Anion Exchange in Saprolite

soil using Cl as a tracer in a 5 mM solution of CaCl2. The AEC ranged from 0.01 to 1.86 cmolc kg 1 with an Many studies have shown that subsoils in the Piedmont region of average value of 0.68 cmolc kg 1. Retardation coeffithe southeastern USA have anion exchange capacity (AEC) and can adsorb anions such as nitrate (NO 3 ). Groundwater discharging to cients were determined from the BTC data and Kd valwells and streams often passes through thick layers of saprolite beues were calculated using the bulk densities and water neath Piedmont soils, but little is known about the AEC of saprolite. contents of the columns. Adsorption coefficients ranged Our objective was to determine the AEC on saprolite samples colfrom 0.01 to 2.43 cm3 g 1 with an average value of 0.69 lected with a split-spoon sampler during the installation of seven cm3 g 1. Retardation coefficients ranged from 1.1 to 6.2 groundwater monitoring wells in north Georgia. Forty four samples with an average value of 2.5. For two Piedmont subsoils, were collected from depths ranging from approximately one to 15 m the values of R were 3.1 and 4.8. They found a positive, below the surface and analyzed for AEC, extractable iron, and pH. linear relationship between Kd and AEC (r 2 0.92) The saprolite thickness was at least 15 m at all seven sites as verified and between R and AEC (r 2 0.93). by drilling an adjacent 7.6-cm hole to bedrock. The average AEC for Piedmont region soils are commonly underlain by 37 saprolite samples was 0.80 cmolc kg 1. This was slightly less than half of the AEC observed in typical subsoil horizons of Piedmont thick layers of saprolite, defined as isovolumetrically soils. There was a significant positive correlation (r 0.64) between weathered bedrock (Pavich et al., 1989). Saprolite forms saprolite AEC and iron oxide content. Taking into account the differas the more soluble minerals weather from bedrock ences in thickness of the soil and saprolite, saprolite had about six leaving behind a porous, brittle material that looks like times the AEC of subsoil. The expected adsorption coefficient (Kd) the parent rock, but can be dug using hand tools. About for NO 3 in saprolite was approximately 0.81 cm3 g 1, based on the 50% of the mapped soils in the Piedmont are underlain relationship between AEC and Kd. Using an estimated bulk density by light-colored (felsic) igneous and metamorphic rocks. and saturated water content based on texture, this corresponds to a Saprolite that forms from felsic rock is typically 15 to retardation coefficient (R ) of 4.4. Models and remediation efforts 30 m in thickness (Pavich et al., 1989). Common soil seinvolving NO 3 in shallow Piedmont groundwater need to account for ries formed from felsic parent material are Appling, anion adsorption in saprolite. Cecil, Helena, Louisa, Louisburg, Madison, Pacolet, Tallapoosa, Vance, and Wedowee (http://soils.usda.gov/ technical/classification/osd/index.html; verified 20 Apr. M studies have shown that acid subsoils of the 2004). Ultisol, Oxisol, acid Alfisol, Andisol, and InceptiThe shallow groundwater system in the Piedmont is sol orders exhibit anion adsorption (Berg and Thomas, an unconfined, two-layer aquifer composed of a zone 1959; Sumner and Davidtz, 1965; Kinjo and Pratt, 1971; of saprolite underlain by fractured bedrock (Fig. 1). McMahon and Thomas, 1974; Black and Waring, 1976a, Studies have shown that the saprolite layer is an integral 1976b, 1976c; Chan et al., 1980; Nkedi-Kizza et al., 1984; part of the groundwater system and acts as an important Wong et al., 1987; Boggs and Adams, 1992; Ishiguro et water storage zone for the deeper fractures (Brackett al., 1992; Katou et al., 1996). Ultisols are common in the et al., 1991, p. 1–160; Cressler et al., 1983; Radtke et al., Piedmont region of the southeastern USA, and Grove et 1986; Rose, 1992). The depth to the water table may be al. (1982), Gillman and Sumner (1987), Bellini et al. as shallow as 2 to 6 m in upland areas during the winter (1996), and Qafoku et al. (1999) have shown that the and the water table elevation generally follows the suranion exchange capacity (AEC) of the unlimed B horiface topography (LeGrand, 1988). Two types of wells zons of soils from this region are typically in the range are common in this region: shallow bored wells that are of 1 to 2 cmolc kg 1. installed to bedrock and draw directly from the saprolite Retardation coefficients in these soils for monovalent storage layer and deep drilled wells that penetrate the anions such as NO 3 and Cl are typically 1.5 to 2.5 for bedrock and draw indirectly from the saprolite layer solution concentrations less than 30 mmolc L 1 (Bellini via bedrock fractures. Recharge occurs throughout the et al., 1996; Gupte et al., 1996). Qafoku et al. (1999) uplands in this shallow groundwater system and travel studied 16 subsoils from the southeastern USA, South times to streams and wells may be quite long. Rose Africa, Australia, Indonesia, Japan, and Hawaii with (1992) measured tritium concentrations in water colvariable charge mineralogy. They measured AEC and lected from Piedmont wells screened in the saprolite breakthrough curves (BTCs) on packed columns of sublayer, streams sampled during baseflow (which would represent discharging groundwater), and springs. He K.A. McVay, Dep. of Agronomy, Kansas State Univ., Manhattan, KS 66506; and D.E. Radcliffe, L.T. West, and M.L. Cabrera, 3111 estimated that at least some of this water was as old as Miller Plant Sciences Building, Univ. of Georgia, Athens, GA 30602. 25 yr. Received 2 July 2003. Original Research Paper. *Corresponding auEvidence suggests that significant amounts of water thor ([email protected]). Abbreviations: AEC, anion exchange capacity; BTCs, breakthrough Published in Vadose Zone Journal 3:668–675 (2004).  Soil Science Society of America curves; CV, coefficient of variation; DCB, dithionite–citrate– bicarbonate. 677 S. Segoe Rd., Madison, WI 53711 USA