HR: 17:05h
AN: H42K-05    [PDF]
TI: Chemical Evolution of Riparian Groundwater and Stream Baseflow at the Panola Mountain Research Watershed, Georgia, USA
AU: * Burns, D A
EM: daburns@usgs.gov
AF: U.S. Geological Survey, 425 Jordan Rd., Troy, NY 12180 United States
AU: Plummer, L N
EM: nplummer@usgs.gov
AF: U.S. Geological Survey, 12201 Sunrise Valley Dr., Reston, VA 20192 United States
AU: McDonnell, J J
EM: Jeff.McDonnell@orst.edu
AF: Oregon State University, Dept. of Forest Engineering Peavy Hall, Corvallis, OR 97331 United States
AU: Peters, N E
EM: nepeters@usgs.gov
AF: U.S. Geological Survey, 3039 Amwiler Dr., Atlanta, GA 30360 United States
AB: Changes in groundwater chemistry along a downstream riparian transect of 19 shallow wells at the Panola Mountain Research Watershed near Atlanta, Georgia are consistent with the release of base cations and silica from the weathering of plagioclase feldspar (in grandiorite bedrock) to kaolinite with minor contributions from the weathering of other silicates and calcite. In contrast, spatial patterns of groundwater chemistry on adjacent hillslopes do not follow a similar pattern and may be controlled instead by the soil-flushing frequency. Concentrations of SiO$_{2}$, Na$^{+}$, and Ca$^{2+}$ in riparian groundwater generally increased down valley, and were highest in a borehole finished in bedrock near the outlet, whereas SO$_{4}$$^{2-}$ concentrations originating from atmospheric deposition decreased down valley. This is consistent with the absence of a bedrock source of S and with removal of SO$_{4}$$^{2-}$ through adsorption and reduction processes. Strong positive correlations were found between the concentrations of these solutes and groundwater age as indicated by CFC and $^{3}$H/$^{3}$He data. The apparent age of groundwater was modern (0 to 1 yr) in the headwaters, 6 to 7 yrs midway down the valley, and 26 to 27 ys in the borehole near the 41 ha catchment outlet. Stream water age was estimated from regression relations between groundwater chemistry and age to be 1.5 years at a 10-ha subcatchment streamflow gage, and 4.5 years at the catchment outlet. A weathering rate for plagioclase feldspar of 6.4 $\mu$mol L$^{-1}$ yr$^{-1}$ was calculated trhough mass-balance geochemical modeling and the age estimates. The mineral-surface area in contact with groundwater during transport down valley was then estimated, and the resulting weathering rate per unit of mineral surface area was calculated. The result was similar to that of a previous study at this site and 3 to 4 orders of magnitude slower than those published in previous laboratory studies of feldspar weathering. The results of this study indicate that an accurate model of the chemical evolution of stream baseflow can be obtained from a fairly simple mass-balance model combined with riparian-groundwater age dating. Spatial patterns of ephemeral groundwater chemistry from hillslopes adjacent to the riparian aquifer are more complex and not easily modeled by a similar approach.
DE: 1010 Chemical evolution
DE: 1045 Low-temperature geochemistry
DE: 1832 Groundwater transport
DE: 1871 Surface water quality
DE: 1886 Weathering (1625)
SC: Hydrology [H]
MN: 2003 Fall Meeting