Groundwater Geochemistry (Hydrogeochemistry) Wan Zuhairi Wan Yaacob (PhD, Assoc. Prof) Program Geologi, UKM

Transcription

1 Groundwater Geochemistry (Hydrogeochemistry) Wan Zuhairi Wan Yaacob (PhD, Assoc. Prof) Program Geologi, UKM

2 Geochemistry Water / rock interactions in unsaturated/saturated zones Geochemistry is important to groundwater studies:- Characterizing the natural system (or groundwater composition) Understanding contaminant migration Designing remediation programs

3 Simple geochemical model Aqueous geochemistry Water/rock interactions To control the groundwater composition and the movement of dissolved constituents Deutch WJ Groundwater Chemistry. CRC Press

4 Fresh water without any dissolved constituents (disequilibrium) Reactions that dissolve gases and minerals and changes the solution composition Dynamic geochemical system consisting (i) Solid phase (ii) Gas phase (iii) Aqueous solution phase disequilibrium

5 Groundwater solution Definitions and concentration units

6 Concentration of solute in solution Milligram per liter (mg/l) Part per million (ppm) mg/l ppm mg/l ppm mg/l = solution density (g/cc) TDS (g/cc) ppm Solution density = g/cc TDS = 10,000 mg/l (0.01 g/cc) Then, 1 ppm = mg/l (0.2 % difference!)

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8 Groundwater solutes Major Ions (concentration > 1 mg/l) Minor ions (concentration < 1 mg/l)

9 Converting measured concentration (mg/l or ppm) to electrical equivalent unit (meq) = 4.6 meq / L (1000 miliequivalents = 1 equivalent)

10 Conversion to meq Electrical balance Electrical balance: Electrical balance for Table 1-2 = +0.5% + (excess cations; insufficient anions) - (excess anions; insufficient cations) Reasonable balance for routine analysis < 5%

11 Several possible reasons that create an electrical imbalance in the data 1. The design of the sampling program neglected a major dissolved species 2. Laboratory error 3. Using unfiltered water samples 4. Precipitation of a mineral in the sample 5. In certain cases, the dissolved species may not correspond to the typical species used in the making the ion balance calculation

12 Groundwater types Classify groundwater based on dominant cations and anions Ca-HCO 3 type (dominant with Ca and HCO 3 ) Displayed graphically by several methods Bar graph Circular diagram Stiff diagram Trilinear or Piper diagram Durov diagrams

13 Bar Diagram Circular Diagram

14 Stiff Diagram Piper Diagram Durov Diagram

15 STIFF DIAGRAM 1. Ion concentrations in meq L -1 are plotted on the horizontal axis. 2. Cations are plotted to the left, anions to the right, of a vertical axis. 3. The data are plotted in four rows and the points are connected to form a polygon. 4. Advantage: each water type produces a distinct shape. 5. Disadvantage: each analysis requires its own plot; only a limited number of data can be shown on a single plot.

16 n e Cre e k, CDA V a l l e y, Id a h o M ine Wat er s Cat io ns m e q / l Anio n s Ca N a + K Cl AD0 0 2 H C O 3 + C O 3 An example of a Stiff diagram drawn for mine waters from the Pine Creek district, Coeur d Alene Valley, ID. The anions are mostly dominated by sulfate, with lesser bicarbonate, whereas the cations are dominated by calcium and magnesium. M g N a + K Cl S O 4 Ca AD0 0 4 H C O 3 + C O 3 M g N a + K Cl S O 4 Ca AD0 0 5 H C O 3 + C O 3 M g N a + K Cl S O 4 Ca AD0 0 7 H C O 3 + C O 3 M g N a + K Cl S O 4 Ca S97-3 H C O 3 + C O 3 M g N a + K Cl S O 4 Ca SP0 0 2 H C O 3 + C O 3 M g N a + K Cl S O 4 Ca SPNEWH C O 3 + C O 3 M g S O 4

17 Stiff pattern Stiff pattern are centered over locations of wells Isocon of TDS

18 PIPER DIAGRAMS 1. Consists of two triangles (one for cations and one for anions), and a central diamond-shaped figure. 2. Cations are plotted on the Ca-Mg-(Na + K) triangle as percentages. 3. Anions are plotted on the HCO 3- -SO Cl - triangle as percentages. 4. Concentrations are in meq L Points on the anion and cation diagrams are projected upward to where they intersect on the diamond. 6. Many water analyses can be plotted on the same diagram and can be used to classify waters.

19 Percentage of cations and anions as percentage of the total (Step 2 and 3) Cations meq/l % of total Anions meq/l % of total Ca Cl Mg SO Na + + K CO HCO - 3 Total 3.18 Total

20 Figure 1-6 from Kehew (2001). Water analyses plotted on a Piper diagram. Cation percentages in meq L -1 plotted on the left triangle, and anion percentages in meq L -1 plotted on the right triangle. Ca = 22.3 % Mg = 13.7 % Na+K = 64 % HCO 3 = 31.3 % SO 4 = 54.5 % Cl = 14.2 %

21 Figure 1-7 from Kehew (2001). Classification of hydrochemical facies using the Piper plot.

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23 M a g n e s i u m ( M g ) Pin e Cr e e k, CDA Va le y, I d a h o M in e W a t e r s S u l f a t e ( S O 4 ) + C h l o r i d e ( C l ) C a l c i u m ( C a ) + M a g n e s i u m ( M g ) M g SO S o d i u m ( N a ) + P o t a s s i u m ( K ) C a r b o n a t e ( C O 3 ) + B i c a r b o n a t e ( H C O 3 ) S u l f a t e ( S O 4 ) AD002 AD004 AD005 AD007 S97-3 SP002 SPNEW An example of a Piper diagram drawn for mine waters from the Pine Creek district, Coeur d Alene Valley, ID. These may be characterized as Ca-Mg sulfatebicarbonate-type waters. Ca N a + KH C O 3 + C3O Cl C a lc iu m ( C a ) C h lo r id e ( C l) % m e q / l /12/2010 C A T I O N S drwzwy A N I O N S

24 0 1, , , , , T o t a l D is s o lv e d S o lid s ( P a r t s P e r M ilio n ) Ca M a g n e s i u m ( M g ) Ke he w ( ) S u l f a t e ( S O 4 ) + C h l o r i d e ( C l ) C a l c i u m ( C a ) + M a g n e s i u m ( M g ) M g SO S o d i u m ( N a ) + P o t a s s i u m ( K ) C a r b o n a t e ( C O 3 ) + B i c a r b o n a t e ( H C O 3 ) N a + KH C O 3 + C3O Cl C a lc iu m ( C a ) C h lo r id e ( C l) % m e q / l 2/12/2010 C A T I O N S A drwzwy N I O N S 5 S u l f a t e ( S O 4 ) An example of a Piper diagram with TDS circles. Plot the radius of TDS using suitable scale (5000 ppm) TDS represents overall salt content of the water

25 karst rivers systems Can you guess the type of aquifer of this groundwater?

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27 Thank you