Open Access

Potential contaminants at a dredged spoil placement site, Charles City County, Virginia, as revealed by sequential extraction

  • Jianwu Tang1Email author,
  • G Richard Whittecar1,
  • Karen H Johannesson2 and
  • W Lee Daniels3
Geochemical Transactions20045:49

DOI: 10.1186/1467-4866-5-49

Received: 23 April 2004

Accepted: 03 November 2004

Published: 07 December 2004

Abstract

Backfills of dredged sediments onto a former sand and gravel mine site in Charles City County, VA may have the potential to contaminate local groundwater. To evaluate the mobility of trace elements and to identify the potential contaminants from the dredged sediments, a sequential extraction scheme was used to partition trace elements associated with the sediments from the local aquifer and the dredged sediments into five fractions: exchangeable, acidic, reducible, oxidizable, and residual phases. Sequential extractions indicate that, for most of the trace elements examined, the residual phases account for the largest proportion of the total concentrations, and their total extractable fractions are mainly from reducible and oxidizable phases. Only Cd, Pb, and Zn have an appreciable extractable proportion from the acidic phase in the filled dredged sediments. Our groundwater monitoring data suggest that the dredged sediments are mainly subject to a decrease in pH and a series of oxidation reactions, when exposed to the atmosphere. Because the trace elements released by carbonate dissolution and the oxidation (e.g., organic matter degradation, iron sulfide and, ammonia oxidation) are subsequently immobilized by sorption to iron, manganese, and aluminum oxides, no potential contaminants to local groundwater are expected by addition of the dredged sediments to this site.

I. Introduction

A former sand and gravel mine site in Charles City County, VA was mined in the 1980s, backfilled with 10 m of sandy mine spoils, and reclaimed to farmland use. The property owner, i.e., the Weanack Limited Land Partners, bulldozed the reclaimed mine-spoils into a diked basin and imported sediments dredged from the upper Potomac River estuary as fill for the excavated basin in order to convert it to high-quality farmland. However, in theory, the addition of these dredged sediments may influence local groundwater quality. First, the back-filling dredged sediments have the potential of becoming a contaminant source because these dredged estuarine sediments may contain contaminants such as heavy metals that could subsequently enter the groundwater. Second, by adding these dredged sediments into the reclamation basin, the in situ conditions could change, potentially liberating toxic heavy metals/metalloids to local groundwater. Possible processes that could be induced by the addition of the dredged sediments include: (1) mixing of pore water from the upper Potomac River sediments with the local groundwater, which will affect ionic strength and pH of the dredged sediment pore water and local groundwater; (2) infiltration of the dredged sediments by rain (pH~ 5.6), which will affect the pH of the dredged sediment pore water; and (3) oxidizing the dredged sediments by exposing them to the atmosphere and/or by local groundwater, which will also affect the redox conditions of the dredged sediment pore water. Therefore, before developing these dredged sediments into agriculturally productive soils, leachate that exits the sediment disposal site must be monitored and traced. To accomplish this, any potential contaminants from the backfilling sediments must first be determined. In this work, a sequential extraction scheme was used to evaluate the mobility of trace elements in sediment and to reveal any potential contaminants that may be mobilized from the dredged sediments.

The mobility of trace elements in sediments, as well as their potential toxicity, depends on their aqueous and solid-phase speciation and the chemical and physical processes to which these elements are subjected. Sequential extraction procedures have been used to identify the "operationally defined solid-phase speciation" of trace elements in sediments, as a means to evaluate their mobility in the natural environments.[16] To ascertain the potential mobility of any contaminants added to a dredged spoil placement site by back-filling with the Potomac River dredged sediments, batch extractions were performed on the local aquifer sediments and on back-filling sediments dredged from the upper Potomac River estuary. The sequential extraction scheme proposed and outlined by Tessier et al.[5] was used to partition trace elements associated with these sediments into the following fractions:

Fraction 1 (Exchangeable phase)

Trace elements, which are weakly associated with clays, hydrated oxides of iron and manganese, and humic acids, and are readily mobilized by changes in ionic strength.

Fraction 2 (Acidic phase)

Trace elements bound to sediment carbonates, which are susceptible to carbonate dissolution as sediments are acidified.

Fraction 3 (Reducible phase)

Trace elements bound to iron and manganese oxides, which will be reduced and solu-bilized under anoxic conditions (i.e., low Eh).

Fraction 4 (Oxidizable phase)

Trace elements in reduced metal complexes or bound to organic matter, which can be released into solution under oxidizing conditions.

Residual phase

Trace elements held within the crystal structure of primary and secondary minerals, which are not expected to be released into the solution over a reasonable time span under natural conditions.

Eight samples of local aquifer sediments and the Potomac River dredged sediments were sequentially extracted using the techniques outlined by Tessier et al.[5] The various batch extraction fluids were subsequently analyzed by inductively coupled plasma mass spectrometry (ICP-MS; Finnigan MAT Element II) for eleven trace elements (Ag, Cd, Sb, Tl, Pb, Cr, Fe, Ni, Cu, Zn, and As). From the analytical results, the percentage of the total concentration of each trace element in each fraction is calculated. The total extractable fractions of trace elements and their "operationally defined solid-phase speciation" are discussed. Finally, any potential mobility of any contaminants from the dredge sediments as in situ conditions change following disposal at the replacement site is evaluated.

II. Study Area

The dredged spoil placement site is located in Charles City County, VA (Fig. 1). The site was mined for sand and gravel and backfilled by 10 m of mine spoils with 1 m of natural soil over the fill. In the fall of 2000, backfill mine spoils were excavated and reconfigured into dikes around a large L-shape basin and subsequently used as a disposal basin to receive the sediments being dredged from the upper Potomac River estuary near Alexandria, VA (Figs. 1 and 2). To the north and east of the site, the slope of the ground is steep, forming a scarp. Numerous monitoring wells were installed at the disposal site (Fig. 2) to monitor groundwater quality and to evaluate the potential flow paths.
Figure 1

Map showing location of study area. Sediments dredged from the Potomac River estuary near Washington, DC (DC) are being reclaimed as farmland at the Weanack study site along the James River estuary near Richmond, Virginia (R). Norfolk, Virginia (N) and state boundaries are also shown.

Figure 2

Locations of L-shaped disposal site (lined), monitoring wells and gauges (solid dots), dredged sediment samples (open rectangles), and the scarp (coarse dash line).

The distribution of aquifers and aquitards at the study site is complex. The complexity stems from the history of cut-and-fill related to the sand-and-gravel mining at this site and the history of stream incision and valley-infilling caused by Pleistocene sea level fluctuations. [79] According to our stratigraphic analyses, at least six different geologic units are thought to play significant roles in controlling the flow of groundwater in the study region (Fig. 3). These geologic units are listed in Table I, and include Mining Fill, Kennon Formation, Tabb Formation, Shirley Formation, Nanjemoy-Marlboro unit, and Aquia-Potomac units. [79]
Figure 3

Schematic geologic map and diagrammatic cross-section of study site. Qs: Shirley Formation; Qt: Tabb Formation; Qk: Kennon Formation. Locations of boreholes (wells) near the transect are shown on the profile.

Table I

Hydrostratigraphic unit at a dredged spoil placement site, Charles City County, VA. [Data are from 2002 annual monitoring report (Whittecar – Ref. 15); unit names from Mixon et al. (Ref. 7), McFarland (Ref. 8), and Johnson et al. (Ref. 9)]

Unit

Description

Mining Fill

Mine spoils placed into old abandoned mine excavation. Commonly 4.6–7.6 m thick, beneath sediment disposal site. Mixed member: poorly sorted mixture of gravel, sand, silt, and clay;deposited by dumping; mostly under north half of refilled mine sites. Stratified member: stratified sands and mud deposited in a lake fromwest to east; mostly under the south end of refilled mine site.

Kennon Formation

Fining upward stream deposit (gravelly sand to silty clay) dominatedby thick clay-rich beds; 1.5–4.6 m thick; fills small recentvalleys carved into Tabb Formation beds.

Tabb Formation

Fining upward stream deposit (cobble beds to silty clay) dominatedby cobbles and gravelly sand; 3.7–6.7 m thick; carpets largevalley carved by the James River; forms broad terrace surface 4.6–6.1 m elevation.

Shirley Formation

Sand and gravel deposit (6.1 m thick) with silty clay cap (1.5 m thick); forms broad terrace at approximately 9.1–12.2 melevation, and terrace remnant beneath Shirley Plantation manorhouse

Nanj emoy-Marlborounit

Sandy silt-clay confining bed; glauconitic; compact; encounteredbeneath Tabb Formation and Mining Fill

Aquia-Potomac units

Layered gravelly aquifers and confining beds (42.7+ m thick)

A flow path map (Fig. 4), generated from hydrologic head data collected monthly at all monitoring wells and staff gauges, indicates that groundwater radiates in nearly all directions from the elevated replacement site. After leaving the bermed area, groundwater passes through highly permeable beds within the Mining Fill and the Tabb Formation. Water draining from the dredged sediments may take as little as one year to more than a decade to reach surface water bodies, depending upon the sediment permeability, distance, and hydraulic gradient along the flow path.
Figure 4

Map of potentiometric surface and illustrative flow lines for head data collected 15 December, 2002.

III. Experiment

A. Sampling

Eight sediment samples were collected from the local aquifers and the back-filling dredged sediments from the upper Potomac River estuary. Samples 1 and 2 are older dredged sediments from the Potomac River, which were filled onto the replacement site between January and March 2001. Samples 4 and 5 represent the more recent filling dredged sediments, which were filled onto the replacement site between January and March 2002. These four samples were collected from the dredged sediments after they were deposited in the reclamation basin (Fig. 2). Samples 6 and 7 belong to the Tabb Formation. Sample 6 represents shallow sediments developed within the Tabb Formation, and was collected from the well SW 40 core at a depth of 1.1–1.7 m (Fig. 2). Sample 7 is from deeper sediments, taken from the well SW44 core at a depth of 2.7–3.4 m (Fig. 2). Samples 8 and 9 belong to the Shirley Formation. Sample 8 represents shallow sediments, collected from the well SW42DV core at a depth of 0.9–1.5 m (Fig. 2), whereas sample 9 represents deeper sediments, obtained from the well SW43 core of a depth of 2.7–4.3 m (Fig. 2).

The sediment samples were stored at 4°C prior to batch extractions. Before batch extractions, sediment samples were dried at ~ 80°C in a forced air oven and then ground with a wooden roller to pass through a 1 mm sieve and homogenized. 0.5 g sediment samples (dry weight) were used for the initial extraction.

B. Leaching procedures and reagents

The methods used for sequential trace element extractions are those discussed in detail by Tessier et al.[5] In the following, we briefly outline the procedure for each extraction step.

1. Fraction 1: Exchangeable phase

A 0.5 g (dry weight) aliquot of sample was extracted into a 50 ml tube with 8 ml of 1 M CH3COONa at room temperature and pH 8.2. The mixture was subsequently agitated on a shaker for 1 h.

2. Fraction 2: Acidic phase

The residue from Sec. III B 1 was leached at room temperature with 8 ml of 1 M CH3COONa adjusted to pH5.0 with CH3COOH. The mixture was agitated on a shaker for 5 h.

3. Fraction 3: Reducible phase

The residue from Sec. III B 2 was extracted with 20 ml of 0.04 M NH2OH·HC1 in 25% (v/v) CH3COOH for 6 h at 96 ± 1°C. The mixture was agitated every 30 min.

4. Fraction 4: Oxidizable phase

The residue from Sec. III B 3 was extracted with 3 ml of 0.02 M HNO3 and 5 ml of 30% H2O2, adjusted to pH2.0 with ultrapure HNO3 (Seastar Chemicals). The mixture was heated to 85 ± 1°C for 2 h with occasional agitation. Then, 3 ml of 30% H2O2 (pH 2.0 adjusted with HNO3) was added and the mixture was heated again to 85 ± 1°C for 3 h with occasional agitation. After the mixture was cooled to room temperature, 5 ml of 3.2 M CH3COONH4 in 20% (v/v) HNO3 was added and the mixture was diluted to 20 ml and agitated for 30 min at room temperature on a shaker.

Between each successive extraction, separation was effected by centrifugation at 10 000 rpm for 10 min. The supernatant was removed and stored in a 50 ml bottle at 4°C until analysis. The residue after each extraction was washed with 8 ml of distilled-de-ionized (18 MΩ cm) MilliQ water (i.e., Q-water). After centrifugation for 10 min, this second supernatant was discarded.

C. Digestion of sediment sample for residual and total trace element analysis

The residue from Sec. III B 4 was first digested in a 50 ml Teflon® beaker with 10 ml of ultrapure HF (Seastar Chemicals). The mixture was subsequently evaporated to near dryness. Then, 10 ml of ultrapure HNO3 (Seastar Chemicals) was added and again, the mixture was evaporated to near dryness. Subsequently, another 10 ml of ultra-pure HF was added and the mixture was evaporated to near dryness. The residue was then dissolved with Q-water and diluted up to 25 ml with Q-water.

Finally, 0.2 g initial sediment sample (dry weight) was digested according to the same procedure for the residual phase and the final solution was diluted to 25 ml with Q-water.

D. Analytical methods

Concentrations of trace elements in extractions, residual phase, and total sediment samples were determined by inductively coupled plasma mass spectrometry (ICP-MS; Finnigan MAT Element II; Dr. Z. Chen, analyst). The detection limits were below 10 ng/L for Ag, Cd, Sb, Tl, and Pb, and below 1 μg/L for Cr, Fe, Ni, Cu, Zn, and As in the extraction fluids. Sequence control blanks and standards prepared from Perkin Elmer multi-element solutions were run regularly during the analyses to monitor the blank level, accuracy, and instrument drift. Analytical precision as relative standard deviation (RSD), estimated from five replicate analyses, is less than 1.0% for Cd, Sb, Tl, Pb, and Ni. For other elements, analytical precision is between 1.0% and 5.2% RSD.

IV. Results and Discussion

A. Concentrations and recoveries

Concentrations of the analyzed trace elements in the extractions, residual phase, and total sediment samples are given in Table II. The sequential extraction procedure provides satisfactory recoveries, most of which are within 80%–120% (Table II).
Table II

Concentrations of trace elements in extractions, residual, and total sediment. (Concentrations are expressed in ng/g of sediment, dry weight, except for Fe, which is in mg/g of sediment, dry weight)

Metal

Sample No.

Fraction 1

Fraction 2

Fraction 3

Fraction 4

Residual phase

Total sediment

Recovery (%)

Ag

1

0.071

0.108

24.282

13.151

295.500

272.972

122.03

 

2

a

0.258

26.423

194.766

491.561

693.362

102.83

 

4

a

a

258.351

1 640.871

ndb

2725.560

nd

 

5

a

0.108

74.332

98.191

348.656

528.108

98.71

 

6

a

a

5.307

1.010

243.375

240.755

103.72

 

7

0.029

a

10.231

2.750

296.623

280.744

110.29

 

8

0.042

0.046

16.736

1.523

255.623

250.221

109.49

 

9

0.062

0.078

9.059

5.319

287.225

279.386

108.00

Cd

1

6.472

48.354

45.847

5.486

101.534

238.342

87.14

 

2

6.275

117.290

85.701

6.849

161.041

303.086

124.44

 

4

43.571

349.965

311.591

106.501

nd

866.235

nd

 

5

39.239

699.384

2223.193

179.158

921.440

5 066.074

80.19

 

6

0.400

1.678

10.813

2.650

91.395

100.167

106.76

 

7

3.998

8.867

64.063

7.605

104.854

174.981

108.23

 

8

0.669

2.506

16.212

2.968

102.618

121.542

102.82

 

9

3.568

8.871

44.786

4.804

98.115

173.394

92.36

Sb

1

5.028

13.786

5.503

4.746

515.786

474.605

114.80

 

2

6.710

28.956

7.883

4.863

483.815

519.791

102.39

 

4

8.204

29.122

12.983

7.455

nd

720.023

nd

 

5

6.312

18.481

7.257

4.735

649.543

632.068

108.58

 

6

1.372

1.554

3.924

2.673

544.855

478.999

115.74

 

7

1.581

1.727

3.974

3.330

532.943

485.434

111.97

 

8

0.517

1.481

5.382

3.878

649.343

644.756

102.46

 

9

0.845

1.681

7.016

4.484

530.655

454.094

119.95

Tl

1

3.188

3.457

15.325

11.274

528.135

621.076

90.39

 

2

3.522

5.351

22.836

17.036

349.515

443.725

89.75

 

4

5.204

7.084

37.733

25.468

nd

569.952

nd

 

5

2.678

4.336

22.136

15.267

406.490

468.313

96.28

 

6

1.246

1.148

22.210

13.809

350.623

398.097

97.72

 

7

1.505

1.265

10.337

9.181

507.974

427.184

124.13

 

8

0.995

0.802

19.498

8.158

396.546

444.724

95.79

 

9

2.806

2.093

19.205

9.198

408.756

400.062

110.50

Pb

1

11.260

1 999.360

9271.597

2 300.046

6 202.777

25 326.599

78.12

 

2

9.060

5 340.496

17 736.960

4 033.203

4 827.587

33 594.686

95.10

 

4

14.420

5 434.827

32 087.274

11519.033

nd

52435.338

nd

 

5

129.663

31 102.512

91 037.459

14546.412

15 745.862

189 134.779

80.66

 

6

4.127

765.766

5 330.865

869.676

4 659.941

15 741.670

73.88

 

7

4.759

615.366

7 478.567

1 773.999

9091.660

18 007.325

105.31

 

8

4.984

1032.451

6 844.099

836.929

5 032.686

18 227.665

75.44

 

9

4.741

792.434

5 562.834

782.768

6638.363

16785.683

82.10

Cr

1

9.250

676.057

4 144.599

3533.393

57449.183

61441.193

107.11

 

2

6.695

904.332

5931.275

4631.480

48271.178

53 827.594

110.99

 

4

8.988

1 302.097

11276.499

9805.180

nd

70 072.496

nd

 

5

9.102

855.133

5 642.895

4 230.704

46 970.601

51 356.518

112.37

 

6

12.017

25.631

3 194.288

1929.951

40 754.522

38091.117

120.54

 

7

7.660

42.031

6 567.750

3 158.592

50 648.430

59 552.998

101.46

 

8

16.475

27.106

4 848.704

1 396.597

43 903.636

51 437.231

97.58

 

9

43.920

15.808

1 888.904

1 545.616

39 454.099

37 642.710

114.09

Fe

1

0.001

0.495

9.124

0.484

27.617

39.577

95.31

 

2

3.3E–5

0.505

10.108

0.737

23.525

31.584

110.42

 

4

4.3E–5

0.326

11.235

1.817

nd

38.392

nd

 

5

4.3E–5

0.606

10.228

0.718

23.674

31.623

111.39

 

6

a

0.001

6.469

0.484

29.853

34.569

106.47

 

7

6.7E–5

0.001

4.964

0.334

17.561

22.729

100.58

 

8

2.6E–4

0.001

8.664

0.356

25.631

36.269

95.54

 

9

1.1E–5

0.002

6.770

0.464

19.486

26.629

100.35

Ni

1

32.781

1 169.294

4853.280

2 054.761

25738.313

33 034.089

102.47

 

2

27.933

1 857.217

6498.011

2316.266

21 107.574

28958.351

109.84

 

4

46.143

1 794.210

9 930.939

3 878.025

nd

35 266.273

nd

 

5

40.443

1 628.284

6 406.030

2813.551

19433.457

30003.451

101.06

 

6

44.151

35.639

1 456.371

1 601.098

19913.320

17795.148

129.53

 

7

186.767

503.479

8233.408

1 412.784

23 452.670

32 684.564

103.38

 

8

58.319

44.355

1 856.858

810.413

22 698.592

23 392.666

108.87

 

9

26.250

114.256

2 307.372

1 272.209

17232.591

16453.127

127.35

Cu

1

53.019

1 188.204

3 758.365

5944.153

17415.774

29436.611

96.34

 

2

49.178

1 773.634

4434.530

8 870.600

15 064.632

27 370.758

110.31

 

4

109.659

3 729.499

11020.389

22 689.878

nd

49 870.074

nd

 

5

34.285

1 427.713

4967.171

7551.255

15315.070

27 577.567

106.23

 

6

15.007

83.844

2938.981

1 822.681

20 064.779

18926.953

131.69

 

7

25.881

932.864

16 554.675

3 301.967

15848.147

33243.183

110.29

 

8

13.493

75.531

3 035.225

990.290

18 085.262

21 602.361

102.77

 

9

19.182

175.868

4095.619

1431.143

11754.839

14271.972

122.45

Zn

1

102.805

2 748.999

16884.517

5534.190

55 179.058

89011.596

90.38

 

2

45.126

15 607.008

42 838.627

7910.451

45 422.619

102402.318

109.20

 

4

300.531

46 232.551

143051.392

32 983.708

nd

254 094.253

nd

 

5

86.802

18314.012

61 859.848

7 562.907

39 928.454

125 067.801

102.15

 

6

6.174

103.943

5 986.605

5 559.082

57 102.705

60814.605

113.06

 

7

37.223

1 530.869

31940.336

9 076.450

55 298.628

89 163.105

109.78

 

8

15.069

133.418

7 826.604

3 432.023

55 492.337

70 172.591

95.34

 

9

7.089

319.206

13 012.595

6488.597

51448.571

59903.783

118.98

As

1

28.328

151.934

1 232.778

34.054

4719.936

5499.051

112.15

 

2

29.745

201.669

1544.199

93.879

4500.311

4431.341

143.74

 

4

19.041

219.711

2713.633

331.686

nd

7 145.773

nd

 

5

47.221

267.051

1 943.043

148.682

5 445.426

5 496.017

142.86

 

6

a

12.706

108.269

a

7 268.973

6212.311

119.08

 

7

34.574

68.575

2 191.545

169.113

10 166.936

10736.104

117.65

 

8

10.537

20.510

154.292

a

7 677.608

8 578.925

91.76

 

9

a

10.850

311.100

a

5 635.792

4 953.477

120.24

aBelow detection limit.

bnd = not determined.

B. Total extractable fractions

The total extractable fractions of all trace elements in the sediment samples from the filling dredged sediments and the local aquifer sediments are summarized in the following and in Table III:
Table III

Total extractable fractions of trace elements in the dredged sediments and the local aquifer sediments. (Note: Because there are no data for the residual phase of Sample 4, percentages of trace elements in the successive extractions were calculated assuming the recoveries of all metals are 100%.)

 

Trace element

The dredged sediments

The local aquifer sediments

  

Recent filling

Older

Deep

Shallow

  

No. 4

No. 5

No. 2

No. 1

No. 7

No. 9

No. 6

No. 8

Total extractable fraction (%)

Ag

69.68

33.12

31.06

11.29

4.20

4.81

2.53

6.70

 

Cd

93.70

77.32

57.30

51.11

44.64

38.73

14.53

17.89

 

Sb

8.02

5.36

9.10

5.33

1.95

2.57

1.72

1.70

 

Tl

13.24

9.85

12.24

5.92

4.20

7.53

9.87

6.91

 

Pb

93.55

89.68

84.89

68.65

52.06

51.83

59.93

63.40

 

Cr

31.96

18.61

19.20

12.71

16.18

8.24

11.24

12.53

 

Fe

34.84

32.79

32.55

26.79

23.18

27.08

18.89

26.03

 

Ni

44.37

35.91

33.64

23.96

30.59

17.75

13.61

10.88

 

Cu

75.29

47.72

50.10

38.59

56.77

32.74

19.50

18.53

 

Zn

87.59

68.75

59.38

31.41

43.51

27.82

16.95

17.05

 

As

45.96

30.64

29.35

23.47

19.51

5.40

1.64

2.34

  1. (1)

    In the dredged sediments, the trace elements Cd, Pb, Cu, and Zn exhibit relatively large total extractable fractions, especially for Cd and Pb, followed by Ni, As, Fe, Ag, and Cr (Table III, Fig. 5). On the other hand, Tl and Sb have low total extractable fractions (Table III, Fig. 5).

     
Figure 5

"Operationally defined solid-phase speciation" of trace elements in the dredged sediments and the local aquifer sediments presented in absolute concentration.

  1. (2)

    Backfilling dredged sediments have higher total extractable fractions for almost all trace elements examined than the local aquifer sediment samples (Table III, Fig. 5).

     
  2. (3)

    Generally, for the trace elements examined, the more recent filling dredged sediments have higher total extractable fractions than the older dredged sediments (Table III, Fig. 5).

     
  3. (4)

    With exception of Tl and Pb, all trace elements examined exhibit higher total extractable fractions in samples of the local aquifer sediments collected from deeper depths than from the shallow depths (Table III, Fig. 5).

     

C. "Operationally defined solid-phase speciation" of trace element

Having discussed the general features of the total ex-tractable fractions of trace elements, we now discuss their "operationally defined solid-phase speciation" in the sediments in detail. The percentages of trace elements in the successive extractions are shown in Table IV.
Table IV

Percentages of trace elements in the successive extractions for both the dredged sediments and the local aquifer sediments. (Note: Because there are no data for the residual phase of Sample 4, percentages of trace elements in the successive extractions were calculated assuming the recoveries of all metals are 100%.)

Trace element

Sample No.

 

Exchangeable

Acidic

Reducible

Oxidizable

Residual

Ag

Recent filling

4

0.00

0.00

9.48

60.20

30.32

 

dredged sediments

5

0.00

0.02

14.26

18.84

66.88

 

Older dredged

2

0.00

0.04

3.71

27.32

68.94

 

sediments

1

0.002

0.003

7.29

3.95

88.71

 

Deep local

7

0.01

0.00

3.30

0.89

95.80

 

aquifer sediments

9

0.02

0.03

3.00

1.76

95.19

 

Shallow local

6

0.00

0.00

2.13

0.40

97.47

 

aquifer sediments

8

0.02

0.02

6.11

0.56

93.30

Cd

Recent filling

4

5.03

40.40

35.97

12.29

6.30

 

dredged sediments

5

0.97

17.22

54.73

4.41

22.68

 

Older dredged

2

1.66

31.10

22.72

1.82

42.70

 

sediments

1

3.12

23.28

22.07

2.64

48.89

 

Deep local

7

2.11

4.68

33.83

4.02

55.36

 

aquifer sediments

9

2.23

5.54

27.97

3.00

61.27

 

Shallow local

6

0.37

1.57

10.11

2.48

85.47

 

aquifer sediments

8

0.54

2.01

12.97

2.38

82.11

Sb

Recent filling

4

1.14

4.04

1.80

1.04

91.98

 

dredged sediments

5

0.92

2.69

1.06

0.69

94.64

 

Older dredged

2

1.26

5.44

1.48

0.91

90.90

 

sediments

1

0.92

2.53

1.01

0.87

94.67

 

Deep local

7

0.29

0.32

0.73

0.61

98.05

 

aquifer sediments

9

0.16

0.31

1.29

0.82

97.43

 

Shallow local

6

0.25

0.28

0.71

0.48

98.28

 

aquifer sediments

8

0.08

0.22

0.81

0.59

98.30

Tl

Recent filling

4

0.91

1.24

6.62

4.47

86.76

 

dredged sediments

5

0.59

0.96

4.91

3.39

90.15

 

Older dredged

2

0.88

1.34

5.73

4.28

87.76

 

sediments

1

0.57

0.62

2.73

2.01

94.08

 

Deep local

7

0.28

0.24

1.95

1.73

95.80

 

aquifer sediments

9

0.63

0.47

4.34

2.08

92.47

 

Shallow local

6

0.32

0.30

5.71

3.55

90.13

 

aquifer sediments

8

0.23

0.19

4.58

1.92

93.09

Pb

Recent filling

4

0.03

10.36

61.19

21.97

6.45

 

dredged sediments

5

0.08

20.39

59.67

9.53

10.32

 

Older dredged

2

0.03

16.72

55.52

12.62

15.11

 

sediments

1

0.06

10.11

46.86

11.63

31.35

 

Deep local

7

0.03

3.24

39.43

9.35

47.94

 

aquifer sediments

9

0.03

5.75

40.37

5.68

48.17

 

Shallow local

6

0.04

6.58

45.84

7.48

40.07

 

aquifer sediments

8

0.04

7.51

49.77

6.09

36.60

Cr

Recent filling

4

0.01

1.86

16.09

13.99

68.04

 

dredged sediments

5

0.02

1.48

9.78

7.33

81.39

 

Older dredged

2

0.01

1.51

9.93

7.75

80.80

 

sediments

1

0.01

1.03

6.30

5.37

87.29

 

Deep local

7

0.01

0.07

10.87

5.23

83.82

 

aquifer sediments

9

0.10

0.04

4.40

3.60

91.86

 

Shallow local

6

0.03

0.06

6.96

4.20

88.76

 

aquifer sediments

8

0.03

0.05

9.66

2.78

87.47

Fe

Recent filling

4

0.00

0.85

29.26

4.73

65.16

 

dredged sediments

5

0.00

1.72

29.04

2.04

67.21

 

Older dredged

2

0.00

1.45

28.98

2.11

67.45

 

sediments

1

0.00

1.31

24.19

1.28

73.21

 

Deep local

7

0.00

0.01

21.71

1.46

76.82

 

aquifer sediments

9

0.00

0.01

25.33

1.74

72.92

 

Shallow local

6

0.00

0.00

17.57

1.31

81.11

 

aquifer sediments

8

0.00

0.00

25.00

1.03

73.97

Ni

Recent filling

4

0.13

5.09

28.16

11.00

55.63

 

dredged sediments

5

0.13

5.37

21.13

9.28

64.09

 

Older dredged

2

0.09

5.84

20.43

7.28

66.36

 

sediments

1

0.10

3.45

14.34

6.07

76.04

 

Deep local

7

0.55

1.49

24.37

4.18

69.41

 

aquifer sediments

9

0.13

0.55

11.01

6.07

82.25

 

Shallow local

6

0.19

0.15

6.32

6.95

86.39

 

aquifer sediments

8

0.23

0.17

7.29

3.18

89.12

Cu

Recent filling

4

0.22

7.48

22.10

45.50

24.71

 

dredged sediments

5

0.12

4.87

16.96

25.78

52.28

 

Older dredged

2

0.16

5.87

14.69

29.38

49.90

 

sediments

1

0.19

4.19

13.25

20.96

61.41

 

Deep local

7

0.07

2.54

45.15

9.01

43.23

 

aquifer sediments

9

0.11

1.01

23.43

8.19

67.26

 

Shallow local

6

0.06

0.34

11.79

7.31

80.50

 

aquifer sediments

8

0.06

0.34

13.67

4.46

81.47

Zn

Recent filling

4

0.12

18.20

56.30

12.98

12.41

 

dredged sediments

5

0.07

14.34

48.42

5.92

31.25

 

Older dredged

2

0.04

13.96

38.31

7.07

40.62

 

sediments

1

0.13

3.42

20.99

6.88

68.59

 

Deep local

7

0.04

1.56

32.63

9.27

56.49

 

aquifer sediments

9

0.01

0.45

18.26

9.10

72.18

 

Shallow local

6

0.01

0.15

8.71

8.08

83.05

 

aquifer sediments

8

0.02

0.20

11.70

5.13

82.95

As

Recent filling

4

0.27

3.07

37.98

4.64

54.04

 

dredged sediments

5

0.60

3.40

24.75

1.89

69.36

 

Older dredged

2

0.47

3.17

24.24

1.47

70.65

 

sediments

1

0.46

2.46

19.99

0.55

76.53

 

Deep local

7

0.27

0.54

17.35

1.34

80.49

 

aquifer sediments

9

0.00

0.18

5.22

0.00

94.60

 

Shallow local

6

0.00

0.17

1.47

0.00

98.36

 

aquifer sediments

8

0.13

0.26

1.96

0.00

97.64

1. Fraction 1: Exchangeable phase

In both the dredged sediments and the local aquifer sediments, exchangeable trace elements are generally found to represent a minor fraction of the total metal concentration of the sediment (Table IV, Fig. 5). The exchangeable fraction is less than 0.5% for most elements. Only Cd has an exchangeable fraction greater than 0.5%, but less than 5.1%, in both the dredged sediments and the local aquifer sediments (Table IV, Fig. 5). Sb and Tl also have more than 0.5% of the exchangeable fraction, but less than 1.3% of the exchangeable fraction in the dredged sediments.

2. Fraction 2: Acidic phase

Although the acid-mobilizable fraction still accounts for a small proportion of the total metal for most of the trace elements in the sediment samples, the filling dredged sediments have higher percentages in fraction 2 than the local aquifer sediments (Table IV, Fig. 5). The higher percentages in fraction 2 are especially evident for Cd, Pb, and Zn, whose acid-mobilizable fractions account for an appreciable proportion of the total metal in the filling dredged sediments: 17.2%–40.4% Cd; 10.1%–20.4% Pb; 3.4%–18.2% Zn; (Table IV, Fig. 5). Cu, Sb, Ni, and As also have 2.5%–7.5% of acidic phase.

3. Fraction 3: Reducible phase

With the exception of Sb and Tl, the reducible fraction of trace elements accounts for a large proportion of the total metal concentrations in both the filling dredged sediments and local aquifer sediments (Table IV, Fig. 5). In general, reducible trace element fractions exhibit higher proportions of the total metals in the filling dredged sediments than in the local aquifer sediments (Table IV, Fig. 5).

4. Fraction 4: Oxidizable phase

With the exception of Sb and As, whose oxidizable fractions are low (less than 2.0% in most of samples, Table IV, Fig. 5) and Cu and Ag, whose oxidizable fraction is larger in the filling dredged sediments (more than 20.0% in most of the dredged sediments, Table IV, Fig. 5), oxidizable fractions also account for 2.0% to ~ 20.0% of the total metals for most of the trace elements in both the filling dredged sediments and local aquifer sediments. In general, the relative mobility in terms of the percentage of the oxidizable trace elements follows the order of the recent filling dredged sediments (placed on site in 2002)>the older dredged sediments (placed on site prior to 2002)>the deep local aquifer sediments>the shallow local aquifer sediments (Table IV, Fig. 5).

5. Fraction 5: Residual phase

For most of the trace elements examined the residual fractions account for the largest proportion of the total metals in the sediments (Table IV, Fig. 5). In some cases, mainly found in the filling dredged sediments, the concentrations of Pb, Cd, Zn, and Cu in the residual phase are lower than those observed in the reducible and oxidizable phases (Table IV, Fig. 5).

D. The potential contaminants revealed by sequential extractions

To become a potential contaminant in drinking water, a particular trace element must be able to be mobilized. The mobility of trace elements in sediments mainly depends upon its solid-phase speciation and the chemical and physical processes to which it is subject. We first discuss the mobility of trace elements examined as related to their "operationally defined solid-phase speciation." Then, we discuss the possible changes of in situ conditions occurring at the replacement site after disposal of the dredged sediments and the effect of these potential changes on the mobility of trace elements examined.

It is generally agreed that the mobility of trace elements decreases approximately in the order of the extraction sequence, i.e., exchangeable>acidic>reducible>oxidizable> residual.[1, 3, 10] The exchangeable phase is weakly associated with the sediments and is the easiest to mobilize by increasing ionic strength of sediment pore water under natural conditions. The acidic phase, bound to carbonate, is also relatively easy to mobilize by lowering the pH of sediment pore water. Many natural processes such as the infiltration of acid rain and the degradation of organic matter can result in slow lowering of the pH of sediment pore water. Trace elements in the reducible and oxidizable phases are thought to be more strongly bound to Fe/Mn oxides/oxyhydroxides and organic matter, respectively (e.g., Tessier et al.[5]), and thus are not easily mobilized under steady-state, natural conditions. Trace elements in the residual phase are strongly bound, or held, within mineral crystal structure, and thus are almost impossible to be mobilized under the natural conditions.

At the dredged spoil placement site, the exchangeable and acidic phases only account for a small proportion of most of the trace elements examined. Indeed, only Cd, Pb, and Zn have an appreciable extractable proportion from the acidic phase in the filling dredged sediments (Table IV). Cu, Sb, Ni, and As also have 2.9%–7.7% of exchangeable and acidic phases (Table IV). Therefore, our data suggest that, of the trace elements examined, Cd, Pb, and Zn are most likely to be mobilized into local groundwater at the replacement site. Cu, Sb, Ni, and As are relatively less mobile than Cd, Pb, and Zn, although small fractions of these elements can still be mobilized if pH changes at the replacement site. Other trace elements, such as Cr, Fe, Ag, and Tl, would not be mobilized unless the redox conditions of the dredged sediments have been changed at the replacement site.

Because the residual phase is difficult to mobilize, the overwhelming dominance of the residual fraction for these trace elements (Table IV, Fig. 5), except Pb, Cd, Zn, and Cu, indicates that trace elements are strongly bound, or held, within mineral crystal structure, and are thus not easily mobilized under the natural conditions. Based on total extract-able fractions (Table III, Fig. 5), the possibility for these trace elements to be mobilized from the dredged sediments may follow the order of Cd, Pb>Cu, Zn>Ni, As, Fe>Ag, Cr>Tl, Sb. Higher total extractable fractions for almost all trace elements examined in the dredged sediments than in the local aquifer sediment samples (Table III, Fig. 5) indicate that the trace elements examined here are more likely to be liberated from the dredged sediments than from the local aquifer sediments under the natural conditions.

The mobility of trace elements ultimately depends upon the changes of in situ conditions occurring at the replacement site. Table V lists monthly groundwater data for pH and conductivity from two upgradient monitoring wells (SDS2 and 3) and two downgradient monitoring wells (SW 30 and 31). Although we have no data for pH and conductivity for the pore water in the dredged sediments from the upper Potomac River, Hall et al.[12] reported some pH and conductivity data for upper Potomac River waters, which are close to the dredged site (Table V). Compared to conductivity for the upper Potomac River water (Table V), the local groundwaters have slightly higher conductivities. Because the exchangeable trace elements only account for a minor fraction of the total element concentration of the dredged sediments, the slight increase in ionic strength (indicated by the slightly higher conductivities) is not sufficient to release trace elements from the dredged sediments into local groundwater at the dredged spoil placement site. Therefore, we do not anticipate any trace element mobilization at this site owing to the change in ionic strength of local groundwaters. However, local groundwaters have much lower pH values than the upper Potomac River waters (Table V). Thus, the changes in pH are notable for the dredged sediments. Moreover, monthly pH data for the local groundwaters indicate that pH slightly decreases with time. Low pH in local groundwaters is a common problem due to oxidation of sulfides in the Shirley and Tabb Formation at old mining exposures. According to analysis data for the dredged sediments by Virginia Cooperative Extension Soil Testing Laboratory at Virginia Polytechnic Institute and State University, there are 2%–5% of carbonates in the dredged sediments. The changes in pH to which the dredged sediments are subject will result in some carbonate dissolution. Considering their acidic fractions, an appreciable proportion of Cd, Pb, and Zn and some Cu, Sb, Ni, and As may be mobilized by carbonate dissolution.

Because the reducible and oxidizable trace elements account for most of the total exchangeable fractions of trace elements in the dredged sediments, we suggest that oxidation of the dredged sediments may mobilize the trace elements examined in this study, as well as other components of the sediments. When the upper Potomac River sediments were dredged and dumped into the replacement site, a considerable portion was exposed to the atmosphere. As they dried the sediments formed coarse polygonal cracks as much as 0.5 m deep. Along these exposed surfaces, the dark bluish gray sediments altered to a pale gray and then to a tan (Fig. 6) as oxidation progressed throughout the new soil profile. According to analyses of sediment samples being barged to the replacement area (analyzed by Gascoyne Labs Inc., Baltimore, MD), dredged sediments contained between 50 and 150 mg/kg dry sediment of sulfate, up to 200 mg/kg dry sediment of sulfide, and up to 2% total organic carbon. Pore water total ammonia (i.e., NH3 + NH4+) in the Potomac River sediments is ~ 27.4 mgN/L, as reported by Schlekat et al.[13] from the same location that the dredged sediments were excavated. When ammonia, organic matter, and iron sulfide (e.g., FeS2) are oxidized in the dredged sediments at the replacement site, the following geochemical reactions may be expected:
Figure 6

Prismatic dredged sediment pulled from approximately 76.2 cm deep in the soil. Note the distinct oxidation of both the surface sediments under the aggregate and the oxidation rind around the aggregate itself. The inbound dredged sediments were all initially the dark bluish black color seen at the center of the aggregate.

2NH4+ + 3O2→2NO2- + 2H2O + 4H+,

NH4+ + 2O2→NO3- + H2O + 2H+,

{CH2O} + O2→H2CO3,

4FeS2 + 15O2 + 14H2O→4Fe(OH)3 + 16H+ + 8SO24-.

These potential reactions would precipitate iron oxides and release hydrogen ions, NO3 + NO2, dissolved inorganic carbon (DIC), and sulfate into local groundwaters. In addition, the incomplete degradation of organic matter (large molecular) in the dredged sediments may also produce some soluble small molecular organic matter. However, the acidity released by these oxidation reactions would be offset by carbonate buffering reactions. Therefore, after deposition of the dredged sediments at the study site, we expect some levels of increase in NO3 + NO2, DIC, DOC, and sulfate concentrations in local groundwaters, but little if any change of pH.

Our monitoring data for local groundwater verify that these potential geochemical reactions have occurred at the study site (Tables V and VI). Table V indicates that pH only slightly decreases in the monitoring groundwaters with time. Table VI shows that, compared to the baseline analyses, NO3 + NO2, and TOC generally increase. Total iron shows higher concentrations at the first post-baseline monitoring analyses but thereafter, iron generally decreases with time in local groundwaters. The oxidation of ammonia, organic matter, and iron sulfides may release the oxidizable trace elements (fraction 4 in sequential extractions) into local ground-waters. However, considering that iron, manganese, and aluminum oxides exhibit strong adsorption affinity because of their reactivity and large specific surface area,[14] we do not anticipate these reactions would result in any significant water quality risk. Instead, the trace elements released by the proposed mechanism are subsequently absorbed by newly produced iron, manganese, and aluminum oxides and become immobile as long as conditions remain oxidic.
Table V

Monthly pH and conductivity in groundwaters from the monitoring wells after the placement of the dredged sediments at the replacement site. Also included are pH and conductivity in upper Potomac River waters. (Note: Data for upper Potomac River waters are from Hall et al. – Ref. 12.)

   

Local groundwaters

 

Upper Potomac River waters

Well

Date

    

3/03

4/03

5/03

6/03

7/03

8/03

9/03

11/03

12/03

pH

Station 1

7.46–9.12

SDS2

6.60

5.64

5.59

5.74

5.30

5.29

5.38

5.36

5.11

 

Station 2

7.29–8.11

SDS3

6.52

5.63

5.62

5.69

5.43

5.45

5.56

5.41

4.87

   

SW30

6.62

5.94

5.78

5.97

5.58

5.58

5.76

6.03

5.99

 

Station 3

7.13–8.76

SW31

6.89

5.71

5.48

5.82

5.26

5.26

5.36

5.29

5.30

Conductivity (μS/cm)

Station 1

140–240

SDS2

187

181

177

184

162

172

173

214

215

 

Station 2

180–260

SDS3

332

139

149

146

153

144

109

147

130

   

SW30

1176

340

265

302

319

226

308

546

546

 

Station 3

150–280

SW31

495

232

234

237

222

230

191

343

247

Table VI

Concentrations of trace elements (mg/L) in groundwaters from the monitoring wells after the placement of the dredged sediments at the replacement site

Trace element

USEPA MCLsa for drinking-water

Baseline analysesb

SDS2c

SDS3c

SW30c

SW31c

   

4/20/02d

10/19/02

10/26/03

4/20/02d

10/19/02

10/26/03

4/20/02d

10/19/02

10/26/03

4/20/02d

10/19/02

10/26/03

Ag

0.1

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

Cd

0.005

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

0.00065

bdl

0.00058

bdl

Sb

0.006

bdl

0.054

bdl

bdl

bdl

bdl

bdl

0.0055

bdl

bdl

0.0071

bdl

bdl

Tl

0.002

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

Pb

0.015

bdl

0.048

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

0.012

0.0052

Cr

0.1

bdl-0.004

0.03

0.0048

0.0035

0.04

0.0098

0.0035

bdl

bdl

bdl

0.015

0.036

0.018

Fe

0.3

2.1–8.1

50

6.3

1.4

5.7

1.7

5

bdl

bdl

bdl

8.7

37

15

Ni

0.1

bdl-0.009

0.028

0.0096

0.0058

0.03

0.012

bdl

bdl

bdl

bdl

0.013

0.026

0.014

Cu

1.3

bdl

0.039

bdl

bdl

0.0064

bdl

bdl

bdl

bdl

bdl

0.0051

0.019

0.0077

Zn

5.0

bdl-0.16

0.088

0.56

0.03

0.16

0.2

0.03

bdl

0.03

0.03

0.062

0.120

0.049

As

0.01

bdl

0.0079

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

NO3 + NO2

11.0+1.0

bdl-0.11

1.1

0.15

bdl

1

2.8

0.55

bdl

bdl

0.18

7.5

4.5

10

Total sulfide

...

bdl

2

bdl

bdl

1.7

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

SO4

250.0

7–76

...

43

33

...

44

26

...

29

55

...

39

43

TOC

...

1.0–3.1

9.4

3.8

1.8

3.7

3.8

3.7

2.1

2.8

3.4

<1.0

1.7

1.0

aUSEPA maximum contaminant levels for drinking water are based on the data provided by Fetter (Ref. 11), except for new As MCL.

bBaseline analyses are the concentration ranges for trace elements in a set of local groundwaters collected prior to placement of the dredged sediments at the replacement site.

cSamples Analyzed by Gascoyne Laboratories, Inc.

dThe collection date of the groundwater samples.

Besides their mobility, the environmental significance of these trace elements is also controlled by their absolute concentrations in the dredged sediments. Figure 5 compares the total concentrations of trace elements and the concentrations of each extractable fraction in the dredged sediments to those in the local aquifer sediments. Pb, Cd, Zn, Ag, and Cu have higher total concentrations and higher concentrations of extractable phases in the dredged sediments than in the local aquifer sediments (Fig. 3). By comparison, As, Cr, Ni, and Sb have similar total concentrations in both the dredged sediments and the local aquifer sediments, but their concentrations in the extractable phase are higher in the dredged sediments than in the local aquifer sediments.

The "operationally defined solid-phase speciation" of the trace elements examined, their absolute concentrations in the dredged sediments, and the changes in in situ conditions indicate that some trace elements, especially Cd, Pb, and Zn, may be released from the dredges sediments. However, because the trace elements released by carbonate dissolution and the oxidation (e.g., organic matter degradation, iron sul-fide oxidation, and ammonia oxidation) are subsequently immobilized by sorption to iron, manganese, and aluminum oxides, these trace elements would not be released into local groundwaters. Therefore, no potential contaminants to local groundwater are expected as revealed by sequential extractions. Our sequential extraction results are generally consistent with our groundwater monitoring results (Table VI). On 8 May, 2000 (prior to placement of the dredged sediments), local groundwater samples were collected from wells around the L-shape replacement site. The analyses of these samples represent the baseline groundwater chemistry at the replacement site. After the dredged sediments were filled into the site, local groundwater samples were collected on different dates and analyzed for the same trace elements (Table VI). The groundwater monitoring data do not clearly indicate that any trace elements were released into local groundwaters from the dredged sediments, because the concentrations of monitoring elements are very low and inconsistent with time. Compared to baseline analyses, only a slight increase was observed in concentrations for some elements in some local groundwater samples. Almost all the concentrations of monitoring elements are below USEPA maximum contaminant levels (MCLs) for drinking water. Only one monitoring well (SDS2) had a high Pb concentration of 0.048 mg/L at the first, post-baseline monitoring analyses (4/20/2002). But later analyses at SDS2 indicate very low levels of Pb in groundwater from the well (Table VI).

V. Conclusion

Our sequential extraction results show that the dredged sediments have higher total extractable fractions for almost all trace elements examined and relatively high concentrations for some trace elements, compared to the local aquifer sediments, which indicates that the trace elements are more likely to be mobilized from the dredged sediments than from the local aquifer sediments. However, the overwhelming dominance of the residual fraction for these trace elements, except Pb, Cd, Zn, and Cu, indicates that trace elements are not easily mobilized under the natural conditions. Moreover, under an oxidizing condition, the trace elements released from the dredged sediments by carbonate dissolution and the oxidation (i.e., organic matter degradation, iron sulfide, and ammonia oxidation) are subsequently absorbed by newly produced iron, manganese, and aluminum oxides and become immobile. No potential contaminants to local ground-water are expected to be released from the dredged sediments.

Declarations

Acknowledgements

This study was funded by Weanack Limited Land Partners, Inc. as part of a grant to W.L.D., G.R.W., and K.H.J. Special thanks go to Charles Carter for advice and historical land-use information. We also wish to thank Dr. Z Chen and Dr. C. Jones for ICP-MS analysis and Ms. J. Daniels, Ms. N. McGinnis, and Mr. W. Bounds for assistance with sample collection.

Authors’ Affiliations

(1)
Department of Ocean, Earth & Atmospheric Sciences, Old Dominion University
(2)
Department of Geology, The University of Texas at Arlington
(3)
Department of Crop and Soil Environmental Sciences, Virginia Polytechnic Institute and State University

References

  1. Banerjee ADK: Environ Pollut. 2003, 123: 95-10.1016/S0269-7491(02)00337-8.View ArticleGoogle Scholar
  2. Feng X, Hong Y, Hong B, Ni J: Environ Geol. 2000, 39: 372-10.1007/s002540050016.View ArticleGoogle Scholar
  3. Li X, Thornton I: Appl Geochem. 2001, 16: 1693-10.1016/S0883-2927(01)00065-8.View ArticleGoogle Scholar
  4. Nyamangara J: Agric Ecosyst Environ. 1998, 69: 135-10.1016/S0167-8809(98)00101-7.View ArticleGoogle Scholar
  5. Tessier A, Campbell PGC, Bisson M: Anal Chem. 1979, 51: 844-10.1021/ac50043a017.View ArticleGoogle Scholar
  6. van Ryssen R, Leermakers M, Baeyens W: Environ Sci Policy. 1999, 2: 75-10.1016/S1462-9011(98)00044-6.View ArticleGoogle Scholar
  7. Mixon RB, Berquist CB, Newell WL, Johnson GH: U.S. Geological Survey, Miscellaneous Investigations Series Map I-2033. 1989Google Scholar
  8. McFarland ER: U.S. Geological Survey, Water Resources Investigations Report 99-4093. 1999Google Scholar
  9. Johnson GH, Ward LW, Peebles PC: Geological Excursions in Virginia and North Carolina: Southeastern Section – Geological Society of America Field Trip Guidebook. Edited by: Whittecar GR. 1987, 189-218.Google Scholar
  10. Hassirson RM, Laxen DPH, Wilson SJ: Environ Sci Technol. 1981, 15: 1378-10.1021/es00093a013.View ArticleGoogle Scholar
  11. Fetter CW: Contaminant Hydrogeology. 1999, Prentice Hall, Upper Saddle River, NJ, 500-Google Scholar
  12. Hall LW, Hall WS, Bushong SJ, Herman RL: Aquatic Toxicology. 1987, 10: 73-10.1016/0166-445X(87)90016-6.View ArticleGoogle Scholar
  13. Schlekat CE, McGee BL, Boward DM, Reinharz E, Velinsky DJ, Wade TL: Estuaries. 1994, 17: 334-10.2307/1352667.View ArticleGoogle Scholar
  14. Davis JA, Kent DB: Rev Mineral. 1990, 23: 177-Google Scholar
  15. Whittecar GR:

Copyright

© American Institute of Physics 2004