Dissolved arsenic release from drinking water distribution system solids

Previous work has shown that arsenic does accumulate in drinking water distribution system (DWDS) solids when arsenic is present in the water. The release of arsenic back into the water through particulate transport and/or chemical release (e.g. desorption, dissolution) could result in elevated arsenic levels at the consumers' tap. The primary objective of this work was to examine the impact of pH and orthophosphate (used for corrosion control) on the chemical release (i.e. desorption) of arsenic from nine DWDS solids collected from four actual utilities in the Midwest. Arsenic release comparisons were based on the examination of arsenic and other water quality parameters in leach water after contact with the solids over the course of 168 hours. Results showed that arsenic did release from solids and suggested that arsenic release was a result of desorption rather than dissolution. Arsenic release generally increased with increasing initial arsenic concentration in the solid and increasing pH levels (in the test range of 7 to 9). Finally, the presence of orthophosphate (3 and 5 mg PO4/L) increased arsenic release at all pH values examined. Based on the study results, utilities with measurable levels of arsenic present in their water should be aware that some water quality changes can cause arsenic release in the DWDS potentially resulting in elevated levels at the consumer's tap.