Water resources of St. Tammany Parish, Louisiana

Introduction

St. Tammany Parish, located in southeastern Louisiana (fig. 1), contains fresh groundwater and surface-water resources. In 2005, about 22.8 million gallons per day (Mgal/d) were withdrawn from water sources in St. Tammany Parish (fig. 2). Almost 100 percent (22.7 Mgal/d) was withdrawn from groundwater, and less than 1 percent (0.06 Mgal/d) was withdrawn from surface water (table 1). Withdrawals for public supplies accounted for 70 percent (16 Mgal/d) of the total water withdrawn (table 2). Withdrawals for domestic use were 28 percent (6 Mgal/d). Generally, water withdrawals in the parish increased from 1960 to 1970, decreased from 1970 to 1985, and again increased from 1985 to 2005 (fig. 2).

Figure 1. Location of study area, St. Tammany Parish, Louisiana.

This fact sheet summarizes basic information on the water resources of St. Tammany Parish, La. Information on groundwater and surface-water availability, quality, development, use, and trends is based on previously published reports listed in the references section.

Year

Figure 2. Water withdrawals in St. Tammany Parish, Louisiana, 1960–2005.

Groundwater Resources

The groundwater resources of St. Tammany Parish, from near surface to deepest, include the Chicot equivalent aquifer system, the Evangeline equivalent aquifer system, and the Jasper equivalent aquifer system (fig. 3). Aquifers in the parish generally dip and thicken to the south. Recharge to the aquifers is from rainfall, leakage from overlying aquifers, and seasonal input from rivers. Discharge from the aquifers is by natural flow into rivers, leakage into underlying aquifers, and withdrawal from wells.

Fresh groundwater (water with a chloride concentration less than 250 milligrams per liter [mg/L]) is present from land surface to about 3,000 to 3,500 ft below National Geodetic Vertical Datum of 1929 (NGVD 29) in most of northern St. Tammany Parish (fig. 1) and to about 2,400 to 3,200 ft below NGVD 29 in southeastern parts of the parish; however, some intermediate sands at depths less than 2,500 ft near Lake Pontchartrain may contain saltwater (water with a chloride concentration that exceeds 250 mg/L). Freshwater from aquifers in St. Tammany Parish is soft (less than 60 mg/L, as calcium carbonate [CaCO3]) and generally does not exceed the U.S. Environmental Protection Agency’s (EPA) 2006 Secondary Maximum Contaminant Levels (SMCLs) for drinking water for chloride, iron, manganese, and dissolved solids. Some aquifers may contain iron or manganese concentrations that exceed the EPA’s SMCLs.

Well-registration records from the Louisiana Department of Transportation and Development (DOTD) indicate that there are about 10,866 active wells screened in the aquifers in St. Tammany Parish, including about 9,740 domestic, 657 public-supply, 431 irrigation, and 38 industrial wells. About 23 Mgal/d of groundwater was withdrawn in St. Tammany Parish in 2005, and most was for public-supply (16 Mgal/d) and domestic (6 Mgal/d) use.

The Chicot Equivalent Aquifer System

The Chicot equivalent aquifer system in St. Tammany Parish consists of two adjacent, near-surface aquifers: the upland terrace aquifer in the northern half of the parish and the upper Ponchatoula aquifer in the southern half of the parish. The base of the aquifer system ranges from about 0 to 500 ft below NGVD 29.

In 2005, about 26 percent (6.0 Mgal/d) of the groundwater used in St. Tammany Parish was withdrawn from the Chicot equivalent aquifer system. Most of the water was withdrawn from the upland terrace aquifer (3.8 Mgal/d) and the upper Ponchatoula aquifer (2 Mgal/d). About 5.3 Mgal/d of the total groundwater withdrawn in this system were for domestic use, and about 0.6 Mgal/d were for public-supply use.

The base of the Chicot equivalent aquifer system ranges from about 0 ft below NGVD 29 in northern St. Tammany Parish to 500 ft below NGVD 29 in the southern parts of the parish. Aquifers in the Chicot equivalent aquifer system typically consist of 50- to 300-ft-thick units of sand and gravel.

The Chicot equivalent aquifer system contains waterbearing units throughout St. Tammany Parish. Aquifers in the system typically yield fresh water that is soft and does not exceed the EPA’s SMCLs (table 3). Water from aquifers in this system generally exceeds the SMCL for iron, and water from the upland terrace aquifer may exceed the SMCL for manganese.

Figure 3. Generalized west-to-east hydrogeologic section through St. Tammany Parish, Louisiana (Griffith, 2003). Trace of section shown on figure 1.

About 9,299 wells are screened in the Chicot equivalent aquifer system and are used for domestic (8,507), public-supply (408), irrigation (363), or industrial (21) purposes. Reported well yields from wells screened in the aquifer system generally range from about 3 to 80 gallons per minute (gal/min).

The Evangeline Equivalent Aquifer System

The Evangeline equivalent aquifer system underlies the Chicot equivalent aquifer system and in St. Tammany Parish consists of, from near surface to deepest, the lower Ponchatoula, Big Branch, Abita, Covington, and Slidell aquifers. In 2005, about 54 percent (12.3 Mgal/d) of the groundwater used in the St. Tammany Parish was withdrawn from the Evangeline equivalent aquifer system. Most of the water was withdrawn from the Slidell aquifer (6.5 Mgal/d), the Abita aquifer (2.7 Mgal/d), and the lower Ponchatoula aquifer (2.3 Mgal/d). About 10.4 Mgal/d of the total groundwater withdrawn in this system were for public-supply, and about 1.2 Mgal/d were for domestic use.

The Evangeline equivalent aquifer system contains waterbearing units throughout St. Tammany Parish. The base of the aquifer system ranges from about 1,800 to possibly about 3,000 ft below NGVD 29 south of Slidell. Aquifers in the Evangeline equivalent aquifer system typically consist of 50- to 200-ft-thick units of medium to very coarse sand.

Freshwater from aquifers in the Evangeline equivalent aquifer system is typically soft and does not generally exceed the EPA’s SMCLs; however, some freshwater may contain iron and manganese concentrations that exceed those SMCLs (table 3). Saltwater is present in the Big Branch aquifer near Lacombe and the Lake Pontchartrain shoreline.

About 1,497 wells are screened in the Evangeline equivalent aquifer system and are used for domestic (1,210), public-supply (213), irrigation (58), or industrial (16) purposes. Reported well yields from wells screened in the Evangeline equivalent aquifer system generally range from about 4 to 300 gal/min.

Water levels in the lower Ponchatoula and Big Branch aquifers are about 20 to 35 ft above NGVD 29 and have declined by as much as about 0.3 ft per year from 1996 to 2005. Water levels in the Abita, Covington, and Slidell aquifers are about 50 to 70 ft above NGVD 29 and have declined by as much as about 1.3 ft per year from 1978 to 2005 (fig. 4).

Figure 4. Water levels in well ST-563 screened in the Slidell aquifer in St. Tammany Parish, Louisiana (see fig. 1 for well location).

The Jasper Equivalent Aquifer System

The Jasper equivalent aquifer system underlies the Evangeline equivalent aquifer system and in St. Tammany Parish consists of, from shallowest to deepest, the Tchefuncte, Hammond, Amite, and Ramsay aquifers. In 2005 about 19 percent (4.4 Mgal/d) of the groundwater used in St. Tammany

Water level, in feet above National Geodetic Vertical Datum of 1929 (NGVD 29)

Parish was withdrawn from the Jasper equivalent aquifer system. Most of the water was withdrawn from the Tchefuncte aquifer (1.9 Mgal/d), the Hammond aquifer (2.1 Mgal/d), and the Amite aquifer (0.4 Mgal/d). About 4.4 Mgal/d (almost 100 percent) was withdrawn for public supply use.

The base of the aquifer system ranges from 2,350 ft below NGVD 29 in northern areas of the parish to as deep as 3,300 ft below NGVD 29 near Covington. Aquifers in the Jasper equivalent aquifer system typically consist of 50- to 250-ft-thick units of fine to coarse sand and some pea gravel.

The Jasper equivalent aquifer system contains waterbearing units throughout St. Tammany Parish. Saltwater is present in some of the aquifers in the system at and to the south of Slidell and Lacombe. Aquifers in the system typically yield freshwater that is soft and does not generally exceed the EPA’s SMCLs; however, some freshwater may exceed the SMCLs for pH, iron, and manganese (table 3).

About 70 wells are screened in the Jasper equivalent aquifer system and are used for public-supply (36), domestic (23), irrigation (10), or industrial (1) purposes. Reported well yields from wells screened in the aquifer system generally range from about 90 to 1,830 gal/min. In 2006, water levels in the Jasper aquifer system generally ranged from about 100 ft above NGVD 29 in the northern part of the parish to about 70 ft above NGVD in the southern part of the parish.

Surface-Water Resources

Lake Pontchartrain and the Pearl, West Pearl, Tchefuncte, and Bogue Chitto Rivers are the primary sources of surface water in St. Tammany Parish. In 2005, about 0.06 Mgal/d of surface water was withdrawn in St. Tammany Parish; about 0.04 Mgal/d were used for livestock, and about 0.01 Mgal/d were used for general irrigation. Other surface water resources in the parish include the Abita and Bogue Falaya Rivers. Although Lake Pontchartrain is a huge potential source of water for St. Tammany Parish, water in the lake is brackish to salty and would require treatment for most uses.

The average discharge for the Pearl and West Pearl Rivers at the town of Pearl River was about 9,470 cubic feet per second (ft/s) (6,120 Mgal/d) for the period 1964–70. Water in the Pearl and West Pearl Rivers is generally fresh, but during periods of low flow, saltwater has intruded 2 to 3 mi upstream from Lake Borgne.

The average discharge for the Tchefuncte River near Folsom was about 159 ft/s (103 Mgal/d) for the period 1944–2007. Water in the Tchefuncte River is generally fresh, but during periods of low flow, saltwater has intruded from Lake Pontchartrain upstream to the City of Covington. Water in the Tchefuncte River generally is soft but may be moderately hard (61–120 mg/L as CaCO3) and slightly acidic (pH less than 6.5 standard units) (table 4).

The average discharge for the Bogue Chitto River near Bush was about 2,000 ft/s (1,289 Mgal/d) for the period 1938–2007. Analyses of water quality samples from the river indicate that the water is typically soft but may be slightly acidic and exceed the EPA’s SMCL for iron (table 4).

References

This fact sheet was published by the U.S. Geological Survey, in cooperation with the Louisiana Department of Transportation and Development (DOTD).

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