Light attenuation – a more effective basis for the management of fine suspended sediment than mass concentration?

Abstract

Fine sediment continues to be a major diffuse pollution concern with its multiple effects on aquatic ecosystems. Mass concentrations (and loads) of fine sediment are usually measured and modelled, apparently with the assumption that environmental effects of sediment are predictable from mass concentrations. However, some severe impacts of fine sediment may not correlate well with mass concentration, notably those related to light attenuation by suspended particles. Light attenuation per unit mass concentration of suspended particulate matter in waters varies widely with particle size, shape and composition. Data for suspended sediment concentration, turbidity and visual clarity (which is inversely proportional to light beam attenuation) from 77 diverse New Zealand rivers provide valuable insights into the mutual relationships of these quantities. Our analysis of these relationships, both across multiple rivers and within individual rivers, supports the proposition that light attenuation by fine sediment is a more generally meaningful basis for environmental management than sediment mass. Furthermore, optical measurements are considerably more practical, being much cheaper (by about four-fold) to measure than mass concentrations, and amenable to continuous measurement. Mass concentration can be estimated with sufficient precision for many purposes from optical surrogates locally calibrated for particular rivers.

Mutual relationships of suspended sediment, turbidity and visual clarity in New Zealand rivers

“…Across all sites, visual clarity is strongly correlated with TSS and turbidity as has been reported by Davies-Colley et al (2014). However, while the R 2 values are relatively high, SE values are also high, suggesting that the relationships are not very precisely predictive.…”

“…Clarity and turbidity were strongly (negatively) inter-related, though TSS is slightly less strongly related to both clarity and turbidity. Davies-Colley et al (2014) have presented mutual scatterplots for each combination of TSS, turbidity and visual clarity across all sites; a scatterplot of TSS and visual clarity shows that, while the correlation is strong, there is still appreciable data scatter (Fig. 1).…”

“…However, TSS measurements were added temporarily in 2011 so as to provide a dataset for relating 'particulate' water quality to mass of particulates . Davies-Colley et al (2014) have reported mutual relationships of TSS, turbidity and visual clarity across multiple sites in the NRWQN. In this contribution we emphasise these relationships within individual NRWQN rivers, and draw conclusions about the value of pairing sediment assays with water quality monitoring and the routine use of optical variables in SoE monitoring.…”

Black disk visibility, turbidity, and total suspended solids in rivers: A comparative evaluation

“…2 is defined for the photopic range of normal human vision; individuals who are color blind will have different sensitivities (Zaneveld and Pegau ). Despite the theoretical and practical advantages of measuring water clarity by the horizontal black disk (Davies‐Colley ), especially when light attenuation by fine sediment is of concern (Davies‐Colley et al ), the method has not been widely used outside of New Zealand, perhaps because of lack of awareness of the method, a scarcity of comparative studies, or perceptions that the black disk is affected by biases similar to the those of the Secchi disk.…”

“…The few investigations associating BDV with turbidity and/or suspended solids concentrations have been specific to a single site (Steel and Neuhausser ), base flow conditions (Davies‐Colley and Close ), routine monthly monitoring data (Davies‐Colley et al ), or across long‐term median values over many sites (Smith et al ). However, Hughes et al () have reported preliminary results from their study comparing OWQ parameters during storm flow.…”