Chemical Hydrogeology

Transcription

1 Physical hydrogeology: study of movement and occurrence of groundwater Chemical hydrogeology: study of chemical constituents in groundwater Chemical Hydrogeology Relevant courses General geochemistry [Donahoe] Aqueous geochemistry [Donahoe] Environmental & Organic geochemistry [Lu] Isotope geochemistry [Aharon] Soil and groundwater remediation [Tick] Contaminant Hydrogeology and Modeling [Zheng] Useful Reference (course website): Hem, J. D., 1985, Study and interpretation of the chemical characteristics of natural water, d ed., USGS Water-Supply Paper 54. OUTLINE Units of concentration Types of chemical reactions Chemical composition of groundwater Water analysis Contaminant transport and geochemical processes Groundwater contamination and remediation Units of concentration a) Weight mass of solute mass concentration volume of solution kg 1000 g g 1 1 m 1000 L L g 1000 mg mg 1 1 m 1000 L L 1 g of solute part per million (ppm) 6 10 g of solution kg 6 g for fresh water, m m 1g of solute mg part per million (ppm) 1 1 m of solution L 1 g of solute part per billion (ppb) g of solution ppm 1

2 Units of concentration Assume the background concentration of dissolved oxygen (DO) is 5 ppb in the above aquifer. Further assume the aquifer is 10,000 m long, 1000 m wide and 100 m thick. b) equivalence(used in chemical reaction calculations) formula weight in g equivalent weight valence e.g. Na + (sodium) =.99 g Ca + (calcium) = g equivalent wt of Na equivalent wt of Ca milliequivalent weight = equivalent weight x 10 - Electronic charge of ion meq # of meq # of mg L # of mg L L L equivalent wt formula wt valance.99 g.99g g 0.04g 1) What s is the total amount of DO in the aquifer? ) What is the mass flux of DO across the right face? To convert mg/l to meq/l meq mg L valance L formula wt To convert meq/l to mg/l mg meq/l formula wt L valance # of moles # of molality 1 kg of solution (1 mole = formula weight in g) To convert mg/l to molality moles 1 L - mg/l10 molality formula weight in g To convert meq/l to molality - meq/l 10 molality valence Ex: convert 0.01 molal NaCl into mg/l Formula weight: Na + :.99 g; Cl : 5.46 g; NaCl: g 0.01 mg L10 x x mg L Types of chemical reactions sufficiently fast equilibrium-controlled reactions, reversible NaCl Na Cl NaX Ca CaX equilibrium approach, simple to apply concerned with terminal state no information on intermediate steps Na

3 Types of chemical reactions Time scales of chemical reactions in groundwater systems insufficiently fast kinetic-controlled reactions, irreversible A C da A dt [first-order decay of A] :kinetic rate coefficient kinetic approach, information on reaction processes rate coefficients difficult to obtain; vary with external conditions LEA valid Equilibrium reactions LEA invalid Kinetically controlled reactions (Domenico and Schwartz, 1990) Composition of groundwater Major cations Ca +, Mg +, Na +, K + Major anions HCO -, Cl -, SO - 4, CO - Together, >90% of the dissolved solids in groundwater

4 Water analysis TDS => total quantity of solids when a water sample is evaporated to dryness CLASS TDS (mg/l) Fresh 0-1,000 Brackish 1,00-10,000 Saline 10, ,000 Brine >100, Specific Conductance => ability of the sample to conduct electricity Hardness =>concentration of Mg + and Ca + in mg/l per equivalent CaCO 0-60 soft Total hardness=.5(ca + )+4.1(Mg + ) moderately soft hard >180 very hard Water analysis (cont.) ph => measure of [H + ], ph= -Log 10 [H + ] Ph=7, natural Ph>7, base Ph<7, acidic Alkalinity =>capacity for the solution to react with and neutralize acid, commonly expressed as equivalent amount of CaCO in mg/l or meq/l Eh => redox potential, a measure of oxidizing or reducing tendency, important in systems with sulfur and iron species because it determines the state of particular species DO => dissolved oxygen, an important factor in biological and microbial studies Graphical presentation of chemical data Collins bar diagram Pie chart 4

5 Graphical presentation of chemical data Stiff plot Piper diagram 5

6 Contaminant Transport Advection Advection Hydrodynamic Dispersion Molecular Diffusion Mechanical Dispersion Chemical Reactions Sorption 1 st -order Radioactive Decay or Biodegradation Refer to the movement of solutes in flowing groundwater with average seepage velocities (v) Usually the dominant process for migration of solutes in the subsurface Hydraulic conductivity (K) of subsurface media is by far the most critical controlling factor K h v l REV Hydrodynamic Dispersion Molecular Diffusion spreading of solute over a greater region than would be predicted solely from average groundwater velocity vectors v t 1 t t Hydrodynamic dispersion refers to the sum of molecular diffusion and mechanical dispersion Spreading of solutes as a result of concentration gradient Fick s law of diffusion C F D * l F : diffusion mass flux D*: diffusion coefficient 6

7 Mechanical Dispersion Velocity Variation as Mechanism of Mechanical Dispersion Spreading of solutes as a result of deviations of actual velocity from average seepage velocity Local Scale Pore Scale Macro-scale (Skibitzke & Robinson, 196) Velocity Variation as Mechanism of Mechanical Dispersion Advective-Dispersive Transport Regional Scales vt vt 1 v Longitudinal Dispersion D L t Transverse Dispersion D T t 7

8 Effects of Advection and Dispersion Sorption Mass transfer (or, partitioning) process between the contaminants dissolved in groundwater (solution phase) and the contaminants sorbed on porous media (solid phase), including absorption (incorporation into the interior of a solid); adsorption (attraction to a surface); and ion exchange Usually considered to be equilibrium-controlled, i.e., the mass transfer process is instantaneous and reversible Retardation Concept advancing contaminant plume appears "retarded" because dissolved contaminants are sorbed on to porous materials, leaving less solute for transport retreating contaminant plume appears "retarded" because sorbed contaminants are de-sorbed into the dissolved phase, leaving more solute behind. 8

9 Linear Sorption Isotherm Retardation Factor Retardation factor Sorbed Conc. C K C d d Dissolved Conc. K : distribution coeff. R 1 b K n d Advection dispersion equation with sorption C C C R D v t x x First-Order Decay or Biodegradation Advective-Dispersive-Reactive Equation C/Co =0. day -1 =0.1 day -1 C C t C C e t 0 t -1 : reaction rate [T ] =0.05 day Time (days) C C C R v RC t x x t 1 ln 0.69 t t 1 1 : half - life [seconds, hours, days, years] 9