Louisiana Department of Environmental Quality researchers estimated 53.2 years as the typical time of travel for one mile in the Sparta, that is, 99.3 feet per year. (10) In calculating typical velocity, they used average values for hydraulic conductivity (the ease with which water flows through the aquifer), hydraulic gradient (head loss per distance of flow) and porosity (percentage of volume through which water can move). The values are averages and do not take into account the facts that discontinuous units differ in flow rates and direction; or that localized areas differ in hydraulic conductivity, hydraulic gradient, and porosity; or that pumping results in increased ground water velocity in areas where (and to the extent that) the hydraulic gradient has been increased by that pumping.Figure 7 is a Louisiana Geological Survey graph depicting typical travel time in Louisiana aquifers.
Water Quality Changes with Distance from Outcrop Area and with Depth
In Mississippi Embayment aquifer water, the dominant minerals are calcium bicarbonate and sodium bicarbonate in shallower parts, sodium bicarbonate in mid-dip, and sodium chloride, especially in deeper parts. (4a)
'The quality of groundwater in Louisiana is generally suitable for public supply, except where dissolved-solids concentrations (TDS) are larger than 1000 mg/L.’ (11a) In the Sparta, the TDS concentration increases from the outcrop area in the west until it reaches approximately 1000 mg/L along the line typically displayed on maps such as Figure 1 of this report as ‘the approximate eastern border of the freshwater Sparta’, also referred to as ’the freshwater/salt water interface’ or ‘the downdip limit of freshwater’. (Ref. 2, p. 7-2, Ref. 6)
Suitability for Use
A United States Geological Survey (USGS) report describes Sparta freshwater as suitable for use without treatment except in some areas where treatment to remove iron may be needed. (3) The Louisiana Department of Environmental Quality describes water from their Sparta monitoring wells as soft. (5) Soft water has less tendency than hard water to cause ‘limescale’, which clogs pipes and decreases the life of water-using equipment.
Monitoring Sparta Water Quality
Louisiana's Department of Environmental Quality (LDEQ) samples water from selected wells in each of the state's aquifers at three year intervals. In aquifer summaries, LDEQ reports water contaminants in terms of EPA-designated Primary Maximum Contaminant Level (PMCL; mandatory, health-related) and Secondary Maximum Contaminant Level (SMCL; unenforceable, generally aesthetic). LDEQ also reports trends. (5) USGS publishes Water Quality Samples for Louisiana: Sample Data.
Results of LDEQ Monitoring of Sparta Water Quality
Results of 14 LDEQ Sparta monitoring wells sampled in 2003 and 2006 were published in the ‘Sparta Aquifer Summary’, 2004 and 2007. (5) Among results were the following:
Water Quality in Excess of Primary Standards in Specific Wells: No primary drinking water standards (PMCL) were exceeded.
Water Quality in Excess of Secondary Standards in Specific Wells: One or more secondary standards (SMCL) were exceeded in some monitoring well:
o Chloride concentrations in excess of SMCL= 250 mg/L were reported in four wells in 2004 and 2007 respectively: in Morehouse Parish (MO-253) 387 mg/L and 373 mg/L; in Union Parish (UN-205) 315 mg/L and 351 mg/L; and in Ouachita Parish (OU-464: 360 mg/L and OU-597: 419 mg/L).
o Iron concentrations in excess of SMCL= 0.3 mg/L (and in excess of 1 mg/L, a standard used in some localities) were reported, in 2007, in a Bienville Parish well (BI-212: 2.17 mg/L) and a Claiborne Parish well (CL-203: 1.380 mg/L).
o The pH of water in the Claiborne Parish well (CL-203) was reported, in 2007, as 6.48 SU, which is slightly acidic, just below the SMCL lower limit for pH = 6.5 SU.
Trends in Sparta Water Quality: From 1995 to 2007 in LDEQ-monitored Sparta wells:
o There was a general increase in specific conductance (related to salinity), salinity, chlorides, total dissolved solids (TDS), iron, and barium.
o There was a general decrease in color and hardness.
o The average TDS concentration increased from 356.5 to 461 mg/L.
o The average chloride concentration increased from 85.8 mg/L to 126.5 mg/L
Increasing Chloride Concentration in Sparta Water is Along the Freshwater/Saltwater Interface
Increasing concentration of chloride in Sparta water is occurring mostly in wells along the freshwater/saltwater interface. (12) Chloride levels are generally stable elsewhere in the Sparta region. (12) A 2008 PowerPoint presentation, ‘Water Quality in the Sparta Aquifer’, by USGS supervisory hydrologist Ben McGee included graphs that demonstrate the same pattern. (13a)
Water Quality Protection
One of the three Sparta aquifer management issues listed by Fenstermaker et al (Section 5a. of this paper) is ‘the management of impervious cover, development, and pumpage in the primary recharge areas, recognizing the need to limit the amount of impervious cover and regulate development of hazardous waste in areas where an aquifer is recharged by direct infiltration of precipitation and runoff. ‘ (6)
A USGS circular describes groundwater vulnerability to contamination as ’a function not only of the properties of the ground-water-flow system (intrinsic susceptibility) (Figure 9) but also of the proximity of contaminant sources, characteristics of the contaminant, and other factors that could potentially increase loads of specified contaminants to the aquifer and(or) their eventual delivery to a ground-water resource.’ The complexities of the behavior of different contaminants in an aquifer system are beyond the scope of this paper, but are discussed in the circular and in an LDEQ paper. (10) Recharge area protection is especially important because, whereas ‘deeper and older ground water tends to be in contact with naturally occurring contamination for long periods of time,’ ‘shallower and younger ground water tends to be more susceptible to current sources of contamination from land surface
Louisiana’s Drinking Water Source Protection Programs
As of this report (March, 2010), the state of Louisiana has no regulations specific to aquifer recharge area protection, but Louisiana DEQ’s Drinking Water Source Programs are designed to protect drinking water supplies. The Wellhead Protection Program originates from the U.S. Congress Safe Drinking Water Act Amendment of 1986. Most of its elements were incorporated into the EPA Source Water Assessment Program (SWAP) of 1996 and Louisiana’s Drinking Water Protection Program. The SWAP program required all states to: (1) delineate a protection area around all public water supply wells and intakes, (2) locate by GPS all wells and intakes in the state and significant potential sources of contamination within the protection areas, and (3) determine the susceptibility of each public water supply to contamination.
The SWAP assessments, completed in 2003, guide LDEQ Source Water Protection Program agents, who work with local citizens on a parish-wide basis to implement Best Management Practices (BMPs) in wellhead protection areas. (13b) The BMPs consist primarily of local people conducting public education programs using SWAP information under the guidance of LDEQ.Figure 10 shows a sign designating a Source Water Protection Area.
Other drinking water supply protection programs are listed in Endnote 6 by agency: LDEQ (programs in addition to SWAP and DWPP), Louisiana Department of Health and Hospitals, EPA , and Louisiana Rural Water Association
Specific storage, storativity, specific capacity, and specific yield are storage properties of an aquifer, referring to the quantity of water that can be released. Storage definitions and some values for the Sparta aquifer in Louisiana are presented in Endnote 4. ‘Storativity ‘ is the subject of sections that follow in this paper.
Release of Water from a Confined Aquifer, like the Sparta, as opposed to an Unconfined Aquifer
An aquifer’s pores make up the space between the aquifer’s sand. In an unconfined aquifer, withdrawing water results in a decline in water table; aquifer pores are drained of water, leaving air to fill in. In a confined aquifer, where water is compressed, withdrawing water results in a pressure drop (decline in potentiometric surface), but aquifer pores remain filled with water. From equivalent unit areas with equivalent change in head, the volume of water released from a confined aquifer is much less than that released from an unconfined aquifer. The concept of storativity is illustrated in Figure 8.
From equivalent unit areas with equivalent change in head, the volume of water released from storage in the Sparta aquifer has been reported as generally about 1,000 times less than that released from the unconfined Mississippi River Valley alluvial aquifer. (1) The alluvial aquifer has greater storativity (Figure 8), as well as greater porosity and hydraulic conductivity. (11)
Importance of the Sparta Aquifer’s Small Storativity
While the Sparta aquifer can produce high quality water, its relatively small storativity, as a confined aquifer, means that large water-level declines over extensive areas are required to achieve the water yields of an unconfined aquifer over a much smaller area. This means that:
Pumping in one location will affect a relatively large area of the aquifer
In the absence of recharge equal to withdrawal, there will be deepening and expansion of the area of drawdown of the potentiometric surface;
Decreased pumping in one location will result in water level rises that become relatively widespread over time.
Yield is a measure of the pumping rate as gallons per minute of a specific well. It is influenced by aquifer characteristics, pumping time, and well construction. Individual wells in the Sparta (excluding those wells located within areas of large drawdowns) generally yield 100 to 500 gallons per minute, with less common rates up to 1,200 gallons per minute. (8)
Specific Capacity is the pumping rate per unit foot of drawdown, tracked over time. The measure is used to identify well and aquifer performance problems. For Sparta wells, the value has been estimated as five to ten gallons per minute per foot of drawdown. (10)
Potentiometric surface is the level to which water in a tightly encased well will rise by hydrostatic pressure or ‘head’. 'Potentiometric surface’, ‘water level altitude’, and ‘hydraulic head’ are often used interchangeably as different expressions of hydrostatic pressure. (9)
Direction of Groundwater Movement Through the Sparta Aquifer
The Sparta is exposed at the surface (outcrop area). It becomes confined as it dips toward the east. In a confined aquifer, water flows downgradient from potentiometric high areas to potentiometric lows. Heavy pumping causes cones of depression in the potentiometric surface and alters the direction of ground-water flow. In 1900 (before widespread pumping, sometimes referred to as ‘pre-development times'), groundwater flow was eastward, indicated by Sparta water level elevations that decreased from 300 feet above mean sea level in Claiborne Parish to 100 feet above mean sea level in Ouachita and Morehouse Parishes. Since development, pumping has induced cones of depression, reversing the flow in Ouachita and Morehouse Parishes westward. (9) (Figure 6)
Speed of Groundwater Movement Through the Sparta Aquifer
Louisiana Department of Environmental Quality researchers estimated 53.2 years as the typical time of travel for one mile in the Sparta, that is, 99.3 feet per year. (10) In calculating typical velocity, they used average values for hydraulic conductivity (the ease with which water flows through the aquifer), hydraulic gradient(head loss per distance of flow), and porosity (percentage of volume through which water can move).
The values are averages and do not take into account the facts that discontinuous units differ in flow rates and direction; or that localized areas differ in hydraulic conductivity, hydraulic gradient, and porosity; or that pumping results in increased ground water velocity in areas where (and to the extent that) the hydraulic gradient has been increased by that pumping. Figure 7 is a Louisiana Geological Survey graph depicting typical travel time in Louisiana aquifers.
The Sparta Sand Formation makes up most of the Middle Claiborne Group, a hydrogeologic unit of the Mississippi Embayment. The sands were laid down as part of the ancient Gulf of Mexico beach system when the Earth was much warmer than it is today, the world ocean was much larger, and the mouth of the ancient Mississippi River was at Cairo, Illinois. The formation extends from southeast Texas, north into Louisiana, Arkansas, and Tennessee, and eastward into Mississippi and Alabama. (1) (Figure 2) An endnote tells more about the Mississippi Embayment.The Middle Claiborne group dates to the Eocene age, which spanned the time period of 55 to 34 million years ago. (2)
Description of the Sparta Sand
‘The Sparta Sand consists of fine to medium sand interbedded with coarse sand, silty clay and lignite. The sands become (thicker) with depth of the aquifer. Laterally, they are discontinuous. The percentage of sand… (depending upon the location within the aquifer) varies from almost completely sand, generally at the base, to fifty percent sand, for example, in an area of Ouachita Parish, where sands are broken with many small clays (layers of clay).’ (3) In north Louisiana and south Arkansas, the Cane River Formation, predominantly marine clay, underlies the aquifer, and the Cook Mountain Formation overlies it. These confining units, of clay, mud, marl, and shale, were depositions of rising seas interrupted by sedimentary rock deposited by streams that emptied into the Gulf of Mexico. (4) (Figure 3)
Description of the Sparta Aquifer in Louisiana
The Sparta aquifer in Louisiana downdips (inclines at an angle downward) from the outcrop area in parts of Bossier, Webster, Claiborne, Bienville, Jackson, and Winn Parishes to the approximate limit of freshwater, which extends from Morehouse south to Caldwell Parish and then southeast to Sabine Parish.(3) (Figure 4)
For most of the Sparta aquifer in Louisiana, the altitude of the top ranges from 200 feet above sea level (recharge area) to 300 feet below sea level (freshwater/saltwater interface)….The altitude of the base ranges from 150 feet above sea level (recharge area) to 1000 feet below sea level (freshwater/saltwater interface)….The otherwise relatively smooth base is interrupted in places by domes created by intrusion of buried salt formations pushing overlying geological units upward. The thicknessof the aquifer ranges from 50 feet (recharge area) to 500 feet (adjacent to recharge area), increasing to 700 feet (freshwater/saltwater interface).‘ (3)
Average Annual Rainfall and Sparta Recharge
Rainfall arrives in the Sparta aquifer directly in outcrop areas, and by water flowing in overlying terrace and alluvial deposits, and by leakage from the Cockfield and Carrizo-Wilcox aquifers.’ (5) ‘In north Louisiana, average rainfall is 56.19 inches per year (range 40 to 80 inches per year), as derived from 1971-2000 data.’ (6) ‘Most rainfall runs off to streams and rivers, or is returned to the atmosphere from plants and soil by processes of evapotranspiration, or discharges as baseflow to streams.’ (4a) ‘(F)ew, if any, studies have been conducted in the (Mississippi) embayment to determine actual recharge rates.’ (7) In their optimization model, McKee, Clark, and Czarnecki fixed a recharge range from 0.39 to 0.77 inches per year for the Sparta region in the outcrop and suboutcrop area of southern Arkansas and north-central Louisiana. (8) Figure 5 shows the general location of the Sparta primary recharge area.