Introduction:
Environmental monitoring is a crucial aspect of assessing the impact of human activities on the environment. It involves the collection of data to evaluate the quality of various environmental components such as air, water, and soil. Different methods are employed to monitor and measure these components, allowing for informed decision-making and the implementation of appropriate measures to protect and preserve the environment.

Explanation:
The question asks which of the following is not an environmental monitoring method. Let's analyze each option to determine the correct answer:

a) Groundwater monitoring: This method involves the regular sampling and analysis of groundwater to assess its quality. Groundwater monitoring is a vital component of environmental monitoring, particularly in areas where groundwater resources are at risk of contamination from various sources such as industrial activities, agriculture, and waste disposal. It helps to identify potential sources of pollution and take appropriate remedial actions.

b) Landfill sealing: While this option does involve a technique related to waste management, it is not a monitoring method per se. Landfill sealing refers to the process of securing and isolating waste in a landfill to prevent the migration of contaminants into the surrounding environment. It is a measure taken during the construction or closure of a landfill to minimize the potential environmental impacts associated with waste disposal.

c) Quality of leachate: Leachate refers to the liquid that percolates through waste materials, often generated in landfills or contaminated sites. Monitoring the quality of leachate is an important environmental monitoring method as it helps assess the level of contamination and the potential risks to surface water and groundwater resources. Regular sampling and analysis of leachate provide insights into the effectiveness of waste management practices and the need for corrective measures.

d) Biodegradation: This option is the correct answer as it does not represent an environmental monitoring method. Biodegradation refers to the natural breakdown of organic substances by microorganisms. While biodegradation is an important process that influences the fate and transport of contaminants in the environment, it is not a specific monitoring technique. Instead, monitoring may involve assessing the rate of biodegradation or the presence of specific microbial populations to understand the potential for natural attenuation of pollutants.

Conclusion:
In conclusion, the correct answer is option 'D' - Biodegradation. This option does not represent an environmental monitoring method but rather a natural process that can influence the fate of contaminants in the environment. The other options, groundwater monitoring, landfill sealing, and quality of leachate, are all important methods employed in environmental monitoring to assess and mitigate potential environmental impacts.