[1] A multiyear flow and conservative tracer test has been carried out in unsaturated mine waste rock to examine the physical mechanisms by which water moves through this coarse, heterogeneous, granular material. The experimental system has a footprint of 8 m  8 m, is 5 m high, and is built on a contiguous grid of 16 zero-tension lysimeters. A chloride tracer was applied during a single rainfall event. Subsequently, the system has been subject to both natural and applied rainfall events in which no further tracer was added. Water flow and tracer transport is monitored using in situ measurements of moisture content, matric suction, and soil water solution samplers. Results demonstrate for transient infiltration conditions the influence and interaction of matrix flow in a heterogeneous granular matrix, preferential flow in macropores, and noncapillary pathways. Tracer migration through preferential flow paths dominates the initial and peak breakthrough concentrations. Point measurements of tracer concentration from in situ solution samplers yield a relatively poor indication of the flux-averaged transport of mass that is recorded at the base of the experiment, in addition to overestimating the stored mass and underestimating residence time.

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[1] Waste rock piles are an outcome of open pit and underground mining operations. Unprocessed low-grade rock is disposed of in piles from several meters high to 100 mþ high. Waste rock piles may still contain sufficient concentrations of metals to be a potential source of pollution. The evaluation of the potential risk involves properly characterizing flow through these piles under unsaturated conditions. The main flow characteristic of the piles is the presence of a large range of grain and pore sizes. Based on data from an instrumented rock pile located in Saskatchewan Canada, unsaturated flow through the pile is modeled as a linear system after separating a fast and a slow component. Water reaching the base of the pile is monitored by 16 contiguous zero-tension lysimeters. The fast component, flowing through macropores, is assumed to be released instantaneously, while the slow component is simulated using a linear-reservoir model that assumes the presence of an interconnected porous matrix. An empirical transfer function (TF) is computed as the ratio of the spectra of signals between the output (basal outflow) and the input (rainfall time series). Determination of a parametric transfer function model provides information on the characteristic time of water storage in the matrix and on the fraction of the water within each subsection of the experimental pile that is channeled through the macropores. An analysis of the output signal at different support scales is performed, indicating the nonlinearity of the macropore fraction scaling processes.Citation: Trinchero, P., R. Beckie, X. Sanchez-Vila, and C. Nichol (2011), Assessing preferential flow through an unsaturated waste rock pile using spectral analysis, Water Resour. Res., 47, W07532,

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High sample electrical conductivity reduces the quality of a time domain reflectrometry (TDR) waveform by the loss of signal amplitude. Two strategies are examined to obtain higher signal/noise waveforms: (i) waveform differencing by remote diode shorting and (ii) covering probe conductors with resistive coatings. Experiments using electrically conductive water solutions (0–5 dS m−1) and three‐rod Zegelin type probes show conventional dual‐tangent waveform analysis and waveform differencing using manual short circuits can accurately determine travel time but the remote diode shorting method can be systematically biased by the electrical properties of the diodes used. Three‐rod Zegelin‐type probes with a high resistance coating on the central rod permit collection of analyzable waveforms for solutions with electrical conductivities at least as high as 70 dS m−1 Dual‐tangent analysis of the raw waveform is found to be more accurate than the remote diode shorting method within water solutions and within silica sand saturated with an electrically conductive water solution. The probe coating creates a nonlinear relationship between the apparent dielectric permittivity estimated using a coated probe and the actual sample apparent dielectric permittivity. Experimental measurements of this relationship can be fitted using an equation of the form for a coaxial cell. A three‐rod coated probe with a single diode at the probe head is a practical means to collect interpretable waveforms in media with high electrical conductivity. However, measurements of travel time alone may not be sufficient to determine water content in soils with high concentrations of dissolved ions in the soil water solution.

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Thermal conductivity (TC) sensors, which provide estimates of matric suction, were used in a field experiment designed to characterize unsaturated water movement through coarse mine waste rock at a mine site in northern Saskatchewan. Two years of monitoring data were used to evaluate long-term TC sensor performance and accuracy. Thermal conductivity sensor output requires corrections of sensor hysteresis and changes in ambient temperature. A correction method for ambient temperature is derived. A comparison of the uncorrected field measurements with the values corrected for both hysteresis and ambient temperature indicates that the magnitude of these corrections can be similar. Corrected TC sensor measurements are compared to measurements of matric suction made using tensiometers. Thermal conductivity sensor response to the initial arrival of a wetting front lagged 1–3 days behind the tensiometer measurements. The TC sensor data tended to overestimate matric suction in the waste rock, when compared to the tensiometer data. Long-term drift in the TC sensors located at depths of 50 cm and below (where the sensors have been continuously exposed to matric suctions less than 20 kPa) has lead to data that are not interpretable using the calibration curves derived prior to sensor emplacement in the waste rock pile.Key words: matric suction, thermal conductivity sensor, hysteresis, temperature.