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Due to the large amount of impervious surface cover, urban areas are at high risk for flooding and, in cities with combined sewer systems, subject to sewer overflow during heavy storm events. The Pennsylvania Department of Transportation (PennDot) is currently reconstructing and expanding parts of Interstate 95 (I-95) through the city of Philadelphia. Due to both federal and local laws, PennDOT must account for the stormwater runoff and minimize outflow to the sewer system. To do so, PennDOT has plans to construct a series of stormwater management practices (SMPs) adjacent to I-95 to control the volumes of highway runoff. In partnership with Villanova University, Temple University has been tasked with monitoring these SMPs, known as bioswales, to provide insight and guidance as the project moves forward and to ensure mistakes aren’t reproduced in future construction. This research is contributing to the overall project goals by testing the application of geophysical monitoring to one of the bioswales known as SMP A. Unlike commonly used point measurements, geophysical surveys are non-invasive and provide extensive spatial coverage. Specifically, this research involves the use of electrical resistivity tomography (ERT), in which a series of cable-connected electrodes are placed in the ground and measure electric potential differences when an electric current is applied. Once processed, the results are a contoured subsurface image of the distribution of electrical resistivity (the inverse of electrical conductivity). If multiple surveys are taken over time, the data can be differenced, known as time lapse inversion, to quantify changes in electrical resistivity. ERT is a favorable for these SMPs as survey results are sensitive to changes in soil moisture and fluid conductivity, which are essential parameters when tracking infiltration and road salt influx at these SMPs. Additionally, the ERT data can be converted to soil moisture values using Archie’s law, which is important for determining soil moisture at points where no sensors are currently placed. We built and installed three ERT survey lines connected to an on-site monitoring station in April 2019 and collected quasi-daily measurements until monitoring seized in November 2021. One way to test SMPs is through a simulated runoff test, in which an SMP is flooded with water from an external source and the SMP’s response is recorded. During September 2020, Villanova University performed an SRT at SMP A, while we performed ERT surveys before, during, and after the SRT to track the infiltration and dry-out cycle. Knowing how long the soil at an SMP takes to recover to pre-storm soil moisture levels is essential in understanding an SMP’s performance and functionality. We were successfully able to capture the wet-up associated with the SRT and the corresponding dry-out period with the ERT data, which showed around a 20% decrease in resistivity when soil sensors indicated saturation. This resistivity change began to decrease and finally reached pre-SRT levels (0 – 5% change) after 68 hours, leading to our estimate of a three day recovery time for SMP A. Interestingly, inflow/outflow measurements at SMP A showed that only 24% of the input water exited the SMP via the overflow drain, meaning the rest of the water remained in the SMP. This discrepancy was solved with our ERT data, which showed that the decrease in resistivity, and therefore increase in soil moisture, was seen at depths beyond the 0.60 m layer of amended fill the SMP contained. Overall, the water was infiltrating past this layer and into the urban soil below. Initially it was thought that the native urban soil would impede infiltration, hence SMP A was designed around this assumption. However, our geophysical results indicate that the native urban soil underlying the SMP has an infiltration rate of 10 cm/hr and is contributing to the overall function of the SMP. This was unknown as previous monitoring was focused on the layer of amended fill material, not the underlying native soil. The relationship between electrical resistivity and soil moisture, fluid conductivity, and porosity is known as Archie’s law, who derived an empirical formula that allows electrical resistivity data to be converted to soil moisture values. However, this equation requires quantifying two parameters, m (also known as the cementation factor) and n, the saturation exponent. Researchers commonly use pre-published values for m and n, or establish site-specific values by fitting Archie’s law to a set of soil moisture and conductivity data. However, as soil is heterogeneous, one set of m and n values may not be accurate across an entire site, especially with the presence of hysteresis, where one soil moisture value can correspond to multiple conductivity values depending on whether the soil is experiencing imbibition or drainage. Additionally, m and n can change over time as soil fabric changes, as well as soil conductivity changes due to the influx of road salt during winter months. In December 2019, we finished installing 16 TEROS12 soil sensors at SMP A, which recorded soil volumetric water content (VWC) and bulk electrical conductivity (bulk EC) every five minutes for nearly two years. These sensors were at six different locations within SMP A at depths of either 0.10 m, 0.30 m, or 0.60 m. We selected 13 storm events and fit Archie’s law to the soil VWC and bulk EC data to get values for m and n. While we were able to find m and n for all events, including events that exhibited hysteresis in soil VWC and bulk EC, each sensor had a different pair of m and n values. This discrepancy was surprising, given that the soil at SMP is a homogeneous, sandy-loam fill with no more than 10% clay. However, even sensors at the same depth show statistically significant differences. We also found that m and n were changing over time, notably m was increasing over time, possibly due to porosity changes. This result indicates that multiple sensors are needed to accurately calculate m and n, even at sites with relatively homogeneous soil. Most notably, the reason why we had success in fitting Archie’s law for every sensor was due to our accounting for changes in porewater conductivity. Most researchers assume a constant value for porewater (fluid) conductivity in Archie’s law. However, we found that not accounting for porewater conductivity changes lead to severe misestimation of soil VWC, even getting physically impossible values (VWC > 1.0 m3/m3) in some cases. Therefore, accounting for changes in porewater conductivity is essential when using Archie’s law. Road salt transport in SMPs is a concern, especially in Philadelphia, which is subject to winter storms and freezing conditions. In some PennDOT SMPs, the presence of road salt in the soil during leaf-out has been suspected to be the cause of stunted plant growth and pre-mature plant mortality. Vegetation is an important aspect of the SMPs, as they provide evapotranspiration pathways, aesthetics, and soil erosion control. Thus, vegetation impairment affects SMP functionality, and plants often need to be replaced, increasing maintenance costs. To track and assess the spatial distribution of road salt, we performed ERT surveys along three lines, with two lines in the topographically lower portion of the SMP, or flood zone, and the other line on the elevated bank parallel to the other lines. All three of these lines had vegetation. In total, we collected 900 ERT surveys from October 2020 to September 2021, sufficiently covering the winter months and growing season. During February 2021, the soil sensors indicated significant increases in conductivity, with sensors ranging from 5.0 – 20.0 mS/cm, compared to pre-winter values of 0.1 – 0.6 mS/cm. The winter ERT surveys show the formation of a shallow conductive (< 10 Ω) layer in the top 0.25 m of soil, and an overall decrease in resistivity of up to 70%. This change decreased over the spring and summer months, indicating that dilute runoff was flushing the salt through the soil column. However, flood-zone ERT data still showed a 20% decrease in resistivity in June when compared to pre-winter data, indicating that road lingered in the soil during the spring and summer months. In May, we began taking bimonthly measurements of plant height, width, and leaf chlorophyll content (SPAD) on plants along the ERT lines, then in July took leaf tissue, root tissue, and root-zone soil samples and analyzed them for sodium content. We found that the plants along Lines 2 and 3 (flood-zone) had statistically significant stunted growth when compared to the plants along the elevated bank, as well as elevated sodium levels (> 400 mg/kg) in root tissue. No detectable sodium was found in leaf tissue samples. The stunted growth and elevated root sodium in the flood-zone plants indicate that early spring storms are not enough to flush out the road salt, and therefore artificial flooding may be required before leaf-out to ensure plant survival. We also suggest planting salt-tolerant plant species in areas of SMPs prone to flooding, such as the topographically lower portions. ERT can also be used to guide the placement of these plant species, as ERT can delineate areas of higher conductivity. / Geoscience

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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京都大学 / 新制・課程博士 / 博士(工学) / 甲第23480号 / 工博第4892号 / 新制||工||1764(附属図書館) / 京都大学大学院工学研究科社会基盤工学専攻 / (主査)教授 立川 康人, 准教授 市川 温, 教授 堀 智晴 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

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