Hydrogeochemistry “Geochemistry of Natural Waters” No wastewater, water resources Class not related directly to social aspects of water Study natural controls of chemistry of rivers, lakes, ground water, oceans etc.

Questions considered: Why do different waters have different chemical compositions? What controls the compositions? This will lead to discussion of chemical reactions between water, rocks, and atmosphere How do compositions vary with setting? How do they vary with time?

Why consider water compositions? Can provide information on basic geological processes: Diagenesis ≡ All chemical (and physical) alteration of solid material (low T and P) Weathering ≡ alteration of mineral phases at earth surface conditions Biological activities in certain environments Exchange with atmosphere (C cycle – global climate change)

Reactants include Water and… Inorganic solids: rocks, sediments, minerals Biota: plants, animals, bacteria, archea Gas phases: Earth’s atmosphere, biologically produced gas (CO2, H2S)

Concentration terminology We will need a way to discuss what is in water Dissolved components (ions, gasses, complexes etc.) Solid components The following is a lot of definitions, terminology, and algorithms to calculate solutes

Dissolved concentrations Fresh water Potable, generally < 1000 mg/L solids per liter of water (TDS) Brackish Non-potable, but < seawater Seawater, salinity 34 to 37 (defined soon) 97% of free water on earth Concentration important threshold for Thermo Saline water/brine > seawater salinity

Total dissolve solids Mass of solid remaining after evaporation of water Note – in GWB, often just “dissolved solids” Bicarbonate converted to carbonate Units of mass/volume (e.g. g/L, kg/L, etc.) Commonly used in fresh water systems

Chlorinity (Cl) Definition Determined by titration with AgNO3 Mass of Cl in one kg of seawater equivalent to Cl, Br, and I in seawater Determined by titration with AgNO3 Precipitate Ag(Cl,Br,I) with indicator AgNO3 calibrated with seawater with known chlorinity (19.374 g/kg) Commonly used in seawater systems

Salinity Operationally – all salts in seawater Originally (Knudsen, 1901) defined as With Cl = 19.374 g/kg measured with AgNO3 Intercept because not all salts measured Precision to 3 decimal places S(‰)=0.03 + 1.805 Cl(‰) From Millero, 2011, Marine Chemistry

Salinity In 1960’s, electrical conductivity of seawater of known Cl More precise, easier to measure Conversion adopted More precise, dropped intercept value Referred to now as “Reference Salinity” SR Units of g/kg or ‰ S(‰)=1.80655 Cl(‰)

Practical Salinity Scale Based on conductivity measurements to determine Reference Salinity (SR) Practical Salinity (SP) is defined as Since ratio, it is unitless Originally had units of PSU (Practical Salinity Units) SP= 35.000 35.165 ∗( SR g/kg )

Absolute Salinity Scale (SA) Defined by SCOR (Scientific Committee on Oceanic Research) IAPSO (International Association of Physical Sciences of the Ocean) Endorsed by IOC (Intergovernmental oceanographic Commission)

Defined as: where dS represents addition in deep ocean water from dissolution of minerals (e.g., calcite, diatoms etc) and organic carbon (e.g., non-conservative solutes) SA g/kg =SR+δS

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

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

Reporting units Need to report how much dissolved material (solute) in water, two ways: Mass Moles (& equivalents) Need to report how much water (solvent) Volume of water, typically solution amount – analytically easy Mass of water, typically solvent amount – analytically difficult

Weight units Mass per unit volume If very dilute solution 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, BUT variable with T, P and X as density changes

1 mole sodium sulfate makes 2 moles Na+ and 1 mole SO42- 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 SO42-

Molal Units Number of molecules (atoms, ions etc) in one kg of solvent Abbreviation: m or mm or mm (little m) Difficult to determine mass of solvent in natural waters with dissolved components not used so often in natural waters Useful for physical chemistry because doesn’t depend on T, P or X (composition of dissolved constituents).

Simple to convert between 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

Example Nitrate a pollution of concern Commonly measured as mass Reported as mass of N in NO3 E.g., g N in NO3 NO3 is measured Weight concentration of NO3 is 4 X bigger than weight concentration of N Molar units of NO3 and N are identical

Mass – Mole conversion Conversion from weight units to molar units Divide by gram formula weight Molar units to weight units Multiply by gram formula weight

Alternative – charge units Equivalents – molar number of charges per volume eq/L or meq/L Used to plot piper diagrams Used to calculate electrical neutrality of solutions

Na2SO4 = 2Na+ + SO42- Calculation: 1 mole of Na = 1 eq Na solution Moles (or millimoles) of ion times its charge Na2SO4 = 2Na+ + SO42- 1 mole of Na = 1 eq Na solution 1 mole of SO4 = 2 eq SO4 solution Although different number of moles, solution is still electrically neutral

Example

Charge Balance SmcZc - SmaZa CBE = SmcZc + SmaZa Electrical neutrality provides good check on analytical error Charge Balance Error – CBE Note: - = excess anions; + = excess cations SmcZc - SmaZa CBE = SmcZc + SmaZa Where: m = molar concentration of major solutes z =charge of cation (c) or anion (a)

Possible causes of errors Significant component not measured Commonly alkalinity – can be estimated by charge balance Analytical error +5% difference OK – acceptable + 3% good 0% probably impossible

Graphical data presentation Cross plots (XY plots) Time series Ternary diagrams Stiff diagram – geographic distribution Piper diagram – comparison of large numbers of samples Many others, not used widely

Stiff Diagrams Cations on left Anions on right Top K+Na & Cl = seawater Middle Ca & HCO3 (alkalinity) most fresh water Carbonate mineral reactions Bottom Mg & SO4 – other major component 4th optional – redox couples Drever, 1997 Depth (ft) Zaporozec, 1972, GW

Piper Diagrams Two ternary diagrams Projected on quadralinear diagram Very useful figure for comparing concentrations of numerous water samples

Construction Convert concentrations to meq/L Use major cations and anions concentrations Cations = Ca, Mg, Na + K Anions = SO4, Cl, HCO3 + CO3 (or alkalinity)

Calculate %’s of each element on ternary diagram For example Ca is: Plot %’s on ternary diagrams Project each % onto diamond diagrams [Ca] *100 [Ca] + [Mg] + [Na + K]

Composition: Ca = 22.3% Mg = 13.7% Na+K = 64% Alkalinity = 31.3% Sulfate = 54.5% Chloride = 14.2%

Santa Fe water chemistry Mixing between three sources of water: 1 = shallow groundwater 2 = deep groundwater 3 = dilute Surface water Moore et al., 2009, J Hydro

Fortunately – computer programs available to make plots for you Stiff diagrams: http://www.twdb.state.tx.us/publications/reports/GroundWaterReports/Open-File/Open-File_01-001.htm Piper plots: http//water.usgs.gov/nrp/gwsoftware/GW_Chart/GW_Chart.html Geochemist workbench http://student.gwb.com/ Download GWB