Tuesday, June 5, 2007
432

Nutrient chemistry and transformation during the final meter of groundwater discharge in seeps adjacent to a groundwater-dominated stream

Frank J. Triska1, John H. Duff1, Alan P. Jackman2, and Ronald J. Avanzino1. (1) Water Resources MS 439, US Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025, (2) Dept of Chemical Engineering, University of California , Davis, Davis, CA 95616

Riparian groundwater chemistry typically differs from channel water because low velocity facilitates biogeochemical transformation.  To examine biogeochemical potential at higher subsurface velocities 11 continuously flowing bank seeps were sampled approximately seasonally 1996-98.  On 5 dates, seep surface chemistry was compared to 1 m depth (4 sites).  Seep chemistry fell into two groups by conductivity.  Group 1 conductivity averaged 474±53µS/cm2 (all sites, seasons) and represented of 10 of 11 seeps; Group 2 averaged 182±29 µS/cm2.  At 5 Group 1 seeps NH4 dominated DIN (NO3/NH4=<0.2) and 5 had higher NO3 (NO3/NH4=0.2-4.0).  Where NH4 dominated, DO was also low (~1mg/L).  Where NO3 was greater, DO was higher (1-4mg/L).  At the single Group 2 seep NO3 overwhelmingly dominated DIN (NO3/NH4=21.5) and DO was continuously high (7-9mg/L).  At the 4 seeps with surface-depth comparisons, 3 were in Group 1.  In the final meter DO and NO3 increased and NH4 decreased, likely from nitrification.  SRP decreased.  SRP decrease was related to ferrous Fe decrease.  At the Group 2 seep NO3 decreased over the final meter, indicating potential denitrification.  In both seep environments groundwater discharge was significantly altered chemically during the last meter of transport despite high flow relative to riparian groundwater.

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