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Turbidity

Turbidity is an indicator of water clarity as measured by the amount of light scattering caused by particles in water, and is an indication of the amount of suspended particles present in a sample. This parameter is regularly used in drinking water treatment process control applications, and testing of source waters is often performed for this purpose. Of substantial importance for surface water early warning applications is the frequent correlation between large increases in turbidity measurements and the potential for adverse changes in microbiological water quality. This relationship results from the fact that both turbidity and microbial concentrations often increase substantially during and subsequent to storm events. As rainwater flows overland into receiving streams and other waterbodies, it picks up both soil particles and microorganisms, and microorganisms will also frequently attach to soil particles. This can be especially problematic in agricultural areas where vegetative groundcover is low and significant amounts of microorganisms exist from animal husbandry operations.

As an example of the application of turbidity for an early warning indicator of increased pathogen concentrations, a New Jersey water utility investigated the correlation of storm events and increased flow, turbidity, and the presence and concentration of the protozoan parasite Cryptosporidium (LeChevallier et al., 1998). The increased flow rate during the Spring rains largely comes from overland runoff, which carries with it soil particles and pathogens. The resulting increases in turbidity were accompanied by increases in Cryptosporidium. Analysis of the data showed that high Cryptosporidium loadings could often be avoided by shutting the intake when turbidity rises above a certain level (e.g., > ~15 ntu). The treatment process at the particular utility where this analysis was performed incorporates on-site raw water storage that allows the intake to be shut down for about 24 hours without interruption in service. As was done for the above example, use of a surrogate such as turbidity should be supported by evidence of a correlation with the parameter(s) of concern in the specific source water area.