I actually looked it up on Wikipedia. I was off a bit on the dates, but here's the gist of it:

Deep injection well

RMA contained a deep injection well that was constructed in 1961. It was drilled to a depth of 12,045 feet (3,671 m). The well was cased and sealed to a depth of 11,975 feet (3,650 m), with the remaining 70 feet (21 m) left as an open hole for the injection of Basin F liquids. For testing purposes, the well was injected with approximately 568,000 US gallons (2150 m³) of city water prior to injecting any waste. The injected fluids had very little potential for reaching the surface or usable groundwater supply since the injection point had 11,900 feet (3,600 m) of rock above it and was sealed at the opening. The Army discontinued use of the well in February 1966 because the fluid injection triggered a series of earthquakes in the area. The well remained unused until 1985 when the Army permanently sealed the disposal well.

So I wonder what "Basin F liquids" were?

It gets better - is a general narrative of what went on at the arsenal. It doesn't state which chemicals went into the well, but does note it was chemical waste.