Effects of ecological roofs in water quality: an experimental study over a humid tropical climate

Although ecological roofs (vegetated or non-vegetated) provide many benefits, it can also leach substances such as nutrients and metals that can affect downstream aquatic ecosystems. Therefore, this work aims to investigate the rainwater quality from ecological roofs in Recife, located in the Northeast Brazil, using local species in a tropical and humid climate. Using four test cells of 1 m2 (one non-vegetated filled with expanded clay aggregate, two vegetated with cactus “Coroa-de-Frade” and grass “Grama Esmeralda”, and one control roof), we analyzed thirteen water quality variables regarding irrigation parameters: pH, electrical conductivity, turbidity, nitrate, ammonia, phosphate, bicarbonate, carbonate, calcium, magnesium, sulfate, potassium, boron, sodium, and sodium adsorption ratio. We simulated rain events controlling its intensity and analyzed a sample of natural rainwater event. All roofs neutralized the pH. Control and clay roofs were source of bicarbonate and calcium, responsible for more alkaline effluents. Carbonate and ammonia were below the recommended limits for irrigation purposes for all roofs. Green roofs were source of nitrate, ammonia, and boron. Neither roofs were source or sink for sulfate and chloride for all analyzed samples. Regarding the natural rainwater experiment, only green roof with Coroa-de-Frade exceeded the recommend irrigation parameters for potassium and phosphate. A post-treatment is required for irrigation purposes. We recommend a first-flush system followed by a filter with sand and activated carbon.