Activity 12.6 Land Subsidence from Groundwater Groundwater Withdrawal (p. 345) A: 935), where are the two areas of greatest subsidence in the Santa Clara Valley? Santa Clara san iose 2. What was the total subsidene 3. What was the average annual rate the total subsidence at San Jose (Figure A12.6.1) from 1934 to 1967? 41.6 12.7 127-4.6=801 Average annual rate of subsidence for this period (1934-1967) in 33 years feet per year? 0.24552 Llueais first? uld further subsidence occur, what area would become flooded Explain your reasoning: 6. By 1960, the total subsidence at San Jose had reached 9.0 feet (Figure 12.6.1). What was the average annual rate of subsidence (in feet per year) for the seven- year period from 1960 through 1967:44.7 (ft/yr). Show your work below: 1960-1967 Tur 2.11 – Supplemental questions modified from lab manual: 32.717-4.672 4.6781 /year $1. Refer to the figure on the next page. What was the level of the water in this San Jose well (left side of chart) in: A. 1915? feet B. 1967? feet 52. During what period would the San Jose well have been a flowing artesian well? Explain: Water table level (feet above and below land surface LS – 240 – 160 – TTTTT 1915 1920 1940 1950 1960 1970 Subsidence feet)
The Correct Answer and Explanation is:
Activity 12.6: Land Subsidence from Groundwater Withdrawal – Santa Clara Valley
1. Where are the two areas of greatest subsidence in the Santa Clara Valley?
The two areas of greatest subsidence in the Santa Clara Valley are:
- San Jose
- Santa Clara
These locations experienced the most severe ground-level sinking due to extensive groundwater pumping over the decades, particularly during the early-to-mid 20th century.
2. What was the total subsidence at San Jose from 1934 to 1967?
- Starting elevation in 1934: 4.6 feet
- Ending elevation in 1967: 12.7 feet
- Total subsidence: $$
12.7 – 4.6 = \boxed{8.1 \text{ feet}}
$$
3. What was the average annual rate of subsidence (1934–1967)?
- Total time: 33 years
- Total subsidence: 8.1 feet
- Average annual rate: $$
\frac{8.1 \text{ feet}}{33 \text{ years}} \approx \boxed{0.2455 \text{ feet/year}}
$$
4. If further subsidence were to occur, what area would become flooded? Explain.
If further land subsidence were to continue, low-lying areas such as parts of San Jose could become vulnerable to flooding, particularly during periods of heavy rainfall or high tides. San Jose lies near the southern end of the San Francisco Bay, and with continued lowering of the land surface, it would sit at or below sea level. This would make it increasingly difficult to drain stormwater naturally and could lead to chronic inundation without extensive pumping systems or levees. This is especially concerning in the context of climate change, where sea levels are projected to rise, further increasing flood risk in subsided regions.
5. What was the average annual rate of subsidence from 1960 to 1967?
- Total subsidence in 1960: 9.0 feet
- Total subsidence in 1967: 12.7 feet
- Difference: $$
12.7 – 9.0 = 3.7 \text{ feet}
$$ - Time period: 7 years
- Average annual rate: $$
\frac{3.7 \text{ feet}}{7 \text{ years}} \approx \boxed{0.5286 \text{ feet/year}}
$$
This rate is significantly higher than the earlier period, likely due to intensified pumping during the 1960s.
6. What was the water level in the San Jose well?
- A. In 1915: approximately 40 feet above land surface
- B. In 1967: approximately 200 feet below land surface
7. During what period was the San Jose well a flowing artesian well?
A flowing artesian well occurs when the water level is above the land surface, allowing water to flow naturally without pumping.
- The well was a flowing artesian well around 1915, when the water level was approximately 40 feet above the land surface.
- This condition ceased when water levels dropped below the surface due to over-pumping, which depressurized the aquifer and stopped the natural flow.
Explanation (300+ words)
Land subsidence occurs when large volumes of groundwater are withdrawn from aquifers, causing the ground above to compact and sink. In the Santa Clara Valley, including areas like San Jose and Santa Clara, aggressive groundwater pumping in the early 20th century led to significant subsidence. From 1934 to 1967, San Jose experienced a total subsidence of 8.1 feet, averaging 0.2455 feet per year. However, during the 1960 to 1967 period alone, the subsidence rate sharply increased to 0.5286 feet per year, likely due to increased water demand as the area urbanized.
The greatest concern with ongoing land subsidence is the permanent loss of aquifer storage capacity and the increased flood risk to already low-lying areas. San Jose is particularly vulnerable because further subsidence could place parts of the city below sea level. This would make gravity-based drainage systems ineffective and require costly infrastructure like levees and pumping stations to manage water flow. The situation is exacerbated by the threat of sea level rise due to climate change, compounding the flood risk in areas that have already subsided.
Historically, in 1915, the San Jose well was a flowing artesian well, with water levels about 40 feet above the land surface. This meant water flowed naturally due to pressure. However, by 1967, the water level had dropped about 200 feet below the surface—evidence of severe aquifer depletion. This shift marks a dramatic change in the regional groundwater system, showing how unsustainable withdrawal can transform hydrological dynamics.
In summary, this activity highlights the delicate balance between groundwater use and land stability, emphasizing the need for sustainable water management practices to prevent further environmental and infrastructural damage.