/or cormorant eggs are stable or declining. Spatial comparisons show that contaminant concentrations in eggs of the above species in areas under the influence of the Area of Concern are equal to or less than those from sites removed from any influence of the St. Lawrence River Area of Concern.
  5. Acceptance for implementation of actions recommended in the Lake St. Francis Fisheries Management Plan, Walleye Fisheries Management Study, Muskellunge Management Strategy and broader St. Lawrence River Restoration plans for these species to help restore, protect and monitor populations of muskellunge, sturgeon and walleye.
  6. Establishment of fish community objectives by the Ministry of Natural Resources and Forestry Lake Ontario Management Unit for Lake St. Francis. These objectives will, when established (and where applicable), be recognized as delisting criteria for the RAP (fish community objectives are currently being finalized).


The sale of angler-caught perch was discontinued in 2004, and a Lake St. Francis Fisheries Management Plan (2007) was developed. This plan encompasses most species in the fish community.

BUI 4- Fish Tumours or Other Deformities

Status: Needs further assessment

Reason for impairment: Studies underway

Fish tumors and other deformities become a BUI when the incidence rates of fish tumours or other deformities exceed rates at unimpacted sites or when survey data confirm the absence of neoplastic or preneoplastic liver tumours in bullheads or suckers.


This Beneficial Use was identified as “possibly impaired” in the RAP Stage 1 and Stage 2 reports. The status was reviewed in 2003-2004 and no definitive conclusion was reached. A reassessment commissioned by Environment Canada was completed and presented to the St. Lawrence River Restoration Council (SLRRC) in September 2008. Liver evaluations of brown bullhead showed low levels of neoplastic lesions (2%). In addition, a comparison of the incidence of both neoplastic and preneoplastic lesions in brown bullhead to the incidence in other Area of Concern sites and reference sites indicated that this BUI is not impaired.

BUI 5- Bird or Animal Deformities or Reproductive Problems

Re-designated from Requires Further Assessment to not impaired in 2007.

BUI 6- Degradation of Benthos

Re-designated from impaired to not impaired in 2007.

BUI 7- Restriction on Dredging Activities

Re-designated from impaired to not impaired in 2007.

BUI 8- Eutrophication or Undesirable Algae

Status: Impaired

Reason for impairment: 

  • Impaired in watershed tributaries; unimpaired in St. Lawrence River
  • Excess nutrients from municipal and rural sources

Delisting Criteria:

  1. Provincial Water Quality Monitoring Network (PWQMN) Reduction: Demonstrate a reduction in phosphorus concentrations at St. Lawrence River tributaries as measured at PWQMN sites.
  2. Lake St Francis: The mean annual Total Phosphorus (TP) concentration in Lake St Francis should not exceed 20 ug/L in waters between the two-metre nearshore contour and the open channel.
  3. Algal Blooms: No evidence of sustained and widespread undesirable algae blooms in the St. Lawrence River, source-specific to the Area of Concern, whether free-floating or attached to surfaces. Site specific occurrences at tributary mouths and in developed areas should be monitored and nutrient control programs put in place.
  4. Create and initiate a plan to improve water quality at the mouth/nearshore areas of Area of Concern tributaries to achieve site-specific annual mean TP concentrations, or in accordance with MCA standards when tributaries are on MCA lands.


The control of nutrient loadings from both point (i.e., municipal, commercial and industrial wastewater discharges) and non-point sources (i.e., runoff from urban, rural and agricultural areas) is a key component to preventing eutrophication and/or the proliferation of undesirable algae in the Area of Concern. A number of successful programs have been implemented and are effectively improving the control of nutrients from point and non-point sources. A review of the Provincial Water Quality Monitoring Network (PWQMN) data from tributaries in the Area of Concern was undertaken in 2009 and re-assessed and confirmed in 2011. The conclusion made was that the data show that at monitoring stations in the Area of Concern, the mean annual TP concentrations are lower in recent years than in the 1980s when monitoring began, indicating that there has been an improvement over time.

An evaluation of water quality in the St. Lawrence River and Lake St. Francis demonstrate that the main portion of water flow in the St. Lawrence River at the Area of Concern originates from upstream sources (Lake Ontario), with Area of Concern tributaries contributing only approximately 0.001% of the water. St. Lawrence River water quality data for total phosphorous (TP) from upstream (Lake Ontario), downstream (Pointe Beaudette, QC) and local Area of Concern locations has improved since the 1970s, with current values consistently well below 20 μg/L. Thus, the St. Lawrence River and off-shore waters of Lake St. Francis meet the water quality guideline.

Monitoring programs undertaken in 2008 and 2009 found potentially toxic algae in some nearshore areas of Lake St. Francis with the occurrence of cyanobacteria. However, the occurrence of potentially toxic algae in the nearshore area of Lake St Francis is not considered site specific to the Cornwall Area of Concern as the occurrence of this type of algae is not uncommon in other shallow, embayment type areas in the Great Lakes basin (e.g., Bay of Quinte, Hamilton Harbour, Sturgeon Bay/Lake Huron, Kingston Inner Harbour, Lake Nipissing, western basin of Lake Erie). The Ministry of Environment, Conservation and Parks (MECP) has protocols in place for responding to incidents involving nuisance algal blooms, and will work collaboratively with community partners to monitor and develop strategies to manage algal related occurrences within the Area of Concern. Upgrades to the wastewater treatment plants in the City of Cornwall and the village of Glen Walter will result in better control of effluent quality and a reduction of nutrient loadings in the nearshore areas of the Area of Concern.

Water quality monitoring undertaken from 2008 to 2010 at tributary mouths found elevated nutrient conditions in these areas. These elevated nutrient levels are associated with current land uses and associated run off. Phosphorus levels in the tributaries have improved and are similar or better than non-Area of Concern sites. Of the 11 tributaries in the Area of Concern, most are close to meeting the delisting criteria. Efforts will continue with rural landowners to encourage adoption of best management practices that will minimize the amount of nutrient export for any given land use.

BUI 9- Restrictions on Drinking Water Consumption, or Taste or Odour Problems

Re-designated from impaired to not impaired in 1997.

BUI 10- Beach Closings

Status: Impaired

Reason for impairment:

  • Bacteria from municipal and rural sources

Delisting Criteria:

  1. At public beaches, no more than 20% of weekly tests (i.e., five sample E. coli geometric means) exceed the Provincial Water Quality Objective during an annual swimming season. In addition, the main/predominant sources of faecal pollution are known; most of these exceedances are associated with local events, such as significant rainfall or high wind periods.
  2. The ‘vast majority’ of body contact water recreation areas in the Area of Concern, other than public beaches, must meet the Provincial Water Quality Objective (PWQO) during the swimming season. Body contact water recreation sites that do not meet these objectives are highly localized and exceedances occur only sporadically (i.e. the Area of Concern does not show widespread evidence of faecal pollution).
  3. For body contact water recreation areas where water quality objective exceedances occur, the main sources of faecal pollution must be identified, pollution control plans must be developed, and these plans must be actively implemented.


At the two public beaches in the Area of Concern, regular testing has shown that E. coli testing meets the PWQOs. The septic system re-inspection program, implemented from 2008-2013, promoted the voluntary care and maintenance of private residential septic systems. Additionally, a number of programs (such as the RAP Tributary Restoration Program, and Environmental Farm Plans) have been effective at improving the management of animal wastes from rural agricultural sources. The Recreational Area Bacteria Working Group (under SLRRC) documented substantial improvements in bacteria (E.coli) levels along the Cornwall waterfront and in Lake St. Francis since the 1980s. The City of Cornwall developed and is implementing a Pollution Prevention and Control Plan that is reducing the number of combined sewers and combined sewer overflow events. A Beach Health Collaborative involving the St. Lawrence Parks Commission, the Raisin Region Conservation Authority, the Eastern Ontario Health Unit and the St. Lawrence River Institute of Environmental Science has been formed with plans to continue to ensure lasting improvements at water recreation areas in the St. Lawrence River. The Eastern Ontario Health Unit undertakes regular monitoring at the bathing beaches.

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BUI 11- Degradation of Aesthetics

Re-designated from impaired to not impaired in 1997.

BUI 12- Added Costs to Agriculture or Industry

Re-designated from impaired to not impaired in 1997.

BUI 13- Degradation of Phytoplankton and Zooplankton Populations

Status: Need Further Assessment

Reason for impairment:

  • Undocumented
  • Possible Zebra mussel problems. 

This BUI would be considered impaired when phytoplankton and zooplankton community structure significantly diverge from unimpacted control sites of comparable physical and chemical characteristics. In addition, this use will be considered impaired when relevant, field-validated, phytoplankton and zooplankton bioassays (e.g. Ceriodaphnia; algal fractionation bioassays) with appropriate quality assurance/quality controls confirm toxicity in ambient waters. A status assessment is currently being undertaken to determine if this BUI is impaired or not.


This Beneficial Use was identified as “possibly Impaired” in the St. Lawrence RAP Stage 2 Report.

Since 1989, mean summer phytoplankton biomass has remained relatively stable at ≈ 200 mg m^3 and has not show a significant change over time. On the other hand, the composition of the phytoplankton community has shown a dramatic change, with Cryptophyceae replacing Chrysophyceae as the dominant taxonomic group. This change in the taxonomic composition may be a result of zebra mussel grazing as evidenced in Lakes Ontario and Erie (Makarewicz et al., 1999; Munawar et al., 1999; 2005; 2006), but would warrant further investigation to determine the specific cause.

Changes in the species composition of the phytoplankton community also yielded important information about the health of the Cornwall Area of Concern. An analysis comparison of eutrophic and oligotrophic indicator species observed in summer 1989 and summer 2004 was performed. A total of 12 eutrophic indicator species were detected in 1989 compared to 2 in 2004. With respect to oligotrophic indicator species, the number remained unchanged at 3 in both 1989 and 2004. The decline in eutrophic species shows that attempts to improve water quality (e.g. reductions in phosphorus loadings) are working.

BUI 14- Loss of Fish and Wildlife Habitat

Status: Impaired

Reasons for impairment:

  • Seaway & dam construction
  • Development along shoreline and in wetlands
  • Habitat modification due to excessive weed growth

Delisting Criteria:

  1. Create and initiate a plan to protect the number, the form and function of coastal provincially significant wetlands, with a goal of “no net loss” in area.
  2. Create and initiate a plan to establish a minimum riparian cover width of 3 meter, or greater, along the St. Lawrence River shores on Akwesasne lands, public lands and private lands.
  3. Create and initiate a plan to improve conditions in Pattingale Creek to enhance fish and wildlife habitat.


Significant habitat protection and management plans have been put in place through the Raisin Region Conservation Authority Natural Heritage Strategy and the Lake St. Francis Fish Habitat Management Plan. In addition, fish habitat along the Cornwall waterfront has been remediated through the Littoral Zone Fish Habitat Improvement Project.