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Priority Substances List Assessment Report for Ammonia in the Aquatic Environment
3.0 Assessment of "Toxic" Under CEPA 1999
3.1 CEPA 1999 64(a): Environment
The environmental risk assessment of a PSL substance is based on the procedures outlined in Environment Canada (1997a). Environmental assessment endpoints (e.g., adverse reproductive effects on sensitive fish species in a community) are selected based on analysis of exposure information and subsequent identification of sensitive receptors. For each endpoint, a conservative Estimated Exposure Value (EEV) is selected and an Estimated No-Effects Value (ENEV) is determined by dividing a Critical Toxicity Value (CTV) by an application factor. A conservative (or hyperconservative) quotient (EEV/ENEV) is calculated for each of the assessment endpoints in order to determine whether there is potential ecological risk in Canada. If these quotients are less than one, it can be concluded that the substance poses no significant risk to the environment, and the risk assessment is completed. If, however, the quotient is greater than one for a particular assessment endpoint, then the risk assessment for that endpoint proceeds to an analysis where more realistic assumptions are used and the probability and magnitude of effects are considered. This latter approach involves a more thorough consideration of sources of variability and uncertainty in the risk analysis.
3.1.1 Assessment endpoints
The bulk of the ammonia emitted in Canada is released to air, with the remainder being released to water. However, because of the rapid and large dilution of ammonia and the high deposition rate, the impacts through the air are not considered to be the main ones. Impacts on water ecosystems are more important from point sources due to the concentrations of ammonia in municipal WWTP effluents and the nature of the toxicity of ammonia to aquatic organisms.
Assessment endpoints include the reduction of growth and reproductive success in a mixed community of aquatic organisms for chronic exposures. The community included eight species of fish, one amphibian and four species of invertebrates. These are the species listed in Table 6, excluding pumpkinseed. The species were selected on the basis of being widespread in large areas of Canada and having at least one good toxicity study done on them. The most sensitive organisms in this community were the scud, Hyalella azteca, sockeye salmon (Oncorhynchus nerka) and the rainbow trout (O. mykiss). Scud are important in an aquatic ecosystem, as they are bottom browsers and act as an important source of fish food. Sockeye salmon and rainbow trout are top-order carnivores highly prized by humans for sport and food. Other assessment endpoints included 10% lethality of the most sensitive aquatic organisms in a community, again rainbow trout, in multiday exposures and 10% lethality of rainbow trout over 12 hours.
Terrestrial plants are the major organisms exposed via atmospheric transport of ammonia. Assessment endpoints for plants are the destruction of leaf material, specifically necrosis, browning and early leaf drop. A review of terrestrial plant toxicity data determined that acute toxicity is generally not a problem with respect to terrestrial plants, as levels of ammonia required to generate an acute toxic response were far higher than the levels documented from Europe to cause adverse effects on terrestrial ecosystems. Most plants require inputs of nitrogen for continued growth and will respond with increased growth rates under very high nitrogen deposition rates. Under conditions of chronic exposure to gaseous and particulate ammonia, reduced drought tolerance was noted as an assessment endpoint that is quite sensitive.
Several sensitive Canadian ecosystems have been identified, in particular sphagnum bogs and conifer forests. Sphagnum bogs are adapted to low nitrogen conditions and do not respond quickly to inputs of nitrogen as ammonia. They can be endangered from other nitrogen-adapted plants, in particular grasses. Conifer forests can be susceptible to reduced frost hardiness and eutrophication when exposed to high levels of ammonia over long periods of time.
A concept also used in the terrestrial toxicity assessment was developed in Europe in response to heavy inputs of nutrients to many ecosystems. This is the concept of the "critical load," a loading of a chemical on an ecosystem that will not cause a deleterious impact (Boxman et al., 1988; Bobbink et al., 1992). Inputs are calculated as yearly loads of the chemical in question. The measurement endpoint is a specific ecosystem, i.e., conifer forests or sphagnum bogs. The assessment endpoints are effects (a shift towards nitrogen-adapted species like grasses) on similar terrestrial ecosystems in Canada.
3.1.2 Environmental risk characterization
3.1.2.1 Hyperconservative assessments
Hyperconservative assessments are presented in Table 9 for four exposure pathways: exposure of freshwater and saltwater fish, exposure of marine benthic organisms from dredging and dumping sediments, and exposure of conifer trees through atmospheric deposition of ammonia.
The EEVs are as follows: for fresh and salt water, a maximum value of 0.68 mg un-ionized NH3/L was detected at one location (sewage from Annacis Island, Vancouver) (Servizi et al., 1978), 3 and for air, 0.56 mg NH3/m3 was detected downwind of a dairy farm in California (Luebs et al., 1973). Application factors of 10 were used for fish due to the large and fairly complete databases; an application factor of 100 was used with dredging operations due to the moderate database on effects and exposure information; and an application factor of 1000 was used for conifer trees because of the relatively poor database on effects.
For the hyperconservative assessment of saltwater dredging operations, an ENEV of 0.008 mg/L was used for Ampelisca abdita (Kohn et al., 1994). The EEV value of 0.177 mg/L is from Sims and Moore (1995) based on average reported pore water concentrations from U.S. Army Corps of Engineers dredging operations in salt water.
3.1.2.2 Conservative assessments
A conservative environmental assessment involves a further analysis of exposure and/or effects to calculate a quotient that is still conservative, but is more "realistic" than the hyperconservative quotient (Environment Canada, 1997a). The EEV is based on typical concentrations or deposition values in the vicinity of sources. The selection of CTVs is more rigorous, taking into account toxicity in organisms that would typically be exposed and matching the length of test exposure to that found in the field. The application factors used may be smaller if an adequate acute toxicity base data set is available (factor of 100) or if threshold sublethal toxicity values are available (factor of 10). However, the ENEV obtained should not be within the range of typical natural concentrations or deposition rates. If the quotient is still greater than one, then a probabilistic risk assessment is warranted, if there are sufficient data. The assumptions inherent in the data and application factor used are examined and minimized where possible, thereby refining the assessment process to generate a more accurate or "real-world" assessment than would be done in a hyperconservative assessment.
3.1.2.2.1 Releases to air
Based on an analysis of the literature (Sheppard, 1999), a critical load of
10 kg N/ha per year may be generally protective for nitrogen-poor sites, such as stands of native vegetation on soils of granitic origin. However, this value is not far above nitrogen deposition rates for remote areas. For example, Shaw et al. (1989) reported deposition of 4.2 kg N/ha per year in a boreal area in central Alberta. Janzen et al. (1997) reported a similar value. In contrast, Barthelmie and Pryor (1999) estimated that agricultural areas in the Lower Fraser Valley annually received from 44 to 105 kg N/ha.
The acute CTV for plants (leaf necrosis, increased sensitivity to cold) after an hour-long exposure to ammonia in air is 25 000 µg/m3 (Van der Eerden, 1982).
Information on responses of plants to gaseous ammonia is sparse. There is a slight possibility of localized impacts on sensitive agricultural crops (in particular vegetables) close to point and area sources of ammonia, and the contribution of airborne ammonia to local water bodies is unknown. Ground-level concentrations of ammonia near agricultural and industrial sources are generally low or sporadic in occurrence and intensity. Because of the absence of Canadian data near point sources, a monitoring and modelling study was conducted by Environment Canada to develop exposure data (McDonald, 1999) using the ISCST3 (Industrial Source Complex Short Term) model at the Agrium Inc. fertilizer facility in Fort Saskatchewan, Alberta. The Agrium Inc. fertilizer plant is one of the major point sources of atmospheric ammonia in Canada. Another modelling run was made to estimate the release and potential impacts of ammonia from a manure fertilizer application to a 1-ha field in summer.
An area around the Fort Saskatchewan site, roughly 7.5 km2, is exposed to a maximum hourly winter concentration of 100 µg/m3. The acute CTV is 25 000 µg/m3 for "general terrestrial" plants. An application factor of 100 is used due to the limited database on effects, but it is reduced from 1000 in the hyperconservative assessment due to the improved exposure estimates.
This quotient (<1) indicates that even for a large point source of ammonia, there is little likelihood of "instantaneous" injury to nearby terrestrial plants with high hourly concentrations of ammonia.
In order to facilitate a direct comparison between the potential influence of a point and an area source on nearby vegetation, the identical conditions were run again replacing the industrial complex with a 1-ha field treated with manure. Typical emission data for a surface application in Ontario were taken from information presented in Section 2.3.2.1. Prior to application, ammonia flux was measured to be less than 0.015 kg NH3 -N/ha per hour (Period A); immediately after application, fluxes of up to 1.2kg NH3 -N/ha per hour were measured (Period B). These flux values dropped off quickly to around 0.1-0.3 kg NH3 -N/ha per hour (Period C) and stayed that way over a period of days, with considerable diurnal fluctuation (Beauchamp et al., 1982). A 2-week period in June 1990 was selected with stable weather that was warm and dry.
The ISCST3 model allows us to determine the concentration of ammonia released during manure fertilization of a field. The 1-hour maximum concentration of ammonia released over a significant area (800 m2 from a fertilized plot of 100 m2) is 100 µg/m3. This concentration could be expected outside the perimeter of a fertilized field as well. The acute CTV of 25 000 µg/m3 is used. An application factor of 100 is used due to the limited database on effects, but it is reduced from 1000 in the hyperconservative assessment due to the improved exposure estimates.
This quotient indicates that for an area source of ammonia (a recently fertilized field using manure), there is little likelihood of an injury to nearby terrestrial plants with high hourly concentrations of ammonia.
In order to improve understanding of the atmospheric fate of nitrogen in the Lower Fraser Valley, two initiatives were undertaken. The first involved updating and improving the ammonia emissions inventory for the region, based on the most recent census data available and improved emission factors. The second initiative was directed towards providing spatial maps of concentration and deposition of nitrogen compounds based on model runs.
The maximum point of deposition during the model exercise was 105 kg/ha per year as NH3 to the surface in the modelled area. A worst-case scenario was used with this deposition rate applied for a full year. Because this deposition is so driven by sources, this amount could vary substantially with season and may be subject to periodically high levels. This could have important terrestrial impacts, as the Dutch have found terrestrial eutrophication impacts on coniferous forests at such deposition rates.
Because the modelling exercise estimated ammonia inputs to the Lower Fraser Valley as a deposition rate, the critical loading rate will be used to estimate potential toxicity. The critical loading rate for sensitive terrestrial ecosystems in Canada is 10 kg N/ha per year based on long-term effects on conifer ecosystems. The modelling in the Lower Fraser Valley provides an EEV of 105 kg N/ha per year from ammonia. No application factor was used for this assessment, as the natural deposition rate is around 4-5 kg/ha per year and the critical load is estimated at only 10 kg/ha per year.
Based on this quotient, there is a definite possibility that conifer forests in the Lower Fraser Valley may be detrimentally affected by ammonia deposition. Unfortunately, there is little information either on the widespread deposition of ammonia or on the effects of ammonia on Canadian terrestrial ecosystems to allow a probabilistic risk analysis to be performed.
3.1.2.2.2 Releases to water
Due to limitations of either exposure or toxicity data, the risk assessment of ammonia proceeded to a probabilistic risk assessment only for releases of ammonia from municipal WWTPs.
The LC10 was chosen as a short-term acute CTV because it is the maximum allowable mortality permitted in the control treatment and therefore defines the accuracy of toxicity testing. For un-ionized ammonia, the LC50 to rainbow trout (O. mykiss) for a 12-hour exposure was 0.74 mg/L, and the LC10 was 0.074 mg/L. The rise of ammonia in fish blood at these water concentrations is rapid. The concentration-lethality relationships are useful for estimating potential effects under these acute conditions when ammonia concentrations are very high. The conservative nature of the LC10 value is demonstrated by the fact that this concentration is bracketed by mortality and non-lethality in longer exposures of 21-120 days and is in the range of sublethal growth effects.
The acute lethality data for invertebrates and fish were evaluated collectively as a community of organisms by plotting the cumulative species response as a proportion of the entire community against concentrations of un-ionized ammonia (WERF, 1996). An ecological risk criterion for lethality can be derived from this distribution of data. Figure 8 is the Aquatic Community Risk Model (ACRM) graph for acute toxicity and is a logistic regression of the concentration-response. It allows prediction limits to be determined for any point on the curve. It must be remembered that each point on this graph is the average response of the species that it represents; in some cases, this is a single toxicity test, and in the case of rainbow trout (O. mykiss), it is an average of 112 toxicity tests. The ecological risk criterion developed is not specific to any particular water body in Canada. To conduct site-specific assessments, a review of each species' presence-absence would be required for each water body under study. This approach was beyond the scope of this assessment.
Figure 8, which uses all the LC50 data from Tables 4 and 5 (fish and invertebrates), indicates that 0.29 mg NH3/L (95% prediction limits are 0.21-0.37 mg/L) would produce 50% mortality in the most sensitive organisms representing 5% of the community. It should be noted that nearly all of the measured LC50 values reported in the literature exceed 0.29 mg/L.
The conservative nature of these estimates is evident when considering that these values are based on constant exposure conditions over a 48-to 96-hour period, conditions that rarely occur in the field. Concentration plumes change in geographical coverage due to variable dilution and currents, and organisms can move in and out of exposure areas over that period of time as part of their natural behaviour. Few aquatic organisms are repelled by ammonia or by municipal wastewater effluents; many, in fact, will be attracted to such effluents due to their supply of organic matter and warmth.
The scientific literature on sublethal ammonia toxicity to invertebrates, amphibians and fish was reviewed in detail and in many cases reanalysed to calculate the EC20 (concentration causing an effect in 20% of the organisms exposed) or IC20 (concentration causing 20% inhibition in exposed organisms compared with the control response) (Craig, 1999). Not uncommon with growth tests is that fry mortality can be as sensitive as, if not more sensitive than, growth per se. The use of the EC20 effect concentration allows comparison of organism sensitivity using the same endpoint and avoids comparison of many different endpoints that often use different statistical methods. The use of the 20% effect level is derived from the use in sublethal bioassay tests of an allowable 20% effect in control organisms due to the difficulty in maintaining a population of organisms over a long period. As with the lethal data, the same community ecological risk criteria were developed using the acceptable sublethal data from the literature reviewed (Table 6).
Figure 8 Acute ACRM OF Canadian freshwater species
Figure 9 represents the chronic ACRM graph developed for Canadian species listed in Table 6. The points on the graph are the geometric means (where they can be calculated) of the EC20 values for that species. The logistic regression of the community response analysis indicates that, at un-ionized ammonia concentrations above 41 µg/L (0.041 mg/L), the most sensitive 5% of the species in an exposed community would be expected to exhibit a 20% reduction in growth or reproduction. The prediction limits on this chronic CTV are 19-63 µg/L due to the relative lack of response data at the lower end of the graph. As with the acute toxicity ACRM, Figure 9 shows the average responses for each species where it was possible to calculate an average. It should also be noted (as shown in Figure 9) that all of the chronic effects values reported in the literature exceed the 0.041 mg/L value.
The acute CTV of ammonia for saltwater fish was determined to be 0.49 mg/L for the winter flounder (P. americanus). This is the most sensitive mean acute toxicity value reported for marine organisms (U.S. EPA, 1989).
Municipal wastewater effluents
For the conservative assessment of ammonia in freshwater lakes and ocean discharges, it was decided to use un-ionized ammonia concentrations measured (a) in Hamilton Harbour, Lake Ontario, from Hamilton and Burlington municipal effluents, (b) in Lake Ontario from Toronto and (c) at the Iona Island deep-sea outfall from the Greater Vancouver Regional District. These are examples of potentially impacted lake and ocean systems (Barica, 1991; IRC Inc., 1997; Gartner Lee Ltd., 1998).
(a) Hamilton Harbour
The maximum concentration of un-ionized ammonia recorded in Hamilton Harbour in 1994 was approximately 0.35 mg/L (Charlton, 1997). This value will be used as the EEV in our lake calculations. The acute CTV of 0.29 mg NH3/L was used. This value is close to the lowest reported acute effects levels for freshwater organisms (0.28 and 0.29 for white perch, M. americana, and mountain whitefish, P. williamsoni, respectively). An application factor of 10 was used due to the large and relatively complete database on fish toxicity.
Figure 9 Chronic ACRM for Canadian species listed in Table 6
The conservative assessment of the acute toxicity of un-ionized ammonia to fish in fresh water generated the quotient:
As this quotient is over one, these concentrations could be acutely toxic to sensitive fish. The toxicity assessment of ammonia released to freshwater aquatic environments should proceed to a probabilistic risk assessment. As Environment Canada conducted a detailed water quality project in 1998, there are sufficient data to allow this assessment to be conducted (Charlton and Milne, 1999).
(b) City of Toronto
The City of Toronto discharges much of its municipal sewage effluent via a pipeline and diffuser array into Lake Ontario at Ashbridge's Bay (see Figure 10). The water is roughly 6 m deep at the diffuser array. In 1998, Environment Canada contracted Gartner Lee Ltd. to conduct an effluent sampling project around the diffuser array to determine the spread of effluent constituents, specifically chloramines and ammonia (Gartner Lee Ltd., 1998). The diffuser array was located and sampled, as well as three samples from farther out in the lake, to establish the background and range of concentrations expected. The longitudinal length of the effluent plume was determined based on maps produced in a modelling exercise, wind direction and conductivity measurements. The down-gradient extent of the effluent was defined by conductivity concentrations within 10% of the background levels.
Figure 10 Discharge of Toronto municipal effluent into Lake Ontario, September 1998
A conservative assessment can be done for this discharge into Lake Ontario. At sites OC, 100A and 100C (top samples), the un-ionized ammonia concentrations were 0.06 mg/L or greater. The acute CTV of 0.29 mg/L was used because the organisms in this area are expected to be exposed over short periods of time, as the plume moves considerably with the wind. An application factor of 10 was used due to the relatively complete database on freshwater fish toxicity.
There appears to be a slight potential for adverse effects from the Toronto Main WWTP effluents. With the uncertainty involved in this calculation and the lack of detailed information on the spatial and temporal extent of these effluents, there is not enough information to continue to a probabilistic risk assessment.
(c) Greater Vancouver Regional District deep-sea outfall
Opened in 1963, the Iona Island WWTP provides primary treatment of wastewater and serves the Vancouver Sewerage Area. As wastewater flows to the plant increased, environmental studies showed that the discharge of effluent across Sturgeon Bank in a shallow channel was degrading portions of the bank. Recommendations for upgrading were made and, in 1988, the Greater Vancouver Regional District commenced operation of the Iona deep-sea outfall, which replaced the previous surface discharge. The new outfall discharges treated effluent at depths ranging from about 72 to 106 m to the Strait of Georgia off Sturgeon Bank.
The plant produces a primary effluent moderately high in ammonia (10 mg/L) with a flow of 567 000 m3/day (Environment Canada, 1997b). This facility deposits roughly 2000 tonnes ammonia/year into the Strait of Georgia. Estimates for initial dilution of the wastewater discharge indicate minimum levels in excess of 100:1 and typical levels of 150:1 at all flows and all discharge depths throughout the year.
Two years of pre-discharge data and 9 years of post-discharge data have been collected. A plume discharge study, including ammonia analyses, was conducted in 1996 following the peak Fraser River flows during a period of high-density stratification at the site. Sampling was done in July and August.
In a 1996 survey, the treated wastewater plume was detected in a north-south corridor up to 1 km north and 4 km south of the outfall diffusers either at a water depth of 55 m or at the bottom. Twenty-nine multidepth water samples were obtained at 10 sampling stations located in the area on 3 days in July 1996. There were no statistically significant differences between in-plume and outside-plume mean concentrations for ammonia in either the 55-m-depth or bottom-water samples. The maximum total ammonia concentration was 0.08 mg NH3 -N/L at 55 m depth at the diffusers, equivalent to 0.0003 mg un-ionized ammonia/L at a pH of 7.4 and a temperature of 9ºC (Bertold, 1999).
A benthic survey in the vicinity of the deep-sea outfall did not detect any anomalies in the benthic or infaunal communities (Stewart et al., 1991).
A concentration of 0.0003 mg un-ionized ammonia/L was detected at the Iona Island outfall, so it was used as the EEV for ocean disposal in this situation. An application factor of 10 was used due to the relative abundance of toxicity data on saltwater organisms. The acute CTV for saltwater fish (winter flounder, 0.49 mg/L) was used due to the likelihood that a benthic fish would not be exposed to the plume over long periods. Based on a maximum detected concentration of 0.08 mg NH3 -N/L (total ammonia), the conservative assessment would be:
Based on this high and rapid level of dilution, there does not appear to be an ecological toxicity hazard from the Iona Island deep-sea outfall.
(d) River discharges: screening
A significant point source of ammonia release to Canadian rivers is municipal WWTPs. This section examines the characteristics of effluent dilution and mixing in rivers at 10 selected municipal WWTPs across Canada. The characteristics of the effluent plumes that develop downstream of these WWTP outfalls provide insight into the spatial extent of potentially toxic zones within the river under different ambient conditions.
The model CORMIX 3.2 was selected for this application since it was suited to the variety of outfall configurations that exist and could be applied with information that was readily available (Doneker and Jirka, 1990; Jirka et al., 1996; Jones et al., 1996). These predictions have not been validated with field data and represent a conservative view of dilution in rivers.
Study sites were selected from across Canada that would typify the types of treatment systems and receiving environments available. The cities chosen were Edmonton, Alberta; Saskatoon, Saskatchewan; Calgary, Alberta; Winnipeg, Manitoba; Guelph, Ontario; Stratford, Ontario; Ottawa, Ontario; Montréal, Quebec; and Edmundston, New Brunswick. The cities chosen represent a mix of treatment types, discharge types and dilution rates. In each situation, average and low-flow assessments were conducted to provide reasonable estimates of the impacts from sewage treatment processes.
Table 10 summarizes the results of the modelling. In this summary, key characteristics of the plume have been identified for ease of comparison and evaluation. The CORMIX 3.2 predictions presented here are based upon average conditions in the river and assume steady flow. This is rarely the case, and therefore the actual plume locations and centre lines are expected to vary considerably with the variations inherent in rivers.
Table 10 Summary of conservative assessment of modelled sewage treatment systems
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Many of the data used came from engineering drawings for each facility as presented in Walker (1998). Municipalities provided the ammonia concentrations in the effluents; the water temperatures at average and low flows were estimated for most locations. By assuming a constant pH of 8 for all of the rivers and flow conditions, the un-ionized ammonia concentration was estimated. The point at which the toxicity estimate was made is the 10:1 dilution point, as modelled by CORMIX. The chronic CTV of 0.041 mg/L was used without an application factor. From this, a conservative estimate can be made of the potential for a chronic impact from ammonia for each outfall.
This exercise indicates that ammonia in sewage effluents from some cities is likely toxic under some conditions, but not under others. The cities of Edmonton, Winnipeg (North and West End plants), Edmundston and Stratford all have potentially toxic plumes of a significant size, under some conditions. Edmonton and Winnipeg generate potentially toxic plumes of a significant size under average conditions simulated. There are sufficient data on the effluents from both of these cities and their rivers to conduct probabilistic risk assessments. Because of the very long distance to the 10:1 dilution point below Calgary (>56 km), there is the possibility of "toxic" conditions prior to this point.
The cities of Saskatoon, Guelph, Ottawa-Carleton and Montréal do not have potentially toxic effluents under the situations simulated here. This is due to ammonia removal processes on the part of Guelph and to ammonia reduction techniques and a wide diffuser in the Ottawa River on the part of Ottawa-Carleton and in the South Saskatchewan River on the part of Saskatoon. Montréal has a weak effluent for a primary treatment system (6 mg/L) and a large dilution capacity in the St. Lawrence River.
Table 11 Summary of conservative assessments for agricultural runoff situations
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Since this work was completed, the cities of Saskatoon and Stratford have installed nitrification systems to remove or alter the form of ammonia they are putting into their local rivers, and they no longer release ammonia concentrations that are toxic under any conditions.
Agricultural runoff
There is no single assessment possible that would cover the many ways in which ammonia could be emitted from an agricultural operation due to the wide variety of such operations across Canada. Therefore, a series of conservative assessments have been conducted for those typical operations where there are data. The results are presented in Table 11.
Application factors of 10 have been used due to the relatively complete database on freshwater fish. In these cases, EEVs have been used from a variety of agricultural situations (see Table 11). Impacts from allowing cattle free access to a small river can be estimated based on Demal (1983) and his study of cattle in the Avon River, Ontario. This is one of the few studies found that estimates ammonia from this source. The short- and long-term impacts from cattle overwintering along a stream can be estimated from the studies by Cooke (1996) and Anderson et al. (1998a), respectively. Both of these studies were conducted in Alberta, where beef cattle are common; no other studies could be found pertaining to eastern Canadian situations. The impacts of ammonia in runoff from winter-applied manure can be estimated from Green (1996). The concentrations used in all of these assessments were the maximum detected.
Conservative analyses of agricultural operations with minimal data that involve cattle and manure handling have shown that some practices (overwintering cow-calf operations near streams, long-term cow-calf operations near streams, feedlot/dairy runoff near streams in springtime, manure fertilization of snow-covered fields near streams) have the potential to cause acute toxicity to aquatic organisms. Unfortunately, there is insufficient information on these types of agricultural systems across Canada to allow this analysis to be continued to a level that would include an assessment of the probability of adverse effects.
Dredged saltwater sediments
Concentrations of ammonia in sediment pore water in dredged material from estuarine and marine sites have been reported by the U.S. Army Corps of Engineers (Gibson et al., 1995) and in the open literature (Sims and Moore, 1995). U.S. data were used since Canadian data were limited. Approximately 21 of the sites were estuarine (salinity = 1-30‰), 5 were marine (salinity >30‰) and 13 were fresh water (salinity <1‰). Where concentrations were represented as total ammonia, un-ionized ammonia was calculated from reported pH, salinity and temperature. When the necessary parameters were not available, the following values were assumed: pH = 7.5, temperature = 20°C and salinity = 20‰ for estuarine systems and 30‰ for marine systems. Conversions were based on the results of a study conducted by Hampson (1977).
In general, the median pore water ammonia concentration reported in the dredged material survey was 0.2 mg NH3/L. Pore water ammonia concentrations for estuarine sites ranged from 0.06 to 1.9 mg NH3/L. Due to the suspension of sediments in the water column through which the sediment falls, a dilution factor of 10 was applied to the reported EEVs.
The receptor organism is winter flounder (P. americanus), which exhibits an average LC50 of 0.49 mg/L. An acute CTV is used, as exposure to ammonia from dumping sediments is expected to be of short duration. An application factor of 10 was used due to the relatively complete database on saltwater toxicity to convert this CTV to an ENEV of 0.049 mg/L.
Data available in the published literature for estuarine and marine conditions presented a range of exposure values for ammonia concentration in sediment pore water. Due to the variation in exposure values among sampling sites, risk quotients were calculated using the median and maximum concentrations from the dredged material survey, 0.2 and 1.9 mg NH3/L, respectively. The assumed pH of 7.5 is too low and the temperature of 20°C is too high for Canadian marine waters (Bertold, 1999), so the average ammonia concentration was recalculated using a pH of 7.8 and a temperature of 9°C. Using the methodology of Spotte and Adams (1983) to calculate a ratio between NH4+ and NH3 in saline water, the median ammonia concentration would be 0.12 mg/L and the maximum would be 1.18 mg/L. Because of the dilution effect that will occur as the dredged sediments fall through the water column, an estimated dilution factor of 10 was applied.
Median
Maximum
Using this method, the acute risk quotients for dredging and dumping ammonia-laden material in a saltwater environment would be <1 using a median concentration of 0.12 mg NH3/L and >1 using the maximum concentration of 1.18 mg/L from dredging surveys done in the United States. The results suggest that the risk of pore water ammonia toxicity in dredged material bioassays is highly variable and depends on the scenario and assumptions considered. The exposure period used to generate the CTV does not adequately match the exposure in the environment, as winter flounder was exposed for 96 hours. It is highly unlikely that benthic fish would be exposed for this time period from dredging operations. Also, the dilution factor of 10 has not been validated, and the physical parameters of saline water (temperature, pH and salinity) can have a major effect on un-ionized ammonia concentrations.
Ammonia is also a natural constituent of sediment. In published literature, a concentration range of 0.17-17 mg un-ionized ammonia/L in sediment pore water was reported to be quite common, and concentrations as high as 430 mg/L have also been reported (Gibson et al., 1995). Calculation of risk quotients using some naturally occurring ammonia concentrations would result in a quotient greater than one.
The conservative assessment of dredged sediments suggests that sensitive pelagic organisms might be harmed by exposure to ammonia liberated from sediments during dredging and dumping, but considerably more work would have to be done to prove that this source of ammonia is harmful in marine environments.
3.1.2.2.3 Other lines of evidence
Ammonia concentrations in interstitial pore waters of sediments are frequently of concern when dredging is to be carried out on the sediments (Schubauer-Berigan and Ankley, 1991). Dredging sediment high in ammonia can liberate considerable concentrations of ammonia to the surrounding water, and redepositing the sediment can also create a hazard. Dredged sediment disposal has been shown to cause toxicity in surrounding waters to Daphnia sp., Polydora sp. and Paleomonetes sp. by DeCoursey and Vernberg (1975). Although these researchers did not take water samples for analysis of ammonia, samples were taken from similar operations in the area. These samples contained up to 5 mg NH3/L at a sediment disposal site and 0.123 mg/L in non-disposal areas.
A major effluent sampling, characterization and effects project was carried out in 1993-1994, called the Joint Industrial-Municipal North Saskatchewan River Study (Golder Associates, 1995). A map of the North Saskatchewan River is illustrated in Figure 11. The project characterized the extent of the effluent plume from the Edmonton WWTP and from a smaller WWTP 20 km downstream. This project formed the basis for most of the plume-specific information used to model the effluent plume for the probabilistic risk analysis on this source. This project also characterized the nature of the benthic invertebrate community above and below the Edmonton WWTP outfall.
Total numbers of invertebrates and taxonomic richness are well-established indicators of environmental quality in rivers. Coupled with the coarse-level taxonomic breakdown of the data, these variables are sufficient to identify the major factor influencing the benthic community of a river. The longitudinal pattern in invertebrate abundance and community composition in the study reach is largely indicative of nutrient enrichment contributed by WWTPs (Golder Associates, 1995). Sewage effluents from Edmonton have altered the zoobenthic community below the discharge from an assembly of clean-water taxa (dominated by species of stoneflies, mayflies and caddisflies) to a less diverse and more abundant fauna characterized by pollution-tolerant taxa (such as oligochaetes and chironomids) (Anderson et al., 1986). Direct effects from nutrients were evident in increased plant growth (measured as chlorophyll a), whereas secondary and synergistic effects contributed to some decrease in dissolved oxygen levels, some increase in biochemical oxygen demand, and cyclic and compositional changes in the zoobenthic community (Anderson et al., 1986).
Dissolved oxygen in the river is always high, with only a slight oxygen sag below the Edmonton discharge. This usually amounts to a 5% decrease in the percent saturation from 75% to 70% in July (equivalent to 5.5 mg/L at the time). Farther downstream from Gold Bar, the dissolved oxygen, as percent saturation, increases from 70% to >80%. A value of 60% saturation would protect natural populations of all organisms in the North Saskatchewan River (Anderson et al., 1986).
Figure 11 North Saskatchewan River in the vicinity of Edmonton
Benthic invertebrate monitoring of the North Saskatchewan River downstream from municipal and industrial outfalls downstream of Edmonton documented severe benthic community alteration. The large increase in oligochaetes (Tubificidae and Naididae) below the Gold Bar WWTP followed by chironomid (Chironomini, Orthocladiniinae, Tanytarsini and Tanypodinae) dominance are typical effects of strong nutrient enrichment. A substantial increase in the abundance of pollution-tolerant oligochaete worms generally occurs in this zone at the expense of sensitive taxa. Farther downstream, the large increase in benthic algal biomass resulting from increased nutrient level results in chironomid dominance, followed by a gradual return of more sensitive invertebrates. Moderately enriched far-field areas may support dense populations of chironomids, net-spinning caddisfly larvae and certain mayfly nymphs. Stonefly nymphs (Chloroperlidae and Perlodidae) generally recover the farthest from WWTP outfalls. The abundance of these invertebrates did not return to upstream levels within 65 km of the study area. Water boatmen (Callicorixa) became abundant in areas with extensive growths of attached macrophytes below the outfall that provided good habitat (Golder Associates, 1995).
Lakes in the Central Plains of southern Alberta, Saskatchewan and Manitoba lie in fertile soils that supply high concentrations of nutrients. Eutrophication is generally the single most important water quality issue (Government of Canada, 1996; Hall et al., 1999).
The Qu'Appelle Valley extends over 400 km from its headwaters near Lake Diefenbaker in western Saskatchewan to its confluence with the Assiniboine River in western Manitoba. The Qu'Appelle River and its tributaries provide water to approximately one- third of Saskatchewan's population, including the cities of Regina and Moose Jaw. Agricultural fields and pastures comprise more than 95% of land use in the drainage basin (Hall et al., 1999). A chain of eight lakes, including two headwater reservoirs and six natural lakes, forms a gradient of trophic status in the valley. These lakes represent a major recreational and economically valuable resource for southern Saskatchewan. They are used for commercial and game fishing, recreation, irrigation, livestock watering, drinking water supply and sewage discharge, in addition to serving as flood control and waterfowl habitat (Munroe, 1986; Chambers, 1989).
Typical of the prairies, the lakes are shallow and hypereutrophic (total phosphorus >300 µg/L) and produce immense blooms of blue-green algae throughout the summer (Munroe, 1986; Kenney, 1990; Hall et al., 1999). Although the lakes are naturally eutrophic, present water quality is considerably worse than before European settlement and intensive agricultural development of the region (Allan et al., 1980; Hall et al., 1999). Growing concern over the continued deterioration of water quality in the Qu'Appelle Lakes in the last 30 years resulted in several federal-provincial studies, which attributed excessive algal and plant growth to high nutrient concentrations in agricultural runoff and municipal sewage discharge. It was estimated that 70% of the phosphorus and nitrogen entering the river basin was from sewage discharged by Regina and Moose Jaw (Munroe, 1986). Regina upgraded its sewage treatment facility to remove phosphorus in 1976, and Moose Jaw diverted all of its sewage to agricultural land through the use of spray irrigation by 1987 (Chambers, 1989).
It is unclear whether the upgrades to the sewage treatment plants have had the desired effect on water quality in the Qu'Appelle Lakes. Although open-water total phosphorus concentrations in the lakes have decreased despite increased annual discharge (Chambers, 1989), recent paleolimnological analysis indicates that nitrogen loading to the Qu'Appelle Lakes is at an all-time maximum (Hall et al., 1999). Similarly, the outflow of the Fishing Lakes exhibits an extremely low ratio of total nitrogen to total phosphorus (2.6:1), which suggests that phosphorus is being retained in the lakes, probably in the sediments. This situation maximizes primary production (Munroe, 1986). This evidence suggests that primary production in the Qu'Appelle Lakes would be nitrogen, not phosphorus, limited. Water quality is, thus, not likely to improve until nitrogen removal technology is instituted at Regina's WWTP or until the phosphorus pool in the sediments is depleted.
