Nitrate and Ammonium Transformation and Fate in Groundwater of an Agricultural Watershed: An Isotope and Geochemistry Approach

• Main Author: Suchy, Martin
• Format: Others
• Language: en
• Published: University of Ottawa (Canada) 2013
• Subjects: Environmental Sciences.; Geochemistry.
• Online Access: http://hdl.handle.net/10393/28922; http://dx.doi.org/10.20381/ruor-13788

The purpose of this study was to characterize the groundwater recharge environment and evaluate the fate and processes affecting agricultural nutrients in groundwater. This consisted of three agricultural sites and one naturally vegetated background site. Naturally occurring stable isotopes (delta18O and delta 2H), which are sensitive to seasonality of recharge, were used in groundwater and precipitation to provide an estimate of recharge and mass loading to the aquifer. The significance of direct infiltration through cultivated corridors where corn is grown (C4 vegetation), versus naturally forested C3 vegetation, which is predominant in the headwater environment, delta 13C analysis of DIC and DOC were conducted. To assess geochemical reactions, isotopes of nitrate and ammonium (delta15N NO3, delta18ONO3 and delta15N NH4), were used to provide insights into the transformation and attenuation processes in the nitrogen cycle. Groundwater delta18O and delta2H results indicate that recharge occurs predominantly in late fall as photosynthetic transpiration declines and in late spring following spring thaw, when vegetation is inactive. The abundance of wormholes observed in an excavated test pit, suggests macro-porosity in the sediments aids the infiltration and contributes to the rapid transmission of the meteoric signal to the water table and thus accounting for the rapid shift in trends observed in the temporal monitoring in the overburden material. Observations of shallow groundwater suggest the presence and concentrations of NO3 and NH4 may be dependent on the type of fertilizer application. The presence and attenuation of nitrate and ammonium varied between the three agricultural sites. There was no one dominant attenuation process throughout, nor were nitrate and ammonium consistent in the unsaturated zone and groundwater. Findings indicate the urea only site has possibly longer retention of NH 4 in soil water, low concentrations of NH4 but the presence of moderately higher NO3 concentrations (up to 20 ppm-N) in shallow groundwater. The infiltration of nitrified NO3 appears to be followed by some level of denitrification, resulting in bedrock aquifer NO3 concentrations below 5 ppm-N. Observations from the agricultural site where liquid manure was the dominant fertilization method, suggest the chronic loading of NH4 in the subsurface. There is possibly less retention of NH 4 in soil water (> 20 ppm-N), resulting in the infiltration of NH 4 (2 ppm-N in piezometers), while NO3 concentrations in shallow groundwater are below detection. Lastly, the mixed application site shows the presence of both NO3 and NH4 at similar concentration in shallow groundwater, with nitrogen attenuation by dilution, denitrification and potentially anammox. Overall, irrespective of the type of fertilization, findings suggest that reactive losses of nitrogen together with crop uptake, greatly limit the total nitrogen flux that is reaching the underlying bedrock aquifer, indicating that agricultural activities do not have an excessive adverse impact on the aquifer. While total nitrate and ammonium concentrations of nitrogen in the unsaturated zone, as sampled in the lysimeters and tile drains reached over 30 ppm-N, concentrations in groundwater were typically <10 ppm-N, and most often <0.5 ppm-N in the bedrock aquifer. The attenuation of high ammonium concentrations in soil water was attributed to plant uptake prior to recharge to the water table, with minor loss by volatilization as groundwater 15N values exhibited slight enrichment. Geochemical and isotopic results from a couple domestic wells exhibited signs of leaking well casings/seals, thus permitting potentially contaminated surface and/or shallow groundwater to enter the well, thereby potentially impacting the groundwater resource. If the proportion of such wells is significant throughout the region, this represents a principle threat to water quality. (Abstract shortened by UMI.)