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Water Resources, Dakota Aquifer

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Summary and Conclusions

The Dakota aquifer system underlies most of the western two-thirds of Kansas and includes sandstone units in the Cretaceous Dakota and Kiowa formations and Cheyenne Sandstone. In southwesternmost Kansas, the underlying Jurassic Morrison Formation, where hydraulically connected, is also considered by state statute to be part of the Dakota system. The Dakota aquifer is unconfined where the aquifer strata outcrop in the eastern portions of the aquifer and where these strata are overlain by alluvial aquifers. The aquifer is also considered to be unconfined where it is overlain by and hydraulically connected to the HPA in south-central and southwest Kansas. Most of the Dakota system in Kansas is overlain by Upper Cretaceous shales and limestones and is considered to be confined.

The Dakota aquifer has been developed as a water-supply source in areas where the groundwater is fresh or only slightly saline and where other more easily obtained water supplies are not available. A total of 2,237 wells with active water rights and active use as of the end of 2011 were determined to produce greater than 5% of their total yield from the Dakota aquifer. Most of these wells are located where the Dakota aquifer directly underlies the HPA in southwest Kansas. Most of the other wells are distributed within the outcrop/subcrop area of the Dakota aquifer from north-central, through central, to south-central Kansas. A much smaller number of wells produce from the confined Dakota aquifer in west-central Kansas. Thirty-nine percent of the 1,555 Dakota wells with water rights in southwest Kansas produce more than 50% of their yield from the Dakota aquifer. About 93% of the 581 Dakota wells in north-central and central Kansas produce all (or greater than 95%) of their yield from the Dakota aquifer. In the 36 counties in which water-right-permitted wells pump all or part of their supply from the Dakota aquifer, the Dakota wells are estimated to comprise 9% of the total of wells with water-right permits in all aquifers. Most (78%) of the water-right-permitted wells that draw part or all of their water from the Dakota aquifer are used for irrigation. Stock, municipal, and industrial wells comprise nearly all of the other uses (9.6%, 8.9%, and 2.2%, respectively) of the Dakota wells.

The mean annual volume of water used from the Dakota aquifer by permitted wells in Kansas is estimated to have been 117,000 acre-ft/yr (1.44 x 108 m3/yr) for the 5-yr period 2006-2010. The use was greatest in southwest Kansas (approximately 86% of the total Dakota aquifer use). The mean annual use for the other regions ranged from approximately 0.5% of the total Dakota use for west-central Kansas, to 2.4% for central, 2.9% for south-central, and 8.1% for north-central Kansas. The county with the greatest estimated annual use from the Dakota aquifer is Stanton County, followed in order of decreasing use by Grant, Hamilton, and Haskell counties, all of which use more than 10,000 acre-ft/yr (1.2 x 107 m3/yr). Counties in southwest Kansas with between 3,000 and 9,000 acre-ft/yr (3.7 x 106 and 1.1 x 107 m3/yr) of Dakota use are Ford, Morton, Kearny, and Stevens counties (in decreasing order). The only counties in the other regions of the Dakota aquifer that exceed 3,000 acre-ft/yr (3.7 x 107 m3/yr) of permitted use are Cloud (4,200 acre-ft/yr [5.2 x 106 m3/yr]) and Washington (3,800 acre-ft/yr [4.7 x 106 m3/yr]) counties. Although water use from the Dakota in north-central Kansas is much lower than in southwest Kansas, the percentage of Dakota use relative to total use from all aquifers is the highest (nearly 20%) of all regions. The percent Dakota use compared to total use from all aquifers for the other regions ranges from 5.2% for southwest, to 2.5% for central, to 2.0% for south-central, and to 0.4% for west-central Kansas.

About 90% of the mean annual use from the Dakota aquifer during 2006-2010 was for irrigation. Most of this use was in southwest Kansas. The use rates for stock and municipal purposes were each nearly 4% of the total volume pumped from the Dakota aquifer. However, municipal use accounted for 41% and 18% of the total use from the Dakota in central and north-central Kansas, respectively.

Variations in the annual rate of water use from the Dakota aquifer are related mainly to climatic conditions because irrigation, stock, and municipal use generally are substantially greater during droughts than during wet periods. For example, in north-central Kansas, almost eight times as much water was used during the drought year of 2002 (15,900 acre-ft/yr [1.96 x 107 m3/yr) than during the very wet year of 1993 (2,070 acre-ft/yr [2.55 x 106 m3/yr]). The relative range between dry and wet years is greater in north-central than in southwest Kansas because during wet years in north-central Kansas, precipitation is completely or nearly sufficient for crop irrigation and lawn growth.

Water use was also estimated for "domestic" wells, which are not required to obtain a permit. The total number of "domestic" wells currently producing most or all of their water from the Dakota aquifer in Kansas is estimated to be more than 11,000 (8,000 for north-central and central Kansas and 3,200 for south-central, west-central, and southwest Kansas). This is about 4.7 times the estimated number of permitted wells that obtain some or all of their source from the Dakota aquifer.

Water use from the Dakota aquifer by "domestic" wells is estimated to be 4,800 acre-ft/yr (5.9 x 106 m3/yr) in central Kansas, 1,500 acre-ft/yr (1.9 x 106 m3/yr) in north-central Kansas, and a combined total of 1,700 acre-ft/yr (2.1 x 106 m3/yr) for south-central, west-central, and southwest Kansas. The estimated use by "domestic" wells ranges from nearly two-thirds of the total water (both permitted and domestic uses) pumped from the Dakota aquifer in central Kansas to only 1% in southwest Kansas. The total "domestic" use (about 8,000 acre-ft/yr [1.0 x 107 m3/yr]) is 6% of the approximately 125,000 acre-ft/yr (1.54 x 108 m3/yr) pumped from the Dakota aquifer by both permitted and domestic wells in Kansas.

The processes of mixing, reactive cation exchange, and mineral dissolution and precipitation have produced a complex range of chemical characteristics for groundwater in the Dakota aquifer. Water quality in the aquifer ranges from very fresh (less than 300 mg/L TDS) to saltwater (greater than 10,000 mg/L TDS). Freshwaters in the outcrop and subcrop portions of the Dakota aquifer in north-central and central Kansas are usually calcium-bicarbonate type waters. Weathering of pyrite in shales in Dakota system strata and concomitant dissolution of any calcite or dolomite present can cause local increases in sulfate, calcium, and magnesium concentrations in Dakota groundwater. Recharge to the Dakota aquifer of groundwater in upper Cretaceous confining strata that was affected by these same processes or by dissolution of gypsum can increase the sulfate, calcium, and magnesium contents of Dakota groundwater. Calcium-sulfate type water can result.

Large areas of the Dakota aquifer contain saline water that is derived from the upward intrusion of saltwater from underlying Permian units, especially the Cedar Hills Sandstone in central and north-central Kansas. Geochemical data indicate that past seawater incorporated in the Dakota sediments has been flushed by groundwater flow. The saltwater intrusion from Permian units is derived from the dissolution of evaporite deposits containing rock salt (halite). Sodium and chloride are the main components of saline water in the Dakota aquifer.

Flushing of saline water from the Dakota aquifer is accompanied by the generation of sodium-bicarbonate groundwater. As freshwater of calcium-bicarbonate or calcium-sulfate types replaces the saline water, exchange of the calcium (and magnesium) in the freshwater for sodium on clays in Dakota strata decreases the calcium (and magnesium) concentration and increases the sodium concentration in the aquifer water. The decrease in calcium and magnesium is accompanied by the dissolution of calcite and dolomite in the sediments, resulting in the increase in bicarbonate that is formed from the dissolved carbonate. Natural softening of the groundwater occurs and sodium-bicarbonate groundwater can be formed in the Dakota aquifer. An increase in the pH of the water is associated with this process.

Fluoride concentrations exceed the secondary MCL for public drinking water (2 mg/L) in parts of the confined and small portions of the unconfined Dakota aquifer and exceed the MCL (4 mg/L) in some areas of the confined aquifer. The high fluoride is derived primarily from the dissolution of calcium minerals that contain fluoride during the generation of sodium-bicarbonate type water. About 10% of the sample records for the Dakota aquifer exceed the MCL for arsenic and the action level for lead; some of the high lead values could be related to lead in plumbing systems and not to natural levels in the Dakota aquifer. Uranium concentration and the radioactivity from radium isotopes and alpha particles exceed the MCL for public drinking waters in a small percentage of Dakota groundwaters. Many other natural constituents and properties in Dakota waters exceed recommended or suggested levels for drinking water, such as TDS, chloride, sulfate, iron, manganese, and ammonium ion concentrations, especially in saline water in the confined aquifer and in groundwaters that have chemically reducing conditions.

The main contaminant from anthropogenic activities in Dakota groundwater is nitrate. Nitrate-nitrogen concentrations exceeding the MCL of 10 mg/L primarily occur in shallow wells in the unconfined aquifer in central and north-central Kansas. Human sources of nitrogen include fertilizer and animal and human waste. Nitrate in or generated from the oxidation of nitrogen in these sources can enter groundwaters by infiltration of water through the unsaturated zone to the water table or by water flowing down the boreholes of wells, especially if the annular space between the well casing and borehole is not sealed or is poorly sealed.

The salinity of groundwater in the Dakota aquifer generally increases with depth. In the western areas of the outcrop/subcrop belt in south-central to north-central Kansas and the parts of the confined aquifer near the boundary with the outcrop/subcrop belt, topographic variations create local hydraulic heads in the Dakota aquifer that are higher than the water levels in the streams in the valleys. This can cause discharge of saline water from the regional flow system deeper in the Dakota aquifer to the stream valleys, creating saline water springs and seeps and increasing the salinity of streams and rivers. Steep salinity gradients can be present in the valleys adjacent to the streams. Thus, care must be taken when drilling wells too deep; shallow freshwater in the Dakota aquifer derived from surface recharge can be underlain by saline groundwater. The greatest salinity changes with depth occur across substantial shale units with appreciable lateral extent that confine or separate aquifer units.

Development of the Dakota aquifer has been dependent on both the hydrogeologic characteristics of the aquifer and the salinity of the groundwater. The substantial variability in both the hydrogeologic characteristics and water quality of the aquifer across Kansas strongly influences the suitability of the aquifer as a source for water supply. Another, more recent factor in the development of the Dakota is the decline in the water table in the HPA where it overlies the Dakota aquifer. Many new wells and wells drilled to replace permitted wells that were completed only in the HPA have been completed in both the HPA and underlying Dakota strata, thereby increasing the percentage of wells completed in both aquifers. An increasing trend has occurred since the latter 1980s in the number of wells with active water rights that have been completed partially or solely in the Dakota aquifer in southwest Kansas. Development is also continuing in the eastern outcrop/subcrop zone of the aquifer.

The KGS, as part of its work on the Dakota Aquifer Program, identified an area of additional nearly fresh to slightly saline waters in upper Dakota strata that could be important for future supplies. The aquifer area is triangular in shape with its base along the south lines of Sheridan and Graham counties and extending into south-central Norton County. Most of this area was previously believed to have waters of greater salinity. Development of this area, as well as other confined regions of the aquifer with greater depths to aquifer units and more saline water than in other parts of the aquifer, could occur in the future depending on water demands for agriculture, municipalities, and industry. Decreases in the cost of desalinization processes could make water use more possible from these areas of the aquifer for municipal and industrial uses.

Water-level data for the confined Dakota aquifer that underlies the HPA near where it is hydraulically connected to the HPA, suggest that water levels in the Dakota, which may have been either slightly above or below those in the HPA during pre-development, are now lower than in the HPA and may be declining at a greater rate than in the HPA. Water levels in the outcrop/subcrop belt of the Dakota, which drop during seasonal pumping, generally recover during the winter, and long-term levels have been relatively stable. Long-term water-level measurements in the Dakota are very limited in the region where it is hydraulically connected to the HPA.

Installation of a well screened solely in the Dakota aquifer and equipped with a continuously recording pressure transducer would be especially valuable for assessing conditions at and future prospects for the index/calibration well site in Haskell County (Butler et al., 2012). Characterization of water levels in the HPA at this site has improved understanding of aquifer responses to pumping and the HPA zones that contribute water to that pumping. For example, a leaky aquifer response has been observed in confined sands and gravels at the base of the HPA at this site (Butler et al., 2013). Continuously monitored water levels from a Dakota observation well would help determine whether most of the leakage to the HPA confined zone is from overlying HPA sediments or underlying Dakota strata. In addition, such water-level data could indicate whether leakage was currently occurring from the HPA to the Dakota or whether that leakage could start as more pumping stress was placed on the Dakota aquifer in the future. Similar monitoring of water levels in the Dakota aquifer elsewhere in southwestern Kansas would be extremely helpful in assessing the Dakota-HPA relationship.

Part of the water use decrease in the Dakota aquifer in Ford and Hodgeman counties from 1975-1982 to 2006-2010 was from decreased yield or abandonment of wells even though long-term water levels appear to have been generally stable across these periods. A probable, important contributing factor was greater maintenance costs for Dakota aquifer wells than for the typical unconsolidated aquifer wells due to the often high iron and manganese concentrations and sometimes corrosive characteristic of Dakota groundwater. As the value of water increases with increasing prices for agricultural products, thereby allowing for greater well maintenance costs, the potential exists for redevelopment of the Dakota in parts of Ford and Hodgeman counties.

This bulletin represents an important step for the assessment of the water resources of the Dakota aquifer in Kansas. Additional efforts recommended for assessment of the future potential of the aquifer include improved determination of the percentage of the Dakota water in mixed aquifer completions for existing wells, updating the percentages for new and replacement wells with water-right permits, and continued estimation of water use based on these percentages and reported water use from the DWR. Further work is needed to determine the depths to which the Dakota aquifer system is usable in southwest and west-central Kansas. Some deep wells in southwest Kansas where the Dakota and High Plains aquifers are hydraulically connected have depths that indicate they penetrate though the entire Dakota system at those locations. This implies that the water in the lower Dakota at those sites is not so saline as to render the well water unusable. However, this is based on water use from the well and not analyses of water samples. Sampling of selected deep wells that obtain a substantial percentage of their yield from the Dakota aquifer in Suitability Area I in southwest Kansas is recommended for further addressing this question. Finally, a selected group of wells across the Dakota in southwestern Kansas should be equipped with continuous monitoring equipment so that a better understanding of the relationship between the Dakota and the overlying HPA can be obtained.

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Kansas Geological Survey, Geohydrology
Placed on web Nov. 12, 2014; originally published Fall 2014.
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