Assessment of geophysical surveys as a tool to estimate riverbed hydraulic conductivity

The goal of this project was to investigate to what extent the vertical hydraulic conductivity (Kv) of a riverbed could be estimated by geophysical methods capable of gathering stratigraphic information of large areas in a relatively short time. Riverbed Kv was investigated at four sites along the Great Miami River. The sites represented a variety of riverbed sediment ranging from clay to cobbles. Estimates of riverbed Kv were obtained using a variety of methods at multiple scales including conventional seepage meter, slug tests and modeling of heat and water flow between the river and aquifer. Estimated riverbed Kv values were compared to stratigraphic information obtained from geophysical (resistivity, continuous seismic, electromagnetism) profiling to assess the usefulness of the geophysical data in predicting Kv values. Seepage meters were used to estimate Kv in the upper 0.3 m of the riverbed and yielded Kv values ranging from 0.11 to 5.3 m/d. Slug tests at depths between 0.45 and 11.2 m yielded Kv values ranging from 0.0284 to 9.46 m/d assuming an anisotropy ratio of 0.1. Modeling of groundwater flow and heat transport provided perhaps the best estimates of Kv over a larger area and longer period of time. Model-derived estimates of Kv agreed well with other methods and ranged from 0.015 to 14.9 m/d. Kv values obtained from conventional seepage, slug tests and temperature modeling compared well qualitatively to the stratigraphy inferred from electrical resistivity surveys at three out of four study sites. Discrepancies associated with the fourth site could be due to the existence of a colmation layer, a layer of a coarse sediment matrix clogged with fine sediment. These findings suggest that geophysical methods cannot be used alone as a method to assess the appropriate ranges of riverbed Kv. This might especially be true where municipal pumping contributes to riverbed clogging. To correlate the resistivity data with hydraulic conductivity, both geophysical and hydrogeological methods should be applied.