Water budget evaluation of an active fly ash dry stack using HELP2

In order to develop a database and a better understanding of how dry stacked fly ash might impact groundwater resources, several studies were initiated at the Bull Run Fossil Plant. Approximately, between 700 to 1100 tons of fly ash that require disposal are produced each year at Bull Run from coal burning operations. This thesis contains the analysis of the water budget distribution in the dry stack which was determined by conducting field studies at Bull Run. This distribution was compared with water budget predictions from the second version of the Hydrologic Evaluation of Landfill Performance (HELP2) Model.

Field investigations included measurements of runoff from the fly ash stack, estimates of evaporation obtained from 24 lysimeters installed at the stack, and determination and analysis of vertical moisture profiles throughout the stack. The amount of runoff was observed to be 4.4% of rainfall between August 1, 1987 to June 8, 1988. As an average, runoff was estimated between 0 and 10 % of rainfall. The lysimeter study, conducted from August 21 to November 3, 1987 produced an evaporation coefficient of 8.2 mm/day^½. This value is a measure of the ability of the soil to transmit water to the atmosphere in the form of evaporation. Evaporation was calculated to be between 50 and 65 % of rainfall. Estimates of storage and leachate were determined based on moisture profiles obtained from borehole drillings made in November, 1987, March 1988, and November 1989. Storage was estimated to be between 25 and 35 % of rainfall and leachate between 1 and 5 %.

Other field investigations and laboratory experiments provided information about the hydraulic properties of fly ash and of the clay beneath the dry stack. Porosity of the clay was determined to be 42.3 %. The average saturated hydraulic conductivity of 3.0 x 10^-5-7 cm/s for the clay. The relationship between volumetric water content and suction was also established for the type of material being dry stacked at Bull Run Fossil Plant.

The HELP2 was used to model the distribution of rainfall into the water budget components from November 1, 1983 to October 31, 1987. Simulations incorporated a combination of 2 approaches for handling the moisture retention characteristics of fly ash, the bottom boundary conditions of the dry stack, and the history of construction of the stack. HELP2 predicted, for the range of combinations simulated, that leachate would vary from 0 to 10 % of rainfall, runoff from 10 to 11 %, storage from 6 to 18 %, and evaporation from 70 to 73 %.

To establish the sensitivity of HELP2 to different parameters, a sensitivity analysis was performed. Among the parameters tested for model sensitivity were meteorological data and stack height, the SCS Curve Number for runoff, evaporation coefficient, and hydraulic conductivity of fly ash.