16 Dissolved Mass in Groundwater

1. 16Dissolved Mass in Groundwater Hydrochemistry

2. Introduction • Water Chemistry: • Origin of water • Uses of water • Water quality (contamination) • Topics: • 16.1 Dissolved constituents in groundwater • 16.2 Types of water analyses • 16.3 Water-quality standards • 16.4 Examples of data collected in chemical surveys • 16.5 Working with chemical data • Case studies : • Madinah Basalt aquifer, Taif Alluvium Aquifer

3. 16.1 Dissolved constituents in groundwater • Rocks, minerals dissolve in water: ions • Cations: positively charged ions (e.g., Ca2+, K+) • Anions: negatively charged ions (HCO3-, Cl-) • Organic compounds, dissolve, form non-charged molecules

4. 16.1 Dissolved constituents in groundwater Examples of dissolution of minerals, liquids, Halite dissolution: NaCl = Na+ + Cl- Calcite dissolution: CaCO3 + H+ = Ca2+ +HCO3- TCE dissolution: TCE = TCEaq

5. Concentration Scales • Molar concentration: Number of moles of a species per liter of solution (mol/L) • mole: formula weight of a substance in grams Example: one-liter solution containing 1.42 g of Na2SO4 has a molarity of (Na2SO4) of: 1.42/(2X22.99 + 32.06 + 4 x 16.00) = 0.010 M • Molal concentration Number of moles per kilogram of solvent (mol/Kg)

6. Concentration Scales • Equivalent charge: number of equivalent charges of an ion per lietr of solution (units: eq/L, meq/L) Equivalent charge = number of moles of an ion multiplied by the absolute value of the charge: Example: 1 M Na+ equals 1 eq/L 1 M Ca2+ equals 2 eq/L • Mass per unit mass Mass of a species or element per total mass of the system (ppm, ppb, mg/kg, ug/kg) • Mass per unit volume (most common): Mass of a solute dissolved in a unit volume of solution units: mg/L, g/L) 1 ppm = 1 mg/kg = 1 mg/L

7. Concentration Scales • Conversion between mg/L to molar concentration: molarity = mg/L x 10-3 ___________________________ formula weight • Conversion between mg/L to meq/L: Meq/L = mg/L ___________________________ formula weight/ charge Example 16.1: The concentration of SO42- in water is 85.0 mg/L. Express this concentration as molarity and meq/L: SOLUTION: • mol/L = 85x10-3 / (32.06 + 4 x 16.0) = 0.89 x 10-3 • meq/L = 85 /(32.06 + 4 x 16.0)/2 = 1.77

8. 16.2 Types of Water Analyses • Minerals • Organic solids • Organic liquids • Gases • Oxygen • carbon dioxide • hydrogen sulfide, • methane Dissolve in groundwater

9. Dissolved Constituents in Groundwater Classified by Relative Abundance (Table 16.1)

10. Dissolved Constituents in Groundwater Classified by Relative Abundance (Table 16.1)

11. Routine Water Analyses Routine: measuring concentration of standard set of most abundant constituents : • Major constituents (except Silicon, Carbonic Acid) • Minor constituents (except Boron, strontium) • pH, TDS (mg/L), Conductance (microS/cm, micromhos/cm) TDS: total quantity of solids when a water sample is evaporated to dryness SC: measure of sample’s ability to conduct electricity Routine analyses: defines almost all dissolved mass, except when water is highly contaminated

12. Example of a routine water analysis(Wadi Al-Arj, Taif, Summer 2002)

13. Specialized Analyses • Trace metals ( Mn, Cr, Cd, Pb, Zn) • Radioisotopes • Organic compounds • Nitrogen-containing species (NO3-, NH4+) • Environmental isotopes • Gases Specialized analyses done for: • groundwater contamination problems • Water-quality assessment • Research • Regulatory issues

14. 16.3 Water Quality Standards Designed to protect public health by requiring that contaminants or naturally occurring constituents in water be less than certain limits • Microorganisms • Disinfection and disinfection byproducts • Inorganic chemicals • Organic chemicals • radionuclides

15. 16.3 water Quality Standards • Primary drinking water standards: • Secondary drinking water standards: • MCL: maximum contaminant level: highest level of a contaminant that is allowed in drinking water (enforceable standards) • MCLG: maximum contaminant level goal: the level of a contaminant in drinking water below which there is no known or expected health risk (targets) Non-enforceable standard EPA Web page

16. 16.5 working with chemical data • Presenting results of chemical analyses: • Abundance or relative abundance • Collins Bar diagram • Stiff pattern diagram • Pie diagram • Piper diagram • Abundance and patterns of change • Graphical/illustrative type diagrams • Statistics

20. Piper diagram

22. Procedure for plotting piper (trilinear) • Convert concentrations from mg/L (ppm) to equivalents • Normalize, cations an anions each separately add to 100 (combining Na + K) • Plot proportions on triangles • Transfer data from triangles to quadrilateral by drawing straight lines • In cation triangle, line is parallel to Mg axis • In anion triangle, line is parallel to SO4 axis • Intersection of the two lines is the location of point on diamond plot

23. Piper diagram, EXAMPLE equivalents proportions 100 100

25. Runoff Groundwater

26. Madinah Basaltic Aquifer

29. Pie chart

30. Stiff diagram

32. Stiff diagram

33. Stiff diagramexample

34. anion cation 3 Na+k Cl 2 Meq/l SO4 Mg 1.5 HCO3 1 Ca 0 Bar diagram(frequency)