AN ASSESSMENT OF THE PERSISTENCE OF CHLORIDE FROM SOURCE WATER TO TAP AND ITS IMPLICATIONS FOR POTENTIAL CORROSION OF METAL PIPES: A PILOT STUDY

Chloride is a conservative constituent of surface and groundwater, occurring naturally from geologic processes and anthropogenically from substances applied to the land surface. Because chloride dissolves so completely, it is difficult to remove from the environment, especially water. This inquiry is not based on a concern about the ingestion of chloride, but its potential role in corrosion of metal infrastructure, including water-distribution systems.

A pilot study is underway in Indiana where concern about lead contamination of drinking water, especially by vulnerable populations such as school-age children, has spurred statewide testing. Along with chloride, sulfate and alkalinity are used to assess potential corrosivity of metal pipes using various indices, such as the chloride-to-sulfate mass ratio (CSMR). Interpretation of CSMR data suggests that CSMR >0.5 in drinking-water distribution systems has increased potential to promote galvanic corrosion of lead solder connections.

We are conducting a statewide assessment of chloride concentrations, trends, and corrosion potential in the source water in Indiana. Preliminary results show that increasing trends in chloride at long-term surface-water stations were observed between 1990 and 2010 at 46% of the sites analyzed. A commensurate increase in sulfate was not observed at most of these sites, suggesting that chloride increases are driving elevated CSMRs in surface water. It is notable that the main stem of the Wabash River is characterized by declining chloride concentration trends, whereas headwater catchments in that basin show increasing trends. This pattern has not been observed in other basins in Indiana, such as the White River Basin.

Although trend data are not available for the groundwater-quality network in Indiana, 48% of unconsolidated wells and 35% of bedrock wells yield a CSMR >0.5. Work is currently underway to ascribe sources, or combinations of sources, of chloride in both surface and groundwater that could enter the drinking-water distribution system in Indiana. Initial work points to road de-icers in urban areas and septic systems and fertilizer (including land-application of manure) in more rural areas.