Using 137Cs and 210Pbex and other sediment source fingerprints to document suspended sediment sources in small forested catchments in south-central Chile

Abstract

A study of the impact of forest harvesting operations on sediment mobilization from forested catchments has been undertaken in south-central Chile. The study focused on two sets of small paired catchments (treatment and control), with similar soil type, but contrasting mean annual rainfall, located about 400 km apart at Nacimiento (1200 mm yr−1) and Los Ulmos (2500 mm yr−1). The objective was to study the changes in the relative contribution of the primary sources of fine sediment caused by forestry operations. Attention focused on the pre-harvest and post-harvest periods and the post-replanting period was included for the Nacimiento treatment catchment. The sediment source fingerprinting technique was used to document the contributions of the potential sources. Emphasis was placed on discriminating between the forest slopes, forest roads and channel erosion as potential sources of fine sediment and on assessing the relative contributions of these three sources to the sediment yield from the catchments. The fallout radionuclides (FRNs) 137Cs and excess lead-210, the environmental radionuclides 226Ra and 40K and soil organic matter (SOM) were tested as possible fingerprints for discriminating between potential sediment sources. The Kruskal–Wallis test and discriminant function analysis were used to guide the selection of the optimum fingerprint set for each catchment and observation period. Either one or both of the FRNs were selected for inclusion in the optimum fingerprint for all datasets. The relative contribution of each sediment source to the target sediment load was estimated using the selected fingerprint properties, and a mixing model coupled with a Monte Carlo simulation technique that takes account of uncertainty in characterizing sediment source properties. The goodness of fit of the mixing model was tested by comparing the measured and simulated fingerprint properties for the target sediment samples. In the Nacimiento treatment catchment the relative contribution from the forest slopes and forest roads increased from 16 to 25% and from 37 to 45%, respectively, after clearcutting. Similar changes in source contributions associated with clearcutting were documented for the Los Ulmos treatment catchment, where the relative contribution of the forest slopes increased from 10.5 to 30% and that of the roads from 10 to 20%. The results indicate that the changes in sediment source are closely related to the disturbance of the catchment by clearcutting, but are also influenced by the amount of rainfall that occurred after clearcutting. They also emphasise the need to implement better management practices during forest harvesting, to reduce the increase in sediment mobilization from catchment slopes and roads, which can result in loss of valuable soil and associated nutrients from the forest floor and cause degradation of the water quality of adjacent streams.