Rae Mackay

A fast algorithm for the estimation of the equivalent hydraulic conductivity of heterogeneous media

Fast upscaling of hydraulic conductivity is a recurrent problem in modeling flow through heterogeneous porous media. We propose a new renormalization technique. It is based on the iterative application of the Cardwell and Parsons (1945) bounds on elementary groups of cells. The combination of the bounds with a heuristic formula allows anisotropy to be taken into account. The new technique is tested and compared with other fast techniques. Among the tested techniques the two most reliable ones are the tensorial renormalization and the new simplified renormalization. The numerical efficiency of the simplified renormalization leads us to recommend it when a diagonal tensor of equivalent conductivity is sufficient.

The use of electrical resistivity tomography in deriving local-scale models of recharge through superficial deposits

Abstract The way in which superficial deposits affect groundwater recharge is often a significant source of uncertainty in groundwater resources and vulnerability assessments. A study of a small catchment in Shropshire, UK, shows how electrical resistivity tomography (ERT), with a degree of borehole control, can be an effective tool for defining the geometry of superficial deposits for purposes of inferring the hydraulic processes controlling groundwater recharge. Major lithological units were mapped to within c. 0.5 m vertically and 5 m horizontally using ERT surveys with a minimum electrode spacing of 2 m. Interpretation was aided by the strong contrast in resistivity between till and glaciolacustrine deposits (20–40 Ω m) and glaciofluvial deposits (generally >100 Ω m) that overlie the Permo-Triassic sandstone aquifer (saturated resistivity 60–145 Ω m) in the study area. A range of local-scale (tens to hundreds of metres) recharge models are presented, based on the findings of the field surveys, and it is shown how existing mapping misses key features of the superficial geology that may be very significant in enhancing or restricting aquifer recharge.

The Effects of Urbanization on Groundwater Quantity and Quality in the Zahedan Aquifer, southeast Iran

Abstract This paper investigates the impacts of urban growth on groundwater quality and quantity in the Zahedan aquifer, which is the sole source of water supply for the city of Zahedan, Iran. The investigation is based on the collection of available historical data, supplemented by field and laboratory investigations. Groundwater levels in 40 wells were measured in December 2000. In addition, 102 water samples were taken in two periods during November and December 2000. Of these, 43 samples were analyzed for major ions, 32 samples were analyzed for nitrogen and phosphorus and the remainder for bacteriological contamination. The water level data show that there has been a general decline since 1977 due to over-abstraction. The magnitude of this decline has reached about 20 m in some places. However, in one area over the same period, a rise of about 3 m has been observed. This occurs as a result of the local hydrogeological conditions of shallow bedrock and relatively low permeability materials down stream of this area that limits the flow of groundwater towards the northeastern part of the aquifer. The general fall in groundwater levels has been accompanied by a change in the direction of the groundwater flow and an overall reduction of the areal extent of the saturated region of the aquifer. The city now has a serious problem such that even if the abstracted groundwater is rationed, water is not available for long periods because the demand far exceeds the supply. The heavy impact of urbanization on the groundwater quality is shown through the observed high nitrate (up to 295 mg/l as nitrate) and high phosphorus values (about 0.1 mg/l as P). Significant changes in the chloride concentration are also observed in two areas: increasing from 100 mg/l to 1,600 mg/l and from 2,000 mg/l to 4,000 mg/l, respectively. Furthermore, the bacteriological investigations show that 33 percent of the 27 collected groundwater samples are positive for total coliform and 11 percent of the samples contained fecal coliforms indicating that local sources are strongly influencing the observed chemical data. Greater depths to groundwater reduce the observation of coliform contamination. In general, the unplanned urban development in Zahedan has significantly degraded the regions water resources and significant actions such as upgrading the sewage waste disposal system, locating other sources of water supply, and strict groundwater management will all be needed to resolve the problems that have arisen.

Determining fracture properties by tracer and thermal testing to assess thermal breakthrough risks for ground source heating and cooling in the Chalk

Abstract Numerous open-loop ground energy systems are under construction or have been constructed in central London. The majority of these systems use the Chalk aquifer as a water source. A significant proportion of the abstracted water must be returned to the aquifer to maintain groundwater levels. If the ground system is to function correctly, the temperature of the abstracted groundwater must not be altered significantly by early thermal breakthrough of the returned water. Groundwater flow within the Chalk is predominantly through fractures and these provide the primary route by which thermal breakthrough might take place. The nature of the fracturing and its impact on the thermal transport beneath a proposed site must be understood to provide confidence that the ground energy system will function correctly. Two tracer and thermal test methods to determine fracture properties are examined. The results for a site with a borehole separation of 100 m suggest that the flow in the Chalk beneath the site is carried by a small number of fractures and that careful modelling and interpretation can provide valuable constraints on the frequency of fractures, their spatial pattern and their hydraulic aperture. The results highlight difficulties for designing within-borehole temperature monitoring systems for fractured aquifer thermal experiments.

Measurement of the Tensile Strength of Organic Soft Rock

Understanding all potential slope failure mechanisms is a pre-requisite for predicting the likelihood of batter movements during excavation in open cut mines. The tensile behavior of soils and rocks may be a significant contributor to a slope failure and must be known in order to quantify the risks of slope failure. The contribution can be particularly significant for Intermediate Geotechnical Materials (IGMs) that possess characteristics of both soils and rocks and where the failure mechanisms are complex due to the interplay between ductile and brittle behavior. Brown coal is such an intermediate geotechnical material. Recent batter movements in the brown coal mines in the Latrobe Valley, Australia have raised doubts about the current understanding of the mechanisms of slope failure in this material. Research is underway to re-evaluate all properties of the brown coal applicable to slope failure. This paper describes the investigation of brown coal tensile strength. There are alternative test methods available to determine the tensile behavior of materials, including direct tensile tests, beam bending tests and Brazilian compression tests. The applicability of each test method is material dependent and, as such, it is necessary to confirm the validity of the methods for each material. Beam bending tests have achieved mixed results for both rocks and IGMs previously. Thus, the present work has explored only the use of Direct tensile and Brazilian test methods. Both methods were implemented using a modified direct shear apparatus and valid test procedures for both test methods were developed. Each test procedure has been verified by Finite Element Modelling (FEM) using ABAQUS 6.12.1 FEM code. The results from the laboratory test methods are in good agreement and show that brown coal is a predominantly brittle material with a peak tensile strength slightly greater than 100 kPa. The finite element analyses confirm that non-uniformity of the tensile stresses during sample loading tends to lead to the underestimation of tensile strength for both tests, but the Brazilian test has less bias for brown coal. It is observed that the rate of loading of low stiffness, low permeability, and saturated samples in the Brazilian test is an important test design parameter for the accurate determination of tensile strength of IGMs in the laboratory.

A Multi-scale Agent-Based Distributed Simulation Framework for Groundwater Pollution Management

Groundwater is like dark matter -- we know very little apart from the fact that it is hugely important. Given the scarcity of data, mathematical modelling can come to the rescue but existing groundwater models are mainly restricted to simulate the transport and degradation of contaminants on the scale of whole contaminated field sites by averaging out the effect of spatial heterogeneity on the availability of the pollutant to the degrading organisms. These coarse-scale mean-field models therefore tend to rely on fitting to data rather than being predictive. Also, they are less suited to incorporate spatial variability and non-linear kinetics and feedbacks. We propose to solve the two mutually exacerbating problems of environmental patchiness and data scarcity by developing a flexible and robust distributed simulation framework that uses an ensemble of small scale simulations running on different processors/computers to scale-up, i.e. to feed the effect of small-scale patchiness into a concurrent site-scale simulation of the dynamics of groundwater pollutant degradation. Our scaling approach solves problem #1 by simulating dynamics also on the small scale where some of the patchiness resides, and problem #2 by enabling rigorous validation of our small-scale model and scaling approach with laboratory data, which are high quality at low cost.

Development of a Methodology to Quantify the Importance of Hydro-Mechanical Processes in Radionuclide Migration Assessments

Abstract A methodology for quantifying the contributions of hydro-mechanical processes to fractured rock hydraulic property distributions has been developed and tested. Simulations have been carried out on discrete fracture networks to study the sensitivity of hydraulic properties to mechanical properties, stress changes with depth, mechanical boundary conditions, initial mechanical apertures and fracture network geometry. The results indicate that the most important (and uncertain) parameters for modelling HM processes in fractured rock are fracture density and rock/fracture mechanical properties. Aperture variation with depth below ground surface is found to be of second order importance.

Review of the historical data characterizing Latrobe Valley brown coal consolidation behaviour

Abstract Victoria, Australia contains 25% of the worlds known brown coal reserves. Much of the coal is located in thick seams overlain by a thin veneer of sands and clays. Three brown coal open-cut mines are presently operating in the Latrobe Valley. As a result of the mining activity, ground movements can be significant within and around the mines. Slope stability has to be managed during all mine development phases. Recent stability issues at some mines have highlighted a need to review the historical geotechnical information and data with a view to guiding future data collection. To support this purpose, the existing understanding of the geomechanical properties of the brown coal has been revisited, with emphasis on impacts of unloading. The coal has low density (specific gravity <1.2) and high water content (>60% by volume). Given this background, historical data for 49 consolidation tests on one of the major mined upper coal seams, the Morwell seam, were extracted from the historical records. Additionally, 44 consolidation tests for interseam materials were also extracted for the same area. These data have been analysed by re-evaluation of the consolidation data. The analysis shows that the behaviour of brown coal differs from that of typical engineering soils, in that at lower load the stress v. strain gradient is higher than for greater loads. At higher loads the stress v. strain gradient is almost constant. These observations indicate that reassessment of the unloading parameters for slope design in the open cuts is required.