COMPARING METHODS TO INVESTIGATE RIVERBED HYDRAULIC CONDUCTIVITY AT SEVERAL WELL FIELDS ALONG THE GREAT MIAMI RIVER, SOUTHWEST OHIO

The vertical hydraulic conductivity (Kv) of a riverbed can be measured or estimated using a variety of methods. In this study, we investigated the riverbed Kv using seepage meters with minipiezometers, temperature modeling and three geophysical methods. These methods were performed at three sites associated with municipal well fields along a 15-km stretch of the Great Miami River in southwest Ohio.

Geophysical methods were performed by the U.S. Geological Survey (USGS) and included continuous seismic reflection profiling (CSP), continuous resistivity profiling (CRP), and multi-frequency continuous electromagnetic profiling (CEP). A global positioning system (GPS) allowed spatial mapping of the profiling results. The CSP and CRP methods provide stratigraphic information to a depth of several meters below the riverbed. Riverbed Kv was also measured directly with seepage meters and minipiezometers, and estimated through temperature modeling. Temperatures and water levels were measured in the river and in monitoring wells over many months, and the data was incorporated into simulations using VS2DH, a two-dimensional groundwater flow and heat transport model.

Preliminary results of the CSP indicated some layering of the glacial and alluvial material. Preliminary CRP results showed variations in material types from sand-and-gravel to clay. The mid- and high-frequency CEP allowed limited interpretation of bottom to sub-bottom material, while the low-frequency CEP proved the best in determining sub-bottom material types and was more amenable to the stream conditions along the entire reach of the river (per. commun., R. Sheets, USGS, 2007). The preliminary results from seepage meters at one of the sites with strong downward gradients showed high spatial variability of riverbed Kv with values ranging from 0.0076 to 0.43 m/d and a geometric mean of 0.061 m/d.

Geophysical profiling does not provide actual values for the riverbeds Kv. However, it can produce continuous geological information on a large spatial scale. This can be used to extrapolate Kv measurements obtained from seepage metering and temperature modeling and then apply them to other parts of the riverbed. Comparison of all these methods should provide an effective way of estimating riverbed Kv on a larger scale than using any single method alone.