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by: Reece Crist II


Reece Crist II
GPA 3.76


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This 29 page Class Notes was uploaded by Reece Crist II on Tuesday October 20, 2015. The Class Notes belongs to GEOL551 at San Diego State University taught by Staff in Fall. Since its upload, it has received 25 views. For similar materials see /class/225293/geol551-san-diego-state-university in Geology at San Diego State University.

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Date Created: 10/20/15
WATER BUDGET OBJECTIVES How to measure or quantify volumes of rainfall and evapotranspiration Calculation of in ows and outflows in a water basin Steadystate vs Unsteady or Transient Scenarios Calculation of ow as a residual of a water balance Errors involved in measurement or estimation technique Use of Spreadsheets for calculations Lecture Notes READING ASSIGNMENT Chapter 1 pgs 810 Chapter 2 pgs 24 37 Fetter A Tale of Two Lakes Module HOMEWORKASSIGNMENTDue 919 Chpt 1 problems 67 9 A Tale of Two Lakes Module Problems 15 From given Mono Lake steadystate water balance model EXCEL file adjust to produce the transient model described in pages 2526 of module predict annual water volume and level changes using model and graph Describe ADDITIONAL RESOURCES Groundwater Recharge Kansas Geologic Survey Bulletin 249 Groundwater Recharge and Water Budgets httpwwwkgskueduHydroPublications2004BulI249contentshtml California Groundwater CA Dept of Water Resources Bulletin 118 httpwwwdroundwaterwatercaqovbulletin1 18 WATER VOLUMES Precipitation and Evapotranspiration Water Volumes Liters 1 cubic meter 1000 I 3785 Igal 1 cubic foot 7480 gal 1 acreft 325851 gal 43560 cubic ft Water Cheaper than dirt for farmers Farmers federal subsidies for water About 5acreft 1 542844 pounds Top soil About 5 per cubic yard 3cubic yard for delivery 1 325 pounds dirt Southern California customer about 31000 gal About 100x rate of farmers Humidity humidity mass ofwater in grams per cubic meter ofair humidity maximum amount of water contained in an air mass 9 saturation humidity is dependent on temperature 9 higher the temperature more water in air mass humidity absolute humiditysaturation humidity 100 at saturation humidity Cooling Air Masses 9 as an air mass with a given amount of water vapor cools it s saturation humidity will relative humidity will increase 9 condensation will occur if the absolute humidity the saturation humidity 9 the temperature at which this will occur to a particular air mass with a particular absolute humidity is called the How can we cool an air mass 9 any lifting of an air mass will cause adiabatic expansion and cooling 9 as air mass moves up in atmosphere pressure is exerted on the air mass ofair mass 9 without any exchange of heat with surrounding air bodies air mass will same amount of heat as originally but now in a larger volume What will cause air masses to lift up 0 Meeting oftwo differing air masses differing temperatures 0 effect lifting of air masses along mountainsides 0 Heating ofair masses will cause expansion and lifting o Afternoon thunderstorms in Midwest and eastern US Measurement of Precipitation US standard rain gauge 8 inches in diameter manually read each day or can be set up with an automatic logging system can get hourly information ocation critical effect of winds trees buildings etc should be close to ground away from trees and buildings basin or catchment all land area sloping toward a particular discharge point Outline of surface water drainage basin occurs along surfacewater boundaries ocean lake or topographic divides Topographic divides highest points ofelevation along edge ofdrainage basin Surface water basins and groundwater basins do not always coincide can have different boundaries Effective Depth of Precipitation average depth of precipitation over a drainage basin must de ne a time period of interest hour day month year if you have a uniform network of rain gauges generally NOT the case simply calculate the arithmetic average Example Fig 24 Fetter if you have a nonuniform network of rain gauges lines precipitation contour map Fig 25 Fetter can use knowledge oftopographic effects 0 air mass lifting up mountainside etc method weighting factor for each rain gage Fig 26 Fetter polygons created to show area represented by each rain gage measure areas of polygons using planimeter or estimate with grid method multiply each rain gage reading by it s area sum all information and divide by total area Example Problem Fetter p 3637 0 Arithmetic average effective uniform depth EUD 270 cm 0 lsohyetal technique EUD 234 cm 0 Thiessen polygon EUD 247 cm EVAPORATION water molecules are continually being exchanged between liquid and atmospheric water vapor when number of molecules going to water vapor exceeds the number of molecules going to liquid evaporation Free water evaporation sometimes computed by a water budget technique often measured by use ofa shallow land pan 4 ft in diameter and 10 in deep unpainted galvanized metal on supports so air can circulate underneath maintain a water level of 7 to 8 inches record daily depth ofwater volume ofwater added and daily precipitation 9 calculate evaporation correct for increased warming on land by using a for your climate and month growing plants are continuously pumping water from the ground to the atmosphere water vapor passed through openings in leaves known as stomata transpiration is higher than evaporation from a land surface however dif cult to differentiate two processes and they are lumped together as evapotranspiration Evapotranspiration water loss that will occur ifthere is no limitation to the amount of water available to the plant from the soil 9 will be upper limit or maximum amount of ET for a plant PET Evapotranspiration amount of ET that actually occurs due to soil water limitations 9 will never exceed PET and is often much less than PET Fig 22 Effect ofdry summer actual ET much less than PET until ppt increases in fall Fig 23 uniform ppt actual ET much closerto PET VOLUMES OF PPT OR ET For both ppt and ET you will have a measurement of length pertime For example 10 inches per year average annual San Diego ppt Or 15 cm per month To obtain the volume of inflow ppt or out ow ET you must multiply the length per time estimate by the area of your basin Example Swimming pool is 20 ft by 100 ft Rate of evaporation is 12 inches per month 1 ft Volume of evaporation out ow per month is 20X100X1 2000 cubic ft per month INFLOWS AND OUTFLOWS Control Volumes or Water Basins Water Flow Rates Rivers cubic ft per second cfs Wells gallons per minute gpm Treatment plants million gallons per day mgd Control Volume or Water Basin define your boundaries swimming pool lake entire water basin ln ows precipitation surface water inflows groundwater in ows artificia import Out ows evapotranspiration from land areas evaportation from surface water surface water out ow groundwater out ow artificia export diversions out of basin groundwater pumping STEADY STATE VS TRANSIENT OR UNSTEADY STATE lf ln ows Outflows no change in the water volume stored in the control volume or basin called storage If no change in storage water volumes and levels will remain constant with time conditions no change in time If ln ows and Outflows are NOT equal there will be a change in storage water volume in basin Could change Surface water ows in streams Surface water levels volumes in streams lakes etc Soil moisture levels in basin Groundwater levels and flows Make sure all your units volume length and especially TIME are the same before doing water balance 9 Make sure estimates are for same time period as sometimes time is not listed in the estimate For example 10 inches ofrain over what Year month day should hopefully say annual rainfall for yearly etc CALCULATION OF COMPONENTS AS RESIDUALS Example given in book is for evaporation in a lake p 25 Lake Hefner ln ows precipitation over surface surfacewater in ow and groundwater inflow Out ows groundwater seepage spillway discharge and pumpage evaporation Change in storage change in water volume Use the water balance equation ln ows Outflows Change in Storage Measure or estimate all inflows and out ows except evaporation and change in water volume of lake 9 calculate evaporation ERRORS IN MEASUREMENTS OR ESTIMATION TECHNIQUES In the example in Fetter forthe lake state that errors for all measurements is less than 10 EXCEPT for groundwater in ows and out ows Very dif cult to accurate measure groundwater inflows and out ows large source of error if estimating a variable as a residual ofthe water budget technique USE OF SPREADSHEETS FOR CALCULATIONS VWI be using EXCEL in the Tale of Two Lakes module See Fetter p 20 21 for review TALE OF TWO LAKES MODULE Go to website to download AQUIFER TESTS Estimation of T K and S for an aquifer CONFINED AQUIFER Graphical solutions to flow equations Theis type curve method Cooper Jacob time drawdown and distance drawdown methods Introduction Previous emphasis Predicting drawdown based on known aquifer parameters Examples You get a call from a client they want to know why their well went dry You need to design a dewatering project You need to predict drawdown from multiple wells or aquifers with hydraulic boundaries Now We will look at another aspect of flow to wells making estimates of aquifer parameters T K S based on eld data aquifer tests 2 basic approaches to the solution Use type curves that represent graphical solutions for our flow equations Use straight line graphical solutions The objective in both methods find T K and S Point to remember all of these methods are for nonequilibrium flow nonsteady state Cone of depression is still expanding Non equilibrium ow in a con ned aquifer Theis method Take the Theis equation solve for T hoh Q Wu 47cT becomes T Q Wu 41Tho h Also need to rearrange our well function solution equation solve for S u r28 becomes S4Ttu 4Tt r So how do we get these numbers from an aquifer test Answer the well function for the Theis solution has been plotted on graph paper Compare this curve to actual eld drawdown data wan 001 l0quot l0 TO K1 105 70quot lu A FIGURE 56 The reverse nonequilibrium type curve Theis Curve for a fully confined aquifer 1000 k i O o 0 01 10 100 1000 10000 Time minutesl Drawdown in feet l 0 FIGURE 57 Eidata plot on logarithmic paper for Thels curvematching technique A Steps to a type curve solution 1 Plot results from an aquifer test on graph paper Use the same scale as the type curve plot Plot drawdown 3 log cycles on y axis scale vs time 4 or 5 log cycles on X axis scale Result should look similar to type curve Note time is plotted in minutes here 2 Overlay type curve and plot aquifer test results light table or transparent type curve helps Carefully slide type curve until it matches shape of pumping test Keep axes parallel don39t twist the type curve to make it match 3 Pick a match point Match point is any intersecting line set on the overlay curve A common choice point represented by Wu 1 and 1u 1 Note any match point should produce similar results 4 Read values for W u 1u ho h and t Time minutes Field data lJIuwdown loot Wu Theis type 001 01 10 10 100 1000 10000 1m lGURE 58 2h of eld data plot to Theis type curve 5 Do necessary conversions so that values from curve t can be plugged into our altered version of Theis39 equation Convert 1u to u Convert t from minutes to days divide by 1440 minday 6 Plug values into Theis39 rearranged equation to solve for T Must know Convert discharge Q units to ft3day if necessary 7 Plug values into Storativity equation to solve for storativity Use the value for T calculated above 8 4 Ttu r2 Must be given a value for r radial distance to an observation well Note you MUST have an observation well to calculate storativity using this mm 8 Sumva T K b m KTb Othevgvaphma su mmnsmthwspmb em esuma mgaqu evpaveme evs mama ethbnum uwcundnmns Cuupev and Jacub shang hne werdvawduwn me md m quotunrethbnum aw m a med aquWev 7 7mm awduwn pan 5 mpunam Wm cumpave m dw ancerdvawduwn mama Tm appvuach usesme m mte senesvyumme Thews mm 522 2th 541 m mt 5a I TVKu r w 05772157Nnon7 7 Q m rR ecugm estha he as evms ave magnmcam lFWe ave deahng Wm may mm pumpmg Cumbmmg same tevms and cunvemng a base m ugs m use Wm uuv gvaphmgrsee pg 173 rThevesuH 2 3 o my 25 n 4 mm 7 m P S Me my mumquot Ms us p uths as a shang m an semHug papa Nam mez 3 s a mm mm cunvevsmn mm natuva ugstu base m ugamhm rTypmaHy amhmeuc scaxe un y aws m we a have 3 cyc e ug sca e cm x aws mumme have m mmmea vemembenu cunvemu days m m uHuwmg equauuns JOUVbU NHO Urawclown I ll Aomoo b 1 TO TOO 1000 Time mini 3 CURE 59 K ervjacub method of solution of pumpingvtest data for a fully confined aquifer Evrjown is plotted as a function of time on semilogarithmic paper New equations for this solution A Equation for transmissivity T 23 Q 4n Aho h Note Aho h refers to the change in h over 1 log cycle B Equation for storativity S 225 T to r2 Note to refers to the time where the straight line intersects the zero drawdown line upper X axis on the graph This is tricky must project the straight part ofthe line backward to find to r distance to an observation well Once again this storativity calculation requires an observation well while estimates of T and K do not need an observation well You must be given Q r You must read A ho h tofrom graph to must be converted to days again C Solve for K if needed TKb This method is only valid for long pumping times MUST CHECK CRITERIA AFTER SOLVING I3 11 E 001 4Tt Your text uses a criteria of less than 005 Check for all early times in the pumping data May find that you should NOT be using some of the early time pumping data to draw your straight line May find out that all your data is too early Notice in graph above howthe early time data do not plot on the straight line lll Nonequilibrium flow Cooper Jacob distance drawdown method Requires data from several wells Plot on semilog paper Wells must be properly spaced forthis method to be effective The ideal spacing have at least 3 wells with distances between the wells that plot on 3 different log cycles Example observation well with 3 wells spaced at 10 ft 100 ft and 1000 ft ter 5 CFOUl lCFVVaLci 0 w i o460ft 2 I 4 i z 6 39E T quotquotquotquotquotquotquotquot i E Si 1 E 10 l l 5 mo 7 m z 88 it l 5 H l L i 16 8 1 m 100 1000 Distance feell A FIGURE 510 Variation of the lacob method of solution of pumping test data for a fully confined aquifer Drawdown is plotted as a function of distance to observation well on semilogarithmic paper New formulas A Equation for transmissivity T 23 Q 21TAhoh Note ho h refers to the change in h over 1 log cycle B Equation for storativity s 225Ttr02 where r0 distance where straight line intersects the zero drawdown axis t some time t into the test in days where drawdown is recorded in all wells Summary These methods give a great regional summary of aquifer parameters T K and S are estimated for the entire area affected by the cone of depression Theis method uses relies on good early time data rst few seconds or minutes Distancedrawdown method uses an especially wide area this is useful for computer models that need a regional estimate of aquifer parameters AND uses later time data SUMMARY FOR CONFINED AQUIFER Either simultaneous data from a series of wells distance drawdown or a series of measurements from one well time drawdown are used to plot a semilog drawdown curve from which the values needed to solve the equations are taken straight line methods or Timedrawdown data is plotted on a loglog plot and then matched to a type curve or series of type curves to derive the unknown parameters type curve method LEAKY CONFINED AQUIFER Type curve method Pot 39 entlometric Sumac e Unscreened aquifer Leaky confining layer Aquifer Confining layer K vertical hydraulic conductivity of leaky confining layer B thickness of leaky confining layer Source of water leaking to confined aquifer is an upper unconfined aquifer in pictured example 0 Drawdown response in leaky confined aquifers o HantushJacob formula ho h ivrui O 4TT B Tb39 Bl lg O K rzS u 4Tt where Wu rB is the well function for leaky aquifer K and b are the hydraulic conductivity and thickness ofthe con ning layer respectively Nonequilibrium 10 e E l 0 E Del 001 10 10 10 101 103 104 105 106 lu A FIGURE 51 l Type curves of leaky artesian aquifer in which no water is released from storage in the J confining layer Source W C Walton Illinois State Water Survey Bulletin 49 796 Match type curve and aquifertests data to get WurB 1u t s and rB Substitute values into HantushJacob equations to obtain T S K UNCONFINED AQUIFER TYPE CURVE MATCHING lLIA 10quot 10 39lO1 103 103 lOquot 1 O1 5 cutquot 0 OO4 0001 s We 001 003 U 1 006 1 rJKviKhtr A 02 39 o 0 0 0 6 2 10 j 2 O 13 3 c 25 l v 3 0 1 i 50 10 Type B curves l0393 l0 1 l0 1 10 lOl 103 103 104 lUB A FIGURE 515 73926 curves for drawdown data from fully penetrating wells in an unconfined aquifer Alrce S P Neuman Water Resources Research 7 7 7975329 42 Used With permISSlOn Type A curves early part of curve match to early data to obtain specific storage Type B curves mid to late part of curve match to later data to obtain speci c yield T estimate from both match points should be similar Obtain vertical K from lambda value match point EFFECT OF PARTIAL PENETRATION OF WELLS rr l 7g39mnce mum POW Co iimsig lawr ell gtUEPH 3 I ll Confin layer A FIGURE 517 Flow lines toward a partially penetrating well in a confined aquifer The problem with having a partially penetrating pumping well is that flow nearthe well will not be completely horizontal as water is pulled upward toward the well opening Hantush has show that this is not a problem if the observation wells are fully penetrating Ifthe observation wells are also partially penetrating then they effect of having a vertical ow component is negligible if the following relationship is true w 15513 pm When designing aquifer tests it is important that these effects be taken into consideration if the pumping well is not going to be fully penetrating SLUG TESTS water in the well by lowering into it a solid piece of pipe called a slug ahhh So that s where An alternative to a pump test is a slug test also called a baidown test In this test the water level in a small diameter well is quickly raised or lowered The rate at which the water in the well falls as it drains back into the aquifer or rises as it drains from the aquifer into the well is measured and these data are analyzed Water can be poured into the well or bailed out ofthe well to raise or lower the water level However perhaps the easiest way to raise the water level in the well is to displace some ofthe the name comes from of the WeH dmcun to make Wm any degree of accuracy wmmmx at start nf taut mmaw head uf aqwrer Zr 77 lxucmiumclrw surface 5 allcr slug mjoc nn A 7 7 7 A pnlcnlxomclllc mime itWilt lrlbrcslugiqiucliun K k N H L b v v v Zr Muulmlu H 412 H r etc and Whetherthe aquer response rs overdamped Water rever recovers m a smooth manner r m mow transrmssrve aquer Tmngs to be oarem about 8mm effect 7 ower hydrauhc oonouowrty materrar orays orrurng mods etc have been e We rr H surge We to 5w an rnoorreot rowvarue of K How Far Will It Go Predicting the Extent of Groundwater Plumes By Kathryn Thorbj arnarson Dept of Geological Sciences San Diego State University OVERVIEW This module will explore the use of a solute transport model to assess the fate of organic contaminants in groundwater An analytical solution to the advection dispersion equation with retardation and transformation will be implemented in Visual Solute Transport VST Upon completion ofthe module students will have gained an understanding possible natural bioremediation or natural attenuation of groundwater contaminants INTRODUCTION Human activities have resulted in numerous groundwater contaminant plumes Implementation of groundwater remediation such as pumping and treatment of groundwater is expensive and can fail to attain the low cleanup levels stipulated by a regulatory agency In the past decade the use of natural bioremediation or natural attenuation has gained interest as a technique for contaminant plume cleanup Natural processes such as biotransformation dispersion volatilization and dilution can attenuate or decrease the contaminant plume size and concentration levels A remediation scenario in which this natural attenuation is monitored can be a low cost and effective cleanup technique A model and extensive monitoring data will be required to convince regulators of the viability of natural attenuation remediation In this module we will describe and use an analytical solution to the solute transport model to assess the natural attenuation potential of some common contaminants PROBLEM Tools are needed for the evaluation of natural attenuation of groundwater contaminants Simple screening models such as analytical solutions can be utilized to assess the potential for natural attenuation Simulations predict the extent and concentration of contaminants in a groundwater plume at a given time and distance from the contaminant source BACKGROUND Groundwater Contamination Two predominant groups of groundwater contaminants are petroleum products and chlorinated solvents Some important differences are found in these two types of pollutants Table 1 Petroleum products have a lower density than water When spilled in suf cient quantities these light nonaqueous phase liquids LNAPLs will move from the ground surface to the shallow areas ofthe saturated zone groundwater Conversely chlorinated solvents are dense nonaqueous phase liquids DNAPLS and when spilled in sufficient quantities will extend beyond the shallow groundwaters into deep regions of the subsurface While these nonaqueous phase liquids can be limited in their mobility from the source or spill zone watersoluble chemicals within these liquids will dissolve into the surrounding mobile groundwaters The resulting contaminated waters emanate from the spill zone outward in the direction of groundwater flow producing a groundwater plume The primary watersoluble components in gasoline which is a mixture of over 100 chemicals are benzene toluene ethylbenzene xylenes BTEX and a gasolineadditive MTBE As BTEX and MTBE dissolve into water from a NAPL mixture the effective solubilities of these components from gasoline are lower than their singlecomponent aqueous solubilities Table 2 The most commonly used chlorinated solvents are tetrachloroethene PCE and trichloroethene TCE All these chemicals will dissolve in water at levels capable of causing health risks and above the regulatory limits or maximum contaminant levels Table 2 Assessment ofthese plumes in the 1980s and 1990s showed another important difference between these contaminant groups In an article titled Where s the benzene the authors pointed out while spills of both chemicals are equally prevalent only the chlorinated solvents have produced large and persistent groundwater plumes Subsequent studies have confirmed the prevalence of smaller stable BTEX plumes emanating from gasoline spill sites Biotransformation of BTEX in the oxygenated shallow groundwaters stops the transport ofthe contaminants past a certain distance from the spill The plume extent is stabilized and controlled by a balance between the solute transport processes advancing the plume away from the spill groundwater advection and dispersion and the biotransformation process Lower rates of biotransformation of MTBE PCE and TCE result in larger and more persistent groundwater plumes Table 1 Ma39or Groundwater Contaminants and Their Properties Petroleum Products Chlorinated Solvents distances from the spill source Type of Light Nonaqueous Phase Liquid Dense Nonaqueous Phase chemical LNAPL density is less than Liquid DNAPL density is liquid water can extend from ground higher than water can extend surface to shallow groundwaters from ground surface to bottom of groundwater aquifers Most Gasoline Tetrachloroethene PCE common Diesel Trichloroethene TCE examples Jet fuel Uses Fuel Metal degreasers dry cleaning Potential Fuel transfer spills leaking Leaking storage tanks Releases storage tanks Primary Benzene B PCE dissolved Toluene T TCE chemicals Ethylbenzene E which will Xylenes X Transformation byproducts form Methyl tert butyl ether MTBE cis dichloroethene DCE groundwater vinyl chloride VC plumes Historic BTEX groundwater plumes tend Very large and persistent Findings to be limited to relatively short groundwater plumes Contaminant Solubility Level ppm ppm Solubility in Gasoline kidney or liver or carcinogen cancer tasteampodor problem or 0005 cancer cancer Solute Transport A solute transport model must simulate all the processes controlling the fate of an organic chemical dissolved in water All chemicals dissolved in moving groundwater will be subjected to advection or movement ofthe groundwater and dispersion or spreading out ofthe groundwater The advection and dispersion processes are primarily controlled by the aquifer properties hydraulic conductivity and its distribution hydraulic gradient In simple models of uniform flow advection is simulated by an average porewater velocity calculated from Darcy s Law v K i e where v average porewater velocity K hydraulic conductivity of aquifer LT i hydraulic gradient in aquifer LL 9 effective porosity of aquifer The dispersion or spreading out of contaminated water results in an increase in groundwater plume size and a decrease in the contaminant concentrations Dispersion is caused by the mechanical mixing of water tortuous pathways through groundwater pores heterogeneities in hydraulic conductivities and chemical diffusion The chemical diffusion component of hydrodynamic dispersion tends to be negligible compared to the mechanical dispersion component In a uniform ow field dispersion will occur in the direction of ow longitudinal dispersion coefficient DL horizontally and perpendicular to the direction of flow transverse dispersion coefficient DT and vertically and perpendicular to the direction of flow vertical dispersion coefficient DV These dispersion coefficients are estimated with the following empirical equations DL ocL v DT ocT v DV xv v where v average porewater velocity LT ocL longitudinal dispersivity of aquifer L ocT transverse dispersivity of aquifer L xv vertical dispersivity of aquifer L The relationship between dispersivity and dispersion coefficient and velocity was found in laboratory solute transport experiments Field estimates of dispersivity have shown a scale effect with larger dispersivities for larger transport distances For this module we will assume constant dispersivity values but obey the following ruleof thumb XL1OOLT OLT1OOLV Many organic compounds have limited water solubility and will tend to adsorb onto aquifer solid surfaces These hydrophobic compounds will have a particular affinity to adsorb onto organic carbon coatings on the sediments The most common model forthe sorption process is an instantaneous reversible process at equilibrium As the compounds adsorb onto sediments and desorb off of sediments there is no permanent loss of contaminant mass through the sorption process However there is a slowing or retarding ofthe apparent advection ofthe sorbed contaminant plume The parameter used to simulate equilibrium sorption is the retardation factor R which will be a function ofthe organic compound and the aquifer sediment R1Kd where pb aquifer bulk density gcm3 19 ps p5 sediment solid density gcm3 9 aquifer porosity Kd sorption partition coefficient cm3g The sorption partition coef cient is a function ofthe chemical s affinity for organic carbon and the amount oforganic carbon present in the sediment The following equation is used to estimate Kd in sediments with more than organic carbon Kd foc Koc where f00 fraction of organic carbon in sediment KOC organic carbon partition coefficient cm3g Biotransformation The biologically mediated transformation biotransformation oforganic compounds generally involves reductionoxidation redox reactions Redox reactions involve the transfer and acceptance of electrons between two compounds The rate oftransformation will be controlled by the chemical type redox conditions ofthe aquifer type and population of bacteria etc In our module this complex process will be simulated by a simple rstorder rate model C C06 1t where Co initial chemical concentration X rstordertransformation rate 1T The transformation process is sometimes described by a chemical s halflife t12 In a firstorder model the relationship between the firstorder transformation rate and halflife is A chemical s transformation rate will vary with differing aquifer redox conditions Aerobic redox conditions will tend to occur in oxygenated high levels of oxygen shallow groundwaters Deeper groundwaters which are farther from the atmospheric oxygen source can tend to be low in oxygen or anaerobic MODEL DETAILS We will be solving the following advectiondispersion equation 3C BC 32C v DL 2 at 3x 3x where C contaminant concentration as a function of xyzt R retardation factor representing equilibrium sorption v uniform porewater velocity LT DL longitudinal dispersion coefficient in the direction of ow ocL v ocL longitudinal dispersivity L X rstorder transformation rate coefficient 1T ln 2 t12 t12 halflife oftransformation rate T ZC This model includes onedimensional advection or uniform groundwater flow onedimensional dispersion equilibrium sorption or retardation firstorder transformation E M PI RICAL DATA Except for MTBE the following firstorder transformation rates are taken from ranges given in Bedient et al 1999 For MTBE aerobic transformation rates have been estimated to be from one to two orders of magnitude lowerthan benzene rates in eld studies Commonly asked questions regarding the use of natural attenuation at federal facilities pjwwwerdllnlqovrescueTopicsAFCEE Technical Guidelines for Evaluating Monitored Natural Attenuation of Petroleum Hydrocarbons and Chlorinated Solvents in Ground Water at Naval and Marine Corps Facilities httpenvironfescnavvmierberb asupportwrk qrparttmna1198pdf PROBLEMS AND PROJECTS For all problems use noted 1 Calculate the solute transport parameters in the following table Show all calculations on a and fill in the answers in the table 2 TRACER TEST Using VST simulate a tracer such as chloride in your aquifer Assume a source concentration of 100 mgI for 10 days Print the graph showing the plume centerline concentrations for 1 10 100 and 1000 days make sure you have the complete plume from near source to far edge What happens to the plume over time What process causes this 3 DISPERSIVITY UNCERTAINTY Using VST evaluate effects ofa range of dispersivities Graph your chloride plume after 100 days oftravel for longitudinal dispersivities of001 m 01 m 1 m and 10 m Print and describe the effects of range of dispersivities What would happen if you overestimated your dispersivity 4 DISPERSION Using VST simulate the concentration history ofthe chloride tracer in a well 100 m downgradient ofthe source using the same range of dispersivities as above 001 m 01 m 1 m and 10 m Print and describe the effects ofthe range of dispersivities If you had to predict when and at what concentration chloride would show up at a well what would happen if you overestimated your dispersivity 5 SORPTION EFFECTS Using VST simulate the concentration history of chloride and all the organic compounds in the table in problem 1 Assume a source leak of 10 days and use either the effective solubility for gasoline components or aqueous solubility for chlorinated solvents as a source concentration see Table 1 Use the longitudinal dispersivity in the table for problem 1 Graph the concentration history in a well 100 m downgradient ofthe source location Graph chloride and 3 to 4 organics per graph to compare How does sorption affect the solute transport 6 BENZENE UNDER AEROBIC CONDITIONS Using VST simulate benzene in your aquifer under aerobic conditions Use the effective solubility in Table 1 as your source concentration Use the aerobic transformation rate for benzene in Table 2 Print the graph showing the plume centerline concentrations for 1 5 10 and 20 days since source has leaked make sure you have the complete plume from near source to far edge At what time does the plume appear to stabilize How far will concentrations above the MCL extend from the source area 7 MTBE UNDER AEROBIC CONDITIONS Using VST simulate MTBE in your aquifer under aerobic conditions Use the effective solubility in Table 1 as your source concentration Use the aerobic transformation rate for MTBE in Table 2 Print the graph showing the plume centerline concentrations for 100 400 1200 and 3000 days since source has leaked make sure you have the complete plume from near source to far edge At what time does the plume appear to stabilize How far will concentrations above the MCL use the cancer health advisory extend from the source area How is this different from benzene REFERENCES Bedient PB S R Hanadi and CJ Newell 1999 Ground Water Contamination Transport and Remediation 2nd edition Prentice Hall PTR Upper Saddle River NJ 604 pp Domenico PA 1987 An analytical model for multidimensional transport ofa decaying contaminant species Journal of Hydrology ThorbjarnarsonKW J lnami and G Girty 2002 Visual solute transport a computer code for use in hydrogeology classes Journal of Geoscience Education Vol 5 p 287291


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