Calibrated Numeric Model for Groundwater Flow Systems
In response to a Colorado Supreme Court ruling, the Colorado Division of Water Resources passed new regulations requiring coal bed methane wells to be permitted as water wells and any water withdrawn from these wells be subject to Colorado’s water appropriation system. If the withdrawn water is tributary to the surface water system a plan to protect senior water rights holders is required (typically a substitute water supply plan followed by a water augmentation plan).
Several coal bed methane operators asked Norwest to evaluate the tributary nature of groundwater pumping from coal bed methane wells in a major coal bed methane producing basin in the State.
Norwest developed a calibrated numeric model of the groundwater flow systems from below the coal bed methane production zone up to the ground surface in order to evaluate the degree of interaction between groundwater and surface water. The model was used to delineate the tributary/ nontributary groundwater boundary in the producing formations and determine the degree of protection needed for existing water rights.
After submittal to the Division of Water Resources Norwest provided expert testimony and support of the model in adjudicatory hearings. The Colorado State Engineer ruled that the model and supporting testimony provided clear and convincing evidence of the tributary/ nontributary groundwater boundary for the basin. The clients were then able to develop a substitute water supply plan based on the modeled impacts to the surface water right holders.
Mine Water Management & Stability Control
Norwest consulted with a mining company that had been experiencing two separate instances of localized instability within a mining area. The first area was characterized as a creep type dump failure caused by placing waste material on what proved to be a weak foundation layer. The second area was characterized as a block type wall failure, which occurred along a weak “aqua putty” layer within the coal seam. Norwest quickly engineered a creative modification to the mine plan to allow for continued mining within the pit.
Norwest developed long-term remediation plans to mitigate the pit wall instability, which included detailed mining and water management plans and permit application submittals. The wall failure remediation plan required mining through two large sediment ponds located in series near the edge of the mine pit backfill, allowing for the pit wall angle to meet the stability requirements. After removing the weak aqua putty layer, a buttress of overburden material was placed to allow the replacement ponds to be reconstructed on a stable backfill surface. An interim water management plan allowed the ponds to be temporarily removed from service during the seasonal dry period.
Norwest developed the detailed permitting and construction plans within a compressed schedule. Approvals were quickly obtained from State and Federal agencies so that implementation of the plan began on time with the exception of the mining and buttress construction which took a little longer than expected. Nevertheless, the interim water management functioned effectively to keep operations going smoothly despite near record rainfalls. Norwest’s efforts were pivotal in maintaining safe mining conditions and meeting coal extraction schedules.
Washington State, USA
Norwest has been developing reclamation and closure designs for mine pits, overburden dumps and coal refuse impoundments at a coal mine in Washington State. Mining was discontinued at one of the mine pits due to geotechnical concerns following a pit side-wall failure in November 2006. As a result of cessation of mining operations in the pit, it was necessary to develop a new reclamation plan for this mine pit. Backfilling of the pit was not a viable option. The plan needed to address geotechnical and hydrologic stability and acid mine drainage (AMD) concerns raised by the regulatory agencies as well as demonstrate that the proposed pit lake would be ecologically viable and meet the water quality requirements for both the pit lake and receiving stream.
Norwest developed a reclamation plan for this pit that provided hydrologic and geotechnical stability and ensured adequate blending of isolated pockets of potentially acid-forming material with alkaline spoils to mitigate AMD and meet post-mining water quality criteria for mine drainage. A principal feature of the reclamation plan was the establishment of a permanent pit lake with a surface area of approximately 217 acres. This included a buttress design to meet minimum stability criteria for the Smith footwall closure configuration. The geotechnical characteristics of the Smith footwall slopes were defined by a combination of inputs including laboratory testing, geological models and back analysis of slope failures. The post-mining topography and drainage was developed using a geomorphic drainage design where feasible and both a terrace and down drain approach for some the remaining areas and a geomorphic drainage design for other portions of the post mining surface. The plans included the removal of the sediment ponds for final reclamation and establishing a lake inlet channel and constructed wetland at the location of the former sediment ponds. The majority of run-off that flows into the lake is routed through a the constructed wetland. The wetland habitat was added as a “polishing” feature for the lake inflow even though the modeling performed in support of the probable hydrologic consequences (PHC) assessment predicted that the lake water quality would be suitable for meeting the needs of aquatic environments, and meeting applicable discharge limits without the need for wetland treatment. The lake outlet structure design included rock weirs and rootwads to provide stable gradient control while providing the necessary fish passage and habitat.
Norwest developed the detailed permitting and design plans within schedule and budget. Approvals were obtained from State and Federal agencies so that implementation of the plan could proceed according to schedule. The elements of the plan address all aspects of permit approval and regulatory requirements. Implementation of the Plan will allow the client to successfully meet the reclamation and post-mining water quality requirements.