Nicholas Ybarra, PE, and Tyler Krueger, PE, showed their finest in a modeling class at PSCU, PSC’s continuing education training session. However, this modeling class was not about haute couture but about engineering landfills.
In “Modeling: It’s Not Just for Magazines,” they explained that it’s not just digging a huge hole for trash. Landfill modeling means providing a design that meets all the regulatory requirements for safely storing trash in a landfill and maintaining that safety threshold.
Landfills can receive waste for over 100 years. One of the priorities of the Texas Commission on Environmental Quality (TCEQ) is to prevent anything harmful from leaking into the aquifers beneath the Earth’s surface. Aquifers provide precious drinking water to communities that are seeing their groundwater dry up.
Preventing leakage
Engineers on solid waste teams design landfills to collect leachate, which is liquid infiltrated through waste that poses a risk to the environment. Engineers use layers of geosynthetic materials that include a geosynthetic clay liner, an HDPE geomembrane liner, and geocomposite to manage leachate collection. The geosynthetic clay liner, placed between the ground and HDPE geomembrane, grows when hydrated to as much as 2 feet thick. This provides a final barrier to prevent leachate from permeating into the ground, if there is a breach in the liner.

Tackling Technology

PSC uses three software programs in its landfill design:
• UNSAT-H is a FORTRAN computer code that simulates one-dimensional flow in soils. It addresses how water, vapor, and heat react to the soil. Engineers basically design the covers to make sure the water balance, the liner, and the soil cover or plant cover thickness on top are right. They calculate climate data, evaporation, plant transpiration, storage, drainage, and rainfall to see how that may affect the landfill and climate change. West Texas projects using UNSAT-H for water balance cover design that have been approved by TCEQ include the Clint Landfill, Greater El Paso Landfill, McCombs Landfill, and West Texas Region Disposal Facility.
• SLIDE by ROCSCIENCE is a computer program that analyzes slope stability, including infiltration, seepage face, and ponding. It analyzes a variety of groundwater flow problems.
• HELP (Hydraulic Evaluation of Landfill Performance) is a two-dimensional modeling program that simulates moisture balance and flow of liquid in landfills. It examines location, evaporative zone depth, the growing season, wind speed on average, solar radiation, rainfall, relative humidity, and temperature. Soil textures come into play, as well as layer thicknesses. Engineers forecast how the landfill will be affected years ahead.


One of the landfill engineers’ main goals is protecting groundwater. They do this by using a liners system which includes geosynthetic clay liner, geomembrane liner, and leachate collection. A typical liner section is shown below. Groundwater is modeled semiannually by each landfill site.
Another type of modeling involves monitoring of air. Some monitoring methods involve taking measurements of gas levels in the air with a series of probes installed around the waste footprint. Samples are taken quarterly by landfill staff. Other gas modeling performed includes annual non-methane organic compounds (NMOC) monitoring. Monitoring emissions from landfill surfaces, flares, or stacks helps staff determine if harmful gases are above limits set forth by TCEQ. All reporting is submitted to TCEQ.

Below are three monitoring models PSC uses for landfills:
• Sanitas – The Groundwater Statistical Analysis uses background data to calculate constituent levels, or different metals or volatile organic compounds that are found in groundwater in groundwater. Using Sanitas involves establishing a background to better track any natural changes in the site’s groundwater and to determine if the landfill is contributing to groundwater contamination. Constituents which that are sampled are set by the Texas Administrative Code.

SCREEN3 – This model examines how the landfill may affect air quality. It provides an easy-to-use method of obtaining pollutant concentration estimates.

• LandGEM – Landfill Gas Emissions Model is an automated software tool that can estimate emissions in the air, such as landfill gas.

PSC also provides services such as planning, permitting, land development, permit modifications, compliance assistance, and closure of a landfill that has reached its capacity.

The complex steps that go into a landfill to make sure it meets regulations involve two priorities: state-of-the-art technology and careful planning by a firm that is on top of current expertise help ensure that each landfill does what it can to protect the environment.
“You model for a worst-case scenario,” Nick said, such as 100-year flood events. “Our main goal is to protect the environment,” he said.