M.S. Al-Suwaiyan, Int. J. Sus. Dev. Plann. Vol. 6, No. 3 (2011) 360–376 EVALUATING AND MANAGING POTENTIAL GROUNDWATER POLLUTION FROM LEAKING USTs AT A COASTAL CITY IN SAUDI ARABIA M.S. AL-SUWAIYAN Civil Engineering Department, King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia. ABSTRACT Leaking underground fuel storage tanks (USTs) are the main source of petroleum hydrocarbon contamina- tion of soil and groundwater. The distribution and fl ow of the leaked hydrocarbon is infl uenced by capillary and gravity forces. Depending on the leaked volume and the soil properties, the hydrocarbon could exist as a residual phase completely held in the soil above the water table or as both free phase product and residual. Various components of gasoline will affect the groundwater quality and each will spread according to its own characteristics. The pollution processes are studied through the development of a combination of two models: a compositional and a potential fl ow model. The former was developed using the mass balance principle and is capable of using the organic compound and the subsurface properties to predict the concentration of the various components of benzene, toluene and xylene (BTX) mixture in a leachate from a site contaminated by a residual mixture. Potential fl ow theory was utilized to model the advective transport of contaminants towards municipal wells. Capture zones were defi ned and travel times for different municipal wells were estimated for a sample study area at a coastal city in eastern Saudi Arabia. Modeling and fi eld survey indicated the high potential for groundwater pollution and concluding that more effort should be directed to promote leak prevention through management strategies that include developing proper design regulations and installation requirements like monitoring wells for new and existing service stations. Keywords: groundwater management, leaking USTs, modeling groundwater pollution, water resources. 1 INTRODUCTION Most countries in the Arabian Peninsula and north and central Africa are situated in the water scarce region according to MENA [1]. Saudi Arabia in particular has essentially negligible surface water which makes its groundwater resources highly important that have to be utilized in an effi cient way in order to sustain its development. This important source is not only being mined but also it is sus- ceptible to pollution. The value of this resource can be reduced drastically if its quality deteriorates. Leakage from underground fuel storage tanks (USTs) is a main source of soil and groundwater contamination. USTs, which are essential parts of all gas stations, represent potential sources of subsurface contamination. Fuel leaks can take place through the tank itself due to corrosion, which is typical for older USTs, or through the associated piping systems, which may take place at both new and old stations. The distribution and fl ow of the hydrocarbon, after a leak, is infl uenced by both capillary and gravity forces. The hydrocarbon can exist in four phases: 1. A residual phase held by capillary and adsorptive forces 2. A vapor phase 3. A free phase product 4. Dissolved in the groundwater The groundwater quality will be affected not only by the dissolved hydrocarbon phase but also by all other phases. Contaminants from all phases will slowly dissolve in the fl owing groundwater as well as recharge water forming a contamination plume, which will spread in the direction of regional © 2011 WIT Press, www.witpress.com ISSN: 1743-7601 (paper format), ISSN: 1743-761X (online), http://journals.witpress.com DOI: 10.2495/SDP-V6-N3-360-376 M.S. Al-Suwaiyan, Int. J. Sus. Dev. Plann. Vol. 6, No. 3 (2011) 361 groundwater gradient. The growth and movement of the contamination plume will be infl uenced by type of contaminant, groundwater velocity and direction, aquifer heterogeneity and subsurface prop- erties including distribution coeffi cients and microorganisms. Contaminant transport in the subsurface environment is not well understood by the public. Many people believe that this environment will degrade many contaminants given enough time. The main fact that contributes to this misconception is the hidden nature of the subsurface. It is well known that contaminants that enter the subsurface will eventually make it to the groundwater aquifer, which could be a drinking water source [2]. During the last three decades, Saudi Arabia, the largest producer and exporter of oil, has under- gone huge and rapid urbanization and agricultural development which cannot be sustained without the availability of enough water. According to the Ministry of Planning [3], the population jumped from 7 million in 1974 to close to 23 million in 2004. Water is not only vital for human life but enough water must be available if we are to sustain the development of natural resources and quality of life. Saudi Arabia has an arid environment with practically no surface water and the main sources of water are groundwater and desalination plants [4]. The country produces and consumes 316 bar- rels of gasoline daily [3], which passes through USTs below service stations scattered around municipalities. The potential for groundwater pollution due to accidental leaks is high given the lack of proper guidelines and regulations during design, installation and monitoring of USTs. This paper presents a modeling study aiming to assess the potential contamination of groundwater by leaking USTs, its possible consequences and expected time frame before municipal water supply is directly affected. This study also reviews that current regulations are practices related to USTs and at the end possible ways to mange this problem and to reduce such risk are presented. 2 FATE OF CONTAMINANTS AFTER A LEAK Assuming static equilibrium in a homogeneous porous media, the distribution of the three fl uids: air, hydrocarbon and water will be infl uenced by the capillary pressure values and the relationship between the capillary pressure and saturation for the system [5]. If we assume that the water satura- tion is controlled by the value of the capillary pressure for the water (wetting fl uid)–oil (non-wetting fl uid), and the total liquid saturation is controlled by the value of the capillary pressure for the oil (wetting fl uid)–air (non-wetting fl uid), it would be possible to plot the water and the total liquid saturation along a vertical section in the contaminated subsurface. Using the fact that the capillary pressure between water and hydrocarbon is zero at their interface in a fully screened well and since the capillary pressure between hydrocarbon and air is zero at the surface of the free product in that well, fl uids’ saturation along the vertical can be generated. Farr et al. [6] as well as Lenhard and Parker [7] showed that the vertical distribution of an hydro- carbon after a spill is expected to be infl uenced by the spill volume, soil properties like displacement head, distribution index and value of residual saturation. These properties refl ect the grain-size dis- tribution in the aquifer material which varies signifi cantly from one location to another. Hydrocarbon properties also infl uence its distribution including density, surface tension, viscosity, solubility and volatility. In general, a hydrocarbon can exist in either of four classes. It can be held by capillary and adsorptive forces in the unsaturated/saturated zone as residual or immobile phase which approxi- mately remain in its place but slowly dissolving part of its mass with any water fl ow. It can also exist as a vapor phase in the unsaturated zone or as free phase near the water table and in monitoring wells. Finally, it can exist as a dissolved phase in groundwater at relatively very low concentrations but note that most of these products are very harmful to humans even at trace levels. Field investigation at this stage could be used for assessing the extent of contamination and knowing the area distribution of the released contaminants. It may involve sampling of aquifer material, 362 M.S. Al-Suwaiyan, Int. J. Sus. Dev. Plann. Vol. 6, No. 3 (2011) construction of wells screened in hydrocarbon zone and wells screened below the water table, which can provide information such as thickness of hydrocarbon in wells, concentration of dissolved contaminants as well as approximate water table elevation. These are the primary data that must be used to evaluate nature and extent of groundwater pollution. Soil samples collected during the fi eld investigation can be taken to the laboratory to get their grain-size distribution which may be in turn used in models such as the one presented by Mishra et al. [8] to generate a fi rst approximation for the hydraulic properties of the subsurface. Hydrocarbon distribution in layered soils was studied by Al-Suwaiyan et al. [9]. A review for estimating spill volume is presented by Saleem et al. [10]. It is well established that monitoring wells are not reliable for spill detection and quantifi cation since in many fi eld cases, leaks are accidentally discovered by detecting free product in utility manholes not by fi nding free product monitoring wells. 3 STUDY AREA The city of Alkhobar is located in the eastern province of Saudi Arabia and sets on the eastern shore of the Arabian Gulf. This area has undergone huge development and population growth along with the oil industry. According to the offi cial data, it has a population of 166,000. The central part of the city (10 km × 10 km) was selected to evaluate the potential groundwater pollution as well as developing strategies for controlling possible pollution. The conditions at Alkhobar are similar to other cities of the same size around the developing world and the general results of the study are expected to apply to some degree on other similar cities.