Nitrate removal in riparian soils and groundwater: Field and laboratory studies

Soil-vegetation systems of riparian zones can impact the concentrations and affect the transport of nonpoint source pollutants found in the shallow groundwater flowing from agricultural land to surface waters of the Chesapeake Bay. Vegetation type, soil characteristics, nitrate concentration, and hydrologic flow patterns control the effectiveness of the riparian zones as contaminant buffer areas. Clarifying the interactive roles between these topographic and vegetative influences was achieved in a field study by imposing three vegetative treatments on riparian zones dominated by either leguminous or non-leguminous trees on Wye Island, Maryland: (1) all vegetation was removed and tall fescue (Festuca arundinacea Schreb.) was planted, (2) only trees were removed, and (3) indigenous plant communities were left intact as the control. Nitrate concentration was measured in the groundwater beneath each vegetative treatment during a 4 year period. The non-leguminous, control riparian zones were found to have lower NO{dollar}sb3sp-{dollar} concentrations than did the fescue treatments. The decrease in NO{dollar}sb3sp-{dollar} concentration was inversely related to the depth to groundwater. In contrast, the black locust trees (Robinia pseudoacacia L.) in the leguminous riparian zone contributed to the NO{dollar}sb3sp-{dollar} in the shallow groundwater, and cutting the trees lowered its concentration. Because insufficient C concentrations were detected in the groundwater to support a totally microbial mediated N-reduction mechanism for the non-leguminous treatments, laboratory experiments were conducted to identify alternative chemical, or abiotic, reduction pathways. Reduction of NO{dollar}sb3sp-{dollar} to NO{dollar}sb2sp-{dollar} was shown to occur in suspensions of synthetic manganese (III,IV) and iron (III) (hydr)oxides under aerobic and anaerobic atmospheric conditions after 24 h equilibration at pH 3.5, 5.5, and 7.0. Thermodynamic predictions showed that the abiotic reduction pathways for NO{dollar}sb3sp-{dollar} reduction are possible. Five B horizon subsoils of Maryland reduced NO{dollar}sb3sp-{dollar} to NO{dollar}sb2sp-{dollar} abiotically during a 1 h equilibration time. Because NO{dollar}sb2sp-{dollar} was easily formed by the oxides and soils, the reactivity of NO{dollar}sb2sp-{dollar} became the focus of the final laboratory research. Natural organic analogues of soil organic matter, hydroquinone and catechuic acid, were reacted with oxide suspensions with known NO{dollar}sb2sp-{dollar} concentrations. Fourier transform infrared spectroscopy identified the formation of N-substituted aromatic species. This nitration reaction that formed nitroaromatic compounds occurs photochemically in the atmosphere using sun light as the energy source for the formation of the essential free radical species. In soil systems, the required free radical formation is through the redox mechanisms of the mineral oxides. Therefore, the (hydr)oxides are the likely substrate that controls both the inorganic NO{dollar}sb3sp-{dollar} reduction, and the generation of the organic free radical precursors to the nitration reaction. The demonstration that abiotic and nitration mechanisms can transform NO{dollar}sb3sp-{dollar} into compounds not yet detected in natural water uncovers a new area of NO{dollar}sb3sp-{dollar} research.