Development of the Gas Push-Pull Test for Landfill Cover Soil Applications

The purpose of this thesis was to develop the Gas Push-Pull Test (GPPT) for landfill cover soil applications to measure H2S oxidation. This thesis begins with an overview of solid waste management in the USA, and describes the alternatives for discarding and handling of solid waste. Also, discussions about the components that makeup a landfill to help resist the exposure of contaminants from the solid waste to the environment is elaborated upon. In addition, the ways in which landfill gas is generated by placing solid waste into landfills along with mitigation techniques to help attenuate H2S is discussed. Then the most suitable test to quantify in-situ rates of chemical or microbial reactions in the vadose zone or unsaturated zone which is the GPPT, is introduced. As a part of the development of the Gas Push-Pull Test (GPPT), chapter three presents a new methodology that allows for sound implementation of the GPPT in diverse subsurface environments such as a landfill to better understand the transport of gaseous components during the GPPT. For this to be accomplished, many GPPTs were run in fine sand and clayey fine sand with non-reactive gases methane (CH4) and sulfur hexaflouride (SF6) with molecular weights of 16.04 g/mol and 146.06 g/mol. With the data from the GPPTs, an equation or function called the correction factor was formulated to correct for the difference in molecular weights. The correction factor was applied to the GPPTs data and was successful in correcting CH4 and SF6 the non-reactive gases to have a molecular weight equal to the reactive gas Hydrogen Sulfide (H2S) with a molecular weight of 34 g/mol. Now one does not have to search for a tracer with equal transport capabilities like the reactive gas, one need only apply the correction factor. Now you are able to account for how much reactive gas was lost because of transport through the cover soils and by reactions within the cover soils. Once the correction factor was applied, the reacted portion can be used to calculate oxidation or reaction rates of landfill cover soils which is shown in chapter four. Also, to optimize H2S reduction in various soil types, it was important to be able to accurately quantify the reaction rate coefficient, k. Therefore, chapter four focuses on the use of lab and field techniques to examine reaction rates of H2S with fine sand, silty fine sand, clayey fine sand, compost and landfill cover soil. First, a series of laboratory flask experiments were conducted to assess the reaction rates of various soil types with no moisture, and then with varying water content from 10% to 60%. Next, the Gas Push-Pull Test (GPPT) was conducted in the field in different soil types to assess reaction rates. The laboratory results showed that the landfill cover soil had the highest reaction rate of 41.87 hr-1 when the soil had no moisture. Whereas compost had the highest reaction rate from 5.84 hr-1 to 9.98 hr-1 when moisture content increased from 10% to 60%, respectively. The laboratory results showed that lab-measured reaction rates of dry soils are strongly related to total iron content. For instance, the landfill cover soil had the highest reaction rate of 41.87 hr-1 with an iron content of 31,000 mg/kg of soil. On the other hand, fine sand had the lowest reaction rate of 1.47 hr-1 with an iron content of 100 mg/kg. The reaction rate with water was also measured to be 1.44 hr-1. It was noticed that water causes the reaction k to decrease for soils that had high k values with 0% water content. Whereby water causes the reaction k of compost to increase. The GPPT well-mixed and plug-flow reactor models' reaction rates for fine sand ranged from 1.63 hr-1 to 3.02 hr-1 and from 0.45 to 2.02, respectively. The GPPT well-mixed and plug-flow reactor models reaction rates for clayey fine sand ranged from 63.80 hr-1 to 144.49 hr-1 and from 47.77 hr-1 to 74.08 hr-1, respectively. Lastly, the GPPT well-mixed and plug-flow reactor models reaction rate values for landfill cover soil ranged from 55.83 hr-1 to 318.18 hr-1 and from 32.69 hr-1 to 110.14 hr-1, respectively. Also, fine sand tested for reaction rates in the flask and with the GPPT was not significantly different because of the homogeneity of the soil. However, the clayey fine sand and landfill cover soil tested for reaction rates was significantly different because of the heterogeneity of the soils. Both the flask test and the GPPT are easy and convenient to perform, but the GPPT is the most reliable because it quantifies in-situ reaction rates. Furthermore, to understand the attenuation of H2S, chapter five looks at lab and field scale studies that were conducted with potential landfill cover soils. For the laboratory experiment a rigid translucent plastic cylinder with a diameter of 5 1/2 inches and a height of 24 inches was constructed. The inside of the column from bottom to top was composed of a 2mm geotextile underlayment, a five-inch layer of course gravel, another two pieces of 2mm geotextile underlayment, twelve inches of 50-50 compost peat (by volume) mixture, and a six-inch air space. Then landfill gas (LFG) was injected into the column to assess the mitigation of H2S. After pumping 28,000 L of LFG was introduced into the column, which indicated the average instantaneous removal efficiency during the monitoring period was 85.7%. Also, the total mass of H2S introduced into the column was approximately 3.12g, the total mass emitted was 0.28g, and the total mass retained by the 1 foot soil mix was 2.84g. Therefore cumulative removal efficiency was 91%. In addition, to check for physical and chemical adsorption in the column, the saturated four-inch bottom layer of the soil mix was removed and tested for total sulfur. The sulfur adsorption capacity of the soil mixture was 2.2 g of total S per kg of dry mass of soil mix. The Michaelis-Menten kinetics parameters to understand the biological oxidation of H2S was determined to be Vmax of 450 nmol/s/kg of dry soil mix and a half saturation constant Km of 30 ppm. Also, a field scale study was conducted by constructing three 65x35 lysimeter test pads at the Riverbend Landfill. The test pads were made up of a composition of local soil and compost. As of October 2014, the average removal efficiency of the test pads is 99%. Test pad II with 6 inches of local soil on the bottom and 12 inches of local soil on the top had the highest H2S removal efficiency of 100%. Whereby test pad I with 18 inches of local soil and test pad III with 6 inches of local soil, both had 99% H2S removal efficiency. Also, continual monitoring of the test pads will persist, because of the increase of LFG flow into the test pads by increasing the orifice plates to 1 inch. In addition, the total sulfur and iron content of the test pads will be examined especially since LFG is still being introduced to the test pads. Lastly the newly developed GPPT will be utilized to study the field scale H2S oxidation of each test pad.