Aquifer Characteristics

Geologic Properties

The Central Oklahoma (Garber-Wellington) Aquifer is hosted in two Permian-age geologic formations: the Garber Sandstone and the Wellington Formation. These units consist of interbedded sandstone, siltstone, mudstone, and shale, deposited in a mix of fluvial, deltaic, and tidal environments (Mashburn et al., 2019). This depositional history has produced a complex and variable subsurface structure that strongly influences groundwater flow and storage.

The Garber Sandstone, which forms the upper portion of the aquifer, is composed primarily of fine- to medium-grained, moderately to well-sorted sandstone, with interbeds of siltstone and mudstone. The sandstone units exhibit moderate to strong cementation, primarily by silica and calcite, which reduces porosity and restricts flow in some areas. The underlying Wellington Formation has a greater proportion of mudstone and siltstone, and fewer, thinner sandstone beds that are generally less hydraulically conductive (Mashburn et al., 2019).

The aquifer is highly heterogeneous, with abrupt vertical and lateral changes in lithology. Sandstone layers are often discontinuous and separated by lower-permeability mudstone, leading to anisotropic flow behavior—groundwater moves more easily in horizontal directions than vertically. This internal variability strongly influences local well yields and recharge patterns (Mashburn et al., 2019).

Porosity in the aquifer is primarily associated with the sandstone units and reflects primary intergranular pore space. The finer-grained mudstones and shales have significantly lower porosity and restrict flow between water-bearing zones. Permeability is highest in the coarser sandstone units, where hydraulic conductivity ranges from 0.3 to 7.1 feet per day, with a median value of 1.5 feet per day, based on aquifer test results from wells deeper than 500 feet (Mashburn et al., 2019).

Although fractures occur in the bedrock, they are not considered a dominant control on groundwater movement. Regional flow is instead governed by the distribution, continuity, and thickness of the sandstone beds, which provide the main pathways for water movement within the aquifer system (Mashburn et al., 2019).

Hydrologic properties

The aquifer exhibits a range of hydrologic conditions depending on location and depth. Near the land surface, particularly where the aquifer is exposed or overlain by permeable materials, it behaves as an unconfined system. At greater depths, and in areas where it is overlain by finer-grained units such as the Hennessey Group, it transitions to a confined or semi-confined aquifer (Mashburn et al., 2019; OWRB, 2019).

It is composed primarily of the Garber Sandstone and Wellington Formation—both heterogeneous sequences of interbedded sandstone, siltstone, mudstone, and shale. These formations result in variable aquifer properties across the basin. Saturated thickness typically ranges from 100 to 600 feet, exceeding 700 feet in some locations, especially in central and eastern Cleveland County (OWRB, 2019).

Hydraulic conductivity values reported from aquifer tests range from 0.3 to 7.1 ft/day, and transmissivity values span from 20 to 900 ft²/day. A test conducted near Norman produced a hydraulic conductivity of 2.4 ft/day and a transmissivity of 220 ft²/day (Mashburn et al., 2019). Storage coefficients are typically low, between 0.0001 and 0.0003, consistent with confined conditions, although localized variation is observed.

The aquifer is also characterized by significant heterogeneity and anisotropy due to changes in grain size, lithology, and bedding. Mudstone-rich intervals reduce vertical permeability, impeding downward flow and enhancing horizontal flow in more permeable sandstone layers. These physical characteristics influence groundwater movement, recharge distribution, and well yields throughout the aquifer system (OWRB, 2019; Mashburn et al., 2019).

Natural Chemical Character of Groundwater

Groundwater in the Central Oklahoma (Garber-Wellington) Aquifer displays significant geochemical variability that reflects both geologic setting and flow system position. The aquifer includes the Garber Sandstone, Wellington Formation, and underlying Permian units such as the Chase, Council Grove, and Admire Groups. Most of the system is unconfined, though it becomes confined in the west by the Hennessey Group (Christenson & Carpenter, 1992).

The dominant water types vary across the aquifer. Calcium-magnesium bicarbonate water is commonly found in the unconfined parts of the Garber Sandstone and Wellington Formation. In confined zones and deeper Permian units, the water transitions to sodium bicarbonate and, at greater depths, to sodium chloride. Sulfate-rich water is associated with the Hennessey Group and may also occur in zones below it (Christenson & Carpenter, 1992). These spatial chemical transitions reflect natural geochemical processes including mineral dissolution, ion exchange, and mixing with pre-existing brines.

Key geochemical reactions influencing groundwater composition include:

• Uptake of carbon dioxide from soil respiration in the unsaturated zone

• Dissolution of dolomite and calcite, especially where carbonate minerals are present

• Cation exchange, where calcium and magnesium are exchanged for sodium on clay minerals

• Dissolution of gypsum, particularly within the Hennessey Group

• Mixing and dispersion of fresh water with older sodium chloride brines (Christenson & Carpenter, 1992)

These reactions occur to varying extents depending on depth, mineralogy, and flow path. Shallow groundwater typically has pH values between 6.0 and 7.25 and is often undersaturated with respect to dolomite and calcite. Deeper groundwater, especially where in equilibrium with dolomite, reaches higher pH levels around 7.5 to 8.5 and exhibits a more evolved chemical signature (Christenson & Carpenter, 1992). In confined parts of the aquifer, cation exchange reduces calcium and magnesium concentrations while increasing sodium, especially in clay-rich zones.

Elevated sulfate concentrations in some wells are attributed to gypsum dissolution, particularly in the Hennessey Group. Chloride concentrations tend to increase with depth and are linked to the dispersion of pre-existing sodium chloride brines trapped within or beneath the freshwater system. Bromide-to-chloride ratios support the hypothesis that these brines are marine in origin (Christenson & Carpenter, 1992). These patterns reflect the long residence time and complex geochemical evolution of groundwater within the aquifer system.

In addition to major ions, groundwater in parts of the Garber-Wellington Aquifer contains naturally elevated levels of trace metals such as arsenic, chromium, selenium, and vanadium. These elements originate from the weathering of aquifer materials—particularly mudstones and shales—and are most commonly detected under reducing conditions in the confined portions of the aquifer. In some locations, concentrations of arsenic and chromium have been observed at levels that exceed EPA drinking water standards (Parkhurst et al., 1994). These trace constituents represent a key consideration for water quality and treatment planning in deeper wells.

Relationship to the Unsaturated Zone

The unsaturated zone plays an important role in controlling the chemical composition of recharge entering the Central Oklahoma (Garber-Wellington) Aquifer. As precipitation infiltrates through the unsaturated zone, it absorbs carbon dioxide produced by microbial activity and root respiration. This uptake increases the partial pressure of CO₂ in recharge water—typically between 0.01 and 0.1 atmospheres—which is significantly higher than atmospheric levels. Stable carbon isotope data confirm that this CO₂ originates within the unsaturated zone rather than from atmospheric or deep subsurface sources (Christenson & Carpenter, 1992).

The elevated CO₂ concentrations result in increased acidity, which enhances the dissolution of carbonate minerals, especially dolomite. This process begins in the unsaturated zone and continues into the upper saturated zone, leading to the development of calcium-magnesium bicarbonate water—typical of the unconfined portions of the aquifer. These reactions strongly influence groundwater pH, buffering capacity, and initial ion composition (Christenson & Carpenter, 1992).

Importantly, geochemical evidence suggests that the unsaturated zone is the primary source of carbon dioxide influencing groundwater composition. There is no indication of additional CO₂ production deeper in the aquifer, either from organic matter degradation or upward migration. This highlights the role of unsaturated-zone processes as the initial driver of groundwater chemistry before recharge reaches the saturated zone (Christenson & Carpenter, 1992).