Studies Of Grain Size Variations In Fluvial Sediments Of The Rio Grande Using Magnetic Techniques

Characterizing the heterogeneity of fluvial sediments in the Rio Grande Valley near El Paso,<br>Texas is important to understanding shallow groundwater flow, groundwater/river exchange and the<br>build-up of soil salinity. We have found the magnetic technique an effective way to rapidly evaluate<br>grain size changes within the river soils. This is in contrast to electrical techniques (resistivity,<br>conductivity) that are strongly affected by seasonal variations in soil moisture and salinity. Medium to<br>coarse-grained sands of the ancestral Rio Grande contain significant (up to 10% by weight) amounts of<br>magnetite, and thus appear as magnetic highs. We have conducted magnetic surveys at four study areas<br>within the Mesilla Valley northwest of El Paso. The sites have a range of sizes (10’s to 100’s of meters<br>in dimension) and geologic complexity (homogeneous channel sands to complex interfingered crevasse<br>splay and flood plain deposits). Borehole grain size analysis, as well as surface geophysical studies (e.g.<br>conductivity, resistivity, seismic, GPR) were conducted at the sites. Our results indicate we can detect<br>small crevasse splay (~2-3 m wide) channels buried at depths of 1-2 m, as well as larger channels (~20<br>m) at depths of ~20-30 m, through simple analysis of magnetic contour maps and magnetic gradients.<br>We believe the technique has the potential for rapidly evaluating the location of channels within the Rio<br>Grande valley, for corroborating surface soil maps and determining potential contaminant flow<br>pathways.