Application of multi-element geochemical methods to identify sediment sources and trace the transport of sediment in small (s4) streams before and after watershed disturbance

• Main Author: Christie, Tara Michelle
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/8905

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 the effects of forestry activities on sediment geochemistry on the Interior Plateau of British Columbia. Samples of sediment were collected from six small streams in 1996, before logging, and in 1997, after clear-cutting. Samples were sieved to - 0.212 mm in grain size and analyzed by multi-element ICP after both a strong acid and a total decomposition. 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. Construction of stream crossings and roadside ditches created new sources of sediment supply to the streams. Although of local origin, the newly exposed and eroding surficial materials contained more fine-grained material and had multi-element geochemical signatures that differed significantly from the mature fluvial sediments found in the stream channel before logging. Input of the new material resulted in geochemical anomalies that extended 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 gave the longest anomalous dispersion trains. 2. Abrasion of zinc from galvanized culverts resulted in zinc anomalies, with concentrations >200 ppm. These anomalies also extended several hundred meters downstream and persisted for at least five years after installation of culverts. The effects of logging roads on sediment geochemistry thus relate to the creation of new sources of local sediment, as well as, to the 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 increases or if changes to the groundwater regime modify input of dissolved elements to the stream channel.