Report Title: Ground-Water-Quality Assessment of the Central Oklahoma Aquifer, Oklahoma: Results of Investigations
Geochemical investigations showed that calcium, magnesium, bicarbonate are the dominant ions in ground water from the unconfined parts of the Garber Sandstone and Wellington Formation. This water chemistry is the result of uptake of carbon dioxide from the unsaturated zone; dissolution of dolomite and to lesser extents, the dissolution of biotite, chlorite, plagioclase, and potassium fledspar; and precipitation of kaolinite. Sodium and bicarbonate are the dominant ions in the Hennessey Group, the confined part of the Garber Sandstone and Wellington Formation, and the Chase, Council Grove, and Admire Groups. The sodium bicarbonate water is derived from the calcium, magnesium, and bicarbonate water by cation exchange of calcium and magnesium with sodium in clay minerals.
The overall quality of water in the aquifer is quite good, based on the small number of sampled wells that exceeded existing water-quality standards. Maximum Contaminant Levels, which are related to public health, were commonly (more that 10 percent of wells) exceeded only for nitrate in wells shallower than 30 meters completed in Permian geologic units and for selenium in wells deeper than 30 meters completed in Permian geologic units.
Although the overall quality of water in the Central Oklahoma aquifer is good, water-quality problems occur in parts of the aquifer. Many public supply wells in central Oklahoma yield water with elevated concentrations of arsenic, chromium, selenium, and uranium. These elements are widely dispersed on solid-phase materials throughout the aquifer and are mobilized under certain geochemical conditions. The quality of ground water under the urban area was found to be significantly different from that of water outside the urban area. Pesticides and volatile organic compounds were much more common in water samples from wells in the urban area than from wells outside the urban area. The types of pesticides and volatile organic compounds found in water samples were consistent with the organic compounds known to be used in the urban area.
The Diagenetic History of Permian Rocks in the Central Oklahoma Aquifer
The abundance, composition, and reactivity of solids in the aquifer reflect the sediment source unit and alteration during deposition, burial, and exposure due to erosion. During deposition of the Permian sediment, soil processes dissolved some framework silicates and precipitated iron oxides, carbonate nodules, and kaolinite. Burial and incursion of evolved seawater resulted in precipitation of dolomite, barite, quartz overgrowths, additional iron oxides, and local accumulations of selenium, uranium, and vanadium. Tertiary and Quaternary erosion exposed rocks in the aquifer to recharge by rainwater. This dilute recharge has dissolved calcite, dolomite, feldspars, and chert, and has precipitated calcite, goethite, manganese oxides, and kaolinite.
Geochemical Characterization of Solid-Phase Materials in the Central Oklahoma Aquifer
Analytical results show that the highest concentration of most elements are in the drill-core samples and the lowest concentrations of the elements are in the outcrop samples. Comparison of geometric means for calcium and magnesium indicate that these elements have been leached from surficial materials. Factor analysis identified element associations directly related to residence sites of the elements and variations in the composition of the solid-phase materials that relate to water quality. In drill-core samples, arsenic is strongly associated with mudstone and samples with enhanced red coloration, indicating adsorption on clay minerals and iron oxides; chromium has a strong correlation with aluminum and, therefore, is strongly associated with high clay-content mudstone and siltstone; selenium and uranium, along with vanadium, are associated with white to pale-green samples that are indicative of reduction zones; uranium also is associated with high clay-content samples.
Two partial dissolution techniques were used to study partitioning of the elements among geochemical phases likely to be affected by environmental conditions. One technique, a two-step sequential procedure, was specifically designed to mimic naturally present high pH-high bicarbonate water and to assess the importance of oxidation processes. The second procedure sequentially extracted elements into five fractions (soluble, ligand-exchangeable, acid soluble, oxidizable, and residual). Appreciable amounts of arsenic, selenium, and uranium were extracted from some samples by the soluble and ligand-exchangeable extractions, indicating that significant portions of these elements reside in phases that are readily available. The extraction results did not clearly explain the sources of readily extractable chromium of the processes involved in chromium mobilization.
The analytical results and partial dissolution studies indicate that the concentration of trace elements in ground water is affected by oxidation, adsorption-desorption, and cation-exchange reactions at mineral surfaces.
Summary of Geochemical and Geohydrologic Investigations of the Central Oklahoma Aquifer
Median values were estimated for the following aquifer properties: transmissivity of the Garber Sandstone and Wellington Formation, 24 to 42 meters squared per day; horizontal hydraulic conductivity of sandstone in the study unit, 1.4 meters per day; ratio of horizontal to vertical hydraulic conductivity, 10,000; porosity of sandstone, 0.22; storage coefficient, 0.0002; recharge rate, 41 millimeters per year.
A finite-difference ground-water flow model was used in conjunction with a particle-tracking model to simulate flow of ground water in the Central Oklahoma aquifer. Results of these models combined with the results of the geochemical investigation show that flow in the Central Oklahoma aquifer has three major components: (1) a shallow, local flow system in the unconfined part of the aquifer with transit times of tens to hundreds of years, (2) a deep, regional flow system in the unconfined part of the aquifer with transit times as much as 5,000 years or greater, and (3) a deep, regional flow system in the confined part of the aquifer, with transit times ranging from thousands to tens of thousands of years. Hydrogeochemical processes have occurred to different extents within these three flow systems in the aquifer. In the shallow, local flow system, the rapid flux of water has been sufficient to remove most of the dolomite, calcite, and exchangeable sodium. In the deep, regional flow system of the unconfined part of the Garber Sandstone and Wellington Formation, the flux of water has been sufficient to remove most of the exchangeable sodium, but sufficient carbonate minerals remain to maintain dolomite and calcite equilibrium. In the confined part of the Garber Sandstone and Wellington Formation and in the less transmissive parts of the unconfined aquifer, including the Chase, Council Grove, and Admire Groups, ground-water flow is slowest, and the flux of water and extent of reactions have been insufficient to remove either the carbonate minerals or the exchangeable sodium on clay minerals.
Arsenic, Chromium, Selenium, and Uranium in the Central Oklahoma Aquifer
The Central Oklahoma aquifer is dominantly interbedded red-bed sandstone and mudstone of Permian age that gently dip west. Major and minor minerals in these rocks that affect water chemistry include mixed-layer illite-smectite, hematite, goethite, gypsum, and dolomite. Low-permeability rocks of the Hennessey Group confine the western third of the aquifer and overlie the sandstone-rich Garber Sandstone and Wellington Formation. Mudstone-rich rocks of the Chase, Council Grove, and Admire Groups underlie the Wellington Formation. Recharge to the aquifer is from precipitation, most of which enters the unconfined part of the aquifer and is discharged into nearby streams; a small part of the recharge enters near the aquifer center and flows along long regional flowpaths through the confined part of the aquifer to discharge into streams after thousands of years.
The base of the aquifer is the contact between fresh ground water and underlying sodium chloride brine that once filled the aquifer. Residual chloride from the brine and sulfate from dissolution of gypsum remain in mudstone-rich, poorly flushed regions of the aquifer. On the basis of spatial variations of water chemistry with depth, geologic unit, and confined or unconfined conditions, the aquifer is divided into six geohydrologic zones including: (1) shallow (less that 300 ft deep) unconfined Garber-Wellington, (2) deep (greater than or equal to 300 ft deep) unconfined Garber-Wellington, (3) shallow confined Garber-Wellington, (4) deep confined Garber-Wellington, (5) shallow Chase-Admire, and (6) deep Chase-Admire.
Concentrations of arsenic, chromium, selenium, and uranium in the ground water generally increase with depth in the unconfined zones of the aquifer and are the highest in samples collected from near the base of freshwater. Higher concentrations are near the aquifer top in confined zones of the aquifer. Water in the shallow zones of the aquifer typically has only low arsenic and chromium concentrations. Water from all three shallow geohydrologic zones contains high selenium and uranium concentrations. Shallow wells with water that exceeds the drinking-water standards for arsenic, chromium, or selenium in unconfined zones of the aquifer are near major streams where old ground water that has traveled along long regional flowpaths discharges, which indicates that such high concentrations develop along long flowpaths. Some shallow wells in unconfined zones of the aquifer with water that exceeds the proposed uranium standard are not near regional discharging streams, which indicates that high uranium concentrations can develop along short, local flowpaths. High concentrations of all four elements are common in the deep confined Garber-Wellington zone with the highest concentrations in the far western part of the ground-water flow system, in distal parts of regional flowpaths, and near the base of the confining layer.
High concentrations of solid-phase arsenic, chromium, selenium, and uranium were found in specific rock types. High arsenic concentrations are in yellow-brown goethite-cemented sandstone, high chromium concentrations are in mudstones and chromium-rich clays, and high selenium and uranium concentrations are in pale greenish-grey vanadium-rich reduced zones. High concentrations of all four elements also were associated with some red iron-oxide grain coatings in sandstone. Sequential extraction results indicated the elements are mobile in oxidizing, high-pH, bicarbonate type water.
Oxidizing conditions, which are indicated by concentrations of dissolved oxygen greater than 1 milligram per liter in most aquifer water, cause arsenic, chromium, selenium, and uranium to be in their relatively soluble, highest oxidation states. Spatial variations in redox conditions are related to mudstone distribution. Recharge through poorly drained clay-rich soils derived from the outcrop of the Chase, Council Grove, and Admire Groups and parts of the confining Hennessey Group is depleted of dissolved oxygen, which locally limits the mobility of chromium and selenium.
Distribution of mudstone in the aquifer also affects the major-element water chemistry as well as the abundance of dissolved arsenic, chromium, selenium, and uranium through decreases in permeability and increases in cation-exchange capacity. Low hydraulic conductivity in mudstones results in less flushing and high residual concentrations of soluble arsenic, chromium, selenium, and uranium in solid phases and ground water than in sandstones.
As water flows into and through the Central Oklahoma aquifer, dolomite and carbon dioxide gas dissolve, and sodium bound to mixed-layer clays is exchanged for dissolved calcium and magnesium from dolomite. This results in water types ranging from calcium magnesium bicarbonate to sodium bicarbonate. Where the water is isolated from soil carbon dioxide in the deep and confined zones of the aquifer, these reactions result in pH values greater than 8.5 that favor desorption of oxidized arsenic, chromium, and selenium from iron oxides. Increased carbonate-ion activity at high pH favors the mobilization of uranium by carbonate complexation. High concentration of selenium and uranium also are found in water with pH less than 8.0 in shallow unconfined zones of the aquifer where abundant mudstone interbeds create a clay-rich vadose zone. In this clay-rich vadose zone, cation exchange and dolomite dissolution occur in the presence of carbon dioxide, resulting in water with a near-neutral pH, but with a high alkalinity. The high alkalinity results in relatively high carbonate-ion activities that favor uranium mobilization. Oxidized selenium desorbs from iron-oxide surfaces at near-neutral pH.
High concentrations of arsenic, chromium, selenium, and uranium are common in specific regions of the aquifer. Individual sandstone layers yield water that exceeds the drinking-water standards for one or more of the elements, whereas vertically adjacent layers yield water of acceptable quality. Analysis of water samples collected from discrete sandstone layers through test wells before installation of public-supply wells could identify sandstone layers with high concentrations of the elements so that these layers can be isolated. Observation of test-hole drill cuttings and onsite measurement of sodium concentration, pH, and alkalinity may indicate which sand layers are likely to yield water that exceeds a drinking-water standard.