Introduction

Bentazone (BTZ) is an herbicide used in rice paddies. BTZ has been detected in the Albufera Natural Park (Spain), recognized as a "Wetland of International Importance". The Albufera lake waters contain variable concentrations of sulfates and chlorides over time. This study aimed to analyze the impact that these two compounds may have on the BTZ ecotoxicity.

Methods

Ecotoxicity was determined using Lactuca sativa seeds according to US EPA OPPTS 850.4200 standard.

First, the individual ecotoxicity of BTZ was determined. Later, 64 samples were prepared using all possible combinations of 0, 300, 600, and 900 mg L⁻¹ of BTZ; 0, 0.8, 1.6, and 2.4 g L⁻¹ of NaCl; and 0, 1.4, 2.8, and 4.2 g L⁻¹ of Na₂SO₄. The maximum concentration for each compound was approximately equal to its individual ecotoxicity threshold.

Results

BTZ exhibits an EC₅₀(5 days) towards Lactuca sativa of 900 mg L^-1, showing a hormetic effect. The toxic effects of a BTZ, NaCl and Na₂SO₄ mixture are generally lower than the individual toxic effects considered additively. A statistical model was obtained to predict the ecotoxicity thresholds for combinations of the three compounds. In general, when the concentration of one compound increases, a lower concentration of the others is necessary for the mixture to be toxic. However, in the presence of NaCl, below 382 mg L^-1 of BTZ, the concentrations of both compounds need to be increased. This is attributable to the hormetic behavior of BTZ. This BTZ concentration decreases as the Na₂SO₄ concentration increases.

Conclusions

EC₅₀(5 days) for BTZ towards Lactuca sativa is 900 mg L^-1. This value decreases when concentrations of Na₂SO₄ and NaCl increase. However, below 382 mg L^-1 of BTZ (a value that is lower as the concentration of Na₂SO₄ increases), the concentration of NaCl must also be decreased.