A hydrogeologist is a specialist in the field of geology who studies the interaction between water and rock structures or soil. A senior hydrogeologist is a lead researcher for an organization in this field. The responsibilities of a hydrogeologist are generally split between two specific focuses, although some study both. The first emphasis is as an environmental geographer who studies the natural process of water interacting with rocks and soil. The second hydrogeological emphasis is related to human health and the protection of groundwater and waterways to ensure they remain free of dangerous contaminants. In both cases, the duties of a senior hydrogeologist are to collect samples, inspect and test the samples, and determine the effect the earth has on the water or vice versa. A senior hydrologist might work for a farm determining erosion effects or on a deep injection well.