A GREEN ROOF IS A NET SOURCE OF PHOSPHORUS IN NORTHEASTERN OHIO

Phosphorus (P) is the key limiting nutrient that leads to eutrophication and harmful algal blooms in Lake Erie and other freshwater systems. Green infrastructure is one increasingly popular solution being used in urban areas to address the water quality issues that urban runoff creates in streams and lakes. Green roofs are a form of green infrastructure designed to decrease and slow down runoff through evapotranspiration and infiltration. The infiltration process in a green roof should decrease P loads because P easily sticks to sediment. Studies across multiple regions have shown, however, that green roofs tend to be net sources, rather than net sinks of P because of fertilization or high P-content in the engineered soil.

We monitored an extensive green roof in northeastern Ohio during 80 storms from June 2015 through October 2016. Event sizes spanned 0.25 to 54 mm. To assess water quality, we collected precipitation and samples from the green roof downspout, where discharge was recorded. We measured phosphate (PO₄^3-) and total P concentrations using a colorimetric assay and a persulfate digestion. Average outflow PO₄^3- concentrations are two times greater than rainfall (inflow) concentrations and outflow total P concentrations are two orders of magnitude more than inflow concentrations. As much as half of the total P outflow concentrations consists of PO₄^3-. However, outflow PO₄^3- concentrations did decrease by ~3x after fertilizer application ceased in 2016. Concentrations are higher during higher intensity storms and lower during low intensity storms, but the loads might be similar. Even though the green roof retained 80% of the volume of water it received during the monitoring period, it still serves as a net source of P to the environment.

One green roof acting as a net source of P is not a priority environmental issue because it will not make much of a difference at the watershed-scale. However, many cities are aggressively promoting green roofs as part of their stormwater management policies. If these policies result in many green roofs that are directly connected to downstream water bodies, they may exacerbate water quality problems in urban areas. Re-consideration of the maintenance and construction of green roofs may be required before further adoption of the practice.