Dosing Alum to Wisconsin Lake Sediments Based on in vitro Formation of Aluminum Bound Phosphate

ABSTRACT

The dose of aluminum (Al) needed to deplete mobile inorganic sediment phosphorus (P) [loosely-sorbed P and Iron (Fe)-bound P] and transform it to Al-P was estimated in vitro in Lake Delavan, Wind Lake, and Bass Bay sediments. The formation of Al-P was logarithmically related to increased Al added as buffered alum. Results of the in vitro treatments were compared with the observed effect on sediment-P composition from the 1991 alum treatment (12 g Al·m⁻²) in Lake Delavan. That 1991 dose resulted in the formation of 2.2 g P·m⁻², measured as Al-P, which was apparently the maximum amount of P that could be adsorbed with that alum dose. Based on in vitro alum additions, 150 g Al·m⁻², twelve times that added, would have been required to nearly deplete mobile P in surface (0–4 cm) sediment of Lake Delavan (16 m). Wind Lake contained less mobile P, so a dose of 80 g Al·m⁻² was required to remove mobile P in sediments from its deep hole (14m), while an addition of only 20 g Al·m⁻² was needed for sediments from its average depth (3 m). Similar to Wind Lake, sediments from the deep hole (6 m) in Bass Bay required 90 g Al·m⁻². These doses were estimated based on a ratio of added Al to adsorbed P of 100:1 (by weight), which was determined in vitro and was consistent for the three lakes. These doses would also be enough to adsorb significant amounts of P moving to the sediment surface from sediment layers deeper than 4 cm and are all less than those determined by water column alkalinity. The mobile sediment-P method could be used to estimate dose based on the 100:1 ratio of Al added to formed Al-P and measured mobile P (loosely-sorbed P + Fe-P) in the top 4 cm of sediment.

Key Words:

• alum-dose
• internal loading
• phosphorus
• aluminum
• sediment