Nutrient chemistry, transformation and release in riparian groundwater seep discharge during the final meter of subsurface transport, Minnesota, USA

摘要

Groundwater discharges into streams either directly through the stream bed or indirectly via bankside seeps. Literature re-ports have long indicated that prior to becoming surface water, riparian groundwater can undergo significant nitrate retention (LOWRANCE et al. 1984, PETERJOHN & CORRELL 1984, JACOBS & GILLIAM 1985, LOWRANCE 1992, HILL 1996), especially where shallow impermeable sediments or other aquicludes force it into biotically active riparian habitats (JACOBS & GILLIAM 1985, JORDAN et al. 1993, CEY et al. 1999, HILL et al. 2000). More re-cently, perfusion studies using intact sediment cores have indi-cated significant nutrient transformations, including coupled nitrification-denitrification during the subchannel discharge of groundwater (SHEВLЕY et al. 2003a). Numerical modeling of these transformations indicated that they can exert significant control on surface water chemistry (SHEIBLEY et al. 2003b). Equivalent nutrient transformations in bankside riparian seeps and their contribution to surface water remain little stud-ied. While riparian seep inputs are rare in most streams, they can be common in groundwater-dominated drainages. GENER-EUX et al. (2002) used mixing models to demonstrate that inter-basin transfer of regional groundwater could contribute sig-nificantly to surface water discharge and chemistry in lowland Costa Rica. At the same site, PRINGLE et al. (1990, 1993), PRIN-GLE & TRISKA (1991) and TRISKA et al. (2006) identified a zone of high soluble reactive phosphorus (SRP) increase in the Rio Salto in Costa Rica based on its presence in adjacent seeps and springs. These seeps and springs emerged along a gradient break and boundary between 2 historic lava flows. We exam-ined nutrient contribution and seasonal nutrient chemistry of eleven riparian bank seeps for 2 years along a 600-m reach of a groundwater dominated stream in north-central Minnesota, U.S.A. Our objective was to determine riparian seep contribu-tion to stream discharge and to identify nutrient transforma-tions as groundwater emerged at the seep surface during the final meter of subsurface transport.