All wastewater from the Black Diamond area is now treated at the regional secondary treatment facility. This transfer action formed the basis for the TMDL which EPA approved. Eutrophic lakes are overly rich in phosphorus, characterized by frequent algal scums, excessive aquatic weeds, and occasional fish kills. A healthy lake in the Puget Sound lowlands should have phosphorus levels no higher than 20 parts per billion. TMDLs require monitoring to test and assure their effectiveness.
King County conducts routine monitoring of phosphorus in Lake Sawyer.
In 1993, after the diversion of wastewater from Rock Creek, the mean phosphorus concentration in Lake Sawyer dropped to 18 parts per billion. Samples collected in 1995 and 1996 showed mean phosphorus of 16 parts per billion. These data show that moving the outfall was effective in surpassing the TMDL goal.