Something's in the Water

I will spare you the grisly details, but these soldiers are violently ill. The nature of their sickness leads me to believe there is an issue with either their food or their water.

I've already checked the crops coming in from the nearby farms, so this only leaves the water, which comes from the local rivers. The best place to collect a sample is from the pools in the plains to the north. Nearly every river runs through the area, so it would be wise to check for contamination there.