Evolution of 3-D Geologic Framework Modeling and Its Application to Groundwater Flow Studies In this Fact Sheet, the authors discuss the evolution of project 3-D subsurface EXPLANATION OKLAHOMA framework modeling, research Aquifers Arbuckle-Simpson in hydrostratigraphy and airborne Arbuckle-Simpson geophysics, and methodologies Trinity used to link geologic and Edwards-Trinity groundwater flow models. Edwards Edwards aquifer recharge TEXAS area boundary Edwards-Trinity Trinity he various geologic processes thatT shape the Earth’s surface and sub- surface all operate in three-dimensional (3-D) space. Within the U.S. Geological Survey (USGS), the greatest needs and Edwards applications for 3-D modeling include 0 125 250 375 500 KILOMETERS the following: (1) visualization of surface 0 125 250 375 500 MILES geology into the subsurface, (2) inter- pretation and verification of the geology and its controlling fault structures, and (3) application of 3-D model data for Figure 1. Location map of the Arbuckle-Simpson and Trinity aquifers and Edwards aquifer investigating geologic issues (Jacobsen recharge area. and others, 2011). Three-dimensional geologic modeling Three-dimensional geologic framework models can be directly converted into of an aquifer can quantitatively depict groundwater flow models, such as MODFLOW. Construction of the 3-D geologic archi- the connectedness of rock units across tecture of an aquifer needs to be the first step in using geologic properties to constrain fault and fracture zones. This model- groundwater flow models. This first-step approach takes much of the subjective guess- ing allows geologists to determine the distribution of geologic units, structural work out of constructing model layers and results in a model that is more realistic and features, and other controlling factors, representative of the actual subsurface hydrologic conditions of the groundwater basin such as porosity and permeability. being modeled. These parameters are complex vari- Several USGS projects, supported by the National Cooperative Geologic Mapping ables that reflect original depositional Program (NCGMP), are using multidisciplinary approaches to reveal the surface and conditions, alteration, dissolution, and subsurface geologic framework of three important groundwater aquifers: the Edwards, dislocation. Geologic 3-D framework Trinity, and Arbuckle-Simpson (fig. 1). In this Fact Sheet, the authors discuss the evolu- modeling also is useful for visualizing tion1 of project 3-D subsurface framework modeling, research in hydrostratigraphy and the interactions of fault and related airborne geophysics, and methodologies used to link geologic and groundwater flow structural features. models.1 The principal sections of this Fact Sheet are as follows: 21 Ratio of the volume of voids in a Geologic Framework of the Edwards and Trinity Aquifers porosity material to the total volume of the 21 porous medium Geologic Framework of the Arbuckle-Simpson Aquifer Rate of flow of a fluid through a 32 permeability porous material 2 Arbuckle-Simpson 3-D Geologic Framework Model to Groundwater Flow Model Freely available USGS computer 3 MODFLOW software to simulate the flow of groundwater through aquifers 43 Arbuckle-Simpson MODFLOW Model 43 U.S. Department of the Interior 4 Fact Sheet 2012–3106 Printed on recycled paper U.S. Geological Survey 4 October 2012 1 3 4 1 Geologic Framework of the Edwards and Trinity Aquifers 2 Geologic Framework of the Arbuckle-Simpson Aquifer The Edwards aquifer in Texas is one of the most productive freshwater The Arbuckle-Simpson aquifer of south-central Oklahoma (fig. 1) encompasses more than aquifers in the world. It has been designated a sole source aquifer by the 850 square kilometers and provides water for public drinking supply, farming, mining, conservation, U.S.3 Environmental Protection Agency (EPA) and is the primary source of water and5 recreation. The aquifer also contains a number of unique freshwater and mineral springs, such as for San Antonio, the Nation’s seventh largest city. Understanding the Edwards is Buffalo spring (fig. 4), in the Chickasaw National Recreation Area. essential for maintaining habitat for several threatened and endangered species. The aquifer’s eastern groundwater basin (Hunton anticline area, fig. 5) has been designated a sole The Trinity aquifer forms the catchment area for the Edwards aquifer, and it 560000 source aquifer by the EPA. Proposed development of water supplies from the aquifer led to concerns intercepts some surface flow above the Edwards recharge area. The Trinity aqui- 555000 that large-scale withdrawals of water would cause decreased flow in rivers and springs, which in turn fer also may contribute to the Edwards’ water budget by subsurface flow across 4 550000 could result in the loss of surface water supplies, recreational opportunities, and aquatic habitat. formation boundaries at considerable depths. Dissolution, karst development, and The Oklahoma Water Resources Board, in collaboration with the Bureau of Reclamation, the 3288000 545000 faulting and fracturing in both of these carbonate-rich aquifers directly control USGS, Oklahoma State University, and the University of Oklahoma studied the aquifer to determine aquifer geometry by compartmentalizing the aquifer and creating unique ground- 540000 the volume of water that could be withdrawn while protecting springs and streams. The USGS water flow paths. 3280000 2 535000 NCGMP, in cooperation Three-dimensional modeling studies of aquifer-bearing units in south-central EXPLANATION with the USGS Oklahoma Texas concentrated on the area’s carbonate-rich hydrostratigraphic units, and Elevation control points 530000 Water Science Center underscored the importance of airborne electromagnetic data for defining new 3272000 Well Picks Geology map relations (colors and other Federal and unit contacts and fault structures (Blome and others, 2006). Two 3-D models 525000 for various units) State agencies, supported discussed5 as follows highlight key controls on aquifer permeability and provide insights on subsurface aquifer processes and aquifer boundaries and interfaces. construction of a 3-D EV The proprietary software EarthVision (EV) was used because of its ability to Figure 2. Three-dimensional model of northern Bexar framework model of the combine various data types and accurately define faulted surfaces while main- County, Texas (Pantea and Cole, 2004). water-producing Hunton taining stratigraphic and structural integrity and complexity. anticline area. Construction The 3-D EV model of northern Bexar County (fig. 2) reveals the subsur- of the model was chal- face geology and groundwater flow units of the Edwards and Trinity aquifers, lenging due to the folded, where water wells range from 60 to over 300 meters in depth. This 3-D model faulted, and fractured nature is based on mapped geologic relations that of the Hunton anticline reflect: (1) Balcones fault zone structures, geology (fig. 5), variable (2) detailed interpretations of 40 principal wells, unit thicknesses (from and (3) gross geometry of the Edwards Group 600 to 2,750 meters), and hydrostratigraphic units derived from prior Figure 4. Buffalo spring, Chickasaw National Recreation Area, south-central interpretations of depositional environments and scarcity of well information. Oklahoma. paleogeography. The 3-D model was also con- structed to determine whether hydrostratigraphic units could be accurately modeled in the sub- surface and to visualize the lateral connections between hydrostratigraphic units of contrasting 97°00' 96°50' 96°40' 96°30' 34°40' EXPLANATION permeability across fault strands. uplift A 3-D EV model of the rock units of the GARVIN Lawrence Model extent of Hunton Chickasaw National MURRAY FFZ anticline area Recreation Area Edwards and Trinity Groups in the north Seco COAL Structures FFZ Franks fault zone Creek area of Medina and Uvalde Counties, Texas PONTOTOC All mapped faults CFZ Clarita fault zone 464000 BFZ Bromide fault zone (fig. 3), was constructed using a variety of digital 3265000 Modeled mapped faults Hickory syncline Modeled inferred faults SFZ Sulphur fault zone datasets, such as: (1) geologic maps, including SSF South Sulphur fault Hunton Geology the current geologic map of the area (Blome and anticline MCFZ Mill Creek fault zone 34°30' CFZ Post-Simpson rocks RF Reagan fault UTM coordinates SulphurSFZ others, 2004); (2) lithologic descriptions, interpre- syncline Simpson Group outcrop 3260000 tations, and geophysical logs from 31 drill holes; SSSSFF Arbuckle and Timbered (3) helicopter electromagnetic geophysical data Belton SFZ Hills Groups outcrop UTM coordinates anticline Mill Creek BFZ Geology outside 3-D (Smith and others, 2003); and (4) known major and syncline MCFZ model extent minor faults in the study area. This model reveals RRFF 472000 County boundaries the complex intersections of both major and minor 3255000 faults in the subsurface, and the impact these faults 34°20' CARTER JOHNSTON have on the continuity of the Trinity and Edwards 0 5 10 KILOMETERS aquifer-forming units. 0 5 10 MILES Body of rock that forms a hydrostratigraphic unit distinct hydrologic unit with Figure 3. Three-dimensional model of the north Seco Creek area of Medina and Uvalde Figure 5. Geology of the Arbuckle-Simpson aquifer’s Hunton anticline area, south-central Oklahoma (outlined in the flow of groundwater Counties, Texas (Pantea and others, 2008). Multiple fault structures (shown in red) blue), and its major and minor fault