Effectiveness of alum in a hypereutrophic lake with substantial external loading

Abstract

ABSTRACTBrattebo SK, Welch EB, Gibbons HL, Burghdoff MK, Williams GN, Oden JL. 2017. Effectiveness of alum in a hypereutrophic lake with substantial external loading. Lake Reserv Manage. 33:108–118.Water quality conditions in hypereutrophic Lake Ketchum improved to mesotrophic conditions after alum treatments in 2014 and 2015. From 2013 pre-treatment conditions, summer mean epilimnetic total phosphorus (TP) declined from 289 to 34 µg/L in 2014 and then to 15 µg/L in 2015 (a total reduction of 95%). Hypolimnetic TP declined 99% overall, and chlorophyll a dropped 70% while there was 135% improvement in transparency over the 2 years. Toxic cyanobacteria blooms, a chronic problem in the lake, disappeared in 2015. Maintenance treatment in 2016 continued water quality improvements. The primary driver of water quality changes was a decrease in internal P loading from a mean rate of 25 mg/m2 per day to zero (negative rate) after the alum treatments. The improvements occurred despite application problems with the 2014 treatment, which had to be halted with only 66% of the planned dose (aluminum: 28 mg/L or 83.7 g/m2) applied. The remaining 34% of the original alum dose plus an annual maintenance water column dose and an additional 15% was applied in March 2015. The water quality improvements were achieved even with continuation of a highly enriched (TP: >450 µg/L) external input from a legacy agricultural source. The changes were consistent with or exceeded predictions of a 2-layer, seasonal mass balance TP model used to evaluate restoration alternatives. These results show that alum treatments can eliminate high internal loading and, when coupled with annual maintenance treatments, provide sustained water quality improvements despite continued external loading.