Thursday, August 18, 2011

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.

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.

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.

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