Resuspension-mediated aluminium and phosphorus distribution in lake sediments after aluminium treatment

Horizontal and vertical distribution of aluminium (Al) in the sediment of six Danish lakes treated with poly-Al chloride in the period 2001-2009 was investigated in 2010 to examine if the added Al was translocated by resuspension. During treatment, Al was applied evenly in the lakes. However, in 2010, recovered Al (in % of dosage) increased with increasing water depth, indicating that Al was redistributed and focused in deeper parts of the lakes. The ratio between recovered Al and Al-bound phosphorus (P) was fairly uniform in the individual lakes independent of depth which suggests that P was also redistributed towards deeper areas. The number of years since treatment had no influence on the horizontal distribution of Al, and in one of the lakes focusing occurred within the first weeks to months after treatment. The vertical Al distribution revealed that the majority of the Al is mixed into the upper 14 cm of the sediment. Thus, in future lake restorations, it is essential not to dose Al in surplus as P settling after the treatment will not necessarily come into contact with the Al floc. Furthermore, Al treatments should be performed in calm periods to avoid resuspension and horizontal redistribution.