Comparison of Turbidity Measurement by Nephelometry and Transmissometry and its Relevance to Water Quality Standards
The standard method for measuring turbidity in United States coastal waters is by nephelometric analysis with Formazin calibration. This study examined relationships between field measurements and various standards, and compared the performance of nephelometry with transmissometry. Turbidity generated during a beach restoration project in Florida was compared with Formazin and marl standards. For each datum, paired readings were taken by nephelometry and transmissometry, and compared using regression analysis. Both instruments measured individual standards in proportion to their concentration over a broad range of turbidity. Turbidity in the field was optically heterogeneous, i.e., more variable than standards, and did not correspond to either instrument using Formazin calibration. Marl predicted turbidity in the field within 95% confidence limits only below 11 nephelometric turbidity units (NTU). The State of Florida's statutory limit on turbidity of 29 NTU corresponds to 4.4 (±1.2) % transmission (%T) using field turbidity data. The use of Formazin to calibrate these instruments at this level underestimated turbidity in the field by about 50% at 29 NTU; marl underestimated field turbidity by about 24%. Weight of silt/clay in the field was linear as a function of percent transmission (Fig. 2b), but not using nephelometric analysis. Marl weight as a nephelometric standard produced a linear response at all concentrations, in contrast to Formazin which failed to produce a consistent nephelometric response at concentrations below 25 mg˙liter–1. Turbidity produced by known weight of Formazin and marl did not correspond using either instrument, and considerably underestimated silt/clay concentrations in the field. Water quality standards are discussed with respect to these findings.
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Document Type: Research Article
Publication date: September 1, 1995
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