Wednesday, August 24, 2011

Your Water Well After the Earthquake

Surprise, Virginia just had an intra plate earthquake measuring a 5.8 on the Richter scale. It has been well documented that earthquakes can have significant effects on water wells. Hydro geologic responses to earthquakes have been known for decades, and have occurred both close to, and thousands of miles from earthquake epicenters. The US Geological Survey has a national network of monitoring wells to study the impacts of earthquakes on groundwater and water wells. An earthquake can cause water wells to become turbid, which is when the water is cloudy or more commonly dirty looking, wells have gone dry or flow has increased, discharge of ground water to streams has increased and new springs have formed, and well water quality have become degraded as a result of earthquakes. Earthquakes can affect your drinking water well.

Aquifers which consist of unconsolidated materials can compact, or become almost liquefied as a result of the seismic energy moving though them during the earthquake, in a process called liquefaction. This results in a compression of the soils and a loss of storage for groundwater, and subsidence on the ground’s surface. 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.

The most common type of observed ground-water response is an instantaneous water-level fall or rise and can occur near or far from the epicenter of the quake without significant change to the rock formation. Recovery to the pre-earthquake water level can be so rapid as to be almost unnoticeable, or it may take as long as several days or months. Water level changes can be large enough to make a well flow to the land surface, or render a well dry. In 1998 there was an earthquake in northwestern Pennsylvania that caused about 120 local household drinking water wells to go dry within 3 months after the earthquake, they never recovered. Very large earthquakes even at great distances can also cause the water table to temporarily rise and fall when the seismic long waves pass through the state and this is the most common type of groundwater response. The 2002 earthquake in Alaska caused a 2-foot water-level rise in a well in Wisconsin, more than a thousand miles from the epicenter.

The shaking associated with an earthquake may cause sand to plug a well screen, and thus reduce the volume of water that can be pumped. Conversely, the shaking can dislodge sand plugging a well screen and cause an increase in the volume of water that can be pumped from the well. In Virginia where well casings typically extend only 50 feet below grade, the shaking or oscillation of the earth may dislodge sand or dirt within the water table that can be captured by the pump. In some cases the well returns to its normal state and the loosened particles can be flushed out of the system but in others the well needs to be serviced to restore former production volume. In an interesting report from the Geological Survey of Japan and the Japanese National Institute of Advanced Industrial Science and Technology (AIST) that groundwater anomalies were recorded several days before the 1946 Nankai earthquake. The reported phenomena were turbid groundwater, decreases of groundwater level or hot spring discharge.

According the US Geological Survey the exact mechanism linking hydro geologic changes and earthquakes is not fully understood. Because an earthquake can cause shifts in the earth and water bearing soil and rock formations, groundwater used for drinking and the private drinking water wells can be affected. 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. Well water can also become cloudy or take on a different color, smell and feel. The water can become contaminated with dirt, minerals and other solids, as well as bacteria due to damage to the casing and grouting. 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. However, if there are any indications of impact the water should be tested to ensure it is still potable.

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