(Key word: slow)
Despite the spring technically being "freshwater" the water springs from its source rife with a murky, oily substance as wee as a noxious green fungus. The resultant "water" is dank and vile; its viscosity causes the spring to flow slow and thick down the natural slope to pool at the bottom, where it slowly drains out.
The "water" can be lighted on fire as if it was oil; if imbibed it can temporarily grant an increase to Strength (an effect of the fungus) and Charisma (from awe and disbelief that anyone would be bold enough to actually drink the fluid). When the effects wear off, the individual suffers a temporary Dexterity and Constitution loss from the resulting sickness.