Investigation of Several Factors Affecting Total Alkalinity Concentration and Its Measurement
Type of DegreeDissertation
Fisheries and Allied Aquacultures
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The optimal endpoint pH for total alkalinity titrations decreased from 5.0 at 10 mg/L alkalinity to 4.2 for 300 mg/L alkalinity or more. The appropriate color changes for bromocresol green-methyl red (BG-MR) and methyl orange (MO) indicators also varied with the initial total alkalinity of samples. Despite differences in pH at endpoints for samples of different alkalinities, when the best endpoint pH , best color of BG-MR and MO, or the endpoint of methyl purple were used in titrations of standard solutions, there were few differences between measured alkalinities and standard alkalinities – the accuracy was better than 3 mg/L. Results of spike and recovery tests on aquaculture pond water samples also revealed that an accuracy of 3 mg/L alkalinity could be achieved on either unfiltered or filtered samples by all four methods of acceptable endpoint detection. Although precision of measurements could not be consistently maintained below 1 mg/L, coefficients of variation for repeated measurements usually were less than 5% for all methods of endpoint detection. Nevertheless, this degree of precision was adequate to achieve good accuracy that is the major concern in water analysis. Variations in alkalinity measurement that could result from improper selection of endpoint pH (or color) were rather small – usually not more than 5 mg/L. In an inter-laboratory comparison of alkalinity determinations on standard solutions, most laboratories reported inaccurate alkalinities. These inaccuracies were greater than possible endpoint variations. It was clear that most of the participating laboratories did not have a satisfactory method of quality control. Total alkalinity and total hardness concentrations in small streams in Alabama have increased from 41.56 mg/L and 43.42 mg/L, respectively, in 1973 to 47.19 mg/L and 52.28 mg/L at present (2014-15). These findings are in agreement with two other studies reporting an increase in the alkalinity of major rivers in the eastern United States. Possible explanations for the increase were given in one study as greater weathering of limestone by acidic deposition, and in the other study, as the result of streams recovering from past acidification. Observations and calculations made in the present study suggest that the dissolution of limestone, calcium silicate, and feldspars has increased in recent years because of rising atmospheric carbon dioxide concentrations resulted in greater alkalinity and hardness. The estimated solubility increase is great enough to explain a major portion of the rise in stream alkalinity and hardness that has occurred in small streams in Alabama since 1973. Large applications (~20t/year) of agricultural limestone in pond FP-14, a large watershed reservoir on the research station, have roughly doubled average annual conductivity and concentration of total alkalinity, total hardness, and calcium hardness as compared to concentrations of these variables in control water sources. The pH of FP-14 discharge is not much greater than for the control, which is an un-limed pond, but pH was never below 6.65 and had an annual average of 7.12 in water released from FP-14. The concentration of water quality variables decreased following large rainfall events and during periods of prolonged rainfall. In general, concentrations were lowest in winter when rainfall tended to be high and evapotranspiration low favoring greater runoff. Because of the effect of rainfall on water quality, calcium hardness concentrations were sometimes so low that they would be expected to adversely impact fish egg survival and hatching at the Fish Genetic Unit (FGU) hatchery. The discharge of FP-14 ultimately reaches pond FP-11 that is the water supply for the Production Research Unit (PRU). Despite having been diluted by considerable runoff from the Research Station Creek (RSC) catchment below FP-14, the total alkalinity of water in FP-11 was about 6-7 mg/L higher than in the control. This should reduce the amount of liming required for the small ponds of the PRU. The pH, conductivity, and concentrations of total alkalinity, total hardness, and calcium hardness in Alabama streams tended to be lowest in the Coastal Plain and Piedmont Plateau, intermediate in the Ridge and Valley, Appalachian Plateau, and Highland Rim, and highest in the Blackland Prairie. Streams seldom had pH below 6, and most pH values ranged between 7 and 8. There were few instances with alkalinity and hardness below 5 mg/L or above 100 mg/L. Conductivity usually was below 500 mho/cm.