Geology and properties of limestone

Geology and hydrogeology

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.

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 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. 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 Øresundsregion.

Limestone geologies are often heavily fractured and can include almost impermeable chert layers and nodules. Figure 2 shows come cores from boreholes from the Akacievej site, which illustrate the heterogeneity of the limestone there. In such fractured limestone geologies, flow predominantly occurs in the fractures due to a much higher hydraulic conductivity as in the limestone matrix, which typically has a very low hydraulic conductivity, but a relatively high porosity (10-40 Vol.-%). Limestone aquifers are typically anisotropic, which means that their horizontal hydraulic conductivity is higher than in the vertical direction. 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 limestone found at the Akacievej site is 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 1-5 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 layer. 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. The orientation of the vertical fractures is often aligned with major faults in the area.

Properties of 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 observed). The following table gives an overview of values that were determined for the Akacievej site:

Overview of hydraulic parameters at the Akacievej site.

Parameter	Value	Comment
K_crushed	5x10^-4 m/s	crushed limestone conductivity
n_matrix	0.10 - 0.20	matrix porosity
Ap	2 mm	typical fracture aperture
K_matrix	1x10^-7 m/s	limestone conductivity
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

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