Citation:

SCHROER, K. L., R. C. THOMAS, D. M. ENDALE, J. W. WASHINGTON, AND V. NZENGUNG. USE OF TRACER INJECTION EXPERIMENTS TO QUANTIFY NITRATE LOSS IN TWO ADJACENT WETLAND STREAMS DRAINING AN AGRICULTURAL FIELD IN THE GEORGIA PIEDMONT. Presented at 2006 USDA-CSREES National Water Conference, San Antonio, TX, February 05 - 09, 2006.

Impact/Purpose:

Improve the scientific understanding of the processes controlling nutrient distributions in surface waters. Produce a suite of enhanced models for characterizing nutrient distributions in surface waters by incorporating improved process understanding in existing models (e.g., WASP), by developing new models (e.g., WHAM, reactive transport), and improving linkages between model components.

Description:

This study investigated the extent to which nitrate was removed from and/or stored in a small wetland depression downgradient of a 10-ha cattle rotational grazing pasture and a 2.5-ha cropped catchment at the USDA-ARS J. Phil Campbell Sr. Natural Resource Conservation Center in Watkinsville, GA. A stream originates at a perennial spring at the base of the pasture and is protected from surface runoff by a berm. An adjacent stream is open to surface runoff from part of the pasture and receives shallow groundwater from the cropped catchment and part of the pasture but does not have a discrete perennial spring. The two streams join approximately 60m downstream. Background NO3--N concentrations in the two stream channels generally decrease from 0.43-0.86mM (6-12mg/L) at the source to 0.04-0.36mM (0.5-5mg/L) at their confluence, with more loss observed along the runoff stream. Daily load calculations and natural chloride concentrations indicate that dilution by ground water flux does not fully account for nitrate loss. We performed four seasonal stream tracer injection experiments in the spring-fed channel and one in the run-off channel. We injected nitrate and bromide at constant rates for 30 minutes and sampled intensively at several sampling locations throughout the day until field measurements of [NO3-] reached pre-injection background levels. In addition, at one control station in each stream, we sampled several times throughout the rising and falling [NO3-] limb for T, pH, DO, SpC, dissolved organic C, Fe (II, III and total), NH4+, and the dissolved gases N2O, CH4, CO2 and H2.

Results show slightly lower percent recoveries for nitrate on the falling time/concentration curves at downstream locations, which indicates non-conservative behavior of nitrate. Stable isotope analysis may be needed to distinguish between denitrification and uptake by plants.

Record Details: