HR: 0800h
AN: H41F-0474    [Abstracts]
TI: Delineating Tracer Test Input Functions from Fluid Electrical Conductivity Logging in Fractured Porous Rocks
AU: * Butler, A P
EM: a.butler@imperial.ac.uk
AF: The Department of Civil and Environmental Engineering, Imperial College London, South Kensington Campus, London, SW7 2BU United Kingdom
AU: Mathias, S A
EM: simon.mathias@imperial.ac.uk
AF: The Department of Civil and Environmental Engineering, Imperial College London, South Kensington Campus, London, SW7 2BU United Kingdom
AU: Williams, A
EM: atw@bgs.ac.uk
AF: British Geological Survey, Maclean Building, Crowmarsh Gifford, Wallingford, OX10 8BB United Kingdom
AB: A radially convergent tracer test was carried out in the Chalk outcrop of Berkshire, UK. Tracer was injected into two boreholes lying 32 m and 54 m from the abstraction hole. One breakthrough curve (BTC) was uni-modal, had a first arrival time of 15 min and a late-time log-log slope (LTS) of 1.7. The other (more distant test) exhibited three distinct peaks, had a first arrival time of just 4 min and a LTS of 2.1. A LTS of 1.5 is indicative of Fickian matrix diffusion (Haggerty et al., 2000). Previously, it has been suggested that LTSs greater than 1.5 are indicative of multiple mass-transfer rates (Haggerty et al., 2000) and/or multiple flow-pathways (Becker et al., 2003). In the case of second test, the latter hypothesis is further supported by the presence of multiple peaks. Both tracers (Fluorescein and Amino G) were combined with a saline solution and a series of fluid electrical conductivity (FEC) logs were performed within the injection wells (during and after tracer injection), along with a suite of more conventional geophysical measurements (including calliper, gamma, temperature and upflow logs). The geophysical measurements allowed the identification of flow horizons. This information was then used in conjunction with an FEC model (similar to that proposed by Tsang et al., 1990) to invert the FEC logs so as to obtain flow rates for each flow horizon. From this, it was found that, in both cases, virtually all of the tracer entered the aquifer through one flow horizon. Furthermore, using the FEC model, it was possible to obtain the continuous time-distribution of the tracer injections. The multiple peaks and enhanced LTS in the second BTC correlated with the form of the estimated tracer injection distribution. The multiple peaks and LTS were not as a result of multiple mass-transfer rates or differential flow-pathways but a function of the non-uniform diameter of the injection well (indicated by the calliper logs) and its various inflow horizons. Such a correlation was less apparent in the former BTC because the advective travel time (as indicated by the first arrival time) was sufficiently long such that the time dependent effects of the injection well were attenuated by matrix diffusion within the Chalk aquifer. The study highlights therefore the importance of understanding tracer test input functions and their elucidation through FEC logging when analysing BTCs , particularly in situations where the advective travel time is comparable with that for the tracer injection.
References: Becker, M. W., and A. M. Shapiro (2003), Interpreting tracer breakthrough tailing from different forced-gradient tracer experiment configurations in fractured bedrock, Water Resour. Res., 39(1), 1024. Haggerty, R., S. A. McKenna and L. C. Miggs (2000), On the late-time behavior of tracer test breakthrough curves, Water Resour. Res., 36(12), 3467-3479. Tsang, C.-F., Hufschmeid, P., Hale, F.V. (1990). Determination of fracture inflow parameters with a borehole fluid conductivity logging method, Water Resour. Res., 26(4), 561-578.}
DE: 1829 Groundwater hydrology
DE: 1832 Groundwater transport
DE: 1847 Modeling
DE: 1894 Instruments and techniques: modeling
SC: Hydrology [H]
MN: Fall Meeting 2005