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

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Highlights

Abstract

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 (3.2 kg 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.

Introduction

The main objectives of the Spanish hydrological plan were to meet water demand and equilibrate and harmonise local and regional development through the increase in resource availability, the protection of water quality and the saving in its use or consumption (Ley 11/2005). In the last decades, pressure on water resources in urban areas is increasing because of growing demand and limited water sources. Then, to meet the growing demand of water for agricultural needs, water resources other than rain or rivers are being considered for irrigation. Among them, and particularly in arid or semiarid regions, treated wastewater from urban effluents has gained increasing interest (Drori et al., 2005, Qadir et al., 2010). Wastewater irrigation offers some attractive environmental and socioeconomic benefits, mainly due to the reduction of effluent disposal in receiving water bodies or nutrient recovery as fertilizers and crop production during the dry season. Additionally the organic matter in wastewater can improve soil aeration, increase water infiltration and soil moisture handling capacity, decrease soil erosion potential, increase soil cation exchange, buffer soil pH and promote the growth of beneficial organisms (Filip et al., 2000, Arienzo et al., 2009). Potential disadvantages are related with pathogenic contamination of irrigated crops and soil (Palese et al., 2009, Vivaldi et al., 2013), as well as effects on soil hydraulic conductivity, permeability or infiltration (Tarchitzky et al., 1999, Lado et al., 2005, Lado et al., 2012, Lado and Ben-Hur, 2009, Fernández-Gálvez et al., 2012), increased soil salinity, undesirable pH values, anaerobic conditions in the root zone and excessive leaching of nutrients and heavy metals (Graber et al., 1995, Ruoss et al., 2008, Arienzo et al., 2009, Travis et al., 2010, Müller et al., 2012).

Application of pesticides (herbicides, insecticides, fungicides) in agriculture is used to promote crop yield through the control of pests, weeds or diseases. For instance herbicides such as pendimethalin (PDM) or chlorotoluron (CHL) are used for the control of many broad-leaved and grass weeds and have been detected in ground and surface waters (Carabias-Martínez et al., 2003, Freitas et al., 2012, Caquet et al., 2013). On the other side insecticides, such as thiacloprid (THC), are employed for the management of insect pests like weevils, leafminers or various beetle species (Tomlin, 2003). Solute transport processes can result in the migration of pesticides from the soil surface, facilitated by water from irrigation or rain, leading to the movement of these chemicals which can contaminate water resources. PDM, CHL and THC display a wide range of physicochemical properties and have been selected as model compounds to better assess the effect of irrigation with wastewater.

Several studies have addressed how the use of treated wastewater affect the behaviour of different pesticides in soils regarding their sorption-desorption (Drori et al., 2005; Gonzalez et al., 2010;Rodriguez-Liebana et al., 2011; Hernández-Soriano et al., 2012, ElGouzi et al., 2012), mobility (Graber et al., 1995, Peña et al., 2011) or degradation (Hernández-Soriano et al., 2009), but mainly in laboratory experiments. The results are contradictory pointing in many cases to a sorption decrease (Graber et al., 1995, Drori et al., 2005), to an increase (Rodriguez-Liebana et al., 2011), or no effect (Gonzalez et al., 2010, Peña et al., 2011, Hernández-Soriano et al., 2012, ElGouzi et al., 2012). The results seem to depend on pesticide physicochemical properties with higher effect for hydrophobic compounds (Rodriguez-Liebana et al., 2011) and less influence on pesticides of medium or high polarity (Peña et al., 2011, ElGouzi et al., 2012), but also on wastewater and soil properties, especially their content in organic carbon (OC) (Ling et al., 2006; Rodriguez-Liebana et al., 2011).

To our knowledge only a few studies have been presented as field experiments (Graber et al., 1995, Müller et al., 2012), but both reports collect soil samples from field plots irrigated for longer or shorter periods with wastewater and perform laboratory assays of pesticide sorption and mobility.

Therefore to fill the gap we present here the results of a field experiment carried out in an agricultural plot located in a plain, the “Vega de Granada” (south eastern Spain), which has been irrigated since the end of the 20th century with treated wastewater from the wastewater treatment plant of Granada, a medium-sized city of approximately 250,000 inhabitants. Water flows through an intricate network of open channel systems reaching numerous agricultural fields. The aim of this work is twofold. Firstly to evaluate how the disappearance of three pesticides applied to the soil is affected by irrigation quality (treated wastewater vs. well water) and to establish whether the addition of an organic fertilizer affects this process. Secondly to assess the effect of the previous treatments on some biological indicators, such as soil respiration or enzyme activities.

Section snippets

Properties of pesticides and irrigation fluids

The behaviour of PDM (N-(1-ethylpropyl)-2,6-dinitro-3,4-xylidine), CHL (3-(3-chloro-p-tolyl)-1,1-dimethylurea) and THC [(Z)-3-(6-chloro-3-pyridylmethyl)-1,3-thiazolidin-2-ylidenecyanamide] was studied in a field assay. For laboratory analysis high purity standards (purity ≥97%) (Dr. Ehrenstorfer GmbH, Augsburg, Germany) were employed. Commercial formulations were used for field studies, UROTAN (Industrial Quimica Key, SA, Tárrega, Lleida, Spain) a 50% w/v SC formulation for CHL, CALYPSO (Bayer

Soil physicochemical properties

Soil pH, EC and OC content did not differ in soil samples from the same treatments taken at the beginning and at the end of the experiment (data not shown), as expected from short periods of irrigation with wastewater and in agreement with other reports (Qian and Mecham, 2005, Kayikcioglu, 2012).

Pesticide sorption

Pesticides were selected because they display a wide range of physicochemical properties and are expected to behave differently in the soil. Sorption isotherms, run using the soil from the plot, showed

Conclusions

Important rainfall events occurred more than 20 days after pesticide application. However, these climatic conditions seem not to have affected pesticide behaviour. Disappearance rates from the upper soil layers were dependent on pesticide properties and always ranged as THC > CHL > PDM. The effect of treatments depended on pesticide hydrophobicity, being negligible for the more polar (THC), while disappearance rates for CHL and PDM resulted in a slightly increased pesticide persistence in surface

Acknowledgements

This work was financed by Project CGL2007-60355 cofinanced by Feder funds. JAR-L thanks CSIC for a JAE-Pre scholarship (cofinanced by ESF) and SEG thanks AECID for a predoctoral grant. The authors acknowledge the assistance of Isabel Cañadas (IACT) and Diego Rojas (Instituto del Agua). The organic fertilizer was kindly provided by Fertilizantes Orgánicos Montaño S.L. José Carlos Romera from the “Comunidad de Regantes de la Acequia Gorda del Genil” is warmly acknowledged for kindly providing the

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      Fitting a second-order model to the kinetic data was found to be unrealistic despite having a high r2 value. As can be seen in Table 3, despite the acceptance criteria established by the European pesticide regulators (r2 ≥ 0.7) [47,48] for a first-order data adjustment, the values of r2 for the accepted model in this work meet the acceptance criteria of r2 ≥ 0.8, in accordance with what was done by Rani & Sud [49], so it is not necessary to use alternative methods for the description of kinetic data. DT50 values (time at which 50% of SP degrades) were calculated as ln(2)/k.

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    J.A. Rodríguez-Liébana and S. ElGouzi have equally contributed to this scientific work.

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