@article{maxwell_winter_birgand_2020, title={Floating treatment wetland retrofit in a stormwater wet pond provides limited water quality improvements}, volume={149}, ISSN={["1872-6992"]}, DOI={10.1016/j.ecoleng.2020.105784}, abstractNote={Floating treatment wetlands (FTW) are an emerging management practice for improving water quality in stormwater wet ponds or other detention basins. Although there is substantial evidence of nutrient reductions by FTW in mesocosm-scale studies, findings from field-scale studies on the water quality benefits of FTW are less conclusive. A medium-sized stormwater wet pond (~0.8 ha) was partially divided using an impermeable liner and monitored for 280 d using a control and FTW treatment (~20–23% FTW coverage) to determine impact of FTW on concentrations of several water quality parameters at the field-scale. Discrete samples and high-frequency, multi-point monitoring were used to assess water quality differences. No significant differences between the two treatments were found for total nitrogen, total Kjeldahl nitrogen, or total phosphorus, based on discrete samples, and in situ monitoring showed dissolved oxygen was significantly lower in the FTW treatment by roughly 2.6 mg L−1. The FTW treatment had significantly higher nitrate concentrations in discrete samples, but this finding was not supported by the high-frequency data collected when considering the full monitoring period. The FTW had significantly lower total suspended solids, based on discrete and high-frequency sampling, and resulted in a 10–43% reduction in TSS (~25% mean reduction), relative to the control. Comparison of subsets of the high-frequency data to discrete sampling data demonstrated how limited discrete sampling can lead to partial information and potentially erroneous conclusions when assessing FTW treatment effect in stormwater, especially when differences in water quality chemistry are small.}, journal={ECOLOGICAL ENGINEERING}, author={Maxwell, Bryan and Winter, Danielle and Birgand, Francois}, year={2020}, month={Apr} }
 @article{maxwell_birgand_schipper_barkle_rivas_helmers_christianson_2020, title={High-frequency, in situ sampling of field woodchip bioreactors reveals sources of sampling error and hydraulic inefficiencies}, volume={272}, ISSN={["1095-8630"]}, DOI={10.1016/j.jenvman.2020.110996}, abstractNote={Woodchip bioreactors are a practical, low-cost technology for reducing nitrate (NO3) loads discharged from agriculture. Traditional methods of quantifying their performance in the field mostly rely on low-frequency, time-based (weekly to monthly sampling interval) or flow-weighted sample collection at the inlet and outlet, creating uncertainty in their performance and design by providing incomplete information on flow and water chemistry. To address this uncertainty, two field bioreactors were monitored in the US and New Zealand using high-frequency, multipoint sampling for in situ monitoring of NO3-N concentrations. High-frequency monitoring (sub hourly interval) at the inlet and outlet of both bioreactors revealed significant variability in volumetric removal rates and percent reduction, with percent reduction varying by up to 25 percentage points within a single flow event. Time series of inlet and outlet NO3 showed significant lag in peak concentrations of 1-3 days due to high hydraulic residence time, where calculations from instantaneous measurements produced erroneous estimates of performance and misleading relationships between residence time and removal. Internal porewater sampling wells showed differences in NO3 concentration between shallow and deep zones, and "hot spot" zones where peak NO3 removal co-occurred with dissolved oxygen depletion and dissolved organic carbon production. Tracking NO3 movement through the profile showed preferential flow occurring with slower flow in deeper woodchips, and slower flow further from the most direct flowpath from inlet to outlet. High-frequency, in situ data on inlet and outlet time series and internal porewater solute profiles of this initial work highlight several key areas for future research.}, journal={JOURNAL OF ENVIRONMENTAL MANAGEMENT}, author={Maxwell, Bryan M. and Birgand, Francois and Schipper, Louis A. and Barkle, Greg and Rivas, Aldrin A. and Helmers, Matthew J. and Christianson, Laura E.}, year={2020}, month={Oct} }
 @article{liu_youssef_birgand_chescheir_tian_maxwell_2020, title={Processes and mechanisms controlling nitrate dynamics in an artificially drained field: Insights from high-frequency water quality measurements}, volume={232}, ISSN={["1873-2283"]}, DOI={10.1016/j.agwat.2020.106032}, abstractNote={Intensive agricultural activities, especially in artificially drained agricultural landscapes, generate a considerable amount of nutrient export, which has been identified as a primary cause of water quality impairment. Several management practices have been developed and installed in agricultural watersheds to reduce nutrient export, e.g. nitrate-nitrogen (NO3-N). Although published research reported considerable water quality benefits of these practices, there exist many unanswered questions regarding the inherent processes and mechanisms that control nitrate fate and transport from drained agricultural landscape. To advance our understanding of processes and mechanisms, we deployed two high-frequency sampling systems in a drained agricultural field to investigate the relationship between agricultural drainage and nitrate concentrations (C-Q relationship). Results indicated that the high-frequency measuring system was able to capture the rapidly changing C-Q relationships at the experimental site, e.g. hysteresis patterns. The 22 identified storm events exhibited anti-clockwise behavior with high variability of flushing/dilution effects. In addition, high drainage flows contributed far more nitrate loading compared with lower flows. For instance, the top 10 % of drainage flow exported more than 50 % of the nitrate lost via subsurface drainage during the monitoring period. Additionally, we observed that animal waste application was the most influential practice to change the C-Q relationship by increasing the size of soil nitrogen pools. The insights obtained from the high-frequency water quality measurements could help provide practical suggestions regarding the design and management of conservation practices, such as controlled drainage, bioreactors, and saturated buffers, to improve their nitrogen removal efficiencies. This subsequently leads to better nutrient management in drained agricultural lands.}, journal={AGRICULTURAL WATER MANAGEMENT}, author={Liu, Wenlong and Youssef, Mohamed A. and Birgand, Francois P. and Chescheir, George M. and Tian, Shiying and Maxwell, Bryan M.}, year={2020}, month={Apr} }
 @article{maxwell_diaz-garcia_jose martinez-sanchez_birgand_alvarez-rogel_2020, title={Temperature sensitivity of nitrate removal in woodchip bioreactors increases with woodchip age and following drying-rewetting cycles}, volume={6}, ISBN={2053-1419}, DOI={10.1039/d0ew00507j}, abstractNote={Woodchip bioreactors are a beneficial management practice with increasing use for the sustainable reduction of nitrate in waters discharged from agriculture and urban landscapes. Previous research has shown an interaction between temperature and carbon quality with respect to microbial respiration, which may affect performance of woodchip bioreactors. This study used two previously published data sets of woodchip bioreactors in Spain and the United States that were exposed to weekly drying–rewetting cycles, to better understand the processes driving changes in temperature sensitivity of nitrate removal. The factor by which nitrate removal increased given a 10 °C increase in temperature (Q10) was used as a metric for temperature sensitivity. Values of Q10 for nitrate removal in both experiments ranged from 1.8–3.1 and generally increased over time as woodchips aged. In field bioreactors, mean nitrate removal rate at temperatures 10–15 °C and 22–27 °C decreased by 36% and 7%, respectively, from the first to second year. Values of Q10 increased with amount of time since resaturation of the woodchips following a drying–rewetting cycle. Dynamic calculations of Q10 showed changes in Q10 were not unidirectional. Subsetting the datasets showed that Q10 was temperature-dependent and varied according to minimum temperature value and total range in temperature. Results suggest temperature sensitivity of nitrate removal was related to short and long-term changes in carbon quality or availability, consistent with the carbon-quality-temperature hypothesis. When sizing woodchip bioreactors, water quality managers should consider that long-term declines in efficiency will be greatest at lower temperatures.}, number={10}, journal={ENVIRONMENTAL SCIENCE-WATER RESEARCH & TECHNOLOGY}, author={Maxwell, Bryan M. and Diaz-Garcia, Carolina and Jose Martinez-Sanchez, Juan and Birgand, Francois and Alvarez-Rogel, Jose}, year={2020}, pages={2752–2765} }
 @article{maxwell_birgand_schipper_christianson_tian_helmers_williams_chescheir_youssef_2019, title={Drying–Rewetting Cycles Affect Nitrate Removal Rates in Woodchip Bioreactors}, volume={48}, ISSN={0047-2425}, url={http://dx.doi.org/10.2134/jeq2018.05.0199}, DOI={10.2134/jeq2018.05.0199}, abstractNote={Woodchip bioreactors are widely used to control nitrogen export from agriculture using denitrification. There is abundant evidence that drying–rewetting (DRW) cycles can promote enhanced metabolic rates in soils. A 287‐d experiment investigated the effects of weekly DRW cycles on nitrate (NO3) removal in woodchip columns in the laboratory receiving constant flow of nitrated water. Columns were exposed to continuous saturation (SAT) or to weekly, 8‐h drying‐rewetting (8 h of aerobiosis followed by saturation) cycles (DRW). Nitrate concentrations were measured at the column outlets every 2 h using novel multiplexed sampling methods coupled to spectrophotometric analysis. Drying–rewetting columns showed greater export of total and dissolved organic carbon and increased NO3 removal rates. Nitrate removal rates in DRW columns increased by up to 80%, relative to SAT columns, although DRW removal rates decreased quickly within 3 d after rewetting. Increased NO3 removal in DRW columns continued even after 39 DRW cycles, with ∼33% higher total NO3 mass removed over each weekly DRW cycle. Data collected in this experiment provide strong evidence that DRW cycles can dramatically improve NO3 removal in woodchip bioreactors, with carbon availability being a likely driver of improved efficiency. These results have implications for hydraulic management of woodchip bioreactors and other denitrification practices.}, number={1}, journal={Journal of Environment Quality}, publisher={American Society of Agronomy}, author={Maxwell, Bryan M. and Birgand, François and Schipper, Louis A. and Christianson, Laura E. and Tian, Shiying and Helmers, Matthew J. and Williams, David J. and Chescheir, George M. and Youssef, Mohamed A.}, year={2019}, pages={93} }
 @article{maxwell_birgand_schipper_christianson_tian_helmers_williams_chescheir_youssef_2019, title={Increased Duration of Drying–Rewetting Cycles Increases Nitrate Removal in Woodchip Bioreactors}, volume={4}, ISSN={2471-9625}, url={http://dx.doi.org/10.2134/ael2019.07.0028}, DOI={10.2134/ael2019.07.0028}, abstractNote={Core Ideas
Nitrate removal in woodchips increased linearly with drying–rewetting duration.
Nitrate removal increased up to 172% in the longest drying–rewetting duration.
Nitrate removal rates increased proportionally with dissolved organic C leaching.
}, number={1}, journal={Agricultural & Environmental Letters}, publisher={American Society of Agronomy}, author={Maxwell, Bryan M. and Birgand, François and Schipper, Louis A. and Christianson, Laura E. and Tian, Shiying and Helmers, Matthew J. and Williams, David J. and Chescheir, George M. and Youssef, Mohamed A.}, year={2019} }
 @article{maxwell_birgand_smith_aveni-deforge_2018, title={A small-volume multiplexed pumping system for automated, high-frequency water chemistry measurements in volume-limited applications}, volume={22}, ISSN={1607-7938}, url={http://dx.doi.org/10.5194/hess-22-5615-2018}, DOI={10.5194/hess-22-5615-2018}, abstractNote={Abstract. An automated multiplexed pumping system (MPS) for high-frequency water chemistry measurements at multiple locations previously showed the ability to increase spatial and temporal data resolution and improve understanding of biogeochemical processes in aquatic environments and at the land–water interface. The design of the previous system precludes its use in volume-limited applications in which highly frequent measurements requiring a large sample volume would significantly affect observed processes. A small-volume MPS was designed to minimize the sample volume while still providing high-frequency data. The system was tested for cross-contamination between multiple sources, and two applications of the technology are reported. Cross-contamination from multiple sources was shown to be negligible when using recommended procedures. Short-circuiting of flow in a bioreactor was directly observed using high-frequency porewater sampling in a well network, and the small-volume MPS showed high seasonal and spatial variability of nitrate removal in stream sediments, enhancing data collected from in situ mesocosms. The results show it is possible to obtain high-frequency data in volume-limited applications. The technology is most promising at the reach or transect scale for observing porewater solute dynamics over daily timescales, with data intervals  < 1 h for up to 12 locations.
                    }, number={11}, journal={Hydrology and Earth System Sciences}, publisher={Copernicus GmbH}, author={Maxwell, Bryan M. and Birgand, François and Smith, Brad and Aveni-Deforge, Kyle}, year={2018}, month={Oct}, pages={5615–5628} }
 @article{birgand_aveni-deforge_smith_maxwell_horstman_gerling_carey_2016, title={First report of a novel multiplexer pumping system coupled to a water quality probe to collect high temporal frequency in situ water chemistry measurements at multiple sites}, volume={14}, ISSN={1541-5856}, url={http://dx.doi.org/10.1002/lom3.10122}, DOI={10.1002/lom3.10122}, abstractNote={The increasing availability and use of high‐frequency water quality sensors has enabled unprecedented observations of temporal variability in water chemistry in aquatic ecosystems. However, we remain limited by the prohibitive costs of these probes to explore spatial variability in natural systems. To overcome this challenge, we have developed a novel auto‐sampler system that sequentially pumps water from up to 12 different sites located within a 12 m radius to a single water quality probe. This system is able to generate high temporal frequency in situ water chemistry data from multiple replicated units during experiments as well as multiple sites and depths within natural aquatic ecosystems. Thus, with one water quality probe, we are able to observe rapid changes in water chemistry concentrations over time and space. Here, we describe the coupled multiplexer‐probe system and its performance in two case studies: a mesocosm experiment examining the effects of water current velocity on nitrogen dynamics in constructed wetland sediment cores and a whole‐ecosystem manipulation of redox conditions in a reservoir. In both lotic and lentic case studies, we observed minute‐scale changes in nutrient concentrations, which provide new insight on the variability of biogeochemical processes. Moreover, in the reservoir, we were able to measure rapid changes in metal concentrations, in addition to those of nutrients, in response to changes in redox. Consequently, we believe that this coupled system holds great promise for measuring biogeochemical fluxes in a diverse suite of aquatic ecosystems and experiments.}, number={12}, journal={Limnology and Oceanography: Methods}, publisher={Wiley}, author={Birgand, François and Aveni-Deforge, Kyle and Smith, Brad and Maxwell, Bryan and Horstman, Marc and Gerling, Alexandra B. and Carey, Cayelan C.}, year={2016}, month={Jul}, pages={767–783} }