Unit 08b : Advanced Hydrogeology Groundwater Chemistry

Groundwater Analysis It is not feasible (or sensible) to attempt to measure all possible constituents present in groundwaters. A “routine” analysis involves measuring a standard set of constituents. The “routine” analysis forms the basis for assessing the suitability of the water for human consumption or various agricultural and industrial uses.

Major Constituents (> 5 mg/L) Calcium Magnesium Sodium Bicarbonate Chloride Sulphate pH Alkalinity TDS Conductivity Total Hardness Silicon Carbonic Acid

Other Reported Results pH is routinely recorded that measures the hydrogen ion concentration (-log[H + ]). Conductivity (electrical) is routinely measured in  S/m (TDS  0.66 E c ). TDS is the total dissolved solids measured as the residue on evaporation. Total alkalinity (expressed as mg/L CaCO 3 ) measures the acid-neutralizing capacity of a water sample. It is an aggregate property, primarily due to the carbonate, bicarbonate, and hydroxide content. Total hardness (expressed as mg/L CaCO 3 ) is another aggregate property of a water sample. Hardness measures the concentration of multivalent cations, particularly calcium and magnesium.

Minor Constituents (0.01 to 10 mg/L) Potassium Iron Ammonium Carbonate Fluoride Bromide Nitrate/Nitrite Carbon Dioxide Oxygen Boron Strontium

Trace Constituents ( < 100  g/L) Li3 Be4 Al13 Sc21 Ti22 Cr24 Mn25 Co27 Ni28 Cu29 Ru44 Ag47 Cd48 In49 Sn50 Sb51 Cs55 Ba56 La57 Ce58 Zn30 Ga31 Ge32 As33 Se34 Rb37 Yt39 Zr40 Nb41 Mo42 W74 Pt78 Au79 Tl81 Pb82 Bi83 Ra88 Th90 U92 Iodide Phosphate

Organic Constituents Humic acid Fulvic Acid Carbohydrates Amino Acids Tannins Lignins Hydrocarbons Acetate Propionate Typically concentrations are very low

Routine Analysis A “routine” analysis usually includes all the major constituents (except carbonic acid). All the minor constituents (except B and Sr that tend to be regarded as trace consituents) are also to be expected.

Example Routine Analysis mg/L Calcium<1 Magnesium<1 Sodium550 Potassium3.5 Iron8.7 Total Hardness8 _______________________ pH7.7 Conductivity23  S/m mg/L Sulphate59 Bicarbonate1315 Chloride45 Fluoride0.25 Nitrite/Nitrate<0.1 Total Alkalinity1078 TDS1321 Balance*1.01 *The balance is the ratio of anions to cations in meq/L

Charge Balance mg/L meq/L Calcium <1 <0.05 Magnesium <1 <0.08 Sodium Potassium Iron Total 24.4 Molarity M mg/L meq/L Sulphate Bicarbonate Chloride Fluoride Nitrite/Nitrate 0.01 Total 24.1 Cations/Anions 1.01

Errors in Analysis Checking the ion balance is good practice to eliminate gross errors Errors mainly arise as a result of: –failure to measure rapidly changing parameters in the field (pH, alkalinity, etc) –poor or no filtration (removal of suspended solids) –improper sample storage (pressure, temperature, buffering, sealing)

Specialized Analysis Additional specialized measurements are sometimes required for specific projects including: –trace metals –nutrients (mainly N and P species) –organics (mainly hydrocarbons, chlorinated hc’s) –stable isotopes ( 14 C, 18 O, 2 H etc) –radionuclides ( 226 Ra, 207 Pb, 208 Po, 231 Th etc) Such analyses (listed above in order of increasing cost) can be very expensive and QA/QC is a significant concern.

Visualizing Chemical Data There are a large number of plots used to visualize ion abundances: –Pie Chart –Collins Diagram (Bar Chart) –Stiff Diagram Other plots are use to group waters and interpret their origins: –Piper Diagram –Fence Diagram

Pie Charts mg/L

Collins Diagram Sample 1 Sample 2 Sample 3 Concentration (meq/L) Na + +K + Mg 2+ Ca 2+ Cl - SO 4 2- HCO 3 - +CO 3 2-

Stiff Diagram Na + +K + Ca 2+ Mg 2+ Cl - HCO 3 2- SO 4 2- meq/L Cations Anions

Piper Diagram CationsAnions Groundwater Facies Ca Mg Na + KHCO 3 + CO 3 Cl SO 4 Ca + Mg SO 4 + Cl HCO 3 + CO 3 Na + K

Plotting on a Piper Diagram Ca Mg Na + KHCO 3 + CO 3 Cl SO 4 Ca + Mg SO 4 + Cl HCO 3 + CO 3 Na + K

Groundwater Facies Calcium-Magnesium Sodium-Potassium Chloride-Sulphate Bicarbonate Bicarbonate-Chloride-Sulphate Chloride-Sulpahte-Bicarbonate Calcium-Sodium Sodium-Calcium CationsAnions

Classification Ca Mg Na + KHCO 3 + CO 3 Cl SO 4 Ca + Mg SO 4 + Cl HCO 3 + CO 3 Na + K Grouping of waters on the Piper Diagram suggests a common composition and origin. Red: Ca-Mg-SO 4 Yellow: Ca-Mg-Na-Cl-SO 4

Facies Mapping Spatial mapping groundwater facies based on classification on the Piper Diagram can help in the visualization of progressive changes in chemistry. A fence diagram is a convenient method of viewing a 3D region as a series of intersecting cross-sections.

Fence Diagram Fence diagrams can be used to show groundwater facies in the same way that lithofacies are displayed. calcium-magnesium calcium-sodium sodium-calcium sodium