Epi-fluorescence micromorphology of saprolite reveals evidence for colloid retention in microscale pore systems

Transport and retention of colloid tracers were evaluated in standard petrographic thin sections, prepared from saprolite (Cr horizons) formed from fractured sedimentary rocks, using epi-fluorescence microscopy. Thin sections prepared from dismantled hydrogeological columns revealed four domains of macropores with apparent apertures ranging from 1 to 500 μm. Colloid retention occurred within both bedding-parallel (BP) fracture macropores and within sets of orthogonal (O), bedding-perpendicular fracture macropores. Fracture macropores were generally partially occluded by illuviated clay coatings and had apertures of at least 5–10 μm, although some macropores were completely infilled with clay. Patchy retention of colloid particles also occurred in association with root pores, which ranged from 50 μm to 3 mm in diameter. Very little colloid retention occurred within the matrix of shale saprolite, and only within 1–2 mm distance from conductive fracture macropores. Sparse colloid retention also occurred in intergranular pores of the sandstone saprolite lithology, and only within 0.5 mm of open fracture macropores. Colloid retention did not occur within BP and O fracture macropores completely infilled by pedogenic clay or Fe/Mn oxides, nor in sandstone saprolite containing extensive pedogenic clay or Fe/Mn oxide secondary pore-fillings. This study demonstrates that epi-fluorescence microscopy is an inexpensive means of investigating colloid retention in macropore systems in saprolite, and compliments visual inspection of experimental columns of soil and saprolite material.