Origin of Dissolved Groundwater Sulphate in Coastal Plain Sediments of the Rio de la Plata, Eastern Argentina |
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| Authors: | Williams S Logan Ronald V Nicholson |
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| Institution: | (1) Department of Geology, George Washington University, Washington, DC, 20052, USA;(2) Department of Earth Sciences, The University of Waterloo, Waterloo, ON, Canada, N2L 3G1 |
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| Abstract: | Groundwater of the coastal plain of the Rio de La Plata, Argentina,contains up to 17 g L-1 SO4 and 37 gL-1 TDS. Some of this SO4 is from paleo-seawater intrusion; however, SO4 : Cl ratios can be>2 : 1, and most of the SO4 must, therefore, have another source. Three possible sources were investigated: gypsum, organic matter,and iron sulphides.
Dissolved SO4 showed δ34S valuesfrom -7 to 0‰, typical values for S from iron sulphides or organicmatter, but distinct from that of seawater (+22‰).
To test whetherthe SO4 was derived from oxidation of reduced S, four 4-mcores were taken from marine sediments of the coastal plain. Two were takenfrom
higher, drier areas where the highest dissolved SO4values were encountered, and two were taken from lower, wetter areas thathad much lower SO4 concentrations. Pore waterSO4, Cl and alkalinity were determined; solids were analyzedfor SO4, sulphide-S and organic-S.
Sulphide-S was the dominant form of reduced S, averaging about0.5% S in the lower interval (2.5–4 m) of the cores. Sulphidewas
absent in the upper 2.5 m in both topographically higher and lowerareas. Sulphate was present in the entire unit in the higher,
drier areas,but almost absent in lower areas. Organic-S was insignificant.
Our model for the origin of dissolved SO4 is: fine-grainedpyrite was oxidized during hotter or drier periods. Some resulting dissolvedSO4 was precipitated as gypsum. Iron from the pyriteprecipitated as FeOOH. Lower, wetter areas formed over time where recharginggroundwater
dissolved most of the gypsum. In higher areas with low hydraulicgradients and high net evapotranspiration, SO4 remained asgypsum and in the dissolved phase.
This revised version was published online in July 2006 with corrections to the Cover Date. |
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| Keywords: | Rio de la Plata La Plata Argentina Postpampeano sedimentary pyrite oxidation sulfur cycling coastal plain sulfur isotopes oxygen isotopes |
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