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Presented at the NABS Annual meeting, Vancouver, British Columbia, 2004
in Biogeochemistry 3
Nitrate mass balance in a karstic, groundwater-dominated stream enriched with agricultural nitrate.
F.J. Triska1, A.P. Jackman2, J.H. Duff1, J. Tesoriero3, and R.J. Avanzino1. 1U.S. Geological Survey, Water Resources, 345 Middlefield Rd. MS 439, Menlo Park CA 94025, 2University of California, Dept of Chemical Engineering, Davis CA 95616, 3U. S. Geological Survey, Water Resources, 3916 Sunset Ridge Road, Raleigh, NC 27607
A base flow nitrate mass balance was determined along a 5 km headwater reach of Beaver Cr. in southeastern MN. Bromide injections determined discharge and estimated groundwater input. Separate rhodamine WT injections estimated tributary inputs. Nitrate was determined at stream stations (5/day), in springs, tributaries and in sub-channel pore water. Discharge increased by 115% (265 L/s) over the reach, mostly in upper sub-reaches. Tributary input was 86.5 L/s and subsurface input 169.9 L/s. Nitrate concentration was high in two large springs (3.32 mg-N/L), and nearly identical to upstream surface water (3.34 mg-N/L). Using spring water to represent karst groundwater nitrate, actual N-load increase (423 mg-N/s) was 295.5 mg-N/s less than predicted. Assuming that sub-channel concentration difference between karst groundwater and sub-channel pore water represents riparian retention, 247.9 mg-N/s (35%) was retained in the riparian zone, and 47.6 mg-N/s (7%) was retained during channel transport. The large role of riparian retention illustrates its importance in N-cycling potential lost by tiling agricultural fields and bypassing riparian areas.
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