$(8)$ You are a regional environmental engineer, working at the Pennsylvania DEP $($Department of Environmental Protection$).$ A recent DMR $($Discharge Monitoring Report$)$ submitted by the facility shows an increase in WET $($Whole Effluent Toxicity$)$ in the effluent from a $5$ MGD treatment plant in your region. The WET value was determined during monthly testing at a commercial laboratory, using Ceriodaphnia dubia. You are sent to the plant to help investigate this increase in WET. The lab technician at the plant runs the $CBO{D}_5$ of the plant influent and effluent three days a week, and maintains historical BOD test data on a spreadsheet. Explain how you could use the BOD lab data to try to learn when this toxicity first occurred.