OVER-PUMPING OF UNDERGROUND AQUIFERS - How Over-Pumping of Underground Aquifers Can Cause Land to Sink - Sucking water from aquifers can cause land to compact over time, often lowering it hundreds of feet in elevation. Specifically, in reference to our drinking water, around half of the US population gets their drinking water from either public or private wells. Water wells are used on a massive scale. They suck water from underground aquifers for use in agriculture or for drinking water. Water is, after all, essential to life, but many civilizations have developed in regions of the world that don't have access to sufficient surface freshwater. Issues arise when the amount of water pumped out of the aquifers is greater than the amount of water flowing into the aquifers. When over-pumping occurs, large swaths of soils underground that previously were saturated with water are now left dried out permanently. All the static and dynamic forces from the land and rock above start adding up and eventually that now-dry soil starts compacting down and down. While this may not seem like a big deal on a small scale, what we've seen in California is the dropping of the surface elevation over a period of years, often by hundreds of feet or meters. This dropping of the ground level is an aspect of a principle called land subsidence. Land subsidence can occur when significant portions of groundwater are pumped out or removed from underground rock and soil.

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Over-Pumping of Underground Aquifers

How Over-Pumping of Underground Aquifers Can Cause Land to Sink

Sucking water from aquifers can cause land to compact over time, often lowering it hundreds of feet in elevation.

By Trevor English

If you're reading this, chances are you don't really have to think about where your drinking water is coming from, how your food was grown, or what effects such processes have had on the environment.

Specifically, in reference to our drinking water, around half of the US population gets their drinking water from either public or private wells.

Water wells are used on a massive scale. They suck water from underground aquifers for use in agriculture or for drinking water.

Water is, after all, essential to life, but many civilizations have developed in regions of the world that don't have access to sufficient surface freshwater.

Take, for example, California and much of the southwestern United States. While this region is one of the most densely populated in the country, it also happens to be one of the most water-scarce.

California has a massive agricultural industry too, meaning the state requires a significant amount of water on a daily and seasonal basis.

For the most part, the states' water has historically come from wells drilled that pump out freshwater from confined and unconfined aquifers under the surface.

This is a fairly common practice globally, but issues arise when the amount of water pumped out of the aquifers is greater than the amount of water flowing into the aquifers.

When over-pumping occurs, large swaths of soils underground that previously were saturated with water are now left dried out permanently.

All the static and dynamic forces from the land and rock above start adding up and eventually that now-dry soil starts compacting down and down.

While this may not seem like a big deal on a small scale, what we've seen in California (and other parts of the world too) is the dropping of the surface elevation over a period of years, often by hundreds of feet or meters.

This dropping of the ground level is an aspect of a principle called land subsidence. Let's take a closer look at just what is occurring.

What is land subsidence?

Land subsidence can occur when significant portions of groundwater are pumped out or removed from underground rock and soil.

The previously water-saturated rock and soil, which is now dry, start compressing under the forces of the rock above it.

Land subsidence is a fairly slow process, and is not generally noticeable on a day to day or minute to minute basis, in the way that says, an earthquake would be.

Localized subsidence is also the principle that describes why a sinkhole or a pothole might occur, only in these cases, it's far more localized land subsidence than the kind that would drop an entire region's elevation.

While dropping the surface elevation in a region may not seem like a big deal, it's actually a rather costly one.

In theory, the dropping of the ground level is fine if it occurs evenly everywhere, but that's not the case.

Land subsidence is a highly variable process, based largely on the soil makeup under the surface.

If one area is comprised mainly of soft clay and the other is mainly of silt, the two areas are going to compact at different rates, even if they took up the same volume when saturated with water.

Since soils are made up of different components and have different shapes and sizes of particles, this also means that different soils can take up various amounts of water.

In gardening, this is one reason why you might buy specialty potting soil – to help the soil retain more or less moisture.

In geology, these different absorption rates impact how engineers design building foundations and wells for underground aquifers.

Back to the core principle here, land subsidence is a big issue because it causes the ground level to sink at highly variable and uneven rates.

When structures are present on the surface, the movement can be enough to crack foundations, collapse bridges, crack underground pipes, and otherwise wreak havoc on civil infrastructure.

Humans have a pretty hard time visualizing something that happens over the span of years though, so I find that when learning about land subsidence, the most effective thing you can do is look at pictures that document its effects.

Perhaps one of the most famous is that of the subsidence that occurred in the California Valley over the span of five decades.

As a warning, this image is going to require some scrolling, but it hopefully underscores just how substantial land subsidence can be.

After seeing that, you hopefully have some grasp on just how substantial land subsidence from over-pumping of aquifers can be.

One might next wonder how this can be tracked and prevented.

While we won't go into that too much in this article, the short answer is that governing bodies track the water levels in wells across a region to monitor whether there is more or less water.

If signs of overpumping are starting to be seen, engineers can either keep pumping and deal with the effects of subsidence, or find another source for water.

The USGS Active Groundwater Level Network includes about 20,000 wells that have
 been measured by the USGS or USGS cooperators at least once within the past 13 months.
The animation shows a daily snapshot of water-level statistics in the network for July 2015,

The gif below is a compilation of USGS well data across the US, indicating water levels in their well network.

This type of data is one of the tools engineers use to figure out how underground water is flowing, refilling, and otherwise behaving.

Engineers will also utilize tools like compaction recording devices to measure the change in sediment layer thicknesses underground.

These tools, rather than measuring water level, allow engineers to measure the soil effects from well pumping in a given area.

The figure below does a pretty good job explaining how this works.

The harmful effects of over-pumping

We've talked a little about how harmful over-pumping can be to buildings and infrastructure, but this isn't the only issue with overpumping, nor have we discussed the full scope of the infrastructure issues that arise.

Land subsidence has many core problems, most of which can be summed up concisely like this – land subsidence can cause:

o  Changes in elevation and slope of streams and surface water systems

o  Damage to infrastructures like roads, bridges, pipes, levees, and sewers

o  Damage to surface buildings

o  Failures of wells

o  The intrusion of chemicals from agriculture

One of these problems is a change to naturally occurring geography, three are changes to infrastructure, and the last is the degradation of water quality.

Let's focus on the change to geography first.

If subsidence changes the path or elevation change across a stream, it can increase its flow rate, causing more erosion in a given area, which can make it harder for certain types of fish and plants to survive, and all of this, in turn, could impact the surrounding ecosystem and even human habitation.

Changes in geography from land subsidence can affect coastal regions too. A house once built far away from high tide elevation may suddenly find itself dangerously close to the water.

Keep in mind this effect is completely different from changes in tide levels caused by climate change.

Going back to the initial list of problems, we've already spent a decent amount of time going over how changes to the ground might impact, damage, or destroy surface and subsurface infrastructure, so I'm mostly going to skip over those effects for further explanation.

What is a new topic though, is the discussion of how land subsidence can actually pollute subsurface water.

Specifically, overpumping from wells can cause concentrations in groundwater of pollutants like arsenic to skyrocket.

Arsenic is a naturally-occurring chemical in nature. Over time, arsenic is transported through rivers and deposited into clay.

This has occurred for millions of years and over time, these clay layers get pushed deeper and deeper into the earth's crust.

What has ended up occurring is a large arsenic concentration in deep clay structures, usually far deeper than wells would ever pump.

However, when wells are overpumped and the soil on top is drained of water, the well starts pulling water from the fine clays underneath, which can have high arsenic concentrations, bringing the arsenic along with it.

This increased arsenic concentration can then pollute crops and drinking water.

Even low levels of arsenic, such as 10 milligrams per liter (8.3 pounds per gallon), are harmful and can cause increased rates of cancer, heart disease, and diabetes.

At the end of the day, land subsidence from over-pumping of subsurface aquifers is a major issue in water-starved regions across the world.

It's also an issue that's not easily solved since, first and foremost, societies need water.

It can be very difficult to switch to less water-intensive crops or growing methods, and you can't just tell people not to drink water or grow crops.

Land subsidence is just one of the fascinating issues that civil and environmental engineers work to solve on a daily basis, and it's one that crucial to the future of many regions across the world.

Trevor English

Writer

Trevor is a civil engineer (B.S.) by trade and an accomplished writer with a passion for inspiring everyone with new and exciting technologies. He is also a published children’s book author and the producer for the YouTube channel Concerning Reality.

https://interestingengineering.com/how-over-pumping-of-underground-aquifers-can-cause-land-to-sink

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SEDIMENT SUPPLY AND THE IMPORTANCE OF BIG RIVERS - Sediment supply is controlled primarily by tectonics and climate. In geologically simple areas, where the basin is fed directly from the adjacent margins and source-area uplift is related to basin subsidence, supply considerations are likely to be directly correlated to basin subsidence and eustasy as the major controls of basin architecture. Such is the case where subsidence is yoked to peripheral upwarps, or in proximal regions of foreland basins adjacent to fold-thrust belts. However, where the basin is supplied by long-distance fluvial transportation, complications are likely to arise. Where the rate of sediment supply is high, it may overwhelm other influences to become a dominant control on sequence architecture. Many sedimentary basins were filled by river systems whose drainage area has been subsequently remodeled by tectonism, and it may take considerable geological investigation to reconstruct their possible past positions. Dynamic topographic processes have generated regional uplifts and continental tilts that have resulted in deep erosion and large-scale continental fluxes of detrital sediment. Much of the detritus derived by uplift and erosion of the Grenville orogen of eastern North America during the late Precambrian may have ended up contributing to the thick Neoproterozoic sedimentary wedges on the western continental margin.

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 Sediment Supply and the Importance of Big Rivers

By OK 

Learning Geology

 

Sediment supply is controlled primarily by tectonics and climate.

In geologically simple areas, where the basin is fed directly from the adjacent margins and source-area uplift is related to basin subsidence, supply considerations are likely to be directly correlated to basin subsidence and eustasy as the major controls of basin architecture.

Such is the case where subsidence is yoked to peripheral upwarps, or in proximal regions of foreland basins adjacent to fold-thrust belts.

However, where the basin is supplied by long-distance fluvial transportation, complications are likely to arise.

Where the rate of sediment supply is high, it may overwhelm other influences to become a dominant control on sequence architecture.

Many sedimentary basins were filled by river systems whose drainage area has been subsequently remodeled by tectonism, and it may take considerable geological investigation to reconstruct their possible past positions.

For example, stratigraphic successions may occur that cannot be related to the evolution of adjacent orogens.

In North America, dynamic topographic processes have generated regional uplifts and continental tilts that have resulted in deep erosion and large-scale continental fluxes of detrital sediment.

For example, much of the detritus derived by uplift and erosion of the Grenville orogen of eastern North America during the late Precambrian may have ended up contributing to the thick Neoproterozoic sedimentary wedges on the western continental margin.

Detailed study of detrital zircons from sedimentary rocks of this age in the western Canadian Arctic indicated that 50% of them are of Grenville age.

A major west-flowing river system was established during the late Proterozoic which transported this detritus some 3,000 km across the continental interior.

Much of the thick accumulations of late Paleozoic and Mesozoic fluvial and eolian strata in the southwestern United States had been derived from Appalachian sources, and this was confirmed by the detrital-zircon.

Tertiary river system draining from the continental interior of North America into Hudson Bay, ultimately delivering sediment to the Labrador Shelf.

This has been supported by the studies of Cenozoic landforms and sediments.

Major river systems may cross major tectonic boundaries, feeding sediment of a petrographic type unrelated to the receiving basin, into the basin at a rate unconnected in any way with the subsidence history of the basin itself.

The modern Amazon river is a good example. It derives from the Andean Mountains, flows across and between, and is fed from several Precambrian shields, and debouches onto a major extensional continental margin.

From the point of view of sequence stratigraphy, the important point is that large sediment supplies delivered to a shoreline may overwhelm the stratigraphic effects of variations in sea level.

A region undergoing a relative or eustatic rise in sea level may still experience stratigraphic regression if large delta complexes are being built by major sediment-laden rivers.

Effects of upstream controls on the development of fluvial graded profiles, fluvial style and the development of nonmarine sequences downstream.

Upstream controls may also be significant in the case of deep-marine deposits.

Major episodes of submarine-fan sedimentation in the North Sea and Shetland-Faeroes basins correlate with pulses of Iceland plume activity, which caused magmatic underplating of the continental margin, and uplift, erosion, and enhanced sediment delivery to offshore sedimentary basins.

A significant example of this long-distance sedimentary control is the Cenozoic stratigraphic evolution of the Texas-Louisiana coast of the Gulf of Mexico.

This continental margin is fed with sediment by rivers that have occupied essentially the same position since the early Tertiary.

The rivers feed into the Gulf Coast from huge drainage basins occupying large areas of the North American Interior.

Progradation has extended the continental margin of the Gulf by up to 350 km.

This has taken place episodically in both time and space, developing a series of major clastic wedges, some hundreds of metres in thickness.

The major changes along strike of the thickness of these clastic wedges is also evidence against a control by passive sea-level change.

Highly suggestive are the correlations with the tectonic events of the North American Interior; for example, the timing of the Lower and Upper Wilcox Group wedges relative to the timing of the Laramide orogenic pulses along the Cordillera.

It seems likely that sediment supply, driven by source-area tectonism, is the major control on the location, timing and thickness of the Gulf Coast clastic wedges.

A secondary control is the nature of local tectonism on the continental margin itself, including growth faulting, evaporite diapirism and gravity sliding.

Variations in deep-marine sediment dispersal in the Gulf of Mexico show very similar patterns to the coastal and fluvial variations.

Large-scale submarine-fan systems are therefore dependent, also, on considerations of long-term sediment supply variation, which may be controlled by plate-margin tectonism, in-plane stress regime and dynamic topography.

In arc-related basins volcanic control of the sediment supply may overprint the effects of sea-level change.

Sediment supply and tectonic activity overprinted the eustatic effects and enhanced or lessened them.

If large supplies of clastics or uplift overcame the eustatic effects, deep marine sands were also deposited during highstand of sea level, whereas under conditions of low sediment input, thin-bedded turbidites were deposited even during lowstands of sea level.

Other examples of the tectonic control of major sedimentary units are provided by the basins within and adjacent to the Alpine and Himalayan orogens.

Sediments shed by the rising mountains drain into foreland basins, remnant ocean basins, strike-slip basins, and other internal basins.

But the sediment supply is controlled entirely by uplift and by the tectonic control of dispersal routes.

For example, the Oligocene Molasse of the Swiss proforeland basin was deposited by rivers flowing axially along the basin, and that these underwent reversal in transport directions as a result of changes in the configuration of the basin and the collision zone during orogenesis.

The shifting of dispersal routes through basins and fault valleys within the Himalayan orogen of central and southeast Asia.

Some of the major rivers in the area (Tsangpo, Salween, Mekong) are known to have entirely switched to different basins during the evolution of the orogen.

Much work remains to be done to relate the details of the stratigraphy in these various basins to the different controls of tectonic subsidence, tectonic control of sediment supply, and eustatic sea-level changes.

Learning Geology is a science website and a community of Earth Scientists that shares geology lessons and host live virtual field tours from all around the world.
With our postings we aim to reach to our fellows with broader public regardless of the boundaries. The postings are done in the great interests of the fellow to learn more and more. The reason for writing is to provide an environment where we can give the best knowledge present just to be shared in the field of Earth and space sciences that we present here. Nobody judge anyone so feel free to feedback and let us know what great ideas you have, be a contributor.  

http://geologylearn.blogspot.com/2015/07/sediment-supply-and-importance-of-big.html

 

FIGHTING SALTWATER INTRUSION INTO GROUNDWATER SUPPLIES - In coastal areas, drawing too much water from underground aquifers draws in saltwater, making the water unfit for consumption or irrigation. As freshwater supplies around the world diminish, human intervention is needed to protect existing sources to slow the onset of water scarcity. One ongoing struggle is to preserve the water in underground aquifers, which cities often rely on for drinking water. Intrusion of saltwater into these underground supplies is rendering them unfit for consumption or irrigation. The rock and soil that form these underground basins are permeable. This allows the aquifers to recharge from precipitation, but when over-extraction changes the aquifer’s pressure, saltwater can seep in. When cities exclusively rely on groundwater as a water source, this is a problem. According to experts from the San Francisco Public Utilities Commission explained to the San Francisco Examiner, San Francisco blends local groundwater with reservoir-sourced water. To keep its groundwater use in check, wells are monitored for both water level and salt content. Agency officials said that if either level should change, it will switch to other wells or stop using groundwater. In nearby Watsonville, the Pajaro Valley Water Management Agency is spending $6.3 million to address the area’s seawater intrusion problems, which are caused by overdrafting.

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Fighting Saltwater Intrusion Into Groundwater Supplies

FLUENCE NEWS TEAM

 

In coastal areas, drawing too much water from underground aquifers draws

 in saltwater, making the water unfit for consumption  or irrigation.

As freshwater supplies around the world diminish, human intervention is needed to protect existing sources to slow the onset of water scarcity.

One ongoing struggle is to preserve the water in underground aquifers, which cities often rely on for drinking water.

Intrusion of saltwater into these underground supplies is rendering them unfit for consumption or irrigation.

The rock and soil that form these underground basins are permeable.

This allows the aquifers to recharge from precipitation, but when over-extraction changes the aquifer’s pressure, saltwater can seep in.

When cities exclusively rely on groundwater as a water source, this is a problem.

According to experts from the San Francisco Public Utilities Commission explained to the San Francisco Examiner, San Francisco blends local groundwater with reservoir-sourced water.

To keep its groundwater use in check, wells are monitored for both water level and salt content.

Agency officials said that if either level should change, it will switch to other wells or stop using groundwater.

In nearby Watsonville, the Pajaro Valley Water Management Agency is spending $6.3 million to address the area’s seawater intrusion problems, which are caused by overdrafting.

This problem is well documented in this and other parts of California, and in coastal New Jersey and Florida, and has been recorded throughout the world for about 100 years.

Island areas such as the Maldives have had ongoing struggles.

Rising Sea Level and Subsidence

In Egypt, a combination of the rising sea level and land subsidence is increasing saltwater intrusion.

With only 660 cubic meters of fresh water a year for each resident, Egypt is one of the world’s water-poor nations.

In comparison, residents of the United States use 9,800 cubic meters of water per capita.

Crop productivity has been decreasing, and with the population of Egypt expected to double within the next 50 years, a lack of both water and food could prove catastrophic.

The global rise in seawater levels — expected to increase between 11 and 88 centimeters during this century — will exacerbate the problem around the world.

Ahmed Sefelnasr, an Assuit University geologist, explained the problem in detail. Not only will the land eventually shift, he said, but also:

[C]limate change may cause variations in rainfall which would affect the natural replenishment of groundwater. […] due to the anticipated reduction in rainfall and surface water resources in arid and semi-arid regions, the reliance on, and exploration of, groundwater resources would increase to substitute for the scarcity of surface water resources and meet the water demands of the various sectors.

Sustainable Water Supplies

If the best solution for the problem is to not use underground water, then what should be done?

Many communities are exploring a combination of options to create more sustainable and resilient water supplies.

These include groundwater management, aquifer recharge, water reuse, stormwater management and capture, and advanced water treatment, including desalination.

The problems are long-term ones. Rosemarie Imazio, head of the Pajaro Valley Water Management Agency board explained to the Santa Cruz Sentinel:

Every bit [of water] we can save and put to a better use without having to draw out of the ground is hugely important. […] We’re chipping away at it a little at a time.

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