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

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

Geology and hydrogeology In eastern Denmark, glacial Quaternary deposits (clay till, sand) are usually on top of the limestone aquifers. The uppermost limestone layer is typically a carbonated sand limestone (also called Københavns Kalk), 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 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 can include almost impermeable chert layers and nodules. The chert can appear as layers that stretch over a distance of tens to hundreds of meters or 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. Note that some of the fractures and crushing is caused by the drilling. In fractured limestone geologies, flow predominantly occurs 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 . Properties of limestone Limestone aquifers are often very heterogeneous and the hydraulic parameters can span over wide ranges. Moreover, they can have a strong spatial variation, both, vertically and horizontally. The limestone matrix has typically a very low hydraulic conductivity, which can be several orders of magnitude higher than the matrix conductivity. 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 about 2 to 10 times as high as 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 [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 fracture aperture [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 [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