Sensing and Geographic Information Systems for Design and Operation of Water Resources , - 0

: (Proceedings of Rabat Symposium S3, April 1997). IAHS Pubf. no. 242, 1997 Oi

The development of a customized GIS for the protection of groundwater resources in Rabat-Kenitra, Morocco

PETER WITT Ecology and Environment, Inc., 999 Third Avenue §1500, Seattle, Washington 98104, USA

M. EL MASSLOUHI, M. OUBALKACE Direction General de l'Hydraulique, Rue Hassan Benchekran, Agabal-Rabat, Morocco

M. A. BENZEKRI, E. JABRI Laboratoire Public d'Essais et d'Etudes, Bd. Moulay Slimane, Casablanca, Morocco

HASSAN HAMDI Ecology and Environment, Inc., 11 Golden Shore Dr., Long Beach, California 90802, USA

SCOTT THORSELL Ecology and Environment, Inc., 368 Pleasant View Drive, Lancaster, New York 14086, USA

Abstract The objective of this project was to demonstrate the use of a customized geographic information system (GIS), which incorporates results of a groundwater flow model, as a decision support tool for the protection of valuable groundwater resources in the Rabat-Kenitra area of Morocco. The study area encompasses approximately 800 km2 containing four major population centres, a major oil pipeline, numerous industries, and several major highways that are well used by large fuel and raw product transport trucks. The customized GIS is a menu-driven system designed to allow non-GIS specialists full use of the system. Two major components of this system are a real time worst-case contaminant transport model and an emergency response decision support system. The GIS-based data layers and routines developed for this project also can be used to aid in the development of resource protection, spill prevention, and emergency response plans.

INTRODUCTION

The project objective was to demonstrate the use of a customized geographic information system (GIS) and a groundwater flow model as decision support tools for petroleum spill contingency planning and response for the Direction General de l'Hydraulique (DGH) of Morocco.

To achieve this, a prototype menu-driven GIS was designed covering the area between Rabat and Kenitra and bounded by the coast on the west and extending to Sidi Yahya du Reharb on the east (Fig. 1). This area was chosen as it coincided with groundwater data available from a previous DGH groundwater study. The approach to developing this system was to make use of available spatially referenced data and to supplement this data where necessary with a limited data collection effort. Data quality issues were not a priority at this stage of the project.

64 Peter Witt et al.

GROUNDWATER FLOW MODEL

The objective was to assimilate the existing data from a 1987 groundwater flow model of the Rabat-Kenitra area into a current, standardized, and user-friendly format of the United States Geological Survey (USGS)-developed MODFLOW code (McDonald & Harbaugh, 1988). The new model should precisely reproduce the steady-state flow regime produced by the 1987 model, include the additional ability to perform particle tracking analyses, and have a limited capability to interact with the GIS-based aquifer protection database also developed under this project.

To meet these objectives, the software program "Visual MODFLOW", produced by Waterloo Hydrogeologic Software, was obtained and registered for the Administration de l'Hydraulique in Morocco. This DOS-based computer software includes the model codes for MODFLOW and MODPATH, is supported by a strong technical support staff, and uses visual or graphically based screens to create and modify groundwater flow models to run under these two codes. The MODPATH modelling code, also developed by the USGS, performs particle tracking analyses based on the results of the MODFLOW groundwater flow model.

"Visual MODFLOW" was also chosen for its compatibility with standardized MODFLOW and MODPATH file formats. For this reason, "Visual MODFLOW" is compatible with and can import such models created in other software programs that use the standardized file format. Ecology and Environment Inc. (E & E) tested this capability using models created in several other modelling software programs (such as individually compiled MODFLOW programs, ModelCAD386, and GMS).

"Visual MODFLOW" offers the convenience of a single software program that includes the two modelling codes and seamlessly provides pre- and post-processing for these codes. The post-processing capabilities of "Visual MODFLOW" include creating iso-contours for head and drawdown, velocity vector arrows for flow direction and gradient, and time dependent particle tracking. Additionally, both ASCII (XYZ format) and AutoCAD DXF file formats can be used for various types of input and output from the program. Hardcopy output is available for numerous types of printers and plotters. File transfers with the GIS was accomplished through the use of DXF and ASCII file formats (Fig. 2).

Groundwater model descritization (or model input summary)

The grid used for the Rabat-Kenitra groundwater model uses a uniform cell size of 1 km2 and consists of one layer of 40 rows and 47 columns. The model has a total of 1880 cells, of which 1146 are active. The remaining cells are designated as "no flow" and are not considered in the model calculations. Where available, all model input data were obtained from the 1987 model report provided by the Administration de l'Hydraulique. Pumping well locations and areas or zones of recharge were input to the model.

Two model parameters required by "Visual MODFLOW" were not directly apparent in the 1987 model report: hydraulic conductivity and porosity. Porosity for the new model was assumed to be 25% (or 0.25%), based on an average of published results for unconfined silty sand aquifers. Hydraulic conductivity was determined

Development of a customized GIS for the protection of groundwater resources 65

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66 Peter Witt et al.

from the transmissivity values provided by the 1987 model and based on an average aquifer thickness of 100 m. An average aquifer thickness of 100 m was used based on an approximation offered by scientists at DGH.

Groundwater model summary

The results of the new steady-state model appear to correlate very well with the results produced by the 1987 model. This is as expected given the duplication of the model grid design and parameter definition. To run the particle tracking model, only two exceptions to the 1987 model required correction. In each case, a pumping well was located within a cell originally designated as a constant head cell for a river boundary. This condition did not allow MODPATH to run until it was corrected by removing the pumping well designation from the cell.

While working with this groundwater flow model, a number of limitations and potential improvements were noted. The primary limitation of the current model appears to be in the model cell size and in particular, the location of wells and constant head boundaries. Given the capabilities of the modelling software, "Visual MODFLOW", many improvements can be made quickly and easily. Such improvements, however, are typically limited by the availability of appropriate data.

GIS DEVELOPMENT

GIS data

The customized GIS was developed on a SUN workstation using the software program ARC/INFO version 7.0.

At the beginning of the project, it was agreed that E & E would use data supplied by DGH in a GIS format. Due to the limited scope of this project, E & E would not be able to perform extensive data quality review and would expend a limited effort to collect or transform data into a GIS format. DGH made available to E & E several base map data layers, which included roads, hydrography, the study area boundary, railroads, and the petroleum pipeline. To supplement these base map coverages, E & E scanned six 1:100 000 topographic maps and developed a seamless georeferenced image of the study area. DGH also provided environmental and cultural data layers such as soils, geology, and landuse. However, due to problems in several of these data layers, E & E was unable to use them as originally intended. For example, the soils data did not cover the entire project area, the landuse data set only had a single landuse classification (forest), and the accuracy of the roads data layer was questionable.

E & E also used a hand-held global positioning system (GPS) with a resolution of 100 m to collect representative data sets. These included underground storage tanks (USTs), pumping stations, and industrial locations. As mentioned previously, these are only representative data sets, are not comprehensive, and should be used only for demonstration purposes. Several other response resource data sets were fabricated to show the full functionality of the GIS.

Development of a customized GIS for the protection of groundwater resources 67

Groundwater vulnerability

E & E developed a GIS-based groundwater vulnerability model that is based on the potential for contaminant migration to the aquifer. This model is based on the DRASTIC system (Aller et al, 1987), and incorporates several different factors, such as soil texture, depth to groundwater, and geology. Within this model, each factor is rated (on a scale of 1 to 10) based on the factor's potential to aid or hinder the migration of contaminants to the aquifer. For example, a clay soil texture would slow the percolation of contaminants through the vadose-zone when compared to a sandy or gravely soil texture. In this case, a lower rating would be applied to the clay texture and a higher rating would be applied to the sandy or gravely texture. Each factor can be weighted to reflect their relative importance or contribution to the vulnerability model. The soils and geology coverages were given a preliminary rating. The depth-to-water coverage was rated after it was developed from a coverage of well locations that had a depth-to-water attribute. To convert a point representation to an area representation, the point coverage was converted to a polygon coverage using a Thessian polygon routine (ESRI, 1991). This routine takes a point and develops areas around each point that are bounded by lines that are equidistant to any other point in the coverage. These three coverages were combined and the ratings for each factor were summed to provide an overall vulnerability coverage (Fig. 3).

Due to some of the data problems already discussed, the vulnerability model is comprised of only three factors and does not cover the entire study area. Originally it was E & E's intent to use this vulnerability coverage (data layer) to aide in the contingency planning/emergency response aspects of this project. In its current state, it should be used for demonstration purposes only. When the missing or incomplete data layers are made available, and DGH hydrogeologists develop appropriate weights and ratings for the vulnerability model, the outputs of the model (a vulnerability coverage) should be used in contingency planning, oil spill prevention, and emergency response activities. For instance, a potable water well's coverage could be overlayed with highly vulnerable areas, with the output being wells that fall within these vulnerable areas (Fig. 4). These wells then could be targeted for a more frequent water quality sampling routine. Additionally, petroleum or hazardous materials storage locations, or industries that handle these materials, that fall within these highly vulnerable areas could be prioritized for spill prevention inspections.

Spill risk areas

The spatial operations and tools intrinsic to the GIS allow the combination of different thematic data sets. A coverage that represents the spatial coincidence of wells and areas where spills are likely to occur provide the response, prevention, and preparedness planners with information that will aid in the development of these plans.

The coverages used in the spill risk areas included: roads, railroads, the oil pipeline, underground storage tank locations, and industrial locations. Each coverage was buffered to a 100 m distance around each feature. These buffered coverages

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Development of a customized GIS for the protection of groundwater resources 69

were then combined to provide an overall risk coverage. The wells coverage was then overlayed with the overall risk coverage, the output of which provides a coverage of wells that fall within these spill risk areas (Fig. 5). These risk features do not represent all potential contamination source risks to the groundwater. Other type of contamination sources such as agricultural runoff, could also be included if a more comprehensive system for water quality protection is desired.

Currently, the spill risk coverages associated with USTs and industrial locations is representative only. However, the spill risks associated with transportation pathways can be used for the contingency planning purposes.

Groundwater modelling

The results of MODFLOW model runs can be easily loaded into the GIS via a customized batch routine. This batch routine takes an ASCII MODFLOW output file that contains piezometric data, and creates a GIS cell-based data layer that is used in modelling the spill path of a potential contaminant. The current data layer in the GIS used in the MODEL SPILL PATH routine, has a cell size of 1000 m. This routine should be used for a cursory look at contamination migration, and will output a worst-case scenario.

MODFLOW also outputs a DXF vector file that presents flow velocities. This file was imported into the GIS and can be drawn on the canvas. This data set can visually present the groundwater gradient at any point in the model.

GIS groundwater response system (GWRS)

The GWRS is designed as a decision support tool for emergency response and contingency planning for the defined study area. This prototype system has several routines that can provide information that will aide in the decision making process during a hazardous material spill. The system is comprised of four main menus and a display canvas. These menus allow the user to select different features to draw on the canvas, change the map extent of the features drawn in the canvas, import MODFLOW data, perform measure operations (i.e. distances), identify specific features, and select from several different analytical operations. These analytical operations are based on the location of a spill event, and provide the user with output in either a data list or map display format. These operations are as follows: enter the spill location, model the potential spill path, display model results, list potentially affected wells based on any model run, search the response equipment inventory database, perform spatial searches for features of interest, and present a listing of the response communication protocol.

FUTURE CONSIDERATIONS

The integration of MODFLOW model outputs and the use of a customized GIS have demonstrated that these tools should be implemented to protect groundwater

70 Development of a customized GIS for the protection of groundwater resources

resources, both in the spill prevention and planning phases and in the response to hazardous waste spills. For the system to be used to its potential, several key data factors, such as depth to groundwater, porosity, and permeability, need to be defined in more detail to allow the development of a more accurate model. The cell size of the groundwater flow model should be reduced for more accurate modelling of contaminant transport runs, and the data sets that define hazardous material storage areas must be augmented to include other potential contaminant sources. With the incorporation of these data layers, the system should be implemented on a basinwide level to protect limited groundwater resources.

REFERENCES

Aller, L., Bennet, T., Lehr, J., Petty, R. & Hackett, G. (1987) DRASTIC: A standardized system for evaluating groundwater pollution potential using hydrogeologic settings. United States Environmental Protection Aeency-t00/2-87-035

Environmental Systems Research Institute (ESRI) (1991) Thessian Polygons. ARC Command References. McDonald, M. G. & Harbaugh, A. W. (1988) A modular three-dimensional finite-difference groundwater flow model.

Techniques of Water-Resources Investigations of the United States Geological Survey, book 6, chapter Al.