Further Analysis of Loss of Ignition in Bioretention Units at the Edison Environmental Center
Citation:
OConnor, T. Further Analysis of Loss of Ignition in Bioretention Units at the Edison Environmental Center. Presented at EWRI World Environmental & Water Resources Congress, Pittsburgh, Pennsylvania, May 19 - 23, 2019.
Impact/Purpose:
In 2009, ORD began a study on bioinfiltration uniits to see if the surface area to watershed area mattered. A study in 2012 indicated that the largest units did not have as robust shrub growth as the smaller units. Through loss on ignition analysis in 2017 and now 2018, there appears to be greater carbon accumulation in the soil of the smaller units. this is part of several studies on the bioinfiltration units that would suggest that over sizing units does not result in better treatment or management.
Description:
Bioretention units (widths 7.1 m) were constructed at EPA’s Edison Environmental Center to evaluate sizing of surface area to watershed area. Three sizes were tested in duplicate with changes in aspect ratio of length from inlet (northern) wall by doubling successive length from smallest (3.7 m) to largest (14.9 m). Each was instrumented for continuous monitoring with water content reflectometers (WCR) and thermistors with data collected since November 2009. The bioinfiltration planting media was comprised of 90% sand and 10% sphagnum peat moss by volume or approximately 99% and 1%, respectively, by weight. A variety of native grasses, perennials and shrubs were planted in the units during May and June of 2010. In late 2012, a survey of shrubs planted in these bioretention units was performed. The published results of the combined analyses of moisture content and size of shrubs indicated the smaller units had superior shrub growth due to the more frequent saturation of the root zone as measured by WCR while the plants in the largest units potentially relied only on direct rainfall. Sediment samples for loss on ignition (LOI) analysis were collected in 2017 in three bioinfiltration units and in 2018 the remaining three were tested. Samples were taken at a depth of 0.15 m and at 0.3 m intervals along the centerline and starting at the north wall of each bioretention unit. Results of the 2017 study were presented at the 2018 EWRI conference. The new 2018 LOI data support previous results. Taken together, results for LOI indicate the largest bioinfiltration units have a flat slope for LOI versus distance from inlets, while slope of remaining smaller rain gardens increases with distance from inlet; however, this increase in LOI abruptly drops off after 4 m. This makes sense for the smallest unit due to physical constraints, i.e. length of 3.7 m, but it is also seen in the next sized unit, i.e., 7.4m length as well. Results of the LOI studies (2017 and 2018) and previous study of shrubs and WCRs (2012) appear to be consistent. The build-up of carbon and greater plant health in the smaller bioretention units appears to be from more frequent inundation, while the largest units potentially deprive plants of moisture and growth potential. Results for the middle sized units (7.4 m long) potentially imply the configuration of the bioretention units for this media and width most likely maximizes at a length of approximately 4 m. Further sampling and analysis of the four smaller bioretention units at different depths and interior locations is planned.