groundwater quality and its significance

Water quality

1. In specifying the quality characteristics of groundwater, chemical, physical, and biological analysis are normally required. A complete chemical analysis of a groundwater sample includes the determination of the concentrations of the inorganic constituent's present; organic and radiological parameters are normally of concern only where human-induced pollution affects quality. Dissolved salts in groundwater of normal salinity occur as dissociated ions; in addition, other minor constituents are present and reported in elemental form. The analysis also includes measurement of pH and specific electrical conductance. Depending on the purpose of a water quality investigation, partial analyses of only particular constituents will sometimes surface. Properties of groundwater evaluated in a physical analysis include temperature, color, turbidity, odor, and taste. Measures of Water Quality

2. Once a sample of groundwater has been analyzed in a laboratory, methods for reporting water analyses must be considered. From an understanding of expressions and units for describing water quality standards can be established so that analyses can be interpreted in terms of the ultimate purpose of the water supply. In a chemical analysis of groundwater, concentrations of different ions are expressed by weight or by chemical equivalence. Total dissolved solids can be measured in terms of electrical conductance. These and other measures of chemical quality are described in the following sections. Concentrations by Weight.  Measures of Water Quality Chemical Analysis

3. Concentrations of the common ions found in groundwater are reported by weight-per-volume units of milligrams per liter (mg/l). The total ionic concentration (or total dissolved solids) is also reported in this manner.  Positively charged cations and negative anions combine and dissociate in definite weight ratios. By expressing ion concentrations in equivalent weights, these ratios are readily determined because one equivalent weight of a cation will exactly combine with one equivalent weight of an anion. The combining weight of an ion is equal to its formula weight divided by its charge. When the concentration in milligrams per liter is divided by the combining weight, an equivalent concentration expressed in milliequivalents per liter ( meq /l) results.  Measures of Water Quality Concentrations by Weight

4. Positively charged cations and negative anions combine and dissociate in definite weight ratios. By expressing ion concentrations in equivalent weights, these ratios are readily determined because one equivalent weight of a cation will exactly combine with one equivalent weight of an anion. The combining weight of an ion is equal to its formula weight divided by its charge. When the concentration in milligrams per liter is divided by the combining weight, an equivalent concentration expressed in milliequivalents per liter ( meq /l) results. Measures of Water Quality Chemical Equivalence

5. Concentrations in milligrams per liter can be converted to milliequivalents per liter by multiplying by the appropriate conversion factor. For undissociated species with zero charge, of which silica is an example in groundwater quality, an equivalent weight cannot be computed.  In application, therefore. it may be expected that of the total dissolved solids in a groundwater sample. The sum of the cations and the sum of the anions when expressed in milliequivalents per liter will be equal. If the chemical analysis of the various ionic constituents indicates a difference from this balance, it may be concluded either that there are other undetermined constituents present or that errors exist in the analysis.   Measures of Water Quality Chemical Equivalence

6. A rapid determination of total dissolved solids can be made by measuring the electrical conductance of a groundwater sample. Conductance is preferred rather than its reciprocal, resistance, because it increases with salt content. Specific electrical conductance defines the conductance of a cubic centimeter of water at a standard temperature of25°C; an increase of 1°C increases conductance by about 2 percent.  Measures of Water Quality Total Dissolved Solids by Electrical Conductance

7. Specific conductance is measured in microsiemens /cm (mS/cm).” Because natural water contains a variety of ionic and undissociated species, conductance cannot be simply related to total dissolved solids. However, conductance is easily measured and gives results that are convenient as a general indication of dissolved solids. An approximate relation for most natural water in the range of 100 to 5000 mS/cm leads to the Equivalencies 1 meq /l of cations = 100 mS/cm and 1 mg/l = 1.56 mS/cm.  Measures of Water Quality Specific Conductance

8. Hardness results from the presence of divalent metallic cations, of which calcium and magnesium are the most abundant in groundwater. These ions react with soap to form precipitates 311d with certain anions present in the water to form scale. Because of their adverse action with soap, hard waters are unsatisfactory for household cleansing purposes. Hence, water-softening processes for removal of hardness are needed.  The hardness in water is derived from the solution of carbondioxide , released by bacterial action in the soil, in percolating rain. water. Low pH conditions develop and lead to the solution of in soluble carbonates in the soil and in limestone formations to convert them into soluble bicarbonates. Impurities in limestone, such as sulfates, chlorides, and silicates, become exposed to the solvent action of water as the carbonates are dissolved so that they also pass into solution. Thus, hard water tends to originate in areas where thick topsoil's overlie limestone formations.  Hardness H T is customarily expressed as the equivalent of calcium carbonate. where HT, Ca, and Mg are measured in milligrams per liter and the ratios in equivalent weights. Measures of Water Quality Hardness