Several years ago I was wondering just how Lake Tahoe was formed. I discovered that geologists believe the Sierra Nevada on the west and the Carson Range on the east were once massive blocks of crystalline rock. These areas were uplifted on both sides of the present day Tahoe Basin. A large down-dropped block created the Basin in between these ranges which were joined in a large U-shape at the south end and open at the northern end. The open north end allowed snowmelt to flow out of the basin toward the east and down to the Great Basin in the area covered by Reno, Nevada today. About 2 million years ago, when large volcanic eruptions were occurring in the area, a large volcano called Mount Pluto poured lava and volcanic mudflows into the northern outlet from the valley blocking the previous outlet and allowing a large lake to form behind it. This lake which was a predecessor to the present Lake Tahoe had a water level about 600 feet higher than the present lake. During the last Ice Age, less than a million years ago, glaciers shaped the surrounding mountains and scoured the landscape, this time depositing a pile of rocks scoured by the moving glaciers as an end moraine blocked the northern end even further. The result was that the outlet of the valley was changed to where the Truckee River outlet now exists at Tahoe City.
The photo at the left illustrates how much the land surface near Mendota, California was lowered because of groundwater pumping from 1925 to 1977. The maximum subsidence at this location is the most recorded anywhere on the planet and totaled more than 29 feet. The total land subsidence at this location in the western San Joaquin Valley was more than 29 feet. This was the result of a water level decline in the area from irrigation pumping which was as much as 600 feet in one area. The land surface decline is a reflection of the compaction of the major confining layer (the Corcoran Clay) and other clay/fine silt layers below the confining layer through which the water level declined.
The layers of unconsolidated deposits below this highly productive agricultural valley extend from the land surface to over 32,000 feet down. The deposits consist of gravels, sands, silt and clays, with over 50% being of the clay/silt compressible material. Over 5,200 square miles of the San Joaquin Valley had at least 1 foot of land subsidence from 1925 to 1977. What occurs when the water level is lowered so much is that the clay layers which are up to 70% water and have no real rock structure become subjected to the weight of the overlying earth above and are squeezed like a sponge full of water. The result is that the water is removed and the material is compacted to a very thin layer which cannot be refilled when the water level rises again. The land surface above these deposits then declines in an amount equal to the total of all the clay layers that are compacted.
The person standing alongside the telephone pole is Dr. Joseph Poland who has been often called the “father of land subsidence”. I will describe this person, who was my mentor on land subsidence, more fully in a future article within this blog.
When I was working in Indiana a local controversy arose regarding the source of high nitrate concentrations that was infiltrating from groundwater into a small tributary and then into a large surface stream. It is known that most elevated nitrate concentrations come from fertilizers or animal and human wastes. Because there was a large swine farm near that location almost everyone believed the nitrate had to come from their operations. However this feed lot was fairly well-designed and the pig wastes were captured and removed so efficiently that the pig farm owner believed it couldn’t be from his operation. Also in that same reach of the tributary were numerous homes along the stream area that were on older septic tanks. So, when they presented this dilemma to me, I suggested to the swine operations that the high nitrate water be analyzed also for caffeine, which had been recently developed as a fairly simple analysis. We moved from the area before I heard any more about this problem. However, from second hand reports I learned that they discovered the high nitrate plume also contained significant caffeine. Since pigs don’t drink much coffee and humans drink lots, the source of the high nitrate was determined to be from the septic tanks.
In the early 1990’s some hydrogeologists needed to know how fast a plume of contaminated groundwater was moving toward Chesapeake Bay. A large area of land near the Bay had been a dumpsite for unexploded munitions from World War II following the war. Apparently these munitions had been buried and forgotten until some rusted through from infiltrating rainfall and exploded, bringing attention to their existence. Fear that the toxic chemicals in the burial site would reach the Chesapeake Bay caused the site owners to investigate. However, because they were aware that drilling sampling wells might cause additional explosions, they had to find another way to determine if the plume was really moving toward the Bay and how fast. The project chief noticed that there was a line of mature trees between the dump site and Chesapeake Bay. Knowing that the plume contained very high concentrations of iron, he took cores from the trees and by analyzing the specific rings representing specific years, was able to determine that the source of water for one tree had a significant rise in iron concentration in 1960. He took additional samples from another tree about 500 feet closer to the Bay from the first tree. This tree had a peaked concentration of high iron in about 1965. The findings indicated that the plume was moving at a rate of about 100 feet per year (500 feet/ 5 years).
Several studies using this tree ring dating method have been conducted in recent years with similar success and many companies are now conducting analyses of tree rings to determine the extent plumes of specific toxic organic chemicals. See publication: “Phytoforensics, Dendrochemistry and Phytoscreening: New Green Tools for Delineating Contaminants from Past and Present” in Environmental Science and Technology dated June 14, 2011 pp. 6218-6226. Also check web site: http://pubs.water.usgs.gov/sir2008-5088, which contains a user guide for this technology.