Geology and properties of limestone

Highlights
Description of limestone geology Properties of limestone aquifers

Fig. 1: Geologic sequence in eastern Denmark. From: Galsgaard et al. (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 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 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 . 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 chert, crushed material and fractures. The upper part of the limestone aquifer may be crushed due to glacial activity. At the Akacievej site, a crushed layer with a thickness of 3-5 m was observed. It has different hydraulic properties than the fractured limestone, f.e. the bulk hydraulic conductivity was lower than in the fractured limestone below. Limestone aquifers are often very heterogeneous and the hydraulic parameters can span over wide ranges and have a strong spatial variation. As with many other geologic materials, 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. Major horizontal fractures can intensify the observed degree of anisotropy. The intact limestone matrix has a very low hydraulic conductivity. When fractures are present, their conductivity is often orders of magnitude higher than the matrix and flow occurs predominantly in the fractures. Different kinds of fractures can be distinguished: horizontal fractures, which are often caused by decompression (related to glacial activity), and vertical/subvertical fractures, typically caused by tectonic activities. Fractures can influence the anisotropic behavior of limestone aquifers, because the groundwater 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 matrix porosity (between 7 and 46 Vol.-% were observed at the Akacievej site ). The porosity of the fracture system (volume fraction of the fractures), where groundwater flow and advective transport often mainly take place, is comparably low (0.5 to 2 Vol.-%). Properties of the limestone at Akacievej (Hedehusene, Denmark) The local geology at the Akacievej site in Hedehusene (eastern Denmark) 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 Determination method [math] K_\text{crushed} [/math] [math] 5\cdot 10^{-4}\ \text{m/s} [/math] crushed limestone (bulk) conductivity slug tests, data from remedial well [math] K_\text{matrix} [/math] [math] 10^{-6} - 5\cdot 10^{-11}\ \text{m/s} [/math] matrix conductivity permeameter tests on subcores, pumping test [math] K_\text{fractures} [/math] [math] 0.6 - 5.4\ \text{m/s} [/math] fracture conductivity determined from hydraulic fracture aperture with the cubic law [math] Ap [/math] [math] 1 - 3\ \text{mm} [/math] typical hydraulic aperture of major horizontal fracture pumping test, packer test [math] n_\text{matrix} [/math] [math] 0.07 - 0.46 [/math] matrix porosity porosimeter tests on subcores, pumping test [math] B [/math] [math] 20 - 25\ \text{m} [/math] aquifer thickness flow logs [math] D_\text{m} [/math] [math] 7.5\cdot 10^{-7} \text{m}^2/\text{s} [/math] (augmented) effective diffusivity of bromide modeling of tracer test, includes effect of neglected fractures and stagnant flow zones [math] k_\text{d} [/math] [math] 0.5-1.0\ \text{L/kg} [/math] sorption coefficient for PCE on limestone sorption tests in lab The sorption behavior for chlorinated solvents on limestone was examined in Salzer (2013) . For chlorinated solvents like PCE, sorption to limestone can be strong ($k_d$ values of $0.5-1 \ \text{L/kg}$ were measured in lab tests). Return to Content