Uptake and loss of chlorpyrifos and atrazine by Juncus effusus L. in a mesocosm study with a mixture of pesticides

Aquatic organisms in agricultural regions typically are exposed to mixtures of agrochemicals, and effects are not adequately predicted from results of single pesticide exposure studies. Thus, a mesocosm study was designed to examine the fate and effects of varying mixtures of three pesticides (chlorpyrifos, atrazine, and monosodium methanearsonate) and mercury-contaminated sediment to the common freshwater macrophyte Juncus effusus L. Exposure doses of pesticides added to mesocosms represented those that might be encountered in a typical runoff or direct spray application. This study reports the uptake and loss of chlorpyrifos and atrazine in the leaves of J. effusus after a single and a repeated dose of the chemical mixture over 94 d. The measured chlorpyrifos levels in leaves were highest on day 1, but levels in both leaves and water dropped rapidly and were at background levels by day 32. Atrazine remained near nominal concentrations in the water through day 16 and reached maximum accumulation in the leaves on day 16. The chemical mixture affected uptake of chlorpyrifos more strongly than atrazine as measured by differences in uptake variability. After the second dose, J. effusus showed a similar uptake pattern for both compounds, although somewhat faster, to that observed after the first dose, with no diminished capacity for uptake of either compound. The greater volatility of chlorpyrifos, the higher solubility of atrazine, and the more efficient transport of atrazine to the root zone are characteristics that are consistent with the differences observed in their uptake and loss behavior.