Calculate the lagoon’s detention time given a surface area of 6 acres, an average depth of 3 ft, and an average daily flow of 0.25 MGD.

To calculate the detention time of a lagoon, you need to first determine the volume of the lagoon and then divide that volume by the average daily flow rate. Detention time is calculated using the formula:

Detention Time (days) = Volume of Lagoon (gallons) / Average Daily Flow (gallons/day)

1. Calculate the volume of the lagoon:

• Convert the surface area from acres to square feet. One acre is 43,560 square feet, so 6 acres equals:

[

6 \text{ acres} \times 43,560 \text{ sq ft/acre} = 261,360 \text{ sq ft}

]

• Calculate the volume using the surface area and average depth. The average depth is given in feet, so the volume in cubic feet is:

[

\text{Volume} = \text{Surface Area} \times \text{Average Depth} = 261,360 \text{ sq ft} \times 3 \text{ ft} = 784,080 \text{ cubic feet}

]

• Convert cubic feet to gallons (1 cubic foot = 7.48 gallons):