HIGH PLAINS AQUIFER
The High Plains aquifer underlies an area of about 174,000 square miles in parts of Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming (fig. 46). Parts of the aquifer are in Segments 2, 3, 4, 8, and 9 of this Atlas; for the most part, the aquifer is within Segments 3 and 4 and is discussed in detail only in the chapters that describe those segments. The High Plains aquifer is the principal source of ground water for the High Plains region, which is one of the Nation's most important agricultural areas. In Nebraska, the aquifer underlies an area of about 63,650 square miles, and in Kansas, it underlies an area of about 30,500 square miles; only these parts of the aquifer are discussed in this chapter.
The High Plains aquifer is named from the High Plains Physiographic Province, an area of flat to gently rolling topography (fig. 47) which is part of a vast plain that slopes gently eastward from the Rocky Mountains. The plain was formed by the deposition of sediments that were transported eastward from the Rocky Mountains by a network of streams. Subsequent uplift and erosion have partly dissected the plain. In places, extensive areas of windblown silt and sand that were derived from channel deposits of the streams are at the land surface. The windblown sand deposits form dunes that cover an area of about 20,000 square miles in central Nebraska. Local dune sands also are common in parts of southern Kansas.
The economy of the High Plains area, which provides a major part of the food supply of the Nation, is dependent on the successful growing of crops. The prevailing method of farming in the High Plains before the drought of 1930 through 1939 was dryland farming. By the 1930's, continuous cropping, primarily by repeatedly planting the cropland in wheat, had depleted the humus that bound the soil. During the drought, the wind blew away much of the remaining pulverized soil as huge clouds of dust, and the area was known as the "Dust Bowl." Ground-water irrigation, which had begun in the late 1800's, was greatly intensified in the 1940's in response to the drought. A second surge in irrigation development followed a severe drought during the 1950's. During 1985, about 10.5 million acres were irrigated on the High Plains in Nebraska and Kansas. Most of the water that supplies these irrigation needs was withdrawn from the High Plains aquifer.
The High Plains aquifer consists of all or parts of several geologic units of Quaternary and Tertiary age. The stratigraphic column in figure 48 shows the formation name, generalized rock type, thickness, and age of the geologic units that compose the aquifer. The Brule Formation of Oligocene age is the oldest geologic unit included in the aquifer. The Brule Formation is the upper unit of the White River Group and is primarily massive siltstone with beds and channel deposits of sandstone. Locally, the Brule includes lenticular beds of volcanic ash, clay, and fine sand. The Brule underlies much of western Nebraska and is included in the aquifer only where it has been fractured or where the formation contains solution openings. Such secondary porosity and permeability are developed only where the Brule crops out or is near the land surface (fig. 49).
The Arikaree Group of Miocene and Oligocene age overlies the Brule Formation and consists primarily of massive, very fine to fine-grained sandstone. Locally, the Arikaree includes beds of volcanic ash, siltstone, claystone, and marl. The Arikaree Group crops out in western Nebraska and pinches out to the south and east as does the White River Group, which includes the Brule Formation (fig. 50). The maximum thickness of the Arikaree is about 1,000 feet in western Nebraska.
The Ogallala Formation of Miocene age is the principal geologic unit included in the High Plains aquifer and is at the land surface throughout most of the extent of the aquifer (fig. 49). The Ogallala consists of unconsolidated gravel, sand, silt, and clay. Locally, it also includes caliche, which is a hard deposit of calcium carbonate that precipitated when part of the ground water that moved through the formation evaporated. The Ogallala Formation was deposited by an extensive eastward-flowing system of braided streams that drained the eastern slopes of the Rocky Mountains during late Tertiary time. The location of the stream system migrated during a long period of time, and the Ogallala Formation was deposited over about 134,000 square miles in eastern Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming.
Unconsolidated deposits of Quaternary age overlie the Ogallala Formation. These Quaternary deposits consist of gravel, sand, silt, and clay, much of which is reworked material that was derived from the Ogallala Formation. Where these unconsolidated deposits are saturated, such as in southeastern Nebraska and south-central Kansas, they compose part of the High Plains aquifer (fig. 49). Deposits of loess (fig. 51) overlie the Ogallala Formation or the unconsolidated Quaternary sediments in some locations. The loess was deposited as windblown material and consists mostly of silt with small quantities of very fine-grained sand and clay. Where the loess is thick, it forms the upper confining unit of the High Plains aquifer. Dune sands of Quaternary age compose part of the aquifer where they are saturated. The dune sands are most extensive in west-central Nebraska where they cover about 20,000 square miles (fig. 49) and attain a maximum thickness of about 300 feet. Saturated dune sands also are part of the High Plains aquifer south of the Arkansas River in southwest and south-central Kansas. The dune sands are highly porous and, therefore, quickly absorb rainfall that recharges the High Plains aquifer. Valley-fill deposits along the channels of streams, such as the Platte and the Arkansas Rivers, also are considered to be part of the aquifer where they are hydraulically connected to it. In such places, the valley-fill deposits directly link the streams to the High Plains aquifer and allow water to move freely between the aquifer and the streams.
The High Plains aquifer is underlain by rocks that range in age from Tertiary to Permian. Rocks of Permian age directly underlie parts of the aquifer in southern Kansas (fig. 52). These rocks are predominantly red shale, siltstone, sandstone, gypsum, anhydrite, and dolomite and locally include limestone and halite (rock salt) as beds or disseminated grains. Partial dissolution of salt and evaporite minerals by circulating ground water has adversely affected the chemical quality of water in the High Plains aquifer where Permian rocks that contain salt beds or saline water are in hydraulic connection with the aquifer. The dissolution of salt beds also has resulted in collapse structures and faulting in the overlying deposits. The Bear Creek and the Crooked Creek Fault Zones in southwestern Kansas (fig. 52) are collapse structures that have formed as a result of partial dissolution of salt beds.
Rocks of Jurassic and Triassic age directly underlie the High Plains aquifer in small parts of southwestern Kansas and western Nebraska. These rocks consist primarily of shale and sandstone, and some of the sandstone beds are permeable enough to yield water to wells. Some irrigation wells in southwestern Kansas withdraw water from the High Plains aquifer and the rocks of Jurassic and Triassic age. For the most part, however, the Jurassic and Triassic rocks have low permeability.
Lower Cretaceous rocks directly underlie the High Plains aquifer in parts of southern Kansas and eastern Nebraska (fig. 52). These rocks are primarily shale and sandstone. The hydraulic properties of the sandstones are highly variable, but Lower Cretaceous rocks provide water for irrigation and other uses in parts of Kansas and Nebraska.
Upper Cretaceous rocks directly underlie the High Plains aquifer in large parts of Nebraska and Kansas. These rocks consist primarily of shale, chalk, limestone, and sandstone of which only the chalk (where it is fractured, contains solution openings, or both) yields quantities of water large enough for irrigation purposes. Elsewhere, Upper Cretaceous rocks have little permeability.
The Chadron Formation that is part of the White River Group of Tertiary age (fig. 48) directly underlies the High Plains aquifer in most of western Nebraska (fig. 52). The Chadron Formation is predominantly clay and silt, both with minimal permeability. The Brule Formation, which also is part of the White River Group, is predominantly siltstone but locally is fractured. Where it contains fracture or solution permeability, the Brule Formation is considered to be part of the High Plains aquifer.
Depth to Water
The depth to water in a particular area is the difference between the altitude of land surface and the altitude of the water table. The generalized depth to water in the High Plains aquifer in 1980 is shown in figure 53. In most places, the water levels shown are lower than those that existed before widespread irrigation withdrawals began. The depth to water in the High Plains aquifer is less than 100 feet in about one-half of the area of the aquifer and less than 200 feet in most of Nebraska and Kansas. The depth to water generally is less near the Platte and the Arkansas Rivers than in areas farther from the rivers because the rivers are hydraulically connected to the aquifer through the stream valley aquifers that parallel the rivers. The water table is between 200 and 300 feet below the land surface in parts of western and southwestern Nebraska and in parts of southwestern Kansas. The depth to water is as much as 400 feet below the surface in a small area in southwestern Kansas where development of the aquifer began earlier than in most parts of Kansas; consequently, water-level declines are greater.
Water in the High Plains aquifer generally is under unconfined, or water-table, conditions. Locally, water levels in wells completed in some parts of the aquifer may rise slightly above the regional water table because of artesian pressure created by local confining beds. The altitude and configuration of the water table of the High Plains aquifer are shown in figure 54. The configuration and slope of the water table are similar to the configuration and slope of the land surface. Water in the aquifer generally moves from west to east, or perpendicular to the contours and in the direction of the arrows shown in figure 54. Water moves in response to the slope of the water table, which typically averages between 10 and 15 feet per mile. On the basis of this average slope and aquifer hydraulic properties, the velocity of water that moves through the aquifer is estimated to average about 1 foot per day.
Where the water-table contours cross streams, the configuration of the contours indicates the relation of the water in the aquifer to the water in the stream. For example, where the contours from 3,200 to 4,000 feet in figure 54 cross the North Platte River in western Nebraska, the contours bend upstream. This upstream flexture indicates that water moves from the aquifer to the stream, and the North Platte River is a gaining stream in this area. By contrast, where the 2,000-foot contour crosses the Platte River in west-central Nebraska, a slight downstream bend in the contour indicates that water is moving from the stream to the aquifer; the Platte River is a losing stream in this area, and the water from the river recharges the aquifer.
In southwestern Kansas, the Bear Creek and the Crooked Creek Fault Zones (fig. 52) have displaced the High Plains aquifer and little or no saturated thickness of the aquifer exists on the upthrown side of the faults. In these areas, the water-table contours shown in figure 54 end abruptly at the faults.
The spacing of the water-table contours is affected by different hydrologic conditions. For example, where contours are widely separated, such as in western Nebraska, the slope of the water table is gentler than where the contours are more closely spaced. Widespread recharge to the aquifer by infiltration of precipitation through dune sands occurs in western Nebraska, and, thus, the slope of the water table is relatively gentle.
Recharge and Discharge
In an undisturbed ground-water flow system, the amount of water that moves into an aquifer (recharge) and the amount of water that moves out of the aquifer (discharge) are equal, and the flow system is in equilibrium. Before development in an unconfined aquifer, such as the High Plains aquifer, the water table of the aquifer and the quantity of water stored in the aquifer vary little in response to changes in precipitation, streamflow, and the amount and types of vegetation.
A ground-water flow system is no longer in equilibrium when the long-term discharge is not equal to the long-term recharge. The altitude of the water table rises when the recharge rate exceeds the discharge rate and declines when the discharge rate exceeds the recharge rate. Withdrawal of large quantities of ground water by wells and redistribution of surface water in ditches and canals, all for irrigation purposes, have changed the natural recharge and discharge of the High Plains aquifer.
Recharge to the High Plains aquifer is primarily by infiltration of precipitation and locally is by infiltration from streams and canals. Some surface water that is applied to crops for irrigation also percolates downward and recharges the aquifer. A small quantity of water from the underlying bedrock moves upward and mixes with water in the High Plains aquifer; this water is also considered to be recharge. The aquifer is recharged at total rates of between 0.05 and 6 inches per year in Nebraska and Kansas. The rates of recharge are highly variable and range from about 0.3 to 20 percent of the average annual precipitation in the dry and wet parts of these States. The greatest rates of recharge by precipitation are in areas where dune sand or other highly permeable material is at the land surface. Recharge by infiltration of streamflow usually is greatest when streamflow is high and, thus, provides a large difference between stream and aquifer water levels.
Natural discharge from the High Plains aquifer is to springs, seeps, and streams and by evapotranspiration. Where the water table is near the land surface, ground water can evaporate directly. Transpiration rates are greatest along stream valleys where deep-rooted salt cedar, willows, cottonwoods, and sedges grow. Where the High Plains aquifer locally is underlain by permeable bedrock and the water table in the aquifer is higher than that in the bedrock, small amounts of water move downward from the aquifer into the bedrock.
Large quantities of water are withdrawn from the aquifer by wells, and in some areas large amounts of water discharge from the aquifer to streams. For example, a study was done during 1975 to determine how much of the flow of the Platte River in Nebraska was derived from ground water. The gain in streamflow within Nebraska was about 3 million acre-feet, most of which was ground-water discharge from the High Plains aquifer to the river. Large quantities of surface water are diverted from the Platte River and used for irrigation; thus, the amount of ground-water discharge to the river probably was significantly greater than the measured gain in streamflow.
Most of the discharge from the High Plains aquifer is by withdrawals from wells, and practically all of the water withdrawn is used for irrigation purposes. Total withdrawals of water from the entire aquifer for irrigation increased from about 4 million acre-feet during 1949 to about 23 million acre-feet during 1978 (fig. 55) then decreased to about 16 million acre-feet during 1990. During 1978, more than 4 million acre-feet of irrigation water was withdrawn in Kansas and about 8 million acre-feet was withdrawn in Nebraska. During 1990, irrigation withdrawals in Kansas were about the same as those during 1978, whereas withdrawals in Nebraska were only about 5 million acre-feet per day.
Saturated Thickness and Well Yield
The saturated thickness of an aquifer is the vertical distance between the water table and the base of the aquifer and is one of the factors that determines the quantity of water that can be pumped from a well. Other factors that affect well yield include well construction and the hydraulic properties of the aquifer. The saturated thickness of the High Plains aquifer in 1980 (fig. 56) ranged from 0 (where the sediments that compose the aquifer were unsaturated) to about 1,000 feet. The greatest saturated thickness is in north-central Nebraska where the aquifer consists of the Ogallala Formation and overlying dune sands. Locally in southwestern Kansas, dissolution of salt in the Permian bedrock that underlies the High Plains aquifer has resulted in collapse features that were filled with younger sediments. These anomalously thick accumulations of sediments coincide with thick sequences of saturated aquifer materials. The average saturated thickness of the entire aquifer in 1992 was about 190 feet. In Nebraska, the average saturated thickness was 340 feet, but in Kansas, it was only about 90 feet.
Changes in the saturated thickness of the High Plains aquifer have resulted from ground-water development. Saturated thickness has decreased in most places (fig. 57), but in two areas in south-central Nebraska, recharge to the aquifer from surface-water irrigation combined with downward leakage of water from canals and reservoirs has increased saturated thickness. In large areas in southwestern Kansas, large-scale irrigation development has decreased the saturated thickness of the aquifer more than 25 percent. Decreases of more than 10 percent in saturated thickness result in a decrease in well yields and an increase in pumping costs because of the increased depth at which the pump must be set in order to lift the water.
The entire High Plains aquifer contained about 21.7 billion acre-feet of saturated material in 1992. The quantity of drainable water in storage in the aquifer can be estimated by multiplying the volume of saturated material by the average specific yield (15 percent). The specific yield of an aquifer is the volume of water that will drain from a unit volume of rock under the influence of gravity alone. Therefore, about 3.25 billion acre-feet of drainable water was in storage in the High Plains aquifer in 1992. About 65 percent of the drainable water in storage in the entire aquifer is in Nebraska, and about 10 percent is in Kansas. The remaining drainable water is in storage in Colorado, New Mexico, Texas, South Dakota, Oklahoma, and Wyoming. Not all drainable water in storage within the aquifer can be recovered for use. The quantity of water that can be recovered varies with location and depends on the lithology, saturated thickness, hydraulic conductivity, and specific yield of the aquifer at that location and on well construction. Water has been almost completely removed from about 8 percent of the formerly saturated aquifer material in Kansas, whereas the quantity of material dewatered in Nebraska is negligible.
The greatest yields of water generally are obtained from wells that are completed in coarse-grained aquifer material in places where the saturated thickness of the High Plains aquifer is great. A generalized map of the potential yield of properly constructed wells completed in the High Plains aquifer is shown in figure 58. The potential yield of wells is greater than 750 gallons per minute in most of Nebraska and large parts of Kansas. A well capable of producing 750 gallons per minute can irrigate 125 acres and effectively supply one center-pivot irrigation system.
Well yields from different formations that compose the aquifer vary. Yields from the Brule Formation typically are less than 300 gallons per minute. Wells completed in the Arikaree Group generally do not yield large quantities of water but might yield as much as 350 gallons per minute in western Nebraska where the saturated thickness is about 200 feet. Well yields from the Brule Formation and the Arikaree Group are greatest where secondary porosity, such as fractures or solution openings, has been developed in the rocks. Well yields from the Ogallala Formation are 1,000 gallons per minute from 100 feet of saturated sand and gravel in many parts of Kansas and Nebraska but are only 100 gallons per minute from 20 feet of saturated sand and gravel in western Kansas.
GROUND-WATER DEVELOPMENT AND WATER-LEVEL FLUCTUATIONS
Development of the High Plains aquifer began in the late 1800's, when windmills were used as a source of power to pump water from scattered irrigation wells. Spurred by the drought of the 1930's, ground-water irrigation expanded rapidly. Development of the High Plains aquifer generally began in Texas, adjacent parts of New Mexico, and in major stream valleys in other States in the 1930's. Widespread development progressed to Oklahoma and Kansas in the 1940's and extended to Colorado, Nebraska, and Wyoming in the 1950's; the aquifer has undergone little development in South Dakota.
In 1949, the total acreage irrigated by ground water in the High Plains was slightly more than 2 million acres (fig. 59), most of which was in Texas. The number of acres irrigated in Kansas and Nebraska expanded greatly in the late 1950's in response to a drought. The amount of irrigated acreage in those States continued to increase until 1978, by which time nearly 170,000 irrigation wells had been completed in the High Plains aquifer; about 23,000 of these wells were in Kansas, and about 59,300 were in Nebraska. Collectively, irrigation wells in eight States pumped about 23 million acre-feet of water from the High Plains aquifer in 1978 to irrigate about 13 million acres. In 1978, water from the High Plains aquifer was used to irrigate about 4.5 million acres in Nebraska and more than 2 million acres in Kansas. The large increase in the number of wells drilled for irrigation between 1952 and 1978 was partially a result of the development of center-pivot irrigation systems during the 1960's. Center-pivot systems, such as those shown from an aerial view in figure 60, are supplied by irrigation wells and have the water-distribution pipes mounted on a wheeled boom that rotates in a circle around the center of the irrigated area. Such systems make it possible to irrigate the rolling terrain of the High Plains. In Nebraska alone, the number of center-pivot irrigation systems increased from about 2,800 in 1972 to more than 27,000 by the end of 1984 (fig. 61).
As the number of irrigation wells increased, the percentage of land that was irrigated also increased. In 1949, the 5 percent or less of land that was irrigated in most of Nebraska and Kansas (fig. 62A), was mostly along river valleys. By 1964, the percentage of irrigated land had increased, and much of the irrigated acreage was away from the rivers (fig. 62B). By 1978, the percentage of irrigated acreage had greatly increased (fig. 62C) as more upland areas were irrigated.
As development of the High Plains aquifer became more extensive, water levels in the aquifer began to decline in some locations. Well yields decreased in some places as a result of the water-level declines. The cost of pumping increased as water levels declined because pumps were set deeper and more energy was required to lift the water an increased distance. The cost of the energy used to pump the water also increased. This increased cost for obtaining irrigation water decreased the profit in growing crops that require irrigation in parts of the High Plains area.
Average annual withdrawals of water from the High Plains aquifer generally are much larger than recharge to the aquifer from precipitation. In places where recharge rates are high, the demand for irrigation water could be as low as twice the average annual recharge; where recharge rates are low, the demand could be more than 100 times the recharge. The quantity of water removed from storage from the entire aquifer between predevelopment conditions and 1980 is estimated to be about 166 million acre-feet, of which about 27 million acre-feet was withdrawn in Kansas. No significant quantity of water has been removed from storage in Nebraska. By 1980, withdrawals of water from the High Plains aquifer had resulted in water-level declines of more than 100 feet in parts of southwestern Kansas (fig. 63). Development of the aquifer in this area began in the 1940's, which is earlier than in most other places in Kansas. Generally, the later development of the aquifer began in an area and the less intense the development has been, the smaller the water-level decline in that area. Water-level rises shown in parts of southern Nebraska are in response to increased recharge of the aquifer by infiltration of surface water applied for irrigation.
The chemical quality of water in the High Plains aquifer is affected by many factors. These factors include the chemical composition and solubility of aquifer materials, the increase in dissolved-solids concentrations in ground water in areas where the water discharges by evapotranspiration, and the chemical composition of water that recharges the aquifer. Ground water generally contains smaller concentrations of dissolved minerals near recharge areas where the residence time of the water in the aquifer has been short, and, thus, dissolution of aquifer minerals has been less. The water generally is more mineralized near discharge areas because residence time has been longer and more dissolution of minerals has taken place.
The dissolved-solids concentration in ground water is a general indicator of the chemical quality of the water. Dissolved-solids concentrations in water from the High Plains aquifer are less than 500 milligrams per liter in most of Kansas and Nebraska (fig. 64) but locally exceed 1,000 milligrams per liter in both States. The limit of dissolved solids recommended by the U.S. Environmental Protection Agency for drinking water is 500 milligrams per liter. Most crops can tolerate water in which the dissolved-solids concentration is 500 milligrams per liter or less. In places with well-drained soils, many types of crops can tolerate water with a dissolved-solids concentration of between 500 and 1,500 milligrams per liter. In southwestern and south-central Kansas, the High Plains aquifer overlies Permian bedrock that contains bedded salt. Where circulating ground water has dissolved some of this salt and the mineralized water has subsequently moved upward into the High Plains aquifer, the dissolved-solids concentration of the water in the High Plains aquifer is greatly increased. Also, dissolved-solids concentrations generally are greater near streams where water from the High Plains aquifer discharges. Ground water near the streams is shallow enough to be transpired by plants or to be evaporated directly from the soil. Concentrations of dissolved solids in the ground water are increased by the evapotranspiration process. Rates of transpiration are greatest where deep-rooted phreatophytes, such as sedges, cottonwood, willows, and salt cedar, grow.
Excessive concentrations of sodium in water adversely affect plant growth and soil properties, and constitute salinity and sodium hazards that may limit irrigation development. Sodium that has been concentrated in the soil by evapotranspiration and ion exchange decreases soil tillability and permeability. Areas of high or very high sodium hazard occur in parts of Kansas. Sodium hazard is evaluated by the sodium adsorption ratio, which relates the concentration of sodium to calcium plus magnesium; if this ratio is high, then the sodium can destroy any clay in the soil and thus affect soil structure. Sodium concentrations in water from the High Plains aquifer in Kansas and Nebraska are shown in figure 65. Concentrations are less than 25 milligrams per liter in most of Nebraska and northern Kansas. Concentrations are greatest in southwestern Kansas where evapotranspiration rates are high and in south-central Kansas where the High Plains aquifer overlies Permian bedrock that contains saline water derived from partial dissolution of salt beds. Sodium concentrations are increased along the Platte and the Republican Rivers where evapotranspiration rates also are high. Salinity and sodium hazards generally are low in Nebraska where the High Plains aquifer primarily consists of sand and gravel, which contain few sodium-bearing minerals.
Excessive fluoride concentrations are a widespread problem in water from the High Plains aquifer. Some of the fluoride is derived from dissolution of fluoride-bearing minerals in parts of the aquifer that contain sand and gravel, such as the Ogallala Formation. Extremely large concentrations (2-8 milligrams per liter) of fluoride are reported where the aquifer contains volcanic ash deposits or where it is underlain by rocks of Cretaceous age. Large concentrations of fluoride in drinking water cause staining of teeth, but fluoride is not a concern in irrigation water.
The generally shallow depth of the water table in the High Plains aquifer makes water in the aquifer susceptible to contamination. Application of fertilizers and organic pesticides to cropland has greatly increased since the 1960's, thus increasing the availability and the amount of potential contaminants available. Increased concentrations of sodium, alkalinity, nitrate, and triazine (a herbicide) have been found in water that underlies small areas of irrigated cropland in Nebraska and Kansas. Of 132 wells sampled during 1984-85 in Nebraska, 43 had measurable concentrations (greater than 0.04 microgram per liter) of the herbicide atrazine. Increased concentrations of 2,4-Dichlorophen-oxyacetic acid (2,4-D, a pesticide) were found in water that underlies rangeland in a small part of the Great Bend area of the Arkansas River in Kansas.
FRESH GROUND-WATER WITHDRAWALS
Withdrawals of fresh ground water from the High Plains aquifer in Segment 3 during 1990 totaled 8,181 million gallons per day (fig. 66). Of this amount, 4,556 million gallons per day was withdrawn in Nebraska. About 97 percent of the total withdrawals, or about 7,900 million gallons per day, was used for agricultural, primarily irrigation, purposes. About 200 million gallons per day was pumped for public supply. Domestic and commercial withdrawals were about 40 million gallons per day, and industrial, mining, and thermoelectric power withdrawals also were about 40 million gallons per day.