THREE-DIMENSIONAL GEOLOGICAL AND GROUNDWATER FLOW MODELING OF DROUGHT IMPACT AND RECHARGE POTENTIALITY IN KHATT SPRINGS AREA, RAS AL KHAIMAH EMIRATE, UAE P. Wycisk1 , M. Al Assam2, S. Akram2, M. Al Mulla2, D. Schlesier1, A. Sefelnasr3, N. B. Al Suwaidi2, M.S. Al Mehrizi2, and A. Ebraheem2 1Institute of Geosciences, Martin-Luther-University, Halle/Saale, Germany 2Department of Soil and Water, Ministry of Environment &Water, Dubai, UAE 3Geology Department, Assiut University, Assiut , Egypt. Institute of Geosciences, Martin-Luther-University, Von-Seckendorff-Platz 3, D-06120, Halle/Saale, Germany peter.wycisk@geo.uni-halle.de Abstract

The available hydrogeological data in the period 1969-2005 for Khatt Basin in the Emirate of Ras Al Khimah, UAE was critically reviewed, analyzed, and then used to build a GIS database for this area. The spatial analysis of GIS was used to analyze this data to determine the quantity and quality of the available groundwater resources in Khatt Basin area as well as to prepare the input data for a recently developed digital 3D geological model. The graphic and analytical capabilities of GIS were used to produce different types of geopotential maps necessary for the sustainable management of the precious groundwater resource in this area. With the GIS database, it was possible to build a pioneering 3D geological model for the area that gives detailed information about the subsurface stratigraphy below any point in the model domain. To improve the scattered geological subsurface information a “true” 3D geological regional subsurface model has been built. The results of the 3D geological structural model were used to assign the model layers’ elevations and their hydrogeological properties in a 3D groundwater flow model which is being developed to determine the impact of the present and planned extraction rates on the quantity and quality of groundwater in the Quaternary Aquifer System which is heavily exploited since the beginning of the 1970’s. The steady state simulation indicated that there is a very close fit between the calculated and observed groundwater contours of 1969 indicating that the Quaternary Aquifer System prior to 1970 was in a steady state condition (i.e. the recharge from Oman Mountain was enough to balance natural and artificial outflow at that time). Keywords: 3D geological model, hydrogeological model, drought impact, Khatt Basin,

Introduction

Successful water resources management in arid regions depends largely on knowledge of the planners and scientists of the available water resources as well as the capability of using mathematical models to predict the consequences of a certain management option in the short and long run. The validity of using these models for water resources management is depending on the availability of a complete and well documented historical environmental database (preferably in GIS). Also, if the geological setting and structural pattern are complicated, the 3D geological modeling technique should be used to determine and resolve these complications prior setting up the 3D structures and parameters for numerical groundwater flow models (Gossel et al., 2004; Wycisk et al., 2005).

In the last forty years, many of groundwater exploration and evaluation studies were conducted in the northern part of the United Arab Emirates. Most of these studies were for consultancy purposes and missing any scientific interest. Due to the fact that the majority of these studies are only documented as internal reports, their use is limited and probably confined to the purpose and organization paid for that particular consultancy job. In an effort to investigate the current situation of water resources in the Northern Emirates region, all the available drilling information and environmental data from 1979 to 2005 were recently critically reviewed and stored in a GIS database. Then the spatial analysis algorithm of GIS enhanced with its graphic capabilities was used to analyze and reinterpret these data and preliminary evaluate the current situation of water resources (availability and management challenges) in the Northern Emirates. The results were used to prepare the input data of 3D geological structures model and subsequently a local scale groundwater flow model. The main objectives of the present study are:

a) Determining the geological and hydrogeological setting of Khatt Spring area; emphasis were given to the impact of the geological structures on groundwater accumulation and flow pattern;

b) Evaluating the impact of drought years and groundwater depletion (caused by intensive irrigation) on the flow and water temperature of springs;

c) Using the newly developed software for digital 3D geological structures modeling which utilizes cross-section based networks for subsurface interpolation in heterogeneous aquifers;

d) Using groundwater flow model as a management tool for the sustainable management of water resources in these areas.

Khatt Springs Area

Khatt springs area is one of the most prominent springs inside UAE and situated at the extreme east of the gravel plains where it meets the foot hills of the Oman Mountain range (Fig. 1). There are two main springs, Khatt north and Khatt south, the flow from which was gauged in February 1966. Khatt north spring emerges about ten yards west of the main limestone foothills; south spring issues direct from the limestone, a bedding plane having been excavated to increase the flow.

The most interesting feature of these springs is the high temperature of the water about 39 ºC as of groundwater may be assumed to increase with depth from a level of about 20 compared with a range of 25-32 ºC for all other groundwater in the northern Emirates. The temperature m below ground level at a gradient of about 1 ºC in 30 m. The difference between the temperature of the spring water and that of groundwater in the surrounding area suggests that

the former issues from a considerable depth; perhaps about 300 m below ground level (Sir William Halcrow & Partners, 1969; IWACO, 1986).

Figure 1: Geological map of Khatt springs area including well locations, the position of the

cross-section (see Fig. 3) and the location of Khatt springs catchments area (modified after IWACO, 1986).

Water flowing out from Khatt springs has been used for traditional bathing and medication till 1979 when it further developed to be a major bathing and recreation site in Ras Al Khaimah Emirate (in year 2006, one five stars hotel was launched in this area). The outflow of pools in addition to water flowing from other sources like small springs and hand-dug wells are drained into an intricate system of lined channels and falajs to supports a substantial cultivation of date palms and other market products (Fig. 2).

Figure 2: Drawdown of up to 65 meters happened in the area studied from 1969-2005.

Major geomorphologic units and agricultural lands are also shown.

Due to the drought conditions prevailed in the period 1998-2004 together with the necessity of intensive groundwater exploitation for irrigation purposes, the outflow from the northern spring was not enough for keeping the temperature in the swimming pools to the desired level due to the relatively long time of water residence time in the pool. This also led to deteriorating the environmental conditions in the recreational park around the pool and the spread of mosquitoes.

Geological and Hydrogeological Setting

The study area lies on the western side of the Northern Oman Mountains (Fig. 1), which are comprised of the Jurassic to Cretaceous Musandam Group limestone. These mountains rise above the western Jiri coastal plain, which consists of late Tertiary to Recent alluvial sediments overlying the late Cretaceous Juweiza Formation (Sir William Halcrow & Partners, 1969). The Juweiza is a flysch-like sequence of marls and shales with varying admixtures of coarse detrital debris of chert, basic igneous rocks, and limestone (Fig. 1).

The area has been subjected to two major tectonic events. These are the thrusting of the Hawasina Formations and the Samail Ophiolite over the Musandam limestone in the Upper

Cretaceous and then the formation of the Oman Mountains due to folding, faulting and thrusting in the mid Tertiary (Fig. 1). This resulted in the formation of the major northeast-trending anticlinal structure through the Musandam Mountains, the Hagab Thrust fault along the western edge of the mountains and the Jiri plain, and the Dibba Zone to the southeast, with the Batha Mahani thrust running along the valley of Wadi Tawiyean. Consequently, the Musandam limestone is strongly faulted, with major trends running northeast (parallel to the Dibba Zone thrusts), north, and northwest (Robertson, 1990).

The drilling information of the available water wells was used to construct several subsurface geological cross sections along different directions. An example of these cross sections is shown in Figure 3.

Figure 3: A subsurface geological cross section in the east- west direction (after IWACO,

1986). Location of the cross section profile is shown in Figure 1.

In figure 3, two hydraulically connected groundwater aquifers can be identified as follows:

1) Quaternary aquifer which is composed of the gravel plain of recent silts and conglomerates and meets the mountain front at a high angle. This layer is overlying the thick Juweiza formation which is generally of low permeability and thus acting as aquitard rather than an aquifer.

2) Carbonate aquifer which refer to the exposed thick Musandam limestone of Jurassic - Cretaceous age in the mountains.

These carbonate layers are composed of well jointed, karst weathered, thin bedded, nodular, fragmental and porcellanous dolomitic limestone and also limestone interbedded with calcareous shales. These beds dip at a very high angle, up to 90º, to the west and have been planed off, probably by marine erosion, at about 600 feet above sea level for about two km into the mountain front. Various north-south trending folds can be seen on the aerial photographs on the western strip of the main mountain massif. At Khatt, these beds are linked to the gravel plain layers in the following ways:

a) as the western flank of a north-south trending anticline; b) as the eastern upthrow side of a north-south trending fault bounding the mountain

front; c) as the ‘nose’ of a thrust of the Mesozoic limestone facies to the west over a

radiolarite-serpentinite fades, as exposed in Wadi Hagil.

The distribution and groundwater potentiality of these aquifers are shown in the hydrogeological map of northern emirates shown in Figure 4.

Figure 4: Hydrogeological map and groundwater potentiality of the study area.

Groundwater Recharge and Drought Environmental Impact

Ruus al Jabal Peninsula is formed mainly of permeable limestone and dolomites, of the order of 3000 m thick (Hudson, 1959). Precipitation entering these beds will be guided by joints and other openings and by the attitudes of the folded beds, which pitch northward outflow of groundwater from these beds will occur either at sea level or through high level overflow

springs occurring where impermeable beds pond back the flow into karst zones or joints as at Khatt and Habhab.

In order to assess the effect of rainfall events, monthly groundwater level data for three observation wells in Khatt area have been selected for the groundwater table analysis. There is a significant variation in the groundwater level in response to recharge events. The maximum groundwater level (approximately 40 masl) was observed on March of 1996 in well Khat-1 which is very close to the southern spring as well as in well RK-14 which is located in Habhab area south of Khatt village (Fig. 5). This also was reflected in the amount of outflow from the northern spring (30 l/s increase; Fig. 6).

The impact of precipitation on the water table level, amount of flow, and temperature of groundwater in the limestone has been studied since 1979 (Figs. 5 & 6). It can be deduced that the precipitation rate has enormous impact on the outflow rate from the spring and can be summarized in Table 1.

Table 1: Impact of 1999-2004 Drought on Khatt Springs. North spring situation in the period 1979-1998 North spring situation in the period 1999-2004

Outflow ranged 10 - 60 l/s Outflow ranged 1 - 30 l/s

Rainfall rate ranged 60 - 450 mm/year Rainfall rate ranged 20 - 66 mm/year

Source water temperature ranged 39 - 40 °C Source water temperature 39 - 40 °C

Irrigation falajs are working without pumping Irrigation falajs are only working with pumping from hand dug wells

During drought years as well as during the summers, over pumping of groundwater for local irrigation purposes has created major cone of depression (Fig. 2) which also lead to a decrease in the springs outflow rates. The following two solutions were suggested to solve this problem.

Figure 5: Water table fluctuations in wells RK-14 and Khatt-1 in response to the amount of

annual precipitation. The water table measurements are shown as monthly values.

Figure 6: Annual flow rate variations in the north and southern springs in response to the

yearly amount of precipitation.

The engineering solution (i.e. deepening the bottoms of pools and the associated outflow falajs) was investigated first but the results indicated that this will only solve problem temporary and thus rejected. Drilling a well which should hit a karst and/or a fracture zone with a special design to prevent mixing of deep hot water with relatively cold water in the alluvial aquifer came out to be the best and sustainable solution for feeding the swimming pools with their need of hot water during drought time (geological solution). The 2D earth resistivity imaging method was used on March 2004 to determine the location of a production well that penetrate the fracture zone and at the same time isolated from the infiltrated return irrigation water. This well was completed on May 2004 and since then it has been used for feeding the swimming pools with hot water during the drought time.

Digital 3D Geological Modeling

Up to now, true 3D modeling of geological structures is not state of the art in regional and local assessment except in the field of economic geology. The required geological information (e.g. drilling information) is not available at least to the desired and appropriate extent for regional statistically based interpolation. The techniques used in most cases for geological 3D modeling are based on statistical or geostatistical interpolation between stratified scattered boreholes. These methods are in adequate in this specific field because it leads to a reduced heterogeneity and an inadequate loss of the “real world” setting of the lithostratigraphic layers. Even the complex structural setting of the Quaternary sediments can not be represented correctly following the geostatistical approach only (Wycisk et al., 2005). Therefore, a first 3D geological model of about 2000 km² was built for Khatt Springs area, southeast of Ras Al Khimah City and is based on a construction of 39 networked cross-sections which are based on 39 borehole records. Beside the drilling information, the following 2D and point information were also used for the 3D-database: a) digital elevation model (DEM), geophysical logging and profiling information, geological and hydrogeological

maps. The developed 3D geological model of Khatt Springs area allows different types of visualization (Fig. 7), calculation and predictions as well as the subsequent operation within hydraulic models. The provided information by the digital subsurface 3D models is of specific need in the fields of groundwater flow modeling of water resources management options as well as for environmental impact assessment of any future development type of this area.

Local-Scale Groundwater Flow Model

Numerous models have been successfully developed for water resources management in clastic aquifers (continuous porous media); where the flow is perfectly laminar flow (Anderson & Woessner, 1992; Slade et al., 1985; Ebraheem et al., 2002 and 2003; Gossel et al., 2004). However, application of numerical models in karst aquifers is more problematic due to the following reasons:

a) Karst aquifers are generally highly heterogeneous. b) Karst aquifers are dominated by secondary (fracture) or tertiary (conduit) porosity and

may exhibit hierarchical permeability structure or flow paths. c) These aquifers are likely to have a turbulent flow component, which may be

problematic in those most numerical models which are based on Darcy’s law, which assumes laminar flow.

d) Despite the fact that modeling of karstic processes is often possible and numerical flow models can sometimes simulate hydraulic heads, ground-water fluxes, and spring discharge, they often fail to correctly predict such fundamental information as flow direction, destination, and velocity (Quinlan et al., 1996)

e) Numerical model in karst aquifers often fail to correctly predict fundamental information for solute transport such as flow direction, destination, and velocity modeling (Huntoon, 1995; Quinlan et al., 1996).

f) Accurate transport predictions require in-depth knowledge of the distribution of the subsurface fracture and conduit systems which is not an easy task. Transport of solutes in fractured rocks is an active research area (Bear et al., 1993).

Figure 7: Constructed 3D-geological model for Khatt Springs area.

The availability of a relatively complete GIS data base of the drilling information for the last forty years as well as the flexibility and modular structure of MODFLOW software enabled us to develop, calibrate, and validate a local scale groundwater flow model for the Quaternary aquifer in Khatt Basin area. Since the karstified aquifer is extending for tens of kilometers with a thickness reaching several hundred of meters (IWACO, 1986) and intensively fractured, it possible to approximate it as equivalent porous media (Larocque et al., 1999; Pankow et al., 1986; Neuman, 1987).

Conceptual Model of the Ground Water System

Schematization of the Aquifer Systems The first step in formulating the conceptual model was to identify the boundaries of the model and its model layers. Geologic information including geological and geomorphological maps (Figs. 1 & 2) well logs, cross-sections (Fig. 3), and statistically-interpolated subsurface stratigraphy using the 3D digital model (Fig. 7) combined with information on hydrogeologic properties (Fig. 4) are gathered to define the hydrostratigraphic units for the conceptual model. As a part of preparing the water budget, the sources of water to the system as well as the expected flow direction and exit points were also identified for the model. The field estimated inflow such as groundwater recharge from precipitation, overland flow, were identified. Outflows such as spring flow, evapotranspiration and pumping were considered in the model conceptualization. Hydrologic information was used to conceptualize groundwater flow system. Hydrologic information like precipitation, evaporation, and surface water runoff, as well as observation well data are also used (Figs. 5 & 6). Water level measurements are used to estimate general direction of groundwater flow, the location of recharge and discharge, and the interaction between aquifers and surface water systems.

Based on well logs, pumping test results, and hydrogeological cross sections, the Quaternary and karstified limestone aquifers were approximated into a six layers model (Fig. 8).

Figure 8: Conceptual model of Khatt springs area in which the subsurface stratigraphy is

approximated into six layers model based on the variations in the vertical hydraulic conductivities.

The model design included setting flow domain and its boundary and initial conditions, development of the grid system, selecting time steps, model layers and their preliminary values of hydraulic parameters, and hydrologic stresses as discussed below (Fig. 9).

Groundwater flow in the main aquifer layers is governed by conditions at the boundaries of the regional system. These conditions are not well defined for catchment area of Khatt Springs. Therefore, in order to model groundwater flow in the Quaternary and karstified limestone aquifers, the modeled area were enlarged to have some cells in the Arabian Gulf in the west (constant head) and some cells in the impermeable argillaceous limestone layer in the southeast (no flow boundary). The boundary conditions also include known flux in northeast part.

The proper characterization of hydrogeological parameters is crucial for model calibration and validation. Point date feature thematic layers of the available records of the hydrogeologic properties (Table 2 & 3) were created using ArcGIS.

Table 2: Pumping tests results of wells in the karstified limestone aquifer.

Well Specific Capacity [m³/hr/m]

Transmissivity [m²/day]

RK-5 23 288 RK-9 50 382

RK-11 27 580 RK-14 67 2800 RK-15 26 140 RK-16 11 66 KhSp-1 39.7

Table 3: Pumping tests results of wells in the Juweiza aquifer.

Name Easting [m]

Northing [m]

Specific capacity

[m3/hr/m]

Saturated Thickness

[m]

Transmissivity [m2/day] Storativity

Hydraulic Conductivity

[m/day]

GP1/1A 394896 2802122 1.8 142 7.6 0.006 0.0535 GP2 388741 2804271 0.7 105 2.7 - 0.025714 GP3 399780 2804621 3 184 6.1 0.0011 0.033152 GP4 395451 2820702 0.2 312 1 - 0.003205 GP6/6A 385858 2782511 29 123 1166 0.02 9.479675 GP8/8A 388718 2822739 7.6 176 270 0.003 1.534091 GP10/10A 393881 2790978 4.1 320 264 0.001 0.825 GP11 384115 2773198 28 166 480 - 2.891566 GP14 401017 2891573 6.2 126 230 - 1.825397 GP15 391717 2792509 5 126 156 - 1.238095 GP16/16A 396682 2798251 2 143 120 0.003 0.839161 GP17 382413 2790957 10 110 110 - 1

The interpolation capabilities of the program SURFER was used to assign an initial value of each property to each grid cell in the model area. A lot of constrains had to be situated prior to the interpolation procedure to avoid the problem of extrapolation. Among these constrains, the appropriate model of griding was firstly chosen by using the variogram kriging function of

SURFER. The variogram is a three dimensional function usually used to match a model of spatial correlation of observed variables. Variogram is a measure of how quickly things change in space and thus it is used to define the weights of the kriging function (Cressie, 1990).

Steady State Simulation

After assigning the model conceptualization (Fig. 8), the appropriate values of hydrogeologic parameters (e.g. hydraulic conductivity, specific Storativity / specific yield), the hydraulic parameters (e.g. porosity), and initial hydraulic head (water table of 1969) were assigned to each grid cell in the model domain (Fig. 9). Then the model was run for the steady state condition using MODFLOW software (McDonald and Harbaugh, 1988) and the simulated groundwater contours were compared into the observed groundwater contours of 1969. A minor modification in the distribution of the hydraulic conductivity parameter spatial distribution was needed to obtain an acceptable fit between observed groundwater contours of 1969 and the simulated steady state groundwater contours. However a deviation of less than 5 meters was measured in the mountainous area in the eastern part of the modeled area where the information about the geological and hydrogeological setting is rather limited. The close fit between the simulated and observed groundwater contours indicated that before 1970, the recharge components of the Quaternary aquifer from Oman Mountain was enough to balance or slightly exceeding the sum of natural and artificial discharge existing at that time and thus the system was in a steady state condition before 1970. The close fit between the two sets of groundwater contours (Fig. 9) is considered as a preliminary calibration of the model and thus the model is being validated by groundwater simulation under transient state for the period 1970 to 1986 as information about water table levels on both years as well as the abstraction rates in this period are available.

Figure 9: Correlation between observed in year 1969 (dashed lines) and simulated (solid

lines) steady state groundwater contours (Coordinate System: UTM).

Results

The drilling information of the drilled wells indicate the presence of two aquifers in the area of Khatt springs, the Quaternary (or shallow) aquifer and the fractured limestone (or deep) aquifer. The deep aquifer in Ruus al Jabal Peninsula is formed mainly of permeable limestone and dolomites, of the order of 3000 meters thick. The Quaternary aquifer is regarded as the main aquifer and is composed of the unconsolidated sediments (mainly alluvium gravel and coarse sand).

Precipitation entering the Musandam limestone beds will be guided by joint and other openings and by the attitudes of the folded beds, which pitch northward outflow of groundwater from these beds will occur either at sea level or through high level overflow springs occurring where impermeable beds pond back the flow into karst zones or joints as at Khatt and Habhab areas (as springs).

Historical observations of outflow from Khatt springs indicated that the amount of flow is strongly affected by the rate and pattern of rainfall. The drought condition prevailed during the period of 1999-2004 has led to a decrease in the amount of flow from the developed northern Khatt spring from 33 liters/second to 5 liters/second which was not enough to support the recreational and irrigational activities in this area.

The hydrological and hydrogeological settings of Khatt Springs area were intensively studied to find a sustainable solution to minimize the drought impact on Khatt spring. The results indicated that drilling of a well hitting a karst zone will be the best solution for feeding the developed spring with its need of hot water. The 2D earth resistivity imaging method was used to determining the location and depth of a karst zone in the nearby of the north spring.

After completing building the GIS database for the studied area, a GIS-based 3D groundwater flow model was developed and calibrated for the steady state conditions. The simulated steady state contours are in a good match with the groundwater contours of 1969 and this indicate that groundwater exploitation at that time was less than or equal to the natural recharge.

The detailed “true” digital geological 3D model developed for the area is an important need of subsequent grid mesh refinement in the developed groundwater flow model and/or any subsequent models for environmental impact assessment and water resources management in the area. The GIS database has been used for linking the geological and numerical groundwater flow model and thus to allow combining several different thematic layers to get valuable answers to environmental questions, e.g. risk assessment of groundwater contamination, fate and pathway prediction, and land use planning concepts. Further hydrogeological investigation and detailed monitoring is needed to evaluate the current status of water resources in the area as well as to get better understanding of the relationship between the karstified limestone aquifer and the Quaternary aquifer in term of hydraulic connection and source of recharge. This is also needed for developing strategies for the sustainable development of these precious natural resources of UAE.

Acknowledgement

We would like to acknowledge Ms. A. Hanf, Mr. P. Stankiewicz, and Mr. Ch. Neumann for their valuable help with figures adjustment and preparation.

References

Anderson, M. P., and W. W. Woessner, 1992, Applied ground water modeling: simulation of flow and advective transport. Academic press, INC. 2-48, pp. 97-120.

Bear, J., C.-F. Tsang, and G. de Marsily, 1993, Flow and Contaminant Transport in Fractured Rock, Academic Press, San Diego, p. 560.

Cressie, N.A.C., 1990, The origin of Kriging. Mathematical Geology, 22, pp. 239-252. Ebraheem, A. M., S. Riad, P. Wycisk, and A.M. Seif El Nasr, 2002, Simulation of present and future

groundwater extraction from the non-replenished Nubian Sandstone Aquifer, SW Egypt. Environmental Geology, 43(1-2), pp. 188-196, DOI 10.1007/s00254-002-0643-7.

Ebraheem, A. M., H. K. Garamoon, S. Riad, P. Wycisk, and A.M. Seif El Nasr, 2003, Numerical modeling of ground-water resource management options in East Oweinat area, SW Egypt. Environmental Geology, 44(4), pp. 433-447, DOI 10.1007/s00254-003-0778-1.

IWACO, 1986, Ground water study. Drilling of Deep Water Wells at Various locations in the UAE. Gossel, W., A.M. Ebraheem, and P. Wycisk (2004). Very large scale GIS Based Groundwater Flow

Model for the Nubaian Sandsone Aquifer in Eastern Sahara. Journal of Hydrogeology, 12(6), pp. 698 -713.

Hudson, R.G., and M. Chatton, 1959), The Musandam Limestone (Jurrassic to lower Cretaceous) of Oman, Arabia, Notes et Mem. Moyen Orient, 7, pp. 69-93.

Huntoon, P.W., 1994, Is it appropriate to apply porous media groundwater circulation models to karstic aquifers? In: El-Kadi, A.I., (Ed.), Groundwater Models for Resources Analysis and Management, 1994 Pacific Northwest/Oceania Conference, Honolulu, HI, pp. 339–358.

Larocque, M., O. Banton, P. Ackerer, and M. Razack, 1999, Determining karst transmissivities with inverse modeling and an equivalent porous media. Ground Water, 37 (6), pp. 897–903.

McDonald, M.G., and A.W. Harbaugh, 1988, A Modular three dimensional Finite-difference Ground-water Flow Model. U.S. Geol. Survey Open-File Report 83-875, USGS, Washington, DC.

Neuman, S.P., 1987, Stochastic continuum representation offractured rock permeability as an alternative to the REV and fracture network concepts. In: Custodio, E., A. Gurgui, and J.P. Lobo-Ferreira (Eds.), NATO Advanced Workshop on Advancesin Analytical and Numerical Groundwater Flow and Quality Modelling, NATO ASI Series, Series C: Mathematical and Physical Sciences, vol. 224. Reidel Publications, Dordrecht, pp. 331–362.

Robertson, A.H.F., 1990, Evaluation of the Arabian continental margin in the Dibba zone, Northern Oman Mountains: in Robertson, A.H.F., The geology and Tectonics of the Oman Region. Geol. Soc. Spec. Pub No 49.

Sir Williams Halcrow & Partners, 1969. Water Resources of Trucial States. Internal report. Ministry of Agriculture and Fisheries, Dubai, UAE.

Slade, R.M., L. Ruiz, and D. Slagle, 1985, Simulation of the flow system of Barton Springs and associated Edwards Aquifer in the Austin area, Texas.USGeol. Surv., Water Resource. Inv. Rep. 85-4299, 49.

Pankow, J.F., R.L. Johnson, J.P. Hewetson, and J.A. Cherry, 1986, An evaluation of con-taminant migration patterns at two waste disposal sites on fractured porous media in terms of the equivalent porous medium (EPM) model. J. Cont. Hydrol., 1, pp. 65–76.

Quinlan, J.F., G.J. Davies, S.W. Jones, and P.W. Huntoon, 1996, The applicability of numerical models to adequately characterize ground-water flow in karstic and other triple-porosity aquifers. In: Ritchey, J.D., and J.O. Rumbaugh (Eds.), Subsurface Fluid-Flow (Ground-water and Vadose Zone) Modeling, American Society for Testing and Materials, ASTM, West Conshohocken, PA, STP 1288, pp. 114–133.

Wycisk, P., W. Gossel, A.Wollmann, H. Fabritius, and T. Hubert, 2005, High resolution digital 3D models as a base of hydrodynamic calculation in heterogeneous aquifers. Proceedings of the 5th International Conference on Aquifer Recharge, 10-16 June, 2005, Berlin, Germany.