Xjyuk

Looking at a wiki page, I see that condensers, which cause rain, cause eco-damage.

I can only presume that is because Planet prefers to be dry, or that rain causes plants which destroy fungus.

Using that logic, I should be afraid to "drill to aquifer" and create a new river. However, the same page does not mention any eco-damage.

Can anyone cite a reference as to whether "drill to aquifer" causes eco-damage or not?

Although the internet has non-authoritative (opinion) answers to your question [see 'terraforming hints', here], the game user manual (starting at the 'ecological risks' section [on page 54]) reads as follows (my comments are in the [] brackets):

'...each technological enhancement constructed on any square in a
base's production radius increases that base's ecological disruption.
In general, the more complex an enhancement (i.e. the longer it takes
to complete), the greater the damage it causes. Also, the higher a
base's mineral production, the greater the chance of ecological
disaster.'

'Your ecological damage ranking, as shown on your base screen [under
the label 'eco damage'] gives a percentage chance on each turn that
Planet will react to your disruptions in the form of uncontrolled
fungus blooms that wipe out all existing enhancements in a given
square, The new fungus also often spawns native life forms.'

'Ecological damage can be minimized (or even eliminated entirely)
through social engineering ['green' is especially good for this] and
secret projects (such as the Pholus Mutagen).'

So the although the user manual seems to imply that any change you make to a square [including drill to aquifer] will cause some ecological disruption, this is not always true. I have found that while building farms, mines, roads, and especially boreholes increases eco disruption, building forest or fungus in squares [non-technological terraforming] tends to decrease eco disruption and some terraforming, like 'drill to aquifer', or raise/lower/flatten terrain are more or less neutral to eco disruption.

In general, if you plant sufficient forest and fungus squares and don't have too high a mineral production in any given city, eco disruption is easily avoided, even if you do a lot of terraforming.