We quantified unsaturated flow beneath playa and adjacent interplaya settings at a site in the Southern High Plains (USA) to resolve issues related to where and how water moves through the unsaturated zone. This is the first study where the data density (39 boreholes) and the variety of techniques used (physical, chemical and isotopic) sufficient to quantify spatial variability in unsatured flow. Water contents, water potentials, and tritium concentrations were much higher and chloride concentrations were much lower beneath playas than in interplaya settings which indicated that playas focus recharge. These results refute previous hypotheses that playas act as evaporation pans or that recharge is restricted to the annular region around playas.Water fluxes estimated from environmental tracers ranged from 60 to 120 mm yr-1 beneath playas and were = 0.5 mm yr-1 beneath natural interplaya areas not subjected to ponding. To evaluate the apparent inconsistency between high recharge rates and thick clay sediments beneath playas, we applied bromide and FD&C blue dye to evaluate flow processes. These applied tracer experiments showed preferential flow of bromide and FD&C blue dye along roots and desiccation cracks through structured clays in the shallow subsurface in playas.

Conclusions
Unsaturated flow is focused beneath playas, as evidenced by high water contents, high water potentials, low chloride and high tritium concentrations in the pore water, high apparent electrical conductivities, and low carbonate contents in the sediments. Water potentials close to zero suggest drainage of water under unit gradient conditions. Low mean chloride concentrations (16 to 29 g m-3) indicate high water fluxes (60 to 100 mm yr-1), which prevent chloride accumulation or flush out previously accumulated chloride. High tritium concentrations to a 29-m depth indicate high water fluxes (~120 mm yr-1). In contrast to the playa setting, unsaturated water movement in interplaya regions not subject to ponding is negligible, as shown by low water contents, low minimum water potentials, high peak chloride concentrations in the pore water, low apparent electrical conductivities, and high carbonate contents in the sediments. Steep upward water potential gradients (=1.5 MPa m-1) in the top 10 m result in an upward driving force for liquid and isothermal vapor movement. At depths greater than ~10 m, water potential gradients are negligible, suggesting drainage of water. Water fluxes estimated from the chloride data were =0.5 mm yr-1 and indicate negligible water fluxes during the past 2,000 to 5,000 yr. Higher water fluxes before that time were =3 mm yr-1 at ~26-m depth.

Frequent ponding of water in playas and the highly structured nature of the clays predispose the system to preferential flow. Applied tracer experiments that included FD&C blue dye and bromide provided visual evidence of preferential flow along interpedal pores, roots, and desiccation cracks in playas. Additional evidence of deep preferential flow was provided by a multipeaked tritium profile beneath a playa.

The primary control on unsaturated flow is surface ponding of water, which occurs in playas and in ditches and which focuses recharge to the underlying aquifer. Macropores in the structured clay in playas allow rapid transport of contaminants through the system and bypass the buffering capacity of much of the unsaturated matrix. Structureless sand layers affect the vertical extent of preferred pathways and may provide a reservoir for volatile contaminants.

Reference
Scanlon, B. R., and Goldsmith, R. S., 1997, Field study of spatial variability in unsaturated flow beneath and adjacent to playas: Water Resources Research, v. 33, no. 10, pp. 2239-2252. [PDF]

February 2003