The Influence of Green Roofs on Runoff Water Quality: A Case Study from Estonia

This paper describes the runoff water quality of Light Weight Aggregates (LWA)-based extensive green roofs and sod roofs in Estonia. Samples were taken from August 2004 to April 2009 from 10 different green roofs to determine what level of water quality may be found. The results presented here show that green roofs influenced water quality to a considerable degree. The runoff water of LWA-based green roofs generally had higher values of pH, BOD7, TP and PO4-P than that from sod roofs. However, COD, TN, SO4 and Ca-Mg salt were higher in the sod roofs than in the green roofs. The results for NH4-N and NO3-N were similar for both roof types. According to the results, the character of the runoff and the contents in the substrate layer at the moment the samples were taken affected runoff quality more than the age and location of the green roof. The use of NPK-nutrients in the substrate or in the soil caused much higher values of COD and concentrations of TP, PO4-P, TN, NH4-N and NO3-N in runoff water than on non-fertilized green roofs. The results of samples taken from the Tartu LWA-based green roof each spring in the period 2005-2009, at a time when the snow had almost melted, showed that concentrations of compounds in runoff water generally decreased gradually. The pH value and Ca-Mg salt concentration were, however, stable, and this was caused by the LWA material.