A pond changes after rainfall

A fictional monitoring team records unusually cloudy water in a pond after heavy rainfall and later finds low dissolved oxygen in several samples. Nearby land includes cultivated fields and a small settlement, but no complete source survey has been conducted. A student says the observation proves one farm caused every effect, while another says rainfall cannot be relevant because it is natural. The team has permission to review existing records and design further monitoring, not to release substances or disturb protected habitat. The exercise supplies no legal standard or measured pollutant load, so the analysis must distinguish plausible pathways from a demonstrated source and causal chain.