The containment pond at the Stennis Space center, Test Stand A-1, is used to hold runoff from contact-cooling water (where water comes into contact with something other than the insides of a clean pipe). You can see it's not particularly deep and does not itself runoff or drain anywhere (except to groundwater and the nearby river).
Google Map view of the Test Stand A-1; the containment pond you're asking about is in the area surrounded by trees above the test stand. (The pipe ending in the dry area above and to the left of the test stand is a Flare, for disposing of excess fuel.)
The test stands use a lot of water for everything from transporting the largest parts by barge, cooling the deflector, even sound dampening (literally, that's not a pun). Most of the steam from the flame trench is the reaction of the fuel, hydrogen and oxygen; they burn clean producing water (steam) and not smoke. The remaining steam is from the heating of the cooling water; heated water that is not evaporated is pumped into the containment pond.
Here's the Stennis Space Center B-2 Test Stand, being tested while under construction:
Here's a typical feed pipe:
Here's a video and webpage of the cooling system being tested at the Kennedy Space Center, Florida. They use a lot of water but it is safely recycled in the form of steam (later becoming rain) or by settling out mild contaminants like rust and paint.
Non-contact cooling water can be returned to the river once it reaches ambient temperature, cooling water that has come in contact with the deflector and even the exhaust, needs to sit in a settlement pond before the water is permitted to leach back into the river.
Watch and listen as Richard Hammond explains how bubbles suppress sound underwater and how (6 minutes later) in air the opposite is demonstrated:
Worth watching twice. See also this question: What is the purpose of the jets of water often under rocket engines during launch?
