Nephelometric Turbidity Units (NTU) is the measure of the amount of light which is scattered due to particles in the water column. These units are equivelant to Formazin Nephelometric Units (FNU).
Formazin Nephelometric Units (FNU) is the measure of the amount of light which is scattered due to particles in the water column. These units are equivelant to Nephelometric Turbidity Units (NTU).

In-Stream Water Quality Monitoring

Before and during dredging, in-stream water quality monitoring will be conducted to assess potential impacts from the dredging. The water quality monitoring will consist of real-time turbidity data and daily total suspended solids (TSS) sampling.

Turbidity is a measurement of water clarity, and is measured by quantifying the amount of light reflected by suspended and colloidal particles. TSS sampling measures the amount of particles which are suspended in the water column, suspended solids typically include silt and clay particles, plankton, algae, fine organic debris, or other particulate matter. Suspended solids are more likely to carry toxic chemicals, and can also negatively affect aquatic organisms, water temperature, and dissolved oxygen levels. TSS measurements will be correlated to turbidity data, such that dredging operator can adjust operations in real-time with turbidity levels.

Turbidity is a natural occurrence in all water bodies and, in addition to dredging, elevated readings can be caused by:

  • Boat Traffic
  • Storm Events
  • Urban runoff
  • Sewer Discharges
  • Bottom feeders (such as carp) stirring up bottom sediments
  • Excessive algal growth

The dredging contractor will measure turbidity both upstream and downstream of the dredging operation. The difference between these two points will be utilized to determine the impacts of the dredging operation on turbidity in an effort to discount natural (background) turbidity. If the difference between upstream and downstream readings is greater than 50 NTU, the dredging contractor is required to take immediate responses. These responses may include:

  • Ensuring that all equipment is functioning properly
  • Slowing dredging operations
  • Pausing dredging until turbidity has been reduced
  • Or installing turbidity curtains around the dredging area (required after three-consecutive days with impacts >50 NTU).

Indiana Harbor & Canal Dredging Water Quality Data

Turbidity monitoring is posted to the dredging contractor’s website in real-time, and can be found at the following link:

 2012 Dredging Season
 2013 Dredging Season
 2014 Dredging Season
 2015 Dredging Season

Other Water Quality Data Sources for Indiana Harbor & Canal

The U.S. Geological Survey (USGS) conducts turbidity monitoring in the Indiana Harbor Canal (click here for the location of the monitoring) at USGS gauge 04092750. This turbidity data is available for daily maximum, daily minimum, and daily mean FNU statistics from March 2011 to the present. Recent USGS monitoring data is considered provisional until the data quality can be verified. The data can be accessed at USGS’s website by following the below link:

While USACE is working with USGS to correlate the dredging turbidity data with the USGS gauge to monitor dredging operations, several factors may affect the two turbidity monitors differently, including but not limited to:

  • The distance of the dredging operation from the USGS gauge. (The dredging schedule and dredging map can be found at the dredging contractor’s website, here).
  • Whether the dredging operation is upstream or downstream from the USGS gauge.
  • Stream flow direction and magnitude.
  • Vessel traffic, discharges, or other factors which could locally affect localized turbidity readings.
  • Turbidity monitor instrument type, calibration, or data collection frequency.

In addition to turbidity data, USGS monitors other water quality parameters at the Indiana Harbor Canal gauge, including chemical parameters and streamflow statistics. Archived data and annual reports can be accessed by the below link: