THE CHALLENGE OF DEVELOPING A NUMERICAL FLOW MODEL OVER VARYING SPATIAL SCALES AND DATA DENSITY IN A STRUCTURALLY COMPLEX AREA WITH A VARYING GRID CELL DISCRETIZATION USING LEAPFROG WORKS AND GROUNDWATER VISTAS

The Eureka Mining District is located in central Nevada, in the geologically complex Basin and Range physiographic region. Mining within the District has been ongoing since the 1860s, with a 1962 estimate of 1.62 million ounces of gold, 39 million ounces of silver, and 625 million pounds of lead produced. Mining in the region is done with both open pit and underground methods, requiring dewatering of the mine where it extends below the water table. Design and operation of mine dewatering systems are typically supported by groundwater flow modeling, starting with a conceptual groundwater model, and ultimately resulting in an analytical and/or numerical groundwater flow model. The models are often constructed using abundant local geological data and interpretation provided by often densely-spaced exploration drilling in and around the ore body. This contrasts with the often sparse regional data further away from the ore body. Bringing these data and interpretation into a numerical groundwater flow model presents significant technical challenges.

This presentation describes a method used for an underground mine project being developed on a historical open pit mine. Two prior existing site-specific geologic models served as the framework for expanding the model domain to the basin edges. Despite having the advantage of the existing detailed geologic data, combining the geologic models and expanding the domain to basin boundaries was challenging and time consuming. The site-specific geology is structurally complex with numerous fault and fracture zones, and overturned beds. The level of detail at the mine site needed to be maintained in order to simulate processes at that scale while extending the geology to the edges of the basin where data were sparse or poorly understood. Adapting to the changing scale was the first significant challenge. A nested model grid discretization is appropriate to capture the level of detail at the site but allows for less detail along the basin margins. This presented the second significant challenge because Leapfrog Works is not compatible with nested grids and it required workarounds to communicate between Leapfrog Works and Groundwater Vistas. While this model development process was challenging, these new workflows can be applied in other situations where the traditional model grids do not readily communicate across modeling platforms.