Laserfiche WebLink
8.0 GROUND -WATER QUALITY -- Continued <br />8.2 pH and Specific Conductance <br />pH and Specific Conductance Vary Among and Within Aquifers <br />pH and specific conductance are easily measurable properties of ground water <br />that allow estimates of the acidity and dissolved - solids concentration. <br />The pH of water is a measure of acidity. A pH <br />of less than 7.0 is acidic; 7.0 is neutral; more than 7.0 <br />is alkaline. Information on the pH of ground water <br />is sparse. The pH of ground water in areas where it is <br />known or can be reasonably inferred from existing <br />data is shown in figure 8.2 -1. The pH of ground <br />water in wells and springs ranges from 6.6 to 9.3. <br />Marked variations in pH occur within distances of <br />less than a mile. Variations in pH cannot be correlat- <br />ed with ratios of sulfate to bicarbonate or chloride to <br />bicarbonate. <br />Ground water in Cretaceous to Tertiary transi- <br />tional and terrestrial formations is more alkaline <br />than in Cretaceous marine formations. The pH of <br />ground water in the coal - bearing Vermejo and Raton <br />Formations most commonly ranges from 7.5 to 8.0, <br />but it exceeds 8.0 in the vicinity of some active or <br />abandoned coal mines. However, it cannot be prov- <br />en that mining raises the pH of ground water since <br />the pH of mine discharge, ranging from 73 to 9.2, is <br />in the same range as that of ground water from <br />undisturbed bedrock. The pH of ground water in the <br />Cuchara and Poison Canyon Formations most com- <br />monly ranges from 7.5 to 8.0 north of the Spanish <br />Peaks and 6.5 to 7.5 south of the Spanish Peaks. The <br />pH of ground water in the Devils Hole and Huerfano <br />Formations, Farasita Conglomerate and Trinidad <br />Sandstone most commonly ranges from 7.0 to 8.0. <br />The pH of ground water in Cretaceous marine for- <br />mations most commonly ranges from 7.0 to 8.0 and <br />74 <br />tends to be slightly greater in shale than in limestone <br />or sandstone. <br />Specific conductance is an indicator of dis- <br />solved- solids concentration; like pH, it is an easily <br />measured property of ground water that can be used <br />to estimate water - quality characteristics which are <br />more difficult and expensive to ascertain. Areas <br />where the dissolved- solids concentration in ground <br />water is known or can be reasonably inferred from <br />analyses are shown in figure 8.2 -2, and graphs of <br />specific conductance versus dissolved - solids concen- <br />tration prepared from these analyses are shown in <br />figure 8.2 -3. These graphs can be used to estimate <br />the dissolved - solids concentrations in parts of Area <br />61 where chemical analyses are not available. From - <br />figure 8.2 -3, it can be seen that for any specific <br />conductance the dissolved - solids concentrations gen- <br />erally will be greatest in shale and limestone forma- <br />tions, less in shale and sandstone formations, and <br />least in sandstone formations and alluvium. <br />The similarity of specific conductance/ <br />dissolved - solids ratios in Purgatoire drainage alluvi- <br />um and sandstone formations suggests that much of <br />the water in the alluvium flowed in from sandstone. <br />In the area sampled, the Purgatoire drainage is <br />underlain by the Poison Canyon and Raton Forma- <br />tions, which contain sandstone and shale. Specific <br />conductance /dissolved - solids ratios indicate that wa- <br />ter from these formations is transmitted principally <br />by the sandstone layers. <br />