Major ion variation and efflux related to discharge in a mafic piedmont province watershed

Abstract

AbstractMajor‐ion variability related to discharge was analysed in a forested 187 km2 mafic Piedmont Province watershed using statistical (both parametric and non‐parametric), graphical (box‐plots) and curve‐fitting (log concentration‐log discharge) techniques. Baseflow alkalinity and base cation concentrations show systematic temporal variations as a result of the influx of additional water that occurs during the late autumn to early spring. Regression analyses indicate that storm‐related discharge and baseflow generated during periods of water surplus are characterized by similar dilution slopes. Mass balance estimates indicate that the additional water, which comprises storm/recession discharge and base‐flow from late autumn to early spring, is between about 30 and 80% as concentrated as summer low‐flow.The thick clay‐rich soil mantle represents a key control on solute concentrations in that it stores water for periods of time sufficient for a high degree of water‐mineral interaction to occur. Hence solute‐discharge relationships (C = aQb, where b is typically < 0) are characterized by relatively low slope values and there is ample acid neutralizing capacity throughout the range of discharge. Owing to the predominance of amphibolite, solute efflux related to rock weathering from the Falling Creek watershed is much greater than other more felsic locations within the region. Statistical analyses (Mest and the non‐parametric Mann‐Whitney‐Wilcoxon test), along with accompanying box‐plot representations, provide a useful method of describing systematic annual hydrochemical variation within streamflow. These methods are particularly effective for those instances in which a long‐term data set exists, but is limited to relatively few sampling periods per year.