Atrazine and simazine are systemic herbicides, transported within plants via the xylem, and accumulated in apical meristems and leaves. They act primarily by binding to specific proteins in the thylakoid membranes of chloroplasts, where they inhibit the Hill reaction (photolysis of water) and thus block photosynthesis. Atrazine also disrupts other enzymic processes. Simazine is absorbed via roots, and atrazine is absorbed via roots and leaves. Both herbicides have many applications in both agricultural and non-agricultural situations, where they may be used selectively or non-selectively.

Concern over the increasing occurrence of atrazine in groundwater in many EC States have led to restrictions on its use in a number of countries.

In the absence of relevant data, it is assumed that only a small part of the atrazine and simazine produced is released to the environment in industrial effluents, via spillage or dumping, and following direct application to water (which is not approved in the UK). The main input is probably associated with diffuse sources, including surface run-off; soil leaching and drainage; drifting of sprays and cleaning of spray equipment.

Hedgecott (1996) concluded that concentrations of atrazine and simazine in the marine environment were likely to be lower because their entry is restricted to river inputs, sewage and industrial discharges, and direct losses from application at coastal sites.

In the UK, atrazine was detected in 5 out of 11 estuaries with a maximum concentration of 0.38 µg l^-1, whilst simazine was detected in 6 of the estuaries with a maximum concentration of 0.39 µg l^-1 (SAC Scientific 1987).

Monitoring data from the National Rivers Authority and the National Monitoring Programme Survey of the Quality of UK Coastal Waters are presented in Appendix D. No water column concentration was found to exceed the EQS value (see Appendix D). Monitoring data were not available for sediments or biota.

The available data suggest that concentrations of atrazine and simazine in UK coastal and estuarine waters do not exceed relevant quality standards derived for the protection of saltwater life.

Hedgecott (1996) reviewed data on the fate and behaviour of atrazine and simazine and concluded that both are stable in the aquatic environment. Their aquatic fate is strongly influenced by their moderate solubilities (33 mg l^-1 for atrazine and 5 mg l^-1 for simazine) and their persistence. Neither compound is volatile and losses to the atmosphere are therefore likely to be minimal (although aerosol losses may result from spraying).

The main routes of removal of atrazine and simazine from water are photo-enhanced hydrolysis to 2-hydroxy derivatives, adsorption onto sediments and degradation by micro-organisms. Adsorption to suspended and sedimented clay and organic particles can remove significant amounts of atrazine and simazine from solution. Correll and Wu (1982) found that, in an estuarine system at equilibrium, about 12% of atrazine was adsorbed on sediments, but suggested that this underestimated adsorption in real estuaries. However, other studies have shown sorption is both rapid and reversible.

Atrazine and simazine are stable in pure solution, with an estimated half-life for hydrolysis of atrazine in sterile, neutral water of 1,800 years (Armstrong and Chesters 1968). In the environment, degradation is enhanced by light and by the presence of organic matter or minerals, in particular humic and fulvic acids. For example, Mansour et al (1985) recorded a half-life of 340 days for the photo-reaction of atrazine with hydroxyl radicals in clean water.

Clearly persistence of triazines in water is dependent upon the local conditions. Atrazine (and probably simazine) appears to be more rapidly removed from saline water than from fresh water. In all waters, simazine appears to be slightly more persistent than atrazine.

In laboratory microcosms with estuarine waters, Jones et al (1982) recorded a much shorter half-life for removal of atrazine in solution of 3 to 12 days. However, in this test, sediment was included and most of the removal was by adsorption, which was increased by the high sediment content and by sediment resuspension when sampling. Atrazine's half-life in the sediment was 15 to 20 days.

An exhaustive literature review on the toxicity of atrazine and simazine to marine organisms has not been carried out for the purposes of this profile. The information provided in this section is taken from existing review documents (Hedgecott 1996). The most sensitive groups of organisms have been identified.

Hedgecott (1996) found few data for saltwater invertebrates or fish. However, based on the information available, animals are much less sensitive than algae or plants. Most of the available data are for estuarine species. For simazine fewer studies are available, but it is likely that its toxicity will be similar to that of atrazine.

As atrazine and simazine are very similar compounds and share a common toxic action in photosynthesising organisms, it is probable that their combined effects will be additive (although this has not been investigated experimentally).

Hedgecott (1996) found acute sub-lethal EC50s of approximately 60 to 100 µg l^-1 atrazine for a number of saltwater algae (Walsh 1972, Hollister and Walsh 1973), with `lowest observed effect concentrations' as low as 50 µg l^-1 (Tchan and Chiou 1977). The effects usually studied are inhibition of photosynthesis, growth or reproduction. Plants show a similar level of sensitivity to atrazine, with acute EC50s in the range 75 to 104 µg l^-1 for inhibition of photosynthesis or growth of many species (Correll and Wu 1982, Jones and Winchell 1984, Jones et al 1986), and a lower concentration of 10 µg l^-1 disrupting metabolism in Zostera marina (Delistraty and Hershner 1984).

In chronic tests, significant growth suppression in the macro-alga Laminaria was caused by 10 µg l^-1 or more of atrazine (Hopkin and Kain 1978) and reproduction of the unicellular algae Nannochloris and Phaeodactylum was significantly reduced by 15 to 50 µg l^-1 (Mayasich et al 1986, 1987). For higher plants, chronic EC50s of 30 and 55 µg l^-1 have been reported for growth and photosynthesis of Potamogeton perfoliatus, respectively (Kemp et al 1985), and a chronic LC50 of 100 µg l^-1 has been estimated for Zostera marina (Delistraty and Hershner 1984). A concentration of 12 µg l^-1 atrazine caused about 50% mortality in the warm-water species Vallisneria americana over 47 days (control mortality 10%; no effect at 1.3 µg l^-1) (Correll and Wu 1982).

In laboratory micro-ecosystems designed to simulate conditions in a tidal saltmarsh, a treatment with 2.2 mg l^-1 atrazine for five days resulted in significant decreases in primary productivity (from 191 to 29 mg C/m5 hour in one test and from 283 to 30 mg C/m5 hour in the second). Measurements of the chlorophyll content of edaphic diatoms on the sediment surface and in the top 5 mm showed that, although there were some significant reductions at certain depths, these were not consistent between the test. There were no obvious changes in species diversity and, although there were some changes in community structure, these were neither consistent nor statistically significant. Attempts to conduct similar studies in the field were unsuccessful as the plastic tubs used to enclose small areas of saltmarsh were found to have more effects than atrazine at the concentrations used.

Effects on saltwater fish and invertebrates are not normally observed at concentrations below the mg l^-1 level for atrazine. However, two particularly low values have been reported: a 96 hour LC50 of 94 µg l^-1(nominal) for the copepod Acartia tonsa; and significantly reduced survival in mysid shrimps chronically exposed to 190 µg l^-1 (measured) (Ward and Ballantine 1985).

Although Hedgecott (1996) found data for simazine more limited than for atrazine, toxicity to saltwater organisms is probably similar to that of atrazine. The most sensitive organisms appear to be algae, with Tchan and Chiou (1977) reporting a `LOEC' of 100 µg l^-1 for photosynthesis of Dunaliella teriolecta. No other adverse responses have been reported at concentrations below 100 µg l^-1.

The only data on the toxicity of simazine toward saltwater animals was information on 48 hour LC50s for adult brown shrimps Crangon crangon, shore crabs Carcinus maenas and cockles Cardium edule in static systems. All above 100 mg l^-1, the maximum test concentration used.

Simazine and atrazine have only low to moderate persistence in sediments and no data on the effects on sediment dwelling organisms could be located.

Atrazine's and simazine's relatively high solubilities (33 and 5 mg l^-1) and relatively low octanol-water partition coefficients (log Kow 2.7 and 2.3) suggest that they will have only low-to-moderate tendencies to accumulate in saltwater biota. Hedgecott (1996) concluded that there were insufficient experimental data to confirm this assertion, with only two studies reported for atrazine and none for simazine.

Jones et al (1986) found that the uptake of atrazine from water by the estuarine plant Potamogeton perfoliatus was a rapid process, reaching equilibrium after about 15 minutes when exposed to 20 to 100 µg l^-1. Depuration when placed in uncontaminated water was also rapid, with 45% released after 2 hours and another 20% removed by two subsequent washes, after which the rate of release declined and a portion remained bound to the plant material. A separate series of tests showed that the highest atrazine concentrations were associated with the shoots rather than the roots with, for example, respective dry weight bioconcentration factors (BCFs) of 85 and 19 when exposed to 10 µg l^-1.

Pillai et al (1979) fed box crabs Sesarma cinereum on cordgrass Spartina alterniflora that had been grown for two days in 0.2 mg l^-1 radio-labelled atrazine followed by three days in clean water. The crabs ingested on average 15 µg atrazine/kg^-1 bodyweight/day^-1 and 46 µg metabolites kg^-1 bodyweight/day^-1, and were fed for 10 days. No behavioural or physiological effects were noted. A total of 0.61 mg atrazine and metabolites kg^-1 bodyweight was consumed, 0.21 mg kg^-1 was egested in the faeces and homogenised crabs at the end of the test contained 0.4 mg kg^-1 (measured as radioactivity). However, the proportion of radioactivity as atrazine (rather than its metabolites) was 24% in the ingested plant matter but only 1.2% in crab tissue and 0.5% in faeces, indicating that the crabs had metabolised much of the atrazine.

The ability of certain plants and animals - particularly crustaceans - to metabolise atrazine suggests that biomagnification of atrazine up food chains is not likely to be extensive, although it may occur in food webs that do not include such `resistant' organisms.

Potential effects include:

• toxic effects to algae and macrophytes at concentrations above the EQS of 2 mg l^-1 (annual average) and 10 mg l^-1 (maximum allowable concentration) in the water column;
• atrazine has been identified as an endocrine disrupting substance and a precautionary approach should be adopted in the control of this substance.

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