Geochemical Modeling of Mineral Precipitation in Class I Industrial Deep Injection Well System
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Project Objective

A confidential utility client needed to evaluate potential performance impacts on a Class I deep injection well system that receives up to 500 gallons per minute (gpm) of inflows from various sources, including extracted groundwater containing coal combustion residuals (CCR) constituents and treated process water from the facility (e.g., flue gas desulfurization blowdown).

Evaluating the level of treatment (e.g., design basis, mass removal requirements) to minimize impacts (e.g., scale formation) to the deep injection well and receiving limestone aquifer was also required.

Geosyntec’s Scope of Services

Geosyntec completed performance impact assessment and level of treatment evaluations using the United States Geological Survey (USGS) PHREEQC program. To simulate chemical interactions between the injectate and the receiving aquifer, Geosyntec developed a static equilibrium geochemical model to assess the potential for mineral precipitation and dissolution. The model simulated equilibrium conditions in a multi-component system that allowed calcium carbonate and calcium sulfate mineral phases to either precipitate or dissolve. The equilibrium model predicted that the mineralogy and porosity of the receiving aquifer was an important data gap that could result in significant model uncertainty if not addressed. To enhance the understanding of aquifer mineralogy within the receiving aquifer, x-ray diffraction (XRD) analysis was conducted on receiving aquifer cores and drill cuttings from each of the four injection wells. XRD results were used to refine assumptions about the mineral composition and porosity of the receiving aquifer.

An enhanced geochemical model (one-dimensional [1D] reactive transport model) was developed to evaluate the potential impact of injectate composition on the host formation. Specifically, the reactive transport model was used to quantify the change in mineral mass (and volume) that could result in a decline in porosity and thereby potentially interfere with injection performance. The reactive transport model simulated constant injection over a 30-year period and predicted at what distances from the injection well mineralogical changes were likely to occur. Select model input parameters, including porosity, injectate geochemistry, and rate of injection, were varied to explore the sensitivity of the injection system and aquifer on scale formation. Similarly, the sensitivity of the system to scaling was assessed by varying the composition of the injectate within a feasible range. The model results were used, in part, to inform development of the design basis for treatment processes prior to deep well injection and for compliance with the Underground Injection Control (UIC) permit.

Notable Accomplishments

Geosyntec prepared geochemical equilibrium and reactive transport models to predict the potential change in porosity in the receiving aquifer during operation of a deep injection well system. Results of the geochemical model identified the treatment criteria (e.g., mass removal requirements) necessary to minimize impacts to and optimize the performance of the deep injection well system.

Project Summary

  • Location: Confidential Coal-Fired Power Plant
  • Client: Confidential Client
  • Project Practice Areas: Water and Wastewater
  • Type of Facility: Confidential Coal-Fired Power Plant
  • Services Provided: Geochemical Equilibrium and Reactive Transport Modeling using PHREEQC, Sensitivity Analysis, Monitoring & Mineralogical Data Evaluation, Data Gap Analysis, Deep Injection Well System Optimization, Treatment Design Basis Development
  • Type of Work: Groundwater Remediation
  • Governing Regulation: State and local