Dual Isotope Analysis of Denitrification in Stormwater Basins
| Autor: | Morgan, Naomi |
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| Jazyk: | angličtina |
| Rok vydání: | 2021 |
| Předmět: | |
| Druh dokumentu: | Text |
| DOI: | 10.34944/dspace/6519 |
| Popis: | 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 M.S. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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