Determination of Appropriate Metrics for Sediment-related Total Maximum Daily Loads (TMDLs)

Project Staff: 

Principal Investigators: Anne Lightbody, Research Associate; Patrick Belmont, Postdoctoral Research Associate; Jeffrey Marr, Research Fellow; Cailin Orr, Postdoctoral Research Associate; St. Anthony Falls Laboratory and Chris Paola, Professor, Department of Geology and Geophysics, University of Minnesota

Funding: 

USGS-WRRI 104B/ CAIWQ Competitive Grants Program

Summary: 

The most common cause of impaired rivers and streams in the United States is sediment pollution. High levels of fine suspended sediment reduce aquatic health in numerous ways, including a reduction in light transmission, interference with aquatic organisms, and reduction in benthic habitat quality following deposition on the bed. There are many ways of measuring suspended sediment levels, including both direct measurements of concentration and indirect measures such as turbidity. Measurements of total suspended solids concentrations are obtained from water samples, which are then filtered and processed in a laboratory. Turbidity, on the other hand, is an expression of the optical property of a sample of water, in which the amount of light scattered by a given water sample is compared with that scattered by a standard sample.

It is possible to develop robust relationships between turbidity and suspended sediment concentrations in a river reach or system where the sediment distribution and its range are spatially and temporally constant. However, when the optical properties of the water and sediment mixture differ, which occurs as a result of changes in watershed slope, soil type, geology, precipitation, and other factors, then turbidity readings will also change, even if the overall sediment concentration remains the same. In many systems, particle size characteristics change rapidly both temporally and spatially, so turbidity does not always correlate well with suspended sediment concentrations.

Sediment impacts on stream systems result from both its effect on water clarity and its physical characteristics. Many states, however, including Minnesota, have a water quality criterion based on turbidity alone. It is possible that stream habitat quality is being reduced by sediment pollution effects that are not captured by an analysis of water clarity alone. In fact, the Minnesota Pollution Control Agency (MPCA) itself notes that much additional work is needed to better define the factors causing aquatic life impairment due to turbidity-associated factors.

Here, we asked the question of what is the most important aspect of sediment pollution. We will test the null hypothesis that turbidity is the most important factor by experimentally manipulating turbidity levels within an outdoor stream ecosystem and observing impacts of turbidity on this system. We will introduce water with different compositions of suspended load (e.g., different proportions of fine sand, silt, mud. and different levels of organic matter and nutrients) but the same turbidity level into the Outdoor StreamLab facility at St. Anthony Falls Laboratory (SAFL), and compare the ecosystem response. Trials will be performed under high flow conditions, which often accompany high turbidity levels and typically exert substantial stress on aquatic ecosystems. The results will indicate whether turbidity (i.e., NTU) most closely correlates with benthic habitat quality, or whether another metric or combination of metrics (e.g., suspended sediment concentration, transparency, net sedimentation, or embeddedness) provides a better understanding of the effect on benthic habitat. These results are needed by federal and State agencies to modify their TMDL program and better protect the water quality of Americas rivers and streams.