Measurement of chemical weathering rates using natural chloride as a tracer

Abstract

The rates of chemical weathering within forested catchments as reported in the literature have been determined principally from input-output budgets, with outputs being measured as streamflow at a weir. In the present study an alternative method has been investigated which uses natural chloride as a tracer and does not require construction of a weir. It is shown that estimates made of weathering release rates of the cations Na +, K +, Ca 2+, Mg 2+ using the chloride tracer method are generally less affected by unmeasured deep seepage losses from the catchment or changes in groundwater storage than those using the weir method. The chloride tracer method could permit estimates of weathering rates to be made more frequently than at present in studies of forest nutrient cycling. Weathering rates were measured in four forested catchments with annual rainfall up to about 2000 mm, lying mostly within the Mount Boss state Forest near Wauchope, NSW. The forest is a mixture of rainforest and moist hardwood forest, parts of which are regenerating after logging. Porphyritic rhyodacite underlies all the catchments and has produced red and yellow acid soils (Dystrochrepts). Cation:chloride ratios in streamwater varied little over time, and showed little if any dependence on streamflow. “Grab” samples could be used in the estimation of weathering rates. The average weathering release rates in the major catchments sampled at Mount Boss for Na, K, Ca and Mg were 10.4, 5.3, 6.0 and 2.2 kg ha −1 yr −1 respectively. The average rock weathering rate for the same catchments calculated from Na release was 450 kg ha −1 yr −1 and from Ca was 340 kg ha −1 yr −1. Theoretical analysis of the effect of forest regeneration on weathering rate estimates from the chloride tracer method shows that the true rock weathering rate should lie between these two figures and was taken as 400 kg ha −1 yr −1.