151.%20Groundwater%20Chemistry

151.%20Groundwater%20Chemistry - Groundwater Chemistry...

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Unformatted text preview: Groundwater Chemistry Lecture 15 Atmospheric Chemistry Dilute solution of the source water Fractional factor: ratio of chemicals in rain Fractional to source water FNa = (Cl/Na)rain/(Cl/Na)sea water FNa F = approximately 1 approximately 0.95 < F > 1.20 0.95 Rainfall chemistry changes as air mass Rainfall moves over different source areas Lecture 15: Groundwater Chemistry Page 1 of 33 Lecture 15: Groundwater Chemistry Acid Rain CO2 gas is soluble in water Dissolved CO2 associates with water CO2 + H2O ↔ H2CO3 H2CO3 ↔ H+ + HCO3– Carbonic Acid Acid Rain SO2 gas is soluble in water gas Dissolved SO2 associates with water SO2 + H2O +O= ↔ H2SO4 H2SO4 ↔ H+ + HSO3– Sulfuric Acid Lecture 15: Groundwater Chemistry Page 2 of 33 Lecture 15: Groundwater Chemistry Groundwater Chemistry on the Hydrologic Cycle Complex Complex geochemical reactions, processes and interactions Non-steady-state Non chemistry Influenced by local Influenced climate Time Affects Unstable chemical system; “early” time - Chemical kinetics control Equilibrium -- Unstable system because of external Factors Lecture 15: Groundwater Chemistry Page 3 of 33 Lecture 15: Groundwater Chemistry Chemical Interference Internal reactions between “A” and “B” produce variable conditions Chemical Processes Hydrolysis — Water Hydrolysis Oxidation — Oxygen/Reduction Oxidation Redox Relationships Redox Carbonation — Calcium Carbonation Chelation — Organic complexing Chelation Cation Exchange — M+ ↔ N+ Cation Dialysis — Organic separation Dialysis Hydration — incorporation of water into crystal Hydration structure Lecture 15: Groundwater Chemistry Page 4 of 33 Lecture 15: Groundwater Chemistry Hydrolysis Most powerful Most weathering process Hydrogen replaces a Hydrogen cation within the mineral Cation becomes Cation chemically mobile Most common Most weathering process of silicate minerals Microcline (KAlSi3O8) ************************ KAlSi3O8 + H2O → HAlSi3O8 + KOH KOH → K+ + OH– Oxidation Oxidation — loss of Oxidation Olivine (MgFeSiO4) an electron causes increase in positive ********************* valence: Fe++ → Hydration: Fe+++ + e– Aerobic conditions — Aerobic 3MgFeSiO4 + 2H2O → Oxygen is the H4Mg3Si2O9 + SiO2 + 3FeO electron acceptor Most powerful Most Oxidization: process for iron (Fe) 4FeO + O2 → 2Fe2O3 compounds Lecture 15: Groundwater Chemistry Page 5 of 33 Lecture 15: Groundwater Chemistry Pyrite Oxidization Pyrite (FeS2) ******************** 2FeS2+7.5O2 +5H2O → 2FeOOH + 4SO4= + 8H+ → 4H2SO4 Pyrite oxidation is the Pyrite major acid-producing reaction Rapid response time Rapid – first noticed (iron rust) Sulfuric Acid Organic Matter Organic Matter (CH2O) ********************** CH2O + O2 → H2O + CO2 CO2 + H2O → H2CO3 Carbonic Acid Lecture 15: Groundwater Chemistry Page 6 of 33 Lecture 15: Groundwater Chemistry Reduction Reduction — gain of an electron causes Reduction decrease in positive valence: Fe++ + 2e- → Fe0 Anaerobic conditions Anaerobic – Nitrate – Sulfate – Carbon Dioxide are electron acceptors are Oxidation-Reduction Redox Reactions For complete reaction both oxidation and For both reduction must occur: Ferrous (Fe++) to Ferric (Fe+++) Oxidation 4Fe++ ↔ 4Fe+++ + 4eReduction O2 + 4H+ + 4e- ↔ 2H2O Lecture 15: Groundwater Chemistry Page 7 of 33 Lecture 15: Groundwater Chemistry Redox Process May be controlled by May microorganisms which act as catalysts Variable throughout Variable ground water system Redox Potential Oxidation potential: Eh (volts) Positive Eh = oxidizing Positive Negative Eh = reducing Negative Lecture 15: Groundwater Chemistry Page 8 of 33 Lecture 15: Groundwater Chemistry Acid — Base (pH) Measure of the amount of available H+ Measure ions in an aqueous solution H2O ↔ H+ + OHpH = -log[H+] log[H+] 0 < pH > 7 = Acid pH = 7.0 = neutral <7 pH > 14 = Base Highly sensitive in the environment – Highly especially to dissolved CO2 and Redox Redox — Acidity Relationships Eh — pH diagram Eh maps species of compounds dissolved in aqueous solutions Lecture 15: Groundwater Chemistry Page 9 of 33 Lecture 15: Groundwater Chemistry Iron Eh — pH Diagram Sulfur Eh — pH Diagram Lecture 15: Groundwater Chemistry Page 10 of 33 Lecture 15: Groundwater Chemistry Carbonation Acidic environment Acidic Excess available H+ Excess H2O + CO2 → H+ + HCO3– Dominate process in Dominate carbonate-rich rocks Calcite (CaCO3) Dissolution H2CO3 + CaCO3 → Ca++ + 2H2CO3– → Precipitation CaCO3↓ + H2O + CO2↑ Chelation Organic complexing Organic with carbon ring structures Associated with humic Associated substances derived from weathering of organic materials Chelate with metals Chelate (Cations) Lecture 15: Groundwater Chemistry Page 11 of 33 Lecture 15: Groundwater Chemistry Cation Exchange Surface chemistry Surface Cation (anion) bound to the surface of a Cation mineral is exchanged with a more active cation. Most active site is within the “double Most layer” around clays Dialysis Physical movement of colloids across a Physical membrane Organic separation Organic Process is used to purify water from salt Process water Lecture 15: Groundwater Chemistry Page 12 of 33 Lecture 15: Groundwater Chemistry Hydration The incorporation of water into crystal The structure Olivine 3MgFeSiO4 + 2H2O → H4Mg3Si2O9 + SiO2 + 3FeO Hydrocarbons Hydrocarbons Complex system of Carbon and Hydrogen Complex compounds Aromatic hydrocarbons Aromatic – – – – – – – Benzene ring Aliphatic Saturated hydrocarbon Paraffin Straight-chain structure StraightBranched-chain structure BranchedStructural isomer Page 13 of 33 Lecture 15: Groundwater Chemistry Lecture 15: Groundwater Chemistry Benzene Ring Six carbon atoms Six Joined in ring structure Joined Alternating single and double bonds Alternating Bonds change positions on the ring Bonds Polycyclic Aromatic Hydrocarbon Polycyclic Two or more benzene rings joined together Two Benzene Structures Lecture 15: Groundwater Chemistry Page 14 of 33 Lecture 15: Groundwater Chemistry Saturated hydrocarbons Straight-chain Branched Chains Structural isomers Same formula Same C5H12 Different Compound Compound Properties Properties Environmental Impact Environmental Lecture 15: Groundwater Chemistry Page 15 of 33 Lecture 15: Groundwater Chemistry Hydrocarbon Degradation Processes Biotic and Abiotic Biotic Oxidation Oxidation Contaminant Transport Vadose Zone Unsaturated Unsaturated Multi-phase system Multi Soil Suction > 0.0 Soil Capillary fringe Capillary Saturated Zone Saturated Saturated Soil Suction = 0.0 Soil Pore Pressure > 0.0 Pore No capillary problems No Lecture 15: Groundwater Chemistry Page 16 of 33 Lecture 15: Groundwater Chemistry Vadose Zone Transport Clay Reactions Colloids < 2 μm Colloids Electrostatic double Electrostatic layer Cation exchange Cation reactions Surface Tension Capillary potential Capillary Function of pore size Function Decreases with moisture Decreases Soil—water—air reactions Soil Preferential Water Movement Short circuiting along Short mud cracks Fingering along root Fingering traces and other features Funneling along Funneling stratigraphic contacts From Fetter, 1993 Lecture 15: Groundwater Chemistry Page 17 of 33 Lecture 15: Groundwater Chemistry Saturated Media Transport Advection Advective transport Advective or Convection Dissolved solids Dissolved carried with flowing ground water “Conservation of mass” Dispersion Longitudinal dispersion Longitudinal Transverse dispersion Transverse Mechanical – mixing Mechanical Diffusion – chemical Diffusion gradient Advective Transport No Mixing Lecture 15: Groundwater Chemistry Page 18 of 33 Lecture 15: Groundwater Chemistry Flow Velocity Average linear velocity (vx) Effective porosity (ne) = interconnected porosity ONLY Effective Hydraulic conductivity (K) Hydraulic Hydraulic gradient (i) Hydraulic Vx = (K/ne)(i) Important in rock—contaminant reactions Important Dispersive Transport Longitudinal mixing and Transverse mixing Lecture 15: Groundwater Chemistry Page 19 of 33 Lecture 15: Groundwater Chemistry Flow at the Microscale Longitudinal Dispersion Transverse dispersion Lecture 15: Groundwater Chemistry Page 20 of 33 Lecture 15: Groundwater Chemistry Contaminant Plume Rock—Contaminant Reactions Dissolved solutes can: be Sorbed on mineral grains be be Sorbed by organic carbon be Precipitate Precipitate undergo Biodegradation undergo undergo Abiotic degradation undergo undergo Redox reactions undergo undergo Cation exchange reactions undergo Lecture 15: Groundwater Chemistry Page 21 of 33 Lecture 15: Groundwater Chemistry Retardation Solute moves slower than the velocity of Solute the ground water because of chemical reactions Retardation Factor Rf = Vw/Vc Retardation – Vw velocity of water – Vc velocity of contaminant Retardation Lecture 15: Groundwater Chemistry Page 22 of 33 Lecture 15: Groundwater Chemistry Retardation Factor Rf = [1 + (ρb/Θ)Kd)] Where: – ρb = Dry bulk density (g/L3) – Θ = moisture content (unsaturated) – Θ = porosity (saturated) – Kd = distribution coefficient (L/kg) Sample Calculation Rf = [1 + (ρb/Θ)Kd)] Given: – ρb = 1.75 g/cm3 – Θ = 0.20 – Kd = 3 ml/g = 3 cm3/Kg = (0.03cm3/g) Solution: – Rf = [1 + {((1.75)/0.20)(3)}] – Rf = [1 + 26.25] – Rf = 27.25 = 27 Lecture 15: Groundwater Chemistry Page 23 of 33 Lecture 15: Groundwater Chemistry Impact on Transport Rf = 27 = Vw/Vc Vc = Vw/27 Vw = (K/ne)(i): How fast does the contaminant move? If: – (ne) = .25 – K = 2.5 X 10-3 cm/s – i = 0.04 Then: – Vw = (K/ne)(i) = (0.0025)/.25)(0.04)cm/s = 4 X10-4 cm/s cm/s And: – Vc = (4X10-4 cm/s)/27 = 1.5X10-5 cm/s Reality Check Is this aquifer truly homogeneous? NO! Lecture 15: Groundwater Chemistry Page 24 of 33 Lecture 15: Groundwater Chemistry Reality Check NO! Variable Transport distance of contaminant Are the water quality data from the wells valid? Soil – Contaminant Reactions Electron microscope views of two different clay minerals are shown to the left. Note the sharp crystal edges in A and the highly irregular crystals in B. Clay B is significantly more reactive than clay A. Lecture 15: Groundwater Chemistry Page 25 of 33 Lecture 15: Groundwater Chemistry Multiphase Transport Nonaqueous phase liquids Nonaqueous – Dense (DNAPL) “Sinkers” – Light (LNAPL) “Floaters” Two fluid system with different physical Two and chemical properties Air—water—NAPL—rock system Air Multiphase System Lecture 15: Groundwater Chemistry Page 26 of 33 Lecture 15: Groundwater Chemistry Multiphase Reactions Vadose zone Air—fluid—pore wall Air interface problem: Capillary Suction Evaporation Evaporation Volatilization Volatilization Saturated zone Variable hydraulic Variable conductivity Variable density Variable Variable viscosity Variable Impact of Saturation Low saturation > Low water surrounds LNAPL High saturation > High LNAPL surrounds water Higher viscosity Higher results in “trapping” of LNAPL Lecture 15: Groundwater Chemistry Page 27 of 33 Lecture 15: Groundwater Chemistry Impact of Water Level Changes Initial Conditions Impact of Water Level Changes Lower water table Lower allows drainage and downward migration Raise water table Raise increases intrapore concentration and trapping of LNAPL’s LNAPL becomes LNAPL trapped within pore spaces and expels water Lecture 15: Groundwater Chemistry Page 28 of 33 Lecture 15: Groundwater Chemistry DNAPL Properties Sink below water column Sink Pass through vadose zone Pass Become mobile at temporary saturation Become Descend to lowest non-permeable unit Descend Can flow counter to regional flow system Can Flow path is controlled by geologic system Flow Common DNAPL’s (70+) Esters Esters Halogenated alkanes Halogenated Halogenated alkenes (x, x-Dichloroethenes) Halogenated Halogenated monoaeromatics Halogenated Polychlorinated biphenyls (PCB’s) Polychlorinated Others (aniline, benzyl chloride, etc.) Others Multi-Component wastes Multi – coal tars – creosotes – heavy oils Lecture 15: Groundwater Chemistry Page 29 of 33 Lecture 15: Groundwater Chemistry Significant Environmental Risk Can migrate along well casings Can Causes desiccation of clay “barriers” Causes Allows vertical migration through barriers Allows Piedmont Springs Grimes County, Texas A Study in Groundwater Geochemistry Donald B. Riley, MS theses, TAMU, 1993 Lecture 15: Groundwater Chemistry Page 30 of 33 Lecture 15: Groundwater Chemistry Piedmont Springs National Historical Site, Grimes County Health resort before and during the Civil War Health Three sulfur springs within 400 feet of each other Three Black spring: strong sulfur -- treat foot problems Black Middle spring: intermediate -- immersion treatments Middle White spring: mild -- internal ailments White White Spring Lecture 15: Groundwater Chemistry Page 31 of 33 Lecture 15: Groundwater Chemistry Local Geology & Geochemistry From Donald B. Riley, MS theses, TAMU, 1993 Examples of Problems The photograph of the well screen pulled, intact, from a 150 foot deep monitor well at a DNAPL contaminated site shows significant amounts of cement on the screen. Lecture 15: Groundwater Chemistry Page 32 of 33 Lecture 15: Groundwater Chemistry The down-hole photograph shows oxidized and decomposed hydrocarbons at the base of the steel casing at a depth of 487 feet, well below the regional water table. Break Lecture 15: Groundwater Chemistry Page 33 of 33 Lecture 15: Groundwater Chemistry ...
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This note was uploaded on 04/16/2011 for the course GEOL 320 taught by Professor Mathewson during the Spring '11 term at Texas A&M.

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