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Changes in Ground-Water Quality near Two Granular-Iron Permeable Reactive Barriers in a Sand and Gravel Aquifer, Cape Cod, Massachusetts, 1997–2000

By Jennifer G. Savoie, Douglas B. Kent, Richard L. Smith, Denis R. LeBlanc, U.S. Geological Survey, and David W. Hubble, University of Waterloo

Water-Resources Investigations Report 03-4309

ABSTRACT

Two experimental permeable reactive barriers (PRBs) of granular zero-valent iron were emplaced in the path of a tetrachloroethene plume (the Chemical Spill-10 plume) at the Massachusetts Military Reservation, Cape Cod, Massachusetts, in June 1998. The goal of the field experiment was to achieve emplacement of a granular-iron PRB deeper than attempted before. The PRBs were expected to create a reducing environment and degrade the tetrachloroethene by reductive dechlorination. The goal of the work presented in this report was to observe temporary and sustained changes to the ground-water chemistry downgradient from the PRBs.
A hydraulic-fracturing method involving injection of the granular iron with a guar-biopolymer and enzyme slurry was used to install the parallel 30- to 33-foot-wide wall-shaped barriers at a depth of 82 to 113 feet below land surface. An acetic acid and enzyme mixture was subsequently injected in wells near the barriers to degrade the guar biopolymer.
Prior to the emplacement, tetrachloroethene concentrations in the Chemical Spill-10 plume at the study area were as high as 250 micrograms per liter. Other water properties in the plume generally were similar to the properties of uncontaminated ground water in the area, which typically has dissolved oxygen concentrations of 250 to 375 micromoles per liter, pH of 5.5 to 6.0, and specific conductance of 60 to 90 microsiemens per centimeter.
Water-quality samples were collected periodically from monitoring wells near the PRBs to determine how the emplacement of the granular-iron walls altered the ground-water quality. In addition, an automated well-sampling device measured temperature, specific conductance, pH, and dissolved oxygen every 1–4 days for 16 months in a well downgradient from the two parallel PRBs.
Temporary increases (lasting about 5 to 6 months) in specific conductance were observed downgradient from the PRBs as a result of the sodium chloride, potassium carbonate, and other salts included in the slurry and the acetic acid and enzyme mixture that was subsequently injected to degrade the guar biopolymer. Temporary increases in the concentrations of major cations (sodium, potassium, magnesium, and calcium) were observed downgradient from the PRBs, as were temporary but substantial increases in the dissolved and total organic carbon concentrations. Methane was detected, sulfate concentrations decreased temporarily, and concentrations of dissolved inorganic carbon increased in samples from wells downgradient from the PRBs.
A sustained (longer than 12 months) reducing environment, in which dissolved oxygen concentrations decreased to zero, the pH increased to about 6.8, and dissolved iron concentrations increased substantially, developed as a result of the oxidation (corrosion) of the granular iron; this zone persisted at least 65 feet downgradient from the PRBs. The pH and dissolved iron concentrations increased with distance from the granular-iron walls. Concentrations of arsenic, cobalt, manganese, and phosphorus increased, and nitrate concentrations were reduced to below the detection limit downgradient from the walls. A sustained decrease of tetrachloroethene concentrations was not observed; however, reductive dechlorination products were observed at wells downgradient from the PRBs during several rounds of sampling.
The emplacement of zero-valent iron in the aquifer to remove tetrachloroethene from the ground water caused changes in the water chemistry that persisted farther downgradient from the PRBs than has been observed at other sites because of the low chemical reactivity of the quartz-dominated aquifer sediments and the low ambient dissolved chemical concentrations in the ground water. The small transverse dispersion in the aquifer and the probable long-term persistence of the iron indicate that the chemically altered zone probably will extend a substantial distance downgradient from the PRBs for a substantial period of time (years); further investigation would be needed to determine this distance.

CONTENTS

Abstract 1

Introduction 2

Permeable Reactive Barriers 2

Geochemical Conditions 5

Acknowledgments 5

Monitoring Changes in Ground-Water Quality 6

Installation of Monitoring Wells 6

Well Selection and Sampling Frequency 6

Ground-Water-Sample Collection and Field Water-Quality Measurements 6

Water-Sample Analysis 9

Ground-Water Quality Downgradient from the Permeable Reactive Barriers 10

Observed Ground-Water Quality 11

Specific Conductance, Sodium, and Chloride 11

Dissolved Oxygen 11

pH and Alkalinity 12

Nitrate, Ammonium, and Sulfate 14

Total and Dissolved Organic Carbon, Dissolved Inorganic Carbon, and Methane 14

Major Cations and Boron 14

Dissolved Iron, Phosphorus, and Trace Elements 15

Tetrachloroethene 16

Changes in Ground-Water Quality 17

Estimated Ground-Water Velocity 17

Effects of Zero-Valent-Iron Corrosion 19

Changes in Concentrations of Ground-Water Solutes 19

Dissolved Oxygen 20

Nitrate and Ammonium 20

Sulfate 21

Dissolved Inorganic Carbon 21

Methane 22

Arsenic, Manganese, Cobalt, and Phosphorus 22

Changes in Chemical Conditions Within the Zone Downgradient from the B-Wall 23

Summary and Conclusions 24

References Cited 26

Appendix 1. The Robowell Automated Sampling System by Kirk P. Smith 77

FIGURES

1, 2. Maps showing:

1. Locations of the granular-iron permeable reactive barrier test site, the Chemical Spill-10 plume, and the altitude of the water table, Cape Cod, Massachusetts 3

2. Locations of monitoring wells, the Robowell sampling system, and the A-wall and B-wall permeable reactive barriers 4

3. Longitudinal section along A-A¢ showing the locations of monitoring wells and the A-wall and B-wall permeable reactive barriers 7

4–9. Time-series plots of:

4. Specific conductance at wells RW 24B-0104 and RW 27-0102A, and dissolved chloride and sodium concentrations at well RW 24B-0104, July 1997 to February 2000 12

5. Dissolved oxygen concentrations and pH at wells RW 24B-0104 and RW 27-0102A, and alkalinity at well RW 24B-0104, July 1997 to February 2000 13

6. Dissolved nitrate, ammonium, and sulfate concentrations at well RW 24B-0104, July 1997 to February 2000 15

7. Dissolved organic carbon, dissolved inorganic carbon including carbon dioxide, and methane concentrations at well RW 24B-0104, July 1997 to February 2000 16

8. Dissolved calcium, magnesium, potassium, boron, and sodium concentrations at well RW 24B-0104, July 1997 to February 2000 17

9. Dissolved iron, manganese, arsenic, cobalt, and phosphorus concentrations at well RW 24B-0104, July 1997 to February 2000 18

TABLES

1. Location coordinates, top-of-casing and well-screen altitudes, water levels, and distances downgradient from A- and B-walls for monitoring wells near the granular-iron permeable reactive barriers, Massachusetts Military Reservation, Cape Cod, Massachusetts page 8

2. Selected inorganic-solute concentrations for quality-control samples page 10

3. Limitations of quantitation and relative precisions and accuracies for inorganic solutes page 10

4. Field water-quality analyses for ground-water samples collected near the granular-iron permeable reactive barriers, 1997–2000 page 31

5. Selected field water-quality analyses for ground-water samples collected by the Robowell system at well RW 27-0102A near the granular-iron permeable reactive barriers, June 1998 to October 1999 page 38

6. Anions and ammonium analyses for ground-water samples collected near the granular-iron permeable reactive barriers, 1997–2000 page 41

7. Carbon species analyses for ground-water samples collected near the granular-iron permeable reactive barriers, 1997–2000 page 49

8. Major- and minor-element analyses for ground-water samples collected near the granular-iron permeable reactive barriers, 1997–2000 page 55

9. Trace-element analyses for ground-water samples collected near the granular-iron permeable reactive barriers, 1997–2000 page 64

10. Tetrachloroethene analyses for ground-water samples collected near the granular-iron permeable reactive barriers, 1997–2000 page 71


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The citation for this report, in USGS format:

Savoie, J.G., Kent, D.B., Smith, R.L., LeBlanc, D.R., Hubble, D.W., 2004, Changes in Ground-Water Quality near Two Granular-Iron Permeable Reactive Barriers in a Sand and Gravel Aquifer, Cape Cod, Massachusetts, 1997–2000: U.S. Geological Survey Water-Resources Investigations Report 03-4309, 84 p.


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