SULPHATE TOXICITY TO FRESHWATER ORGANISMS AND MOLYBDENUM TOXICITY TO RAINBOW TROUT EMBRYOS/ALEVINS

Transcription

1 SULPHATE TOXICITY TO FRESHWATER ORGANISMS AND MOLYBDENUM TOXICITY TO RAINBOW TROUT EMBRYOS/ALEVINS Trevor D. Davies, M.Sc., 1, 2 Janet S. Pickard, B.Sc., 3 and Ken J. Hall, Ph.D. 1 1 Institute for Resources, Environment and Sustainability, University of BC, Vancouver, BC V6T 1Z3 2 Environmental Division, Nova Scotia Power Inc., Halifax, NS B3J 2W5 3 BC Research Inc., 3650 Wesbrook Mall, Vancouver, BC V6S 2L2 ABSTRACT The guidelines for sulphate and molybdenum used for the protection of aquatic life may be overly protective. The current BC guidelines for sulphate and molybdenum are set at 100 and 1 mg/l, respectively. Key toxicity tests used in the development of the guidelines were replicated to determine whether results of tests reporting extremely low toxicity endpoints were artefacts or indicators that these substances can be more toxic under specific environmental conditions. Tests on sulphate were conducted using the amphipod, Hyalella azteca; the cladoceran, Daphnia magna; striped bass, Morone saxatilus; and the aquatic moss, Fontinalis antipyretica. An early life stage test using rainbow trout, Oncorhynchus mykiss, was conducted to examine molybdenum toxicity. The results of these tests indicated that the key studies used to justify the current conservative guidelines do not accurately assess sulphate or molybdenum toxicity. These findings indicate that the water quality guidelines for the protection of aquatic life for both molybdenum and sulphate need to be re-examined. INTRODUCTION Sulphate Toxicity to Freshwater Aquatic Organisms Sulphate is ubiquitous in freshwater environments and frequently acts as the main sulphur source for plants and bacteria. Sulphate levels in freshwater aquatic environments are usually low with most lakes and rivers in British Columbia having sulphate concentrations between 23 and 30 mg/l (Singleton 2000). However, in general, organisms have a relatively high tolerance for sulphate. Some BC lakes have natural sulphate levels in excess of 3000 mg/l; however, most concentrations are below 580 mg/l (Environment Canada 1984 as cited in Singleton 2000). Invertebrates tend to exhibit higher sensitivity to sulphate than do most fish species. For example, Mount et al. (1997), reported 48-hour LC50s for fathead minnow (Pimephales promelas) and Ceriodaphnia dubia of >7960 and 3080 mg/l Na 2 SO 4, respectively. The current Ambient Water Quality Guidelines for Sulphate for the Protection of Aquatic Life has set a discharge limit of 100 mg/l sulphate as a maximum concentration which should not be exceeded at anytime (Singleton 2000). This guideline was based on a relatively limited number of studies that investigated the effects of sulphate on various invertebrates, fish species and aquatic macrophytes. While the majority of the studies determined that sulphate has a low toxicity to most organisms, three studies found that sulphate was highly toxic to their study organisms, striped bass larvae, Morone saxatilus

2 (Hughes 1973), an aquatic moss commonly found in BC, Fontinalis antipyretica (Frahm 1975), and the amphipod, Hyalella azteca (PESC 1996). The studies reporting extreme sulphate sensitivity were replicated to assess their validity in the use of water guideline development to ensure that the sulphate guideline is based on reproducible and defensible data. In addition, some studies reported a reduction in sulphate toxicity in waters of increasing hardness. Therefore, the replication of the studies investigated this phenomenon. Additional studies were undertaken using Daphnia magna to further explore the phenomenon of sulphate toxicity reduction in waters of increasing hardness and investigate the relative contribution of calcium and magnesium, which make up water hardness, to decreasing sulphate toxicity. Molybdenum Toxicity to Rainbow Trout Molybdenum is an important element in the production of alloy steels, lubricants, and chemicals. Water quality investigations near several molybdenum mining sites in BC have found dissolved molybdenum from to 0.22 mg/l in background water with a range of to 11.4 mg/l at sites down stream of mine discharges (Jones 1999). A review of the US EPA ECOTOX database and the primary literature indicates that there is little information about the toxicity of molybdenum to aquatic organisms, and particularly to fish. The current BC Water Quality Guidelines for Molybdenum for the Protection of Aquatic Life (Swain 1986) has set a discharge limit of 2 mg/l molybdenum as a maximum concentration which should not be exceeded at anytime. This guideline is largely based on fathead minnow toxicity tests, whose 96-h LC50 values in waters of hardness 20 and 400 mg/l as CaCO 3 were misreported as 42 and 260 mg/l Mo (added as MoO 3 ) respectively (Tarzwell & Henderson 1956 in Swain 1986). The corrected values are 70 and 370 mg/l Mo in the respective water hardness solutions (Tarzwell & Henderson 1956). Moreover MoO 3, is not representative of molybdenum found in aquatic environments. Sodium molybdate (Na 2 MoO 4 ) is the appropriate toxicant because the disassociated molybdate ion is the predominant form of molybdenum in aquatic systems of ph >5 and oxidation conditions (Eh) >0.2 volts. Therefore, all of the studies examined here used molybdate (added as Na 2 MoO 4 ) as the toxicant. The toxicity of molybdenum to early life stages of rainbow trout (fertilized eggs through to alevins) is uncertain because different studies have reported toxicities ranging from 0.73 (Birge 1978) to >30mg/L Mo (Mcdevitt et al. 1999). McConnell (1977) reported 96-hour LC50s in static tests of 800 mg/l and 1320 mg/l to rainbow trout that were 20 mm and 55 mm long, respectively. Furthermore, as part of the same study, a one-year exposure test starting with eyed eggs indicated no significant reduction in hematocrit, growth or mortality up to molybdenum exposures of 17 mg/l.

3 Studies using other salmonids have also reported a high tolerance to molybdate. Pickard et al. (1999) reported a 30-day LC50 to cutthroat trout (Oncorhynchus clarki clarki) to be >90 mg/l Mo. Hamilton & Buhl (1990) reported 96-hour LC50s to be in excess of 1000 mg/l to both coho salmon (Oncorhynchus kisutch) and chinook salmon (Oncorhynchus tshawytscha) fry. Reid (2002) reported a 96-hour LC50 in excess of 2000 mg/l Mo using 1 2 g kokanee salmon (Oncorhynchus nerka Kennerlyi). The Canadian Water Quality Guidelines for the Protection of Aquatic Life: Molybdenum was derived using the lowest available chronic toxicity value of 0.73 mg/l Mo (Birge 1978) and multiplying it by a safety factor of 0.1 (CCME 1999). As the LC50 value reported by Birge (1978) is extremely low in comparison to other reported values, two molybdenum studies were undertaken. One replicated the original Birge (1978) study to assess the studies reproducibility, and the other followed accepted Environment Canada test procedures in order to gain further insight into molybdenum toxicity using accepted experimental methodology. This paper summarizes the results of Davies (2002) study of sulphate and molybdenum toxicity to aquatic organisms, and compares them to the studies that were used to develop the current water quality guidelines. METHODS All tests were conducted as static renewal tests with the exception of the 48-hour Daphnia magna bioassay, which was static with no water renewal. Sulphate Studies Striped Bass (Morone saxatilus) Environment Canada or the US EPA has not developed a standard protocol for this test organism. Therefore, the US EPA Methods for Measuring the Acute Toxicity for Effluents and Receiving Water to Freshwater and Marine Organisms, EPA/600/4-90/027F, 4-day static renewal method for fathead minnow was used. Sulphate concentrations of 0, 250, 500, 750, 1000, 2000, and 4000 mg/l were tested in three different hardness treatments (25, 100 and 250 mg/l as CaCO 3 ). The test had two replicates (Test #1 and Test #2).

4 Aquatic Moss (Fontinalis antipyretica) Since a standard protocol for this test organism was not available, a new 21-day static renewal method was developed. This investigation examined three characteristics to assess the toxicity of sulphate on F. antipyretica: dry weight, shoot growth and chlorophyll levels. Sulphate concentrations of 0, 200, 400, 600, 800, 1000, and 1500 mg/l were tested in three different hardness treatments (19, 26 and 105 mg/l as CaCO 3 ). Hyalella azteca and Daphnia magna The Environment Canada test methods, Biological Test Method: Acute Lethality Test Using Daphnia spp. EPS 1/RM/11 (48-hour exposure) and Biological Test Method: Test for Survival and Growth in Sediment Using the Freshwater Amphipod Hyalella azteca EPS 1/RM/33 (4-day water only exposure) were used for these tests. Sulphate concentrations up to 4000 mg/l were tested in hardness treatments (between 25 and 250 mg/l as CaCO 3 ). Two 48-hour D. magna tests were performed with Ca/Mg molar ratios of 0.7, 3.8 and 6.9 (expressed as molar ratios) while maintaining water hardness levels of 25 and 100 mg/l as CaCO 3. Two 4-day H. azteca tests were conducted with the Ca/Mg molar ratios of 0.7, 3.8 and 6.9 at a hardness of 100 mg/l (as CaCO 3 ). Molybdenum Studies Rainbow Trout (Oncorhynchus mykiss) Two static renewal tests were performed to examine the effects of molybdenum on early life stages of rainbow trout. The first test was a replication of the previous study conducted by Birge (1978) using the same water chemistry found in Birge et al. (1980) (hardness approximately 100 mg/l as CaCO 3 ) and the second test followed the Environment Canada test method using soft water of 42 mg/l as CaCO 3, Biological Test Method: Toxicity Test Using Early Life Stages of Salmonid Fish (Rainbow Trout), EPS 1/RM/28 (Environment Canada 1998). Both tests were started with freshly fertilised rainbow trout eggs and terminated 7 days after 50% of the control fish had hatched. The first test used the molybdenum concentrations of 0.5 to 400 mg/l, while the second test used concentrations of 100 to 1500 mg/l molybdenum.

5 RESULTS Striped Bass Poor survival in all control and low sulphate exposures were observed in all three hardness treatments in all tests. Mean control survival was zero in half of the control treatments with the maximum control treatment survival of slightly less than 35% in one high hardness treatment. However, as sulphate concentration increased, survival tended to increase in all hardness treatments with survival being >70% in most treatments above 1000 mg/l sulphate in the second test. Overall survival in the second test was generally better in than in the first test. Aquatic Moss Dry weight, shoot growth and chlorophyll levels were compared to control values in the 21-day experiment. The lowest observable effect level (LOEL) was observed by a reduction in the mean chlorophyll levels at 400 mg/l sulphate in soft water (19 mg/l as CaCO 3 ) and the moss appeared dead at exposure above 800 mg/l sulphate. In a replication of that experiment, the moss showed sensitivity to sodium sulphate exposure in soft water (26 mg/l as CaCO 3 ) but did not display a significant reduction in mean chlorophyll levels until the highest concentrations of 1500 mg/l sulphate, when the moss appeared dead. However, in water of 100 mg/l hardness no significant reduction in chlorophyll levels was observed up to the maximum sulphate exposure of 1500 mg/l. Hyalella azteca PESC (1996) reported 96-hour LC50s of 205, 3711, and 6787 mg/l sulphate at water hardnesses of 25, 100, and 250 mg/l (as CaCO 3 ). The results of the current study gave comparable results at the two highest water hardnesses (2971 and 4864 mg/l sulphate, respectively). Repeated attempts at replicating the soft water test of the original study using the same water chemistry failed. One LC50 of 491 mg/l and one no observable effect concentration (NOEC) of 453 mg/l sulphate were calculated using accepted water chemistry outlined in accepted test methods in Environment Canada (1997). Daphnia magna Sulphate toxicity was reduced in waters of increasing hardness with 48-hour LC50s for the 25, 50, and 75 mg/l hardness treatments (as CaCO 3 ) of 957, 1768, and 3155 mg/l sulphate, respectively. Changing Ca/Mg molar ratios also affected toxicity (Table 1).

6 Table 1. Effect of changing calcium and magnesium ratios on sulphate toxicity to the cladoceran Daphnia magna at water hardness of 25 and 100 (as CaCO 3 ). Values expressed are 48-hour LC50. Two replicates were done at each ratio level. Values in brackets correspond to LC50 range. Ca:Mg Ratios Hardness ( ) 1571( ) 1993 ( ) Hardness ( ) 3839( ) 4418( ) Rainbow Trout In the first test, the NOEC was 200 mg/l Mo, while the lowest observable effects concentration (LOEC) was 400 mg/l Mo. A valid LC50 value could not be estimated from this data. Furthermore, the mortality at the highest molybdenum concentration was likely due to contamination of the replicate set rather than molybdenum toxicity. A second test was performed at higher concentrations with the aim of determining the LC50. As in the first test, an LC50 could not be calculated since 50 % mortality did not occur in any of the replicates up to 1500 mg/l molybdenum. The highest mortality statistic that occurred was an LC20 of 1425 mg/l molybdenum. The NOEC and LOEC were 750 and 1000 mg/l, respectively. DISCUSSION Sulphate Toxicity to Aquatic Organisms The current Ambient Water Quality Guidelines for Sulphate for the Protection of Aquatic Life of 100 mg/l (Singleton 2000) is based on studies that do not accurately assess the toxicity of sulphate to aquatic organisms. Replication of the studies used to develop the guideline produced results that were markedly different than those of the original studies. A summary of the discrepancies between the primary guideline criteria and the replicated studies is as follows: 1. Hughes (1973) reported a 4-day LC50 of 250 mg/l sulphate (added as sodium sulphate) using striped bass larvae. The current study had low survival in control groups; however, survival dramatically increased in treatments with additions of sodium sulphate. The best survival was observed in the sulphate in excess of 4000 mg/l sulphate. The extremely low survival observed in the control water and low sulphate exposures in all water hardness treatments suggest that the

7 low salinity waters used in the control treatments was unsuitable for larval striped bass survival. Winger & Lasier (1994) and Morgan & Rasin (1981) investigated striped larvae survival in low salinity waters and had similar results. The high sensitivity of striped bass larvae to sulphate as reported by Hughes (1973) was likely due to the unsuitability of control and dilution waters of low salinity rather than toxicity to sulphate. Therefore, sulphate toxicity or the reduction in sulphate toxicity in waters of increasing hardness could not be assessed in larval striped bass. 2. Frahm (1975) reported an extremely low toxic threshold of 100 mg/l sulphate using the aquatic moss, F. antipyretica. However, the study used K 2 SO 4 that has been shown to be extremely toxic to fish, plants, and invertebrates. Therefore, it is likely that the reported high toxicity was due to potassium rather than sulphate. The replicated study used Na 2 SO 4, which is more suited to assessing sulphate toxicity because the sodium ion is relatively innocuous. A toxic threshold (using chlorophyll content as an endpoint) of approximately 800 mg/l sulphate in soft water (hardness 19 mg/l as CaCO 3 ) was determined. No observable effect in waters of 100 mg/l hardness up to sulphate concentrations of 1500 mg/l were observed. 3. Toxicity tests using H. azteca performed by the Pacific Environmental Science Centre (PESC 1996) reported a 4-day LC50 of 205 mg/l sulphate (added as Na 2 SO 4 ) in soft water (25 mg/l as CaCO 3 ) and low sulphate toxicity in medium and hard water (4-day LC50s of 3711 and 6787 mg/l sulphate in well water of 100 mg/l and 250 mg/l as CaCO 3, respectively). Multiple replications and modifications to the experimental methods used in the PESC (1996) study indicated that increased toxicity observed in the soft water treatments may have been due to a deficiency in chloride ions in the test waters used in PESC (1996). A 4-day LC50 resulted in a NOEC of 453 and a second test of 491 mg/l sulphate (added as Na 2 SO 4 ) in soft water (25 mg/l as CaCO 3 ) was obtained from the current study. It was observed that sulphate toxicity was greatly reduced in waters of increasing water hardness. The differences between these results suggest that the toxicity data used to derive the water quality criteria should be re-examined based on these findings. Molybdenum Toxicity to Rainbow Trout The results of the current study indicate that rainbow trout are not as sensitive to molybdenum as reported by Birge (1978). Whereas the current study determined a NOEC of 200 mg/l in the first test, and 750 mg/l in the second test, Birge (1978) reported a 29-day LC50 of 0.73 mg/l. The reason for the discrepancy between the two studies is unknown. However, other tests using Rainbow trout and other

8 salmonids are comparable with the current study indicating low toxicity of molybdenum. Therefore, it is likely the results of the original study were spurious and are not an accurate assessment of molybdenum toxicity. Calcium/Magnesium Ratios The composition of the water hardness in terms of the Ca/Mg molar ratios appears to have an appreciable affect on sulphate toxicity. The reduced toxicity in waters of increasing Ca/Mg ratio indicates that the calcium rather than the magnesium component of water hardness plays a more substantial role in reducing mortality of D. magna in sulphate in waters of increasing hardness. Specifically, approximately 50% more sodium sulphate was required to achieve the LC50 in high calcium waters with a Ca/Mg ratio of 7.0 compared to waters of high magnesium content with a Ca/Mg ratio of 0.7. Most natural waters have Ca/Mg ratios substantially above 0.7; and therefore, toxicity tests investigating sulphate toxicity in waters with low Ca/Mg ratios should be viewed as conservative evaluations of sulphate toxicity. CONCLUSIONS AND RECOMMENDATIONS Sulphate Toxicity to Aquatic Organisms The results of the current study suggest that the current water quality guidelines are overly conservative and a regulatory framework incorporating the hardness of receiving waters can safely be adopted. A guideline that takes into consideration the water hardness of receiving waters is presented in Table 2. Table 2. Proposed maximum allowable sulphate discharge limits for the protection of aquatic life at difference receiving water hardness levels. Water Hardness (as CaCO 3 ) Maximum allowable discharge (as SO 2-4 ) Less than 50 mg/l 200 mg/l 50 to 100 mg/l 300 mg/l Above 100 mg/l 400 mg/l A guideline should be protective of most aquatic environments, but flexible to site-specific conditions. Incorporating water hardness as a factor in the determination of a sulphate guideline will prove more useful to interpreting potential sulphate toxicity to a variety of environments.

9 Molybdenum Toxicity to Rainbow Trout The two experiments examining the toxicity of molybdenum to rainbow trout (Oncorhynchus mykiss) indicate that molybdenum is likely less toxic than previously reported (Birge 1978). The replication of Birge et al. (1980) found that molybdenum caused low toxicity to rainbow trout with two experiments failing to reach an LC50 concentration with molybdenum concentrations up to 400 and 1500 mg/l. Research and Development Needs Toxicity tests are done in order to provide information about the potential effects that a contaminant may have on ecosystems. A basic lack of understanding of the specific mechanisms of how ions are toxic to organisms and the ameliorative effects between ions is prevalent throughout the majority of the literature about ion toxicity. This makes it difficult to gain any understanding of ion toxicity and interactions beyond what can be gained from single salt investigations. This is confounded by toxicity endpoints being reported as a mass per litre (ie mg/l) rather than the more meaningful ion concentration (moles). Experimental design needs to adopt using molar units especially when examining complex solutions with multiple toxicants so that relative contributions to toxicity can be more easily determined. The use of chronic rather than acute tests to investigate the effects of ion toxicity to organisms needs to be adopted. The potential reductions in growth and reproductive success that may be caused to organisms under osmotic stress are difficult to assess with acute studies. Chronic studies would also provide insight into any adaptive response that organisms may be able to utilize under osmotic stress. ACKNOWLEDGEMENTS Thanks are given to the general staff at BC Research Inc. for guidance, use of lab space, equipment and materials. BC Ministry of Environment (Smithers), through the Pacific Environmental Science Centre generously donated water analysis on the test solutions. The Science Council of British Columbia, Brenda Mines, Endako Mines, Highland Valley Copper, Inmet Mining Corp., Placer Dome Inc. and Taseko Mines provided financial assistance for this project. We would like to thank Wilf Schofield of the Botany Department at the University of British Columbia for assistance in the location and experimental design of the moss experiment as well as Susan Harper and Paula Parkinson for the use of the spectrophotometer in the environmental engineering lab in the department of civil engineering.

10 REFERENCES Birge, W.J Aquatic toxicology of trace elements of coal and fly ash. In: Energy and Environmental Stress in Aquatic Systems, J.H. Thorp and J. W. Gibbons, eds. Department of Energy Symposium Series CONF Birge, W.J., J.A. Black, A.G. Westerman and J.E. Hudson Aquatic toxicity tests on inorganic elements occurring in oil shale. In Oil Shale Symposium: Sampling, Analysis and Quality Assurance, US EPA Report #600/ CCME (Canadian Council of Ministers of the Environment) Canadian Water Quality Guidelines for the Protection of Aquatic Life: Molybdenum. Davies, T Sulphate toxicity to the freshwater organisms and molybdenum toxicity to rainbow trout (Oncorhynchus mykiss). Masters Thesis, Department of Resource Management and Environmental Studies, University of British Columbia. Environment Canada Biological Test Method: Acute Lethality Test Using Daphnia spp. EPS 1/RM/11. July Environment Canada Biological Test Method: Test for Survival and Growth in Sediment Using the Freshwater Amphipod Hyalella azteca. EPS 1/RM/33. December Environment Canada Biological Test Method: Toxicity Tests Using Early Life Stages of Salmonid Fish (Rainbow Trout). EPS 1/RM/28 Second Edition. July Frahm, J.P Toxicity tolerance studies utilising periphyton. Toxitoleranzversuche an Wassermoosen. Gewasser Und Abwasser. 57/58: (GERMAN) Hamilton, S.J., and Buhl, K.J Acute toxicity of boron, molybdenum, and selenium to fry Chinook salmon and coho salmon. Arch. Environ. Contam. Toxicol. 19: Hughes, J.S Acute toxicity of thirty chemicals to striped bass (Morone saxatilis). Louisiana Wildlife and Fisheries Commission : Jones, C.E Molybdenum in the environment: An overview of implications to the British Columbia mining industry. Proceedings of the 1999 Workshop on Molybdenum Issues in Reclamation. W.A. Price, B. Hart, and C. Howell, (eds.) Bitech Publishers Ltd., Richmond, BC. pp McConnell, R.P Toxicity of rainbow trout under laboratory conditions. In: Molybdenum in the environment. Vol 2. The geochemistry, cycling, and industrial uses of molybdenum,. W.R. Chappell and K.K. Peterson (eds.). Marcel Dekker, Inc. New York, pp McDevitt, C.A., Pickard, J., Andersen, K., and Eickoff, C Toxicity of molybdenum to early life stages of rainbow trout in on site bioassays.. Proceedings of the 1999 Workshop on Molybdenum Issues in Reclamation. Price, W.A., Hart, B., and Howell, C. (eds.) Bitech Publishers Ltd., Richmond, B.C. pp Morgan, R.P. & V. James Rasin Jr Temperature and Salinity effects on Development of Striped Bass eggs and Larvae. Transactions of the American Fisheries Society 110:

11 Mount, D.R., D.D. Gully, J.R. Hockett, T.D. Garrison & J.M. Evans Statistical models to predict the toxicity of major ions to Ceriodaphnia dubia, Daphnia magna and Pimephales promelas (fathead minnows). Environmental Toxicology and Chemistry 16: PESC Analysis of Laboratory Bioassays of Sulphate (Unpublished). Pickard, J., McKee, P., and Stroiazzo, J Site specific multi-species toxicity testing of sulphate and molybdenum spiked mining effluent and receiving water.. Proceedings of the 1999 Workshop on Molybdenum Issues in Reclamation. Price, W.A., Hart, B., and Howell, C. (eds.) Bitech Publishers Ltd., Richmond, B.C. pp Reid, S. D Physiological impact of acute molybdenum exposure in juvenile kokanee salmon (Oncorhynchus nerka). Comparative Biochemistry and Physiology, Part C. Vol 133, pg Singleton, H Ambient Water Quality Guidelines for Sulphate, Water Management Branch, Ministry of the Environment, Lands and Parks, Victoria, BC. Swain, LG Water quality criteria for molybdenum: Technical appendix. Ministry of Environment, Land and Parks, Resource Quality Section, Water Management Branch, Victoria, BC. Tarswell, C.M., and Hendersen C Transactions of Seminar on Sanitary Engineering Aspects of the Atomic Energy Industry, Sanitary Engineering Center, TID US Environmental Protection Agency Methods for Measuring Acute Toxicity of Effluents and Receiving Water to Freshwater and Marine Organisms, 4th ed. EPA/600/4-90/027F. Office of Research and Development, Cincinnati, OH. Winger, P.V. & P.J. Lasier Effects of Salinity on Striped Bass Eggs and Larvae from the Savannah River, Georgia. Transactions of the American Fisheries Society 123: