Introduction and background

This Wiki is part of the Region H Limestone Project, a collaboration between DTU Environment at the Technical University of Denmark and Region H about the fate of contaminants in fractured limestone aquifers. The limestone project aims at improving our understanding of contaminant transport in fractured limestone aquifers and to identify and develop appropriate tools for the assessment and remedial planning of contaminated sites. The project involves a combination of field and lab work, and modeling of field data.

The goals of the Region H Limestone Project are to

• enhance the conceptual understanding of the behavior of contaminants in fractured limestone aquifers, which are a major drinking water resource in Denmark and other countries
• develop and test appropriate mathematical models for the quantitative description of processes, e.g. for risk assessment or the planning of a remediation strategy
• test and evaluate field methods for the determination of relevant hydraulic data and transport properties, which are a prerequisite for modeling
• test and compare sampling and analysis methods for the characterization of contaminants (distribution in the aquifer, localization of DNAPL)
• contribute to the development and evaluation of contaminant remediation methods

This Wiki summarizes the major outcomes of the project and is an information source about contaminant transport in fractured limestone aquifers.

Background

Map showing the location of the Akacievej field site, which was used as selected as a representative test site.

Limestone aquifers are important drinking water resources. In Denmark, for example, about one third of the drinking water is abstracted from limestone aquifers. Limestone aquifers provide usually high-quality drinking water, which does not need much treatment before it can be supplied to the consumers. However, these aquifers are often threatened by pollutants from contaminated sites. Hence, the understanding of the fate and transport behavior of contaminants from contaminated sites in limestone aquifers is important. This is challenging because limestone aquifers are often heavily fractured and may contain chert layers and nodules, resulting in a complex flow and transport behavior, which is difficult to predict.

Standard methods used for unfractured sites (e.g. sandy aquifers) may give poor or misleading results and more advanced techniques, which can give information about processes in the fractures and in the limestone matrix, are required. Different field methods have been developed in the past to better characterize the flow field and transport behavior in fractured aquifers. Moreover, specialized methods for determining aquifer parameters and estimating the contaminant distribution in fractured aquifers have been shown to be useful. This Wiki aims at giving an overview of recent field methods and modeling tools that and at evaluating their applicability.

To enhance the conceptual understanding of the behavior of contaminants in fractured limestone aquifers a representative field site with a PCE contamination in limestone was selected. The field site is located west of Copenhagen (see map) called Akacievej in the following. It was used to conduct a pumping test combined with multiple tracer injections under forced gradient conditions and to test different depth-discrete contaminant sampling methods. This gave some important insights and the impacts on the conceptual understanding of processes in fractured limestone are discussed in this Wiki and in an accompanying report. This Wiki gives an overview and evaluates some useful methods for the determination of relevant field data and for the assessment of contaminant plumes in fractured limestone aquifers. The focus is on dissolved chlorinated solvents (PCE, TCE etc.) as contaminants.

Another focus of this Wiki is on modeling techniques for contaminant transport in fractured limestone aquifers. Numerical models are important tools for the planning and interpretation of field work and is essention for the prediction of the contaminant behavior in the aquifer and for the planning of remedial actions. It is, however, important to select an appropriate model, which accounts for the fractured nature of the aquifer and does not oversimplify the studied system. Then, it can be used a very useful tool to improve the conceptual understanding and to guide with decision support for risk assessment and remedial site management.

Structure of the Wiki

The Limestone Wiki is composed of two major parts:

1. Data acquisition and conceptual model development
2. Modeling contaminant transport in limestone

The first part focuses on field methods to obtain data about the geology and hydrogeology and the contaminant distribution. It will be shown, how data from field and lab measurements contributes to enhance a conceptual model of a contaminated limestone site.

The second part of this Wiki aims at giving an overview of modeling tools that can be used for the interpretation and prediction of flow and transport processes in fractured limestone aquifers (Chapter Model concepts). Fracture flow and transport models with different complexity are available for the modeling of contaminant transport, ranging from simple spreadsheet tools to advanced models that incorporate the fracture geometry. The models will be compared for their suitability to simulate field data and to represent typical features of fracture flow and transport based on a field example. The steps how to build up a numerical model will be exemplified, and recommendations for a good modeling practice will be made.

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