Monitoring Atrazine, Hydroxyatrazine and Deethylatrazine in an Agricultural Plot Irrigated with Wastewater throughout a Crop Cycle.

In the Mezquital Valley, Mexico, crops have been irrigated using untreated municipal wastewater (WW) for more than 100 years. Additionally, the herbicide atrazine has been applied to maize crop for weed control. In the monthly irrigation of maize crop, a water table of 200 mm is applied to soil. Out of the total WW applied during the irrigation, 20% is lost by runoff, 50% is retained by the soil, and the remaining 30% leaches towards subsoil. The irrigation of croplands by flooding with WW has resulted in a stable shallow aquifer, which supplies water to the area. The aim of this work was to understand the atrazine mobility in the system.

The monitoring of atrazine was performed on an agricultural plot during the crop cycle. For this monitoring, soil samples from 0-15 and 25-40 cm depth were collected in seven sampling points distributed within the plot. Additionally, percolation water samples were collected from seven shallow piezometers installed at the contact between the soil and the volcaniclastic sediments, within the plot. Lastly, groundwater samples were obtained from a deep piezometer aside the plot. Sorption batch type experiments were conducted to evaluate the adsorption capacity of the soil for atrazine and its metabolites.

The studied soil showed soil higher affinity for atrazine (Kd=5 L/kg) than for hydroxyatrazine and deethylatrazine (Kd=1.3 L/kg). For the three compounds, the value of desorption coefficient was higher than that observed for adsorption. The presence of atrazine in soil and percolation water was detected until three months after being applied. The hydroxyatrazine was detected only in soil samples from the first and third irrigation event, while deethylatrazine only was found in percolation and groundwater until the second irrigation event.

The high water volume applied for the irrigation, explains the mobility of the compounds despite the soil's retention capacity.