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

Erosion of fine sediment and its delivery to streams pose significant issues for freshwater quality and  downstream receiving environments. Increased sediment delivery can lead to negative impacts due to changes to visual clarity (VC) and nutrient levels, which can degrade freshwater and marine environments. Most research on sediment in catchments focuses primarily on the total mass or quantity of sediment in relation to erosion, sediment transport, and deposition. In contrast, ‘quality’ aspects, notably particle size as it affects water quality, are not often evaluated, particularly in terms of their erosion source. This is problematic, as the physical qualities of sediment, which strongly affect environmental behaviour and influence water quality, may vary across catchments, geological parent materials, and erosion processes. Here, we assess the extent to which sources, defined spatially according to erosion process and geological parent material, may be discriminated, and classified by their sediment-related water quality (SRWQ) attributes. This involved 1) evaluating variability in SRWQ attributes across different sources; 2) reclassifying sources to the minimum number needed to adequately represent variation in attributes; and 3) assessing the potential influence of erosion sources on instream VC.

Erosion sources were sampled across two New Zealand catchments representing six types of erosion and eight parent materials. Sample measurements focused on particle size, organic matter content, and light beam attenuation (which is convertible to VC). Particle size attributes included three size fractions (<0.063mm, 0.063 – 2mm, and >2mm), particle size distribution (PSD) attributes (mean, D₁₀, D₅₀, and D₉₀, based on both surface area (sur) and volume (vol) distributions). Organic matter related attributes included the percentage of particulate organic carbon (POC), particulate organic nitrogen (PON), inorganic suspended solids (InorgSS), and volatile suspended solids (VSS). Given its importance for predicting VC, light beam attenuation coefficient (beam- ) was measured and converted into beam-  to use as a SRWQ attribute.

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 sediment concentration (SC), the impact of erosion source on VC became most evident ranging from 2.6 to 5.6 m at a SC of 5 g m^-3. These findings showed 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.