On the relation between steep monoclinal flexure zones and steep hydraulic gradients.

Steep hydraulic gradients are found in association with steep monoclinal flexures. However, the physics of the reduction of the hydraulic conductivity, which is responsible for the steep gradients, has seldom been studied. We present results of hydrological and mechanical modeling aiming to study the effect of such steep hydraulic gradients demonstrated in the Judea Group Aquifer system, Israel. The hydrological configuration of steep dips and anisotropy between flows parallel and perpendicular to the bedding planes was simulated using the FEFLOW code. It exhibited a situation whereby part of the flow is oblique to the bedding planes and therefore some steepening of the hydraulic gradients occurred due to actual conductivity reduction. However, this reduction is not enough to account for the steeper gradients observed. The effect of a deep-seated reverse fault under the monocline on the permeability distribution within the structure was examined by numerical mechanical simulations. It exhibited a compressional stress distribution in the steep part of the monocline, which, due to shortening and closure of joints and voids, is presumably responsible for a significant pressure-induced permeability reduction. This process by itself in a layered structure, including interlayering of thin marl layers, could be responsible for the steep hydraulic gradients in the steep part of the monocline.