This human population developed in a mild climate region, with fair abundance of food, water and shelter. However, though they have the skills and the means to explore forth, the population is unable to leave the region, because as soon as it does, it becomes "homesick".
No, not only missing mommy and friends, but really because their bodies are unable to synthetize a particular substance which is vital for their well-being and that is plenty available in their home wells and springs water, but not elsewhere.
Therefore, as soon as they leave the region and have no access to the local water (except some reserve of water that they can carry along), they simply get sick.
They initially report fatigue, difficult to focus and communicate, then they further develop difficulties to coordinate their movements until even breathing becomes difficult and death is inevitable.
If they fail to return home they perish within a few days. Only few of them have been reported to survive more than two weeks away.
We are not able to examine the water in loco, can you help understanding what could this substance be?
Important I am more interested in a substance which can be water soluble and essential for life. I am open to adapt the symptoms and/or the time span for the death, if a suitable substance is given in the answer.