ABE 325 2021 HWK Wetlands and Water Quality - KEY

ABE 32500 Soil and Water Resources Engineering 1 Wetlands and Water Quality Homework (9 pts) 1. (2 pts) Water Quality - The Secchi disk depth for a lake was measured at 1.8 m. What is the trophic status and the TSI for the lake? 𝑇𝑇𝑇𝑇𝑇𝑇 = 60 − 14.4 ln( 𝑇𝑇𝑆𝑆 ) = 60 − 14.4 ln(1.8 𝑚𝑚 ) = 51.5 The lake is “Eutrophic” 2. (3 pts) Wetlands - The following data were obtained for the wetland at ACRE. Precipitation was 985 mm and evapotranspiration was 700 mm for the year. Surface inflows to the marsh totaled 35,430 m 3 and surface outflows were 92,200 m 3 . Subsurface tile drainage inflows were 52,350 m 3 . Determine the change in storage for this 1.2 ha wetland. If the depth of water in the wetland was 0.3 m at the start of the year, what was the depth at the end of the year? Estimate the residence time. In order to compute a mass balance, all quantities have to be converted to the same units. Precipitation and ET can be converted to units of volume: P = 985 mm × 1.2 ha = 11,820 m 3 ET = 700 mm × 1.2 ha = 8400 m 3 Now apply Equation 12.1: Inflows = Outflows + Δ Storage 11,820 + 35,430 + 52,350 = 8400 + 92,200 + Δ Storage Δ Storage = -1018 m ß = 0.085 m lost depth over the 1.2 ha wetland Depth at the end of the year was: 0.3 m - 0.085 m = 0.215 m Use the average depth over the year and Equation 12.2 to estimate the residence time: 𝑹𝑹 𝒕𝒕 = wetland volume ∑ inflows = 𝟎𝟎 . 𝟓𝟓 ( 𝟎𝟎 . 𝟑𝟑 + 𝟎𝟎 . 𝟐𝟐𝟐𝟐𝟓𝟓 )( 𝟐𝟐𝟐𝟐𝟎𝟎𝟎𝟎𝟎𝟎 ) 𝟐𝟐𝟐𝟐 , 𝟖𝟖𝟐𝟐𝟎𝟎 + 𝟑𝟑𝟓𝟓 , 𝟒𝟒𝟑𝟑𝟎𝟎 + 𝟓𝟓𝟐𝟐 , 𝟑𝟑𝟓𝟓𝟎𝟎 = 𝟎𝟎 . 𝟎𝟎𝟑𝟑𝟐𝟐 yrs = 𝟐𝟐𝟐𝟐 . 𝟑𝟑 days