Because the Sparta, as a confined aquifer, has relatively small storativity, the response of water levels to heavy pumping stress is substantial and occurs over a large area. Long-term pumping has resulted in regionally extensive water-level declines and change in location of potentiometric lows from natural groundwater discharge areas in pre-development years to pumping centers in northern Louisiana and southern Arkansas. (8)
Groundwater flow, potentiometric surface, and water level declines
In predevelopment times (around 1900) well water levels were well above the top of the Sparta Sand. (15) As early as the 1940's, substantial declines in water levels were documented in Union and Jefferson Counties in Arkansas. (1) Figure 16 shows simulated declines of greater than 260 foot over a century at the location of two Sparta wells in Louisiana.
In 1960, USGS, in cooperation with Louisiana Department of Transportation and Development, began reporting water withdrawals and usage data every five years (14), allowing analysis of trends. Declining potentiometric levels within the Sparta due to pumping have been plotted on maps for more than 35 years. (Ref. 12 and Sec. 7.a.1. of this paper) By 1965, withdrawals had formed cones of depression at Minden, Jonesboro-Hodge, Monroe, Bastrop, and Farmerville. (15) Since 1980, the deepening and expanding cones of depression in Monroe, Bastrop, and Farmerville have coalesced, forming a trough between El Dorado and Monroe regions. (6) Figure 17 shows average water level declines from one to four feet per year, increasing eastward.
Figure 18 is a USGS hydrograph of a Lincoln Parish Sparta well (L-26). Over 42 years (October 1967 to October 2009), the water level in L-26 declined 71 feet (an average of 1.7 feet per year). Water levels in two Ruston wells declined at a rate of 2.8 feet per year over 44 and 46 years; another declined 3.2 feet per year over 41 years. (2, p.26)
‘The effect of heavy pumping can be remarkable locally. An aquifer test near El Dorado resulted in an approximate 5.7 foot water level decline 2,400 feet from the pumping well after 3 days of pumping at a rate of about 460 gallons per minute.’ (1)
Dewatering and Potential Compaction of Sands
By 1997, the potentiometric surface in the Sparta aquifer was below the top of the Sparta Sand in much of Webster, Claiborne, Lincoln, Bienville, and Jackson Parishes, and parts of Ouachita, Union, and Bossier Parishes. (9) Figures 19 and 20
‘Excessive dewatering of the Sparta aquifer and overlying confining units can lead to irreversible compaction (subsidence), reducing its ability to be recharged and its water-yielding capacity….(and) reducing the rate at which water can move through the aquifer.’ (1)
A USGS Fact Sheet ‘The Sparta Aquifer: A Sustainable Water Resource?’ describes the process of compaction. ‘[The stress of rock and water mass acting downward] is borne by the granular skeleton of the aquifer matrix (effective stress) and the fluid pressure of water in the pore spaces. When the fluid pressure is reduced, the effective stress increases….If water level declines below the top of a confined aquifer, the aquifer becomes unconfined…at that location and the fluid pressure becomes zero transferring all the stress to the aquifer matrix. Aquifers and confining units containing significant amounts of fine-grained materials, as the Sparta aquifer does, are most susceptible to compaction.‘ (1) (Figure 21)
Notable subsidence had not been documented in the Sparta aquifer when the USGS Fact Sheet was published. (1) Interpretable data is required. Meyer, Meyer, LeCroix, and Hixson in the Sparta Groundwater Study remarked, ‘It is important for the long-term preservation of the aquifer to restore the water level to the top of the aquifer.’ (2)
Advance of the Freshwater/Saltwater Interface Westward and Upconing of Salt Water in the Sparta
When large scale pumping of water out of the Sparta reduces the hydrostatic pressure, saltwater can fill in, either upconing locally or moving the salt water interface westward into the freshwater region. (2)
The 'Water Quality' section of this paper (section 3.d.) describes increasing salt concentration in Sparta water. Briefly, over twelve years ending 2007, average chloride concentration rose from 85.8 mg/L to 126.5 mg/L in LDEQ monitored wells (5); in three of 14 wells, the chloride concentration exceeded the EPA secondary standard for drinking water (5); a 2009 study shows that chloride concentration of Sparta water along the freshwater/saltwater interface continues to increase (12). Figure 22 shows increasing chloride concentration in a Winn parish well over 30 years.
Upconing of brackish water because of extreme drawdowns has resulted in increased chloride concentrations in some Union County, Arkansas Sparta wells (1) and possibly a Union Parish well (OU-205) that had a chloride concentration of 351 ppm in 2006. (5)