There are two metrics to consider for solid waste: total weight (tonnage) and weight per athlete. Contact your waste hauler in advance of the event and let them know you want to receive the total weight of each type of waste your event produces: landfill/incineration waste, recycling, and compost. Typically they can provide you with the data within a week.
When you get the data, convert tons to pounds (lbs) and divide the pounds by the total number of finishers (don't use registrants) to get the weight per athlete. Weight per athlete is a key statistic you'll want to use for year-to-year and event-to-event comparisons. And specifically, weight of landfill waste per athlete.
From the total weight figures, calculate the percentage of each type of waste: landfill, recycling and compost. You can use the percent figures for year-to-year and event-to-event comparisons.
If you can get tonnage from last year's event from the waste hauler you can use that as your baseline for comparisons. Otherwise, consider this year's measurement your baseline. Focus on opportunities next year to improve recycling and compost as a percent of the total waste and reducing the landfill waste per athlete.