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Dundas, Canada

Worthington S.R.H.,Worthington Groundwater | Smart C.C.,University of Western Ontario
Carbonates and Evaporites | Year: 2013

Defining protection areas around wells and springs used for water supply currently relies almost exclusively on model-derived groundwater velocities. Tracer testing provides actual groundwater velocities but is seldom undertaken. The reticence to undertake tracing can partly be attributed to the dilemma in selecting a tracer mass that can be unambiguously identified without unacceptable contamination of the water. A large number of ad hoc equations have been proposed to evaluate the mass of tracer required for tracer tests between sinking streams and springs or between wells but the accuracy of such equations has not been assessed. Here, a meta-analysis is undertaken of 211 natural-gradient sinking stream to spring tests and 44 forced-gradient well to well tests in carbonate aquifers, mostly using fluorescent dyes. The pertinent variables are mass (M), discharge (Q), linear distance (L), peak concentration (c) and travel time (t). Simple linear regression of log-transformed combinations of trace variables has been used to generate bivariate power relations. Regression analysis shows that the two equations first proposed by Martel and by Dole had the highest correlations, with the best fit for sink to spring tests being M/c = 23(LQ)0.97 and M/c = 0.76(tQ)0.99, respectively, using base SI units (m, s, g). For the well to well tests M/c = 3,100(LQ)0.97 and M/c = 4.1(tQ)1.02. Equations using travel time are marginally better fits, but are impractical for most applications. Well to well tracer tests exhibit much greater variability than sink to spring tests reflecting variations in preferential flow. © 2013 Springer-Verlag Berlin Heidelberg. Source

Worthington S.R.H.,Worthington Groundwater | Smart C.C.,University of Western Ontario | Ruland W.,Citizens Environmental Consulting
Journal of Hydrology | Year: 2012

Preferential flow through solutionally enlarged fractures can be a significant influence on travel times and source area definition in carbonate aquifers. However, it has proven challenging to step beyond a conceptual model to implementing, parameterizing and testing an appropriate numerical model of preferential flow. Here both porous medium and preferential flow models are developed with respect to a deadly contamination of the municipal groundwater supply at Walkerton, Ontario, Canada. The preferential flow model is based on simple orthogonal fracture aperture and spacing. The models are parameterized from borehole, gamma, flow and video logs resulting in a two order of magnitude lower effective porosity for the preferential flow model. The observed hydraulic conductivity and effective porosity are used to predict groundwater travel times using a porous medium model. These model predictions are compared to a number of independent estimates of effective porosity, including three forced gradient tracer tests. The results show that the effective porosity and hydraulic conductivity values closely match the preferential flow predictions for an equivalent fracture network of ∼10. m spacing of 1. mm fractures. Three tracer tests resulted in groundwater velocities of hundreds of meters per day, as predicted when an effective porosity of 0.05% was used in the groundwater model. These velocities are consistent with a compilation of 185 tracer test velocities from regional Paleozoic carbonate aquifers. The implication is that carbonate aquifers in southern Ontario are characterized by relatively low-volume dissolutionally-enlarged fracture networks that dominate flow and transport. The porous matrix has large storage capacity, but contributes little to transport. Numerical models based on much higher porosities risk significantly underestimating capture zones in such aquifers. The hydraulic conductivity - effective porosity prediction framework provides a general analytical framework for a preferential flow carbonate aquifer. Not only is the framework readily parameterized from borehole observations, but also it can be implemented in a conventional porous medium model, and critically tested using simple tracer tests. © 2012 Elsevier B.V. Source

Worthington S.R.H.,Worthington Groundwater
Journal of Hydrology | Year: 2015

Transport in bedrock aquifers is complex because there is often substantial flow through fractures, and the apertures and interconnectivity of these fractures are usually uncertain. Single-porosity numerical models often give satisfactory results for simulating flow. However, simulating transport is more challenging and results based on single-porosity assumptions can yield inaccurate results. Seven cases are reviewed where travel times were found to be unexpectedly short. Results show that dual-porosity flow is common, with advective flow through fracture networks and immobile storage in the matrix. However, in some cases a dual- or multiple-permeability (or porosity) approach provides better simulations of aquifer behavior. Fracture porosity of bedrock aquifers is usually <1%, resulting in rapid groundwater velocities in many aquifers. Overestimation of the effective porosity is the most common reason for the overestimation of travel times. Residence times of artificial tracers in bedrock aquifers are typically two to three orders of magnitude less than residence times of environmental tracers because the latter are retarded by matrix diffusion. Fourteen diagnostic tests for determining the appropriate conceptual model for bedrock aquifers are described. © 2015 Elsevier B.V. Source

Worthington S.R.H.,Worthington Groundwater
Bulletin of the Geological Society of America | Year: 2015

Carbonate aquifers are some of most challenging to characterize because dissolution can greatly enhance permeability, but its effects are often difficult to determine. This study analyzes data from caves, wells, and tracer tests to explore the extent of solution channel networks and the factors that influence their development. The nonlinear dissolution kinetics of calcite, mixing of waters with different CO2 concentrations, and unstable dissolution fronts all promote the development of solution channels, which are widespread in unconfined carbonate aquifers. Fractures are important for guiding channels at a local scale, but hydraulic gradients are the dominant control at a regional scale. Channels provide continuous, large-aperture pathways that result in rapid groundwater flow. Small channels are much more abundant than large channels, and often account for most of the permeability measured in wells. Caves represent the largest channels; they are more common in limestone than in dolostone, and the development of caves rather than smaller channels is also favored where there is sparse fracturing, low matrix porosity, and the presence of sinking stream recharge rather than percolation recharge. Solution channel networks have fractal properties, and their presence explains why carbonate aquifers have higher permeability than aquifers in any other rock type. © 2014 Geological Society of America. Source

Worthington S.R.H.,Worthington Groundwater | Alexander E.C.,University of Minnesota
Earth-Science Reviews | Year: 2016

The permeability of bedrock aquifers varies by more than four orders of magnitude between different lithologies, but the reasons for this large range remain unexplained. In this review, we examine the role that weathering plays in enhancing the permeability of the five major hydrolithologies, represented by limestone, basalt, granite, sandstone and shale. In limestone aquifers, rapid dissolution kinetics and congruent dissolution result in widespread permeability enhancement. Weathering is usually focused along fractures, and feedbacks between flow and dissolution result in self-organization into networks of channels that discharge at springs. Caves represent prominent examples of weathering. In silicate aquifers, slower dissolution kinetics and incongruent dissolution make it more difficult to predict permeability enhancement. However, positive correlations between permeability and both the solute concentrations and the dissolution rates of the five major lithologies suggest that weathering is a major factor that enhances permeability in silicate as well as in carbonate aquifers. This explains why the largest springs occur in the most permeable lithologies, why groundwater velocities > 10 m/d are common, and why microbial contamination is more common in bedrock aquifers than in unconsolidated sediments. Differences in weathering rates explain why limestone is much more permeable than shale, and why mafic igneous rocks such as basalt have higher permeabilities than felsic igneous rocks such as granite. Weathering appears to play an important role in enhancing permeability in most bedrock aquifers. © 2016 Elsevier B.V. Source

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