Pore water chemistry of acid sulfate soils: Chemical flux and oxidation rates

Abstract

Peeper pore water profiles above the oxidation front in acid sulfate soils (ASS) revealed unexpected differences in the elevation of the concentration maxima for chemical element species. These reflect the differing chemical and transport processes occurring in the oxic soil above, and the anoxic zone below, the oxidation front. Transport below the oxidation front is diffusive and by using the measured profiles of dissolved Li, liberated by acid hydrolysis of clays, it is possible to assess the period over which oxidation has occurred at this site. This time, ca 60 years, is consistent with the initial engineered drainage of the site ca. 100 years. But this current diffusion process may also be superimposed on earlier weather-driven oxidation events. It is estimated that 78–90 tonnes of H 2SO 4/ha have been exported from the McLeod's Creek catchment. The pore water profiles of “background” species, namely Cl, Na, K, Mg and Sr, increased monotonically with decreasing elevation in the ASS profile, suggesting upward diffusion, from estuarine-formation pore waters at lower elevations in the profile, towards fresher surface waters. Ion ratios of “background” species relative to chloride reveal sources of these species close to the oxidation front, as expected from acid hydrolysis of clays. Ion ratios also showed depletion of K above the oxidation front. The relative concentration of “background-corrected” Na, Ca, and Mg at the oxidation front suggests that smectite is the major clay mineral in the oxic soils. The monotonic profile of Cl reveals an upward diffusion. The overall Cl flux is calculated as 660 kg/ha/y, implying a time scale of exposure of these soils to fresh surface waters of 2000 years.