Robert D. Shannon

EVALUATION OF A SECOND DERIVATIVE UV/VISIBLE SPECTROSCOPY TECHNIQUE FOR NITRATE AND TOTAL NITROGEN ANALYSIS OF WASTEWATER SAMPLES

A method for nitrate analysis based on second derivative UV/Visible spectroscopy was developed by Simal et al. (1985: Simal J., Lage M. A., and Iglesias I. (1985) Second derivative ultraviolet spectroscopy and sulfamic acid method for determination of nitrates in water. J. Assoc. Analyt. Chem. 68, 962-964) and Suzuki and Kuroda (1987: Suzuki, N. and Kuroda R. (1987) Direct simultaneous determination of nitrate and nitrite by ultraviolet second-derivative spectrophotometry. Analyst 112, 1077-1079), and later modified for the analysis of total nitrogen in aqueous samples of varying nitrate:organic nitrogen ratios (Crumpton et al., 1992: Crumption W. G., Isenhart T. M. and Mitchell P. D. (1992) Nitrate and organic N analyses with second-derivative spectroscopy. Limnol. Oceanogr. 37, 907-913). The procedure uses the second derivative of the absorption spectrum for nitrate (NO3-), which has a peak at approximately 224 nm that is proportional to the NO3- concentration. Samples for total N analysis are first oxidized to NO3- by persulfate digestion. The objectives of this study were to: (1) test the accuracy and precision of the second derivative method through the use of NIST-traceable wastewater check samples; (2) determine whether the second derivative method for nitrate analysis can be used for wastewater samples and whether the method compares favorably with other currently used nitrate analysis methods; and (3) use the method to analyze wastewater samples containing a range of nitrate and total nitrogen concentrations. Our results indicated that the method needed to be modified to include a longer digestion time (60 min) and dilution of samples prior to digestion (if needed). With the modified method, nitrogen recoveries were not significantly different (P > or = 0.05) from samples with known N concentrations. In addition, nitrate concentrations in constructed wetland and wastewater samples analyzed by both second derivative spectroscopy and ion chromatography were not significantly different. Total nitrogen concentrations in wastewater samples also compared favorably to the same samples analyzed by Kjeldahl digestion. The method is faster, simpler, requires smaller sample volumes, and generates less waste than many EPA-approved methods of N analysis, and may offer a suitable alternative to current methods for analysis of nitrate and total N in wastewater samples.

A process‐based model to derive methane emissions from natural wetlands

A process-based model has been developed in order to calculate methane emissions from natural wetlands as a function of the hydrologic and thermal conditions in the soil. The considered processes in the model are methane production, methane consumption and transport of methane by diffusion, ebullition and through plants. The model has been tested against data from a three-year field study from a Michigan peatland. The interannual and seasonal variations of the modelled methane emissions and methane concentration profiles are in good agreement with the observations. During the growing season the main emission pathway proceeds through plants. Ebullition occurs whenever the water table is above the soil surface, while diffusion is only significant in the first 15 days after a drop of the water table below the peat surface.

Effect of seasonal changes in the pathways of methanogenesis on the δ13C values of pore water methane in a Michigan peatland

The δ13C value of pore water methane produced in a Michigan peatland varied by 11‰ during the year. This isotopic shift resulted from large seasonal changes in the pathways of methane production. On the basis of mass balance calculations, the δ13C value of methane from CO2 reduction (average = −71.4 ± 1.8‰) was depleted in 13C compared to that produced from acetate (−44.4 ± 8.2‰). The dissolved methane at the site remained heavy (approximately −51‰) during most of the year. Tracer experiments using 14C-labeled CO2 indicated that during January 110 ± 25% of the methane was produced by CO2 reduction. Because of low-methane production rates during the winter, this 13C-depleted methane had only a slight effect on the isotopic composition of the methane pool. In early spring when peat temperatures and methane production rates increased, the δ13C value of the dissolved methane in shallow peat was influenced by the isotopically light methane and approached −61‰. Peat incubation experiments conducted at 15°C in May and June (when the peat reaches its maximum temperature) indicated that an average of 84 ± 9% of the methane production was from acetate and had an average δ13C value of −48.7 ± 5.6‰. Rising acetate concentrations during April-May (approaching 1 mmol L−1(mM)) followed by a rapid decrease in acetate concentrations during May-June reflected the shift toward methane production dominated by acetate fermentation. During this period, dissolved methane in shallow peat at the site returned to heavier values (approximately −51‰) similar to that produced in the incubation experiments.

Controls on methane production in a tidal freshwater estuary and a peatland: methane production via acetate fermentation and CO2 reduction

Rates of total methane production, acetate fermentation andCO2 reduction were compared for two different wetland sites. On aper-liter basis, sediments from the White Oak River estuary, a tidal freshwatersite in eastern North Carolina, had an annual methane production rate (53.3mM yr−1) an order of magnitude higher thanthat ofBuck Hollow Bog (5.5 mM yr−1), a peatlandinMichigan. Methane was produced in the White Oak River site on an annual basisbyboth acetate fermentation (72%) and CO2 reduction (28%) in a ratiotypical of freshwater methanogenic sites. Competition for acetate bynon-methanogenic microorganisms in Buck Hollow peat limited methane productionfrom acetate to only a few months a year, severely impacting annual methaneproduction rates. However, when acetate was available to the methanogens in thepeat during early spring, the percentage of methane production from acetatefermentation (84%) and CO2 reduction (16%) and rates of totalmethaneproduction were similar to those of the White Oak River sediments at the sametemperature. Rates of CO2 reduction and acetate fermentationconducted at both sites at various temperatures showed that Buck Hollow peatmethane production was also limited by a colder temperature regime as well asdifferences in the response of the CO2 reducing and aceticlasticmethanogens to temperature variations.

Quantifying wetland methane emissions with process-based models of different complexities

Bubbling is an important pathway of methane emissions from wetland ecosystems. However the concentration-based threshold function approach in current biogeochemistry models of methane is not sufficient to represent the complex ebullition process. Here we revise an extant process-based biogeochemistry model, the Terrestrial Ecosystem Model into a multi-substance model (CH 4, O2, CO2 and N2) to simulate methane production, oxidation, and transport (particularly ebullition) with different model complexities. When ebullition is modeled with a concentrationbased threshold function and if the inhibition effect of oxygen on methane production and the competition for oxygen between methanotrophy and heterotrophic respiration are retained, the model becomes a two-substance system. Ignoring the role of oxygen, while still modeling ebullition with a concentration-based threshold function, reduces the model to a one-substance system. These models were tested through a group of sensitivity analyses using data from two temperate peatland sites in Michigan. We demonstrate that only the four-substance model with a pressure-based ebullition algorithm is able to capture the episodic emissions induced by a sudden decrease in atmospheric pressure or by a sudden drop in water table. All models captured the retardation effect on methane efflux from an increase in surface standing water which results from the inhibition of diffusion and the increase in rhizospheric oxidation. We conclude that to Correspondence to: J. Tang (tang16@purdue.edu) more accurately account for the effects of atmospheric pressure dynamics and standing water on methane effluxes, the multi-substance model with a pressure-based ebullition algorithm should be used in the future to quantify global wetland CH4 emissions. Further, to more accurately simulate the pore water gas concentrations and different pathways of methane transport, an exponential root distribution function should be used and the phase-related parameters should be treated as temperature dependent.

Sedimentation basin retention efficiencies for sediment, nitrogen, and phosphorus from simulated agricultural runoff

Simulated agricultural runoff, amended with sediment, nitrogen, and phosphorus, was passed through an experimental sedimentation basin. A series of six sequential runoff events was run through the basin for each of two treatments. The treatments consisted of one-day and three-day detention times, created using a perforated riser outlet structure. Effluent concentrations were monitored for total suspended sediment and various forms of nitrogen and phosphorus. For all runs, an average of 94% of the sediment, 76% of the nitrogen, and 52% of the phosphorus added to the inflow were retained by the basin. The three-day treatment retained significantly more sediment than the one-day treatment (p = 0.02). The majority of the sediment, nitrogen, and phosphorus was released within the first 12 h during the three-day runs and the first 4 h during the one-day runs.

EFFECTIVENESS OF SEDIMENTATION BASINS THAT DO NOT TOTALLY IMPOUND A RUNOFF EVENT

Twelve simulated 35–mm, 2–yr, 24–hr runoff events from a denuded 0.4–ha construction site in Pennsylvania were introduced into a 51–m 3 sedimentation basin. Each event consisted of a 100–m 3 inflow hydrograph that contained a sedigraph with 454 kg of soil. The basin’s sediment–capture ability was evaluated during four sediment removal/dewatering control treatments, each designed to dewater the basin in 24 hrs. Additionally, these small–basin sediment capture results were compared to those obtained from a 142–m 3 large basin previously charged with identical hydrographs and sedigraphs. The perforated riser treatment in the small basin discharged 37.5 kg of the injected 454 kg of soil through the combined principal and emergency spillways, in comparison to 31.8 kg discharged from the large basin, where only the principal spillway was used. The skimmer treatment in the small basin discharged 26.4 kg of the injected 454 kg of soil through the combined principal and emergency spillways, in comparison to 15.0 kg discharged from the large basin. On average, the small basin discharged 150% more sediment than the large basin. The addition of trenched–in geotextile barriers oriented perpendicular to the primary flow direction in the basin did not cause a significant improvement in the capture efficiency of either basin or for either of the principal spillway configurations.