Hydrochemical Data Presentation, Visualization, and Interpretation

Presentation, Visualization, and Interpretation of Hydrochemical Data

Hydrochemical data can be presented in several forms:

• Tables: Data expressed in mg/l, meq/l, or mMol/l.
• Graphs: Using various diagrams and plots.

Graphical Presentation/Visualization of Hydrochemical Data

• Piper Tri-linear Diagram:
  • Hydrochemical data in units of meq/l.
  • Percentage composition of major ions considered (Na, K, Ca, Mg, Cl, HCO3, CO3, SO4, NO3).
  • Groundwater types/facies and reservoir rock types can be interpreted.
  • Grouping of waters suggests common composition and origin.

Piper Diagram

The Piper diagram is a trilinear diagram used to represent the relative concentrations of major cations and anions in water samples. It consists of two triangles, one for cations and one for anions, which project onto a central diamond-shaped field. The position of a water sample on the diagram indicates its hydrochemical facies.

• Cations: Ca, Mg, Na + K
• Anions: CO3 + HCO3, SO4, Cl

Piper Diagrams Indicating the Dominant Hydrochemical Facies

• Calcium type
• Magnesium type
• Sodium type or potassium type
• No dominant type
• Sulfate type
• Chloride type
• Bicarbonate type

Water Types

Classification of Groundwaters after Furtak and Langguth (1967):

• Normal earth alkaline water
  • With prevailing bicarbonate
  • With prevailing bicarbonate and sulfate or chloride
  • With prevailing sulfate or chloride
• Earth alkaline water with increased portions of alkalis
  • With prevailing bicarbonate
  • With sulfate and chloride
• Alkaline water
  • With prevailing bicarbonate
  • With prevailing sulfate-chloride

Schoeller Semi-Log Diagram

• Data plotted in meq/l.
• Used for visual comparison of the composition of different waters.
• Parameters: Ca, Mg, SO4, HCO3

Stiff Diagram

• Helps rapid visual comparison between waters of different origin/sources.
• The larger the area of the polygonal shape, the larger the concentrations of the various ions.
• Data plotted in meq/l.

Other Diagrams

• Pie chart (plotted in meq/l): Visual comparison of hydrochemical data.
• Histogram
• Scatter Plot
• Ludwig Langelier Plot
• Radial Plots
• Durov Diagrams

Durov Diagram Fields (Lloyd and Heathcoat, 1985)

Durov diagrams are composite diagrams that combine two trilinear diagrams (Piper diagrams) with a central square field. They are useful for classifying water types and identifying hydrogeochemical processes.

• Ion exchange
• Reverse Ion exchange
• Simple dissolution or mixing

Interpretation of Hydrogeochemical Processes from Durov Diagram

1. HCO3 and Ca are dominant: Frequently indicates recharging water in limestone, sandstone, and other aquifers.
2. Water dominated by Ca and HCO3 ions with presumed association with dolomite if Mg is significant. However, an important ion exchange is presumed if the Na is significant
3. HCO3 and Na are dominant: Indicates an ion exchanged water, although the generation of $CO_2$ at depth can produce HCO3 where Na is dominant under certain circumstances.
4. $SO<em>4$ dominates, or anion discriminant and Ca dominant, Ca and $SO</em>4$ dominant, frequently indicates a recharge water in lava and gypsiferous deposits, otherwise a mixed water or water exhibiting simple dissolution might be indicated.
5. No dominate anion or cation: Indicates water exhibiting simple dissolution or mixing.
6. $SO_4$ is dominant or anion discriminant and Na dominant; which is a water type that is not frequently encountered and indicates probable mixing influence.
7. Cl and Na are dominant: Which is frequently encountered unless cement pollution is present otherwise the water may result from reverse ion exchange of Na-Cl waters.
8. Cl is a dominant anions and Na dominate the cations: Which indicates that the ground waters can be related to reverse ion exchange of Na-Cl waters.
9. Cl and Na dominant: Frequently indicate end-point waters.

Bivariate and Multivariate Variation Diagrams

• Variation diagram of various ion pairs.

Variation Diagram of Na versus Cl

Gibbs Plot

Gibbs plot of TDS versus cation and anion ratio to understand factors that control the observed hydrochemical composition

Multivariate Statistical Analysis: Principal Component Analysis

Principal component analysis component plots

Hydrochemical Facies and Hydrochemical Evolution of Groundwater

• As water flows through an aquifer it assumes a diagnostic chemical composition as a result of interaction with the lithologic framework.
• In hydrochemistry, the term hydrochemical facies is used to describe the bodies of groundwater in an aquifer that differ in their chemical composition.

Hydrochemical Facies are a Function of:

• Lithology, solution kinetics, and flow patterns of groundwater in the aquifer.
• Hydrochemical facies can be classified on the basis of the dominant ions in the facies by means of the trilinear diagram.

Vertical Hydrochemical Evolution and Sequences

1. Upper Zone
   • Active ground water flushing well-leached rocks.
   • Water low in TDS.
   • Dominant minerals dissolved: $CaCO<em>3, CaMg(CO</em>3)_2$
   • Dominant ion: $HCO_3^-$
   • Dominant cations: $Ca^{2+}, Mg^{2+}$
2. Intermediate Zone
   • Less active GW circulation with continuing dissolution.
   • Water higher in TDS.
   • Dominant minerals dissolved $CaSO<em>4 · 2H</em>2O, CaSO_4$
   • Dominant ion: $SO_4^{2-}$
   • Dominant cations: $Ca^{2+}, Na^+$
3. Lower Zone
   • Sluggish GW flow.
   • Minimal flushing, soluble minerals present.
   • Water high in TDS.
   • Dominant mineral dissolved: NaCl (halite)
   • Dominant ion: $Cl^-$
   • Dominant cation: $Na^+$

Regional Hydrochemical Evolution Sequence (Chebotarive Sequence)

$HCO<em>3^- → HCO</em>3^- + SO<em>4^{2-} → SO</em>4^{2-} + HCO<em>3^- → SO</em>4^{2-} + Cl^- → Cl^- + SO_4^{2-} → Cl^-$

• The longer the residence time, the higher the concentration of dissolved ions in groundwater.
• Groundwater tends to evolve chemically toward the composition of sea water during the course of flow.
• Note that minerals precipitate out of solution in the reverse order of their solubilities; the order of precipitation is:
  1. Calcite ($CaCO<em>3$) and dolomite ($CaMg(CO</em>3)_2$)
  2. Gypsum ($CaSO<em>4 · 2 H</em>2O$) and anhydrite ($CaSO_4$).
  3. Halite (i.e. common salt, NaCl)
  4. Potassium and magnesium salts