East Branch Pecatonica River
Stream & Floodplain Biogeochemistry
Occurrence and generation of nitrite in ground and surface waters in an agricultural watershed
Sponsored by the Wisconsin Groundwater Coordination Council and Wisconsin Water Resources Institute
Streams throughout southern and central Wisconsin carry the signal of extensive agricultural activity in the form of high nutrient concentrations. In past studies, we have routinely found high concentrations of nitrogen (N) in stream water, mostly in the form of nitrate (NO3-). We also found that these N-rich streams contain low levels of nitrite (NO2-), a form of N that is an intermediary in several biological reactions, as well as being a form of N that can be toxic at relatively low concentrations (Stanley and Maxted 2008). Thus, my current work at the East Branch Pecatonica River is intended to identify the locations and the processes that are likely to be responsible for generating NO2- in stream water. This has involved measuring concentrations of different forms of nitrogen in groundwater and streamwater stream reaches before and after restoration and conducting a series of experiments to measure the environmental factors that favor NO2- production. Preliminary results indicate that prior to discharging into the stream channel, groundwater is NO2- poor indicating that NO2- is likely generated within the channel. Further, laboratory experiments point to reduction of NO3- within sediments as the likely place and process responsible for NO2- formation. This summer, we will be investigating how this NO2- formation varies among sediment type (e.g., fine silt vs. sand) to test the hypothesis that most NO2- is generated in fine, organic-rich silts that are abundant in unrestored reaches. If this hypothesis is supported, it would suggest that the restoration activities at the East Branch should minimize the production of this potent nitrogen byproduct.
Prior to restoration, woody debris from the riparian zone in the East Branch Pecatonica was abundant in the channel (photo, left) and was extremely effective at trapping sediments. As a result, sediments in unrestored (“treatment”) reaches are finer-grained than in restored reaches, and the layer of sediments (measured as the depth a steel rod can be inserted into the sediments, or Depth to Refusal) is significantly greater in unrestored (“treatment” and “reference” reaches) reaches as well. Data from Lottig & Stanley (unpublished). We hypothesize that this thick layer of fine sediments is an ideal environment for nitrite generation in nitrogen-rich streams such as the Pecatonica.
Stanley, E.H. 2008. Potential sources of nitrite in southern Wisconsin agricultural streams. Annual Summer Meeting of the American Society of Limnology and Oceanography, St. John’s, Newfoundland.