Solute transport and chemical weathering in a tropical forest saprolite, Puerto Rico

¹Dept. of Geosciences, Penn State University, University Park, PA 16802, USA

²U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA, 94025-3591, USA

³U.S. Geological Survey, GSA Center, Suite 400-15, 651 Federal Drive, Guaynabo, Puerto Rico, 00965-5703, USA

Abstract

Fluxes of chemical weathering products from the Río Icacos watershed, Puerto Rico, are among the highest documented in the world. The extremely high fluxes have been attributed to the high average annual temperature (22° C) and high annual precipitation (4300 mm) in the watershed. To compare weathering rates in the Rio Icacos with those from similar watersheds, it is desirable to estimate the rates of individual reactions. In order to quantify solute movement in the saprolite, a pair of LiBr tracer tests have been conducted: one on a relatively flat ridgetop where vertical flow is dominant, and one on a characteristically steep slope. Soils at both sites are 0.5 m deep and underlain by a deep saprolite depleted with respect to most primary aluminosilicate minerals. The only primary aluminosilicate mineral in the saprolite that is releasing Mg and K to solution is biotite. The bedrock in the area is quartz diorite. Results from the ridgetop indicate that the downward linear velocity of pore water is on the order of 2 m per year. This value is similar to the annual base flow corrected for volumetric water content, which is 1.8 m per year. Using the observed increases with depth of Mg and K concentrations in pore waters, biotite appears to be weathering at a rate of 10-16 to 10-14 mol Mg per m² biotite per s, depending on how the surface area of biotite is determined. Results from the slope test indicate that subsurface flow is primarily horizontal.

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