Municipal Landfill Remediation and Groundwater Management
Trihydro provided environmental, surveying, engineering design, and construction quality assurance services for the Sheridan Landfill Remediation Corrective Measures Project in Sheridan, Wyoming at the City’s Old and Existing Landfills. At the Old Landfill, groundwater conditions, saturated waste, and areas of surface-water infiltration required a clearer understanding of how water was moving through the waste mass before an appropriate corrective measure could be designed.
Trihydro combined groundwater and landfill gas monitoring, land surveying, high-resolution site characterization, and conceptual site model development to define site conditions and evaluate corrective measures. Monitoring activities included both the old and new landfills, while corrective measures investigation and design focused on the Old Landfill.
Understanding Landfill and Groundwater Conditions
Trihydro completed surveying and environmental monitoring to support site investigation and remedial design. Services included:
- Verifying the Old Landfill permit boundary
- Staking and surveying test pit and borehole locations
- Completing topographic surveys of the Old Landfill
- Monitoring and reporting landfill gas
- Conducting quarterly groundwater monitoring and reporting
- Performing statistical analyses supporting the development and evaluation of site-specific groundwater protection standards
Trihydro used small unmanned aerial systems (sUAS) to complete high-resolution topographic mapping of the more than 50-acre site in less than four hours. The resulting data provided a detailed basis for site characterization and engineering design.
To further define waste limits, saturated waste, and subsurface conditions, Trihydro conducted test pit investigations and high-resolution site characterization (HRSC) using a Geoprobe equipped with an Optical Image Hydraulic Profiling Tool (OIHPT). The integrated tool provided continuous subsurface data, including hydraulic response and electrical conductivity, that field geologists interpreted alongside field observations to characterize subsurface materials, identify water-bearing intervals, and estimate hydraulic conductivity.
Together, the surveying, monitoring, and subsurface investigation data helped Trihydro refine the conceptual site model (CSM) and better understand the relationship between buried waste, groundwater, and saturated waste zones.
Evaluating Groundwater Remediation Alternatives for Landfill Corrective Measures
With a clearer understanding of subsurface conditions, Trihydro evaluated several approaches for intercepting impacted groundwater at the Old Landfill, including:
- Interceptor trenches
- Horizontal wells
- Vertical extraction wells
The evaluation also considered options for managing extracted groundwater, including discharge to the City’s sanitary sewer system and management through evaporation ponds.
The refined CSM provided the technical foundation for comparing these alternatives and selecting a corrective measure suited to the landfill’s groundwater and waste conditions.
Groundwater Extraction System and Evaporation Pond Design
Trihydro designed the selected corrective measure, which includes downgradient groundwater extraction wells, force main infrastructure, two evaporation ponds, and site grading improvements.
As part of the design process, four test wells were installed to further characterize saturated waste and support step-drawdown and constant-rate pumping tests. Pump-test data were used to estimate hydraulic properties of the saturated waste mass and support capture-zone analyses.
The analyses helped determine extraction well locations and pumping rates needed to capture impacted groundwater and reduce the potential for offsite migration. Estimated pumping rates were then incorporated into water-balance calculations used to size the evaporation ponds.
Because of space constraints, the two evaporation ponds were designed over the existing waste mass. The liner system consists of:
- A geocomposite gas collection layer
- A 60-mil linear low-density polyethylene (LLDPE) secondary liner
- A geonet leak detection layer
- A 60-mil LLDPE primary liner
The pond design minimizes excavation and reduces the potential for contact with buried waste.
Trihydro also designed grading improvements for portions of the Old Landfill where settlement of historical trench fill had created low areas that collected surface water. Imported fill was used to improve surface drainage and reduce infiltration into the waste mass.
Construction Quality Assurance for Landfill Remediation
Trihydro prepared construction drawings, technical specifications, and a Construction Quality Assurance (CQA) Plan to support public bidding and construction.
During construction, Trihydro provided onsite CQA services to document that the corrective measures were constructed in accordance with the project technical specifications and CQA Plan.
From site characterization through construction, Trihydro connected field data, groundwater analysis, and engineering design to implement a site-specific corrective measure. The integrated approach provided the technical foundation needed to manage impacted groundwater, improve surface drainage, and reduce the potential for offsite migration.
