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Subsidence due to Groundwater Extraction
In the Santa Clara Valley, California

by Aubrey Weese

General Geology of Region and Aquifer

Figure 1. Location Map of the Santa Clara Valley Ingebritsen, S.E and Jones, David R U.S. Geological Survey

The Santa Clara Valley is a long, narrow trough that extends about 90 miles southeast from San Francisco (Ingebritsen 15). On the east of the trough are the foothills of the Mount Hamilton-Diablo mountain range, and on the west of the trough are the foothills of the Santa Cruz mountains. The entire region is characterized by northwest-southeast trending folds and faults. Some of these structures were caused by the collision between the Farallon and North American plates in ancient times, and some were caused by the San Andreas fault. About 1.9 million people live in the basin, nearly half of them in the city of San Jose. The climate in the area is classified as Mediterranean, with modest precipitation in winter and very dry summers. The average annual rainfall in the Santa Clara Valley floor 14.42 inches, roughly the same as Los Angles (Sweeny 2). Eighty percent of this rainfall occurs during the wet season (November to April). In this area and most of California, there is an annual moisture deficit, which means that the total evaporation and transpiration of water exceeds the total precipitation in a typical year. Only when there is a water surplus is water available to recharge aquifers. The rainfall also varies greatly from year to year in the area and droughts are common. The mountains surrounding the valley receive much more rain than the valley floor (about 40 inches per year). Runoff from the mountains is the major source of recharge water for the aquifers (Planert 5). There are two main types of rock formations in the Santa Clara county which are related to groundwater. The first is the consolidated rocks, which contian little water, and the second are the water-bearing series. The consolidated rocks are exposed in the Santa Cruz and Diablo mountains around the valley as well as in the foothills. They also underlie the entire valley at depths ranging from 100 feet to over 1,500 feet, forming the bottom of the ground basin. These rocks are primarily from the Franciscan Complex, and are composed of marine sediments and intrusive igneous rocks, ranging in age from Jurassic to late Tertiary. They do not hold much water because they have very low permeability (Robie 11). The water-bearing series of rocks have high permeability because they are made of unconsolidated or semi-consolidated clay, silt, sand and gravel. They can be split into two formations: the older Santa Clara formation and the younger valley alluvium. The Santa Clara formation is Plio-Pliocene in age, and rests on top of the consolidated rocks, being found at depths of a few feet to over 200 feet underground. It is a continental deposit that has been folded and faulted. Its grains decrease in size and it becomes less permeable with depth. The sediments also become more permeable as one moves toward the eastern side of the valley, and most of the wells are located there for this reason (Robie 12).

The Valley Alluvium is the most important water bearing unit. It is made of coalescing alluvial fans that are Cenozoic in age. The fans have been deposited by streams and rivers, such as the Guadeloupe River, which drain the adjacent highlands and meander across the valley floor into the San Francisco Bay. It has alternating layers of sand and gravel (which act as aquifers) and silt and clay (which act as aquitards). The alternation of sand and clay is believed to have been caused by a periodic rise and fall in sea level. The sand and gravel has been deposited by streams when sea level was low, and the silt and clay was deposited by the ocean when it filled the valley (Robie 12).

The main ground water basin below the Santa Clara valley belongs to a group of basins called the "Coastal Basins," all of which are structural depressions in the rock that were formed by folding and faulting (Planert 14). It is made of three interconnected subbasins. The Santa Clara subbasin is in the northern part of Santa Clara county. The northern part of this basin is overlain with a series of clay layers, creating what is called a "pressure zone" or a "confined zone." The confined zone is the source of most groundwater extraction. The southern part of the basin is not overlain with clay, and that portion is called a "forebay" or an "unconfined zone." The forebay is the zone of ground water recharge (Santa Clara Basin Watershed Management Initiative). The Coyote subbasin is entirely unconfined and drains into the Santa Clara subbasin. The Llagas subbasin is in the southern part of the county, and has a confined zone in its southern part.

Figure 2. Santa Clara Valley Groundwater Basins
Watershed Characteristics Report
Santa Clara Basin Watershed Management Initiative

These three subbasins are all filled with gravely alluvial fans deposited by streams. Water filters through this gravel down into the deeper confined aquifer in the central part of Santa Clara Valley. The thick clay unit above this lower aquifer is located at about 150 feet underground throughout the basin. The entire basin has a vast storage capacity, supplying roughly 50% of the counties water, which amounted to165,000 acre-feet in 2000 (Santa Clara Basin Watershed Management Initiative). An acre-foot is the volume of water that will cover an area of one acre to a depth of one foot.

History of Land Use

The first settlers arrived in the Santa Clara valley in 1777. The valley was given its name by the founding of Mission Santa Clara de Asis along the Guadeloupe river by Franciscan padres. Later that same year, Pueblo de San Jose was founded nearby, and became the first civil settlement in California (Robie 4). Even at this early stage, water was a problem. Missionaries in 18th century had to construct crude aqueducts that would transport water from the nearest streams and rivers to the adobe missions (Miller 2). The first well was drilled in the early 1850's to access groundwater. The water was pumped up from the wells by windmill pumps, or flowed on its own under artesian conditions. Most of the wells in the area were artesian at this time, because of the natural hydrogeology of the valley. The unconfined recharge zone of the aquifer is located at higher elevations in the foothills, and the confined zone that wells exploit is located at lower elevations on the valley floor. By 1865, close to 500 artesian wells had been dug and this number eventually rose to more than 2,000. Many of them were left uncapped, which let to a substantial waste of water.

Figure 3. Hydrogeology of Artesian Wells Ingebritsen, S.E and Jones, David R U.S. Geological Survey

The first main economic land use in the valley was cattle ranching. Then, the Gold rush came and made the population skyrocket from 14,000 in 1848 to 380,000 in 1860 (Miller 2). Farmers who came saw the agricultural riches in the Santa Clara Valley. Cattle ranching gave way to grain farming and then eventually to orcharding as Pierre Pellier discovered that the climate and soil was good for raising prunes. Soon fruit trees of many kinds (apricots, plums, cherries and pears) were springing up everywhere, and the valley became known as the dried fruit and canning fruit center of the world, and the "Valley of Heart's Delight." The only problem with this was that fruit orchards need to be irrigated and require tremendous amounts of water. Farmers had to use ground water to do this. By 1920, two-thirds of the valley was irrigated, and farmers were pumping around 40,000 acre-feet out of the aquifers per year. New wells were being drilled at the rate of 1,700 per year (Ingebritsen 17). By 1930, the ground water levels in the area had fallen to 80 feet below the ground surface. The wells that had once been artesian now had to be pumped. The situation continued to worsen as more people moved into the area and it became progressively more urban. By 1960, industry outpaced farmers almost 2 to 1 in water use. The water extraction rates had risen to around 100,000 acre feet per year (Miller 1). The ground water supply reached an all-time low of feet 235 feet below ground level in 1964. Today, California still leads the United States in agricultural and municipal water use (USGS Water Resources of California).

Figure 4. Annual Maximum Groundwater Depth in San Jose
Ingebritsen, S.E and Jones, David R
U.S. Geological Survey

Because of this massive usage of groundwater, between 1920 and 1965 the valley subsided about 4 inches per year - a total of about 13 feet (Miller 1). The maximum subsidence is in downtown San Jose, where the surface elevation has decreased from 89 feet above sea level in 1910 to 84 feet above sea level today (Ikehara 1).

Figure 5. Subsidence in the Santa Clara Valley as of 1962
Poland, J.F. and Ireland, R.L
U.S. Geological Survey

Causes and Consequences of Subsidence

Land subsidence is the decrease in elevation of the land surface because of loss of support underground. It occurs on some scale in almost every state in the United States. Most of the time is it caused by human activity, i.e., pumping fluids out of the ground such as water, oil or gas. It can also be caused by the collapse of an underground mine, dissolution of limestone (sinkholes), dewatering of organic soils or peat, and first time wetting of very dry low-density soils (hydrocompaction). But by far the major cause is excessive withdrawal of groundwater from underground aquifers (Leake 1).

This kind of subsidence has occurred in the Santa Clara Valley, as well as in many other southwestern regions. It occurs in these areas because they get low amounts of precipitation and, therefore, have low amounts of available surface water, so the inhabitants must turn to sources underground. Table 1 shows maximum subsidence measurements for various regions in the southwest as of 1997.

Table 1. Maximum Subsidence Measurements for Southwestern States as of 1997
Leake, S.A. Human Impacts on the Landscape
U.S. Geological Survey

Although it is not the most dramatic example of subsidence in the US, the Santa Clara valley is unique in being the first place the problem was discovered and tied to ground water extraction (Ikehara 1). In 1928, O.E. Meinzer of the U.S. Geological Survey first realized that aquifers were compressible, and Karl Terzaghi came up with a theory about how that happened while working at Harvard. He called it the "one dimensional consolidation theory" (USGS Water Resources of California).

The theory states that aquifers become compressed as water is pumped out of the pore spaces between the grains in sandy or gravely lawyers. As large amounts of water are withdrawn over long periods of time, it reduces the water pressure in these beds. If there are clay or silt beds nearby (aquitards) this reduced water pressure will begin to slowly draw water out of the clay. This reduces the water pressure in the clay as well, and causes it to lose its means of support. Clay is a compressible material and when it loses its support it will compact. When the clay beds underground compact, the land surface will lower permanently. The process can be halted by the addition of new ground water, but it cannot be reversed, because the clay will not decompress (Leake 1).

The fact that the Santa Clara valley was subsiding became generally known in 1933, when benchmarks that had been established in 1912 were resurveyed and found to have subsided 4 feet. Because of this finding, the U.S. Coast and Geodetic Survey erected a series of benchmarks in stable bedrock all around the edges of the Valley. These benchmarks were used to map subsidence and were monitored closely between 1934 and 1967.

In 1952, Joseph Poland headed a study carried out by the US Geological Survey to research the discrepancy in elevation measurements. They discovered that the subsidence was correlated with unprecedented growth in the use of ground water, and they also confirmed and expanded upon Terzaghi's theory of compression (USGS Water Resources of California).

Santa Clara county is also the first place where mitigation procedures were put into place to stop subsidence. These measures were necessary because land subsidence causes many problems. The decrease in elevation of certain parts of the land can change the slope of streams, disrupting or even reversing their flow. Many structures below and above land can be damaged, such as bridges, roads, storm drains, sewers, pipes, and wells. In coastal areas there are additional problems caused by high tides moving further inland and salt water intrusion into the groundwater (Leake 2). Subsidence in Santa Clara Valley has caused all these problems.

For example, in the southern end of the San Francisco Bay, 17 square miles of land sank below the high tide level, and dikes and levees had to be built to prevent flooding. This sinking changed the gradient of the streams flowing into the bay, and they had to be artificially raised to prevent them from changing direction and salt water flowing in (Ingebritsen 18). This salt water intrusion, which was detected in wells as far as 10 miles inland, greatly altered the stream channel habit around the bay. The intrusions would have been much worse if it were not for the impervious clay strata located over the major aquifers. In addition, the increase in the amount of land below sea level has reduced the extent of tidal marshes and the habitat associated with them.

Subsidence Mitigation Measures

Concern over ground water problems in the area began as early as 1913, when farmers asked the federal government for relief from the increased cost of pumping ground water as the water level continued to lower. In 1919 the Farm Owners and Operators Association presented a resolution to the Santa Clara County Board of Supervisors asking for dams to be built to supplement existing water supplies, and in 1921 a report was made showing that more water was being pumped out of the ground than nature could ever replace. In 1929, the Santa Clara County Water Conservation District was created to deal with these problems (Santa Clara Basin Watershed Management Initiative). By 1936 they had built six dams to capture runoff water from winter storms instead of letting it run into the San Francisco Bay. This runoff is released into natural streams that have permeable streambeds, and also into percolation ponds. Percolation ponds are artificial ponds that have been built in recharge zones of the aquifer where water will seep down into the aquifer through gravels and sands. Two more reservoirs were completed in 1952 (Sweeny 2).

Figure 6. Location of South Bay and San Felipe Aqueducts Sweeny, Frank. SiliconValley.com

During this same time, the county began to import water from outside because they realized that measures to recharge the aquifer from natural sources were not sufficient. In 1951 they began importing water from San Francisco. The city of San Francisco obtains water from the Tuolumne River watershed in the Sierra Nevada mountains. Water from this and from the Calaversa Reservoir is delivered to Santa Clara County by the Hetch-Hetchy aqueduct. It feeds directly into the water distribution system at sufficient pressure to avoid the need for pumping. The maximum contract limit is 6.57 million gallons per day (City of Santa Clara Plans and Ordinaces - 5.4: Water Resources). Since 1965, Santa Clara has also been receiving water from the South Bay Aqueduct which runs from the Sacramento-San Joaquin River Delta under the State Water Project. The Federal San Felipe Water Project began in 1987, supplying water via the Santa Clara Conduit. This imported water supplies half the water Santa Clara county uses. One fourth of it is put into percolation ponds and injection wells for groundwater recharge (Ingebritsen 20). The water district favors recharging water into the ground rather than simply using it directly for various reasons. One is that as the water seeps into the ground it is naturally filtered by the rocks and purified enough to be pumped directly into the water supply, instead of having to be sent to treatment plants. Another reason is that it is more cost effective than constructing storage and conveyance systems for the water. This recharge has stopped subsidence and has caused the water table to rise. It has risen 30 feet in the past ten years, from 40 feet below ground surface in 1990 (approximately sea level) to less than 10 feet (flowing artesian) in 1999. Some capped and long-forgotten wells near the bay began to leak and were discovered because of this.

The Santa Clara county water district now closely monitors ground water withdrawal. The amount of water that can be withdrawn without causing a recurrence of subsidence is called the "safe yield." The water district has done and is doing many studies to determine exactly what this is. It is important to know this because groundwater is used as an emergency backup supply in times of drought. The Water District would like to be able to increase the amount of ground water they are using, but they have to do so while staying within in the limit they have set of no more than 0.01 feet of subsidence per year. The subsidence is monitored very closely at several sites using surveying, extensometers, and recently (as of 1999) INSAR (Interferometric Synthetic Aperture Radar) technology as well (Ikehara 1).

Per-capita use of water in the area today is as little as 1/5 the level it was in the agrarian past of the 1920's (Ingebritsen 20). This is partly due to the fact that water conservation and waste water reclamation are encouraged highly in Santa Clara Valley. Meeting of future water needs hinges on conservation, but it can only do so much, and environmentalists now say there can be no more reservoirs, pipelines or aqueducts built. In addition, the water supplied by the South Bay Aqueduct is limited and may have to be reduced because of subsidence problems that are beginning to crop up in that area. It is a real problem, and the truth of the matter is that "ultimately, the limit on Santa Clara County's growth may not be traffic congestion, air pollution or the outrageous cost of housing. It may be the finite amount of water [they] can get" (Sweeny 4).

Bibliography

"Environmental Quality Element" City of Santa Clara Plans and Ordinances. July28, 1992. http://cho.ci.santa-clara.ca.us/40725.html#5-0

Ikehara, Marti E. "1999 Project Descriptions: Testing the Viability of Utilizing Interferometric Synthetic Aperture Radar to Monitor Land Subsidence in the Santa Clara Valley." USGS Water Resources of California. March 9, 1999. http://ca.water.usgs.gov/projects1999/ca526.html

Ingebritsen, S.E. and Jones, David R. "Santa Clara Valley, California. A case of arrested subsidence." U.S. Geological Survey, Menlo Park, California.

"Land Subsidence in California." USGS Water Resources of California. April 24, 2001. http://ca.water.usgs.gov/sub/

Leake, S.A. "Land Subsidence from Ground Water Pumping" U.S. Geological Survey. Human Impacts on the Landscape. July 10, 1997. http://geochange.er.usgs.gov/sw/changes/anthropogenic/subside/

Miller, Forrest. "Sinking State." The Changing American West PRISM. April, 1996. http://www.journalism.sfsu.edu/www/pubs/prism/apr96/14.html

Planert, Michael and Williams, John S. "California and Nevada Ground Water Atlas." USGS Water Resources. September 22, 2000. http://ca.water.usgs.gov/gwatlas/

Reymers, Vanessa and Hemmeter, Tracy. "Ground Water Management Plan." Santa Clara Valley Water District. July, 2001.

Robie, Ronald B. "Evaluation of Groundwater Resources: South San Francisco Bay" DWR BULLETIN No. 118-1. California Department of Water Resources. December, 1975.

Sweeny, Frank. "Projects Nurture Valley." SiliconValley.com. December 21, 1999. http://www.siliconvalley.com/news/special/orchards/ten.htm

"Watershed Characteristics Report." Santa Clara Basin Watershed Management Initiative. San Jose, California. May, 2000.

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