Turbidity is a measure of water clarity that expresses how much light passes through the water column. It is dependent upon the amount of suspended particles (e.g., sediment, algae, bacteria) and colored organic matter present. Clear water is critical for the growth and survival of fish, crabs, and other aquatic organisms.

High turbidity in the Chesterville Branch of the Chester River. Photo by Ron Melcer.

How is it measured?

Turbidity was measured year round. Data from 2012-2017 was used for the 2019 report card. Data from 2013-2018 was used for the 2020 report card. Data from 2014-2019 was used for the 2021 report card. Turbidity is the amount of particles in the water measured as NTUs. The proportion of time that turbidity was above the threshold at each station was calculated. Station scores were averaged to HUC12. HUC12 scores were area-weighted to the reporting region scores.

Threshold Levels

Multiple thresholds for turbidity were determined by using government standard values; 3.0 nephelometric turbidity units (NTUs) is a value EPA recommends as a pass/fail level, and 10.0 NTUs was a level determined by MD DNR (US EPA 2000; Kilian et al. 2006). By anchoring 3.0 as the 4 value and 10.0 as the 1 value, equal intervals were used to determine the remaining threshold levels.*

Score Turbidity Thresholds (NTUs)
5 <3
4 ≥3 - <4.75
3 ≥4.75 - <6.5
2 ≥6.5 - <8.25
1 ≥8.25 - <10
0 ≥10

*EcoCheck (2013)