The influence of erosion sources on sediment-related water quality attributes

Suspended fine sediment has a significant impact on freshwater quality variables such as visual clarity (VC). However, freshwater quality is related to the attributes of the catchment sources contributing fine sediment to the stream net-work. Here, the extent to which an array of sources defined spatially according to erosion process and geological parent material may be discriminated and classified based on sediment-related water quality (SRWQ) attributes that poten-tially affect VC was examined. Erosion sources were sampled across two New Zealand catchments representing six types of erosion and eight parent materials. Erosion source measurements focused on particle size, organic matter con-tent, and light beam attenuation (which is convertible to VC). The source data were analysed to: 1) evaluate source variability using a combination of Kruskal-Wallis and principal component analysis; 2) reclassify sources using a Ran-dom Forest model; and 3) demonstrate how erosion source affects VC for a range of theoretical sediment concentra-tions (SC) using a simple empirical model. The results indicate that SRWQ attributes show significant variation across erosion sources. The extent to which attributes differed between sources often related to whether there was a strong association between a specific erosion process and parent material. The 19 a priori source classifications were reduced to 5 distinct sources that combined erosion process and parent material (i.e., bank erosion-alluvium; mass movement-ancient volcanics; mass movement-sedimentary; surficial erosion; gully-unconsolidated sandstone). At low SC, the impact of erosion source on VC became most evident ranging from 2.6 to 5.6 mat SC of 5 g m-3. These findings show how catchment sources of sediment, in addition to sediment concentration, influence VC, and highlight the need to consider quality as well as quantity of material supplied to stream networks when planning erosion control.

Automated summary

Suspended fine sediment has a significant impact on freshwater quality variables such as visual clarity. This study examined the influence of erosion sources defined by erosion process and geological parent material on sediment-related water quality attributes, particularly visual clarity. The findings highlight the need to consider both the quality and quantity of sediment when planning erosion control.