Katherine H. Baker

Selecting a Green Roof Medium for Water Quality Benefits

Green roofing is a best management practice for urban areas where land for traditional stormwater practices is unavailable. However, it is unknown how effective they are at treating stormwater. The two primary research objectives are the following: 1. To develop an effective media for green roofs that will improve roof runoff quality while maintaining the known water retention benefits of green roofs, and 2. To demonstrate that green roofing will generate lesser pollutant loadings into urban runoff than traditional roofing materials. In Phase I (currently ongoing), several green roof media (formulated from commonly-used expanded minerals, stormwater filter media, and organic matter) are being evaluated for their abilities to retain the pollutants from a synthetic acid rain. The samples are being analyzed for metals, nutrients, pH, and conductivity. The hypothesis is that one media will be “better” at pollutant removal and permanent retention. In Phase II, once the optimized media has been selected, it will be fieldtested on a green roof. The water quality of the runoff from the green roof will be compared with the runoff quality from a traditional roof. Field testing will address two objectives: 1. The media is capable of supporting the green roof plants, and 2. The anticipated removals (including water retention) are actually seen in the field. Phase I results on the mineral portion of the future media mix demonstrates all media were able to neutralize the acid rain. Comparing the media for pollutant removal and retention, the expanded shale was best able to retain phosphorus, ammonia, and metals from the synthetic acid rain. Phase I results on the mineral and additive combinations show the expanded shale and granulated activated carbon mix to be the most effective at pollutant removal and retention.

Selecting a Green Roof Media to Minimize Pollutant Loadings in Roof Runoff

Green roofs are a best management practice for urban areas where land for traditional stormwater practices is unavailable. The two primary research objectives are the following: (1) To develop an effective media for green roofs that will improve roof runoff quality while maintaining the known water retention benefits of green roofs, and (2) To demonstrate that green roofing will generate lesser pollutant loadings into urban runoff than traditional roofing materials. In Phase I (currently ongoing), several green roof media (formulated from commonly-used expanded minerals, stormwater filter media, and organic matter) are being evaluated for their abilities to retain the pollutants from a synthetic acid rain. The samples are being analyzed for metals, nutrients, pH, and conductivity. The hypothesis is that one media will be better at pollutant removal and permanent retention. In Phase II, once the optimized media has been selected, it will be field-tested on a green roof. The water quality of the runoff from the green roof will be compared with the runoff quality from a traditional roof. Phase I results on the mineral portion of the future media mix demonstrates all media were able to neutralize the acid rain. Comparing the media for pollutant removal and retention, the expanded shale was best able to retain phosphorus, ammonia, and metals from the synthetic acid rain. Phase I results on the mineral and additive combinations show the expanded shale and granulated activated carbon mix to be the most effective at pollutant removal and retention.

Nutrient leaching from disturbed soil horizons.

Hydrologic cycle restoration is the primary objective of stormwater management. Infiltration and bioretention systems composed of engineered and/or native soils are preferred tools for achieving this objective while also providing pollutant removal. However the disturbance of native soils can cause releases of nutrients and suspended solids in the early life of these systems. To limit the potential of replacing one problem with another, a better understanding of the behavior of soil components as they contribute to water transport and pollutant treatment is needed. This project investigated the ability of the various soil horizons in a Wharton silt loam and Leetonia loamy sand (Pennsylvania) to treat runoff from simulated storm events. Both soils were collected intact, but the Wharton silt loam had to be air-dried and the columns repacked when soil shrinkage caused bypassing of runoff along the walls of the laboratory columns. This process is similar to the disturbance of soil by bioretention construction and provided a unique opportunity to evaluate resulting nutrient releases. The effluent water from this reconstructed silt loam had elevated concentrations of total nitrogen (leaching > 100 mg/L of N initially from all soil horizons) and of total phosphorus from the organic horizon (∼1.5 mg/L) during application of the first 0.6 m of stormwater runoff. A release of calcium (∼500 mg/L) from all soil horizons also occurred, likely due to the destruction of cement bonds between soil aggregates. Potassium was also released from the O-horizon of the disturbed silt loam (∼30 mg/L) but leveled off after 0.2 m of applied runoff.

GREEN ROOFS - A BMP FOR URBAN STORMWATER QUALITY?

The focus of this research is the impact of green roofs on urban stormwater quality and builds on a laboratory study that evaluated several potential green roof media to produce a mix that produced the “best” overall reduction in pollutants from simulated rainwater. Because of the limitations of the laboratory testing, this second phase (field test) was performed. A second purpose was to determine if this actual media mix would support plant life with minimal maintenance. The third question raised by the researchers was whether the green roof, at the low concentrations often seen in many pollutants in wet and dry deposition, could remove these pollutants. If not, was the green roof at least “chemically neutral” (e.g., did not remove or add pollutants to the runoff, such as those added to rainwater by passage over a galvanized metal roof?) The field study showed that, during the early life of these roofs, water quality from the green roofs was comparable to the control roofs and better, with the exception of phosphorus and added color, to that of the galvanized roof. The increase is color is believed to be from washout of the organics in the media. Plant development on these

Pollutant Transport within the Vadose Zone of Natural Soils: With Focus on the Interactions of Individual Soil Horizons

Increasing impervious surfaces in the urban landscape has created a need for stormwater management practices that control both volume and peak flow rate. Infiltration systems are preferred because they address both issues. However, the potential for groundwater contamination is a concern when siting an infiltration practice. In addition, soil is not homogeneous and its variety of layers provides a heterogeneous environment for pollutant removal. This study, started in November 2007 and to be completed by October 2008, is using undisturbed natural soil columns of a Wharton silt loam and Leetonia loamy sand to treat stormwater runoff from roofing, parking lots and sidewalk. This research is a temporal study of runoff treatment and soil accumulation as a function of soil horizon and resultant soil chemistry. Early water results show leaching of total nitrogen and removal of total phosphorus by all soil horizons of both soil types. Potassium and sulfate removal has been seen in the AB/A1A2 and OAB/OA1A2 soil horizon columns of both soils but may only be temporary. The lower horizons and the entire profile are able to retain the leaching potassium and sulfate from the organic horizon. All soil horizons of both soil types have lowered the pH of influent stormwater and increased conductivity, turbidity, color, and hardness.