COMPARISON OF METHODS FOR ANALYSIS OF SILT/CLAY RATIO IN THE JAMES RIVER ESTUARY

In fall 2017, a joint project by the Army Corps of Engineers and the Coastal Hydrodynamics and Sediment Dynamics lab at VIMS used laser diffraction particle size analysis (LDPSA) and pipette analysis, respectively, to determine the clay to silt ratio of sediment samples from dredging sites in the James River Estuary in Virginia. Ultimately, the clay-to-silt ratio of these samples will be used to determine river bottom erodibility and the potential impacts of dredging on the estuary, as well as where to dispose of dredging spoils. The use of different methods led to contrasting results, with the LDPSA yielding approximately 80% silt and 20% clay and the pipette method yielding about 40% silt and 60% clay. Due to this disparity, a follow-up study was conducted at VIMS in summer 2018 to examine the clay-to-silt ratio of a subset of the James River dredge-site samples using four different analytical techniques: 1) pipette analysis and 2) hydrometer, both of which employ sediment settling principles; and 3) LDPSA and 4) laser in-situ scattering and transmissometry (LISST), both of which utilize laser diffraction. Sub-samples from a box core were taken at 1-cm intervals from 1-10 cm depth. Each depth sample was analyzed by all four methods.

For the samples analyzed, the summer 2018 study found that the percentage of silt was higher than the percentage of clay for each method; however, the LDPSA and LISST found a significantly higher silt percentage (~80%) compared to that determined by the pipette and hydrometer methods (~55%). The LDPSA and LISST results showed greater reproducibility (i.e., they were more precise) whereas the pipette analysis and hydrometer were less precise. The sediment settling principle on which pipette analysis and hydrometer are based has been utilized far longer than laser diffraction, and sediment settling is generally considered to be the accepted standard for determining the clay-to-silt ratio. Because the clay-to-silt ratio has important implications for sediment erodibility and transport, further work is needed to resolve the observed disparities. An additional control on sediment cohesion is the mineralogy of the fine particle fraction. Ongoing analyses of the James River dredge-site samples using x-ray diffraction aim to provide mineralogy data to compare with the particle size results.