Bioretention basins are a stormwater control method implemented in urban areas to curtail runoff and pollution; however, recent studies show inconsistent nitrate (NO3-) removal, and in many cases average nitrate concentrations in basin outflow are higher than inflow. Microbial denitrification to promote nitrate removal can be enhanced by using underdrains in basin design that provide anoxic conditions. This study examines the impact of basin design and storm characteristics (precipitation intensity and antecedent dry period length) on microbial denitrification efficacy. Three basins in the Philadelphia area were selected for storm sampling: a large (~0.6 ha) wet basin without internal water storage, a small (~0.02 ha) basin without internal water storage, and a medium-sized (~0.1 ha) basin with internal water storage and a raised underdrain. In addition, three laboratory bioretention columns with underdrain configurations at the bottom, middle, and top of an internal water storage zone were sampled under steady-state and transient flow conditions. Samples collected as time series and grab samples during storm events were analyzed for nitrate concentrations and nitrate isotopes. Because microbes preferentially consume lighter nitrate isotopes (14N and 16O), stable isotope analysis offers an indication of denitrification. Stormwater outlet nitrate concentrations were lower than the inlet in the large suburban basin, similar to the inlet in the small suburban basin, and higher than the inlet in the urban basin. Differences in storm intensity and dry periods did not appear to increase or decrease nitrate concentrations in any basin, suggesting that basin design is a more dominant factor. The values of δ15N and δ18O in basin samples showed stormwater mixing without denitrification in all three basins. Only in the basin with water internal storage were periods of denitrification in samples observed, based on heavier δ15N and δ18O ratios. In laboratory studies, a lower underdrain configuration is preferred to promote denitrification based on heavier isotopic ratios and enrichment calculations. Bioretention columns had the largest enrichment factors (up to -5.3‰ ɛ 15N and -5.0‰ ɛ 18O) during steady-state flow. Lower enrichment factors associated with the low-intensity storm (-2.6‰ ɛ 15N and -1.3‰ ɛ 18O) show that transient flow disrupted denitrification rates. Field enrichment factors were greater than those in the columns (up to -11.9‰ ɛ 15N and -7.4‰ ɛ 18O). Even though nitrate decreased consistently over three storms, isotopic ratios did not exhibit these denitrification trends until at least eight hours after the onset of the storm events. Therefore, decreases in nitrate concentration alone are an unreliable assessment of denitrification efficacy. This study suggests that isotope analysis should be considered to better understand the conditions that promote denitrification. / Geology

Urbanized areas with increased amounts of impervious surfaces alter hydrologic systems by increasing stormwater runoff, decreasing infiltration, and reducing vegetation cover and evapotranspiration. Modeling hydrologic systems here is especially difficult due to the increased impervious land cover, which makes predicting processes such as urban streamflow and flooding challenging. By understanding the drivers of hydraulic processes, such as soil characteristics, bedrock depth, and land use, the quality and accuracy of models can be improved. The goal of this study was to use soil moisture loggers and electrical resistivity tomography (ERT) along the Pennypack Creek (Philadelphia, PA) to evaluate soil infiltration and bedrock depth in urban areas to ultimately access their impact on critical zone modeling. ERT was also used to validate or dispute recent seismic interpretations. Four study sites adjacent to Pennypack Creek were selected based on variations in underlying geology: Triassic basin sedimentary rock (Lukens), Paleozoic mafic gneiss (Meadow), Piedmont mica schist (Pine Road), and coastal plain weathered down to mica schist (Rhawn Street). Soil moisture sensors were installed at each site to a depth of up to 50 cm. ERT surveys were conducted at Pine Road and Rhawn Street sites. High infiltration variation at Pine Road and Meadow indicated macropores, which create preferential flow paths whereas low infiltration variation at Rhawn Street and Lukens indicated compaction associated with their land use (public parks). Comparing field capacity data to USDA soil type maps indicated the soil type was not a good predictor and in situ sampling was needed to estimate soil properties. ERT demonstrated bedrock was not shallow at the streambed as predicted by the seismic inversion and showed the need to corroborate depth to bedrock from seismic surveys beneath streams with resistivity inversions. Structure beneath the streambed was particularly noisy for the seismic surveys due to the flow of stream water. This study demonstrates that an accurate critical zone model, especially at urban sites, must rely on in situ investigation of hydrologic parameters based on land use, rather than assumptions of parameter values based on the underlying geology or soil type. / Geology

Compaction of urban soil where stormwater infrastructures are built reduces infiltration, vegetation growth, and stormwater treatment capacity. Biochar—a carbonaceous porous material produced by pyrolysis of organic waste – can be used as a soil amendment to improve the function of stormwater infrastructure in addition to the proven benefit of increased pollutant removal. However, the benefits depend on the biochar’s properties such as particle size distribution and concentration. Further, because biochar’s particle size distribution is altered by compaction, the hydraulic functions of compacted biochar amended soil is unknown. Herein, we examined the effect of biochar concentrations (0-6% w/w) and particle sizes (unsieved, sieved to < 2mm, and to < 0.5 mm) on water retention and saturated or unsaturated hydraulic conductivity of compacted stormwater media amended with biochar. Our results show the particle size of biochar plays a critical role in whether or not compaction is alleviated: while increasing concentration of unsieved biochar increased hydraulic conductivity up to 3% biochar, increasing concentration of fine biochar (< 2 mm) resulted in consistent decline in hydraulic conductivity under compaction. The results indicate that large biochar particles can effectively dissipate the compaction energy, while the fine biochar under compaction increased clogging by generating more fines that occupy the pores. Water retention improved regardless of the size distribution of added biochar, indicating that addition of biochar would reduce the irrigation requirement to maintain plant health in dry climate or water-stressed conditions. Overall, the results indicate that biochar addition can be effective in mitigating the negative impacts of compaction on stormwater infrastructures, depending on the proportion of coarse biochar. / Geology

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