THE DRILLERS LOG CHALLENGE: MODELING GEOLOGIC FEATURES WITH LIMITED DATA SETS

Typical groundwater projects require aquifer characterization with less than optimal hydrogeologic data sets, consisting primarily of drillers logs of varying (and typically limited) quality. The resulting challenge is to combine conceptual geologic models with the generalized well logs to produce useful numerical models. In this study, we assessed over 10,000 drillers' logs from the Tuolumne River fluvial fan, located around Modesto, California, in order to model the influence of coarse-grained incised valley fill (IVF) deposits on regional-scale hydraulics. Previous studies (Weissmann, et al. 2002, 2004, 2005) indicated that these coarse-grained IVF deposits (1) consist of a fining upward succession with a thick (4-8 m) gravel/cobble base fining upward to thick coarse sands that are ultimately capped by finer grained sediments; (2) are approximately 1.5-2 km wide; (3) have a depth of approximately 30 m near the fan apex thinning to 2-3 m on the distal fan; and (4) extend the length of the fans (approximately 40-60 km). Because these coarse-grained units cause significant change in drilling character, drillers usually identify the thick basal gravel/cobble units of the IVFs with some accuracy. Thus, we searched the drillers logs for thick (>3m) gravel deposits. Identified wells were then classified into 4 groups based on the character of overlying sediments reported in the log, where rank 1 wells matched the conceptual IVF thickness and facies character and rank 4 only showed thick gravels. By mapping rank 1 and 2 wells and evaluating depths of IVF base from these wells, we distinguished four possible IVF trends in the Tuolumne River fan. These were explicitly incorporated into groundwater models adapted from a previously developed USGS model. Simulation results indicate that the IVF deposits significantly influence groundwater flow patterns by: (1) creating a large zone of flow convergence near the fan apex; (2) providing a regional-scale preferential pathway for ground-water flow; (3) increasing overall dispersion; and (4) reducing average groundwater residence times. These findings may be significant for developing artificial recharge projects using the IVF deposits, as well as understanding contaminant fate and transport from regional non-point sources.