Land Subsidence
Introduction
Of all water that reaches the surface of the earth from all types of precipitation, some runs off as stream flow, some is evaporated from land and water surfaces, and some is transpired by vegetation. It is the purpose of this exercise to consider some aspects of what happens to the rest of the water - that which enters the ground. The water may remain in the ground from a period of days to thousands of years.
Because of increased demand on groundwater supplies, many places in the world today are experiencing groundwater related problems.
What is Groundwater?
Some precipitation infiltrates the ground and percolates downward through voids (pores, fractures, crevices, and other spaces) in the soil and rock. The water in these voids is called groundwater. Porous, water-saturated layers of sand, gravel or bedrock through which usable groundwater flows are called aquifers. Any area of land through which water passes downward or laterally into an aquifer is called a recharge zone. Some materials are very impermeable to water infiltration, such as clay, shale or dense igneous bedrock, and are called aquicludes. (See figure 1)
Figure 1
There are two types of aquifers: confined and unconfined. An unconfined aquifer forms when groundwater collects above a layer of relatively impermeable rock or compacted clay, and the top of the water represents the water table. A confined or artesian aquifer forms when groundwater is sandwiched between two aquicludes. This type of aquifer is completely saturated with water under great pressure and when a well is drilled into the confined aquifer, sometimes water is able to rise to the surface without pumping. This type of well is called a flowing artesian well system. It is the confined aquifer that is associated with land subsidence.
Land Subsidence
Mexico City, Tokyo, Houston, Las Vegas, several areas of Arizona and California are experiencing serious problem as a result of land subsidence. Land subsidence or sinking of the land surface can be due to the removal of underground water (groundwater mining). When many wells are drilled into the aquifer, pumping removes water from the aquifer and lowers the hydrostatic pressure (water pressure). This reduced hydrostatic pressure in the pore spaces of the sediments result in the compaction of the aquifer and in the gradual lowering of the land surface. If the water is replaced in the aquifer the increased hydrostatic pressure will return the beds of sands and gravel to their original form, but once compacted the clays and silts of the confining bed will not expand to their original thickness resulting in a lower surface elevation. (See figures 2 and 3.)
Figure 2
Land subsidence can cause problems such as flooding along coastal areas, as well as structural damage to buildings, highways, and dams.
Subsurface mineral mining, petroleum mining, and a natural process called chemical (solution) weathering of certain bedrock material can also result in land subsidence. Central Florida, Kentucky, Tennessee, and a small area in Michigan have underlying limestone bedrock material. This limestone bedrock is highly soluble and forms great underground caverns. In certain cases, the land surface subsides until it reaches a point of failure and caves in. The resulting hole is called a sinkhole and can reach great depths.
Although we can't increase the earth's supply of water, we can manage what we have more effectively to reduce the impact and spread of water resource problems. Problems such as aquifer depletion, land subsidence, saltwater intrusion, and groundwater contamination are growing concerns in the United States as well as many developing countries.
Figure 3
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Subsidence in the Santa Clara Valley, California
The first place in the United States where land subsidence due to groundwater removal was recognized was in the Santa Clara Valley in California. The valley is a large, structural trough filled with nearly 2,000 feet of fined-grained sands. Below the depth of 200 feet the groundwater is confined in these fine- grained aquifers by layers of clay. In the past, wells near Santa Clara flowed (artesian wells), but pumping for irrigation resulted in a lowering of the water pressure surface by 150 to 200 feet by 1965. The water level decline and subsequent surface subsidence continued, and, except for a recharging of the aquifer between 1938 and 1947 due to the construction of surface reservoirs, was an increasingly serious problem until a groundwater recharge program brought it to a halt around 1971.
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Table 1
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Table 2
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Year
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Water level in well at San Jose
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Year
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Subsidence (ft) of benchmark at San Jose
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1915
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+8
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1915
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0.0
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1920
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-32
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1920
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0.3
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1925
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-40
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1934
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4.6
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1930
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-68
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1935
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5.0
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1935
|
-100
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1936
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5.0
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1940
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-73
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1937
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5.2
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1945
|
-50
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1940
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5.5
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1947
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-45
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1945
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6.0
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1950
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-125
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1947
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6.0
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1955
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-115
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1948
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8.0
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1960
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-120
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1955
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9.0
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1965
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-160
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1963
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11.1
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1967
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-195
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1967
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12.7
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1. Plot the data in Table 1 on the graph paper provided showing the change in water level in the well at San Jose.
2. On the same graph, plot the data in Table 2 showing the subsidence of the land surface as recorded at the known benchmark in San Jose.
3. What was the total subsidence at San Jose from 1934 to 1967?
4. What was the average annual rate of subsidence from 1934 to 1947?
5. What was the average annual rate of subsidence from 1947 to 1967?
6 . How do the subsidence rates compare
A. During which time period was the average subsidence rate higher (land sinking quickly)?
B. During which time period was the average subsidence rate lower (land sinking slowly)?
C. How does the rate of subsidence correspond with the level of the water in the well during these two time periods?
Land Subsidence and Well Levels at San Jose, California 1915-1967
Time period "A" 34-47 Time period "B" 47-67
7 . Impermeable materials such as clay, silt, shale, and dense igneous bedrock form confining subsurface layers called?
8. Permeable materials such as sand, gravel, and porous sedimentary rock such as sandstone and limestone form underground layers through which water flows are called?
9. Land subsidence can occur as the result of:
10. How does a sinkhole form?
11. Once subsidence has occurred, the land surface can be raised to its original elevation with proper groundwater recharge. (Circle one)
a. True
b. False