HIGH-RESOLUTION CHARACTERIZATION OF PREFERENTIAL FLOWPATHS IN THE THICK UNSATURATED ZONE OF THE NORTH CHINA PLAIN

Direct-push electrical conductivity (EC) logging has become a commonly used tool for high-resolution characterization of saturated sediments. We evaluated its ability to discern lithologic variations in the thick unsaturated zone that has resulted from extensive groundwater withdrawl on the North China Plain (NCP). Efforts to understand recharge and chemical loading to the NCP aquifer near the Tiahang Mountains are complicated by the presence of a complex network of fluvial and alluvial channels. These well-sorted, fine- to coarse-sand filled, Holocene paleochannels dissect the fine-grained diluvial and loess-mantled piedmont sediments. EC logs were obtained in the upper 20 m of the unsaturated zone using a Wenner array EC probe with 0.015 m vertical resolution. Low total dissolved solids (<0.07 wt%) in adjacent core samples demonstrated that the influence of fluid chemistry on soil EC is minimal. Strong correlations between EC and fine sediments (r² = 0.74) and between soil moisture and fine sediments (r² = 0.78) allowed high EC values to be used to identify fine-grained stratigraphic features. Well-sorted, fine- to coarse-sand layers were identified in grain-size analysis of seven continuous soil cores, verifying the presence of the paleochannels. The depths of these layers correspond with baseline conductivity values in adjacent EC logs, in sharp contrast with over- and underlying fine-grained layers. The collection of 50 EC logs in the 1200 km² study area allowed for refinement of paleochannel margins, depths, and thicknesses predicted in previous studies. The mobility and high-resolution capability of the direct-push EC approach enable rapid access to more detailed information on the distribution of preferential flow paths in NCP sediments than is possible using conventional driller's log and map surveys.