Introduction to Groundwater Chemistry

1. Introduction to Groundwater Chemistry October 04, 2010

2. Units of Measurements • Common units are (1 mg/L = 1 ppm = 1000 ppb). • ppm = 1 part in 1,000,00 (106) parts by mass or volume • 2. Molar concentration (molarity) = moles of solute per liter of solution. • molarity = moles of solute/ liter of solution • 3. Molality = moles of solute per kilogram of solvent (not solution). • molality (M) = moles solute/kg of solvent • molality = (mg/l) *0.001/formula weight in grams • 4. Equivalent weight of a substance is the amount of that substance which supplies or consumes one mol of reactive species. An element's equivalent weight is its atomic weight divided by its valence. • meq/l = (mg/l)/valence of ion

3. Units of Measurements In order to convert the mass concentration to an equivalent concentration the following mathematical relationship is used: (mass concentration) * (ionic charge) / (molecular weight) = (equivalent concentration) For example, a water with a calcium concentration of 120 mg/L would have the following calcium equivalent concentration: (120 mg/L) * (2 meq/mmol) / (40 mg/mmol) = (5 meq/L)

4. Types of chemical reactions in water 1. Reversible reaction can reach equilibrium with their hydrochemical environment. The simplest aqueous reaction is the dissociation of an inorganic salt. If the salt is present in excess, it will tend to form a saturated solution: NaCl == Na+ + Cl- If the solution is undersaturated more salt will dissolve. If it is supersaturated, salt will crystallize. 2. Irreversible reaction: one way reaction, A+B ---- C+D

5. Law of mass action Law of mass action: the reaction will strive to reach equilibrium. cC + dD == xX + yY Equilibrium constant (K) = [X]x [Y]y / [C]c [D]d

6. Major ion chemistry More than 90% of the dissolved solids in groundwater can be attributed to: Na, Ca, K, Mg, SO4, Cl, HCO3, and CO3 These ions are usually present at concentration greater than 1mg/l. Silica, SiO2, a nonionic species, is also present at concentrations greater than 1mg/l. Direct analysis can be done for the first six ions. Bicarbonate and carbonate concentrations are found by titration with acid to an endpoint with a pH of about 4.4. pH, Temperature, and specific electrical conductance are usually made at the time the sample is collected. Other naturally occurring ions that may be present in amount of 0.1mg/l to 10 mg/l include iron, fluoride, strontium, and boron.

7. Major ion chemistry Iron and nitrate are typically included in water-chemistry studies, with fluoride, strontium, and boron being less commonly reported. Total dissolved solids (TDS) can be determined by evaporating a known volume of the sample and weighing the residue. TDS can be estimated by summing the concentrations of the individual ions.

8. Ion Exchange Sites Location selection Under certain conditions, the ions attracted to a solid surface may be exchanged for other ions in aqueous solution. The ion-exchange process can be conceptualized as the preferential absorption of selective ions with contaminant loss of other ions. Ion-exchange sites are found primarily on clays and soil organic materials, although all soils and sediments have some ion-exchange capacity. A general ordering of cation exchangeability for common ions in groundwater is: Na > K > Mg > Ca Ex. Sodium adsorption ratio (SAR): SAR = Na / [ (Ca+Mg)/2 ]0.5 If SAR between 2 and 10, little danger from sodium If SAR between 7 and 18, medium hazards If SAR between 11 and 26, high hazards

9. Presentation of results of chemical analysis A cation-anion balance is usually performed as a check on the chemical analysis. This is accomplished by converting all the ionic concentrations to units of equivalents per liter. The anions and cations are summed separately, and the results are compared.

10. Charge Balance • For any solution, the total charge of positively charged ions will equal the total charge of negatively charged ions. Net charge for any solution must = 0 • Charge Balance Error (CBE) • Tells you how far off the analyses are (greater than 5% is not good, greater than 10% is terrible…)

11. Stiff Diagram Characterizes Water Chemistry Analyses in meq/l are plotted on four parallel horizontal lines. Concentrations of up to four cations and anions can be plotted, one each to the left or right of the center zero axis. Resulting points are connected to give an irregular polygon pattern. Stiff patterns can be a relatively distinctive method of showing water-composition similarities and differences.

12. Schoeller Diagram Line Plot That Characterizes Water Semi-logarithmic diagram that represents major ion analyses in meq/l. Demonstrates different hydrogeochemical water types on the same diagram. Number of analyses plotted at one time is limited. Actual parameter concentrations are displayed.

13. Schoeller Diagram Average concentration of major anions and cations of groundwater, surface-runoff, and rainfall

14. Piper Diagram Shows Groupings of Water Types Major ions are plotted as cation and anion percentages in meq/l in two base triangles. Total ions are set to equal 100%. Data points in the two triangles are projected to central diamond. Allows comparison of a large number of samples. Shows clustering of samples and water type.

16. ASARCO Data