بسم الله الرحمن الرحيم تلوث المياه الجوفية الناتج عن التسربات من خزانات الوقود المدفونة الخيار بين الوقاية والعلاج

Groundwater Contamination from Leaking UST’s: Prevention versus Restoration Mohammad Al-Suwaiyan Civil Engineering Department King Fahd University of Petroleum & Minerals

Presentation Layout  Introduction  Complexity of groundwater Contamination Assessment  need for modeling  associated difficulties, uncertainties

1,400,000,000,000,000,000 m 3

1,400,000,000,000,000,000 m % 37,500,000,000,000,000 m 3

1,400,000,000,000,000,000 m % 37,500,000,000,000,000 m 3 23% 8,300,000,000,000,000 m 3

1,400,000,000,000,000,000 m % 37,500,000,000,000,000 m 3 23% 8,300,000,000,000,000 m 3 1.5% 126,250,000,000,000 m 3

Distance to sun 150,000,000 km

5 million

Background  Ground water a main source of potable water  Contamination of ground water  Hydrocarbon leaks and associated problems

Laws related to GW contamination  Legislations related to air or SW pollution were enacted in 1940’s and 50’s  1980 CERCLA or superfund act was passed establishing $ 15 billion to pay for clean up of abandoned hazardous waste sites which can be recovered by EPA from responsible parties  What is the reason for the time lag?

Leaking Tank Residual hydrocarbon Mobile hydrocarbon Ground-water flow GAS

Leaking Tank Residual hydrocarbon Mobile hydrocarbon Ground-water flow GAS

Leaking Tank Residual hydrocarbon Mobile hydrocarbon Water table Ground-water flow Hydrocarbon components dissolved in ground-water GAS Saturated zone Vadose zone

How Can a Hydrocarbon Exist  Residual phase held by capillary & adsorptive forces  Vapor phase  Free phase  Dissolved in groundwater

Typical Phase Distribution After Spill

Important Notes  Due to the hidden nature of the subsurface environment many think it will degrade any contaminant given enough time.  Contaminants that enter the subsurface will eventually make it to the aquifer.  Remediation and cleanup of subsurface became an important issue.

Factors Influencing Fluids Distribution Soil Hydraulic Properties  Displacement head  Pore size distribution index  Residual saturation

Factors Influencing Fluids Distribution Continued Fluid Properties  density  Surface tension  Viscosity  Solubility  Volatility

How is a spill discovered?  Accidental detection  Product could appear in a monitoring well  Is using monitoring wells effective measure?

Water Oil Air Monitoring Well

Problems associated with using MW to detect leaks  Free product may not appear in MW even with significant LNAPL in formation  LNAPL thickness in MW varies with the condition of the water table  Sudden appearance and disappearance of LNAPL in MW is observed in the field

Conclusion Monitoring wells are easy to install and to use but they are not reliable methods to detect and quantify LNAPL leaks

Remediation Project  Control source  Complete site characterization  Design remediation process & system  Evaluate effectiveness

Remediation Project  Control source  Complete site characterization  Design remediation process & system  Evaluate effectiveness  CAN ANY OF THE ABOVE BE DONE WITHOUT THE HELP OF MODELING?

Modeling needs 1) Water flow model 2) Dissolved pollutant model 3) Model to handle various phases

Using water mass balance and Darcy equation a Water flow model can be developed However it has limitations and its results will influence performance of next models Water flow model

Groundwater modeling flow equation K exhibits large variation that is impossible to represent which leads to non accurate velocity field

Dissolved pollutant model Using mass balance for each species and including Various influencing processes a model capable of prediction distribution of dissolved pollutant model can be developed However it has limitations and its results will influence design and operation of treatment system

Contaminant Transport modeling transport of dissolved contaminants Processes that can be included : dispersion, advection, reactions, Sorption kinetics.. How close can we represent the actual processes? Velocity field from flow model is input to this model.

Contaminant Transport Mechanisms  Advection : contaminant is carried with flowing water  Flow velocity  Solubility  Hydrodynamic Dispersion : spreading of contaminants  Concentration gradient  heterogeneity

Attenuation Mechanisms  Adsorption  Retardation factor  Reaction  Decay  Oxidation/reduction  Biodegradation

Modeling various phases This is the most difficult part of modeling but it can Be done as shown in the next slide, However modeling real life cases is impossible i.e. modelingit has limitations and its results will influence design and operation of treatment system

Modeling Different Phases Contaminants as distinct phases For each phase similar equation will have to be solved along With proper auxiliary and boundary, initial conditions

Conclusion 1 Modeling is an essential tool for aquifer assessment & remediation

Conclusion 2 Results obtained by modeling are rarely accurate which directly affects the effort for remediation

Conclusion 3 Verifying model prediction are practically impossible due to the nature of the subsurface environment

Subsurface restoration

Remediation Priority  Stop bleeding & Control source  Know subsurface & figure out extent  Develop & implement cleanup plan

Sources  leaking tanks  Free product (Mobile phase)  Residual phase

Subsurface & Extent  challenging and difficult  hidden nature  inherited heterogeneity  Naturally will involve heavy modeling

Cleanup planning  mobile phase  Pump-and-treat  Number of wells  Locations  Flow rates and time variations All are selected through modeling

Cleanup planning  residual phase (potential sources) SVE  Through promoting mass transfer to vapor phase then extraction  How close can actual processes be modeled  Enhancing biodegradation IBD  How close can the behavior of microorganisms be modeled Operation variables are selected through modeling

Cleanup process complicating factors  Initial moisture distribution  Natural heterogeneity  process may trigger worse situation through creation of new channels which facilitate contaminant transport

Supporting reports NRC committee on groundwater cleanup alternatives investigated 77 sites in which treatment processes were conducted for a long time NRC committee on groundwater cleanup alternatives investigated 77 sites in which treatment processes were conducted for a long time Only 8 sites reached cleanup goals Only 8 sites reached cleanup goals In some cases the situation became worse In some cases the situation became worse

Case of Brunswick,N.J.  Subsurface contamination by a computer company was discovered  After 6 years of Cleanup the process was stopped thinking it was complete  After 3 years contamination came back worse than ever before

Conclusion  Characterization, analysis, design and operation of cleanup systems relies heavily on modeling  Subsurface clean-up process is usually difficult, costly and has very low success rate

Recommendation  More effort should be given to prevention  Meaningful regulation to the process of design, installation, monitoring and maintenance of USTs should be developed

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