Simple models of the relationship between well and spring flows in basalt aquifers

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Zemansky, G.M. 2007 Simple models of the relationship between well and spring flows in basalt aquifers. Lower Hutt, N.Z.: GNS Science. GNS Science report 2007/26 16 p. + 1 CD

Abstract: The Northland Regional Council requested assistance from GNS Science with regard to the question of finding technically appropriate but simple ways to estimate the effects of groundwater takes on spring flows in the basalt aquifers that exist within that Region. This report was prepared in response. Basalt aquifers are fractured rock systems in which groundwater flows primarily through the fracture network. However, to some degree the unfractured blocks of the matrix also contribute. Such dual porosity systems are by nature complex and the information to accurately model them seldom exists. However, the simplification of modeling them as equivalent porous media is frequently used and some data to support this approach at the larger scale of capture zones has been developed. Similarly, delineating the source area of springs in basalt aquifers is also complex. However, unlike a pumping well, it can reasonably be assumed that the source area of a spring will be upgradient of the location of the spring. Simple mathematical models of capture zones for wells are available that can be used to define both the capture zone for the well and the source area of the spring. Two such analytical models are presented in this report: (1) the steady state model of Grubb (1993); and (2) the transient model of McElwee (1990). These can be used in a regulatory approach to assist in considering the implications of consenting a well in the vicinity of a spring within the circumstances of a basalt aquifer. Such a regulatory approach is proposed and an example of its application given. Use of these models requires limited site-specific information, some of which can be estimated from knowledge of the circumstances of the well and spring. Since these models involve simplifying assumptions (e.g., that the aquifer is homogeneous and isotropic and the well is fully penetrating and producing only horizontal flow), this approach cannot be considered precise. However, it may be sufficient to provide a good first cut for relatively small wells and springs in many cases. It cannot replace more detailed site-specific analysis where larger wells and springs are involved and proposed uses constitute major projects. (auth)