Evaluation of Nutrient Concentrations in Runoff Water from Green Roofs, Conventional Roofs, and Urban Streams

Abstract

This study evaluated nutrient concentrations in runoff water from conventional roofs, green roofs, and urban streams, focusing on the impacts of compost addition at the industry standard of 15% by volume to green roofs at installation. Water samples were collected during selected rainfall events (n = 9) during calendar year (CY) 2008 and from the urban stream approximately monthly during baseflow conditions. Water samples were analyzed for ammonium-nitrogen (NH4-N), nitrite-N (NO2-N), nitrate-N plus NO2-N (referred to as NO3-N), total N (TN), soluble reactive phosphorus (SRP), total P (TP), and total organic carbon (TOC). The concentrations of SRP, TP, TN, and TOC were significantly greater in runoff from the green roofs that received compost during installation than from the conventional roofs or the green roof without compost addition, while the NH4-N, NO2-N, and NO3-N concentrations in stormwater runoff were generally not significantly different across the conventional or green roofs. Nutrient concentrations in the study streams, except for TOC, generally increased with the percentage of urban and pasture land use in the stream catchment, and the exponential relationship was generally strongest (higher R2) for NO3-N, TN, and P. Nutrient concentrations in stormwater runoff from the green roof without added compost were within the range observed across the study streams. However, nutrient concentrations in stormwater from the green roofs with compost were more variable when compared to the selected streams, and P concentrations were significantly greater in stormwater from the compost-amended green roofs compared to that measured in the study streams. The data collected in this study provide evidence that compost applied at the industry standard of 15% by volume to maintain plant growth is contributing to increases of nutrients in stormwater runoff 17 to 23 months after installation. Future studies should focus on compost additions to green roofs that maximize plant growth and survival while minimizing nutrient (particularly P) loss in stormwater.