Geology and properties of limestone

2018-02-13T21:12:51Z

Hmco: tiny "improve english usage" edits.

{| class="side1"
!Highlights
|-
|
* Description of limestone geology
* Properties of limestone aquifers
|}

[[File:GeologicSequenceGEUS2014.png|thumb|25em|Fig. 1: Geologic sequence in eastern Denmark. From: GEO and GEUS (2014).]]
[[File:BoreholeCores.png|thumb|25em|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.]]

{| class="body1"
|
== 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 a 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 <ref name="GeoGeus2014"> GEO & GEUS (2014), ''Strømning og stoftransport i kalklagene på den københavnske vestegn. Geologisk og hydrologisk vidensopsamling og typemodel.''</ref> (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 chert, 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.
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.
The 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.
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 <ref name="GeoGeus2014" />.
Limestone has a relatively high porosity (between 7 and 46 Vol.-% were observed at the Akacievej site <ref name="Broholm2016"> Broholm et al. (2016a), ''Sammenligning af niveauspecifikke prøvetagningsmetoder for vurdering af knocentrationsfordeling i kalkmagasin: Akacievej, Hedehusene.'' Technical University of Denmark, DTU Environment</ref>), 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 [[The Akacievej field site|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 | '''Data acquisition and field methods''' ]] and [[ Transport parameters and contaminant data | '''Determination of transport parameters and contaminant data''']].
Table 1 gives an overview of values that were determined at the Akacievej site.

{| class="wikitable"
|+ 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) <ref> Salzer, J.P. (2013), ''Sorption capacity and governing parameters for transport of chlorinated solvents in chalk aquifers'', Master Thesis, DTU. </ref>.
For chlorinated solvents like PCE, sorption to limestone can be strong (kd values of 0.5-1 L/kg were measured).

{{Return}}
|}
[[Category:Introduction]] [[Category:Geology]]

Limestone - User contributions [en]

Geology and properties of limestone