# Introduction to Groundwater Chemistry October 04, 2010.

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Introduction to Groundwater Chemistry October 04, 2010

Units of Measurements 1.Common units are (1 mg/L = 1 ppm = 1000 ppb). ppm = 1 part in 1,000,00 (10 6 ) 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

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)

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

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

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.

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.

Sites Location selection Ion Exchange 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

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.

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…) Charge Balance

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.

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.

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

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.

Station Name Bicarbonate Alkalinity As CACO3 Calcium (Ca) TOTAL Carbonate Alkalinity As CACO3 UNKNOWNChloride (Cl)Magnesium (Mg)Potassium (K)Sodium (Na)Sulfate (SO4) EP-114 400 603< 1 214 222 164 471 3160 EP-116 356 284< 1 390 55.5 45.5 766 2000 EP-119 384 157< 1 400 64 52.5 699 1310 EP-12 876 215< 1 203 67.8 12.1 891 1250 EP-122 304 182< 1 197 78.9 49.9 560 1340 EP-13 348 423< 1 946 67.2 87.4 2560 4610 EP-131 520 100< 1 212 36.2 14.2 650 1210 EP-132 294 221< 1 413 44.6 45.3 752 1540 EP-133 416 234< 1 324 66 49.4 1170 1990 EP-135 500 609< 1 991 176 16.4 1340 3060 EP-49 502 556< 1 288 130 242 1400 4320 EP-51 206 933< 1 4830 772 55 1910 2270 EP-58 860 533< 1 829 225 226 1890 4480 EP-62 362 165< 1 415 68 59.7 829 1290 EP-68 460 292< 1 575 119 14.6 798 1740 EP-75 464 467< 1 299 289 662 3020 9240 EP-77 503 65.3< 1 120 18.3 16.2 604 906 EP-81 146 89.4< 1 33.8 34.6 10.3 105 382 EP-85 316 121< 1 156 52.4 49.7 507 1000 EP-85 310 121< 1 155 52.3 45.9 514 964 SEP-1 260 126< 1 378 33.7 14.6 468 627 SEP-1 250 125< 1 361 33.8 14.6 467 568 SEP-10 320 194< 1 994 101 20.4 1440 1770 SEP-11 340 166< 1 659 72.8 30.6 874 1360 SEP-12 306 143< 1 663 61.7 27.6 880 1280 SEP-13 322 142< 1 651 61.9 24.9 876 1360 SEP-2 338 163< 1 677 71.8 29.2 863 1490 SEP-3 218 105< 1 369 25.5 13 432 500 SEP-4 290 141< 1 638 61.4 22.6 845 1250 SEP-6 222 108< 1 370 26.3 13.2 446 487 SEP-7 216 106< 1 362 24.9 13.1 435 504 SEP-9 194 82.9< 1 475 18 14.5 435 424 ASARCO Data