The draft of an EPA investigation of fracking in Pavillion, Wyoming has just been released. It's not yet peer-reviewed, this will happen after the draft has been available for 45 days for public comment.
They measured an unusually alkaline pH level, which could be caused by strong base used during fracking
The high pH measured in the deep monitoring wells was unusual and
Presence of hydroxide alkalinity suggests strong base addition as the
causative factor for elevated pH in the deep monitoring wells.
They also found high concentrations of methane, which they determined to be of thermogenic origin. Thermogenic means it was produced under conditions of high heat and pressure, as opposed to biogenic methane produced directly by bacteria.
The monitoring wells produce ground water near-saturated in methane
at ambient pressure, with concentrations up to 19.0 mg/L.
Similarity of δ 13 C values for methane, ethane, propane, isobutane,
and butane between gas production and monitoring wells and plots of δ
13 C-CH4 versus δD -CH4 (Figure 18b) and δ 13 C-CH4 versus C1/(C2
+ C3) (Figure 18a) indicate that light hydrocarbons in casing and dissolved gas in deep monitoring wells are similar to produced gas
and have undergone little oxidation or biodegradation. These
observations combined with radiocarbon analysis of CH4 (< 0.2%
percent modern carbon) obtained from gas in casing of both MW01 and
MW02 indicate that methane in deep monitoring wells is of thermogenic
Those are only two points of evidence in the report, there is more in the full report. Based on these observation they state in their conclusion
While each individual data set or observation represents an important
line of reasoning, taken as a whole, consistent data sets and
observations provide compelling evidence to support an explanation of
data. Using this approach, the explanation best fitting the data for
the deep monitoring wells is that constituents associated with
hydraulic fracturing have been released into the Wind River drinking
water aquifer at depths above the current production zone
The report presents some convincing evidence that components of the fracking fluids are released, though the authors state:
However, further investigation would be needed to determine if
organic compounds associated with hydraulic fracturing have migrated
to domestic wells in the area of investigation.
The contents of this report are also nicely summarized in the article "How the EPA linked "fracking" to contaminated well water" from Scott K. Johnson at Ars Technica.