Once considered to behave conservatively in drinking water distribution systems, trace metal and radiological contaminants participate in a variety of dynamic reaction and transport mechanisms contributing to their accumulation and intermittent release. Only within the past decade has the industry become aware of the ability of certain common solids — primarily iron and manganic oxides — to concentrate trace inorganic contaminants.
The mechanisms and extent of these associations, and the potential for sustained periods of elevated contaminants at customer taps as a result of remobilization and release processes, is substantially greater than previously recognized.
This was a major finding from “Assessment of Inorganic Accumulation in Drinking Water System Scales and Sediments (WRF 3118),” a study of 27 utilities nationwide conducted by U.S. EPA, Water Research Foundation, Washington and Iowa universities, Confluence Engineering Group LLC, and HDR Inc. The research was prompted by several isolated instances in which systems compliant with applicable maximum contaminant levels (MCLs) for trace contaminants were found to experience protracted episodes of elevated concentrations of elements such as arsenic, copper, lead, radium, and radon.
EPA's regulatory framework assumes public health can be ensured by monitoring for trace inorganics at system entry points. But in the instances referenced above, contaminant levels had risen by as much as four orders of magnitude (10,000 times) by the time the water reached customer taps. In addition, the prolonged duration of the episodes raised concerns about potential health impacts of sub-chronic exposure to contaminants at levels barely above their respective MCLs.
Finally, the issue isn't necessarily the contaminants' presence but the potential for their remobilization back into the water. Once accumulated, these solids are susceptible to destabilization and mobilization through a variety of hydraulic and chemical events.
Solids mobilization is often accompanied by perceptible degradation in aesthetic water quality, a common source of customer complaints but basically considered innocuous. While iron- and/or manganese-based solids cause the discoloration, rarely is it recognized as worthwhile to analyze for the trace inorganic contaminants we now know are associated with the elevated iron and manganese solids.
Contaminants of concern
Inorganic contaminants of particular concern to accumulation-and-release phenomena include regulated trace metals (arsenic, barium, beryllium, cadmium, chromium, copper, mercury, lead, antimony, selenium, thallium), naturally occurring uranium, and isotopes of radium and radon. Vanadium and nickel are also being considered for regulation.
These contaminants are regulated with MCLs developed to protect against adverse health effects associated with long-term ingestion at low concentrations. However, a review of toxicity summaries and health advisory reports has shown that they may also cause or contribute to serious acute and/or subchronic nausea, vomiting, and damage to organs and organ systems when consumed at levels above their respective MCLs for relatively short time periods.
Here's another concern: These contaminants co-accumulate. Therefore, the incidence and severity of adverse health impacts may increase depending on the number and interactivity of contaminants.
Solids represent a substantial reservoir of trace metals in close proximity to the consumer. The chart below presents the range of trace metals contamination found in distribution system scales and sediment from the study's 27 utilities. The whisker plots show the average, the 90th percentile, and the extremes of trace metal mass normalized to the mass of the sediment and scale. In some cases, barium and lead levels approach 1% of sediment mass, a concentration factor of approximately four orders of magnitude.
Factors affecting chemical release
Many utilities rely on multiple supply sources. The dynamic use of multiple sources, especially those of dissimilar quality, can create chemical disequilibria capable of promoting contaminant release. Examples include:
Converting or adding a treatment process also can alter the finished quality of source water. Examples include (1) pH adjustment practices; (2) implementing or optimizing corrosion control treatment; (3) implementing or converting disinfection; and (4) other physiochemical processes that affect ionic distribution, such as ion exchange, coagulant implementation or conversion, and alkalinity supplementation.
Applying a new pH target can contribute to scale instability, dissolution, and/or a “long-term” desorption scenario capable of mobilizing a host of contaminants.
Applying orthophosphate helps control corrosion and stabilize lead and copper components. But because it reacts with a number of metals including aluminum, calcium, and lead, orthophosphate contributes to mineral phase transformation as well as competition-based desorption of some trace metal contaminants.
Changes in disinfection practices can also impact the stability of contaminant sinks, primarily due to the influence of disinfectant residual on oxidation-reduction potential. Examples of changes that may present an issue include:
Considerable gaps remain in our understanding of which contaminants accumulate, what accumulation levels are of concern, specific factors significant to controlling these phenomena, and what measures utility managers can take to manage potential risks.
Given the issue's complexity and need for more information on behavior in actual water systems, consider investigating the scope of your operation's potential challenge. But don't just analyze levels of trace inorganics. Collect and analyze solids as well, especially during episodes of discoloration; and check out customer complaints.
— Steve Reiber (firstname.lastname@example.org) is technical lead of drinking water quality for consulting firm HDR Inc.
For more about the report “Assessment of Inorganic Accumulation in Drinking Water System Scales and Sediments (WRF 3118)” click here.