| Summary: | 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.
|
|---|
