The Arkansas River in southeast Colorado and southwest Kansas is one of the most saline rivers in the U.S. Most of the dissolved salts in the river water entering Kansas remain in the river corridor and contaminate the ground waters in the alluvial and High Plains aquifers. The Upper Arkansas River Corridor Study was formulated and conducted as a Kansas Water Plan project to evaluate the fate and transport of the dissolved solids that are contaminating the ground-water resources in the river corridor. The evaluation addressed needs described in the Kansas Water Plan for determining the relationships among river flow and quality, ground-water levels and quality, and water use in the corridor area in the Upper Arkansas River Basin.
The primary sources of the dissolved constituents in Arkansas River water are from soils and bedrock in Colorado. The dissolved salt concentration in the river water greatly increases across eastern Colorado as evapotranspiration from ditch diversion and storage systems consumes water, while the dissolved salts remain in the residual water. Dissolved solids contents in low flows of the Arkansas River can exceed 4,000 mg/L at the Colorado-Kansas border. The major dissolved constituents in the river water, in the order of decreasing mass concentrations that usually occur, are sulfate, sodium, bicarbonate, calcium, magnesium, chloride, and silica. Sulfate concentration has ranged from 700 to 2,600 mg/L and averaged between 1,900 and 2,000 mg/L during the last couple of decades. The range in chloride content has been about 40-200 mg/L during that period. The water is usually extremely hard. Fluoride and boron can exceed 1.0 and 0.75 mg/L, respectively the maximum limits recommended for irrigation water, in saline low flows. Selenium usually exceeds 5 µg/L, the chronic toxicity level for aquatic life. Nitrate concentrations are always relatively low (usually 1-3 mg/L as nitrate-nitrogen).
The salinity of the Arkansas River in eastern Colorado began to increase soon after the start of substantial irrigation diversions in the 1870's. Since the start of continuous records of both water quality and discharge at the Colorado-Kansas state line in 1963, the salinity of moderate to high flows has continued to increase. During the last decade, the salinity of the river water at the state border has decreased with increasing river discharge at flows greater than 200 cfs. However, even the highest flows that have occurred during the last decade have been saline.
After entering Kansas, the salinity of Arkansas River water does not change significantly as it passes through southwest Kansas. Most of the current spatial variations in the salinity within southwest Kansas are due to the differences in quality at the state line that are transferred downstream. The average river salinity in southwest Kansas prior to surface-water and ground-water irrigation in Colorado and Kansas decreased downstream as a result of dilution by freshwater discharge from the High Plains aquifer. Substantial declines in the water levels of the High Plains aquifer, particularly since the 1970's, changed the average flow condition from one of ground water discharging to the river to one in which river water recharges the aquifer. As ground-water level declines in the High Plains aquifer extended from the Kearny-Finney counties area eastward along the river corridor in southwest Kansas, the location where the dilution was significant slowly moved farther downstream. Today, the dilution point is east of Dodge City.
The increase in salinity of the ground waters in the river corridor derives primarily from infiltration of saline waters from the Arkansas River. The seepage occurs from the river channel into the alluvial aquifer and then into the underlying High Plains aquifer, and also from below irrigation canals, ditches and fields irrigated with the river water. Dissolved solids contents in ground waters unaffected by the river water are as low as less than 300 mg/L. These waters are primarily calcium-bicarbonate in chemical type. With increasing salinity, the water chemistry changes to sodium-sulfate type to reflect the river-water influence. The TDS concentration ranges to over 4,000 mg/L in ground waters affected by seepage of saline water from the river channel, ditches, and fields irrigated by the river water. Sulfate concentration ranges from less than 30 mg/L in the freshest waters to over 2,700 mg/L in the most saline ground waters. The chloride concentration is less than 10 mg/L in the freshest ground water and is usually less than 300 mg/L in the most saline water affected only by saline river water and ditch irrigation. Saline waters with chloride levels substantially greater than 300 mg/L or with relatively low sulfate/chloride ratios in comparison with other ground waters impacted by river water derive additional chloride from waste sources. The high calcium and magnesium contents of the ground waters made saline by river-water seepage make the waters extremely hard. Nitrate-nitrogen concentrations range from less than one mg/L to over 30 mg/L. There are both fresh and saline ground waters with nitrate-nitrogen levels exceeding the drinking water standard of 10 mg/L. The high nitrate in the ground waters is not from the Arkansas River but generally from agricultural sources.
In general, current sulfate contents of the Quaternary alluvial aquifer decrease eastward from Hamilton County through Ford County. The sulfate values typically exceed 2,000 mg/L in the ground water in the alluvial valley in Hamilton County. Sulfate concentrations in most of the alluvial aquifer from Kearny County through Finney County range between 1,500 and 2,000 mg/L. In Gray County, the area with 1,500-2,000 mg/L sulfate content narrows within the alluvial valley to be mainly near the Arkansas River. The band of 1,500-2,000 mg/L sulfate concentration in the alluvium along the river extends to near Dodge City. A zone of ground water with 1,000 to 1,500 mg/L sulfate concentration follows the alluvium near the river past Dodge City. In general, ground water salinity in the alluvial aquifer near the edges of the alluvium in Gray and Ford counties is substantially less than near the river.
The present sulfate concentration in the High Plains aquifer is greater than 1,000 mg/L in the ground water underlying most of the Quaternary alluvium in Kearny County and under substantial parts of the alluvial aquifer in Finney County. The area with ground waters containing over 1,000 mg/L sulfate extends into part of the ditch irrigation service area to the north of the alluvial valley and east of the Amazon canal in Kearny and Finney counties. South of the Arkansas River in eastern Kearny and western Finney counties, an area of elevated sulfate concentrations (greater than 100 mg/L) extends south of the alluvial aquifer boundary. Essentially all the saline water in the High Plains aquifer in the Arkansas River corridor in Gray and Ford counties underlies the Quaternary alluvium. A band of ground water with greater than 500 mg/L sulfate content extends from southeast of Garden City through most of Gray County. In Ford County, only isolated areas of the High Plains aquifer (primarily in the Dodge City area) contain greater than 500 mg/L sulfate concentration. The ground water in the High Plains aquifer south of areas affected by the river water is fresh along the river corridor, with sulfate concentrations less than 50 mg/L. Ground water in the upper Dakota aquifer underlying the upper Arkansas River corridor is fresh.
Substantial thicknesses of clay layers underlying parts of the alluvial aquifer and within much of the High Plains aquifer retard the downward movement of saline water from the alluvium and irrigated areas. However, gravel packs of large capacity wells (mainly irrigation wells) without grout seals or in which the grout seals are not deep enough to seal off shallow saline water can allow flow across the clay layers. The wells allowing gravel-pack flow include actively used, plugged, and abandoned wells. Abandoned, unplugged wells with corroded casing can allow direct flow down the casing opening to the water table. Cross flow of shallow aquifer water to deeper zones can explain the substantial variations in salinity and nitrate concentrations observed for the High Plains aquifer.
The pumping of large-capacity wells in the High Plains aquifer after 1900 produced local, shallow cones of depression in the aquifers. The cones of depression resulted in saline alluvial water moving down into parts of the underlying High Plains aquifer. As increased numbers of high-capacity wells were installed in the High Plains aquifer, the local cones of depression in the water levels coalesced. During the 1970's, ground-water levels declined in most of the aquifer in the study area. The water-level declines became regional and dropped below the shallow zones of the High Plains aquifer. This caused a substantial increase in the movement of saline water from the alluvial aquifer into the underlying High Plains aquifer, and an increase in the movement of shallow saline water in the High Plains aquifer into the deeper aquifer. The regional declines in water levels in the High Plains aquifer also changed the direction of ground-water flow in the river corridor. Saline ground water began not only to move from the river into the alluvial aquifer and down into the underlying High Plains aquifer but also to migrate away from the river and alluvial valley.
After the 1970's, increasing amounts of river water seeped into the aquifers. The net baseflow additions to the river changed to net flow decreases even after accounting for the diversions for ditch irrigation. The river seepage is much greater downstream of where the High Plains aquifer underlies the alluvial aquifer (east of the Bear Creek Fault zone) than in the alluvial valley from the Colorado-Kansas border to western Kearny County. During the 1980's and 1990's, recharge from the ditch service areas, coupled with decreased pumping of ground water when ample supplies of river water were available for irrigation, increased ground-water levels in eastern Kearny and western Finney counties underlying the ditch irrigation areas north of the Arkansas River. However, the large water-level declines in the High Plains aquifer south of the river have produced an even greater hydraulic gradient from the river valley to the south than was generated in the 1970's. The observed salinity distribution in the High Plains aquifer in the river corridor indicates that local pumping effects are enough to cause a substantial increase in the travel distance of saline ground water.
The largest mass of sulfate that enters and leaves the corridor is in the river water. Mass budgets for sulfate in the river water provide a good estimate of the sulfate mass accumulating in the river corridor in southwest Kansas. The mass budget, coupled with a volume budget for ground water in the corridor, indicates that in about 50 years river water seepage has the potential to contaminate all of the High Plains aquifer underlying 500 mi2 (1,300 km2) of the corridor to a sulfate concentration exceeding 1,000 mg/L. This concentration is four times the recommended level for drinking waters and twice the level proposed by the U.S. EPA as a maximum concentration for public water supplies.
The recharge from the river channel and the ditch irrigation system to the underlying alluvial and High Plains aquifers exceeded 100,000 acre-ft during the late 1990's when the river flows were greater than average. The river water that recharges the aquifer, even if saline, is a valuable resource in terms of quantity. The recharge of the High Plains aquifer by river water seeping from the river channel and infiltrating below the ditch-irrigated areas is on a scale similar to other recharge projects in the United States costing many millions of dollars. As water treatment technologies become more cost efficient, desalinization of a local supply might become competitive with distant transport of freshwater if water levels continue to decline at substantial rates in the High Plains aquifer. The larger the flow from Colorado, the greater the potential recharge and the lower the salinity of the river water. Thus, management of water storage and releases in Colorado are important to the amount of aquifer recharge and contamination of Kansas aquifers.