16 Dissolved Mass in Groundwater Hydrochemistry

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

16.1 Dissolved constituents in groundwater Rocks, minerals dissolve in water: ions Cations: positively charged ions (e.g., Ca 2 +, K + ) Anions: negatively charged ions(HCO 3 -, Cl - ) Organic compounds, dissolve, form non- charged molecules

16.1 Dissolved constituents in groundwater Examples of dissolution of minerals, liquids, Halite dissolution: NaCl = Na + + Cl - Calcite dissolution: CaCO 3 + H + = Ca HCO 3 - TCE dissolution: TCE = TCEaq

Concentration Scales 1. 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 Na 2 SO 4 has a molarity of (Na 2 SO 4 ) of: 1.42/(2X x 16.00) = M 2. Molal concentration Number of moles per kilogram of solvent (mol/Kg)

Concentration Scales 3. 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 Ca 2+ equals 2 eq/L 4. Mass per unit mass Mass of a species or element per total mass of the system (ppm, ppb, mg/kg, ug/kg) 5. 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

Concentration Scales Conversion between mg/L to molar concentration: molarity = mg/L x ___________________________ formula weight Conversion between mg/L to meq/L: Meq/L = mg/L ___________________________ formula weight/ charge Example 16.1: The concentration of SO4 2- in water is 85.0 mg/L. Express this concentration as molarity and meq/L: SOLUTION:  mol/L = 85x10 -3 / ( x 16.0) = 0.89 x  meq/L = 85 /( x 16.0)/2 = 1.77

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

Dissolved Constituents in Groundwater Classified by Relative Abundance (Table 16.1) Trace (< 0.1 mg/L) Minor mg/L Major (  5 mg/L) CopperPlatinumAluminumBoronBicarbonate GoldRadiumArsenicNitrateSilicon LeadRubidiumBariumCarbonateCalcium LithiumSilverBeryllium Potassium Sodium ManganeseCobaltBromideFluorideChloride NickelSeleniumCadmiumStrontiumSulfate PhosphateThoriumCeriumIronMagnesium PlatinumZincChromiumCarbonic Acid UraniumVanadiumTungsten

Dissolved Constituents in Groundwater Classified by Relative Abundance (Table 16.1) Organic Compounds (Deep) Organic Compounds (shallow) Acetate Humic Acid Propionate Fulvic acid Carbohydrates Amino acids Tannis Lignins Hydrocarbons

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

Example of a routine water analysis (Wadi Al-Arj, Taif, Summer 2002) Mg/LParameterMg/LParameter ND Total Iron 7.3 pH (25 C) 274 Bicarbonate 1610 TDS 340 Chloride 2300 Conductivity 430 Sulfate 265 Sodium 46 Nitrate 12 Potassium 12 Phosphate 168 Calcium 134 Carbonate 38 Magnesium ND Ammonia ND Mercury 1 Arsenic 1000 Total Bacteria

Specialized Analyses Trace metals ( Mn, Cr, Cd, Pb, Zn) Radioisotopes Organic compounds Nitrogen-containing species (NO 3-, NH 4+ ) Environmental isotopes Gases Specialized analyses done for: groundwater contamination problems Water-quality assessment Research Regulatory issues

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

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

16.5 working with chemical data Presenting results of chemical analyses: Abundance or relative abundance 1. Collins Bar diagram 2. Stiff pattern diagram 3. Pie diagram 4. Piper diagram Abundance and patterns of change Graphical/illustrative type diagrams Statistics

Wadi Al-Arj Samples Runoff samples Well samples Average TDS Conductivity (μS/cm pH T (oC) Sodium Potassium Calcium Magnesium Sulfate

Chloride Nitrate Bicarbonate Pb Iron Zn ND12 Mercury (ppb) 0.01 Co (ppb) As (ppb) 4633 Coliform Bacteria (per 100 ml) 52.8 BOD

Piper diagram

Procedure for plotting piper (trilinear) 1. Convert concentrations from mg/L (ppm) to equivalents 2. Normalize, cations an anions each separately add to 100 (combining Na + K) 3. Plot proportions on triangles 4. 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 SO 4 axis 5. Intersection of the two lines is the location of point on diamond plot

Piper diagram, EXAMPLE 2.8 meq/lHCO340Ca 234SO415Mg 45Cl120Na 20K 2.8HCO31.996Ca 4.87SO41.234Mg 1.27Cl5.22Na 0.511K equivalents HCO3Ca SO4Mg ClNa + K proportions 100

Runoff Groundwater

Madinah Basaltic Aquifer

Pie chart

Stiff diagram

Stiff diagram example

Bar diagram (frequency) Na +k Mg HCO SO 4 Cl Ca cation anion Meq/l