Modeling Methane Emissions and Oxidation

  We used our large dataset of static flux chamber measurements to develop geo-spatial methods to extend point measurements of methane emissions to total landfill emissions. Abichou et al. (2011), Yuan et al (2009), Abichou et al. (2008) described a numerical model that combines water and heat flow with a gas transport and oxidation model. The gas transport and oxidation model is able to use dynamic parameters associated with water content and temperature and incorporates dynamic methanotrophic activity. Gas transport and oxidation can be estimated from the cover design, management practices, and daily climatic conditions. Daily volumetric water content and temperature are generated at several depths by the unsaturated flow module. The gas transport module uses these dynamic results to simulate methane transport and oxidation during each day for the average climatic conditions along with the methane oxidation capacity of the cover. Geostatistical techniques are then used to generate two dimensional random fields of soil properties with porous zones or windows representing the presence of cracks. The generations of these random fields statistically represent the spatial variation of soil air permeability and gas diffusion coefficient. The methane oxidation properties of the cover were subject to spatial variation. The results is: An integrated gas model, which couples the climate model and soil environmental model with the gas model and furthermore use it to assess the effects of soil heterogeneity, on methane oxidation in landfill cover soils. 

The model comprises four major modules:

· A climate generation module: generation daily minimum and maximum air temperature along with daily rainfall based on the landfill site geographical location. 

· A random field generation module generating the relative soil properties as random field in order to account for the heterogeneity of soil properties, e.g. saturated hydraulic conductivity, porosity, and methane oxidation capacity. This module also introduces cover soil defects for different degree of landfill cover maintenance scenarios. 

· A soil environmental simulation module which used the data generated in the first two steps and simulates the soil moisture and temperature based on the landfill site location and soil texture information and the climate generation.

· A core computational module, a pseudo 3-dimensional gas model based on [Yuan et al., 2009, Abichou et al. 2010], which simulates the concentration and flux of the gas components including methane in the landfill cover soil. 

· A mapping module that generates three dimensional maps of soil properties with space, gas concentration with space, gas flux with space, and degree of oxidation with space.

A numerical model (LandSEM Model) was developed by a research team at Florida State University. The model combines water and heat flow with a gas transport and oxidation model. The following few extra details on LandSEM: (Figure 3)

Climate Generation Module

The climate simulator is a module adopted from models GlobalTempSIM and GlobalRain [Spokas and Forcella, 2009], including an air temperature simulation and precipitation simulation. The air temperature generation predicts the daily average air temperature based on a 30-year (1961-1990) temperature records and calculate the daily maximum and minimum air temperature. The global rainfall module estimates daily rainfall patterns for a yearly cycle based on a 30-year (1961-1990) global average monthly precipitation records. The information obtained from these modules together with the soil texture information serve the soil environmental simulator as input. 

Random field generation

Soil properties vary from point to point due to the soil texture or sometimes the cracks and fissures or “hot spots” in landfill covers. The spatial variability of soil properties suggest that it is natural to describe them as a random function of position, i.e., as a random field [Przewlocki, 2000]. In order to account for the horizontal heterogeneity of soil properties such as the saturated hydraulic conductivity, a 2-dimensional autocorrelated random field generation module is developed.

Climate and Soil environmental simulators

The soil environmental simulator is adopted from the Soil Temperature and Moisture Model (STM2) [Spokas and Forcella, 2009] which estimates soil temperature and moisture content based on minimum information including the soil texture, air temperature, and precipitation. These climate and soil property models have been validated by field data [Spokas and Forcella, 2009; Spokas et al., 2009]. Refer Spokas and Forcella [2009] for detail information of these modules.

Output of Model:

The outputs of LandSEM are the concentrations of daily CH4, O2, CO2, and N2 at every node of the modeled soil profile. These concentrations are then used to calculate the mass flux of each of the gases into or out off of the bottom and top boundaries of the soil profile. The methane fluxes into the bottom and out of the top of the soil profile are then used to determine the daily oxidation rate and daily percent methane oxidation.