1999 GSA Annual Meeting -- Denver, Colorado
1999 GSA Annual Meeting -- Denver, Colorado
Author(s): King, Lance J., and Breit, G.N., USGS, MS973, Denver, Colorado 80225, gbreit@usgs.gov; and Olsen, H.W., CSM, Golden, CO 80401.
Keywords: conductivity, smectite, alluvium, dispersion,
Substantial reductions in the hydraulic conductivity (HC) of sediment can result from dispersal of clay by the sequential flushing of saline, sodium-rich water and dilute water. This effect was studied experimentally on recent alluvium of the Canadian River near Norman, Oklahoma. Alluvium at this site is fine to medium grained sand with minor mud layers. Most sand contains <6 wt.% clay. Five separate core samples of sand were each sequentially flushed in a triaxial cell with 1) simulated local ground water, 2) 0.1N NaCl, 3) 1N NaCl, and 3) either local ground water or 0.01N NaCl followed by deionized water. Flow through the sand approximated the hydrologic gradient at the study site. Experiments lasted from 2 to 12 weeks and hydrostatic pressure across the samples was monitored continuously. Effluent solutions were analyzed to determine ion concentrations, pH, and the amount and composition of suspended solids. Hydraulic conductivity decreased substantially in experiments in which at least 15 pore volumes of 1N NaCl passed through the sample. The maximum measured decrease in HC was from 4.38 x 10-3 cm/sec to 2.05 x 10-5 cm/sec. Onset of decreased HC corresponded to effluent ionic strengths <0.01N NaCl. The amounts of suspended solids increased coincident with the initial drop in HC; maximum values of suspended solids ranged from 0.04 to 1.2 g/L. Suspended solids include smectite, illite-smectite, chlorite, kaolinite and quartz. The increased double-layer thickness and swelling of the smectite particles disrupt the fine particle aggregates resulting in the dispersion of all fine particles. Clay-bound exchangeable cations measured at the end of the experiments are mainly Ca and Mg rather than Na. The replacement of exchangable Na is attributed to the dissolution of carbonate mineral grain coatings. Continued decline of HC at the termination of the experiments suggests that fine particles continue to relocate independent of the amount of sodium on exchange sites. The observed decreases in HC demonstrate the significance of clay dispersion to aquifer properties, remediation, and migration of particle bound contaminants.
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