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Although it is well established that background geochemical patterns in stream sediments generally depend on drainage basin geology, little is known of the effects of watershed disturbance on such patterns. Here we report on the effects of forestry activities on sediment geochemistry on the Interior Plateau of British Columbia. Stream sediment samples were collected from six small streams in 1996, before logging, and in 1997 after clear-cut logging. Samples were sieved and the −212 μm fraction analysed by multi-element ICP after both strong acid and total decomposition procedures. Field and analytical duplicates, and samples from unlogged control sites were used to estimate variations not associated with logging activities. Results show that prior to logging each stream had a distinct multi-element geochemical signature related to drainage basin geology. There was no overall effect of logging on these signatures within or downstream of the cut-blocks — probably because unlogged buffer zones along the stream channels prevented the development of new sediment sources. Logging roads did, however, cause local changes of sediment geochemistry in two ways. (1) Abrasion of zinc from galvanized culverts results in zinc anomalies, with concentrations > 200 ppm. These anomalies can extend several hundred metres downstream and persist for at least five years after installation of culverts. (2) Construction of stream crossings and roadside ditches create new sources of sediment supply to the streams. Although of local origin, the newly exposed and eroding surficial deposits contain more fine-grained material and have multi-element geochemical signatures that differ significantly from those of the mature fluvial sediments found in the stream channel before logging. Input of the new material results in geochemical anomalies that can extend several hundred metres downstream from the source. In this study Co, Mg and Ni were found to have the largest concentration differences between sediment sources and sediments, and hence to give the longest anomalous dispersion trains. The effects of logging roads on sediment geochemistry thus relate to creation of new sources of local sediment as well as to introduction of exotic materials into the stream. No geochemical response could be attributed to the clear-cut forest block. Such changes might develop eventually if soil erosion increased or if changes to the groundwater regime modified input of dissolved elements to the stream channel. |
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