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Nephelometric turbidity is an optical index for the side scattering of light caused by fine particles suspended in water. When a mixed composition of suspended inorganic and organic materials, including dissolved organic material, is present, turbidity measurements can be affected by the different optical properties of the organic and inorganic materials present, and different turbidimeters are more or less sensitive to these influences. Two different methods of nephelometric turbidity measurement were assessed (using instruments confirming to two different turbidity standard methods: EPA 180.1 and ISO 7027). We investigated the influence of particulate organic matter and coloured dissolved organic matter on relationships between turbidity and suspended sediment concentration for rivers in diverse Otago catchments, in the South Island of New Zealand. The presence of organic matter and dissolved colour affected turbidity measurement owing to light absorption; however, turbidity measurement following the ISO 7027 standard, which specifies near infrared radiation at wavelengths where organic absorption is very weak, was less affected by organics. As a result, rating equations between suspended sediment and turbidity may be significantly different with ISO 7027 compared with EPA 180.1 methods.

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Turbidity, an index of light side-scattering, depends on the mass concentration of suspended sediment (SS) within water. Turbidity of river water is regulated by the presence of suspended particulate matter and is used to identify visual changes in response to SS. We used data from the New Zealand National River Water Quality Network, to calculate "specific turbidity" (K; turbidity normalised to mass concentration of suspended particulates). Specific turbidity is shown here to be an effective metric to assess the effect of suspended material composition and particle size distribution of suspended particulate matter over different landscape characteristics, and the effect on SS-turbidity relationships. Of the catchment characteristics considered in our study, specific turbidity was most responsive to lithological factors, and relatively insensitive to land use and soil parameters. Decreasing particle size has a positive linear response to K, attributed to the higher proportion of ultra-fine particulate material that is generated by certain lithologies, underscoring the lithological influence on K. SS-turbidity relationships, therefore, vary spatially across New Zealand's national record of water clarity, and K is considered a useful index for intercatchment comparison of SS than turbidity alone.

No abstract