Response of a first-order stream in Maine to short-term in-stream acidification

An experimental short-term acidification with HCl at a first-order stream i n central Maine, USA was used to study processes controlling the changes in stream chemistry and to assess the ability of stream substrate to buffer p H. The streambed exerted a strong buffering capacity against pH change by i on exchange during the 6-hour acidification. Streambed substrates had subst antial cation and anion exchange capacity in the pH range of 4.1 to 6.5. Th e ion exchange for cations and SO42- were rapid and reversible. The speed o f release of cations from stream substrates was Na1+ > Ca2+ > Mg2+ > Aln+ > Be2+, perhaps relating to charge density of these cations. Ca2+ desorption dominated neutralisation of excess H+ for the first 2 hr. As the reservoir of exchangeable Ca diminished, desorption land possibly dissolution) of Al 3+ became the dominant neutralising mechanism. The exchangeable land possib ly soluble) reservoir of Al was not depleted during the 6-hour acidificatio n. Sulphate adsorption during the acidification reduced the concentration o f SO42- in stream water by as much as 20 mu eq L-1 (from 70 mu eq L-1). Des orption of SO42- and adsorption of base cat ions after the artificial acidi fication resulted in a prolongation of the pH depression. The streambed had the capacity to buffer stream water chemistry significantly during an acid ifying event affecting the entire upstream catchment.