The City of Maple Valley implemented a swimming beach monitoring program in 2008 to test the swimming beach at Lake Wilderness for bacteria known to be associated with risk to human health. In 2008 Maple Valley Council passed Resolution R-08-592 which authorized an interlocal Surface Water Technical Agreement with King County Department of Natural Resources Division (WRLD) to provide professional swimming beach sampling and laboratory analysis 2008 through 2012.
The swimming beach monitoring program provides for professional collection of water samples from the Lake Wilderness swim beach on each Tuesday morning over a nineteen week period, typically May through September. Water samples are transported by King County staff to the King County Environmental Laboratory where they will be tested for Fecal Coliform and E. coli, bacteria known to be associated with human health risk. King County will post the data on their website along with data of other area lakes participating in the program.
Bacterial counts are measured in colony-forming units (CFU) per 100 milliliters (ml.) of sample. Typical low bacteria counts found in high quality water are less than 50 – 100 CFU per 100 ml. or (50-100 CFU/100 ml.). When laboratory analysis indicate that the bacteria counts in a single sample are elevated greater than 1000 CFU/100 mL the swim beach would be re-sampled to out rule sampling or analysis error and if the second sample substantiates the elevated bacteria count, Seattle King County Health Department would contact the City and recommend that the City close the beach. The beach would remain closed until the water samples analysis from the next regularly scheduled Tuesday sample indicate that the bacterial contamination level was within the range where human health risks were no longer an issue. The presence of fecal coliforms and E. coli can be an indicator of sewage pollution in the water; however, other mammals and waterfowl also contribute to this type of bacteria and higher than normal bacteria levels can also be the result of weather related storm events that can carry animal feces or goose feces into the lake.