Design and Evaluation of Eutrophication Control Measures for the St. Paul Water Supply

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Abstract
In 1984, intensive lake and watershed studies were undertaken by St. Paul Water Utility to identify causes and remedies for taste-and-odor episodes that had plagued the supply in recent years. Water is derived from the Vadnais Lake Chain, and fed mainly by diversions from the Mississippi River and runoff from local watersheds. Early phases of the study implicated algal blooms in the storage lakes as the immediate cause of the taste-and-odor problems. Historical control schemes, including routine copper sulfate applications and dosing of various chemicals at the water treatment plant, had been largely unsuccessful. Following three years of collecting baseline data control measures were implemented to address the basic problem of nutrient enrichment. These controls targeted sources of phosphorus including diversions from the Mississippi River, runoff from urban watersheds, and recycling from lake bottom sediments. Specific control measures included: (1) selecting supply sources based upon chemical factors – phosphorus, silica, iron; (2) injecting iron chloride to inactivate orthophosphorus in waters pumped from the Mississippi River; (3) using wet detention ponds to reduce phosphorus loadings in runoff from urban watersheds; (4) aerating the hypolimnion – plus iron chloride injections in 1988. This paper discusses the design and evaluation of control measures and observed lake responses over the 1984–88 monitoring period. Results indicate substantial progress has been made toward reducing nutrient enrichment levels in the supply lakes as well as undesirable impacts on the water supply; these include taste-and-odor, trihalomethanes, and water treatment costs. The ongoing monitoring program will track lake responses and control (success) over a wider range of hydrologic conditions.