Urban effluents, including discharges of treated and untreated wastewater, overflows of sanitary and storm sewers, and surface water runoff, affect both human and ecosystem health. The effluent components can be chemical, physical, or biological in nature, and their impacts include changes in aquatic habitats and species composition, decreases in biodiversity, impaired use of recreational waters and shellfish harvesting areas, and contaminated drinking water (Table 3). These impacts all lead to a less valuable environment, a less prosperous economy, and, ultimately, a diminished quality of life.
Municipal wastewater effluents are responsible for the degradation of several ecosystems across the country. Impacts may arise from an increase in nutrient loads, decreased levels of dissolved oxygen, and releases of toxic substances, many of which can bioaccumulate and biomagnify in aquatic wildlife. Physical changes to the environment can also occur, including thermal enhancement, increased water flow, leading to potential flooding and erosion, an increase in suspended solids, and the release of floating debris to the country’s waters.
Nutrient enrichment
One of the most widely recognized and studied environmental effects of municipal wastewater
effluents is nutrient enrichment (Welch 1992). Some nutrients, particularly phosphorus and nitrogen,
are essential for plant production in all aquatic ecosystems. However, an oversupply of nutrients can
lead to the growth of large algal blooms and extensive weed beds. Such a condition is known as
eutrophication, and it degrades aquatic ecosystems in a number of ways.
In lakes where large algal blooms are present, the death of the vast numbers of phytoplankton that make up the blooms may smother the lake bottom with organic material. The decay of this material can consume most or all of the oxygen dissolved in the surrounding water, thus threatening the survival of many species of fish as well as bottom-dwelling vertebrates and invertebrates. Some algal blooms, in both lakes and marine coastal areas, also contain substances that are poisonous to both humans and wildlife.
In rivers and streams, the addition of nutrients tends to encourage the growth of periphyton, the stringy algae that grow on rock surfaces, and rooted aquatic plants. Excessive enrichment, however, can result in deoxygenation of the water and a consequent decline in the productivity of periphyton, as well as reductions in populations of bottom-dwelling invertebrates and fish and losses of some species.
In marine coastal waters, nutrients stimulate the rapid growth of phytoplankton and larger varieties of algae, which reduces the amount of light reaching seagrasses on the bottom. As the seagrasses, which stabilize the bottom sediments, die off, the water becomes more turbid and increasingly inhospitable to bottom plant life. Meanwhile, the phytoplankton, which float near the surface where there is greater light exposure, thrive and continue to multiply. With the disappearance of the seagrasses, many fish and bottom-dwelling organisms lose an important element of their habitat and are no longer able to survive.
The net effect of eutrophication on an ecosystem is usually an increase in the abundance of a few plant types (to the point where they become the dominant species in the ecosystem) and a decline in the number and variety of other plant and animal species in the system. Sportfish are among the species most often lost when water bodies become eutrophic. Probably one of the best known examples of a eutrophied lake in recent years, and its subsequent recovery, is Lake Erie (Box 2). However, local eutrophication problems remain a concern in several Canadian Great Lakes communities. Most rivers in the populated regions of Canada also show signs of nutrient enrichment downstream of municipal wastewater outlets or areas of intensive agriculture. In addition, periodic fish kills in Halifax Harbour have been linked, in part, to phosphorus inputs from raw sewage.
| Lake Erie is one of the most widely recognized examples of how an aquatic ecosystem can be
damaged by excessive nutrient loadings and how controls on nutrient inputs can lead to its
restoration. The damage began in the 1800s, when soil erosion as a result of the clearing of
land for agriculture and settlement increased phosphorus loadings to the lake. Another, more
dramatic, rise in phosphorus loadings began in the 1940s, as more and more people were
connected to sewage systems that discharged to the lake and detergents with high phosphorus
content came into use.
Concern about the persistent foam from detergents, the increasing degradation of visible water quality, and other environmental problems prompted authorities to undertake scientific studies of the causes and impacts of pollution in the lake. In 1970, a binational study confirmed a link between increasing concentrations of nutrients, particularly phosphorus, and the appearance of nuisance algae. To resolve the problem, modelling exercises suggested that phosphorus loadings would have to be reduced from roughly 28 000 tonnes a year to about 11 000 tonnes. In 1972, with the signing of the Canada–U.S. Great Lakes Water Quality Agreement, the two countries agreed to take steps to reduce phosphorus loadings to the recommended level of 11 000 tonnes per year. Four main strategies were employed to achieve this target:
By the mid-1980s, the total phosphorus load to Lake Erie had been reduced by more than 50%. Since then, it has continued to oscillate around the recommended level of 11 000 tonnes annually. |
Depletion of dissolved oxygen
Although nutrients in wastewater contribute to oxygen depletion through eutrophication, other
constituents of wastewater effluents do so more directly. Wastewater effluents contain large
quantities of organic solids, and the bacterial breakdown of this material and the oxidation of
chemicals in it can consume much of the dissolved oxygen in the receiving water. The amount of
oxygen consumed by decay processes over a period of days is known as the BOD in a laboratory
analysis of the effluent. Oxygen consumed over a couple of hours through chemical reactions is
known as the chemical oxygen demand of the effluent.
Since dissolved oxygen is essential to most aquatic life, oxygen depletion can have serious effects on aquatic life (Box 3). These effects may be immediate and short-term or may extend over months or years as a result of the buildup of oxygen-consuming material in the bottom sediments (Hvitved-Jacobsen 1982).
The amount of oxygen that can be dissolved in water depends on water temperature, elevation above sea level, and salinity. More oxygen can dissolve in cold water than in warm water. Similarly, fresh water holds more oxygen than salt water, and water at lower elevations (where the air pressure is greater) holds more oxygen than water at higher elevations. Harmful episodes of oxygen depletion often occur during summer when the water is warm and can hold less oxygen. However, serious depletion episodes can also occur in winter when ice cover on rivers and lakes prevents the replenishment of the water’s dissolved oxygen from the air (Chambers and Mills 1996). In Canada, many northern icecovered rivers may be vulnerable to the effects of wastewater effluent on winter oxygen levels.
| In the 1980s, studies were carried out to assess the impact of sewage from the Iona Island
treatment plant on fish and the receiving environment of Sturgeon Bank in British Columbia’s
Fraser River estuary. Prior to 1988, effluent from the treatment plant was discharged at high
tide onto the intertidal area of the bank. At low tide, the effluent was conveyed seawards across
extensive sandflats by a dredged channel that extended more than 6 kilometres into the Strait
of Georgia. A rock jetty paralleled the effluent channel on its north side for about 4 kilometres
and effectively restricted the dispersion of effluent to the southern portion of the bank. When
the bank was submerged by the tide, the oxygen demand of the effluent and sludge beds in
the vicinity of the outfall progressively reduced the dissolved oxygen in the receiving waters.
The dissolved oxygen depression frequently extended more than 4 kilometres into the Strait of
Georgia, but tended to be close to the jetty at low tide. Many organisms encountering this
oxygen-deficient water became stressed or were killed. As both bottom-dwelling species such as flounder and halibut and pelagic or upper-water species such as herring were affected, it was clear that oxygen depletion had occurred throughout the water column. Fish in oxygen-depleted waters typically rise to the surface to breathe and in doing so become easy prey for predatory birds. Herons and gulls on Sturgeon Bank usually congregated around oxygen-deficient waters where fish could be found at the water’s surface. Many dead flatfish of different age classes were found on the intertidal sandflats of Sturgeon Bank. In addition, catches of flatfish began to decline in the fishing area adjacent to the Fraser River just after the Iona Island treatment plant came into operation (Birtwell 1996). The realization of these significant ecological impacts led in 1988 to the extension of the Iona outfall diffuser beyond the estuary and into the Strait of Georgia, thus eliminating the old discharge point on Sturgeon Bank. Scientists have since studied the re-establishment of aquatic life in the vicinity of the old outfall and measured changes in water and sediment quality. Several improvements have been seen, and oxygen concentrations in the water above the sediments have recovered from the low levels experienced when the outflow was on the bank (Environment Canada 1998a). |
Low dissolved oxygen levels affect the survival of fish by increasing their susceptibility to disease, slowing growth, hampering swimming ability, altering feeding, migration, and reproductive behaviour, and making them less adept at avoiding predators. Extreme oxygen depletion results in rapid death. Low dissolved oxygen levels can also affect fish indirectly by reducing the populations of organisms that they eat (Alberta Environmental Protection 1996).
Long-term reductions in dissolved oxygen concentrations can result in changes in species composition. An increase in food supply in the form of more detritus tends to lead to a less diverse assortment of bottom fauna, dominated by worms and midges. This tends to favour bottomfeeding fish such as suckers and carp. In Lake Erie, for example, the populations of cisco, whitefish, walleye, sauger, and blue pike declined drastically over the 40-year period when loadings of nutrients to the lake were increasing. The total fish catch, however, did not decline. Instead, the catch of more desirable species was replaced with such species as carp, buffalo fish, freshwater drum, and rainbow smelt (Welch 1992).
Direct toxicity to wildlife
The toxic impacts of municipal wastewater on wildlife may be acute and occur within a short period
of time, or they may be cumulative and appear only after an extended period of time (Hvitved-
Jacobsen 1986; Harremoes 1988). Acute impacts from treatment plant effluents are generally caused
by high levels of ammonia and chlorine, high loads of oxygen-demanding materials, or toxic
concentrations of heavy metals and organic contaminants. Cumulative impacts result from a gradual
buildup of pollutants in the receiving water or in its sediments and biota and become apparent only
after accumulation exceeds a certain threshold. Because of the complexity and variability of
municipal effluents, however, and the variety of environmental factors that affect their biological
activity individually and in combination, it is not easy to arrive at broad generalizations about the
toxicity of municipal wastewater effluents (Welch 1992; Chambers et al. 1997).
Laboratory toxicity tests using planktonic algae, zooplankton, and fish have been conducted for effluents from many Canadian treatment plants to determine the level at which concentrations become lethal or cause physiological or behavioural changes.9 Although organisms differ in their responses to complex effluents (and to specific substances within these effluents), un-ionized ammonia has been shown to be the most frequent cause of toxicity in municipal wastewater effluents. Municipal treatment plants are, in fact, the leading quantifiable source of ammonia entering aquatic ecosystems throughout Canada.
Freshwater organisms are most at risk from exposure to ammonia (Environment Canada 2000). Some of the most sensitive species include rainbow trout, freshwater scud, walleye, mountain whitefish, and fingernail clams. Aquatic insects and micro-crustaceans are more resistant to ammonia, although there is a large variation in sensitivity among aquatic insects (Environment Canada 2000). The major impact of ammonia in aquatic ecosystems is likely to occur through chronic toxicity to fish and bottom-dwelling invertebrates, resulting in reduced reproductive capacity and reduced growth in the young.
The zone of impact from the toxic components of municipal wastewater effluents varies considerably with discharge conditions, such as river flow rate, temperature, and pH. For example, waters most at risk from municipal wastewater-related ammonia are those that are routinely basic in pH with a relatively warm summer temperature combined with low flows. Under estimated average conditions, some municipal wastewater discharges could be toxic for 10–20 kilometres from their point of release. Severe disruption of bottom flora and fauna has been noted below municipal wastewater discharges, and normal bottom conditions may not resume until as much as 20–100 kilometres from the discharge site.
Bioaccumulation and biomagnification of contaminants
Substances that are found only in low or even barely measurable concentrations in water can
sometimes be found in very high concentrations in the tissues of plants and animals. This is due
to a phenomenon known as bioaccumulation. Bioaccumulative substances tend to be very stable
and long-lived chemically and are not easily broken down by digestive processes. Many of them
are more soluble in fat than in water and therefore tend to accumulate in fatty tissues rather than
being excreted from the body. A limited number of these contaminants can undergo a further
phenomenon whereby their concentrations increase even more dramatically by being passed
up the food chain from prey to predator. During this phenomenon, each predator receives the
contaminants that each of its prey has accumulated in a lifetime and passes its own accumulation
on when it is eaten by predators at the next level in the food chain. This process is called
biomagnification; because of it, concentrations of a persistent toxic substance in an animal at the
top of the food chain, such as a herring gull or a beluga whale, can be 10 million times greater than concentrations in the water.
Because of these processes, even very low concentrations of certain substances in wastewater are of concern. Examples of persistent, toxic, bioaccumulative substances that have been detected in municipal wastewater include PCBs, dioxins and furans, organochlorine pesticides, and mercury and other heavy metals. Only a few metals and organic chemicals, such as mercury and DDT, are known to biomagnify throughout food webs, even though many substances can bioaccumulate. The effects of bioaccumulating substances on wildlife are well documented and include reduced reproductive success, physical deformities, tumours and lesions, reduced growth rates, and impairment of the central nervous system (Box 4). Although there are several other sources of persistent, bioaccumulative, toxic substances in the environment, including industrial discharges and deposition of atmospheric contaminants, municipal wastewater remains one of the most significant (Government of Canada 1996).
| High concentrations of many persistent, toxic, bioaccumulative substances have been found
in top predators in various regions across Canada. One notable example is the St. Lawrence
population of the beluga. Since 1885, when there were approximately 5000 St. Lawrence
belugas, the population has dwindled to somewhere between 300 and 700 individuals. As a
result of this decline, the beluga has been placed on the species at risk list of the Committee on
the Status of Endangered Wildlife in Canada. The decline of this population has been attributed, in part, to high levels of contaminants in the fatty tissues of the whales. PCBs, DDT, and mirex concentrations are, respectively, 25, 32, and 100 times higher in St. Lawrence male belugas than in Arctic-population males. These contaminants come chiefly from prey species, particularly the American eel, which migrates from the highly urbanized Great Lakes and Upper St. Lawrence region. American eels are thought to be the source of all the mirex (a flame retardant and pesticide whose use is now banned) and up to 50% of the other toxic chemicals in the whales. These high levels of contaminants are thought to be responsible for decreased reproductive success, the appearance of rare diseases, and suppressed immune systems in the belugas (Beland et al. 1993; Beland 1996). |
Physical changes to receiving waters
Thermal enhancement
Because aquatic life forms have characteristic temperature preferences and tolerance limits, an
increase in the average temperature of a water body can have important ecological effects. These
include changes in the variety and abundance of species as well as enhanced algal growth (Welch
1992). Municipal wastewater effluents can be a source of thermal enhancement because they are
usually warmer than the water bodies that they empty into. Warm urban surfaces such as roads
and rooftops, for example, add heat to rainwater as it runs off these surfaces and flows into storm
or combined sewers. Further warming may occur in runoff control facilities, particularly stormwater
ponds with extended detention times. In fact, studies have shown that, in summer months,
stormwater pond effluent might be up to 10°C warmer than the inflow (Schueler 1987). Effluent
from wastewater treatment plants may also contribute to thermal enhancement. Temperature
enhancement becomes more noticeable during periods of low flow, particularly when the effluent
is discharged into standing water bodies.
Increased water flow
Flow is one of the most important physical factors affecting the structure of aquatic habitats.
Increased or more variable water flow from urban runoff and wastewater effluents can cause
habitat changes in any receiving water. However, the most serious impacts occur in small urban
creeks. Urbanization increases the volume of surface runoff by reducing the infiltration of rainwater
into the ground and reducing evapotranspiration from vegetation. Urban drainage systems also
provide better conveyance channels that can remove surface runoff at a faster rate and thus
increase peak runoff flows.
The environmental effects of increased wastewater flows include bank erosion and flooding, erosion of stream- or riverbeds, and washouts, all of which result in habitat degradation (Schueler 1987; Borchardt and Statzner 1990). Some flow impacts, such as flooding and washout, are instantaneous, while others, such as changes in the physical structure of the stream and the resulting loss of habitat, are long-term. The broader ecological impacts can include changes in the food web and losses of critical species. Fishing is the most affected beneficial water use (Lijklema et al. 1993).
Increased suspended solids
Suspended solids occur naturally in surface waters as a result of erosion, transport of material from
the lake or river bottom, and tributary inflows. They are also added by erosion caused by human
activity and by effluents. Municipal wastewater effluents are responsible for a long-term continuous input of suspended solids to the environment.
Suspended solids released into receiving waters, mainly from stormwater or CSO discharges, can cause a number of direct and indirect environmental effects, including reduced sunlight penetration (and consequently reduced photosynthesis), smothering of spawning grounds, physical harm to fish, and toxic effects from contaminants attached to suspended particles (Horner et al. 1994). The growth and survival of some species may also be affected, either through direct effects (e.g., abrasion of sensitive tissues) or through indirect effects caused by changes in the food web or interference with dispersal or migration (e.g., the blockage of zones of passage). Such effects can manifest themselves on various time scales. A single large rainfall or runoff event can cause significant immediate impacts, but generally the long-term effects are more important.
Floating debris
Our rivers, lakes, and oceans contain an astonishing amount of debris from human sources. Debris
that originates on land includes plastic bags, fast food containers, pop cans, plastic chip and candy
bags, coffee cups, cigarette butts, tampons, condoms, and plastic ring six-pack holders. If this
debris is carried to a treatment plant, it is generally screened out.
Marine mammals and seabirds are particularly at risk from this material. Plastic bags floating on the water’s surface resemble the jellyfish that are eaten by many species of fish, dolphins, and turtles. Death can result from a blocked digestive tract, from toxic by-products produced by the digestion of some plastics, or through starvation from a false sense of being full. Wildlife entangled or snared in plastic debris face starvation, exhaustion, infection from wounds, or drowning.
Even though the oceans would seem to have an infinite capacity to disperse and absorb such materials, ocean currents tend to concentrate them in areas where currents meet. One such area is the northern Sargasso Sea in the Atlantic Ocean, which is a favourite spawning place for fish. It is difficult to determine how much debris is present in any given ocean area, but one study estimated that 8 tonnes of debris wash up on the shores of Sable Island, off the Nova Scotia coast, every year. About 92% of this material is plastic. On the west coast, Fisheries and Oceans Canada has estimated that between 100 000 and 500 000 pieces of debris are floating in British Columbia’s coastal waters.
Although MWTPs screen out solid material in raw sewage, municipal wastewater effluents are still a significant source of debris in the environment. Stormwater and CSOs are major contributors; in many of Canada’s coastal areas, however, the still-widespread practice of discharging raw sewage directly to the oceans provides a large and constant inflow of floating debris.
In Canada, the cost of treating health problems related to water pollution is estimated at about $300 million per year (Health Canada 1997). Canadians may be exposed in a variety of ways to chemicals and pathogens in water. They may ingest small amounts of pollutants in their drinking water, absorb contaminants through their skin while bathing or swimming, or inhale airborne droplets or vapours while showering. They may also ingest food, such as fish and shellfish, that has been contaminated by waterborne pollutants (Health Canada 1997). In addition to such human health impacts, pollution from wastewater effluents can reduce the social and economic benefits that we derive from the use of water. These impacts include periodic closures of urban beaches, closures of commercial fisheries because of fish and shellfish contamination, and aesthetic problems (with associated losses in tourism income).
Contamination of drinking water and waterborne diseases
Waterborne diseases caused by bacteria, viruses, and protozoa are the most common health
hazards associated with drinking water (and recreational waters) in Canada (Health Canada 1997).
Human and animal wastes are the main sources of these microbial contaminants. Most
municipalities treat and disinfect water used for drinking; thus, widespread outbreaks of
waterborne infections are rare. Even so, isolated incidents of microbial contamination of drinking
water in Canada from CSOs, stormwater, and inadequately treated sewage have been reported
(Box 5). These are usually associated with either poorly functioning water treatment facilities or the
complete lack of such facilities and a dependence on good-quality raw water.
| Most reported outbreaks of waterborne disease in Canada are due to the protozoa Giardia and
Cryptosporidium. Protozoa are capable of surviving for long periods of time in the aquatic
environment as dormant cysts or oocysts and are generally more resistant to chlorination than
pathogenic bacteria or viruses.
Giardia causes giardiasis, which is a long-lasting gastrointestinal disease. Fecal contamination from wild and domestic mammals has often been implicated in water-related outbreaks of giardiasis. Despite the potential for disease transmission by animals in Canada, most waterrelated outbreaks have been traced back to human sewage contamination (Health Canada 1998). In 1988 and 1989, five outbreaks of giardiasis from contaminated drinking water, involving 18 people, were reported in Canada. Since then, further outbreaks have occurred. Those related to sewage contamination include outbreaks in Temagami, Ontario, in 1994 and Dauphin, Manitoba, in 1996. The latter incident involved over 30 confirmed cases of giardiasis (Government of Manitoba 1997). The potential for giardiasis outbreaks is greater in northern regions, since cold water and ice cover provide ideal conditions for the proliferation of parasites (Yukon Department of Renewable Resources and Environment Canada 1996). Cryptosporidium is even more resistant to chlorination than Giardia. In 1996, an outbreak of cryptosporidiosis, an intestinal illness similar to giardiasis, was reported in Kelowna, British Columbia, where an estimated 15 000 people became ill. Heavy rains and snowmelt in the spring may have contributed to the outbreak. It has also been suggested that unusual wind conditions reversed the normal flow patterns in Lake Okanagan and pushed the sewage discharge back towards the city’s water intake. Cryptosporidiosis can be fatal in people who have weakened immune systems, such as AIDS patients. Health Canada has indicated that the true incidence of waterborne diseases is likely much higher than reported, as the majority of cases involve mild, flu-like symptoms that do not require medical treatment (Health Canada 1997). |
Ironically, another potential human health risk associated with municipal wastewater effluents results from the use of chlorine as a disinfectant in both wastewater and drinking water treatment. The use of chlorination to disinfect drinking water began in Canada around 1916. The provision of chlorinated water from this point on virtually eliminated typhoid fever, cholera, and other waterborne diseases and was one of the great achievements of public health policy in Canada during the 20th century. Unfortunately, chlorine’s potent oxidizing power causes it to react with naturally occurring organic material in raw water to produce hundreds of chlorinated organic compounds, referred to generically as chlorination by-products (CBPs). These by-products were first reported in drinking water in 1974. The most common CBPs are called trihalomethanes (THMs), a group of chemicals that includes chloroform, bromodichloromethane, chloro-di-bromomethane, and bromoform. Canadians may be exposed to THMs by drinking chlorinated water or beverages produced with chlorinated water, by inhaling airborne THMs released from tap water, or by absorbing THMs directly through the skin, particularly during showers (Health Canada 1997). Although only a few CBPs have been tested so far, the evidence suggests that they may pose a significant risk of cancer, particularly bladder cancer, to humans (Wigle 1998).
In addition to the health risks associated with contaminated water, communities may have to deal with taste and odour problems caused by large accumulations of algae. Additional filtration may provide a remedy for these problems, but at increased cost to the municipality (Anderson and Quartermaine 1998). The City of Toronto, for example, recently spent $6 million to install granulated carbon filters at its four filtration plants to deal with algae-related odour problems.
Water degradation and recreational water uses
Nearshore recreational areas can be easily contaminated by bacteria and other pathogens that are
present in CSOs, stormwater, and poorly treated sewage. Contact with microbially contaminated
waters may cause gastrointestinal disorders and minor skin, eye, ear, nose, and throat infections.
E. coli and/or fecal coliforms are generally used as indicators of contamination by pathogens that cause such diseases as hepatitis B, enteritis, cholera, and typhoid fever (Box 6). The current federal guideline for recreational water quality states that between 1 and 2% of recreational water users would be at risk of gastrointestinal illness at an E. coli (or fecal coliform) concentration of 200 per 100 millilitres (Health and Welfare Canada 1992). Many of the provinces and territories, however, have their own guidelines for recreational water quality.
| Fecal coliforms include several species of bacteria that naturally inhabit the guts of humans and
animals. Because they are expelled from the gut in feces, they eventually end up in sewage and
urban runoff. Some fecal coliforms, such as certain strains of E. coli, can be pathogenic — that
is, they can cause disease (Health Canada 1997). Other disease-causing bacteria, viruses,
and protozoa, originating from infected individuals, can also be transmitted to water bodies
through wastewater discharges. Fortunately, the more advanced types of wastewater
treatment, especially those with disinfection (e.g., ultraviolet radiation or chlorination),
are effective at reducing pathogen numbers in the final effluent. Identifying and enumerating all the disease-causing viruses, bacteria, and protozoa in wastewater on a regular basis would require an extraordinary amount of time, labour, and money (Droste 1997). However, if fecal coliforms are present in the water, one can assume that other pathogens that have passed through human and animal digestive systems will also be present. Thus, municipal and provincial/territorial authorities measure fecal coliform levels to estimate the degree to which water is contaminated by fecal pathogens. Fecal coliforms are especially useful for this purpose because they generally occur in high numbers in wastewater, can easily be identified and counted, and have been correlated with the presence of other pathogens (Geldreich 1978; Droste 1997). In Canada, coliform counts are used to determine whether beaches should be open for recreation, whether water is fit for consumption, and whether shellfish growing areas should be open for harvesting. Although the total fecal coliform count has historically been the most widely used indicator, other bacterial indicators, such as E. coli and fecal streptococci counts, are now more commonly used in Canada. |
Beaches are closed by local authorities when contaminant levels exceed guideline thresholds and remain closed, often for several days, until contaminant levels have returned to safer values. It is difficult to obtain comprehensive beach closure data on a national level due to differences in data collection methods by the municipalities. However, some beach closure data do exist. For example, between 1986 and 1994, 44% of Ontario’s Great Lakes beaches, most of them on Lake Ontario, were subject to closure notices at one time or another (Edsall and Charlton 1997). During the year 2000 swimming season in Manitoba, 46 beaches were monitored and 5 (11%) exceeded recreational guidelines at least once. Beach closures in Canada occur most frequently after heavy rainfalls.
Large quantities of algae can also interfere with recreational uses and reduce the aesthetic appeal of the shoreline. Algal blooms can cause increased water turbidity and discoloration, unpleasant odours, excessive fouling of fishing gear, and foaming along coastlines. In places where the nuisance species Cladophora has taken hold, long strands that break off in late summer and during storms can accumulate along shorelines to a thickness of a metre or more. The accumulations make swimming undesirable, and subsequent decay generates noxious amounts of ammonia, which may render adjacent properties unusable and lower their value. Increased plant growth can also cause problems for boaters.
A more serious threat from algal growth comes from certain species of blue-green algae that produce potent toxins that can damage the liver or nervous system. These toxins have also been blamed for animal poisonings in Alberta, Saskatchewan, Manitoba, and Ontario. Although the foul appearance and odour of the water deter people from drinking it, accidental exposures may occur during recreational activities, such as swimming, canoeing, and sailing.
Other wastewater problems that interfere with recreational uses include floating debris, which diminishes the aesthetic appeal of a shoreline area and makes it less attractive to tourists (Box 7), and stresses from increased water flow, suspended solids, BOD, and thermal enhancement, which can diminish the abundance and variety of fish in an area and hence its potential for sport fishing. In Nova Scotia, for example, the Survey of Recreational Fishing reported that the total number of recreational fish caught by anglers declined by nearly 1.7 million fish, or 45%, between 1990 and 1995. This resulted in a $5.5-million decline in total recreational fishing expenditures on food and lodging, transportation, and fishing services between these years. The disposal of untreated municipal wastewater effluents was partly responsible for these declines (Wilson 2000a).
| The Norwegian Sky, the second largest cruise ship in the world at 76 000 tonnes, recently
visited St. John’s, Newfoundland, and contributed over $200 000 to the local economy. Visits
to the harbour by large ships are now possible because of the widening of the harbour
entrance in the Narrows. However, St. John’s appeal as a tourist destination is somewhat
compromised by the release of 120 million litres of raw sewage and stormwater runoff into the
harbour every day from the surrounding municipalities. Much of this is deposited on the
harbour floor. When the organic waste is decomposed by anaerobic bacteria, highly odorous
hydrogen sulphide gas accumulates. When large ship propellers churn up the sediment, the
gas is released and can cause some people to become ill from the smell. Tour boat operators also report that tourists are displeased when they spot wastes (condoms, sanitary napkins, tampons, toilet paper, and other flushable material) in the water, both inside St. John’s Harbour and while travelling along the coast. There is no doubt that sewage pollution in Canada’s coastal communities is having a significant negative impact on the tourism industry. |
Contamination of shellfish growing areas
The marine coasts of Canada support a shellfish industry that had a total landed value of over
$1 billion in 1997 (Statistics Canada 2000). Unfortunately, this industry may not achieve its full
potential, because large areas along both the Atlantic and Pacific coasts are closed to harvesting
as a result of sewage contamination or the presence of dangerous levels of toxins and pathogens
from both natural and human sources. Shellfish consist of crustaceans, such as lobsters and crabs,
and bivalve molluscs, such as clams, mussels, and oysters. It is the consumption of bivalve molluscs
that poses the greatest threat to human health. Because bivalve molluscs filter large volumes of
water to extract suspended food particles, any harmful bacteria, viruses, and toxic substances
present in the water can be concentrated in these organisms to much higher levels than occur in
the surrounding waters.
Municipal wastewater effluents and urban runoff contribute to three types of pollution that affect shellfish: chemical pollution, bacteriological pollution, and pollution from natural biotoxins found in toxic forms of algae. Most closures of shellfish harvesting areas in Canada are the result of bacteriological pollution, while natural biotoxins account for the next largest number of closures. Only a few shellfish fisheries have been closed specifically because of chemical contamination. In those cases, dioxins and furans, pesticides, and mercury and other metals were the principal contaminants involved.
Bacteriological contamination is usually associated with the discharge of urban runoff or sewage effluent that has not undergone disinfection. Shellfish in areas exposed to these discharges can become contaminated with fecal bacteria, and consumption of these shellfish can lead to illnesses such as gastroenteritis, salmonellosis, typhoid fever, cholera, and hepatitis (Menon 1988; Nelson 1994; Nantel 1996).
Contamination from natural biotoxins occurs in both fresh and salt water when nutrients from sewage discharges, for example, stimulate the growth of toxic species of microscopic algae. The toxins produced by these algae can reach undesirable concentrations when large masses of them form what are known as algal blooms. These toxins become increasingly concentrated along the food chain as the algae are consumed by shellfish and other marine life. Although the shellfish are only marginally affected by the toxins, a single clam can accumulate enough toxin to kill a human adult (Anderson 1994). In Canada, three serious forms of poisoning from algal contamination have occurred: paralytic shellfish poisoning (PSP), amnesic shellfish poisoning (ASP), and diarrhetic shellfish poisoning (DSP) (Health Canada 1997).
PSP is caused by toxins produced by the dinoflagellate species Alexandrium fundyense. PSP toxins may occur in lobsters, clams, oysters, and mussels. Although PSP episodes are rare in Canada, with only a few cases reported per year, PSP continues to be a problem in three regions of the country: the St. Lawrence estuary, the lower Bay of Fundy, and the entire coast of British Columbia (Health Canada 1997).
ASP is caused by domoic acid, a toxin produced by tiny algae called diatoms, which can occur in intense blooms. In the world’s only confirmed outbreak of ASP, which occurred in November and December of 1987, more than 100 Canadians became ill and 3 people died after eating contaminated mussels from Prince Edward Island.
DSP is the result of toxins produced by species of the dinoflagellate genus Dinophysis. DSP toxins occasionally occur in clams and mussels. In 1990, the first reported outbreak of DSP in North America occurred in Nova Scotia after 13 people ate contaminated mussels. Since then, there has been one other confirmed episode of DSP, but the actual number of cases is likely much higher, as the symptoms can easily be confused with those of stomach flu (Health Canada 1997).
In response to concerns about shellfish contamination from algae and other sources, the Government of Canada developed the Canadian Shellfish Sanitation Program and the Canadian Shellfish Water Quality Protection Program. The main aims of these programs are to ensure that growing areas for clams, mussels, oysters, scallops, and other bivalve molluscan shellfish meet approved federal water quality criteria, that sources of pollution discharges to these areas are identified, and that all shellfish sold commercially are harvested, transported, and processed in an approved manner. Shellfish are now routinely tested for phytoplankton toxins that could be a serious threat to human health.
Closures of harvesting grounds have seriously limited the economic potential of all of Canada’s major shellfish fisheries. On the coast of British Columbia, for example, there were 246 shellfish closures due to contamination by pathogens under the Fisheries Act as of July 1999, encompassing an area of about 1050 square kilometres. Multiple pollution sources accounted for the largest area of closures, followed by sewage outfalls, agriculture/hinterland drainage, boat sewage discharges, urban runoff (including septic seepage), and pulp mill pollution (Environment Canada 1999d). The area of B.C. coastline closed to shellfish harvesting has increased substantially since Environment Canada began routinely assessing water quality for shellfish consumption in the early 1970s. Only part of this increase can be attributed to expanded monitoring activities, however.
In Quebec, of the 196 shellfish zones that were evaluated in 1999, 114 (58%) were permanently closed and a further 21 (11%) were closed from June 1 to September 30 (Environment Canada 1999e). Private residences, municipal sewage treatment plants, and agricultural runoff were responsible for the 114 zones that were permanently closed. Municipal sewage was also directly responsible for the closure of 34 of the 190 soft clam and blue mussel harvesting areas in Quebec (Nantel 1995).
On the Atlantic coast (excluding Quebec), nearly 36% or 2092 square kilometres of the areas surveyed as suitable for direct harvesting of shellfish were closed in 1995 (Statistics Canada 2000). In 1999, the closed area was nearly the same, 2065 square kilometres (Menon 2000). Losses to the local economy have been estimated at about $10–12 million.
The risk of harvesting shellfish from polluted waters increases with proximity to highly urbanized or agricultural areas. The pollution conditions are often aggravated by rainfall, which can result in sewage-contaminated runoff or effluent from overloaded sewage treatment systems reaching the shellfish beds. Areas that are near towns, villages, and other human habitation are often closed year-round.
Contamination of fisheries
Several toxic substances are known to accumulate in fish, and provincial/territorial authorities
routinely issue advisories about safe consumption limits for species caught in certain areas. Five
contaminants or groups of contaminants account for most of these advisories: mercury, PCBs,
mirex/photomirex, toxaphene (a pesticide), and dioxins (OMOE 1999). Although these
contaminants come from a wide variety of sources, all of them have been detected in municipal
wastewater effluent.
There are also concerns about the effects of algal toxins on the finfish aquaculture industry. As caged fish cannot avoid areas where there are blooms, fish kills could result from the direct uptake of toxins, deoxygenation of the surrounding water, or clogging of the fishes’ gills. Phytoplankton blooms are already a threat to the $100-million aquaculture industry in the Bay of Fundy (Percy 1996), and water temperatures and phytoplankton populations are now regularly monitored in an effort to prevent any problems.
Wild fish kills can also result when toxins from blooms are passed through the food web. Anchovies in B.C. waters, for example, have been known to be affected by domoic acid. Hundreds of tonnes of herring were also poisoned on the Atlantic coast in 1976 and 1979 by PSP toxins accumulated through the food web.
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