Analysis of Ground Water Flow and Deformation in the Vicinity of DUSEL Homestake

The Deep Underground Science and Engineering Laboratory (DUSEL) is an underground facility planned for the workings of the former Homestake gold mine in the northern Black Hills, South Dakota. The mine workings cover several km2 in plan and extend to a depth 2.4 km. The area is underlain by Proterozoic metamorphic rocks that were deformed into regional-scale folds whose axes plunge approximately 40o to the SSE. A conceptual model and preliminary numerical analysis of the hydrogeology of the area indicates that permeability depends on effective stress, with values ranging from 0.1 mD at a depth of 2 km to 100 mD at depths of 100m. A deep ground water flow system is contained within a surface-truncated ellipsoid roughly 8 km by 4 km in plan view and 5.5 km deep with its short-axis aligned to the strike of the workings. The deep flow system consists of a zone of relatively rapid flow from the ground surface to the workings overlying the southern part of the mine, and a much larger ellipsoidal zone extending up to several km from the workings where water has been removed from storage. Numerical analyses of the ground water flow and poroelastic deformation in the vicinity of Homestake DUSEL have been refined by sharpening the 3-D resolution of important features. Mine workings have been resolved into three to four major regions where relatively large densities of rock were removed. These mined regions are shaped roughly like plunging ellipsoids with minor axes of several hundred m and major axes up to more than 2 km. They are treated in the simulations as highly permeable regions with an average elastic modulus significantly less than the intact rock; e.g. like soft, permeable inclusions. Recent field investigations and evaluation of the mine database indicate the presence of a relatively large fault that strikes approximately N20W, roughly parallel to the mined out regions. The Homestake fault dips 60NE, cutting the top of one mined region and extending beneath and adjacent to another mined region. It extends at least 1.5 km along strike and dip, with a center roughly 1.5 km deep along the western side of the mine. The model includes large-scale folds, and in particular it includes the geometry of the Yates member at the core of the Lead Anticlinorium. Topography and the stream drainage network are also included. The findings generally confirm previous results of Murdoch et al. [Eos Trans. AGU, 90(52), Fall Meet. Suppl., Abstract H23E-1009], but the finer resolution from the new analyses provides improved detail on the distribution of hydraulic head and stress state in the vicinity of the workings. Including the poroelastic effect shows that dewatering of the mined workings may cause larger deformations than were previously expected, and it also indicates that dewatering should affect the distribution of stresses on the Homestake fault.