Organizing GIS Data for Hydrogeological Applications

The paper describes the hydrogeological mapping process at the State Geological Survey of Baden-Würtemberg, Germany, and its representation in the survey's information system infrastructure. GIS is the basic software component in this process. It has the task to store and process geodata and their attributes, as well as to integrate the other software components, that are involved in the mapping process. These are RDBMS, INTRANET applications and specific hydrogeological software like groundwater simulation models.

A 7-step hydrogeological mapping process is introduced and described from an organizational and from a technical view. It is illustrated with an application example. The paper focuses on a conceptual model for storing mapping results as thematic information layers in ArcInfo data sets.

Introduction

Hydrogeological mapping is one of the main tasks of the State Geological Survey of Baden-Württemberg, Germany. Hydrogeological maps are an instrument for state-wide planning of the administration. The maps have special significance for all environmental tasks concerning waste disposal sites, groundwater supply and groundwater protection. Hydrogeological maps describe the main features of groundwater circulation systems by means of individual thematic maps. Up to now these maps exist for the most important porous media aquifers or aquifer systems in the state of Baden-Württemberg. The mapping procedures were driven by individual tasks and the specific hydrogeological conditions of the study areas. Therefore, every mapping project has produced different kinds of thematic maps. The demand for further hydrogeological mapping in other parts of the state and for periodical updating of existing maps require a harmonization of the mapping process itself and the documentation of its results.

The mapping process produces a huge quantity of geodata sets. To create, use and update this data sets in a flexible and comfortable way, digital mapping techniques are applied and information systems are set up. Due to the great variety of data types and scientific requirements for data processing, a lot of different software products are used. Among these are relational database managements system, geographic information systems and a great variety of specific hydrogeological software. This specific software includes statistical and geostatistical techniques, simulation models, 3D visualization, hydraulic and hydrochemical evaluation techniques and graphic applications. As all data are characterized by their location, a GIS is the central component within such a software environment.

In this context, the basic tasks of a GIS are the administration and visualization of spatial data and their attributes, as well as geocoded pre- and postprocessing of data for 3D analysis, simulation and other hydrogeological evaluations. GIS acts as an integration platform and offers the possibility of a consistent, interactive user environment.

The paper focuses on the use of GIS for the hydrogeological mapping and evaluation process at the State Geological Survey and its implementation in the survey's information system infrastructure. A conceptual model for storing spatial hydrogeological data in ArcInfo data sets is described.



The hydrogeological mapping process

The goal of a hydrogeological map is the detailed description of an aquifer or an aquifer system. Usually it contains:

A basic task in the mapping process is the interpretation of point data and their interpolation to linear and spatial objects in vertical and horizontal projections. The key parameter within the mapping process is the geometry of aquifers, i.e. the volume defined by their horizontal and vertical borders. If this body is derived from mapped borders, hydraulic and hydrochemical parameters from representative wells are assigned. Additionally, fluid flux at aquifer boundaries is calculated and the balance terms of groundwater budget are set up. Results are documented in an information system or as maps and used for further evaluations, e.g. with simulations models, which also work as plausibility check for mapping results. Hydrogeological mapping as a 7-step process is described in Table 1.



Table 1: The 7-step hydrogeological mapping process with links to application examples
1 Screening and updating the hydrogeological database (point and spatial data)
2 Visualizing the geological records of boreholes (hydrogeological interpretation) and time series of parameters
3 Building series of cross sections across the study area (vertical projection of 3D problem)
4 Interpreting and interpolating hydrogeological borders in the study area (horizontal projection of 3D problem)
5 Deriving hydrogeological bodies from borders; Assigning hydraulic and hydrochemical parameters to bodies; Defining boundary conditions for fluid flux into and out of bodies
6 Documentation and metadata administration in an information system
7 Cartography, 3D visualization and further evaluations, e.g. simulation models


Information System Architecture

The 7-step process of hydrogeological mapping is reflected in the architecture of the survey's information system. The system includes the RDBMS ORACLE, the GIS ArcInfo - ArcView, a variety of software packages for visualization (SGI Explorer, NCSA Polyview), modeling (MODFLOW, FEFLOW), graphics (Adobe Illustrator) and HTML browsers (NETSCAPE). The representation of the mapping process steps in the information system components is shown in Fig. 1.



Figure 1: Representation of the 7-step mapping process in the information system architecture

Figure 1: Representation of the 7-step mapping process in the information system architecture




GIS Data Model for Thematic Information Layers

A key term in the mapping process is the thematic information layer. It combines all data, which belong to a thematic theme, e.g. phreatic groundwater surface, bottom of phreatic aquifer, outcrop of hydrogeological units at the surface. The thematic information layer consists of mapping results and a link to primary data, which were used to create these results. Usually, these primary data are points of the survey's point database. The information layers are stored as ArcInfo data sets, following the principle, that all geodata of the same type are put together in one layer. That means all point objects are stored in a point coverage, all linear objects are stored in a line coverage and all area objects are stored in a polygon coverage. If the thematic information layer describes continuous data, a grid is interpolated and stored with the data set. In case of hydrogeological borders, the grid attribute is the z-value (m a.sl.) of this border. In case of a parameter distribution, the grid attribute is the parameter value (e.g. k-values in the case of permeability, chloride- or nitrate values in the case of hydrochemistry). A scheme of this data model is presented in Fig. 2. Table 2 gives some examples of typical thematic information layers and their objects, which are created in the hydrogeological mapping process.



Figure 2: GIS data model for storing thematic information layers

Figure 2: GIS data model for storing thematic information layers


Table 2: Examples of thematic information layers and their spatial objects
MAP: Hydrogeological map of the Rhinegraben aquifer system

Thematic Information Layer:
Phreatic groundwater surface
  • POINT OBJECT: boreholes, wells
  • LINE OBJECT: contour lines
  • AREA OBJECT: border of phreatic aquifer in the study area, border of study area
  • SURFACE: z-values of groundwater surface

Thematic Information Layer:
Bottom of phreatic aquifer
  • POINT OBJECT: boreholes, wells
  • LINE OBJECT: contour lines, faults
  • AREA OBJECT: border of phreatic aquifer in the study area, border of study area
  • SURFACE: z-values of aquifer bottom

Thematic Information Layer:
Permeability of phreatic aquifer
  • POINT OBJECT: pumping test site, slug test site
  • LINE OBJECT: contour lines
  • AREA OBJECT: border of phreatic aquifer in the study area, border of study area
  • SURFACE: values of permeability (k-values)


Application Example

The formalized mapping process is explained by an application example. It is a part of the hydrogeological map of the Upper Rhinegraben valley. The valley bears the most important aquifer system in the state. It is a porous media groundwater system of quaternary age. The different steps of the mapping process are illustrated by screendumps.

Step 1:
The survey's hydrogeological point database (ADB) contains all data, that refer to point objects. These are base data (identification of the point, etc.) borehole descriptions (filter screnes, geological records, etc.) and (hydro-)chemical, (geo-)physical and isotopical measurement data. The ArcView screendump shows boreholes and wells of the hydrogeological point database around a public water supply. The tables display basic information about borehole identification and a subset of the geological information stored with these points.

Step 2:
Data from the ORACLE point database are evaluated and visualized by a variety of software packages. ArcView is a platform, that allows to integrate these results in a consistent and interactive user environment. The ArcView screendump shows results from the geological profiling software and time series from GNUPLOT.

The geological record in the left window show separate geological layers in different colors. The hydrogeological interpretation of these layers is shown by text and markers on the right side of the profile. The markers show the borders of hydrogeological units. The text is an abbreviation for the hydrogeological border, (Fl11, Fl15, etc.) or the hydrogeological body (ooOKL, ZH1, etc.). The hydrogeological interpretation is used for vertical (step 3) and horizontal (step 4) interpolation of hydrogeological bodies.

Step 3:
Vertical projection of the hydrogeological bodies is applied by interpreting and interpolating profile data. The left window of the ArcView screendump show the section lines of the study area. The upper right window show the uninterpreted visualization of a cross-section. The lower right window shows interpretation results and the interpolation of hydrogeological bodies in vertical projections. The cross-sections are stored as ADOBE ILLUSTRATOR graphic files.

Step 4:
The ArcView screendump shows the horizontal projection of the bottom of the phreatic aquifer. It is stored as thematic information layer, which consists of point objects/primary data (wells, boreholes), line data (contour lines), area data (study area, border of phreatic aquifer in the study are) and a surface grid. All components of the thematic information layer are shown in the left window. The other windows show each individual component in a separate window.

The hydrogeological borders are used to define hydrogeological bodies. A critical point in the interpolation process is the exact match of the individual layers of a body, especially when using a 2D data model.

Step 5:
The ArcView screendump shows example of hydrogeological evaluations. Thickness is calculated by grid overlay. The permeability layer is created by evaluating pumping test data and assigning them to hydrogeological bodies.

Step 6:
The results of the mapping process are documented and stored in an information system. The INTRANET application for searching and visualizing the mapping results distinguish between basic geographic information (e.g. topography, administrative borders), state-wide mapping projects (e.g. geological base maps) and project data (specific evaluations). The screendump shows the user interface for searching the database with the NETSCAPE browser.

Step 7:
Mapping results are further evaluated by groundwater models, 3D visualization software and other software packages. The screendump shows a visualization of the basic hydrogeological layers in the study area as a 3D animation with SGI Explorer.

Conclusions

Hydrogeological mapping on a state-wide scale is supported and documented by an information system. The 7-step process of hydrogeological mapping is reflected in the information system architecture. Thematic information layers for hydrogeological themes are the main result of the process and form the most important part of the information system. As they are stored in GIS data sets, GIS is the basic software component of the system.

The information system provides fast and comfortable tools for querying and screening geodata and their attributes. Different hydrogeological problems and tasks are solved by applying scientific methods within the GIS environment. It is achieved in a very economic way.

Günter Sokol
GIS Project Manager
State Geological Survey of Baden-Württemberg
Alberstraße 5
79104 Freiburg, Germany
Phone: ++49 761 204 4426
Fax: ++49 761 204 4438
E-mail: sokol@gla.uni-freiburg.de


Dr. Ralph Watzel
Hydrogeologist
State Geological Survey of Baden-Württemberg
Alberstraße 5
79104 Freiburg, Germany
Phone: ++49 761 204 4423
Fax: ++49 761 204 4438
Email: watzel@gla.uni-freiburg.de