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114 lakes treated with aluminum (Al) salts to reduce internal phosphorus (P) loading were analyzed to identify factors driving longevity of post-treatment water quality improvements. Lakes varied greatly in morphology, applied Al dose, and other factors that may have affected overall treatment effectiveness. Treatment longevity based on declines in epilimnetic total P (TP) concentration averaged 11 years for all lakes (range of 0-45 years). When longevity estimates were used for lakes with improved conditions through the end of measurements, average longevity increased to 15 years. Significant differences in treatment longevity between deeper, stratified lakes (mean 21 years) and shallow, polymictic lakes (mean 5.7 years) were detected, indicating factors related to lake morphology are important for treatment success. A decision tree developed using a partition model suggested Al dose, Osgood index (OI, a morphological index), and watershed to lake area ratio (related to hydraulic residence time, WA:LA) were the most important variables determining treatment longevity. Multiple linear regression showed that Al dose, WA:LA, and OI explained 47, 32 and 3% respectively of the variation in treatment longevity. Other variables (too data limited to include in the analysis) also appeared to be of importance, including sediment P content to Al dose ratios and the presence of benthic feeding fish in shallow, polymictic lakes.

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Pilgrim, K.M. and P.L. Brezonik. 2005. Evaluation of the potential adverse impacts of lake inflow treatment with alum. Lake and Reserv. Manage. Vol. 21(1):78-88.Laboratory and field investigations at Fish Lake in Eagan, MN, and Tanners Lake in Oakdale, MN, were conducted to evaluate the potential adverse effects of using alum to treat lake inflows. Detention ponds are used at both sites to promote settling of alum floc before discharge to the lake. Tests in a 1.5 m column designed to estimate settling in detention ponds showed nearly complete Al settling by 6 h. Final residual Al concentrations were affected only slightly by dose. Average concentrations of total Al entering Fish Lake were 346 and 458 µg Al L -1 at doses of 1 and 8 mg Al L -1 , and average in-lake concentrations were 70 and 152 µg L -1 . Aluminum settled in Fish Lake at a rate of 74 m y -1 . Average Al concentrations in Tanners Lake inflow were higher, but Al concentrations in the lake were lower because of its greater size. The settling pond at Fish Lake protected benthic invertebrates in the lake, but floc accumulation during treatment at 8 mg Al L -1 eliminated nearly all invertebrates in the pond. Concentrations of Al that showed no observed mortality effects (NOECs) were assembled from literature, and a measure of unbound, positively charged Al species (Al lm ) was compared to the NOECs to evaluate potential aquatic toxicity from alum treatment. The potential risk of aquatic toxicity should be negligible if treated water entering the lake has a pH > 6.0.