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You are standing above the Little Arkansas River, just as it meets with the Arkansas River. This lesson is actually about the aquifer which lies beneath this spot, but I won’t make you burrow down into the earth. However, the Little Arkansas does obtain about eight percent of its total flow from the Equus Beds aquifer.
What is an Aquifer?
An aquifer is a body of saturated rock through which water moves, so it must be both permeable and porous. It often includes rock types such as sandstone, conglomerate, fractured limestone and unconsolidated sand and gravel. An aquifer is filled with moving water and the amount of water in storage can vary from season to season and year to year. Ground water may flow through an aquifer at a rate of 50 feet per year or 50 inches per century, depending on the permeability. But no matter how fast or slow, water will eventually discharge or leave an aquifer and must be replaced by new water to replenish or recharge the aquifer. Thus, every aquifer has a recharge zone or zones and a discharge zone or zones.
Formation
The Equus Beds aquifer is part of a regional system known as the High Plains aquifer system and it derives its name from Equine fossils found in unconsolidated deposits underlying the entire region. It was formed during the Quaternary Period between 700,000 to 1,000,000 years ago by depositional, erosional and structural processes. Erosion from through-flowing rivers and subsidence from the solution of portions of the underlying Hutchinson Salt member reshaped the ancestral land surface. Deep troughs were cut into bedrock. Later deposition by streams, rivers and wind filled the channels with unconsolidated deposits of clay, silt, sand and gravel. These deposits were later saturated with water from rainfall, rivers and streams to form the groundwater reservoir known as the Equus Beds aquifer.
Depth to groundwater below land surface ranges from less than ten feet to 110 feet. Depth to water in the northern portion of the District is greater and ranges from 40 feet to 110 feet; in the southern portion it is less, ranging from less than ten to 40 feet. And groundwater velocity is very slow, ranging from 300 to 500 feet each year.
Recharging
Recharge is the continuous process that adds water directly to the aquifer and several natural sources of water make up the total recharge supply for the Equus Beds aquifer. Precipitation contributes the greatest amount via infiltration. Annually, 30 inches of precipitation falls on the land surface overlying the Equus Beds aquifer. Due to geologic, hydrologic and climatic conditions, about 80 to 90 percent of this precipitation will either drain into streams or rivers, evaporate back to the atmosphere or be used by plants and people.
Factors which affect the rate which precipitation is recharged include surface topography, vegetation, temperature, evaporation, soil properties, subsurface conditions and depth to aquifer’s water table. Throughout the southern two-thirds of the District, recharge conditions are favorable with 20 percent or six inches of precipitation returned to aquifer annually. In the northern one-third, conditions are less favorable and only 10 percent or three inches of annual precipitation recharges the aquifer.
Discharging
Groundwater discharges are continuous natural and man-made processes including outflow, baseflow, evaporation, transpiration, and withdrawals by wells.
The Equus Beds aquifer loses water to adjacent aquifers in the southeastern and extreme northern portions of the District, as groundwater moves laterally from the aquifer.
Baseflow is groundwater that seeps, flows or is naturally discharged from the aquifer to rivers or streams. These conditions exist when the water table is higher than the water level in the rivers or streams. The Little Arkansas River obtains about eight percent of its total flow from baseflow, amounting to 18,000 acre-feet per year discharged from the aquifer to the river.
In areas where the water table intersects or is near land surface, groundwater evaporates from the water surface or through soil pores and transpires from plants with roots that intercept the water table.
Presently, about 1,620 wells withdraw water from the aquifer for industrial, municipal, irrigation, recreational and stockwater uses. A myriad of small yielding domestic wells also tap the aquifer.
Water Use
The Equus Beds aquifer is the primary water source for most of the population living above it. It spans four Kansas counties and covers approximately 900,000 acres. It has supplied 60% of Wichita’s water needs for over 50 years, in addition to providing water for surrounding towns throughout the region.
In 1997, Wichita started a study on the feasibility of artificially recharging the aquifer. It created two recharge sites: one in Halstead, the other in Sedgwick. These sites diverted excess water from the Little Arkansas River. After the water was treated, it was inserted in the aquifer via basin, trench and injection wells. During its testing period (1997 -2002) the artificial recharge contributed 3% of the municipal use by Wichita. The feasibility study ended in 2002, but recharge continues when the steam flow of the Little Arkansas River permits.
To Log this Cache
1) What is the width of the Little Arkansas River at this point?
2) Estimate the water flow speed. How does this compare to the groundwater flow in the aquifer?
3) Approximately how far below the surface does the aquifer lie?
4) What is the main source of recharge for the aquifer?
Additional Hints
(No hints available.)