Chemical dilation and the dual porosity of humified bog peat

Abstract

Hydrochloric acid and calcium and sodium chloride salt solutions were passed through humified bog peat cores of 1 m length to determine values for the material properties governing solute transport: hydraulic conductivity, effective porosity, and dispersivity. Chloride passed through the bulk peat cores about twice as fast as predicted from chloride breakthrough curves measured by selective-ion probes inserted into the peat 30 cm downgradient from the source. With continued experimentation, chloride dispersion increased. Chloride also passed through the peat up to 12 times slower than predicted by conservative advection. The retardation of the chloride fronts, the faster bulk transport, and the asymmetry of the breakthrough curves all indicate that humified bog peat is a dual porosity medium. Solutes both move preferentially through active macropore spaces and diffuse into dead pore space in the peat matrix. Neither the effective porosity nor the dispersivity of the peat could be precisely determined from the breakthrough curves because the hydraulic conductivity and discharge rates through the peat increased as much as five times when the chloride tracer solutions passed through the cores. In initial experiments, the shapes of the breakthrough curves were generally consistent with dispersivities of the order of centimeters, similar to that observed for granular soils at the same 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 tracer solutions with organic-acid functional groups may cause macropores to dilate and micropores to close off.