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)

Groundwater Contamination

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