Conservative and Reactive Solutes

1. Solute Transport Conservative and Reactive Solutes Conservative do not react with soil / groundwater Chloride is a good example Reactive Sorbed onto mineral grains as well as organic matter

2. Retardation Slows the rate of transport

3. Surfaces of solids can possess an electrical charge Particularly true of clays, tend to possess excess negative charge Cations more likely than anions to be adsorbed Divalent ions more strongly adsorbed than monovalent ions HCO3- SO4 2- NO3- Size of ion matters if too large not adsorbed

4. One dimension advection - dispersion with sorption b  C  t 2C x2 C  x C*  t DL vx = - - dispersion advection sorption • DL = coefficient of longitudinal hydrodynamic dispersion • C = solute concentration in liquid phase • vx = average linear groundwater velocity • t = time • b = bulk density of aquifer • = porosity (saturated aquifer) C* = amount of solute sorbed per unit weight of solid

5. Direct linear relationship between amount of solute sorbed onto solid (C*) and the concentration of the solute (C) C* = KdC C* = mass of solute sorbed per dry unit weight of solid (mg/kg) C = concentration of solute in solution in equilibrium with the mass of solute sorbed onto the solid (mg/L) Kd = distribution coefficient (L/kg) C* Slope of linear isotherm = Kd C

6. One dimension advection – dispersion with sorption b  C  t 2C x2 C  x C*  t DL vx = - - C* = KdC Substitute into advection – dispersion equation b  C  t 2C x2 C  x (KdC)  t DL vx = - - C  t b  2C x2 C  x (1 + Kd) DL vx = - b  (1 + Kd) = rf = retardation factor

7. If solute is reactive, it will travel slower than groundwater rate due to adsorption vc = vx / [1 + (b / ) (Kd)] = vx / rf Linear isotherm has no upper limit to amount of sorption What if data don’t fit linear?

8. Freundlich isotherm Nonlinear relationship C* = Kf Cj If you plot C* vs C … data will be curvilinear Linearize the data by plotting log … Log C* = j log C + log Kf C* = mass of solute sorbed per bulk unit dry mass of soil C = solute concentration Kf, j = coefficients

9. Plot of log C* vs log C … straight line Slope is j Log C* Log C* = j log C + log Kf intercept log Kf Log C

10. Plug into advection – dispersion equation b  C  t 2C x2 C  x (KfCj)  t DL vx = - - bKfj C j-1  2C x2 C  x C  t DL vx = - (1 + ) Retardation factor for Freundlich sorption isotherm If j = 1 this becomes the linear isotherm Still no upper limit

11. Langmuir Sorption Isotherm Limited number of sorption sites When all sorption sites filled, no more sorption 1 12 C C* C 2 = + C = equilibrium concentration of the ion in contact with soil C* = amount of ion adsorbed per unit weight of soil 1 = adsorption constant related to the binding energy 2 = adsorption maximum for the soil (mg/kg) ( ) 12 b  = rf 1 + (1 + 1C)2

12. ( ) 12 b  = rf 1 + (1 + 1C)2 If you plot C* verses C will have curved shape that reaches a maximum C* C C C* If you plot C/C* vs C data will plot on straight line C 2 = reciprocal of the slope 1 = slope of line divided by intercept

13. Effect of retardation on solute transport Lower peak value and peak arrives later

14. DNAPL (Denser)

15. Density of Contaminant

16. Transport With Water If Denser than Water

17. Leaking Gas Tanks