We Just Don't Know

Except we kinda do know - or at least we're getting to where we have to be starting to think there has to be something to it. There's always whatever small probability that it could be a case of post hoc ergo propter hoc, but c'mon - at what point do we just have to say those damned hippies were right all along?
From The Rocky Mountain Arsenal:

Deep well injection for liquid waste has been safely used for many years at sites throughout the United States without documented damage to human health or the environment. After an extensive study of deep injection wells across the country by the U.S. Environmental Protection Agency (EPA), it was concluded that this procedure is effective and protective of the environment.
The Rocky Mountain Arsenal deep injection well was constructed in 1961, and was drilled to a depth of 12,045 feet. The well was cased and sealed to a depth of 11,975 feet, with the remaining 70 feet left as an open hole for the injection of Basin F liquids. For testing purposes, the well was injected with approximately 568,000 gallons of city water prior to injecting any waste. However, when the Basin F liquids were actually introduced, the process required more time than anticipated to complete because of the impermeability of the rock. The end result was approximately 165 million gallons of Basin F liquid waste being injected into the well during the period from 1962 through 1966.
The waste fluid chemistry is not known precisely. However, the Army estimates that the waste was a more dilute version of the Basin F liquid which is now being incinerated. Current Basin F liquid consists of very salty water that includes some metals, chlorides, wastewater and toxic organics. From 1962 -- 1963, the fluids were pumped from Basin F into the well. From 1964 -- 1966, waste was removed from an isolated section of Basin F and was combined with waste from a pre-treatment plant, located near Basin F, and then pumped into the well. The waste from the pre-treatment plant was generally a solution containing 13,000 parts per million sodium chloride (salt), with a pH ranging from 3.5 to 11.5. The organic content of the solution was high but is largely unknown.
The injected fluids had very little potential for reaching the surface or useable groundwater supply since the injection point had 11,900 feet of rock above it and was sealed at the opening. The Army discontinued use of the well in Feb. 1966 because of the possibility that the fluid injection was triggering earthquakes in the area. The well remained unused for nearly 20 years.
In 1985 the Army permanently sealed the disposal well in stages. First, the well casing was tested to evaluate its integrity. Any detected voids behind the casing were cemented to prevent possible contamination of other formations. Next, the injection zone at the bottom 70 feet of the well was closed by plugging with cement. Additional cement barriers were placed inside the casing across zones that could access water-bearing formations (aquifers). The final step was adding Bentonite, a heavy clay mud that later solidified, to close the rest of the hole up to the ground surface.

I was 14, living in Arvada Colorado in August 1967, and I remember very clearly at the time that nobody wasn't convinced the earthquakes were being caused by the US Army pumping tons of toxic waste into those wells.  The Army didn't deny it although they were pretty cagey about what they were willing to say about it in public.

Historic Earthquakes

Denver, Colorado
1967 08 09 13:25
Magnitude 5.3

The main damage occurred in Northglenn, a northern suburb of Denver, but minor damage occurred in many area towns. At Northglenn, concrete pillars were damaged at a church; foundations, concrete floors, and walls cracked; windows broke; and tile fell at a school. At one residence, a piano shifted about 15 cm and a television set overturned. Some bricks fell from a chimney in downtown Denver, damaging a car. This was the largest of a series of earthquakes in the northeast Denver area that were believed to be induced by pumping of waste fluids into a deep disposal well at the Rocky Mountain Arsenal. The Colorado School of Mines recorded more than 300 earthquakes from this zone during 1967. Felt north to Laramie, Wyoming, south to Pueblo, west to Vail, and east to Sterling.

Cracks in highway overpass pillar in the Denver, Colorado, area caused by the August 9, 1967, earthquake. (Photograph by the Denver Post.)

And then this, from Up With Chris Hayes:

Indeed, energy independence–and the economic opportunities that come with it–may be an admirable goal. But then there’s this: fracking is causing earthquakes. Federal scientists presented a new study this week to the American Geophysical Union that suggests natural gas drilling is the likely culprit behind a skyrocketing number of earthquakes in the Raton Basin in Colorado and New Mexico. From 1970 to 2001, there were just five earthquakes of magnitude 3.0 or greater in that region. Then, companies began injecting what’s called “wastewater fluid” from natural gas drilling into the Earth. After that, from 2001 to 2011, there were a total of 95 earthquakes of magnitude 3.0 or greater–an increase of 1,900%. Scientists from the U.S. Geological Survey concluded in their report that “the majority, if not all of the earthquakes since August 2001 have been triggered by the deep injection of wastewater related to the production of natural gas from the coal-bed methane field.”

This has to stop.