Irrigation of a Mediterranean soil under field conditions with urban wastewater: Effect on pesticide behaviour

•Wastewater affects the field behaviour of three pesticides in a Mediterranean soil.•Irrigation with wastewater tends to slow down pesticide decay.•Organic fertilizer added to soil reduces wastewater effect on pendimethalin persistence.•Wastewater irrigation and organic fertilizer application could cause soil salinization.•The addition of wastewater decreases the soil microbiological potential. The use of wastewater for irrigation in arid or semiarid regions, where shortage of fresh water restricts agricultural production, has been well established in the last decades. However, the effect of this irrigation practice on the retention on soil of pesticides applied for pest control has been mostly approached by laboratory assays. More realistic approaches are needed because soil pollution with pesticides may cause a serious risk to the environment and to human health due to direct exposure or through the food chain. Therefore in this study a field experiment was run with three pesticides with contrasting properties: the insecticide thiacloprid and the herbicides chlorotoluron and pendimethalin. Three different treatments were considered: non-amended soil irrigated with well water (WL), with treated wastewater (TW) and organic-amended soil (3.2kg per plot) irrigated with treated wastewater (F+TW). Pesticide decay kinetic models showed that chlorotoluron residues were adequately explained by the bi-exponential first-order equation whereas the other two pesticides were fitted to a single first-order equation. The short-term pesticide persistence (DT50) was always higher for TW, reflecting that irrigation with TW slightly increased pesticide persistence in surface soil. Addition of an organic fertilizer (F+TW) did not modify the long-term thiacloprid persistence (DT90) with respect to irrigation with TW, decreased that of pendimethalin (almost 10 days) but increased that of chlorotoluron (12 days). The biological soil indicators measured showed contradictory results, with soil respiration increasing and dehydrogenase activity decreasing with TW irrigation.