Using Secchi tube readings in stream water quality assessments

Water quality standards

Sediment is one of the main pollutants in Minnesota rivers and streams, and can harm fish and aquatic insects, degrade aesthetic and recreational qualities, and be expensive to treat for drinking water purposes. For these reasons, Minnesota places limits, or water quality standards, on the amount of Total Suspended Sediment (TSS) allowed in rivers and streams. When there is too much TSS in a river, water quality standards are exceeded and the river in question is placed on a list of impaired waters. The list is then used to target resources toward protection and improvement efforts.

Measuring and estimating TSS

Lab analysis of TSS from water samples collected in the field is the most accurate method for determining sediment levels in rivers, but it can be time consuming and expensive. Measuring stream transparency, or water clarity, with a Secchi tube (S-tube) is a quick and easy, cost effective alternative that has been shown to reliably predict TSS levels. S-tube readings can therefore serve as a surrogate for TSS. While direct TSS measurements are generally preferred, S-tube data sets are often more robust, and their relative strength is considered in water quality assessments to determine compliance with TSS standards.

River map of MinnesotaBecause S-tube measurements are not perfect surrogates, a margin of safety is applied. First, there are unique S-tube threshold values for different River Regions across the state (see details in the map and table), and for specific portions of the Red and Lower Mississippi Rivers. Second, to increase certainty in determining if a river exceeds or meets the TSS standard, S-tube values must be below a certain value, or threshold, to exceed the standard; and above another distinct threshold to meet the standard. Values between the two thresholds are not considered in water quality assessments. Previously, one threshold was identified, with values below the threshold indicating an exceedance of the standard and values above the threshold indicating the standard is met.

Evaluating compliance with TSS standards

The standards for TSS (mg/L) and S-tube (cm) are:

Region or river
Assessment season for these waters is April - September
 

 TSS (mg/l)

 S-tube (cm) below this value

Exceeds standard

 S-tube (cm) = or above this value 

Meets standard

All Class 2A Waters

10

55

95

Northern River Nutrient Region as Modified for TSS

15

40

55

Central River Nutrient  Region as Modified for TSS

 

30

25

35

Southern River Nutrient  Region as Modified for TSS

65

10

15

Red River Mainstem – Headwaters to Border

100

5

10

A stream is considered in exceedance of the standard for TSS/S-tube if:

  • the standard is exceeded more than 10 percent of days during the April through September assessment season as determined from a data set that provides an unbiased representation of conditions over this period, and
  • there are at least three such measurements exceeding the standard.

A stream is considered in compliance with the TSS/S-tube standard if:

  • the standard is met at least 90% of the days during the assessment season.

Meeting the standard generally requires at least 20 suitable measurements from a data set that gives an unbiased representation of conditions over at least two different years. However, if it is determined that the data set adequately targets periods and conditions when exceedances are most likely to occur, a smaller number of measurements may suffice.

S-tube measurements that fall between the two relevant threshold values for a given region or river are considered indeterminate, and are not used in calculating overall percentages. If a stream satisfies neither the criterion for exceeding nor meeting the standard, the stream is considered to have insufficient information regarding TSS levels.