Rocky Mountain Regional Hazardous Substance Research Center

Project 5 Abstract

Evaluation of Hydrologic Models for Alternative Covers at Mine Waste Sites

Investigator(s): Dr. Charles D. Shackelford; Dr. Craig H. Benson

Institutions: Colorado State University and the University of Wisconsin-Madison

EPA Project Officer: Mitch Lasat

Project Period: October 1, 2002 - September 30, 2005

The conventional approach to final cover design is to use a compacted clay layer and/or a geomembrane (polymeric sheet) as the primary barrier to flow. This approach is impractical at many mine sites because of expense, the lack of suitable clay borrow sources, and the potential for damage by frost and desiccation. As a result, alternative approaches to final cover design are being investigated. Although the underlying principle of alternative covers (e.g., monolithic covers, capillary barrier covers) is simple, predicting their hydrologic performance is fraught with difficulties that introduce appreciable uncertainty regarding the accuracy of predictions, and only limited effort has been devoted towards ensuring that predictions made with hydrologic models accurately represent field conditions. Comprehensive studies have not yet been conducted because hydrologic data from full-scale instrumented alternative covers have only become available within the last two years. The proposed research will represent the first comprehensive evaluation of five different hydrologic models (HELP, UNSAT-H, Vadose/W, Hydrus-2D, and LEACHM) commonly used for the design of cover systems based on high quality field data from large-scale test facilities of alternative covers that have been constructed and monitored at 12 sites in the US.

Objectives: The proposed study has four objectives: (1) a baseline assessment and comparison of the algorithms in existing hydrologic models when applied to a variety of meteorological conditions, (2) an unbiased critical assessment of the predictive capabilities of existing hydrologic models for covers using field data, (3) improvement of the hydrologic model (or models) that have the most promise so that predictions made with the model are accurate, and (4) incorporation of additional algorithms in the model that can be used to assess the impact of long-term processes such as plant secession, pedogenesis, and climatic change.

Approach: The first objective will be achieved by performing comparative simulations with each hydrologic model to define differences in output obtained from the five models for different types of alternative covers and different locations with different climatic conditions. The second objective will be achieved by performing simulations using a subset of the five models based on the results of the first objective for each test facility using data representative of field conditions as input. The third objective will be achieved by modifying the algorithms with deficiencies that were identified in the first two objectives to accurately reflect field behavior based on the detailed and comprehensive data collected at the field sites. This effort may require substantial re-writing of portions of the codes and incorporation of additional algorithms. The fourth objective will involve adding algorithms to the model (or models) for simulating long-term effects that may influence the hydrologic behavior of covers. The characteristics of these algorithms will be based on the short-term pedogenetic and vegetative changes observed at the field sites, as well as the knowledge that has been gained from natural analog studies.

Expected Results: The key deliverable from this study will be an improved, easy-to-use, and field-verified model for long-term assessment of alternative covers at a variety of sites and climatic conditions. This study will also leverage Rocky Mountain Regional HSRC funds by making use of extensive investments that have been made by three federal agencies (USEPA, USDOE, and US Army).