Tracer Studies of Solute Flow Pathways in Caliche and Related Clayey Soils of Central Texas

Caliche soils of Central Texas are currently undergoing considerable residential development. However, little information is available on solute and water transport through these soils, or their suitability for wastewater disposal and bioremediation.

Brilliant Blue FCF and Br^- tracers were used to characterize water and solute pathways through caliche and related clayey soils. Brilliant Blue is believed to be a better proxy for movement of larger organic molecules, while Br^- is considered a better tracer of water transport in soil. Tracers were applied simultaneously to surface and subsurface horizons of 8 selected sites. Vertical and horizontal sections were excavated through each application site, resulting in 20 vertical and 14 horizontal sections. Bromide alone was also applied to 12 sites, producing 27 vertical sections. Brilliant Blue pathways were photographed, followed by spraying a chemical indicator and photographing Br^- pathways. Stained areas were then determined by image analysis.

In most cases, vertical penetration of Br^- was deeper than Brilliant Blue (1.0 m and 0.7 m, respectively). Cemented Bk, Bkm, Cr and R horizons acted as aquitards, resulting in considerable lateral movement of both tracers beyond the direct infiltration zone. On average, Br- moved to the lateral distances of 0.3 m, while Brilliant Blue moved to about 0.2 m beyond the application area.

While Brilliant Blue demonstrated preferential flow along macropores, Br^- moved throughout the soil matrix in “piston” flow pattern. Bromide penetrated into hard limestone fragments to the extent of several centimeters, while many soft limestone fragments were completely stained by Br- indicator. This suggests that significant volume of limestone rock clasts, together with soil, was involved in water transport and thus contributed to soil ability to serve as bioremediation and moisture storage media.