Geologic Framework and Processes of Pontchartrain,

94° 93° 92° 91° 90° 89° loss and are critical environmental concerns N Arkansas Mississippi within the Pontchartrain Basin. U.S.

Geological Survey scientists and 33° collaborators have conducted a multidisciplinary study of lake-bottom Jackson sediments, sediment transport, and distribution of contaminants to assist Louisiana future remediation efforts. r Alabama e iv 32° R INTRODUCTION i p p i s and adjacent in s i Pontchartrain Louisiana form one of the larger in s s the Gulf Coast region. The drains the i Basin

M Pontchartrain Basin (fig. 1), an area of over Texas 31° 12,000 square kilometers situated on the eastern Lake side of the Mississippi delta plain. In Pontchartrain Louisiana, nearly one-third of the State population lives within the 14 parishes of the basin. Over the past 60 years, rapid growth and Baton development within the basin, along with Rouge natural processes, have resulted in significant New 30° environmental degradation and loss of critical Orleans habitat in and around Lake Pontchartrain. Human activities associated with pollutant discharge and surface drainage have greatly 100 KILOMETERS affected the water quality in the lake. This change is evident in the bottom sediments, 29° which record the historic changes of the lake. Also, land-altering activities such as logging, Figure 1. Lake Pontchartrain and the Pontchartrain Basin are located in Louisiana and Mississippi. , and control in and around the lake have led to shoreline and loss of . The effects of pollution, shoreline geologic past. The sedimentary layers below Shoreline and wetland change.—To erosion, and wetland loss on the lake and the lake bottom were imaged using a radar- determine shoreline and wetland change, his- surrounding areas have become a major like technique called high-resolution seismic torical charts and aerial photographs were ana- public concern. profiling. Sidescan-sonar images of the lake lyzed digitally. These analyses were compared To better understand the basin’s origin floor also were collected. These images are with recent surveys and charts. Because of this and the processes driving its development similar to aerial photographs of the land. work, scientists can use information on shore- and degradation requires a wide-ranging Sediment samples were collected and their line change over the past 100 years to predict study involving many organizations and compositions were analyzed to guide interpre- areas of future change. personnel. When the U.S. Geological tations of seismic profiles. Water circulation.—Water circulation in Survey (USGS) began the study of Lake Sediment characterization.—The sedi- Lake Pontchartrain was characterized by wind, Pontchartrain in 1994, information on four ment samples were also used to measure the current, , turbidity, salinity, and temperature topics was needed: (1) geologic framework, natural concentration of chemical elements measurements. Satellite images were studied to (2) sediment characterization, (3) shoreline known as trace metals. Trace metals today are observe water quality over time. Water circula- and surrounding wetland change over time, associated with pollution; knowing the amount tion, especially waves, resuspends polluted bot- and (4) water circulation. of trace metals present before humans affected tom sediments, and knowledge of circulation is Geologic framework.—The geologic the area allows the level of pollution to be key to predicting where the contaminants are like- framework is an assessment of how the determined. From surface sediments, scientists ly to be concentrated. The objective was to con- sediments accumulated in the area during the can also locate modern areas of contamination. struct a computer model of circulation patterns.

U.S. Department of the Interior USGS Fact Sheet FS–025–00 U.S. Geological Survey February 2000 90° 35' 90° 25' 90° 15' 90° 05' 89° 55' 89° 45' 89° 35' 89° 25'

Explanation Seismic survey 0 10 KILOMETERS L a k e P o n t c h a r t r a i n Sidescan-sonar survey Bathymetric survey

Bottom sediment sample 20' ain 010°

Box core survey MILES 30 Vibracore survey

Pearl The River 10' ° 30

ne rg o B ke

La 00' ° 30 50' ° Figure 2. Location of remote-sensing and sediment sampling surveys of Lakes Maurepas, Pontchartrain, and Borgne. 29

COLLABORATORS INVESTIGATIONS USGS J. Kindinger (project leader) Between 1994 and 1998, surveys were Studies include the following: conducted in Lakes Pontchartrain, Maurepas, C. Demas (water quality) J. Flocks (geochemistry) and Borgne by the USGS, in cooperation Sediment Sample Data with private, State, and Federal institutions C. Holmes (age determination) • 100+ Vibracores in lakes J. List (modeling) (see box, figs. 2 and 3). The acquired data are • Data from 100+ U.S. Army Corps of critical in characterizing the lake system and F. Manheim (geochemistry) Engineers boreholes across lake M. Marot (age determination) in providing information for environmental • 80+ box cores in lakes management of the basin. C. Polloni (data base, visuals) • 1,700 bottom sediment samples R. Signell (modeling) R. Stumpf (remote sensing) Geologic Analyses D. Willard (palynology) • Sediment composition S. Williams (framework) • Stratigraphy (layering) • Palynology (pollen and spores) EXTERNAL • Sediment ages Coalition to Restore Coastal Louisiana (CRCL) • Trace-metal concentrations M. Davies (environmental) • Models of water and atmospheric Environmental Protection Agency (EPA) circulation N. Thomas (biology) Lake Pontchartrain Basin Foundation (LPBF) Remote Sensing C. Dufrechou (director) • 650 line km of single- J. Waters (project manager) seismic profiling National Oceanic and Atmospheric • Five sidescan-sonar grids Administration (NOAA) • Four bathymetric surveys (>200 km) K. Clemens Dennis (environmental) • Continuous wave, turbidity, salinity, and U.S. Army Corps of Engineers (USACE) temperature measurements (five sites up D. Britch (geology) to five months each) University of Georgia • AVHRR (advanced very high J. Noakes (geochemistry) resolution radiometer) satellite imagery University of New Orleans (UNO) S. Penland (geology) Figure 3. Most of the surveys were conducted from the USGS Research Vessel G.K. Gilbert, M. Porrier (biology) depicted here acquiring a Vibracore. M. Roth (computers)

2 GEOLOGIC FRAMEWORK

18,000 years ago 6,000 years ago 5,000 years ago 4,500-2,000 years ago Present day A Prairie Terraces B C D E Complex Pearl River Lake Pontchartrain Pontchartrain Embayment Wisconsinan Pine St. Bernard Delta New Orleans Island exposed surface Gulf of Mexico

Figure 4. Evolutionary model for basin development from 18,000 years ago (A) to today (E).

An evolutionary model (fig. 4A–E) for sediment transport began developing a barrier- ing the St. Bernard Delta lobe (on which the the Lake Pontchartrain area was developed beach system (fig. 4B). As sea level continued city of New Orleans is located), enclosing the by using seismic and sedimentologic data. to rise, the barrier system was bypassed, form- embayment (fig. 4D). Modern stabilization and Following the late Wisconsinan sea-level low- ing the Pine Island Barrier trend, while flood- subsidence of the delta lobe began about 2,000 stand (~18,000 years ago), the sea rose and ing created the Pontchartrain Basin embayment years ago, developing Lakes Maurepas, flooded the area (fig. 4A, B). By 6,000 years (fig. 4C). By 4,500 years ago, the Mississippi Pontchartrain, and Borgne and creating the ago, sea-level rise had slowed, and longshore River moved into the area and began deposit- land surface seen today (fig. 4E).

SEDIMENT CHARACTERIZATION Figure 5. Pollen and trace-metal abundances in (Vibracore) Ipon 97–1. AND CIRCULATION MODELING Note: ppm=parts per million, wt%=weight percent.

Depth Lithology Depth Evidence for environmental change can (cm) (cm) be seen in a Lake Pontchartrain sediment core (fig. 5). Percent abundances of selected pollen taxa and trace-metal concentrations show that 100 100 about 120 centimeters (4 feet) beneath the Higher Higher Higher Higher (Pigweed family) lake bottom, pollen assemblages changed from clays Colonization and land clearing began and pine/oak (common Gulf Coast) to herbaceous silts species that indicate human colonization (Aster family) (Cattails) 200 (Sawgrass family) 200 (Grass family)

and land clearing. At the same level, the concentrations of trace metals increased, also Typha

an indication of human activity. Poaceae Asteraceae

Depositional patterns in Lake Pontchartrain Cyperaceae 300 300 were determined from current and circulation modeling and from rates of sediment deposition clays Chenopodiaceae/Amaranthaceae (fig. 6). The current model was based on 0 20 40 60 0 10 0 0 0 0 15 25 35 16 24 32 2 3 4 0.1 0.5 0.9 Concentration of Percent abundance Copper (ppm) Lead (ppm) Iron (wt%) Cadmium (ppm) prevailing wind speed and direction and pollen/gram x 103 of pollen bathymetry. Current direction and magnitude (size of arrows) show a predominantly northwest flow from the eastern side of the lake (Rigolets) during a prevailing northwest wind, which sets up gyres of circulation in the central part of the lake. Recent sediment deposition (darker red areas) appears to coincide with areas of decreased circulation (short arrows).

Wind direction Rigolets

Figure 6. Circulation and sediment accumulation patterns in Lake Pontchartrain. Current direction and magnitude (size of arrows) were generated from a computer model based on bathymetry and wind speed and direction. Areas filled by shades of red show beryllium-7 (Be7) isotope activity obtained from lake-bottom sediment samples. Darker red areas have higher concentrations of Be7, which indicate recent N sediment deposition because Be7 forms in the atmo- sphere and dissipates rapidly with a half-life of 53 W E days. Light-blue areas lack Be7. S

3 AVHRR IMAGERY

Images of Lakes Pontchartrain, Maurepas, flood-control structure was opened, and the the lake as a result of the diversion of floodwa- and Borgne were derived from data collected water from the Mississippi River was diverted ters. The sediment-laden water enters from the by the advanced very high resolution radiome- through Lake Pontchartrain to avoid potential Bonnet-Carré Spillway in the west, travels ter (AVHRR) onboard polar-orbiting weather flooding of New Orleans. The images in figure through the lake, and exits via the Rigolets into satellites. In March 1997, the Bonnet-Carré 7 illustrate the increase in suspended material in and the Mississippi Sound.

Figure 7. Turbidity in Lake Pontchartrain after the Bonnet-Carré Spillway opening. The darker red has more suspended sediment.

Bonnet-Carré Bonnet-Carré Bonnet-Carré Spillway Spillway Spillway March 6, 1997 March 23, 1997 April 7, 1997

WAVE MODELING

Resuspension and transport of bottom surface and of bathymetry. This action can be (fig. 8C, D). Results indicate that although sediments due to wave action are important simulated by using the numerical wave-pre- waves may be higher in the central portions physical components of sediment distribution diction model HISWA (Hindcasting Shallow of the lake, increased bottom orbital veloci- in Lake Pontchartrain. Wave height and Water Waves). Figure 8 shows the HISWA ties at the shorelines drive resuspension and associated bottom orbital velocity (current predictions for significant wave height transport of fine-grained sediment. motion) are functions of wind stress at the (fig. 8A, B) and maximum orbital velocity

Figure 8. Wave modeling for Lake Pontchartrain shows the increase in wave height and lake bottom orbital velocity (turbulence) generated as south winds increase from 23 to 39 knots.

A B C D Increasing Increasing Increasing Increasing wave height 23 kts. wave height 39 kts. orbital velocity 23 kts. orbital velocity 39 kts.

SHORELINE/WETLAND CHANGE

Having information about shoreline erosion Erosion rates are derived from comparison of Wetlands have been lost to urbanization and wetland change is crucial for improved recent measurements with historic data. and natural submergence. Changes include management in the Pontchartrain Basin. In the Where the shoreline has not been stabilized by cypress swamps converting to freshwater past 60 years, more than 76,000 hectares sea walls or other structures, erosion averages marsh, freshwater marshes converting to higher (188,000 acres) of land in the basin have been 2 to 4 meters per year. salinity marsh, and complete submergence. lost due to a complex suite of causes (fig. 9).

Figure 9. Shoreline erosion rates around Lake Pontchartrain. For further information, please contact: Erosion Rate (m/yr) Jack L. Kindinger 0-1 James G. Flocks 1-2 U.S. Geological Survey 2-4 600 4th Street South 4+ St. Petersburg, FL 33701 Stabilized Telephone: 727–803–8747 x3018 Fax: 727–803–2032 E-mail: [email protected]

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