Sachin Karan

Dynamic streambed fluxes during rainfall‐runoff events

Observed 3-D temperature distributions within a streambed were used to analyze the effects on exchange fluxes between groundwater and the stream during rainfall-runoff events. By combining a dense vertical and lateral monitoring network of streambed temperatures with coupled surface/subsurface 3-D flow and heat transport modeling, we demonstrate how temperature can be used directly as a calibration target. Three model setups with different hydraulic conductivity distributions were evaluated in an optimization approach using temperature and hydraulic head data. The hydraulic conductivity distributions were based on slug test surveys within the streambed and aquifer. A detailed characterization of the hydraulic conductivity of the streambed and aquifer is needed to accurately simulate observed temperatures. Hence, the most sensitive parameters, the vertical hydraulic conductivity and the thermal conductivity, were calibrated within different conductivity zones of the heterogeneous model. Simulated exchange fluxes across the streambed showed variations up to a factor of four within just a meter. Such differences may not have been correctly predicted using 1-D heat transport models due to lateral conduction amongst the different flow paths. During the rainfall-runoff event, fluxes decreased substantially (−50%) due to a decrease in the hydraulic gradient with increased stream stage. Although no flow reversals were observed during the studied conditions, it is possible that these can occur during larger rainfall-runoff events. We show that with the current sampling and modeling techniques, 3-D temperature data can be used to estimate dynamic exchange in heterogeneous flow fields encountered in the field.

Groundwater flow and heterogeneous discharge into a seepage lake: Combined use of physical methods and hydrochemical tracers

Groundwater discharge into a seepage lake was investigated by combining flux measurements, hydrochemical tracers, geological information and a telescopic modelling approach using first two-dimensional (2D) regional then 2D local flow and flow path models. Discharge measurements and hydrochemical tracers supplement each other. Discharge measurements yield flux estimates, but rarely provide information about the origin and flow path of the water. Hydrochemical tracers may reveal the origin and flow path of the water, but rarely provide any information about the flux. While aquifer interacting with the lake remained under seemingly steady state conditions across seasons, a high spatial and temporal heterogeneity in the discharge to the lake was observed. The results showed that part of the groundwater flowing from the west passes beneath the lake and discharges at the eastern shore, where groundwater springs and high discharge zones (HDZs) are observed at the lake bottom and at seepage faces adjacent to the lake. In the 2D cross-section, surface runoff from the seepage faces delivers 64% of the total groundwater inputs to the lake, and a 2 m wide offshore HDZ delivers 13%. Presence of HDZs may control nutrient fluxes to the lake. This article is protected by copyright. All rights reserved.

Air/water/sediment temperature contrasts in small streams to identify groundwater seepage locations

The need to identify groundwater seepage locations is of great importance for managing both stream water quality and groundwater sourced ecosystems due to their dependency on groundwater borne nutrients and temperatures. Although several reconnaissance methods using temperature as tracer exist, these are subjected to limitations related to mainly the spatial- and temporal resolution and/or mixing of groundwater and surface water leading to dilution of the temperature differences. Further, some methods, e.g., thermal imagery and fiber optic distributed temperature sensing, although relative efficient in detecting temperature differences over larger distances, these are labor-intensive and costly. Therefore, there is a need for additional cost-effective methods identifying substantial groundwater seepage locations. We present a method expanding the linear regression of air and stream temperatures by measuring the temperatures in dual-depth; in the stream column and at the streambed-water interface (SWI). By doing so we apply metrics from linear regression analysis of temperatures between air/stream and air/SWI (linear regression slope, intercept and coefficient of determination), and the daily water temperature cycle (daily meantemperatures, temperature variance and the mean diel temperature fluctuation). We show that using metrics from only single-depth stream temperature measurements are insufficient to identify substantial groundwater seepage locations in a head-water stream. Conversely, comparing the metrics from dual-depth temperatures show significant differences; at groundwater seepage locations, temperatures at the SWI, merely explain 43-75% of the variation opposed to ≥91% at the corresponding stream column temperatures. In general, at these locations at the SWI, the slopes ( 6.5 ∘C) are substantially lower and higher, respectively, while the mean diel temperature fluctuations (<0.98 ∘C) are decreased compared to remaining locations. The dual-depth approach was applied in a post-glacial fluvial setting, where metrics analyses overall corroborated with field measurements of groundwater fluxes and stream flow accretions. Thus, we propose a method reliably identifying groundwater seepage locations along streambeds in such settings.