On the trail of nitrogen to quantify N removal from contaminated aquifers
In the early 20th century Fritz Haber developed a process to create reactive ammonia, which the chemical company BASF scaled up to industrial level production by 1910. To fuel the agricultural revolution, BASF established a chemical industry in Leuna thanks to syngas sources needed to make the nitrogen fertilizers. The site was rapidly expanded, becoming one of the biggest chemical industrial complexes in Germany in the last century. However, there's another side to this story. The spills, accidental discharges, etc … from the industry in Leuna have persisted over the last.
Figure 1. Leuna industrial area, photographed October 2013, Naomi S. Wells
Unfortunately, this problem is not unique to Leuna: EU states have over 100.000 groundwater sites that have been found to be too contaminated for human consumption. In order to make sure that the measures taken to prevent the spread of contamination into adjacent waterways, it's important to understand both the biological and the hydrological factors that control its spread.
How can we solve it?
The microorganisms living in the soil and groundwater are capable to remove nitrogen pollution (known as natural attenuation). However, it's difficult to measure the rates that these processes are happening in groundwater. For instance, a measured concentration decrease could also be caused by rainfall (dilution) or mixing of different source plumes below ground, this means that more information is needed in order for measurements to determine whether or not these microorganisms actually did something.
In the nature, the ammonia molecule undergoes many different transformations changing from one form to another as illustrated on the figure (N-cycle).
The major transformations of nitrogen are nitrification (yellow area) and denitrification (green area), while new evidence shows that anaerobic ammonium oxidation (anammox; pink area), and dissimilatory reduction of nitrate to ammonium (DNRA), nitrifier-denitrification, co-denitrification (not shown) can play important roles under certain conditions. Environmental conditions dictate which processes are energetically favourable for microbes to perform
How to quantify N removal from contaminated aquifers?
Dr. Naomi S. Wells is an experienced researcher on ADVOCATE Project working on the quantification the importance of in situ nitrogen cycling for the remediation of contaminated groundwater megasites. She is developing "isoflux" type models to improve estimations of nitrogen loss pathways and rates within complex contaminated aquifers.
Addressing this question, to quantify N removal, there is a promising avenue: the use of multiple N isotopes and the detection of microbial populations for developing sensitive indicators of in situ transformations. This includes measuring the isotopic composition of both oxygen and nitrogen on NO3-(δ18O-NO3-and δ15N-NO3-); as well as newer techniques to measure the isotopic composition of NO2-(δ15N-NO2-and δ18O-NO2-) and ammonium (δ15N-NH4+). Variations in all of these species are being used to identify the N removal hotspots that would be missed by measuring only NO3-isotopes and the isotopic composition of ammonium.
Naomi Wells and her colleagues from the department Catchment Hydrology at the UFZ are carrying out a study on site, where they are measuring the distribution of all N isotopes across the aquifer in Leuna, and analysing how these change in conjunction with concentrations over time (see below figure).
Figure 2. Caption: water samples collected from the Leuna site being prepared for isotopic analysis. Note the distinct colours of samples from various locations across the contaminant plume! (Photo credits: Naomi S. Wells)
Preliminary results revealed a seasonal development of N attenuation hotspots along the plume fringe. The broad correlation of these hotspots with redox transition zones and changes in key microbial populations showed that N removal in groundwater may be much more variable than has traditionally been assumed. And, while coupled nitrification and denitrification did seem to dominate the biological removal of ammonium from Leuna, at least two hotspots of anammox activity were identified within the contaminant plume.
To learn more about Naomi Wells's expertise here are her latest papers
Wells, N., Baisden, W., & Clough, T. (2015). Ammonia volatilisation is not the dominant factor in determining the soil nitrate isotopic composition of pasture systems Agriculture, Ecosystems & Environment, 199 , 290-300 DOI:10.1016/j.agee.2014.10.001
Wells, N., Clough, T., Johnson-Beebout, S., & Buresh, R. (2014). Land management between crops affects soil inorganic nitrogen balance in a tropical rice system Nutrient Cycling in Agroecosystems, 100 (3), 315-332 DOI:10.1007/s10705-014-9644-7
Wells, N., Clough, T., Condron, L., Baisden, W., Harding, J., Dong, Y., Lewis, G., & Lear, G. (2013). Biogeochemistry and community ecology in a spring-fed urban river following a major earthquake Environmental Pollution, 182 , 190-200 DOI:10.1016/j.envpol.2013.07.017