COMPARISON OF TWO SUSPENDED SEDIMENT ANALYSIS METHODS

Grand Valley State University (GVSU) is a liberal arts university located on 1,322 acres to the west of the Grand River in western Michigan. Deep ravines have been eroded into the glacial sediments on which the campus is built resulting in a ravine system east of campus which flows into the Grand River. Since the construction of the GVSU campus, beginning in 1960, the amount of impermeable surface has increased resulting in increased storm water runoff and peak flows in the ravines. Lag time between the start of a rain event and the arrival of runoff in the ravine system has decreased.

Erosion caused by the increased peak flow and decreased lag time has resulted in widespread slope stability problems, some near campus buildings and structures. In fall 2013, a prominent historic bridge was temporarily closed due to unstable footings. Runoff measurement, sampling, and water quality analysis in the ravines has been ongoing since 2006. Over 900 samples have been analyzed for total suspended sediment (TSS), using a modified EPA method ESS 340.2. In an effort to find a more efficient method for determining sediment loads samples previously analyzed using the EPA method were reanalyzed using a modified version of the ASTM Standard Test Method D 3977-97 (suspended solid concentration (SSC)).

The SSC method is simpler and faster, especially when samples have high sediment concentrations. The TSS method allows preservation of the sample, is more widely used by wastewater treatment facilities, and is more compatible with performing other water quality measurements. The SSC method requires fewer steps and there is less chance for analytical error while processing samples, however it requires Total Dissolved Salts (TDS) data for comparison with TSS data.

In order to compare the two analysis methods a random subsample from 600 previously analyzed samples taken from 2011 to 2013, as well as non-randomized samples collected in 2014 were reanalyzed. Preliminary results comparing the TSS and SSC method for random samples and non-random samples suggest that the TSS method underestimates the suspended sediment content, particularly for lower sediment concentrations. The TSS method may be less accurate at lower concentrations due to sampling bias resulting from sample size; and the potential for analytical errors due to scale precision.