No abstract

No abstract

Groundwater flow in karst terrains is difficult to map because it can be concentrated through conduits that do not necessarily coincide with the surface features. We applied electrical resistivity (ER) and self-potential (SP) techniques at three sites to locate an inferred trunk conduit feeding a major spring in the Inner Bluegrass region of Kentucky (USA). Royal Spring is the primary water supply for the city of Georgetown; the upper part of its basin coincides with the Cane Run watershed. ER profiles (972 m total length) were measured using a dipole–dipole electrode configuration with 2- to 3-m spacing. SP measurements were taken along those ER lines and an additional test profile (230 m) using one stationary reference electrode and another roving electrode at a fixed interval. The SP technique has been used by many researchers to detect the electro kinetic potential generated by groundwater flow. The low resistivity of water in the conduit, as compared to the high background resistivity of limestone bedrock, was the ER exploration target. A negative SP anomaly corresponded to a low ER anomaly for most of the profiles, but a few are not comparable. Although SP data collected over multiple days along the test profile differed significantly, they showed similar trends. Field drift in SP data was found to be highly sensitive to temperature changes during the time of measurement. Although the overall trends of the final SP profiles for different dates were similar, the SP magnitudes varied with the amount of precipitation and the average soil temperature. The low-resistivity anomalies in the 2D inverted sections and corresponding negative SP anomalies at two sites (Berea Road and Kentucky Horse Park) encountered water-filled conduits, although mudfilled voids encountered during drilling at University of Kentucky Agricultural Research Farm sites suggest that these may be tributary conduits rather than the trunk conduit.

Focused groundwater discharge to streams is problematic at contaminated sites because high fluxes can limit natural attenuation in the hyporheic zone. However, information on location, spatial evolution, and temporal persistence of springs in unlithified sediments over multiyear time scales is limited. We examine discharge at point (~1‐m) to reach (~300‐m) scales along a stream that intercepts trichloroethene and technetium‐99 plumes from a Superfund site. During 2011 to 2012, we seasonally monitored stream and spring flow and contaminant concentrations, along with probing streambed temperatures on a grid in winter and summer, building on prior monitoring during 1999 to 2002. Baseflow measured by both gauging and dye dilution generally increased with distance downstream, and stream and spring discharge varied seasonally, from minima in October to January to maxima in February to June. Thermal anomalies identified by probing occupied approximately 3% to 6% of the reach and typically coincided with visible springs or seeps. Locations of anomalies were similar to those identified in summer 2002, although some orifices disappeared and others emerged. Vertical groundwater fluxes calculated from probing tended to be less than net fluxes calculated from stream discharge, perhaps in part because the assumption of one‐dimensional, steady‐state flow in calculating point fluxes was simplistic. Maximum contaminant concentrations and fluxes decreased between 1999 to 2001 and 2011 to 2012 as a result of partial capture by an upgradient pump‐and‐treat system. Our findings confirm that springs in unlithified sediments can remain stationary within a few meters over decadal time scales, and seasonal variability in discharge can be greater than decadal variability.

The Lesser Himalayan Belt in the northern margin of Indian Shield preserves abundant sedimentary and magmatic records related to the assembly and breakup of the Columbia supercontinent. However, the Palaeoproterozoic tectonic history of the northern margin of the Indian Shield and its position in the Columbia supercontinent are still controversial. Based on detailed geological investigations in the Dailekh area of western Nepal, we conducted zircon U-Pb dating and Lu-Hf isotopic analysis for meta-granitoids that intruded into the Lesser Himalayan Crystalline and Metasediments. The Dungeshworl granitic augen gneiss, meta-monzogranite, and tonalitic gneiss yielded crystallization ages of 1,829 ± 12 Ma, 1,856 ± 11 Ma, and 1,852 ± 11 Ma respectively, which demonstrate that the deposition limits of both the Lesser Himalayan Crystalline and Metasediments are the Palaeoproterozoic. Zircon ε Hf (t) values of these meta-granitoids range from À3.99 to +0.18, with corresponding depleted mantle two-stage model (T DM2 ) ages of 2.41-2.62 Ga, indicating that they formed by reworking of Neoarchean to Palaeoproterozoic crust. Synthesizing existing geochronological, petrochemical, and zircon Lu-Hf isotopic data for metamorphic sedimentary and igneous rocks from the entire Lesser Himalayan Belt, we suggest that the Palaeoproterozoic magmatism in the northern margin of Indian Shield may be formed within subduction-related tectonics responded to the assembly of Columbia supercontinent.