The Arkansas River in southeastern Colorado and westernmost Kansas is one of the most saline rivers in the United States, and contains more than twice the average dissolved-solids concentration of the river near Hutchinson, Kansas, where the river receives saltwater intruding from underlying bedrock. Consumption of water by evapotranspiration in Colorado has substantially decreased the flow and greatly increased the salinity of the river water entering Kansas. Ground-water levels have declined in the High Plains aquifer in southwest Kansas from decreased recharge from the river and consumptive pumping from the aquifer. Not only have ground-water supplies declined, but also Arkansas River flow entering Kansas from Colorado is lost in the river stretch from the state line to Dodge City. The amount of saline river water entering the subsurface is large. During 1988-1990 the mean total annual flow of the river was over 150,000 acre-feet with an approximate average of 3,600 mg/L of total-dissolved solids. Most of this entered the subsurface. In addition to salinity, the concentrations of many other dissolved constituents are high. Thus, poor quality water is constantly infiltrating to and contaminating the ground water in the alluvial aquifer. The water of the High Plains aquifer (primarily Ogallala aquifer) underlying the alluvium has also been found to contain saline water derived from downward infiltration of the alluvial ground waters. Ground-water declines in the main part of the High Plains aquifer have not only allowed downward leakage of alluvial ground water, but have also decreased the amount of fresh subsurface flow to the alluvium that can dilute salinity and other constituent concentrations. Nitrate concentrations have also been observed to be increasing in many well waters in the river corridor. However, the nitrate content of the river is not as high as the concentrations observed in some of the ground waters in the corridor.
Although the general character of the saline river water and ground-water contamination were known before this study, the understanding of the system was insufficient to determine clearly the water-quality variations in Arkansas River water, the fate and effects of the saline river water on the ground waters in the river corridor, and the links among decreased flow in the Arkansas River, increased levels of water contamination in the aquifers, and lowered ground-water levels. Prediction of future changes in the quality of the ground-water resources is dependent on a good comprehension of these relationships. An improvement in the knowledge of the system functions is necessary for forecasting the changes that would occur based on different water-use and management actions.
History and Description of the System
Before settlement of the Great Plains in the 1800's, Arkansas River flows carried dissolved solids primarily derived from Cretaceous bedrock in southeastern Colorado and southwestern Kansas near the state line. The dissolved solids inputs entered the river from tributaries and ground-water discharge. During low and moderate flows, the river waters were probably slightly saline as they entered Kansas. As the flows passed through southwest Kansas, they could either recharge to or receive discharge from the alluvial aquifer, depending on the amount of flow and the ground-water levels in the aquifer. High-flow conditions following dry periods would be times of greater recharge. However, during flood conditions, the Arkansas River waters were more dilute and could have recharged the alluvial aquifer with fresher waters. The bank storage would then have been returned to the river as discharge in later dry periods after partially mixing with waters in the alluvium. Water in the alluvial aquifer would have been diluted by fresh ground-water discharge from the High Plains aquifer. The amount of this discharge would have increased towards the east as recharge of the High Plains aquifer, as well as of the alluvial aquifer, became greater with the increase in average annual rainfall.
Beginning in the 1860's, farmers dug small ditches along the Arkansas River in Colorado to divert water for crop irrigation. Larger canals were then constructed for extensive diversion systems starting in the 1870's in Colorado and the 1880's in southwest Kansas. Evapotranspiration of the diverted waters increased the salinity of ground-water recharge from the fields and irrigation return flows. Studies in the 1960's and 1970's indicated that the dissolved-solids content exceeded 1,500 mg/L in the alluvium of the Arkansas River valley from the state line to Gray County and in parts of the High Plains aquifer adjacent to the alluvium. Part of the dissolved solids in ground waters underlying diversion ditches and the fields irrigated by the ditch waters was attributed to concentrated river waters. Ground-water levels rose in the vicinity of the canals and ditches due to leakage.
Large increases in ground-water withdrawals from pumping wells began in the 1950's. The consumptive use caused general decreases in subsurface water levels in large areas of southwest Kansas, including those adjacent to the Arkansas River. The Division of Water Resources declared a moratorium on application permits for water appropriations in 1977 along the Arkansas River corridor in southwestern Kansas due to the concern of coupled ground-water table declines and decreasing streamflows. The U.S. Geological Survey investigated the relationships, including simulation with computer models, and published the results in the early- to mid-1980's. In 1986, the Division of Water Resources established an Intensive Groundwater Use Control Area (IGUCA) along the Arkansas River valley from the Colorado state line to the Ford-Edwards county line east of Dodge City.
Irrigation from diversions of Arkansas River water in southeastern Colorado has increased the salinity of the water flowing into Kansas since the 1870's due to evapotranspiration concentration. Regulation of river flows by the John Martin reservoir beginning about 1950 further increased the salinity problem through increased diversions and consumptive loss by evapotranspiration in southeastern Colorado. The consumptive use also decreased total flows into Kansas, and the reservoir operation reduced the magnitude of more dilute high flows. The evapotranspiration caused increases in many dissolved constituents to undesirable levels for water use.
The total flow volume in the Arkansas River at Coolidge, Kansas, near Colorado state line averaged 152,000 acre-feet during water years 1988-1990 (October, 1987 to September, 1990). At an average total-dissolved-solids concentration of approximately 3,570 mg/L for this period, the mean annual load of dissolved solids is 640,000 metric tons. Most of the saline water contaminates the ground water in southwestern Kansas under present conditions because the amount of water flowing past Dodge City is a minor amount of the total flow when averaged over several years.
At the start of this project in 1995, the ground water in the alluvial valley was already saline and was thought to be decreasing in quality as suggested by studies of Southwest Kansas Groundwater Management District No. 3 (GMD3). The average dissolved-solids concentration of ground water at depths less than 150 feet in the IGUCA was in the range 2,000-2,300 based on data of the GMD3 and the KGS. Ground waters in the IGUCA at greater depths averaged about 1,500-1,700 mg/L. The additional, constant input of water that is more saline than the ground water existing in the IGUCA can only lead to contamination of water supplies as the saline water flows farther outwards and to greater depths in the system.
Depths to water in the alluvium are shallower than in the upland areas of the High Plains aquifer, resulting in faster response of the alluvial ground water to surface contamination than in the High Plains aquifer. Increasing nitrate concentrations in the alluvial aquifer do not appear to be mainly from infiltration of Arkansas River water entering Kansas because the river nitrate levels are generally lower than the ground-water concentrations. Nitrate contents are also increasing downstream of the area where the river waters have provided substantial recharge in the last two decades. Thus, nitrate increases appear to be related to other surface sources.
The overall goal of the project is to provide research and data that can be used to maintain the availability of freshwater supplies from the High Plains aquifer in the Arkansas River corridor between the Colorado state line and Dodge City.
At the beginning of the project in 1995 the basic objectives were derived from the water-quality and ground-water decline issues and objectives in the subsection on the Arkansas River Corridor Subbasin in the Upper Arkansas Basin section of the Kansas Water Plan:
- Water-Quality Issue: Document the fate and effects of contaminated Arkansas River flows on the alluvial, Ogallala, and Dakota aquifers in the river valley.
- Ground-Water Decline Issue: Clearly establish the links among decreased flow in the Arkansas River, increased levels of water contamination in the alluvial, Ogallala, and Dakota aquifers, and lowered ground-water tables.
During the project, Kansas developed state objectives for water resources for the year 2010 in the Kansas Water Plan. The 2010 objectives have been further modified during the last two years. This project is supplying research and data that will help evaluate whether Kansas is meeting the following 2010 objectives related to water quality in the Kansas Water Plan:
- By 2010, reduce the average concentration of bacteria, biochemical oxygen demand, dissolved solids, metals, nutrients, pesticides and sediment that adversely affect the water quality of Kansas lakes and streams.
- By 2010, reduce the average concentration of dissolved solids, metals, nitrates, pesticides and volatile organic chemicals that adversely affect the water quality of Kansas ground water.
- By 2010, ensure that water-quality conditions are maintained at a level equal to or better than year 2000 conditions.
The main component objectives include:
- Characterize the water quality of Arkansas River water entering and flowing in southwest Kansas and document the fate of the river water.
- Determine the extent of ground-water contamination in the Arkansas River corridor from the Colorado-Kansas state line to Dodge City, including the nature and location of the saline-freshwater transition zone.
- Evaluate the present and possible future migration of saline river water that has intruded to the High Plains aquifer and is contaminating fresh ground waters.
- Evaluate the implications of the results to possible management and protection strategies for ground-water availability and quality in the river corridor in cooperation with other agencies.
- Prepare data bases, geographic information coverages, publications, and a web information site for state and local agency, industry, and citizen use to achieve technology transfer and for educational use.