In January of 1996, Three Sisters Spring failed some kind of health department muster. It seems that there's a law that all public water (TSS is in Ouachita State Park), in order to be listed as potable, has to be filtered and irradiated with UV. That's pretty standard treatment for drinking water, although it kind of seems to defeat the whole point of the mineral spring experience.
So the authorities did what I think is the reasonable and preferable thing. They bypassed the water treatment apparatus and just allow the water to run right out of the spring boxes into a trough, pretty much just like in the old days (the old days in this case being the late '60's) when I used to go on fishing trips here with my family. So drink at your own risk. The water hasn't been treated, but Europeans have been drinking it right out of the ground for a hundred years, and the Indians for thousands of years before that.
From time to time the springs have been exploited commercially, either as the focus of a health resort or a source for bottled water. People have been drinking the curative water from these springs since before there was a lake here, and even today mineral water pilgrims fill bottles and jugs from Three Sisters and carefully number the jugs so that they'll know which spring filled which jug.
I could not distinguish the three springs by taste, and they all precipitated a coppery orange flock in their respective troughs; so it's possible all three springs gush from the same aquifer. Nevertheless, by custom the three different waters are taken for three different sets of complaints, and here they are. I got this list from the Visitors Center at Lake Ouachita State Park, and the flyer states that this list was once painted on one of the previous springhouses that used to cover the Three Sisters.
Brights disease, Diabetes, Dropsy, Pus in Kidney, Bladder and Urethra, Cystitis, Enlarged Prostrate (sic) Gland, Paralysis, Stones in Kidneys and other Urinary troubles, Change of Life and Female irregularities, Insomnia, Anemia, High or Low Blood Pressure, Gout, Hyperacidity, Rheumatism, Arthritis, etc.
Chronic Constipation, Chronic Indigestion, Catarrh of Stomach, Excessive Acid, Gastritis, Ulcerated Stomach, Poor Assimilation and Elimination, Low Blood Pressure, Gall Stones, Mucus Colitis, etc.
Highly Astringent, Quite Constipating, Used only for Flux, Diarrhea, Dyssentery, Cholera-Infantum and Kindred troubles, bath for Diabetic Sores and Eczema, Granulated and Sore Eyes, Catarrh of the Head, Nose and Sinuses, etc.