Stormwater performance of a full scale rooftop farm: Runoff water quality

•The rooftop farm had a neutralizing effect on acid rain.•Conductivity and color of runoff were negatively affected by rooftop farming.•Runoff macronutrient concentrations were negatively affected by rooftop farming.•Only P and K concentrations in the farm runoff were higher than EPA guidelines.•Changes in nutrient management practices would reduce nutrient leaching.

A number of benefits have been attributed to green roofs including food production. However, little research has been done to quantify the effects of rooftop farming practices on green roof benefits. The impact of rooftop farming on stormwater runoff is especially important, considering the different nutrient management practices on ornamental and agricultural green roofs. In order to advance knowledge on this potential impact, runoff water quality from a full-scale rooftop farm in Long Island City, Queens, New York, was monitored and compared to runoff water quality from a suite of extensive, sedum green roofs, also located in New York City. Samples of runoff water and rain were collected and analyzed for pH, electrical conductivity, turbidity, apparent color, suspended solids, nitrate-N, ammonium-N, phosphorus, potassium, calcium, magnesium, sodium, boron, iron, manganese, copper, zinc, nickel, aluminum, arsenic, barium, cadmium, chromium, and lead. Results indicate that runoff from all green roofs is higher than the average pH of incoming acid rain water, although the pH of runoff from the rooftop farm was slightly lower than that of the extensive green roofs. The conductivity, apparent color, and suspended solids concentrations of runoff from the rooftop farm were higher than those of the extensive green roofs, but not higher than values reported in the literature on agricultural runoff. The concentrations of nitrate-N, phosphorus, potassium, calcium, and magnesium in runoff from the rooftop farm were also higher than those of runoff from the extensive green roofs. Measured values of nitrate-N, calcium, and magnesium were not higher than concentrations reported elsewhere in the green roof literature or in agricultural literature; nor where they higher than EPA guidelines for water quality. Measured values of phosphorus and potassium were found to be higher than EPA guidelines. However, changes in nutrient management practices would help reduce these values.