Fracture Control of Ground Water Flow and Water Chemistry in a Rock Aquitard

Abstract

There are few studies on the hydrogeology of sedimentary rock aquitards although they are important controls in regional ground water flow systems. We formulate and test a three-dimensional (3D) conceptual model of ground water flow and hydrochemistry in a fractured sedimentary rock aquitard to show that flow dynamics within the aquitard are more complex than previously believed. Similar conceptual models, based on regional observations and recently emerging principles of mechanical stratigraphy in heterogeneous sedimentary rocks, have previously been applied only to aquifers, but we show that they are potentially applicable to aquitards. The major elements of this conceptual model, which is based on detailed information from two sites in the Maquoketa Formation in southeastern Wisconsin, include orders of magnitude contrast between hydraulic diffusivity (K/S(s)) of fractured zones and relatively intact aquitard rock matrix, laterally extensive bedding-plane fracture zones extending over distances of over 10 km, very low vertical hydraulic conductivity of thick shale-rich intervals of the aquitard, and a vertical hydraulic head profile controlled by a lateral boundary at the aquitard subcrop, where numerous surface water bodies dominate the shallow aquifer system. Results from a 3D numerical flow model based on this conceptual model are consistent with field observations, which did not fit the typical conceptual model of strictly vertical flow through an aquitard. The 3D flow through an aquitard has implications for predicting ground water flow and for planning and protecting water supplies.

Reply

“…The presence of bedding plane fractures is supported by ground water geochemical data (Figure 7 in Eaton et al [2007]). …”

“…Maquoketa Formation, we also emphasize the role of bedding-plane fractures in the three-dimensional flow system (Figures 2 and 10 in Eaton et al [2007]) and present an innovative application of thin model layers in our MOD-FLOW model ( Table 1 in Eaton et al [2007]). The presence of bedding plane fractures is supported by ground water geochemical data (Figure 7 in Eaton et al [2007]).…”

“…We thank Dr. Batu for his comments on our paper (Eaton et al 2007) on three-dimensional ground water flow in the Maquoketa Aquitard. We agree that the classic analytical methods for leaky aquifer analysis, developed by Neuman and Witherspoon (see reference citations given by Batu) and others, are still relevant.…”

Vertical heterogeneities of hydraulic aquitard parameters: preliminary results from laboratory andin situmonitoring

“…This could partially explain why the petrophysical data are, in the worst case, two orders of magnitude higher than vertical K deduced from in situ measurements. This influence of fractures was described by Eaton et al (2007) for a larger scale of characterization of the impact of laterally extensive bedding-plane fracture zones on vertical hydraulic head gradient. Our study emphasizes the influence of those discontinuities on the estimation of hydraulic parameters and the choice of the methodology of characterization.…”

A numerical investigation to illustrate the consequences of hydraulic connections between granular and fractured-rock aquifers

“…The depleted aquifer is discharged, whereas the supplied aquifer is recharged. This case of hydraulic exchange between two aquifers via an aquitard is well known and documented (see for example Eaton et al 2007), and was analytically resolved by Hantush (leaky aquifer model: Hantush 1960). Another type of case, albeit less reported and frequently ignored, consists of two directly superimposed aquifers, one of a granular type and the other of a fractured rock type, with hydraulic connections existing between them (Fig.…”