Measurements of Sedimentation Rates in Caddo Lake
Jerry Lisantti, Associate Professor of Physics
Dept. of Physics and Engineering
Centenary College of Louisiana
Shreveport, LA 71134
Caddo Lake which straddles the Texas-Louisiana border is the largest naturally formed lake in Texas. It is an area of beauty and serves as a large wetlands area for various plants and animals. It is one of 15 wetlands areas in the United States identified by the Ramsar convention "as a wetlands of international importance especially as a wildfowl habitat."
Caddo Lake was formed in the late 1700's as a product of the great log rafts that formed on the Red River during this time. This log jam was eventually cleared in 1873 and the lake region began to slowly drain. In the early part of the 1900's oil was discovered in the region and it was determined that an oil field extended into the lake and in 1914 a dam was constructed on the east end of the lake so as to raise the water levels so that oil drilling equipment could be floated onto the lake. Thus, Caddo Lake became the first place in the world in which oil was drilled for over a water body.
This lake has a history of being studied by geologists most recently for a prosed dredging of deep channels so as to form part of the waterway for the Red River lock and dam project.
Centenary College Geology Professor Mary Barrett has been studying the complete sediment history of the lake in order to reconstruct its origins, development and its possible future. Part of this study involves the measurement of modern sedimentation rates. This document discusses our method for carrying out such work and recent results.
The modern sedimentation rates are studied by examining the amount of radioactive 137Cs in the sediment as a function of depth of the sediment. 137Cs is not a naturally occurring radioisotope, it is generally created in the fission of U or Pu isotopes. During thermonuclear tests that occurred in the atmosphere during the 1950's and early 60's this isotope was produced and then transported over the globe. This occurred since these tests transported material into the stratosphere and the material was distributed over the globe. One assumes that the Cs that eventually reaches the ground was distributed uniformly. Atmospheric weapons tests ceased in 1963 due to the Atmospheric test ban treaty signed that year by the major nuclear powers. Thus one assumes that no more 137Cs was deposited in the atmosphere after that year. 137Cs has a half life of 30.2 years so it is still present in the natural environment. By measuring the activity of 137Cs as a function of depth of the sediment one can obtain a rate of sedimentation that has occurred from 1963 until the present. If the activity is plotted versus the depth of the core what is observed is a fairly constant count rate as a function of depth and at a particular depth the activity drops off in a smooth manner until a low level is reached. Where the activity drops off is where the amount of 137Cs produced reached a maximum. It drops off since this corresponds as a backwards time projection from 1963 until 1952 (the time of the first American thermonuclear test) The point where the activity reaches its maximum and drops off is assumed to correspond to 1963 so the amount of sediment in the sample above that point is how much sediment has occurred since 1963. Thus a modern sedimentation rate can be obtained.
The measurement of each core takes approximately 30 to 45 days so site selection is important. Various locations have been studied by Dr. Barrett and from these locations our sites were selected. Cores are taken with a 2" PVC pipe. The part of the coring device in which the core is taken is attached to a ribber holder that can be tighten to the sample holder. This is attached to another 2" PVC pipe of various length which has a fixture at the end in which to put an endcap onto. The pipe is driven down into the sediment to a depth of 18 to 24". The top of the coring device is then capped which forms a seal to hold the sediment. The pipe is quickly raised to the surface and a cap is put on the bottom of the core section. The sample pipe is removed from the coring device and its top is also capped. Thus one has a 2" PVC pipe approximately 2' long that contains the sediment.
Care is taken to keep the core upright. The core is then frozen to prepare it to be cut into sections. Once frozen the core is cut into 2 cm sections using a bandsaw. Each core section is then placed into a pyrex petri culture dish. These cores are then placed in an oven and heated to approximately 150C for about one day to remove organic material. Before counting the sample is placed in a mortal and pistal and ground to a fairly uniform powder whose mass is then measured. The sample is placed inside a Pb brick castle with the sample approximately 1 cm from a HpGe detector. The sample is counted for 3 to 4 days and then peak fit using the acquisition computer. The software GAMMAVISION is used for both acquisition and data analysis. The absolute efficiency of the detector is not known so data is reported in the form of counts/min/g (counts per minute per gram of material).