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Summary and Conclusions
Conversion from range land dryland farming to intensive cultivation under irrigation with water from Cedar Bluff Reservoir upset the natural equilibrium of the semiarid hydrologic environment of the area in and adjacent to the Cedar Bluff Irrigation District No. 6, west-central Kansas. Progressive changes in the chemical quality of ground water beneath the irrigated land and in water in the Smoky Hill River adjacent to the district depend on the composition and configuration of the geohydrologic system as well as on the composition, distribution, and consumption of the irrigation water.
The irrigated area is underlain by unconsolidated fluvial deposits of Pleistocene age that overlap and fill channels eroded in relatively impermeable limestone and shale of Cretaceous age. The Cretaceous rocks function chiefly as a lower boundary for ground-water circulation; however, included soluble minerals are a source of ions in the ground water.
The texture and related physical properties of the fluvial deposits and overlying soils vary widely, both vertically and laterally, but permeability is generally adequate to permit infiltration, deep percolation, and circulation of excess rainfall and irrigation water. Deposits of sand and gravel near the bedrock surface are the chief aquifers.
The water table in the valley-fill deposits under most of the irrigated land slopes gently to the south and east toward the Smoky Hill River. Erosional remnants of Cretaceous rock in some places retard the flow of water from the aquifer under the high terraces to the aquifer under the flood plain. Relief on the erosional surface affected the thickness and textural characteristics of the overlying sediments at the time of deposition, and is a significant factor in the thickness of the saturated section, the direction and rate of movement of ground water, and the distribution of waters of differing chemical composition.
During the period 1964-71, annual rainfall ranged from about 17 to 29 inches (43 to 74 cm). The mean was about 22 inches (56 cm), about 75 percent of which occurred during the growing season when consumptive use by crops exceeded rainfall. As the moisture deficiency was overcome by irrigation, an average of from 5 to 14 inches (13 to 36 cm) of excess water was available for leaching and recharge in the irrigated area. Part of the precipitation during the remainder of the year, as well as seepage from the irrigation canal and laterals, augmented the excess water available to percolate below the root zone and into the ground-water reservoir.
The specific conductance of the irrigation water generally increased progressively from about 800 to 980 micromhos per centimeter at 25°C between 1964 and 1971. Excepting a progressive increase in sulfate at the expense of bicarbonate, the relative proportions of the ions in the calcium-sulfate type water remained nearly constant. Sodium comprised less than 17 percent of the cations and the sodium-adsorption ratio (SAR) was less than 0.8. With moderate leaching, the water is well suited for irrigation of field crops in medium-textured soil.
Seasonal fluctuations in a generally rising water table reflected infiltration of excess water into and drainage from the aquifers beneath and adjacent to the irrigated land. Springs and marshes developed in some topographic depressions, but the water table under most of the irrigated land remained well below the root zone.
Based on rainfall, consumptive use, and the quality and quantity of the applied irrigation water, the average specific conductance of infiltrated water on the irrigated land should range from 1.2 to 3.2 times the specific conductance of the irrigation water. If seepage losses from the canal and laterals were included, the ratios would range from 1.1 to 1.9. To facilitate evaluation of the effect of the irrigation water on the quality of sampled water, the specific conductance and concentrations of the ions in well waters, soil extracts, and surface water are expressed as concentration ratios to corresponding quantities in the irrigation water.
The locally variable chemical composition of natural waters in the area was determined chiefly by soluble minerals in the soil and bedrock. With the addition of excess water from irrigation, the specific conductance of most of the well waters increased and calcium and sulfate became the predominant ions. Increases in the concentrations of sodium and chloride in many of the wells were disproportionately higher with respect to the other ions than would be predicted from simple mixing with irrigation water or from estimates based on consumptive use. Widespread distribution of significant amounts of chloride in transient storage in the aquifer suggested a more extensive and abundant source than the irrigation water or the known localized oil-field and livestock operations in the area.
Detailed information from three experimental plots that typify the varied geologic conditions beneath the irrigated land shows that the rate and magnitude of continuing changes in the quality of ground water under the irrigated area, and eventually in drainage to the river, are dependent on locally variable conditions that are not readily generalized. Annual applications of irrigation water exceeded the irrigation requirement for all three plots.
Plot 1, located near the northern boundary of the high terrace, is underlain by predominantly fine-grained colluvial and fluvial deposits with lenses of sand and gravel. The thickness of unconsolidated sediments increased from about 10 to 70 feet (3 to 21 m) in a southeastward direction. As a result of wide variation in the depth of the well screens and inhomogeneity of the aquifer, changes in water levels in the 14 observation wells were neither simultaneous nor of equal magnitude. Infiltrated calcium-sulfate type irrigation water evidently mixed with and displaced other types of water to the south and east. Tracer dye injected at the surface appeared in well waters down gradient, but residues of chlorinated hydrocarbon pesticides applied generously to crops remained in the upper foot of the soil profile.
Sodium and chloride in some well waters increased to concentrations far higher than in the irrigation water during the first two years of irrigation. Subsequent decreases, and convergence with time of the concentration ratios for the specific conductance and concentrations of most of the ions toward a common value less than two, indicated depletion of soluble salts in the soils and aquifers. The quantity and average concentration of chloride in the ground water decreased from 1965 to 1971, but the persistence of high concentrations of sodium in some well waters may represent continuing displacement of sodium from the fine-grained sediment by ion exchange for abundant calcium in the percolating waters. Significant increases in the concentrations of nitrate in 1969 probably reflect transport of fertilizer nitrogen below the root zone by excessive applications of irrigation water during the preceding growing season, but concentration ratios for potassium, another plant nutrient, were consistently less than one.
Plot 3 is located on the high terrace about 1.5 miles (2.4 km) south of and down the ground-water gradient from plot 1. The saturated zone lines entirely within relatively homogeneous sand and gravel that underlies 10 to 15 feet (3.0 to 4.6 m) of silt. From 1965 to 1971, the local water table rose about 5 feet (1.5 m) to a minimum recorded depth to water of 29 feet (8.8 m). In 1971, response of the water level to deep percolation in the center of the field lagged the start and finish of the irrigation by about 19 days.
The predominant ions in 11 closely spaced wells varied widely in 1965, but calcium and sulfate were the predominant ions in all the well waters by 1969. Progressive southeastward migration of water types, increases in specific conductance, and reduction of sodium-adsorption ratios in the well waters indicated infiltration to the aquifer of excess water carrying salts from the irrigation water and soil and the movement of ground water toward the river.
After 1968, the concentration ratios for specific conductance remained relatively constant between 1.5 and 2, but the ratios for chloride reached maximums of more than 10 (more than 260 mg/l) in 1969. Lack of a conspicuous tendency for the concentration ratios of the ions to converge toward a common value suggests that changes caused by irrigation in the plot are superimposed on and masked by changes caused by lateral migration of ground water from adjacent irrigated areas. Concentration ratios for sulfate and potassium consistently less than one indicate loss of those ions to plants and soil or dilution by waters containing lower concentrations of those ions than in the irrigation water. As in plot 1, leaching of fertilizer probably caused a significant increase in the concentration of nitrate in most of the well waters after 3 years of irrigation.
Plot 2 is located on a low terrace overlying a buried alluvial channel about 600 feet (180 m) north of the Smoky Hill River. Measured water levels fluctuated within a range of about 2 feet (0.6 m) at a depth of about 16 feet (4.9 m). Salts from the irrigation water and calcium and bicarbonate from the sandy alluvium seem to be the main sources of ions in the well waters. Other soluble salts probably were washed away before irrigation began. The concentration ratios for specific conductance ranged between 1.2 and 1.6, slightly lower than was predicted from estimates of rainfall and consumptive use. Concentration ratios for nitrate were variable, possibly as a result of the nitrogen-fixing activity of alfalfa. As was observed in the other plots, concentration ratios for potassium were less than one.
There was no evidence of significant leaching of sodium and chloride in the soil as was observed for the other plots. After 1968, the concentration ratios for chloride were less than one. The chemistry of the ground water beneath the plot may closely resemble the chemistry of ground water beneath the irrigated area (and drainage to the river) after prolonged application of irrigation water.
Particle-size analyses show that the soil profiles to a depth of 128 inches (325 cm) in plots 1 and 3 consist mainly of silt and clay and are similar to the soil profiles in adjacent nonirrigated areas. Chemical analyses of soil saturation extracts revealed leaching and translocation of soluble salts by percolating water. Calcareous zones in the profiles are continuing sources of calcium and bicarbonate in percolating water; therefore, the concentrations of those ions depend chiefly on the solubility of calcium carbonate. Concentration ratios less than one for specific conductance above the calcareous zone probably represent leaching by rainfall.
In plot 1, the concentration of soluble sodium in the extracts was only slightly lower, but the concentration of chloride was much lower in the irrigated than in the nonirrigated soil profile. In plot 3, high concentrations of sodium and chloride appear to have been displaced downward by excess water. High concentration ratios for nitrate near the surface of irrigated plots 1 and 3 reflect application of anhydrous ammonia; high ratios near the base of the sections probably represent fertilizer that was transported below the root zone by excess water.
Calcium, magnesium, and sulfate from the irrigation water apparently augmented the natural accumulation of salts in the upper part of the irrigated profile. At depths greater than 4 feet (1.2 m), the salt content of the irrigated profiles generally was less than for the corresponding nonirrigated profiles. If differences in the soluble salt content of the profiles actually represent leaching of salts by excess irrigation water, the concentrations of the drainage from plot 1 would have increased by about 240 mg/l for chloride and 40 mg/l for sodium during the period 1965-71. Comparable increases for plot 3 would be about 110 mg/l for both sodium and chloride.
The soluble cation content generally represented less than 10 percent of the extractable cation content of the profiles; the calculated increases for sodium based on differences in the extractable ion contents would be greater. Naturally accumulated salts leached from the soil profile apparently supplied enough sodium and chloride to account for the observed changes in the ground water beneath the plots.
The content of nitrate in the irrigated soil profiles was greater than for the nonirrigated profiles in all plots. In plot 1, the difference was equivalent to about 178 kilograms of nitrate (40 kg of nitrogen) per acre, mainly below the root zone. The increase, which probably is related to increased concentrations of nitrate in the well waters, may indicate excessive or poorly timed application of fertilizer, water, or both.
Because the approximately 6000 acres (2,400 ha) of irrigated land normally contributed an insignificant part of storm runoff from the 220 square miles (570 km2) of drainage area to the reach of the Smoky Hill River adjacent to the irrigation district, the effects of irrigation on the river were most conspicuous during periods of low flow. However, additional study of the quality of direct runoff from the district over a wide range of discharge is warranted.
Sixteen seepage-salinity surveys of the reach revealed progressive changes in the quantity and quality of low flow in the main stem, of tributary inflow, and of seepage directly into the main stem between 1964 and 1971. Accelerated drainage of ground water from the irrigated acreage progressively raised the concentration of chloride in a downstream direction to values far higher than chloride in the irrigation water. However, the concentration of sulfate generally decreased in a downstream direction from values higher to values lower than the irrigation water.
After 1965, net gains between successive stations during the surveys consisted mainly of ground-water discharge from the irrigated area north of the river. However, the rate of inflow per mile varied from reach to reach in response to local variations in the distribution of irrigation water and in the geohydrology.
Net gains in the irrigation reach that comprised from 30 percent of the inflow to the river between the fish hatchery and the downstream station (334.4) in 1964 to about 80 percent in 1967. From 1968 to 1971, the percentages ranged between 62 and 69 percent despite wide and relatively random variation in antecedent rainfall. The lowest net gain of 1.40 cfs (0.04 m3/s) was measured in October 1964. After 1967, net gains fluctuated between 8 and 10 cfs (0.23 and 0.28 m3/s) in the spring and fall, respectively, in response to fluctuations in the drainage of excess water from the irrigated acreage.
Net gains consist of measured tributary inflow plus net seepage. In April 1964, seepage was the major component of net gains. As water levels under the irrigated acreage on the high terrace rose in response to irrigation, tributary inflow comprised a larger percentage of net gains in the irrigation reach. In general, the concentrations of the ions in net seepage gains (excluding sodium) exceeded corresponding concentrations in tributary inflow. Net seepage losses represent unmeasured withdrawals of water or ions from the river or from tributaries downstream from data-collection stations. Cumulative net seepage losses for nitrate in the irrigation reach accompanied gains for water and the other major ions during all surveys. During the last four surveys, net seepage losses of from 45 to 53 kilograms of nitrate per day were recorded. Interception and consumption of more highly concentrated tributary inflow by phreatophytes on the flood plain and selective consumption of nitrate by terrestrial and aquatic plants probably caused the losses.
The chemical discharge in net gains to the irrigation reach (consisting mainly of irrigation return flow) increased from year to year between 1964 and 1971, but the rate of increase diminished as the system seemed to approach equilibrium. Calcium and sulfate were the predominant ions in the net gains, but the percent sulfate was lower and the percent sodium and chloride were higher than in the irrigation water. The specific conductance and concentrations of the ions generally increased progressively after 1964, but were not directly related to the rate of water discharge.
The specific conductance of water discharge in the irrigation reach increased from 880 to 900 micromhos per centimeter at 25°C in the fall of 1964. In the fall of 1971, the specific conductance in the same reach increased from 1,070 to 1,230 micromhos per centimeter at 25°C. The concentration ratio for specific conductance increased from 1.01 in 1964 to 1.15 in 1971.
Concentration ratios for calcium in net gains were similar to those for specific conductance, but the ratios were less than one for sulfate and potassium during 1965-71 and for magnesium during the last three years. Relatively small changes or declines in the disproportionately high concentration ratios for sodium and chloride after 1967 may indicate depletion of a source of those ions supplementary to the irrigation water. Readily apparent similarities of the changes in the chemistry of net gains to changes in the ground water show that the quality of low flow in the river depends to a large extent on the ions in transient storage in the aquifers receiving excess water from irrigation. Maximum concentrations of the ions in the ground water probably exceed maximums that will occur in the Smoky Hill River as a result of irrigation.
Chemical analyses of discrete samples representing a wide range of discharge at four stations on the river adjacent to the district generally substantiated information from the seepage-salinity surveys. The mean specific conductance at the three downstream stations and the specific conductance corresponding to a given rate of water discharge increased from year to year. Disproportionately high ratios for sodium and chloride and low ratios for sulfate characterized the waters containing drainage from the irrigated land.
At the station downstream from the district near Schoenchen (334.4), concentration ratios for the mean specific conductance increased from 1.08 in 1966 to 1.25 in 1971, slightly less than corresponding ratios for net gains in the irrigation reach during the seepage-salinity surveys. Successively higher values of specific conductance generally prevailed for progressively longer periods during water years 1966-71, but the discharge equalled or exceeded 50 and 90 percent of the time (Q50, Q90) remained relatively constant from year to year because drainage from the irrigated land sustained base flow during dry periods.
The concentrations of the ions corresponding to measured values of daily mean specific conductance that were equalled or exceeded 10, 50, and 90 percent of the time (E10, E50, E90) during each water year were calculated using polynomial regression equations relating the concentrations of the ions to the specific conductance of discrete samples. Although the concentrations of some ions, notably sodium and chloride, increased appreciably as a result of irrigation, they remained well below recommended maximums for drinking water. The concentrations of dissolved solids and sulfate exceeded recommended maximums even before irrigation began.
Estimates of the quantity and quality of annual net drainage (drainage excluding waste and storm runoff) from the irrigated land based on releases to the river and on the data collected at the gage near Schoenchen (334.4) are remarkably similar to net gains between the fish hatchery and the gage during the seepage-salinity surveys. Reasonable estimates evidently can be made using either of the two methods. During periods of stable streamflow, the estimated average net drainage from the district ranged from 10.2 cfs (0.29 m3/s) per day in 1966 to 15.1 cfs (0.43 m3/s) per day in 1970.
The quantity and quality of net drainage from the district also was estimated from the distribution and consumptive use of precipitation and irrigation water. Annual values for excess water on the irrigated acreage were generally similar to the magnitude of net drainage estimated from the continuous records at the gage near Schoenchen. However, the chemistry of net drainage, and comparison of cumulative values indicates that excess input of irrigation water to the system (including seepage from the canal and laterals) is a more realistic estimate of net drainage than is the excess water on the irrigated acreage. The predicted values for specific conductance and the concentrations of dissolved solids and calcium were similar to corresponding values determined by the other two methods, but the concentrations of sodium, chloride, bicarbonate, and nitrate were much lower.
During the period 1966-71, net input of chloride to the system was less than net drainage, whereas net input of magnesium, potassium, and sulfate delivered in the irrigation water exceeded net drainage. From 1969 to 1971, more sodium, bicarbonate, and nitrate were carried out of the system in net drainage than was delivered in the irrigation water.
In 1971, significant quantities of soluble salts remained in transient storage in the aquifers beneath and adjacent to the irrigated land. With continued irrigation the supply of natural salts in the soil and aquifers will be depleted and the quality of drainage from the irrigated land will depend chiefly on the quality, distribution, and consumption of the irrigation water. If conditions do not change drastically, the composition of ground water under the irrigated land and drainage to the river will eventually become more uniform and similar to the irrigation water. However, the concentrations of the major ions probably will be from 1.5 to 2 times higher than in the irrigation water. Phreatophytes, which have proliferated in response to the increased availability of water may significantly reduce recharge to the ground-water reservoir from excess water. Use of water for irrigation and augmentation of base flow by drainage from the irrigation district have not caused serious degradation of the chemical quality of streamflow in the Smoky Hill River.
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Kansas Geological Survey, Geohydrology
Placed on web Nov. 2012; originally published 1975.
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