Nitrogen fertilizer companies refuse to admit that it is a pollution source because they use only inorganic mercury instead of methyl mercury in the production process. They may use upstream, local surface water and downstream water to determine mercury in water, and compare it with tap water, industrial water used by nitrogen fertilizer companies and sewage discharged by them to prove that their production process has not caused mercury pollution. In addition, they may also measure and estimate the mercury content in their raw materials and chemical raw materials to prove that these raw materials do not contain a lot of mercury.

How to confirm this problem needs to start from the following angles:

1. Continuous collection and detection of water samples and soil samples: water samples and soil samples are regularly collected near the production area and downstream area of the nitrogen fertilizer company to detect mercury content and other possible pollutants. Through long-term continuous monitoring, it can be judged whether the production activities of nitrogen fertilizer enterprises have caused mercury pollution.

2. Investigate the raw materials and production processes used by nitrogenous fertilizer enterprises: Understand all raw materials and production processes used by nitrogenous fertilizer enterprises, including a large number of raw materials and all production links, and ensure that any links that may lead to mercury pollution are not ignored.

3. Understand the local environmental background: When evaluating whether the nitrogenous fertilizer company is a pollution source, we also need to consider the local environmental background. If the mercury content in the local environment is already high, even if no additional mercury is introduced into the production activities of the nitrogen fertilizer company, the mercury content in its products may be high.

4. Establish and maintain a good enterprise environmental management system: Nitrogen fertilizer enterprises should establish and maintain an effective environmental management system, regularly assess the impact of their production activities on the environment, and take necessary measures to reduce possible pollution.

Please note that the above contents are for reference only, and the specific verification process needs to be adjusted and implemented according to the specific situation.