Perchlorate in Arizona: Occurrence Study of 2004

Conclusions of the Water Quality AssessmentPerchlorate is present in certain areas of the state but generally levels are well below Arizona’sHBGL. Comparing the 2004 sampling results with earlier sampling efforts indicates that remedialactions at the source in Nevada have resulted in a slow but steady decline in concentrations inboth surface water and groundwater along the Colorado River as well as in those areas usingColorado River water. Surface water concentrations at the Wilmer Pumping Plant, intake to theCentral Arizona Project, continue to decrease over time. It is expected that these levels willcontinue to decline as perchlorate releases to Las Vegas Wash are reduced.In 2003, Flow Science conducted perchlorate modeling for Metropolitan Water District toestimate how long it will take Colorado River perchlorate concentrations to reach target levelsunder various control strategies and hydrological conditions. The report assumed the 90%capture of all perchlorate sources to Las Vegas Wash and predicted perchlorate concentrationlevels at the Colorado River Aqueduct intake (in the same vicinity as the CAP intake at theWilmer Pumping Plant) to reach 4 ppb by mid-2004 and 2 ppb by mid to late 2005. The averageannual concentration in the river at this location is 4.8 ppb (4.5 ppb at Wilmer).While concentrations are declining, the impacts of the drought and seasonal variations onperchlorate concentrations are unknown. Generally, the drought has resulted in less waterentering Lake Mead and reservoirs in Arizona that receive Colorado River water. This hasresulted in overall decreases in lake volumes. If the drought continues, perchlorateconcentrations in Lake Mead and other reservoirs may not decline as much as expected andcould possibly increase.- 24 -