Occurrence, distribution, and fate of pesticides in an intensive farming region in the Brazilian semi-arid tropics (Jaguaribe River, Ceara)

This study is the first to assess herbicide levels in sediment of the Jaguaribe River, which is located in one of the largest agricultural areas of the tropical Brazilian semi-arid region (Ceara state), with the goal of evaluating the occurrence of herbicides as well as their geographical distribution pattern and environmental fate in this river. Sediment collection was performed in October 2012, and nine points were selected located along the river to its mouth, and the levels of atrazine (ATR), alachlor (ALA), bromacil (BRO), ethalfluralin (ETHA), fluridone (FLU), metolachlor (METO), norflurazon (NOR), simazine (SIM), tebuthiuron (TEB), and trifluralin (TRI) were determined. Sediment from each site was characterized by granulometric analysis and total organic carbon (TOC), humic acid (HA), and fulvic acid (FA) contents. ALA, BRO, ETHA, FLU, NOR (a prohibited compound), and TEB were detected in all samples analyzed, whereas herbicides ATR, METO, SIM, and TRI were not detected at any of the sampling sites. The total herbicide concentrations (a(herb)) (the sum of six herbicides detected) per sampling sites ranged from 174.86 to 348.84 ng g(-1) (fluvial zone) and 111.43 to 419.35 ng g(-1) (estuary zone). The results showed that the sum of ETHA concentrations at all sampling points (I (ETHA)) pound (1687.47 ng g(-1)) was the most abundant herbicide, followed by I (BRO) pound (201.43 ng g(-1)), I (ALA) pound (41.73 ng g(-1)), I (TEB) pound (24.41 ng g(-1)), I (FLU) pound (1.33 ng g(-1)), and I (NOR) pound (0.43 ng g(-1)). The physicochemical properties of ATR, METO, and SIM favor their dispersion in the aqueous fraction; thus, the lack of these herbicides in the sediments is due to environmental partitioning. The environmental and geographical distribution pattern indicates that the estuarine zone is a sink that controls the fate of TEB, ALA, and FLU herbicides. Differently from expected results, data based on non-linear Spearman's coefficient show that herbicides with high log K (ow) and K (oc) values were more likely to be found in the sediment inorganic fraction, whereas herbicides with high solubility and low K (oc) values were dispersed between both the organic and inorganic fractions of sediments.