Evaluation of the Potential Adverse Effects of Lake Inflow Treatment with Alum

Abstract

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 (Allm) 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.