The Influence of Water Hardness under Turbid and Non-turbid Conditions upon the Acute Toxicity of Paraquat to Fathead Minnows (Pimephales promelas)

Abstract

The static acute 96 hr LC50 of paraquat [1,1'dimethy1-4,4'- bipyridinium ion (dichloride salt)] for fathead minnows (Pimephales promelas) was determined in reconstituted water. Secondly, the effect of a suspended solid, bentonite clay, upon the 96 hr LC50 was tested. Finally, the effects upon paraquat toxicity of using water of increased hardness in the presence and absence of bentonite were evaluated. The 96 h static acute LC50 of paraquat as an emulsifiable concentrate for fathead minnows was 10.2 mg/L (95% confidence limits: 8.6-12.3) of the dissolved paraquat cation in water with a hardness of 100 mg/L as CaCO₃. The addition of 5 mg/L bentonite (montmorillonite clay) to water of 100 mg/L nominal hardness decreased the acute toxicity of paraquat by a factor of 2.2, yielding a 96 h LC50 of 22.4 mg/L (21.0-23.8) unadsorbed paraquat. Adsorbed paraquat was not quantitatively measured. Increasing the hardness to a nominal 200 mg/L decreased the acute toxicity by a factor of 2.8, to 28.4 mg/L (24.4-34.5) free paraquat. The addition of 5 mg/L bentonite to the harder water did not significantly alter the toxicity of paraquat [96 h LC50 = 31.7 mg/L, (27.3-35.3)]. The toxic action of the herbicide occurred above a threshold concentration of 20.2 mg/L of dissolved paraquat. This effect may have been due to calcium, magnesium (water hardness), and sodium ions (desorbed from bentonite) competing with paraquat cations for absorption sites on the gills of the fish. Fish sensitivity to paraquat was found to vary when fish from different batches of eggs were tested under similar conditions. Toxicity symptoms were observed and recorded for non-lethal through lethal doses of paraquat. The toxicity symptoms observed for a lethal dose included: a rapid ventilatory rate; rapid pectoral fin movement; a redness around the head and gills; and swimming erratically and sluggishly near the water's surface. Though not studied quantitatively, dissolved paraquat had persisted in covered non-aerated test solutions (pH 7.8-8.3) for up to one week after testing. This evidence of paraquat persistence and the threshold effect induced by the calcium, magnesium, and sodium ions suggests a potential hazard to aquatic flora and fauna should large amounts of paraquat be accidently introduced into a pond environment.