The detection of harmful substances you mentioned should be qualitative detection, that is, whether the sample contains some harmful substances (or it does, but it is within a safe range). There are thousands of toxic and harmful substances in the world, and it can be said that any sample taken contains harmful substances more or less. There are so many harmful substances that it is impossible to detect everything. It should be targeted. Therefore, whether the sample contains some harmful substances should be analyzed and judged first, and then according to the preliminary conclusion, targeted experiments should be carried out to detect whether it contains (if the content is detected, it is a quantitative problem). For example, the water in farmland, you can analyze the possible pesticide components in the water, and further investigate what components are, and then design experiments to verify them; For the wastewater from nickel plating plant, you can analyze it. The wastewater contains toxic and harmful metal ions, unlike the pesticide components in farmland water. Therefore, in your thesis, you should first write out the general methods and steps of analysis, judgment and experiment for detecting toxic and harmful substances, and then analyze them with specific samples, so that you have something to write. Remember to give examples.

I guess you study science, and science should be supported by experiments. You can look up books related to environmental monitoring and make a plan. This is just to give you an idea of how to proceed. You can communicate with your tutor, hoping to help you. Because I changed my career, my answer is only based on my impression and nostalgia for environmental engineering. I hope you will succeed.