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A three-dimensional (3D) geological model of known and potential aquifers as well as enclosing materials in the Fargo-Moorhead region of North Dakota and Minnesota was required for a current water supply assessment coordinated by U.S. Department of the Interior, Bureau of Reclamation and their partners. A multi-agency team funded by Reclamation therefore compiled, interpreted, and integrated existing geologic mapping at a scale of 1:200,000 for an area (Figure 1) extending from 46 to 47.5 degrees north latitude and from 98 to 95 degrees west longitude, an area 190 km east to west and 170 km north to south. These maps were merged with drillhole data (Figure 2) for 30,000 sites, 60 Quaternary stratigraphic control sites, and new subsurface correlations to produce a 3D geological model extending down to the top of Precambrian rocks, compiled at a scale comparable to that of a 1:500,000 map. This new 3D geology and an accompanying USGS compilation of the quantitative properties of selected major aquifers will provide the framework necessary to conduct subsequent water resource analyses. To construct the 3D geology, drillhole cross-sections were constructed using ArcView 3.3, ArcGIS 9.0, and custom extensions. Thirty-four regional cross sections were constructed at 5 km spacing. Drillhole logs and surficial geology were displayed along the locus of each cross section to facilitate correlation. Four smaller areas encompassing important aquifers, ranging from 1,300 to 3,200 square km in area, were chosen for more detailed analysis. East-west cross sections were constructed in these areas using 0.5 to 3 km spacing. Creating these cross sections in ArcView allowed sand and till boundaries to be correlated by overlaying shapefiles in the same view window. A series of points for each stratum were extracted from the cross-section shapefile lines and converted into X, Y, & Z values. These points as well as cross-sections scanned and registered in 3D space were then used as the basis for construction of surfaces and volumes in GOcad software (Figure 3). Modeled strata were constructed for a total of 36 lithostratigraphic units (Figure 4), including Quaternary sediments as well as Cretaceous, Ordovician and Precambrian rocks, and these are now being prepared for further application to regional groundwater inventory, to help ensure long-term water supply for the region.

3D geological modeling in support of ground water inventory in the Fargo-Moorhead Region, Minnesota and North Dakota