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NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

The Association of Arsenic With Redox Conditions, Depth, and Ground-Water Age in the Glacial Aquifer System of the Northern United States

U.S. Geological Survey Scientific Investigations Report 20075036

By Mary Ann Thomas


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Abstract

More than 800 wells in the glacial aquifer system of the Northern United States were sampled for arsenic as part of U.S. Geological Survey National Water-Quality Assessment (NAWQA) studies during 19912003. Elevated arsenic concentrations (greater than or equal to 10 micrograms per liter) were detected in 9 percent of samples.

Elevated arsenic concentrations were associated with strongly reducing conditions. Of the samples classified as iron reducing or sulfate reducing, arsenic concentrations were elevated in 19 percent. Of the methanogenic samples, arsenic concentrations were elevated in 45 percent. In contrast, concentrations of arsenic were elevated in only 1 percent of oxic samples.

Arsenic concentrations were also related to ground-water age. Elevated arsenic concentrations were detected in 34 percent of old waters (recharged before 1953) as compared to 4 percent of young waters (recharged since 1953). For samples classified as both old and methanogenic, elevated arsenic concentrations were detected in 62 percent of samples, as compared to 1 percent for samples classified as young and oxic.

Arsenic concentrations were also correlated with well depth and concentrations of several chemical constituents, including (1) constituents linked to redox processes and (2) anions or oxyanions that sorb to iron oxides.

Observations from the glacial aquifer system are consistent with the idea that the predominant source of arsenic is iron oxides and the predominant mechanism for releasing arsenic to the ground water is reductive desorption or reductive dissolution. Arsenic is also released from iron oxides under oxic conditions, but on a more limited basis and at lower concentrations.

Logistic regression was used to investigate the relative significance of redox, ground-water age, depth, and other water-quality constituents as indicators of elevated arsenic concentrations in the glacial aquifer system. The single variable that explained the greatest amount of variation in the data was redox. Multivariate models that included a redox variable overestimated the percentage of samples with elevated arsenic concentrations because, even though elevated arsenic concentrations were associated with strongly reducing samples, not all strongly reducing samples had elevated arsenic concentrations.

Arsenic concentrations and redox conditions differed among four broad areas of the glacial aquifer system. For the East, Central, and West-Central north areas, there was a trend of increasing arsenic concentrations that corresponded to an increase in reducing conditions. For the West-Central south area, arsenic concentrations in oxic samples were higher than for the other areas, possibly because of high concentrations of orthophosphate, which is linked to desorption of arsenic from iron oxides under oxic conditions.

The observed differences in arsenic concentrations among broad areas of the glacial aquifer system were generally consistent with a conceptual model developed by Smedley and Kinniburg1, who studied or reviewed studies of widespread arsenic contamination in Bangladesh, India, China, Vietnam, Hungary, Argentina, northern Chile and the Southwestern United States.

_________________
1Smedley, P.L., and Kinniburgh, D.G., 2002, A review of the source, behaviour, and distribution of arsenic in natural waters: Applied Geochemistry, v. 17, p. 517–568.


Contents

Abstract

Introduction

Background

Previous Studies

Methods

Description of the Study Area

Arsenic Concentrations and Related Factors in the Glacial Aquifer System

Arsenic Concentrations

Redox Conditions

Well Depth and Well Type

Ground-Water Age

Other Water-Quality Properties

Relative Significance of Factors Related to Arsenic

Regional Variation of Arsenic and Redox Conditions Within the Glacial Aquifer System

Regional Framework of the Glacial Aquifer System

Comparison of Arsenic and Related Factors Among Framework Areas

Relation to Conceptual Model for High-Arsenic Environments

Summary and Conclusions

References Cited

Figures

1–3. Maps showing:
1. Location of wells in the glacial aquifer system sampled for arsenic by the National Water-Quality Assessment Program,
      1991–2003.
2. Bedrock beneath glacial deposits.
3. Thickness and texture of glacial deposits near land surface.
4. Graph showing requency of arsenic concentrations in samples from the glacial aquifer system.
5. Map showing arsenic concentrations in samples from the glacial aquifer system.
6–11. Graphs showing:
    6. Frequency of estimated redox conditions of samples from the glacial aquifer system, and arsenic concentrations for samples in
    each redox category.
    7. Relation of arsenic concentration and redox condition to well depth for the three types of wells sampled.
    8. Relation of arsenic concentrations to estimated ground-water age determinations from chlorofluorocarbons, sulfur hexafluoride,
    or tritium/helium-3 methods.
    9. Relation of arsenic concentrations to estimated ground-water age determinations from tritium concentrations.
    10. Relation of arsenic concentration and redox condition to well depth for young and old waters.
    11. Percentage of samples with elevated arsenic concentrations for young and old waters in three redox categories.
  12. Map showing framework areas of the glacial aquifer system.
13–15. Graphs showing:
    13. Arsenic in relation to redox and well-depth relations for framework areas of the glacial aquifer sysem.
    14. Comparison of arsenic concentration and redox-related characteristics among framework areas of the glacial aquifer system:
     percentage of samples with elevated arsenic concentrations, percentage of samples with strongly reducing conditions, median
     concentrations of dissolved organic carbon and median concentrations of ammonia.
    15. Comparison of oxic samples among framework areas of the glacial aquifer system: percentage of oxic samples, median arsenic
     concentrations in oxic samples, median orthophosphate concentrations in oxic samples, and median pH in oxic samples.

Tables

  1. Concentrations of water-quality constituents used to classify redox conditions of samples from the glacial aquifer system.
  2. Results of Spearman’s rho tests for the 12 water-quality constituents that have the best correlations with arsenic.
  3. Relative significance of individual variables as indicators of elevated arsenic concentrations, based on results of univariate logistic
    regression.


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Suggested Citation:


Thomas, M.A., 2007, The association of arsenic with redox conditions, depth, and ground-water age in the glacial aquifer system of the Northern United States: U.S. Geological Survey Scientific Investigations Report 20075036, 26 p.




For more information about activities of the USGS National Water-Quality Assessment (NAWQA) Program, visit the NAWQA home page.



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