Chemistry of growth medium and leachate from green roof systems in south-central Texas

Abstract

Installation of intensive and extensive green roofs is becoming popular for reducing runoff from impervious surfaces in many cities around the world. Most studies on runoff quality from green roofs have been conducted in cooler northern climates. We examined the losses and gains of nutrients, cations and selected anions in planted and unplanted growth medium and compared these to initial growth medium (IGM) typically used for green roof modules in south-central Texas. Water extracts of growth medium and leachate from three replicates of unplanted growth medium and three planted species (Sedum kamtschaticum, Delosperma cooperi and Talinum calycinum) were examined. During the first 6 months after establishment we observed high losses of nitrate (25 to 44 mgkg −1 ), dissolved organic carbon (DOC: 155 to 190 mgkg −1 ) and nitrogen (DON: 9.0 to 11.2 mgkg −1 ) and orthophosphate-P (1 to 2 mgkg −1 ). Average leachate concentrations based on four rain events 6 months after establishment ranged from 0.3 to 6.6 mgL −1 in planted modules and 6.3 mgL −1 in unplanted modules for nitrate-N, 38 to 42 mgL −1 in planted modules and 32 mgL −1 in unplanted modules for DOC, 2.1 to 3.1 mgL −1 in planted modules and 2.1 mgL −1 in unplanted modules for DON and 0.27 to 0.37 mgL −1 in planted modules and 0.40 mgL −1 in unplanted modules for orthophosphate-P. We suggest that after the establishment of green roofs, leachate losses may contribute some runoff concentrations of nitrogen, carbon and phosphorous in urban areas.

Effect of Green Roof Configuration and Hydrological Variables on Runoff Water Quantity and Quality

“…Similar results were obtained for organic matter (Figure 8), phosphorus, and coliforms, with concentrations for GRs that were significantly higher than the control cases' concentrations (Table 3). These are similar to results presented in previous studies, indicating GRs as a source of organic matter [7,46], phosphorus [22,24,25,47,48], pathogens [48], and total dissolved solids [1,30], as well as the pH neutralization effect from GRs [1,22,25,26,31,[49][50][51][52]. GRs had significantly higher concentrations for total Kjeldahl nitrogen (TKN), nitrates, and nitrites than the control cases, confirming the findings of Aitkenhead-Peterson et al [46], Greogoire and Clausen [53], Moran et al [54], and Whittinghill et al [55].…”

Green Roof Ecosystems

“…This increase in organic matter agrees with the Köhler and Poll (2010) study discussed above and with the findings of Getter et al (2007) who reported that organic matter nearly doubled from 2.33 to 4.25 % in just 5 years where the primary component of the substrate was heatexpanded slate. Similarly, Getter et al (2009b) reported that the amount of carbon sequestered on shallow sedum based roofs increased with age and that 100 g C/ cm 2 (57.8 oz/in 3 ) were sequestered during the first 2 years after installation of 6 cm (2.4 in) deep plots.…”

“…Zhang et al (2007) indicate that green roof "soil erosion" induced by winds decreases with higher levels of plant cover. Roots bind plant masses to the substrate, thus providing a windbreak from erosive forces.…”

“…One might therefore expect a negative relationship between plant presence and P runoff. Some studies do indeed indicate that plant presence reduces both TP and PO 4 3− concentrations and fluxes (Table 5.1) and that plant species identity may (Beck et al 2011) or may not Aitkenhead-Peterson et al 2011) be important in altering P in runoff. However, it is still unclear what plant-mediated processes are responsible for observed patterns.…”

Science Walden: Exploring the Convergence of Environmental Technologies with Design and Art

“…Aitkenhead-Peterson et al mentioned that phosphorus can be released from the soil layer in a green roof. However, the concentrations of NH4+ and NO3− decrease when a green roof system is adopted [13]. Figure 9.…”