Open-file Report - 614
A Three-Dimensional Hydrogeologic Model from the Pecos Slope to the Southern High Plains, Southeastern New Mexico

Understanding and managing groundwater resources has become more important than ever, especially in New Mexico, which relies on groundwater more than any other state in the southwest. Accurate information on the depth to groundwater, water quality, and thickness of the state’s aquifers is needed for a variety of reasons, including: improving well drilling success, informing regional water planning, and reducing oil and gas well interference with fresh water zones. Currently, this type of data is found in regional maps, or buried in databases, and extrapolating the data to determine site specific conditions can be tedious and, oftentimes, inaccurate. The New Mexico Bureau of Geology and Mineral Resources is investigating the feasibility of using the robust suite of spatial analysis tools available in ArcGIS to create publicly available, three dimensional maps of the major aquifers in the state to fill this data gap. The southeastern portion of New Mexico, specifically the Pecos Slope region of the Sacramento Mountains and the lower Pecos River valley, was selected for this initial study of 3-D mapping due to its wealth of geologic data, including thousands of well logs from oil and gas exploration. The major aquifers in this region are divided up as the Pecos Valley Alluvium, the Southern High Plains Aquifer System, and the Permian Aquifer System. The water resources in this area are in high demand and under intense active management: groundwater is relied upon to support large agricultural and dairy/cattle industries, as well as a booming oil and gas industry; surface water use is strictly limited by the terms of the Pecos River Compact, further highlighting the need for this project.

This collection includes 3 parts. First, work by Colin Cikoski, provides an in-depth look at the feasibility of using ArcGIS to build a digital, three-dimensional hydrogeologic model. There are several 3D geologic modeling software packages, ultimately, however, ArcGIS was selected for this study due to its widespread use in several fields, the ability to utilize the final data with free or open source software, and the ease of importing the final data files into other modeling software. In addition to focusing on a model system that was widely available, the project was designed to create a model that would minimize modeler bias and maximize objective utilization of a wide variety of input data. The geologic surfaces were created through an iterative process of developing regional surfaces, comparing that surface to raw control points to calculate local deviance models, then incorporating the deviance model into the next iteration of the surface. Uncertainty was quantified through each of the stages of the model building and combined to create uncertainty maps. The results of Cikoski’s analysis show that the modeling method is unbiased and faithfully captures complex large and medium scale geologic structures. The model is reflective of the current understanding of the geology of the area and had reduced accuracy with increased distance from control points.

The second part of the collection, by Marissa Fichera and Ethan Mamer, is the application of the completed geologic model to calculate aquifer extents and volumes. Defining freshwater aquifer boundaries is more complicated than looking at the geologic contacts alone, as hydraulic properties and water quality often vary spatially within the same formation. To constrain these variables, water quality and water level information from the USGS, the New Mexico Office of State Engineer, and the Texas Water Development Board were compiled into a relational database and plotted in 3-dimensions. Water quality was broken into two zones, fresh to low-salinity brackish (below 3,000 mg/L TDS) and brackish (above 3,000 mg/L TDS). Water level surfaces were contoured from control points measured in wells between 2010 and 2019. After creating these water quality and water level zones within the geologic model, relevant hydraulic properties were applied to estimate current extractable volumes. The result of this analysis estimate the Pecos Valley Alluvial Aquifer volume of 4–6 Million Acre-Feet (Maf), Southern High Plains Aquifer System volume of 20–50 Maf, and Permian Aquifer System volume of 90–260 Maf.

The third part of this collection is an ArcGIS map package containing digital hydrogeologic model data, including raster surfaces, and contours of the subsurface elevations of geologic units and relevant hydrologic surfaces. Basal surfaces of the following geologic units are included in the map package: the Pecos Valley Alluvium, the Ogallala Formation, Lower Cretaceous strata, upper and lower Dockum Groups, upper and lower Ochoan strata, the Artesia Group, the San Andres Formation, and the Yeso Formation. Aquifer extents, water table elevation and saturated thickness contour maps, and estimated basal aquifer extents are also included.

While great care was taken to create an accurate and user friendly set of maps and model layers, this data should still be used with caution. The Pecos Slope model has a horizontal resolution of no more than 1km and is intended primarily for visualization and communication, and for regional or preliminary studies. Users of this model should be aware of the levels of uncertainty in each surface and conduct site specific studies, especially in structurally complex areas, as needed.

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