THE USE OF MEASURED AND CALCULATED ACIDITY VALUES TO IMPROVE THE QUALITY OF MINE DRAINAGE DATASETS

摘要

The net acidity of a water sample can be measured directly by titration with a standardized base solution or calculated from the measured concentrations of the acidic and basic components. For coal mine drainage, the acidic components are primarily accounted for by free protons and dissolved ferrous iron, ferric iron, aluminum, and manganese. The base component is primarily accounted for by bicarbonate. A standard calculation is: Acid~(calc) =50*(2*Fe~(2+)/56 + 3*Fe~(3+)/56 + 3*Al/27 + 2*Mn/55 + 1000*10~(-pH)) -Alkalinity, where acidity and alkalinity are measured as mg/L CaCO_3 and the metals are mg/L. Because these methods of estimating acidity are derived by independent laboratory procedures, their comparison can provide a valuable QA/QC for AMD datasets. The relationship between measured and calculated acidities was evaluated for fourteen datasets containing 1,484 sample analyses. All datasets consisted of samples collected from mine drainage discharges or polluted receiving streams. The datasets were variable in nature, ranging from watersheds where most of the discharges contained alkalinity to ones where all of the discharges were acidic. Good relationships were found to exist between measured and calculated acidities. The average acidity measurement was 239 mg/L CaCO_3 and the average acidity calculation was 226 mg/L CaCO_3. Linear regressions were calculated for individual datasets and for the entire dataset. The linear regression for the entire dataset was: Acid~(calc) = 0.98 * Acid~(meas) - 8, r~2 = 0.98. The good correlation between calculated and measured acidity is the basis for an easy and inexpensive QA/QC for AMD data. Substantial variation between measured and calculated acidities can be used to infer sampling or analytical problems and allow data corrections, when appropriate.