Geology and properties of limestone

Highlights
Description of limestone geology Properties of limestone aquifers

Fig. 1: Geologic sequence in eastern Denmark. From: GEO and GEUS (2014).

Fig. 2: Borehole cores from the Akacievej field site with flint inclusions and fractures. Note that some of the fractures and crushing that can be seen in Figure 2 are caused by the drilling.

Geology and hydrogeology In eastern Denmark, glacial Quaternary deposits (clayey till, sand) are usually on top of the limestone aquifers. The uppermost limestone layer is typically a calcarenitic limestone (also called Københavns Kalk), followed by bryozoan limestone. The calcarenitic limestone is rather evenly and horizontally layered and may contain flint layers and nodules. The bryozoan limestone, in contrast, typically shows bank structures and few to no flint inclusions. A good description of the geologic stratification in the greater Copenhagen area is given in the Report by GEO and GEUS (in Danish). Figure 1 shows the typical krono-, bio- and lithostratigraphy in eastern Zealand (Denmark) and in the Øresundsregion. Limestone geologies are often heavily fractured and especially the calcarenitic limestone includes almost impermeable chert layers and nodules. The chert layers can stretch over distances of tens to hundreds of meters or they can occur as loose inclusions embedded in the limestone. Figure 2 shows some borehole cores from the Akacievej site, which illustrate the heterogeneity of the limestone with flint layers, crushed material and fractures. Limestone aquifers are often very heterogeneous and the hydraulic parameters can span over wide ranges and have a strong spatial variation. Limestone aquifers are typically anisotropic, which means that their horizontal hydraulic conductivity is about 2 to 10 times as high as in the vertical direction. The limestone matrix has typically a very low hydraulic conductivity, which can be several orders of magnitude lower than the conductivity in the fractures. In fractured limestone geologies, flow occurs predominantly in the fractures, because they provide a much higher hydraulic conductivity than the limestone matrix. Two different kinds of fractures can be distinguished: horizontal fractures often caused by decompression, and vertical/subvertical fractures, typically caused by tectonic activities. Especially the vertical and subvertical fractures can influence the anisotropic behavior of limestone aquifers, because the dominating flow is guided through the fractures. Due to that it is possible, that the main flow direction differs from the overall hydraulic head gradient, and the direction of the major fractures has to be considered. The orientation of the vertical fractures is, as a rule of thumb, often aligned with major faults in the area . Limestone has a relatively high porosity (between 15 and 40 Vol.-%), whereas the fracture porosity is comparably low (0.5-2 Vol.-%). Properties of the limestone at Akacievej The local geology at the Akacievej site is described here. Based on a set of different measurements and modeling interpretations, hydraulic parameters and transport parameters were determined for the Akacievej site. The employed field methods are described in the Chapters Data acquisition and field methods and Determination of transport parameters and contaminant data. Table 1 gives an overview of values that were determined at the Akacievej site. Tab. 1: Overview of parameters determined at the Akacievej site or for comparable limestone sites. Parameter Value Comment [math] K_{crushed} [/math] [math] 5\cdot 10^{-4} \text{m/s} [/math] crushed limestone conductivity [math] K_{matrix} [/math] [math] 10^{-7}\ \text{m/s} [/math] limestone conductivity [math] n_{matrix} [/math] [math] 0.10 - 0.20 [/math] matrix porosity [math] Ap [/math] [math] 2\ \text{mm} [/math] typical aperture of major horizontal fracture [math] B [/math] [math] 20 - 25\ \text{m} [/math] aquifer thickness [math] D_m [/math] [math] 7.5\cdot 10^{-7} \text{m}^2/\text{s} [/math] effective diffusivity (modeled) [math] k_d [/math] [math] 0.5-1.0\ \text{L/kg} [/math] sorption coefficient for PCE on limestone The sorption behavior for chlorinated solvents on limestone was examined in Salzer (2013) . For chlorinated solvents like PCE, sorption to limestone can be strong (kd values of 0.5-1 L/kg were measured). Return to Content