Melinda J. Friedman, Andrew S. Hill, Steve H. Reiber, Richard L. Valentine, and Gregory V. Korshin

OBJECTIVES:

The objective of this research effort was to investigate the accumulation of regulated inorganic contaminants and naturally occurring radionuclides in distribution system piping scales and accumulated sediments. The research emphasis was on regulated metals, metalloids, and radionuclides, specifically antimony, arsenic, barium, cadmium, chromium, nickel, lead, radium, selenium, thallium, uranium, and vanadium.

BACKGROUND:

The potential for the accumulation and intermittent release of trace inorganic and radio-logical contaminants within water distribution systems has gained attention in the drinking water community over the past several years. These phenomena have been shown to result in at-the-tap concentrations of certain contaminants (e.g., arsenic, radium) that far exceed their entry-point levels and in some cases, their respective drinking water standards. The non-conservative nature of certain inorganic and radiological contaminants has raised concerns about public health risks and limitations of the Safe Drinking Water Act monitoring framework for these contaminants. The collection of native deposit materials from utility water systems and examination of occurrence of a broad array of trace inorganic and radiological contaminants within these deposits is a critical first step towards developing a better understanding of the behavior of these contaminants within water distribution systems.

APPROACH:

The project approach consisted of three tasks: (1) collection and evaluation of existing data and information; (2) development and implementation of a field study program; and (3) evaluation and interpretation of the data generated as part of this research investigation. The study was structured and performed in such a manner as to build upon previous findings and fill in the most apparent data gaps.

RESULTS/CONCLUSIONS:

Occurrence Trends

Trace inorganic and radiological contaminants were found to occur in distribution system deposits at widely-varying levels. Median occurrence levels for the various trace contaminants assessed spanned three orders-of-magnitude. Occurrence distributions of individual contaminants spanned, on average, four orders-of-magnitude. A series of operationally-defined occurrence bins were created to enable a comparison of the relative occurrence trends and to prioritize the trace elements based on their tendency to accumulate.

To identify potential concentration increases that could result from deposit mobilization, Conceptual Exceedance Factors (CEFs) were defined as the ratio of the theoretical concentration increase of a trace contaminant in water to its respective drinking water standard. The magnitude of the CEF provides an indication of the ability of released trace contaminants to approach and possibly exceed drinking water standards. The use of CEFs is a robust, risk-based prioritization approach that takes into account the variability in both occurrence/prevalence levels and chronic toxicity levels of these contaminants. Under this prioritization approach, contaminant emphasis would be ranked in accordance with the magnitude of the CEF, i.e., arsenic, lead, radium, vanadium, nickel, cadmium, barium, chromium, uranium, thallium, antimony, and selenium.

Influencing Factors

Trace contaminant occurrence trends were examined in the context of sample-specific finished water quality and deposit composition with regard to “major” matrix elements, e.g., iron, manganese, calcium, phosphorus. Based on similarities in accumulation trends and contaminant behavior, the trace elements can be broadly-divided into two groups-- trace metal cations and anionic compounds. Trace metal cations include barium, lead, nickel, and radium isotopes. These elements have a strong affinity for hydrous manganese oxides (HMOs) and an apparent affinity for phosphate precipitates and/or phosphate surface groups. Their accumulation by adsorption/co-precipitation mechanisms is typically enhanced under conditions of elevated pH and when potentially competitive cations (e.g., Ca, Mg) are present at low levels. Trace anionic compounds include the oxoacids arsenate, chromate, and vanadate, as well as complexes of uranyl. These compounds have a strong affinity for HMOs and hydrous ferric oxides. Their accumulation by adsorption/co-precipitation mechanisms is typically enhanced under conditions of reduced pH and when potentially competitive anions (e.g., carbonate, phosphate, silicate) are present at low levels.

APPLICATIONS/RECOMMENDATIONS:

The findings from this study have been framed in a manner to provide clear direction for future research as well as guidance for drinking water utilities. Regarding the latter, a three-step strategy has been developed to help utilities assess and control the potential for accumulation and release in their systems.

Step 1 - Assess Existing Conditions and Vulnerability

Step 1 focuses on developing an understanding of existing system conditions that may pose a risk for trace contaminant accumulation and release. It is recommended that each utility conduct a self-assessment to gauge its vulnerability to these phenomena, assessing system data and information in the following areas: distribution system pipe material, extent of deposit accumulation, and water quality conditions.

Step 2 - Address Existing Deposits

It is possible that a significant amount of deposit material and contaminants may have accumulated within certain portions of the distribution system. The utility should focus its immediate resources on removing loosely-adhered solids from the distribution system and implementing measures to stabilize adhered solids to the extent possible. Both of these are essentially “release-management” techniques. Of particular importance are the recommendations that (1) utilities apply unidirectional flushing rather than conventional flushing approaches for removal of loose deposits, and (2) assess/implement treatment measures to address water quality/chemistry variations associated with the blending of different sources.

Step 3 - Reduce Contaminant and Solids Loading

To ultimately reduce the potential for ongoing contaminant accumulation, utilities are encouraged to implement engineering measures to improve treatment of inorganic contaminants and solids that can act as substrates (e.g., Fe, Mn).

Despite the numerous findings and conclusions resulting from this investigation, there remain considerable research gaps to be addressed in order for the drinking water industry to develop a more complete understanding of accumulation and release phenomena and associated potential public health risks. Several recommendations for future research are provided in the report.

RESEARCH PARTNER:

U.S. Environmental Protection Agency

PARTICIPANTS:

Sixteen utilities participated in this project.