BASE CATION CHEMISTRY OF STORM RUNOFF IN A FORESTED HEADWATER WETLAND

Hydrologic data were combined with analysis of a natural isotope, oxyg en 18, and solutes (Ca, Mg, K, Na) to examine saturation overland flow and stream base cation dynamics during storms in a headwater swamp. L aboratory experiments were also used to study the interaction of mixtu res of throughfall and groundwater with swamp substrates in zones of s aturation overland flow. Increased streamflow was associated with decr eased concentrations of Ca, Mg, and Na, whereas K concentrations remai ned constant, or increased in some storms. A comparison of chemical an d isotopic hydrograph separation indicated that Ca, Mg, and Na were co nservative in storms with instantaneous event water contributions <25% and were reactive in storms with a larger event water component. Labo ratory data suggested that these cations were only released rapidly fr om swamp substrates when mixed with water containing >40% throughfall (event water). Stream K concentrations appear to be controlled by the mixture of throughfall and groundwater in saturation overland flow dur ing occasional storms with high throughfall K concentrations. Laborato ry data suggested that for most storms which have throughfall K concen trations of <1 mg L-1, K was released as overland flow interacts with swamp substrates, resulting in constant stream K concentrations.