Impact of hydraulic conductivity on water quality and microbial ecology of rain gardens

Rain gardens are a form of sustainable urban drainage which lower flood risk and reduce environmental contamination from stormwater.  A combination of processes including filtration, sedimentation and microbial metabolic processes work to remove contaminants from the stormwater.  In this study we examined the impact of hydraulic conductivity of raingarden soil on raingarden performance, exploring its impact on the removal of contaminants from the stormwater, as well as microbial community composition and function.  This was undertaken as part of a large scale project to install raingardens across the city centre of Glasgow, thus improving the city’s climate resilience.   The study utilized four raingardens fed real stormwater from a busy road.  All raingardens tested reduced contaminant concentrations in the stormwater, and reductions in contaminant concentrations were greatest when pollutant levels in the input water were higher.  Importantly, road salting in the winter did not cause dissolved metals to be released from the raingardens.  DNA was extracted from waters and soils for microbial community and function analysis using Illumina 16S sequencing and a bioinformatics suite.  A diverse community of bacteria capable of hydrocarbon degradation and metal resilience were found in stormwaters and raingarden soil.  Notably, the taxonomic evenness and overall diversity of the stormwater microbial community was increased as it passed through the raingarden. Furthermore, the raingarden soil displayed a greater functional richness compared to the input waters.  This demonstrates that the microbes in the raingardens can undertake a greater range of functions than those in the untreated stormwater, and highlights the importance of the raingarden bacteria in treatment of contaminants.   Microbial community composition and function showed little difference between rain gardens and PERMANOVA analysis identified that hydraulic conductivity had no significant impact on functional Beta diversity in the soil.  Overall, in this study, hydraulic conductivity did not appear to have a significant impact on microbial community composition, nor on the removal of contaminants by the raingarden, with all raingardens performing similarly well.