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Diel redox cycling and its impact on inorganic nitrogen in an engineered wetland designed for water treatment
Reed, Ariel P.
Reed, Ariel P.
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2019
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Abstract
The Prado Wetland Basin located near Orange County, CA consists of experimental, unvegetated, wetland cells that were designed and implemented to remove incoming nitrate (NO3-) from the diverted Santa Ana River, an anthropogenically-impaired drinking water source. NO3- is a federally-mandated compound that causes human disease and eutrophica- tion of waterbodies. The sediment, a.k.a. biomat, within these wetland cells is instrumental in nitrogen transformations and removal and consists of photosynthetic diatoms, bacteria, and archaea. The processes by which this biological consortium removes NO3- remain elusive. In order to assess NO3- removal in these experimental cells, surface water samples were collected at the inlet and outlet of a mature, open-water wetland cell with a hydraulic residence time of one day during June and September of 2018. Physical and chemical pa- rameters, such as pH, dissolved oxygen (DO), nitrogen oxides, and dissolved metals, revealed that the wetland water chemistry changes on a day versus night basis, or by a predictable, diel pattern. Since the biomat is a key facilitator in nitrogen transformation and removal within the wetland cell, inorganic nitrogen species within the biomat porewater were quantified at various depths during day and night conditions. Porewater sampling revealed a diel pattern in NO3- and nitrite (NO2-) at more surficial biomat depths, as well as the presence of oxidized nitrogen species at deeper biomat depths, depths assumed to harbor extremely reduced conditions. The presence of intermediate, inorganic nitrogen species in the surface and porewater provide clues as to the fate and removal of nitrogen within the wetland. Stable nitrogen isotope tracer experiments using 15NO2- were performed to quantify ni- trogen transformation and removal pathways on a day versus night basis. Of the measured biochemical reactions, all reactions were determined to occur faster at night than during the day. Coupled nitrification-denitrification, a metabolic process that converts aqueous nitrogen to gaseous nitrogen, was identified as the dominant nitrogen removal pathway during both the day and night. An additional pathway that does not contribute to the net removal of aqueous nitrogen was identified. More research is needed to quantify other nitrogen cycling pathways not addressed here. The identification and quantification of nitrogen transformation rates and documenta- tion of diel changes in surface and porewater chemistry on a day versus night basis within these experimental, unvegetated, wetland ecosystems can better inform wetland design and operation, leading to water quality and ecosystem optimization.
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