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), Strøming og stoftransport i kalklagene på den københavnske vestegn.

Fig. 2: Borehole cores from the Akacievej field site.

Geology and hydrogeology In eastern Denmark, usually glacial Quaternary deposits (clay till, sand) are on top of the limestone aquifers. The uppermost limestone layer is typically a carbonated sand limestone, followed by a bryozoan limestone. The carbonated sand limestone is rather evenly and horizontally layered and may contain flint layers and nodules. The bryozoan limestone, in contrast, has typically bank structures and less to no flint inclusions. A good description of the geologic stratification is given in the Report (in Danish). Figure 1 shows the typical krono-, bio- and lithostratigraphy in eastern Zealand (Denmark) and the Øresundsregion. Limestone geologies are often heavily fractured and can include almost impermeable chert layers and nodules. Figure 2 shows some borehole cores from the Akacievej site, which illustrate the heterogeneity of the limestone with flint layers, crushed material and fractures. Note that some of the fractures and crushing is caused by the drilling. In fractured limestone geologies, flow predominantly occurs in the fractures due to a much higher hydraulic conductivity as in the limestone matrix. The fractured limestone can contain different kind of fractures: horizontal fractures often caused by decompression, and vertical and subvertical fractures, typically caused by tectonic activities. Especially vertical and subvertical fractures can have a major influence on the anisotropic behavior of limestone aquifers, because the flow happens predominantly along the fractures. Due to that, it is possible that the main flow cannot simply be identified from the head isolines, but the direction of the major fractures has to be considered. The orientation of the vertical fractures is often aligned with major faults in the area. Fig.3: Geology at the Akacievej site. The geology at the Akacievej site is depicted in Figure 3 and consists of a carbonated sand limestone (København Kalk) on top of a bryozoan limestone (Stevns Klint formation), both with a strongly varying hardness. Due to glacial activity, the uppermost 1 to 5 meters of the limestone are crushed. Most of the crushed limestone is in the carbonated sand limestone, but at some places it also penetrates into the bryozoan limestone. The bryozoan limestone at Akacievej does not show strong bank structures and is rather even. Properties of limestone Limestone aquifers are often very heterogeneous and the hydraulic parameters can span over wide ranges. Moreover, they can have a big spatial variation, both, vertically and horizontally. The limestone matrix has typically a very low hydraulic conductivity, and the hydraulic conductivity in the fractures can be several orders of magnitude higher than in the matrix. Limestone has a relatively high porosity (8-40 Vol.-%), the porosity of the fractures is comparably low (0.5-2 Vol.-%). Limestone aquifers are typically anisotropic, which means that their horizontal hydraulic conductivity is higher than in the vertical direction. Based on a set of measurements, 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 for the Akacievej site. Tab. 1: Overview of hydraulic parameters at the Akacievej site. Parameter Value Comment K_crushed 5x10^-4 m/s crushed limestone conductivity K_matrix 1x10^-7 m/s limestone conductivity n_matrix 0.10 - 0.20 matrix porosity Ap 2 mm typical fracture aperture B 20 - 25 m aquifer thickness D_m 7.5x10^-7 m^2/s effective diffusivity k_d 0.5-1.0 L/kg 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