Nitrogen and Redox Speciation in Fresh-Water Systems
Figure 1: USDA research area (PDF, 1 pp., 101 KB, about PDF).
The United States Department of Agriculture Agricultural Research Service (USDA/ARS) owns, maintains and has instrumented several agricultural research watersheds at the
J. Phil Campbell, Sr., Natural Resource Conservation Center near Watkinsville, GA (Figure 1). At this site, we are collaborating with USDA/ARS researchers Dinku Endale and Steven Norris on several projects, and they grant us access to their site for several other studies. The nitrogen at the USDA site is elevated locally by waste from about 300 head of cattle and most of our work is centered around understanding the fate of this excess nitrogen. This work is being conducted in support of ERD's goal to develop predictive models for the transformation and fate of nutrients in the environment.
I. Kinetic Control of Oxidation State at Thermodynamically Buffered Potentials in Subsurface Waters:
Redox speciation in subsurface environments is not well understood. Near-equilibrium of selected redox couples has been shown for some environments such as in acidic mine drainage and the pores of marine-system floors, however, the limited data for the majority of terrestrial subsurface settings often are discrepant. In an effort to understand better the redox speciation of typical terrestrial subsurface environments, a field study has been initiated consisting of a monitoring program as well as one-time sampling at other locations.
Working with USDA/ARS researchers Dinku Endale and Steven Norris, we monitored an agricultural spring (Figure 2), for two years and sampled several other locations to develop our understanding of how best to characterize oxidation state and modes of control on oxidation state. These data were subjected to thermodynamic modeling to define concentration-rate-limited kinetics. Because nitrogen has several oxidation states, this work is highly relevant to our understanding of its environmental fate.
Figure 2: Spring SpW2 instrumented by USDA personnel with a flume for gauging flow and an Isco sampler for automated sampling. The cattle pasture is in the background. We sampled this spring monthly for about two years and analyzed these samples for several redox-sensitive species.
II. Forms of Nitrogen in Subsurface Waters:
In nitrogen-contaminated river systems, nitrate usually is the dominant species, but other species such as nitrous oxide, ammonium and organic nitrogen can be elevated as well. The biologic availability of nitrogen species is thought to vary among these species. For example, organic nitrogen has been reported to persist in river systems relative to nitrate.
About 50% of the flow in most rivers is discharged from groundwater systems. Consequently, the forms of nitrogen in groundwater affect speciation of impacted rivers. Yet there are few data on the forms on nitrogen in groundwater systems. We are quantifying nitrogen speciation is several subsurface settings and are modeling redox controls on this speciation.
Among interesting findings, we measured dinitrogen in water drawn from a well in a cattle pasture. After correcting for partitioning, we calculate a dinitrogen fugacity of 1.12 atm which is high compared to an atmospheric fugacity of 0.78 atm. Since 1.12 atm exceeds total atmospheric pressure, depths below the water table for which hydrostatic pressure is less than 1.12 atm are supersaturated with respect to dinitrogen. As such, dinitrogen might well be exsolving from solution at depths of <150 cm below the water table, where P(hydrostatic) < 1.12 atm if bubble-nucleation energy can be overcome. A manuscript reporting this work is in progress.
III. Controls on Denitrification in Wetland Settings:
Wetland areas commonly are recognized as sinks for excess nitrogen; however, the mode of these decreases is not well understood. Possible mechanisms for these nitrogen decreases in wetland systems include: 1) denitrification; 2) uptake by plants; and 3) dilution by low-nitrogen groundwater recharge to the wetland.
In collaboration with doctoral student Kathy Schroer and Professor Valentine Nzengung of University of Georgia, we are monitoring changes in speciation in a wetland, and performing tracer studies (Figures 3-5) to elucidate controls on nitrogen speciation at the wetland scale.
Figure 3: Dye injection at Spring SpW2. |
Figure 4: EPA, USDA, and UGA researchers monitor dye migration. |
Figure 5: Research team measures dye concentration with a portable field fluorometer. |
| IV. Controls on Nitrogen Speciation in Agricultural Streams and Ponds:
Headwater streams comprise a large fraction of flow and nitrogen load to larger, impacted river systems. In order to support future modeling efforts for calculating total maximum daily loads (TMDLs), we are working with Earl Hayter of ERD, Athens to document concentrations and forms of nitrogen in the stream/pond draining the USDA/ARS facility in Watkinsville, GA (Figure 6). Our work so far suggests that farm ponds can play a major role in reducing excess agricultural nitrogen. |
Figure 6: EPA researcher collects samples at Spring SpW2. |
Questions? Contact John Washington.