Iron, Clay Mineralogy, and Water Infiltration in West Texas Playa Lake Soils.

Playa lakes in the Southern High Plains (SHP) of Texas act as recharge sites for the underlying Ogallala aquifer.  In a playa basin near Plainview, Texas, soil profiles were described and soil samples collected at 5 sites from basin center to edge.  Soil samples were physically and chemically analyzed at Texas Tech and at USDA-NRCS Lincoln, Nebraska.  The playa soils were classified as Randall (fine, montmorillonitic thermic, Udic Pelluderts) and Lofton (fine, mixed thermic Torrertic Argiustolls).  At the surface, clay contents ranged from 55 to 76% and organic carbon contents ranged from 1.4 to 3.0%.  Water infiltration rates ranged from 0.5 to 4.6 cm/hr, were greater in winter (moist soil surface) than in summer (dry soil surface), and generally increased toward the playa edge where clay contents were lowest.  The clays consisted of ~20% kaolin minerals, illite, and a disordered expandable phase (smectite, illite/smectite).  The <2µm cation-exchange capacity decreased from 60 to 50 cmol/kg after Li+ saturation and (250 °C) heat treatment.  This suggests tetrahedral sites dominate the expandable mineral (beidellite?) layer charge.  Ferrous and total Fe contents were determined on total clay (<2µm), coarse clay (0.2-2µm), and fine clay (<0.2µm) fractions.  Total Fe was 5.5% in the <2µm clay, 6.0% in the fine clay, and 2.9% in the coarse clay.  Ferrous Fe was <2% of total Fe in the <2µm clay and the coarse clay contained most of ferrous Fe, probably in illite. Tests for reduced conditions based on soluble ferrous Fe in SHP playas are usually negative.  Grey soil colors indicate little if any free Fe oxides are available for reduction to dissolved ferrous Fe.  The playas have sufficient organic matter for reduced conditions when flooded, but lack free Fe oxides needed for a positive α,α-dipyridyl test.