CHEMICAL DILATION AND THE DUAL-POROSITY OF HUMIFIED BOG PEAT

Hydrochloric acid and calcium and sodium chloride salt solutions were passed through humified bog peat cores of 1 m length to determine valu es for the material properties governing solute transport: hydraulic c onductivity, effective porosity, and dispersivity. Chloride passed thr ough the bulk peat cores about twice as fast as predicted from chlorid e breakthrough curves measured by selective-ion probes inserted into t he peat 30 cm downgradient from the source. With continued experimenta tion, chloride dispersion increased, Chloride also passed through the peat up to 12 times slower than predicted by conservative advection. T he retardation of the chloride fronts, the faster bulk transport, and the asymmetry of the breakthrough curves all indicate that humified bo g peat is a dual porosity medium. Solutes both move preferentially thr ough active macropore spaces and diffuse into dead pore space in the p eat matrix. Neither the effective porosity nor the dispersivity of the pear could be precisely determined from the breakthrough curves becau se the hydraulic conductivity and discharge rates through the peat inc reased as much as five times when the chloride tracer solutions passed through the cores, In initial experiments, the shapes of the breakthr ough curves were generally consistent with dispersivities of the order of centimeters, similar to that observed for granular soils at the sa me length scale (meters), whereas they 'flattened' considerably during later experiments, suggesting dispersivities in the range of tens of centimeters. We hypothesize that the interaction of the chloride trace r solutions with organic-acid functional groups may cause macropores t o dilate and micropores to close off. (C) 1997 Elsevier Science B.V.