People living in the vicinity of shale gas drilling have reported foul smells in their tap water. In some instances gas well pipes have broken, resulting in leakage of contaminants into the surrounding ground.

One small town in Pennsylvania called Dimock, for example, has been devastated by fracking. Cabot Oil & Gas drilled dozens of wells in Dimock. Sadly, problems with the cement casing on 20 of those caused contamination of local water wells, driving down property values and causing sickness. In some cases, levels of methane in some Dimock water wells are so high that homeowners are able to set water aflame as it comes out of their taps. In April, 2010, state environmental regulators fined Cabot $240,000, and ordered it to permanently shut three wells and install water-treatment systems in 14 homes within 30 days or face a $30,000 a month fine. Cabot's more than two-dozen pending drilling applications were also put on hold.

The violations seen in Dimock are not uncommon in Pennsylvania. A 2010 report issued by the Pennsylvania Land Trust Association found that the state has identified 1,435 violations by 43 Marcellus Shale drilling companies since January 2008. Of those, 952 were identified as having or likely to have an impact on the environment.

Texas’ Barnett shale region is another area where fracking is booming. In August, 2010, an air sampling in the Texas town of Dish by Wolf Eagle Environmental "confirmed the presence in high concentrations of carcinogenic and neurotoxin compounds in ambient air near and/or on residential properties." In June 2010, tests by the Texas Railroad Commission showed arsenic, barium, chromium, lead and selenium in a residential water well in Dish. The tainted water turned up at a home in Dish shortly after a nearby gas well was drilled.

Results of air testing by the commission released the same month detected benzene concentration, 37 parts per billion, at a Devon Energy complex on Jim Baker Road between the towns of Justin and Dish. The highest benzene reading overall, 95 ppb, was detected at a Stallion Oilfield Services commercial disposal well in Parker County. All six facilities that state inspectors revisited are within about 1,000 feet from people’s homes.

Some environmental and human health concerns possibly associated with hydraulic fracturing may include the potential mishandling of solid toxic waste, potential risks to air quality, potential contamination of ground water, and the unintended migration of gases and hydraulic fracturing chemicals to the surface within a given radius of drilling operations. The potential costs associated with possible environmental clean-up processes, loss of land value and human and animal health concerns are undetermined. New technological advances and appropriate state regulators are working to study and safely implement the process.

A well blowout in Clearfield County, PA on June 3, 2010, sent more than 35,000 gallons of hydraulic fracturing fluids into the air and onto the surrounding landscape in a forested area. Campers were evacuated and the company EOG Resources and the well completion company C.C. Forbes have been ordered to cease all operations in the state of Pennsylvania pending investigation. The Pennsylvania Department of Environmental Protection has called this a "serious incident".

In April, 2010, the state of Pennsylvania banned Cabot Oil & Gas Corp. from further drilling in the entire state until it plugs wells believed to be the source of contamination of the drinking water of 14 homes in Dimock Township, PA. The investigation was initiated after a water well exploded on New Year's Day in 2009. The state investigation revealed that Cabot Oil & Gas Company "had allowed combustible gas to escape into the region's groundwater supplies."

Injection of fluid into subsurface geological structures, such as faults and fractures, reduces the effective normal stress acting across these structures. If sufficient shear stress is present, the structure may slip in shear and generate seismic events over a range of magnitudes. Subsidence is not directly caused by hydraulic fracturing but may occur after considerable production of oil or ground water. Subsidence occurs over reservoirs whether they have been subject to hydraulic fracturing or not because it is a result of producing fluids from the reservoir and lowering the reservoir pore pressure. The subsidence process can be associated with some seismicity. Reports of minor tremors of no greater than 2.8 on the Richter scale were reported on June 2, 2009 in Cleburne, Texas - the first in the town's 140-year history. It must be noted that no scientific studies have been conducted that can substantially prove that conventional hydraulic fracturing is directly related to large scale seismic activity.

One use of hydraulic fracturing is in stimulating water wells. In that case, the fluid used may be pure water (typically water and a disinfectant such as bleach). Another use of hydraulic fracturing is to remediate waste spills by injecting bacteria, air, or other materials into a subsurface contaminated zone.

It has been reported that the hydraulic fracturing industry has refused to publicly disclose, allegedly due to intellectual property concerns, the specific contents of the fluids employed in the fracturing process. A "NOW on PBS" episode aired in March, 2010, introduced the documentary film "Gasland". The filmmaker claims that the chemicals include toxins, known carcinogens and heavy metals which may have polluted the ground water near well sites in Pennsylvania and Colorado. The film also makes a case for explosive gases entering private potable water wells, causing "flammable water."

A 2008 newspaper report states that medical personnel were inhibited in their treatment of workers injured in a fracturing accident because they did not know which specific chemicals were used. In the article, a nurse claimed she may have been exposed to the unknown chemicals on the patient's clothes.

In the United States, a 2004 Environmental Protection Agency (EPA) study concluded that the process was safe and didn't warrant further study, because there was "no unequivocal evidence" of health risks, and the fluids were neither necessarily hazardous nor able to travel far underground. That study, however, was not intended as a general study of hydraulic fracturing, but only of its use in coalbed methane deposits, and the study did not consider impacts above ground. The EPA report did find uncertainties in knowledge of how fracturing fluid migrates through rocks, and upon its release service companies voluntarily agreed to stop using diesel fuel as a component of fracturing fluid, due to public concerns of its potential as a source of benzene contamination. With critics claiming that Bush administration officials influenced the 2004 EPA study, the U.S. Congress has requested that the EPA undertake a new, broader study of hydraulic fracturing. The report is due to be released in 2012.