The American Mathematical Society has studied the various dimensions of the application of mathematics to the study of environment (http://www.ams.org/books/psapm/032/psapm032-endmatter.pdf). The University of East Anglia has a special school for Environmental Mathematics within which it examines these topics: ocean, atmosphere and climate modelling; wave modelling and volcano modelling. In addition to research, it offers PhD projects in the subject (https://www.uea.ac.uk/mathematics/research/applied/environmental). University of Exeter also has a special focus on this subject (http://www.exeter.ac.uk/undergraduate/degrees/mathematics/mathsmsci-environment/). Grantham Institute, part of the Imperial College London discusses ‘Seven ways maths can save the world’ (https://granthaminstitute.com/2016/05/31/seven-ways-maths-can-save-the-world/). The Faculty of Environmental Science and Technology, Okayama University says “To improve the quality of life and create an environment in which human beings live in harmony with nature, it is necessary to evaluate the effects of human activities and predict the results under various conditions”, which calls for that mathematics and statistical tools along with computing techniques (http://www.est.okayama-u.ac.jp/contents/emsen.html). Scientific Computing World has an interesting discussion on Mathematics and the Environment through the work of Louis Gross, a mathematical ecologist (https://www.scientific-computing.com/feature/mathematics-and-environment). The Centre for Applications in Natural Resources Mathematics of The University of Queensland aims “to develop and apply mathematical and statistical theory to produce tools that directly impact the management of fisheries, forestry, water security, conservation, pest and disease management, and adaptation to global changes” (https://smp.uq.edu.au/research/centres/carm). The Department of Forest Resources Management of The Swedish University of Agricultural Sciences engages in research in mathematical statistics applied to forest sciences (https://www.slu.se/en/departments/forest-resource-management/sections/mathematical-statistics-applied-to-forest-sciences/). The National Center for Replacement, Refinement and Reduction of Animals in Research is engaged in research in applying mathematics to 3Rs problem (https://www.nc3rs.org.uk/applying-mathematics-3rs-problems).
One of the applications of mathematics in environment is in modelling. One of the earliest books (1999) in this area is titled ‘Modeling the environment: an introduction to system dynamics models of environmental systems’ by Andrew Ford (https://searchworks.stanford.edu/view/4104319). Envision, an institution committed to developing next generation leaders in environmental sciences has set up a research project (as a PhD) in ‘Geospatial modelling the spread of antimicrobial resistance in the environment’, to focus attention on the ‘spread of AMRs in the environment’. It says further: “Antimicrobials and antimicrobial resistant genes (ARGs) and organisms have sources in agriculture and wastewater treatment plants (WWTP), which are spread on land through slurry, manures or sewage sludge, or released directly into rivers. Soil and water polluted by antimicrobials and resistant bacteria can impact crops, animals and humans”(http://www.envision-dtp.org/2016/geospatial-modelling-the-spread-of-antimicrobial-resistance-in-the-environment/). (Please check if the project is open when you read this article)
The Oxford Martin Programme on the Future of Food has set up a project on ‘Modelling the Relationship between the Food System and Health, Development, and the Environment’ (http://www.futureoffood.ox.ac.uk/project/modelling-relationship-between-food-system-and-health-development-and-environment). There are private institutions such as Indeco which focus on modelling environment-economy relationships (http://www.indeco.com/ideas/eemodel/).
We could go on but you can find out more based on your areas of interest. People study the impact of various water recycling technologies, patterns in agriculture and their impact on the environment, impact of upstream mining on downstream water and the health of people settled in the area surrounding the downstream. Fortunately, most field work produces large amounts of data. Assuming the integrity of the data, analysis can reveal a lot of hidden aspects. Literally, there is no limit to what you can study about the environment using mathematics.