Estancia Basin
Geology and Groundwater
A groundwater community
Thirty miles east of Albuquerque, agricultural communities in the Estancia basin survive on a limited groundwater resource. Since the mid-1900s, farming families have depended on large-scale groundwater pumping to supply their crops, while ranchers have, for even longer, relied on windmill-driven groundwater wells to keep their livestock watered. As the Albuquerque metro area continues to grow, communities along the east flank of the Manzano Mountains will need more and more groundwater supplies as well.
However, with scarce rain falling on the valley (~14 inches per year) and only a little more falling on neighboring highlands, recharge to the aquifer is limited. With little rainwater coming into the aquifer, the extensive reliance on groundwater has taken a toll on the resource. Groundwater levels in parts of the central basin have declined over 80 feet since the 1950s, and the stored volume of water is estimated to have decreased by over a million acre-feet.
(data source: adapted from results of the statewide water assessment )
The survival of the communities depends on the survival of the groundwater resource, but managing a resource that cannot be seen or measured directly is wrought with challenges. Since we cannot directly observe the groundwater system, we instead need to model the system, by first constructing a conceptual idea for how and where water flows through the subsurface from recharge areas to lakes, streams, springs, and wells. Constructing such a model begins with first understanding the geology.
Geology
Understanding the groundwater of an area requires collecting and interpreting the surface geology and by using subsurface well data. The surface geology of the Estancia basin consists of rocks and sediment as old as about 1.7 billion years old.
(data source: adapted from the Geologic Map of New Mexico )
Precambrian
older than 541 million years
The oldest rocks are crystalline basement rocks found in the Rattlesnake and Pedernal Hills and the west flank of the Manzano Mountains. Precambrian rocks in this region are composed largely of metamorphic rock types, having been subject to intense heat and pressure through millions of years of history. These rocks are generally poor producers of groundwater, but locally yield water from highly fractured areas.
Pennsylvanian
323 to 299 million years old
Pennsylvanian-age limestones, shales, and sandstones lie on top of the Precambrian rocks along the east flank of the Manzano Mountains and tilt eastward under the Estancia valley floor. These rocks formed in or near to ancient oceans that flooded much of western North America about 300 million years ago. In some localities, the limestones have caves and open fractures that can yield substantial amounts of groundwater.
Permian
299 to 260 million years old
Permian-age mudstones, sandstones, and limestones ring the southern and eastern areas of the basin. As Pennsylvanian-aged oceans gradually retreated through Permian time, Permian rocks accumulated in many settings, from oceans to beaches to river floodplains. The Permian-aged Glorieta Sandstone and sandstones in the Yeso Group produce water in the center and southwestern part of the basin, respectively, but good water production is localized. Most Permian rocks are dry or poor water producers.
Triassic to Cretaceous
247 to 75 million years old
Triassic to Cretaceous sandstones and mudstones are found at the north end of the basin, east of the basin, and along some fault zones. The older Triassic and Jurassic rocks were deposited in continental settings like floodplains and dunefields, while the younger Cretaceous rocks accumulated within and along the margins of another shallow sea. These rocks locally produce water in fractured zones, but are generally poor water producers in this area.
Early to Middle Tertiary igneous rocks
48 to 28 million years old
Tertiary crystalline rocks occur to the northwest and southeast of the basin in the San Pedro and Gallinas Mountains, where they intrude into older sedimentary rocks. These rocks are only sporadically found through the area, associated with a nearly statewide north-trending belt of intrusive rocks that extends southward into Texas. These rocks are largely poor water producers but may yield water in well-fractured zones.
Mid-Tertiary to Quaternary sediments
<~ 20 million years old to recent
Finally, the basin floor is blanketed by young sands, gravels, and muds washed out of the surrounding highlands that have accumulated in the valley. These deposits are usually uncemented and have open pore space that are capable of carrying and producing substantial groundwater.
Subsurface geology
The surface geology also reflects the subsurface, three-dimensional geology. With help from drill hole data, where available, the surface geologic map can be used to interpret the nature of the subsurface.
A three-dimensional model for the central portion of the Estancia basin has been constructed that provides a window into the subsurface geology of the basin.
Crystalline basement rocks in the Manzano Mountains underlie the entire area at depth. Pennsylvanian and Permian rocks overlie the basement as an eastward-thickening wedge, with each group of rocks enlarging to over 1500 ft thick before thinning again eastward over the uplifted basement rocks underlying the eastern margin of the Estancia valley. Relative to the bedrock, the Quaternary sediments along the valley floor are a thin skiff, nowhere thicker than about 400 ft. This basin fill material (thin yellow unit on image) is the primary aquifer in the region.
From south to north, the Permian and Pennsylvanian rocks below Chupadera Mesa thicken and thin beneath the valley floor, locally pinching-out over some basement rock highs. At the very north end, Permian and Mesozoic rocks thicken northward in the subsurface, with each group of rocks enlarging to over 2000 ft thick. Most of this complexity in the subsurface is hidden by the thin blanket of Quaternary sediments.
Well Data
Well data provides information on where good quality, accessible groundwater occurs in a basin. Over 9,000 well records (shown here as blue points) exist for the Estancia basin, concentrated between Estancia and Edgewood. General trends in groundwater resource location and aquifer quality can be established by examining available depth-to-water, well depth, and estimated production rate data.
Depth to Water
Contours are feet below ground surface
Groundwater is shallowest along the valley floor, specifically along Arroyo de Manzano and around isolated playa lakes in the southeast part of the basin. Groundwater is deepest in the northeast portion and southern end of the basin. Topography, geologic setting, and precipitation patterns mostly control these trends. Groundwater originates in the Manzano Mountains as rainfall and snowmelt, then flows downslope to the east to the center part of the basin. In the drier bedrock of Chupadera Mesa, the Pedernal Hills, and the northeastern mesas, groundwater occurs much deeper in fractured bedrock aquifers.
Well Depth
Contours are feet below ground surface
A map of typical well depths shows similar trends, with shallow wells common along the valley floor and Arroyo de Manzano, and deeper wells along the southern and northeastern margins of the basin. The depth to which a well is drilled is determined by several factors, including the depth to water, the thickness of the freshwater zone, the permeability of the aquifer, and the purpose of the well. Along the valley floor, shallow groundwater is available from productive sediment aquifers, and deep wells are unnecessary. In bedrock areas, however, deeper wells may be needed to tap certain productive water-bearing units, or to compensate for poor production rates by providing a longer screened interval.
Estimated Well Production Rates
Contours are gallons per minute
Aquifer productivity can be estimated from driller’s well production rate estimates. These estimates indicate high production wells occur along the valley floor, while the lowest yields occur in the bedrock areas along the southern, western, and northeastern margins of the basin. These trends reflect the geology of the aquifer units occurring through the area: the loose sediment aquifer along the valley floor is highly permeable and results in high-yield wells, while the fractured rock aquifers beneath Chupadera Mesa and in the Manzano Mountains are substantially less permeable and result in lower production rate wells. Some particularly well-fractured, or possibly karstic (cavernous), rocks appear to provide ample production rates, however.
Groundwater
Within an aquifer, groundwater is a continuous, connected, slow-flowing volume. How that resource travels through the subsurface is determined by the geology of the aquifer. Geologic data and well data are together used to determine groundwater flowpaths, from recharge areas to natural discharge areas and groundwater wells.
Contour lines are water table elevations (in feet above mean sea level)
Groundwater Flow
Just as surface water flows downhill, so too does groundwater flow “downhill,” flowing from higher water table elevations to lower water table elevations. This map shows contours of the elevation of the water table determined from the depth-to-water well data. Based on the water table elevations in the Estancia basin, groundwater flows mainly toward the playa lakes at the center and southeastern ends of the basin. This flow direction is to be expected: rainfall and snowmelt in the Manzano Mountains are likely sources of aquifer recharge, and groundwater naturally flows from recharge areas to discharge areas. The playa lakes are natural discharge points, where groundwater returns to the surface to evaporate back to the atmosphere. Groundwater most likely flows mainly through sediment aquifers occurring along stream channels and the valley floor, although some groundwater flows through fractured bedrocks, particularly the Pennsylvanian limestones.
Hydrogeologic model
The base of known, utilized groundwater resources can be estimated from well depth data, and the most productive aquifers can be determined from the geology and production rates. Use the slider bar to overlay the known extent of usable water on the subsurface hydrogeology. Groundwater resources are most commonly found in the basin-floor sediment and the locally-cavernous Pennsylvanian limestones, but can also be found in fractured zones in the less permeable Permian rocks.
Hydrogeologic models provide the framework for understanding, predicting, and developing strategies to address hydrologic challenges. Models illuminate the groundwater flow paths and connections between recharge areas, discharge areas, and wells, and can be used to predict the effects of future resource development, droughts, or of resource conservation strategies. Understanding the subsurface geology and its controls on groundwater flow and occurrence, therefore, provides for informed resource management.
Groundwater is a precious shared resource that much of New Mexico depends on, especially in the Estancia valley. Balancing current needs and proposed developments against the limits of the resource requires careful study, community discussion, and informed management strategies. It is no small task, but with adequate study and planning we can achieve this balance and prolong the resource for future generations.
