Effects of reclaimed water irrigation in the soil-plant system of Madrid urban parks

Laburpena

Water scarcity in many regions of the world is one of the greatest crises that mankind needs to address. The steady increase of water consumption and the decrease of available water resources are the main factors that determine it. This scenario leads to a paradigm shift: circular economy, where waste will become resources. Therefore, water reuse plays a fundamental role in circular economy. Reclaimed water irrigation has become a long-standing practice over the world, especially among water deficit areas such as Spain. One of the first cities in Spain that started using reclaimed water to irrigate its urban parks was Madrid. Since the beginning of the 2000s, the City Council has developed a vast system of pipes and deposits to irrigate most of its parks with reclaimed water. Despite its great advantages, water reuse could involve some risks. Reclaimed water irrigation, more mineralized than drinking water, may produce adverse effects in soils and plants, including the presence of pathogens and chemical contaminants as well as salinization, impacts on soil structure and effects on vegetation. Therefore, the main objective of this Doctoral Dissertation was to assess the effects of reclaimed water irrigation in the soil-plant system of urban parks of Madrid (Spain). To that end, this research was carried out along six successive years (2012–2017) in two public urban parks of Madrid: Emperatriz María de Austria Park and Garrigues Walker Park. Both were irrigated with reclaimed water since 2002 and 2012, respectively. Furthermore, two plots from each park were irrigated with reclaimed water (RW) and two other plots with drinking water (DW). Samples of irrigation water, soil solution, soil and leaves of four plant species –cedar, grass, hackberry and Photinia– were taken for further analysis. Results achieved showed that reclaimed water of Madrid was adequate for irrigation according to international water quality standards. However, the use of reclaimed water to irrigate urban parks was potentially leading to a modification of some soil properties. The park which has been irrigated with reclaimed water for 15 years showed a slight soil salinization (EC > 2 dS m-1 ). Furthermore, there was a steady increase of Cl− (157%), Na+ (180%), SAR (127%) and EC (69%) in soils that were irrigated for 5 years with reclaimed water. Whereas in plots irrigated with drinking water significant lower values (p < 0.05) for these parameters were observed. Likewise, it caused an increase of microaggregate stability in the topsoil, while macroaggregate stability decreased after RW irrigation in the top and the deepest layer. Soil penetration resistance was significantly higher (p < 0.05) and infiltration rate was lower in the RW plot. On the other hand, soil porosity results showed that there was no influence of the kind of irrigation water used. Furthermore, there was no soil sodification in RW plots and lower values of micronutrient concentration in soils were obtained when compared with other studies on reclaimed water irrigation in urban parks. Regarding vegetation, Cl (%) leaf content was significantly higher (p < 0.05) in those hackberries and Photinias irrigated with RW in comparison with those irrigated with DW. However, for Na (%) leaf content, there were only significant differences for cedars irrigated with RW versus DW in PGW. In most of the cases salt concentration in leaves were below the threshold when plants start to show injuries. Thus, no major salt stress symptoms were observed. Likewise, foliar micronutrient content of cedar leaves was nearly always within the optimum parameters. The statistical analysis carried out showed no correlation between micronutrient concentration in irrigation water and cedar leaves, thus cedar decline cannot be attributed to the input of these micronutrients by reclaimed water. Moreover, irrigation with reclaimed water led to a grass biomass increase (on average of 66%), mainly due to the high proportion of nutrients received through the irrigation water, which acted as a fertilizer. Reclaimed water irrigation also contributed to a significant increase in nutrient removal by grass. In conclusion, prolonged reclaimed water irrigation may be altering the features of the soilplant system of urban parks. For that reason, and in order to avoid future problems, the use of reclaimed water in urban parks irrigation should be continuously monitored. One of the recommended measures proposed is to use an adequate leaching requirement (10%) in order to wash out the excessive salt accumulation in parks irrigated with reclaimed water. Eventually, the transfer of these research results to municipal managers may contribute to a better management of reclaimed water irrigation in urban parks, with the aim to prevent the likely appearance of adverse symptoms on sensitive plants.