Mobility of atrazine in soils of a wastewater irrigated maize field

The behavior of atrazine has been studied mainly in laboratory experiments, seldom directly in the field, and even less in agricultural plots irrigated with wastewater. In the Mezquital Valley, central Mexico, maize (Zea mays L.) crops have been irrigated with untreated municipal wastewater for more than 100 years. The herbicide atrazine has been applied to maize for weed control for 20 years. The research objectives of this study were: (i) to monitor the persistence and migration of atrazine and its main metabolites (hydroxyatrazine (HyA) and deethylatrazine (DEA)) in a wastewater irrigated maize field along a cropping cycle; (ii) to assess their sorption behavior in batch experiments; and (iii) to determine the degradation of atrazine under field conditions. Soil samples (0-15 and 25-40 cm) were collected before and after three irrigation events within a 2 ha plot. Additionally,percolation water was collected below the rooting zone and down to 5.0 m depth. Atrazine degradation was studied by calculating dissipation rates considering field data. Atrazine was detected in soil and percolation water during the first two irrigation events after application. HyA was detected only in the soil samples from the first and third irrigation events (similar to 0.08 mg kg(-1)) while DEA was found only in the percolation water of the second irrigation event (0.03 mg L-1) and also in the groundwater (similar to 0.02 mg L-1) from a deep piezometer. Batch experiments showed, that the soil has a higher affinity for atrazine (Kd = 5 L kg(-1)) than for HyA and DEA (Kd = 1.3 L kg(-1)). The atrazine half-life value was 16 days under field conditions. The moderate filter capacity of the soil and the relatively fast degradation rates seem to prevent the transport of atrazine and its metabolites into the unsaturated zone. Nevertheless it is recommended to establish a monitoring program with more narrow time intervals during the first days after atrazine application. We concluded that the large wastewater volume applied for irrigation explains themobility of these compounds.