Controls on the geochemistry of rare earth elements along a groundwater flow path in the Carrizo Sand aquifer, Texas, USA
Introduction
Rare earth elements (REE) are of interest to chemical hydrogeologists because of their potential as sensitive tracers for studying groundwater–aquifer rock interactions and, in some cases, for tracing groundwater flow (Smedley, 1991, Fee et al., 1992, Johannesson et al., 1997a, Johannesson et al., 1997b, Dia et al., 2000, Möller et al., 2000). Because pH, redox conditions, and the major solute composition of groundwater change and evolve along groundwater flow paths owing to chemical weathering reactions and microbial processes occurring in aquifers (e.g., Champ et al., 1979, Hamlin, 1988, Edmunds et al., 2003), it is expected that groundwater REE concentrations, their relative distributions across the lanthanide series (i.e., fractionation), and their speciation will reflect these biogeochemical changes in aquifer conditions. Although, the REEs are known to be strongly controlled directly or indirectly by pH, redox conditions, and aqueous and surface complexation reactions (Goldstein and Jacobsen, 1988, German and Elderfield, 1989, Dia et al., 2000, Tang and Johannesson, 2003, Tang and Johannesson, in press, Quinn et al., 2004), much less effort has focused on how changing pH, redox conditions, and solution composition in aquifer systems affects dissolved REE concentrations and fractionation (Johannesson et al., 1999, Duncan and Shaw, 2003). Furthermore, remarkably little is known about the biogeochemical processes that affect REE concentrations and fractionation along groundwater flow paths. Some investigators, for example, have argued that groundwaters obtain aquifer rock-like REE fractionation patterns towards the end of flow paths and, hence, proximal to discharge zones (Banner et al., 1989). Others suggest, however, that groundwaters inherit aquifer rock-like signatures in the recharge zones, and that with subsequent flow down-gradient, additional chemical weathering, adsorption, and solution complexation reactions modify groundwater REE concentrations and fractionation patterns (Johannesson et al., 1999, Johannesson et al., 2005, Tang and Johannesson, 2005). As redox conditions are known to strongly influence REE cycling in the marine environment (Moffett, 1990, German et al., 1991, De Carlo et al., 1998), it is reasonable to expect that changing redox conditions along groundwater flow paths may be accompanied by profound changes in REE concentrations and fractionation patterns (e.g., Leybourne et al., 2000). Microbial metabolism is chiefly responsible for the general decrease in oxidation–reduction potential reported to occur along groundwater path in aquifer systems containing sufficient organic matter (Champ et al., 1979, Lovley and Goodwin, 1988, Chapelle and Lovley, 1990). Consequently, with careful measurement of the appropriate ancillary geochemical measurements, groundwater–aquifer flow systems may provide a unique environment to study microbially mediated biogeochemical cycling of REEs. In this study, the geochemical behavior of REEs along the groundwater flow path in the Carrizo Sand aquifer of southern Texas is described and possible controls on their geochemical behavior are discussed.
Section snippets
Hydrogeology of the Carrizo sand aquifer
The Carrizo Sand aquifer crops out in a band that nearly parallels the Gulf Coast in southeastern Texas (Fig. 1). The aquifer dips towards the southeast and contains fresh water at depths as great as 1500 m (Pearson and White, 1967). In the study area (i.e., Atascosa and McMullen Counties, Texas), average annual precipitation ranges from 46 to 89 cm and average annual temperature ranges from 21° to 23 °C (Hamlin, 1988). The composition of the chiefly unconsolidated, Eocene Carrizo Sand is
Field sampling and field parameter measurement
All high density linear polyethylene (HDPE) sample bottles were cleaned using trace metal clean techniques before being transported to the field site (see Johannesson et al., 2004 for details). Groundwater samples were collected from wells along the groundwater flow path in the Carrizo Sand aquifer in Texas (October 2002 and June 2003) for analysis of REEs, major solutes, and dissolved organic carbon (DOC). Alkalinity was titrated in the field during these field campaigns using a digital
Groundwater composition along the flow path
Field measurements (i.e. pH, alkalinity, temperature, conductivity, dissolved oxygen, Eh, iron speciation, and H2S concentrations), DOC, major solute compositions, and the redox-sensitive trace elements (i.e., U and Re) for groundwaters from the Carrizo Sand aquifer are presented in Table 1. The concentrations of REEs in groundwaters from the Carrizo Sand aquifer are presented in Table 2. Our data indicate that groundwaters from the Carrizo Sand aquifer are chiefly Ca–Na–HCO3–Cl type waters at,
Redox conditions along the flow path
Geochemical analyses of groundwaters from the Carrizo Sand aquifer demonstrate that the redox conditions change along the flow path. The changing redox conditions are clearly evident in the fluctuating Fe species concentrations, dissolved H2S concentrations, and the concentrations of redox-sensitive trace elements, U and Re (Table 1; Fig. 2). It should be noted that U and Re behave conservatively in oxygenated water, occurring as the uranyl oxycation (UO22+), which forms stable carbonate or
Conclusions
Redox conditions change along the flow path in the Carrizo Sand aquifer. Eh and other redox indicators (i.e., DO, Fe concentrations and speciation, H2S, and redox-sensitive trace elements U and Re) indicate that in the region of the aquifer proximal to the recharge zone, groundwaters exhibit fluctuating redox conditions (i.e., oxic with localized, more reducing, i.e., “suboxic” zones). However, from roughly 10 km down-flow, and thereafter, groundwaters are reducing and exhibit a progressive
Acknowledgements
The authors are especially thankful to Dr. L.D. James, Hydrologic Sciences program director at NSF for making the study possible as part of NSF grant EAR-0303761 to K.H. Johannesson. The authors would like to express gratitude to Dr. M.I. Leybourne at the University of Texas at Dallas for use of his ICP-OES and ICP-MS for some of these analyses, Dr. D.J. Burdige at Old Dominion University for DOC analyses, Dr. A. Kruzic at the University of Texas at Arlington for use of the IC, and Drs. Z. Chen
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