Ember: Fire and Wind
At over 11 miles wide by 23 miles long, the richly-colored waters of Viridian Lake look inviting and refreshing. The truth, however, is that the lake water has such a high concentration of salt and minerals that it is toxic to most creatures.
The lake gets its color from tiny algae that feed on the dissolved minerals (the algae also accounts for the chance of disease transmission). A purify food and drink spell has no effect on the water, since the algae itself retains the salty minerals and toxicity (the spell has no effect on living creatures). A few strangely-colored plants grow along the shore in places, and people have reported unusual amphibians living in the lake, as well.
A small group of gnomes has set up shop on the West bank of the lake. Using crystal lenses, glass tubes, and a few bound mephits, the gnomes have created a small, profitable, water purification operation. While their intentions are generally benign, the gnomes have drawn the ire of a pair of druids that have lived at the lake for decades.