Spatial Variability in Unsaturated Zone Flow...and Implications for Contaminant Transport, Southern High Plains, Texas

Analysis of spatial variability in unsaturated flow is critical to evaluating contaminant transport in areas of focused or preferential flow. This study examined spatial variability at a variety of scales from focused flow through playas to preferential flow in macropores. A total of 39 boreholes from 7 playa and adjacent interplaya settings on the Southern High Plains of Texas were sampled for hydraulic and hydrochemical parameters. To evaluate preferential flow in surficial sediments, we applied bromide and organic dyes. Results of this study show that recharge is focused beneath playas, as evidenced by high water contents, high water potentials, low chloride and high tritium concentrations in pore water, and low carbonate contents in sediment. Water fluxes estimated from chloride and tritium data ranged from 60 to 120 mm yr-l. In addition to focused recharge beneath playas, applied tracer experiments showed preferential flow of bromide and FD&C blue dye along roots and desiccation cracks through structured clays in playas. A multipeaked tritium profile to 29-m depth also indicated preferential flow. Frequent ponding in playas and the structured nature of the clay sediments predispose this system to preferential flow. In contrast to playas, unsaturated water flux in natural interplaya settings under current climatic conditions is negligible, as indicated by low water contents, low minimum water potentials, upward water potential gradients, high peak chloride concentrations in pore water, and high carbonate contents in sediment. Upward water potential gradients suggest net upward water movement in the top 5 to 10 m of the sediment. Chloride profiles suggest very low water fluxes (10.1 mm yr-l) during the past 2,000 to 5,000 yr. Spatial focusing of recharge beneath playas and preferential flow through macropores greatly increase the transport rate of contaminants through the unsaturated zone.
Bridget R. Scanlon
Richard S. Goldsmith
William F. Mullican III

Scanlon, B.R., Goldsmith, R. S., and Mullican, W. F. III, 1997, Spatial Variability in Unsaturated Flow Beneath Playa and Adjacent Interplaya Settings and Implications for Contaminant Transport, Southern High Plains, Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 243, 68 p.

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The University of Texas at Austin, Bureau of Economic Geology
Report of Investigation

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