Earthquake Strikes Mexico

At 11:29 AM last Tuesday a strong earthquake struck the state of Oaxaca, Mexico. This quake was preceded by two milder earthquakes that stuck the area a 4.9 magnitude quake struck a 9:17 PM the evening before and a 4.3 magnitude quake struck at 5:07 AM. The big earthquake was followed by a series of aftershocks that measured at 4.6 magnitude earthquake at 12:31 PM and 5.0 magnitude two hours later, 4.6 magnitude a couple of hours after that. Tuesday night there was a 5.4 magnitude and 4.5 magnitude. Wednesday morning a 4.1 magnitude earthquake was recorded. Those are just the greater than 4 magnitude earthquakes that have been recorded an unknown number of unknown mild tremors have also occurred.

The severity of an earthquake is expressed in whole numbers and decimal fractions, and have evolved over time. The magnitude of an earthquake, usually expressed by the Richter Scale, is a measure of the amplitude of the seismic waves, but does not effectively describe the destructive power of large earthquakes. The moment magnitude of an earthquake is a measure of the amount of energy released - an amount that can be estimated from seismograph readings. The intensity, as expressed by the Modified Mercalli Scale, is a subjective measure that describes how strong a shock was felt at a particular location, which is why the USGS collects comments and location from people who felt the earthquake.

The Richter Scale, named after Dr. Charles F. Richter of the California Institute of Technology, is still the best known scale for measuring the magnitude of earthquakes. The scale is logarithmic so that a magnitude 7 earthquake is 10 times as large as a magnitude 6. Earthquakes with a Richter value of 6 or more are commonly considered major; great earthquakes have magnitude of 8 or more on the Richter scale. This earthquake falls in between major earthquake and great earthquake at 7.4 magnitude.

This is a very seismically active area. In 2017 an earthquake with a magnitude 8.2 struck the area. Historically, several significant earthquakes have occurred along the southern coast of Mexico. Located on top of three tectonic plates, Mexico is one of the world's most seismically active regions. Most of Mexico sits on the westward moving North American plate, but this area is also a subduction zone where two plates converge, and one plate is thrust beneath the other. This process results in earthquakes and volcanoes. The largest earthquakes on Earth occur in these areas.

The Pacific Ocean floor south of Mexico is being carried northeastward by the underlying Cocos plate at a geologically rapid pace of about 50 millimetres a year. Because oceanic crust is relatively dense, when the Pacific Ocean floor encounters the lighter continental crust of the Mexican landmass, the ocean floor is subducted beneath the North American plate creating the deep Middle American trench along Mexico's southern coast.

Brownish or Dirty Well Water after the Earthquake

Brownish or Dirty water is a common occurrence after earthquakes. An earthquake can cause water wells to become turbid, which is when the water is cloudy or more commonly dirty looking, and well water quality have become degraded as a result of earthquakes. Earthquakes can affect the water level in your well. To see if your well has been impacted, you will have to empty your pressure tank and see what pumps out of the well. Turbidity could move through the system and pass in a short period or not depending on the specific geology, soil type and hydro geology. Aquifers are water-bearing subsurface soil and rock formations that can be effected by seismic activity. In bedrock formations, for instance, the well will be drilled until it hits a fracture or crevice that holds water. Earthquake shocks can increase the permeability of the aquifer rocks and cause the water level to fall with gravity through the more permeable materials and the water will fall to a lower level leaving the well dry. http://pubs.usgs.gov/fs/fs-096-03/

There are also plumbing problems that can cause brownish water and do not forget that earthquakes can cause the fittings to the septic tank to fail and a well could become impacted by a leaking septic tank (or several leaking tanks in the neighborhood). First, check your plumbing, check the screens and aerators in your sinks and then verify that both the hot water and cold water are both discolored. If the hot water only is discolored then the problem might be with rust the hot water heater that was shaken loose, simply drain it. After determining that the brown water is coming from the cold water tap also, it is still possible that there is rust in the plumbing fixtures or the piping, but it would typically manifest in only one sink or tub and not uniformly throughout the house (unless the rust is in the main water pipe from the well).

After rust in the household fixtures there are four likely causes for well water to be brown or brownish, surface infiltration, soil fines having shaken loose are being pulled into the well, water level dropping or iron (and/or manganese) in the water. Earthquakes can cause a change in water, either by loosening fine grains of silt and soil, loosening minerals or lowering the water level. According the US Geological Survey there is no rhyme or reason to which wells will be impacted by an earthquake, but time might restore your well. Run your well thought your hoses to the yard for a couple of hours. Let the well rest for a couple of hours and then run the hoses again. See if there is sediment or only color with the water. If no sediment appears, you probably are only pumping fines and the well should clear after several rounds of pumping and rest. If you are pumping sediment, it might be a loss of water level. Wait and see if the well recovers. According to David Helms of the US Geological Survey in Richmond, who is still analyzing the data, USGS monitoring wells have shown significant impact. The example he gave me was the Reston well which experienced a sudden drop in groundwater level yesterday and though it recovered by this morning, the water level was lower than the pre-event level. It is unknown if the groundwater level will recover fully and a lowering of the water in a well can cause the well to basically pump mud.

Surface infiltration of water is due to impaired pump and casing system, but is unlikely to be the cause of sudden brownish water after an earthquake. A leaking septic system could also impact water quality. Testing the well for bacteria would determine if the water were safe to drink. A bacteria test checks for the presence of total coliform bacteria and fecal coliform bacteria. These bacteria are not normally present in deeper groundwater sources. They are associated with warm-blooded animals, so they are normally found in surface water and in shallow groundwater(less than 20-40 feet deep). Most bacteria (with the exception of fecal and e-coli) are not harmful to humans, but are used as indicators of the safety of the water. An inspection of the well and pump system might visually locate any obvious flaws but the presence of coliform surface bacteria would certainly identify where to begin looking. The most likely source of brown water after the earthquake is from the well itself. It is typical in Virginia not to have well casing beyond 40-50 feet deep. The Balls Bluff Siltstone and red clay common to this area does not typically need a casing. The most common modern well installation is to have a pump that installed in the well and looks a little like an outboard motor on a stick. While the most common source of brown water is soil fines shaken loose in the earthquake, there can be other causes. Changes in water level or supply could result in the pump pulling up a bit of mud or the pump could have wracked a bit and is hitting the side of the well hole. So that water that suddenly turns brown may indicate a problem with the well structure or water level. Turn on your hoses and put your hands on the well cap, if you feel any vibrations or hear a sound you probably have a pump problem. Also listen at the well pipe into the house.

The final source of brown water is iron (and/or manganese) in the water. As rain falls or snow melts on the land surface, and water seeps through iron-bearing soil and rock, iron can be dissolved into the water. In some cases, iron can also result from corrosion of iron or steel well casing or water pipes. Iron can occur in water in a number of different forms. Iron is harmless, but can affect taste and use of water. The earthquake might have shaken loose minerals or rust in your system. This source of brownish color might pass through the system like soil fines, but could require a greensand filter.

Groundwater Impacts from Geological Events

The US Geological Survey has been studying the impacts of earthquakes on groundwater. For years they have been monitoring groundwater wells to observe any noticeable changes coinciding with earthquakes. The most common effect on groundwater from earthquakes is an instantaneous water-level increase or decrease. Recovery to the pre-earthquake water level can be so rapid that no change is detected. http://va.water.usgs.gov/earthquakes/index.htm

These spikes have been observed to occur thousands of miles from the earthquake epicenters. Most of the time, these spikes are transitory with no lasting consequences for groundwater supply or quality. In rare cases the water supply is permanently changed- wells have dried up, some wells have seen flow increase, springs have been created and springs have dried up. Groundwater quality has also been observed to change as a result of earthquakes. The response is not predictable and surprisingly far reaching. The USGS monitoring well in Christiansburg, Virginia has been observed to react to earthquakes over 10,000 miles away.

Responses of water levels in wells to earthquakes are influenced by many factors including the magnitude and depth of the earthquake, distance from the epicenter, and the type of rock that surrounds the groundwater. The depth of the well, whether the aquifer is confined or unconfined, and well construction also influence the degree of water-level fluctuations in wells in response to seismic waves. Some aquifers may even act as resonators, which may amplify the response. However, there is not a complete understanding of mechanism of impacting groundwater wells and how a well will be impacted. http://va.water.usgs.gov/Gw_FS_2008.pdf

In hydraulic fracking on average 2.8 million gallons of chemicals and water is pumped into the shale formation at 9,000 pounds per square inch and literally cracks the shale or breaks open existing cracks and allows the trapped natural gas to flow. While geologists and engineers believe that there is little risk that the fracking “water,” a mix chemicals and water, will somehow infiltrate through the shale and the thousands of feet to reach the groundwater reserves though a fissure created by the fracking, there are other routes of contamination and impact.

Much is not known about the impacts of hydraulic fracturing. Shale fracking is usually at depths of approximately 9,500 feet well below the drinking water aquifer. Horizontal drilling and multi-stage fracking are used to collect this gas, which differs from the hydraulic fracturing techniques historically used. Modern fracking is not only deeper, it also uses substantially more freshwater an average of 2.8 million gallons rather than the up to 100,000 gallons and much lower pressure used in older forms of fracking. There are many unknowns with respect to the environmental and long-term impacts on groundwater supply and hydrology. Currently, the US Environmental Protection Agency (EPA) is studying the impact of hydraulic fracturing on water resources, but they are focusing on the potential to directly pollute the drinking aquifer, not looking at potential changes in the groundwater hydrology.

Millions of gallons of water are used to fracture each well. Using fresh water to fracture a well is an unsustainable use of water resources, when you consider that there are reported to be over 5,000 permits to drill hydraulic fracturing wells next year in Pennsylvania alone. That would be 14 billion gallons of water withdrawn from the water table and injected into the shale. Once the fracking process is complete, anywhere from 30-60% of the fracking water comes back to the surface as flowback. This means that each well produces a million or more gallons of wastewater which will have to be treated and disposed of. Land application, untreated release to rivers, processing through waste water treatment plants are inappropriate methods for disposal and potential sources of pollution to our water supplies. The impact and fate of the 40-70% of fracking water that does not flowback is unknown.

Errors in natural gas well construction or spills during injection can occur and lead to drinking water contamination. Fluids can spill before they are injected and fluids recovered from fracturing can contaminate surface waters. Additionally, drilling into the subsurface can create pathways for fracking fluids or natural gas to find its way into water supplies, if grouting isn’t properly done and the well properly constructed. It should be noted also that the horizontal sections of the wells are not cased in cement and, introduces a potential point where contamination can originate. Hydraulic fracturing should continue slowly. A limited number of wells should be installed with careful monitoring of local and regional groundwater supplies as well as verification of proper well construction and wastewater recycling.