A new discrete model to better consider tracer distribution along boreholes during the Finite Volume Point Dilution Method.

The Finite Volume Point Dilution Method (FVPDM) is a single-well tracer experiment which has been successfully used in many hydrogeological contexts to quantify groundwater fluxes. During continuous injection of tracer into a well, the tracer concentration evolution measured within the tested well directly depends on the groundwater flow crossing the well screens. Up to now, the FVPDM mathematical formulation used to simulate the tracer concentration evolution measured in the tested well assumed perfect homogenization of the tracer along the tested interval, which is a reasonable assumption in many cases. However, when FVPDM are performed in long-screened boreholes or in very permeable aquifer materials, the recirculation flow rate imposed to ensure mixing is suspected to be too low to perfectly homogenize the tracer. In order to assess the effect of non-perfect mixing on FVPDM results, we introduce here a new discrete model that explicitly considers the recirculation flow rate. The mathematical developments are validated using field measurements, and a sensitivity analysis is proposed to assess the effect of the mixing flow rate on tracer concentration homogenization within the well. Results confirm that, when the recirculation flow rate applied is not high enough compared to the groundwater flow rate, the tracer distribution is not uniform in the tested interval. In this case, the use of the classical analytical solution, commonly used to interpret the concentration evolution leads to highly overestimated groundwater fluxes. The discrete model introduced here can be used instead to properly estimate groundwater fluxes and assess the tracer distribution within the tested interval. The discrete model offers the possibility of interpreting field measurements conducted under non-perfect mixing conditions and increases the range of fluxes that can be investigated through FVPDM.

An ADTS Toolbox for Automatically Interpreting Active Distributed Temperature Sensing Measurements.

Active distributed temperature sensing (ADTS) experiments are very useful to provide in-situ and distributed estimates of thermal conductivities of the subsurface and of groundwater flows. However, the data interpretation can be seen as difficult considering the large amount of data collected along a heated fiber-optic cable and the lack of associated tools for their automated analysis. In this context, we developed an automated routine program for the interpretation of ADTS measurements: the ADTS Toolbox. It contains several codes written in MATLAB that calculate, for each measurement point located along a heated section, both the thermal conductivity of the surrounding material and the groundwater flux. In addition, it provides uncertainties on the estimated thermal conductivities and fluxes according to the temperature resolution (noise) or to errors on temperature measurements. By offering the possibility of automatically interpreting ADTS measurements, the ADTS Toolbox facilitates the use and interpretation of ADTS experiments for characterizing at high resolution the groundwater flows distribution and for imaging the thermal conductivities variability.

A Comparison of Different Methods to Estimate the Effective Spatial Resolution of FO-DTS Measurements Achieved during Sandbox Experiments.

For many environmental applications, the interpretation of fiber-optic Raman distributed temperature sensing (FO-DTS) measurements is strongly dependent on the spatial resolution of measurements, especially when the objective is to detect temperature variations over small scales. Here, we propose to compare three different and complementary methods to estimate, in practice, the "effective" spatial resolution of DTS measurements: The classical "90% step change" method, the correlation length estimated from experimental semivariograms, and the derivative method. The three methods were applied using FO-DTS measurements achieved during sandbox experiments using two DTS units having different spatial resolutions. Results show that the value of the spatial resolution estimated using a step change depends on both the effective spatial resolution of the DTS unit and on heat conduction induced by the high thermal conductivity of the cable. The correlation length method provides an estimate much closer to the value provided by the manufacturers, representative of the effective spatial resolutions along cable sections where temperature gradients are small or negligible. Thirdly, the application of the derivative method allows for verifying the representativeness of DTS measurements all along the cable, by localizing sections where measurements are representative of the effective temperature. We finally show that DTS measurements could be validated in sandbox experiments, when using devices with finer spatial resolution.