Chlorpyrifos in freshwater and marine water
Toxicant default guideline values for protecting aquatic ecosystems
Extracted from Section 8.3.7 ‘Detailed descriptions of chemicals' of the ANZECC & ARMCANZ (2000) guidelines.
The default guideline values (previously known as ‘trigger values’) and associated information in this technical brief should be used in accordance with the detailed guidance provided in the Australian and New Zealand Guidelines for Fresh and Marine Water Quality.
Description of chemical
Organophosphorus pesticides are derivatives of phosphoric, phosphonic, phosphorothioic, or phosphonothioic acids, comprising many chemicals with a wide range of uses (WHO 1986). They exert their acute effects in insects, fish, birds and mammals by inhibiting the acetylcholinesterase (AChE) enzyme, but may also have a direct toxic effect (WHO 1986).
Chlorpyrifos (CAS 2921-88-2) is a phosphorothioate organophosphorus (OP) pesticide, developed by DowElanco, with a non-systemic mode of action through contact ingestion or breathing vapour (Tomlin 1994). Its IUPAC name is O,O-diethyl O-3,5,6-trichloro-2-pyridyl phosphorothioate, formula is C9H11Cl3NO3PS and molecular weight is 350.6. Chlorpyrifos is only slightly soluble in water to 1.4 mg/L at 25°C (Tomlin 1994) and its log Kow is 4.7 (Tomlin 1994). The current analytical practical quantitation limit (PQL) for chlorpyrifos in water is 0.1 µg/L (NSW EPA 2000).
Uses and environmental fate
Chlorpyrifos has a wide range of insecticidal uses against Diptera (two-winged flies and mosquitoes), Homoptera, Coleoptera (beetles), Lepidoptera (moths), Isoptera, Blattelidae (cockroaches), Muscidae, in a wide variety of locations. In Australia, chlorpyrifos has over 1200 registered uses, on around 100 crops (NRA 1997a), including cotton, wheat, vegetables and fruit, as well as an insecticide in pasture, machinery, stored grain and hides, turf plus home, garden and pet uses. An important use of chlorpyrifos is against subterranean termites (NRA 1997a), where it replaced the persistent organochlorine pesticides, and it is one of the most widely used insecticides in Australia.
Chlorpyrifos degrades more rapidly in alkaline pH and the DT50 varies from 1.5 d at pH 8 (25°C) to 100 days at pH 7 (15°C) (Tomlin 1994). Racke (1993) has reviewed the environmental fate of chlorpyrifos, including a number of field studies. Chlorpyrifos at overspray levels dissipated completely within 21-30d in ponds and estuaries. Sediments contributed to most of the persistence, as it was removed rapidly from the water column. It degrades in the presence of copper and other chelating metals. Bioconcentration factors for fish continuously exposed to chlorpyrifos from embryonic to juvenile stages varied from 58 to 5100 (Racke 1993). NRA (2000) reviewed the environmental risk of chlorpyrifos and reported BCF figures to fish up to 1400. However, it was reported that depuration was rapid (≤ 2 days) in clean water.
As an OP pesticide, chlorpyrifos inhibits the acetylcholinesterase (AChE) enzyme in organisms. The oxon metabolite (from initial oxidation of the P=S bond) is more toxic. Chlorpyrifos had very high toxicity to most groups except marine molluscs and algae. Barron and Woodburn (1995) have reviewed the ecotoxicology of chlorpyrifos. Invertebrates are most sensitive to chlorpyrifos.
Freshwater fish: 16 species, 48 to 96-hour LC50 of 1.3 (Cyprinus carpio) to 542 µg/L, although outlying figures were reported for Ictalurus (806 to 810 µg/L) and Gambusia (1018 µg/L) (lower figures for both species were included in the 16 species). Chronic NOEC for Pimephales promelas: 32 to 216-day mortality (0.63 to 3.2 µg/L); 30-day development (1.3 µg/L); 7 to 136-day growth (0.012 to 3.900 µg/L; geometric mean = 0.63 µg/L: the 0.012 µg/L figure may be anomalous, as several other figures for the same species and test duration of 1.2 µg/L were reported); 200 to 216-day reproduction (0.27-1.2 µg/L; geometric mean = 0.57 µg/L).
Freshwater crustaceans: 14 species, 48 to 96-hour LC50 or EC50 immobilisation, 0.06 to 6.00 µg/L, although outlying figures were reported for crayfish Procambarus clarki of 21-23 µg/L and the crab, Oziotelphusa senex senex, of 300 to 700 µg/L. Geometric means for the sensitive species were 0.08 µg/L (Gammarus pulex), 0.12 µg/L (Ceriodaphnia dubia), 0.13 µg/L (Hyalella azteca), 0.14 µg/L (G. lacustris), 0.21 µg/L (G. pseudolimnaeus), 0.3 µg/L (Daphnia longispina). Cladocerans (water fleas) were most sensitive. Chronic NOEC figures for two species; D. magna, 21-day reproduction, 0.056 µg/L and mortality of 0.06 to 1.00 µg/L, giving an ACR of 10; D. pulex, 25-day reproduction 0.065 µg/L. A 10-day mortality NOEC for H. azteca (amphipod) was 0.09 µg/L.
Freshwater insects: 13 species, 48 to 96-hour LC50 of 0.2 to 10.0 µg/L. 11 species had LC50 values below 1 µg/L. A 9-day LC50 of 0.97 µg/L was reported for Neopleas striola (backswimmer).
Freshwater molluscs: one species; 96-hour LC50 of 2.7 µg/L.
Freshwater rotifer: one species, 48-hour LC50 of 360 µg/L [outlying figure of 12,000 µg/L, AQUIRE (1994). Branchionus calyciflorus had a 72-hour NOEC reproduction of 200 µg/L.
Freshwater ciliate: NOEC, growth, 72 hours for Tetrahymena, 330 µg/L.
Freshwater algae: six species, 7-day NOEC (growth) of 10-100 µg/L.
Marine fish: 10 species, 48 to 96-hour LC50 of 0.25-7 µg/L. [Outlying figures for three additional species: Cyprinodon variegatus (140 to 270 µg/L); Fundulus sp. (470 µg/L); and Opsanus beta (68 to 520 µg/L).]
Marine crustaceans: seven species, 48 to 96-hour LC50 of 0.035 to 5.400 µg/L. NRA (2000) reported a NOEC for growth impairment of 0.0046 µg/L for a mysid.
Marine molluscs: one species, Crassostrea gigas, 48 hours, growth and development of 34 to 2000 µg/L.
Marine algae: three species, 48 to 96-hour EC50 (growth) of 140 to 300 µg/L, although 48-day NOEC (growth) on six species was 1200 to 10,000 µg/L.
Mesocosm studies: Up to 27 mesocosm studies have been reported on chlorpyrifos. Unfortunately, most of these did not satisfy the criteria for acceptance of mesocosm data, and only eight could be considered for an initial investigation. The problem with many of these eight was insufficient number of treatments and/or insufficient replication, as well as insufficient breadth of taxa. For some, the lowest concentration produced an effect, giving a LOEC but not a NOEC figure. LOEC figures reported for zooplankton were 5 µg/L (Mani & Konar 1988); 1.7 µg/L (Lucassen & Leeuwangh 1994); 1.5 µg/L (Roberts et al. 1973); 0.5 µg/L (Brazner et al. 1989) and 0.37 µg/L (Nelson & Evans 1973). Other studies produced LOEC values: phytoplankton (1.2 µg/L; Brown et al. 1976); for insect colonisation, caddis abundance and fish mortality (1.15 µg/L; Macek et al. 1972); for transient reduction in bug abundance (0.03 µg/L; Giddings 1993, Biever et al. 1994); and for elevated community respiration (0.004 µg/L; Butcher et al. 1977).
Three studies produced NOEC figures: 0.06 µg/L (after pulse exposure to stream macroinvertebrates) (Pusey et al. 1994); 0.065 µg/L for D. pulex in ponds (van Wijngaarden & Leeuwangh 1989); and 0.1 µg/L for overall community effects for zooplankton and macroinvertebrates in ditches (van den Brink et al. 1996). These are similar to the chronic NOECs for invertebrates. At this stage, it was not considered appropriate to use these data to derive a guideline figure, although the figure derived from the laboratory data is 6 to 10-fold lower than the lowest of those mesocosm NOECs.
Australian and New Zealand data
The only Australian dose-response data was for the waterflea C. dubia, with a 48-hour LC50 of 0.25 µg/L, which is similar to data for other cladocerans. Abdullah et al. (1993) found that a concentration of 1 µg/L was lethal to the shrimp Paratya australiensis in 24 hours but 0.1 µg/L reduced AChE levels by 28%. Exposure to 0.05 µg/L for 28 days resulted in moribund shrimp with acetyl cholinesterase (AChE) depression over 85%. Repeat exposures to 0.1 µg/L at 7-day intervals gave increased AChE depression.
Factors that effect toxicity
Toxicity of chlorpyrifos to Oncorhynchus mykiss and P. promelas increased 2 to 15 times with a 16°C increase in temperature (Johnson & Finley 1980). Toxicity of chlorpyrifos to cutthroat trout, Salmo clarki, increased by 3 times when the pH was increased from 7.5 to 9.0 (Johnson & Finley 1980). Patra et al. (1995a) found almost a four-fold increase in the 96-hour LC50 to the silver perch, Bidyanus bidyanus, over the range of 15 to 35 ⁰C. LT50 values for three species of Australian fish exposed to 1.5 µg/L chlorpyrifos decreased at 35°C. Critical thermal maximum temperatures for three Australian fish species (Patra et al. 1995b) decreased significantly for fish exposed to sublethal levels (3.5 to 5.0 µg/L) of chlorpyrifos.
Naddy et al. (2000) reported that chlorpyrifos toxicity to D. magna varied with exposure duration and intervals between pulses. Daphnids exposed to two 12-hour pulses of chlorpyrifos at 0.5 µg/L showed around 85% mortality regardless of the pulse interval (3 to 14 days). However, they were able to survive if exposed to two 6-h pulses if separated by at least 3 days.
There was a comprehensive dataset for chlorpyrifos. Rotifers and ciliate 72-hour figures were included in the chronic data.
A freshwater high reliability trigger value of 0.01 µg/L was derived for chlorpyrifos using the statistical distribution method with 95% protection. The 99% protection level is 0.00004 µg/L. A marine high reliability trigger value of 0.009 µg/L was derived for chlorpyrifos using the statistical distribution method with 95% protection. The 99% protection level is 0.0005 µg/L, which is currently unachievable analytically.
Chlorpyrifos has the potential to bioaccumulate but also depurates rapidly. Given this moderating factor the authors consider that the 95% protection figure should provide sufficient protection from acute and chronic toxicity and from bioaccumulation in slightly-moderately disturbed systems. If local data suggest that bioaccumulation is an issue at the specific site, the 99% protection level may be used but both marine and freshwater 99% figures are well below analytical detection levels. The 95% freshwater protection level is also around the detection limit, so any detection of chlorpyrifos may exceed the trigger values for marine and freshwater systems.
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