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Hydrogeochemistry

Hydrogeochemistry. “Geochemistry of Natural Waters” No wastewater, water resources Study chemistry of rivers, lakes, ground water, oceans etc. Questions considered:. Why do different waters have different chemical compositions? What controls the compositions?

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Hydrogeochemistry

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  1. Hydrogeochemistry • “Geochemistry of Natural Waters” • No wastewater, water resources • Study chemistry of rivers, lakes, ground water, oceans etc.

  2. Questions considered: • Why do different waters have different chemical compositions? • What controls the compositions? • How do compositions vary with setting? • How do they vary with time?

  3. Why consider these questions? • Diagenesis • Chemical (and physical) alteration of solid material (low T and P) • Material: rocks, sediments, minerals, plants, animals, bacteria • Often involves gas phases

  4. Why consider these questions • Hydrology and Hydrogeology • Variations in chemical composition can be used to understand (map) flow paths • Flow can alter chemistry • Diagenesis and hydrology are linked

  5. Hydrologic cycle • Free water distribution • 96% in oceans • 3% in ice • 1% in ground water • 0.01% in streams and lakes • 0.001% in atmosphere

  6. The hydrologic cycle – this figure is for water How would dissolved mass be included in this?

  7. Reservoirs = location of water (e.g. lake, ocean, river etc.) • Flux = motion of water between reservoirs • Units = mass per time (per area) • Hydrologic cycle = closed loop of the flux of material

  8. Flux = can be motion of any material (e.g. water, solutes, students in room) • Flux only applicable if cycle is steady state • Not changing through time • Input = output

  9. For systems in steady state – can consider the “Residence Time” • Average time that material is in reservoir • Definition T= A/J Where: A = abundance (not concentration of material J = flux of material (into or out of)

  10. System not in steady state called transient • Best defined by “Response time” • The amount of time for mass to change to certain value • Typically doubling or halving. • Sometimes considered “e-folding time” • Amount of time for exponentially growing quantity to increase by a factor of e. • Exponential decay = time to decrease by a factor of 1/e

  11. Quantification of hydrologic cycle – a box model

  12. A more complicated (complete?) box model

  13. A natural system: Suwannnee River What are the values water and mass for each box? Abundance in reservoir What are values for arrows? Fluxes

  14. Quick discussion of chemical changes in hydrologic cycle • Rain • Streams • GW • Meteoric vs non-meteoric water

  15. Chemical (and Isotopic) composition of water • Natural water always in contact with soluble material – air, sediments, rocks, organic matter • Consequence – no natural water is “pure”

  16. Importance • Dissolution of gases (e.g., CO2) • Dissolution of solid phases –porosity • Precipitation of solid phases –cements • Coupled with hydrologic cycle - controls flux of material

  17. Rain water chemistry Na+ concentrations • What might be the most likely source for Na and Cl? • How could you test to see if this hypothesis is true? • What are implications if this is true, e.g. what and where are other sources? Cl- concentrations

  18. Relative concentrations, Rainfall Pollution – H2SO4 Gypsum dust Excess Ca, Mg, less Na and K from oceans SO4 matches pH – H2SO4 SO4 matches Ca SO4 marine influence – dimethyl sulfide

  19. Temporal variations • During storm • Rain starts salty, becomes fresher during strom • O and H isotopes also change during storm • Snow melt initially saltier & lower pH • change in melting temperature

  20. Rainfall not the only mechanism to deposit material from atmosphere to land surface • Aerosol – suspension of fine solid or liquid in gas (e.g. atmosphere) • Examples – smoke, haze over oceans, air pollution, smog

  21. Dry deposition – aerosols • Dissolution of gases and aerosols by vegetation and wet surfaces • Sedimentation of large aerosols by gravity • Occult deposition • More general term - Dry deposition plus deposition from fog • Dry and Occult deposition difficult to measure

  22. Atmospheric deposition of material called “Throughfall” • Sum of solutes from precipitation, occult deposition, and dry deposition • A working definition • Data Available • National Atmospheric Deposition Program • napd.swsl.uiuc.edu

  23. Compositional changes resulting from throughfall – NE US Open box – throughfall composition Closed box – incident precipitation composition

  24. Hydrology/hydrogeology • Atmospheric deposition leads to surface and ground water • Variety of processes alter/move this water: • Evaporation • Transporation (vegetative induced evaporation • Evapotranspiration

  25. Movement across/through land surface • Overland flow – heavy flow on land surface • Interflow – flow through soil zone • Percolate into ground water

  26. Conceptualizaton of water flow Important to consider how each of these flow paths alter chemical compositions of water Through- fall

  27. Examples of changing chemistry • Plants • Provide solutes, neutralize acidity, extract N and P species • Soil/minerals • Dissolve providing solutes • Evaporation • Increase overall solute concentrations • Elevated concentrations lead to precipitation • Salts/cements

  28. Stream Hydrology • Baseflow • ground water source to streams • Allow streams to flow even in droughts • Augmentations of baseflow • Interflow, overland flow, direct precipitation • Result in flooding • Chemical variations in time • caused by variations in compositions of sources

  29. Bank storage • Flooding causes hydraulic head of stream to be greater than hydraulic head of ground water • Baseflow direction reversed • Water flows from stream to ground water • Hyporheic flow • Exchange of water with stream bed and stagnant areas of stream • Nutrient spiraling – chemical changes in composition because changing reservoir

  30. Stream compositions • Generally little change downstream • Short residence time in stream • Little contact with solids • Changes usually biologically mediated • Nutrients (N, P, Si) uptake and release (Nutrient spiraling) • Pollutants • Chemistry changes with discharge • Chemistry changes with exchange of GW and SW

  31. Ground water • Unconfined example • Porosity – fraction of total solid that is void • Porosity filled w/ water or water + gas • Vadose zone – zone with gas plus water (unsaturated – can be confusing term) • Phreatic zone – all water (saturated zone) • Water table – separates vadose and phreatic zone

  32. Ground water flow • Flow through rocks controlled by permeability • Water flows from high areas to low areas • Head gradients • Water table mimics land topography • Flow rate depends on gradient and permeability

  33. Confined aquifers • Regions with (semi) impermeable rocks • Confining unit • Confined aquifers have upper boundary in contact with confining unit • Water above confining unit is perched • Level water will rise is pieziometric surface • Hydrostatic head

  34. Effects of confined aquifers Perched aquifers, springs, water table mimic topography GW withdrawal lowers head

  35. Other types of water • Meteoric water – rain, surface, ground water • Water buried with sediments in lakes and oceans • Formation waters • Pore waters • Interstitial water/fluids • Typically old – greatly altered in composition

  36. Other water sources • Dehydration of hydrated mineral phases • Clays, amphiboles, zeolites • Metamorphic water • Water from origin of earth – mantle water • Juvenile water • Both small volumetrically; important geological concequences

  37. Concentrations/Units • Need common way to describe dissolved components • Many ways to do this: • Solutes: mass (e.g. g, kg) or moles • Solvent: amount of solvent or solution • Geology usually reported by mass – units of analyses • Chemical calculations always by moles

  38. Concentration terminology • Total dissolved solids (TDS) – mass of solid remaining after evaporation of water • Bicarbonate converted to carbonate • Units of mass (e.g. g, kg, etc.)

  39. Salinity – similar to TDS except quantities of Br and I replaced with Cl • Operational definition • Cl titration includes Br and I • Salinity reported as ratio of electrical conductivity to standard • Originally “Copenhagen seawater” • Now KCl standard • Ratio so dimensionless (commonly ppt, ‰, PSU, nothing)

  40. Chlorinity • Determined by titration with AgNO3 • Definition • Mass (g) of Ag necessary to precipitate Cl, Br, and I in 328.5233 g of seawater • Total number of grams of major components in seawater: • gT = 1.81578*Cl(‰) • S(‰) = 1.80655*Cl(‰)

  41. Water salinity • Fresh water • Potable, generally < 1000 mg/L TDS • Brackish • Non-potable, but < seawater • Seawater, salinity 34 to 37‰ • Saline water/brine > seawater salinity

  42. Other measures of TDS • Refractive index • Amount of refraction of light passing through water • Linearly related to concentrations of salts • Conductivity/resistivity • Current carried by solution is proportional to dissolved ions

  43. Conductivity • Inverse of resistance • Units of Siemens/cm • 1 Siemen = 1 Amp/volt = 1/Ohm = 1 Mho • Conductance is T dependent • Typically normalized to 25º C • Called Specific Conductivity

  44. Reporting units • Need to report how much dissolved material (solute) in water, two ways: • Moles • Mass • Need to report how much water (solvent) • Volume of water, typically solution amount • Mass of water, typically solvent amount

  45. Molar units • Number of molecules (atoms, ions etc) in one liter of solution • Most common – easy to measure solution volumes • Units are M, mM, µM (big M) • Example Na2SO4 = 2Na+ + SO42- 1 mole sodium sulfate makes 2 moles Na and 1 mole SO4

  46. Molal Units • Number of molecules (atoms, ions etc) in one kg of solvent • Abbreviation: m or mm or mm (little m) • More difficult to measure weight of solvent, not used so often • Difficult to determine amount of solvent in natural waters with dissolved components

  47. Why use molar units? • Reaction stoichiometry is written in terms of moles, not mass CaCO3 = Ca2+ + CO32- 100 g = 1 mole 40 g = 1 mole 60 g = 1 mole Simple to convert between mass (easily measured) and moles

  48. Mass – Mole conversion easy • Based on Avagadro’s number = 6.022 x 1023 • 1 Mole is Avogadro’s number of stuff • Defined by number of atoms in 12 g of 12C

  49. Example • Nitrate a pollution of concern • Commonly measured as mass • Reported as mass of N in NO3 • N is the element of concern • If not specified, concentrations could be very different • Moles of NO3 and N are identical

  50. Alternative – Weight units • Mass per unit volume • For example: g/L or mg/L • If very dilute solution • Mass per unit volume about the same as mass per mass • 1L water ~ 1000 g, variable with T, P and X

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