ABSTRACT

Faults play a critical role in a wide variety of hydrogeological phenomena ranging from the role of faults in metalliferous ore-formation in sedimentary basins to the assessment of the risk of the spread of contaminated groundwater in faulted aquifer systems. This study focuses on fault zone permeability within poorly consolidated sediments using an exceptionally-rich field data set which includes high resolution geological data, and multi-decadal records of hydraulic head observations from a dense network of several hundred observation wells from the Lower Rhine Embayment that forms the southernmost Roer Valley Rift System (at the border area between Germany, the Netherlands and Belgium, Europe). As part of this study, thermal, isotopic, and geochemical data are collected from these wells, some of which cut through fault zones. A high resolution three-dimensional finite-element model of faults in the Lower Rhine Embayment will be developed to test new and existing conceptual models of fault permeability. One of the main questions is whether faults can act as conduit-barrier systems in which vertical fluid flow is enhanced while horizontal flow is impeded. Such a system could exist as a result of a strongly anisotropic permeability in the fault zone. We hypothesize that geochemical data along faults in the Roer Valley Rift System can serve as robust tracers which can document enhanced vertical fluid flow along faults. We will test a new algorithm that we developed to calculate fault permeability anisotropy in layered sedimentary sequences as a function of fault throw, effective stress and the clay-content of the lithologies flanking the fault zone. One of the main objectives of this study is to test this algorithm as well as the existing Fault Seal Analysis method developed in the petroleum industry.

Results of the project will provide an improved understanding of factors controlling groundwater flow within faulted sedimentary aquifers. Many urban areas in the southwestern US are situated in extensional tectonic settings (e.g. Las Vegas, Albuquerque) and rely for their water supply upon siliciclastic aquifer systems cut by numerous faults. Given the increased demands on existing groundwater sources, the project will directly contribute to an improvement in the sustainable development of groundwater resources in extensional basins. The methods developed for fault seal prediction and fault property inclusion in groundwater flow models will improve the accuracy and efficiency of groundwater flow predictions and ultimately groundwater management. The risks of contaminant flow are becoming more acute given the increased usage of groundwater and associated commercial developments within urbanized areas. The knowledge and methods generated by this project will help improve risk assessment exercises associated with contaminant transport within aquifers containing faults (e.g. Yucca Mountain).

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