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05.2.waterpollt2.aos104f08.sld9

05.2.waterpollt2.aos104f08.sld9 - Lakes and Reservoirs...

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B Surface Waters and Acidification 1 B Water Pollution II Surface Water Eutrophication Acidification of Lakes ! Chemistry ! Observations of Ion Concentrations ! Mobilization of Metals 2 B Surface Water Quality In 1972, when the Clean Water Act was enacted, ~90% of surface water in the US wasn’t fishable or swimmable (one or the other or both)! As of 1992, 38% of rivers and 44% of surface water in the US was not fishable or swimmable, due to nutrients, pesticides, oxygen demanding wastes, metals, etc. 2001: 35% of rivers and 45% of lakes 3 B Eutrophication: A natural lake aging process that can take 1000’s of years. ! Accumulation of silt and organic matter eventually fills in lakes ! Process can be accelerated by human activities and is currently affecting about half of US lakes. Oligotropic Lake: Young lake with few nutrients and low plant productivity. Lakes and Reservoirs 4 B • Euphotic zone: The upper layer of a lake where plants produce more oxygen than they consume by respiration. • Profundal zone: The layer below the euphotic zone. ! At the euphotic/profundal boundary, the light is ~1% of the sunlight hitting the lake surface. 5 B Most effective way to control eutrophication: Remove the limiting nutrient In fresh water, nitrogen (as nitrate) is usually not limiting, since a) it is deposited from the polluted atmosphere and b) blue-green algae, which (together with rhizobial bactieria) can fix N 2 from the atmosphere, are common in eutrophying lakes. 6 B Nitrate: N 2 bond is too strong for most organisms to use. A chemical representation of algal photosynthesis: 106CO 2 + 16NO 3 ! + H 3 PO 4 2 ! + 122H 2 O + 18H + " C 106 H 263 O 110 N 16 P + 138O 2 mass N mass P = 16 ! 14 31 = 7.22 7 B N/P > 10, water is phosphorous limited. N/P < 5, water is nitrogen limited. 8 B Phosphorus of less than 0.01 mg/L generally does not promote excessive algal growth. Writing the mass balance: addition of P = removal of P S = rate of addition of P from a point source C = Concentration of P in the lake C in = incoming stream flow v s = the settling rate of P (m/s) A = surface area of the lake Rearranging we have: QC in + S = + v s AC C = + Q + A 9
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B Example 1 A phosphorus-limited lake with a surface area of 80 × 10 6 m 2 is fed by a 15 m 3 /s stream that has a phosphorus concentration of 0.01 mg/L. In addition, effluent from a point source adds 1 g/s of phosphorus. The phosphorus settling rate is 10 m/yr. What is the phosphorus concentration, and will it be contributing to eutrophication? C = QC in + S Q + v s A = 15 m 3 s ! " # $ % 0.01 mg ! ! " # $ % 10 3 g mg ! " # $ % = 0.15 g s = 1 g s 10 B = 10 m/yr 365 day/yr ( ) 24 hr/day ( ) 3600 s/hr ( ) = 3.17 ! 10 " 7 m/s = + + = 0.15 g/s + 1 g/s 15 m 3 /s + 3.17 ! 10 " 7 m/s ( ) 80 ! 10 6 m 2 ( ) = 0.028 g m 3 = 0.028 mg ! 11
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05.2.waterpollt2.aos104f08.sld9 - Lakes and Reservoirs...

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