Water-Level Surface in the Chicot Equivalent Aquifer System in Southeastern Louisiana, 2009

By

Dan J. Tomaszewski

2011

Prepared in cooperation with the 

LOUISIANA DEPARTMENT OF TRANSPORTATION AND DEVELOPMENT

OFFICE OF PUBLIC WORKS, HURRICANE FLOOD PROTECTION AND INTERMODAL TRANSPORTATION

WATER RESOURCES PROGRAMS

SCIENTIFIC INVESTIGATIONS MAP 3173

Chicot Equivalent aquifer system—SHEET 1 OF 2

Introduction

The Chicot equivalent aquifer system is an important source of freshwater in southeastern Louisiana. 

In 2005, about 47 million gallons per day (Mgal/d) were withdrawn from the Chicot equivalent aquifer 

system in East Baton Rouge, East Feliciana, Livingston, Tangipahoa, St. Helena, St. Tammany, 

Washington, and West Feliciana Parishes (figs.1 and 2; Sargent, 2007). Concentrated withdrawals exceeded 

5 Mgal/d in Bogalusa, the city of Baton Rouge, and in northwestern East Baton Rouge Parish (fig. 1). In 

the study area, about 30,000 wells screened in the Chicot equivalent aquifer system were registered with 

the Louisiana Department of Transportation and Development (LaDOTD). These wells were constructed 

for public-supply, industry, irrigation, and domestic uses (Z. Bolourchi, Louisiana Department of 

Transportation and Development, written commun., 2009). Most of the wells (about 27,500) were 

registered as domestic-use wells and are small-diameter (4-inch casing or smaller), low-yielding wells. 

Total withdrawal from the Chicot equivalent aquifer system for domestic use was estimated to be 12 

Mgal/d in 2005 (Sargent, 2007).

This report documents the 2009 water-level surface of the Chicot equivalent aquifer system in 

southeastern Louisiana. The report also shows differences in water-level measurements for the years 1991 

and 2009 at selected sites. Understanding changes and trends in water levels is important for continued use, 

planning, and management of groundwater resources. The U.S. Geological Survey (USGS), in cooperation 

with the LaDOTD, conducted this study of the water-level surface of the Chicot equivalent aquifer system 

in Louisiana as part of an ongoing effort to monitor groundwater levels in aquifers in Louisiana.

The study area (fig. 1) is bounded by the Mississippi-Louisiana State line to the north and east. The 

southern boundary is the approximate southern extent of East Baton Rouge, Livingston, Tangipahoa, and 

St. Tammany Parishes. The western boundary is the Mississippi River.

The cooperation of municipal, industrial, and private well owners during data collection is greatly 

appreciated. Special thanks go to Zahir “Bo” Bolourchi, Director, Water Resources Programs, Louisiana 

Department of Transportation and Development, for providing water-well registration data and assistance 

in the publication of this report.

Methods

Water-level data were collected by USGS personnel by using steel or electric tapes and pressure 

gages. Measurements were made by using equipment that is quality assured. Field procedures included 

making a second water-level measurement to verify the first and to ensure that the water level was static. A 

land-surface elevation was estimated for each site by using quadrangle maps with 5- or 10-foot (ft) 

contours. Although not shown on the map, water-level data reported by drillers and contained in LaDOTD 

well-registration files were used as ancillary data. Water-level data from LaDOTD were used to refine the 

conceptualization of the water-level surface with respect to land surface. Digital-elevation model data 

(Gesch and others, 2002; Gesch, 2007) were evaluated in the recharge area to configure water-level 

contours with land surface. Water-level data collected during this study are stored in the National Water 

Information System (accessible at http://nwis.waterdata.usgs.gov/la/nwis/gwlevels).

Hydrographs showing water-level data since 1990 were selected from 

long-term water-level network wells. The selected hydrographs had the most 

complete (no large time periods of missing measurements) datasets for the period 

1990–2010 and were spatially distributed across the study area. Water levels at 

these sites were measured by USGS personnel in cooperation with personnel from 

the LaDOTD and the Capital Area Ground Water Conservation Commission. An 

average rate of water-level decline for the period 1990–2009 was determined for 

each hydrograph by using a simple least-squares linear regression (Golden 

Software, Inc., 2007).

The difference between 1991 and 2009 water levels in the aquifer system 

was evaluated by using water-level data collected at 29 sites. These sites were 

measured in 1991 for mapping the water-level surface in the Chicot equivalent 

aquifer system in St. Helena, Livingston, Tangipahoa, St. Tammany, and 

Washington Parishes (Walters, 1995) or as part of a USGS observation network 

in East Baton Rouge Parish.

Hydrogeology

The Chicot equivalent aquifer system (figs. 3 and 4) consists of southerly 

dipping unconsolidated deposits of silt, sand, and gravel separated by 

discontinuous units of clay and sandy clay (Walters, 1995). The aquifer system is 

regionally extensive and extends north and east of the study area into Mississippi. 

West of the study area, the Chicot equivalent aquifer system is truncated by the 

Mississippi River alluvial aquifer system (Griffith, 2006). At the approximate 

southern boundary of the study area, locally named aquifers in the Chicot equivalent 

aquifer system transition into the New Orleans aquifer system. South of the study area, 

the aquifers contain large areas of saltwater, although limited quantities of freshwater 

are available (Tomaszewski, 2003).

In the northern half of the study area (the area north of a line drawn along the northern boundaries of East 

Baton Rouge, Livingston, and St Tammany Parishes), the system contains large amounts of sand and gravel. 

Previous reports have referred to aquifers in this area as the upland terrace aquifer or shallow aquifer (Nyman 

and Fayard, 1978; Tomaszewski, 1988). The northern half of the study area contains higher land-surface altitudes 

(hilltops) where precipitation infiltrates through permeable sands into the aquifer system—the recharge area. 

Groundwater in the recharge area has higher water-levels than in the south because of the higher elevation of 

land surface.

South of the recharge area (fig. 1), the Chicot equivalent aquifer system dips below younger deposits and 

is confined by clays (fig. 4). In the approximate southern half of the study area, aquifers in the system are 

generally referred to as the shallow, “400-foot,” or “600-foot” sands of the Baton Rouge area, and in Tangipahoa 

and St. Tammany Parishes, as the shallow and upper Ponchatoula aquifers (fig. 3).

The base of the Chicot equivalent aquifer system is about 100 ft above National Geodetic Vertical Datum 

of 1929 (NGVD 29) near the Mississippi-Louisiana State line. In northern parts of the study area, sands in the 

aquifer system are near land surface and coalesce with underlying sands. The aquifer system dips southward; the 

base of the aquifer system is about 0 ft NGVD 29 in central parts of the study area and 600 ft below NGVD 29 

in southern areas. The aquifer system generally contains 100–200 ft of combined sand 

thickness; however, sands can be locally missing. The aquifer system thickens 

southward, and thickness reaches 400 ft or more in southern areas (Griffith, 2003).

Previous investigations have shown that the Baton Rouge fault affects water 

levels in the Baton Rouge area (Whiteman, 1979; Tomaszewski, 1996). The effects of 

faulting on water levels were not evident during this investigation because water levels 

in wells south of the fault were not measured in the Baton Rouge area. In Livingston, 

Tangipahoa, and St. Tammany Parishes, thick beds of sand north and south of the 

Baton Rouge fault are present (Griffith, 2003, plates 4, 5, and 7) and may allow 

unrestricted groundwater flow across the fault in the aquifer system.

Wells in the recharge area are shallower than wells in the downdip confined 

zones. Wells constructed in the recharge area generally have total depths that range 

from 70 to 190 ft below land surface (10 and 90 percentiles) and have a median depth 

of 110 ft (Z. Bolourchi, Louisiana Department of Transportation and Development, 

written commun., 2009). Depths of wells in the southern half of the study area 

generally range from 100 to 480 ft below land surface (10th and 90th percentiles) and 

System Series

Stratigraphic

unit

Hydrogeologic units

Aquifer system

or

confining unit

Aquifer or confining unit

St. Tammany, Tangipahoa, and

Washington Parishes

North South

Q

u

atern

ary

Holocene Mississippi

River and

other alluvial

deposits

Near-surface

aquifers or

surficial

confining unit

Mississippi River

alluvial aquifer

No regionally extensive

hydrogeologic units

P Shallow aquifer leistocene

S

o

uthern

Hills regio

n

al

a

quifer system

Chicot

equivalent

aquifer

system or

surficial

confining

unit

Unnamed

Pleistocene

deposits

Shallow sands

Upland terrace aquifer

“400-ft” sand

“600-ft” sand

Upland terrace

aquifer

Upper

Ponchatoula

aquifer

Sequence of aquifers and clay confining units

? ?

East Baton Rouge, East Feliciana,

Livingston, St. Helena, and

West Feliciana Parishes

1990 1995 2000 2005

0

10

20

30

40

50

YEAR

Livingston

Washington

Tangipahoa

St. Tammany

East Baton Rouge

East and West Feliciana

St. Helena

Parishes

ANNUAL WITHDRAWAL, IN MILLION GALLONS PER DAY

Lake 

Pontchartrain

Lake 

Maurepas

APPROXIMATE SOUTHERN EXTENT OF RECHARGE AREA

12

5

5

91° 30 ́ 91° 00 ́ 90° 30 ́ 90° 00 ́

30° 00 ́

30° 30 ́

31° 00 ́

25.28

0.21

3.31

0.83

5.99

4.22

7.18 0.02

5.99

EXPLANATION

5 STUDY AREA BOUNDARY

TRACE OF SECTION—See figure 4

Registered well used for water withdrawal.

 Data source: Louisiana Department of 

 Transportation and Development 

Area where more than 5 million gallons per day (Mgal/d)

 of water was withdrawn during 2005 (from Sargent, 2007).

 Number is withdrawal rate in Mgal/d.

Total rate of water withdrawal, in Mgal/d, from the Chicot

 equivalent aquifer system in the parish during 2005.

Modified from Louisiana Oil Spill Coordinator’s Office (2007)

A

A’

A A’

WEST FELICIANA

EAST

FELICIANA

ST. HELENA

EAST 

BATON ROUGE

LIVINGSTON

TANGIPAHOA

ST. TAMMANY

WASHINGTON

BATON 

ROUGE

Mississippi River

BOGALUSA

A R K A N S A S

M

I S S I S S I P P

T

I

E

X

A

S

G U L F O F M E X I C O

Study Area

Boundary

LOUISIANA

MILES

KILOMETERS

0 10 20

0 10 20

Baton Rouge Fault

Figure 3. Correlation of hydrogeologic units in the Chicot equivalent aquifer system in southeastern Louisiana (modified from Stuart and 

others, 1994).

Figure 1. Location of the study area and locations of wells constructed in the Chicot equivalent aquifer system in southeastern Louisiana. 

Figure 2. Withdrawals from the Chicot equivalent aquifer system in selected parishes of southeastern 

Louisiana, 1990–2005 (from Sargent, 2007).

D O TD

APED TR MENT OF TRANSPORTATION AND DEVELOPMENT

Figure 4. Generalized hydrogeology of the Chico equivalent aquifer system in southeastern Louisiana (modified from Griffith, 2003). 

-1200

-800

-400

400

-1200

-800

-400

400

0 5 10 MILES

NGVD 29 NGVD 29

FEET A A’ FEET

VERTICAL SCALE GREATLY EXAGGERATED

NORTH SOUTH

CLAY CONFINING UNIT

CLAY CONFINING UNIT

FRESHWATER AQUIFER MOSTLY SAND

SEQUENCE OF AQUIFERS

AND

CLAY CONFINING UNITS

CHICOT EQUIVALENT AQUIFER SYSTEM

RECHARGE AREA

CONFINED AREA

GENERALIZED LAND-SURFACE PROFILE

WASHINGTON PARISHST. TAMMANY PARISH

NATIONAL GEODETIC VERTICAL DATUM OF 1929 (NGVD 29)

0 5 10 KILOMETERS