Green Roofs: Water Recycling Effect on Water Quality and Soil Chemistry

Green roofs can improve urban environments by reducing the amount of runoff and in some cases improve the quality of runoff leaving urban areas. Water quality and soil chemistry of four green roofs in Houston that harvest and recycle rainwater and irrigation runoff were investigated. The green roofs vary in age from 2.5 to 8 years. Continuous recycling of water is expected to decrease soil and water quality and in turn decrease plant productivity. Soil cores, irrigation samples, and runoff samples were collected and analyzed. Preliminary irrigation samples indicate that the sodium absorption ratios (SAR) are below 2 which are not concerning. Electrical conductivity is relativley high (450, 450 475, 300 µS/cm respective of age) in the irrigation water compared to the soil (all below 200 µS/cm). Soil organic matter percentage was rather high for a green roof substrate ranging from 15% to 18%. Other nutrient concentrations in the soil do not appear to be high enough to become problematic. We will collect more data to accurately quantify the effects of the water recycling system on soil chemistry, water quality and plant physiology of the green roof system under a wide range of environmental conditions.