U.S. Department of the Interior 

U.S. Geological Survey 

Ground-Water Flow and Contaminant 

Transport at a Radioactive-Materials 

Processing Site, Wood River Junction, 

Rhode Island 

U.S. Geological Survey Professional Paper 1571 
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Ground-Water Flow and Contaminant 

Transport at a Radioactive-Materials 

Processing Site, Wood River Junction, 

Rhode Island 

By BARBARA J. RYAN and KENNETH L. KIPP, JR. 

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1571 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITI, Secretary 

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director 

The use of firm, trade, and brand names in this report is for 

identification purposes only and does not constitute 

endorsement by the U.S. Government. 

Library of Congress Cataloging in Publications Date 

Ryan, Barbara J. 

Ground-water flow and contaminant transport at a radioactive-materials processing site, Wood River 

Junction, Rhode Island I by Barbara J. Ryan and Kenneth L. Kipp, Jr. 

p. em. -- (U.S. Geological Survey professional paper : 1571) 

Includes bibliographical references. 

1. Radioactive pollution of water--Rhode Island--Wood River Junction Region. 2. Scrap metal 

industry--Waste disposal--Environmental aspects--Rhode Island--Wood River Junction Region. 

3. Radioactive pollution of water--Pawcatuck River Watershed (R.I. and Conn.) 4. Scrap metal 

industry--Waste Disposal--Environmental aspects--Pawcatuck River Watershed (R.I. and Conn.) 

I. Kipp, Kenneth L. II. Series. 

TD427.R3R93 1997 

628.1'685--dc21 

97-7722 

CIP 

ISBN 0-607-86634-9 

For sale by U.S. Geological Survey, Information Services 

Box 25286, Federal Center 

Denver, CO 80225 
CONTENTS 

Abstract.................................................................................................................................................................................. 1 

Introduction ... . . . ...... ...... ........ .. . . ............ ... .. . ..... ... .. .... .............. .. .. .......... .... .. ... . ..... ........................ .. ........ .... ............................ 2 

Purpose and Scope....................................................................................................................................................... 2 

Description of Study Area........................................................................................................................................... 2 

Acknowledgments ....................................................................................................................................................... 2 

Site History and Investigations.............................................................................................................................................. 4 

Previous Investigations................................................................................................................................................ 5 

Current Investigation .. . . . . ... . ... . . .. ... . . . . .. . . . . ... ......... ...... ... . . ............. ... ...... ..... .......................... ............. .......... .......... .. ...... 6 

Field Techniques................................................................................................................................................ 6 

Laboratory Techniques...................................................................................................................................... 12 

Hydrogeologic Setting........................................................................................................................................................... 14 

Geology....................................................................................................................................................................... 14 

Aquifer Characteristics and Boundaries...................................................................................................................... 14 

Potentiometric Surfaces and Ground-Water Flow....................................................................................................... 18 

Ground-Water Quality .................. ......................................................................................................................................... 28 

Background Water Quality.......................................................................................................................................... 28 

Contamination Plume .................................................................................................................................................. 29 

Chemical Characteristics . .... ... .. . ............... .. . ..... .......... .. ............... ...... ... .... .. .. ........ .. .. . ............................... ......... 29 

Location and Configuration............................................................................................................................... 30 

Strontium-Ion Exchange.............................................................................................................................................. 49 

Numerical Simulation of Ground-Water Flow and Contaminant Transport......................................................................... 53 

Approach and Limitations........................................................................................................................................... 53 

Simulation of Ground-Water Flow.............................................................................................................................. 54 

Theory and Assumptions................................................................................................................................... 54 

Areal-Flow Model ................................................................................................................................... 56 

Cross-Sectional-Flow Model................................................................................................................... 59 

Parameter Estimation......................................................................................................................................... 61 

Discussion of Simulation Results...................................................................................................................... 63 

Areal-Flow Model................................................................................................................................... 63 

Cross-Sectional-Flow Model................................................................................................................... 67 

Proposed Ground-Water Development.............................................................................................................. 70 

Simulation of Contaminant Transport......................................................................................................................... 75 

Theory and Assumptions................................................................................................................................... 75 

Nitrate Transport................................................................................................................................................ 76 

Strontium-90 Transp9.rt ..................................................................................................................................... 77 

Discussion of Simulation Results...................................................................................................................... 84 

Summary and Conclusions.................................................................................................................................................... 85 

Selected References............................................................................................................................................................... 87 

Appendix 1. Modifications and input-file specifications for the computer code for twodimensional-flow simulation................................................................................................ On diskette 

2. Areal-flow model input and output data files............................................................................... On diskette 

3. Cross-sectional-flow model input and output data files............................................................... On diskette 

Contents Ill 
FIGURES 

I. Map showing locations of study area and geohydrologic section A-A', Wood River Junction, 

Rhode Island......................................................................................................................................................... 3 

2. Detailed sketch of the radioactive-materials processing site, I979 ........ .... .. ...... .... ........ .... ... . ... . .... .. . .. ... .... ... . ... . .. 4 

3. Map showing location of observation wells with local well numbers ... .... .... .... ........ .... .... .... .... .. ... . .. . ... ...... .. . .. ... 8 

4. Hydrogeologic section A-A' showing locations of screened intervals for observation wells............................... II 

5. Schematic of hydraulic potentiomanometer ......................................................................................................... 12 

6, 7. Maps showing: 

6. Location of differential-head piezometric data-collection sites ... . . .. . .. . .... .... .... .... .... .... .............. .. .... .... .. . ... 13 

7. Generalized surficial geology of the study area and location of geohydrologic section B-B' .................... 15 

8. Generalized geohydrologic section B-B' ........................................................................................ ...................... 16 

9,IO. Maps showing: 

9. Altitude of bedrock surface······················································································!································· 17 

I 0. Configuration of the water table based on average measured water levels in observation 

wells, January through December I983 ....... .. .. ..... .. . ...... .. .... .. . . .. . . .... ..... .. . .... .... .... ..... ... . . .. .. .. ..... .. . . . . . . ... . . .. .. 19 

II. Geohydrologic section showing configuration of average ground-water potentiometric head 

based on average measured water levels in observation wells, January through December 1983 .... .................. 20 

12-15. Graphs showing: 

I2. Altitude of potentiometric surface in selected observation wells near the Pawcatuck 

River screened at different depths, September 198I through December 1983 ........ .. .. .. . . . . . . . ... .... ... . . . . . .. . . . . 21 

I3. Seasonal fluctuations of water levels in observation well CHW-51 0 near the till 

boundary, observation well CHW-538 near the radioactive-materials processing 

plant, and observation well RIW-641 near the Pawcatuck River, October 1981 

through December I983 ... ........ ......... ... ... ..... .. . . .... .... ........ .... ... . .... ...... .. .... .... .... .... ... .. . . . . . . . . . .. .... .. .. .. . . . . . . . ... . . 22 

I4. Altitude of water levels in the Pawcatuck River near streamflow-gaging station and 

observation well CHW-567, and in observation well CHW-567 during 215 hours, 

Dec~mber 1-9, 1982................................................................................................................................... 22 

I5. Relation between precipitation, stage of the Pawcatuck River, and water levels in 

observation wells CHW-538 in upland area near the radioactive-materials processing 

plant and RIW-64I near the Pawcatuck River, January 1981 through December 1983 ............................ 23 

16. Map showing altitude and configuration of the water table during a low-flow period, 

December 1982 ..................................................................................................................................................... 24 

17. Geohydrologic section showing altitude of ground-water potentiometric head during a 

low-flow period, December 1982 ... ............ .... ........ ... . ...... .. ...... .. .... ... . .... ..... .. . .... .... .... ..... ... . . . . .. . . . . . . . . . . . . .. . .. . . ... . . .. . . 25 

18. Map showing altitude-and-configuration-of-the-water-table during a high-flow period, 

April 1983-. .. . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 

19. Geohydrologic section showing altitude of ground-water potentiometric head during a 

high-flow period, April 1983 ................................................................................................................................ 27 

20. Block diagram showing generalized view of contaminated ground water at the radioactivematerials processing site....................................................................................................................................... 31 

21-25. Maps showing: 

21. Areal distribution of nitrate concentrations in ground water at the radioactivematerials processing site, September and October 1982 ...... ........ .... .... ........ .... ... . .... .... .... .... .... .... .. . . .... ...... 32 

22. Areal distribution of strontium-90 concentrations in ground water at the radioactivematerials processing site, September and October 1982 .. .... .... .... .... .... .. . . .... .... .... .... .... .... .... .... .... .. ... . . .. .. . .. 33 

23. Areal distribution of nitrate concentrations in ground water at the radioactivematerials processing site, April and May 1983 . ... ...... .. ........ ........ .... .... .... .... .... .... .... .... .... .... ... . .. . ... . . . ... .... .. 34 

24. Areal distribution of nitrate concentrations in ground water at the radioactivematerials processing site, January 1984 ..................................................................................................... 35 

25. Areal distribution of strontium-90 concentrations in ground water at the 

radioactive-materials processing site, January 1984 .................................................................................. 36 

IV Contents 
26-31. Geohydrologic sections showing: 

26. Vertical distribution of nitrate concentrations in ground water at the radioactivematerials processing site, September and October 1982 ............................................................................ 37 

27. Vertical section without exaggeration of nitrate concentrations in ground water 

at the radioactive-materials processing site, September and October 1982 ............................................... 38 

28. Vertical distribution of strontium-90 concentrations in ground water at the 

radioactive-materials processing site, September and October 1982. ........................................................ 39 

29. Vertical distribution of nitrate concentrations in ground water at the radioactivematerials processing site, April and May 1983 .......................................................................................... 40 

30. Vertical distribution of nitrate concentrations in ground water at the radioactivematerials processing site, January 1984 .............................................................................................. .... ... 41 

31. Vertical distribution of strontium-90 concentrations in ground water at the 

radioactive-materials processing site, January 1984 ....................................................................... ........... 42 

32. Map showing areal distribution of electromagnetic and specific conductance at the 

radioactive-materials processing site, April1982 ................................................................................................ 43 

33-37. Graphs showing: 

33. Relation between precipitation and concentration of gross beta emitters in 

observation wells CHW-538 near the radioactive-materials processing plant 

and RIW-641 near the Pawcatuck River, April1982 through January 1984 ............................................. 44 

34. Relation between depth below land surface and concentration of gross beta 

emitters in selected wells near the Pawcatuck River, June 1981 through January 1984 ............................ 45 

35. Relation between depth below land surface and concentration of gross beta emitters 

in selected wells in the swamp at the radioactive-materials processing site, January 1983 

through, January 1984 .......................................................................................... ................................ .... ... 46 

36. Relation between depth below land surface and concentrations of gross beta emitters 

in selected wells near the radioactive-materials processing plant, January 1982 through 

January 1984............................................................................................................................................... 47 

37. Relation between concentrations of strontium-90 and gross beta emitters in wells 

near the Pawcatuck River, strontium-90 concentrations and specific conductance in wells 

near the radioactive-materials processing plant, and specific conductance and nitrate 

concentrations in all wells........................................................................................................................... 49 

38. Schematic of conceptual model of the hydrologic-flow system........................................................................... 55 

39-40. Maps showing: 

39. Grid and boundary conditions for the areal-flow model............................................................................. 57 

40. Altitude and configuration of aquifer bottom for areal-flow model........................................................... 58 

41. Geohydrologic section showing grid and boundary conditions for the cross-sectional-flow model................... 60 

42-43. Maps showing: 

42. Areal distribution of horizontal hydraulic conductivity for the areal-flow model ..................................... 62 

43. Configuration of simulated water table and measured water table based on average 

measured water levels in observation wells, January through December 1983 ......................................... 66 

44. Geohydrologic section showing configuration of simulated and average measured ground-water 

potentiometric head, January through December 1983 ..................................................................... ............ ....... 68 

45. Map showing generalized path of ground-water flow and calculated traveltimes along the axis of 

maximum contamination in the plume................................................................................................................. 71 

46. Geohydrologic section showing generalized paths of ground-water flow and calculated 

traveltimes at the top, middle, and bottom of contamination plume.................................................................... 72 

Contents V 
47-49. Maps showing: 

47. Configuration of simulated water table under steady-state conditions for a projected 

pumping rate of 0.25 million gallons per day west of the Pawcatuck River.............................................. 73 

48. Configuration of simulated water table under steady-state conditions for a projected 

pumping rate of 1 million gallons per day between the Pawcatuck River and Meadow 

Brook Pond ................................................................................................................................................. 74 

49. Configuration of simulated water table for a projected pumping rate of 1 million gallons 

per day after 1,000 days of pumping between the Pawcatuck River and Meadow Brook 

Pond ............................................................................................................................................................ 75 

50-52. Graphs showing: 

50. Calculated nitrate-cleanout times at the radioactive-materials processing site.......................................... 77 

51. Calculated strontium-90 clean out times at the radioactive-materials processing site................................ 81 

52. Dimensionless strontium-90 and total ion concentration ·profiles for selected dimensionless 

times at the radioactive-materials processing site...................................................................................... 83 

TABLES 

1. Chemicals used in recovery or waste-treatment processes at the study site, Wood River 

Junction, Rhode Island ........................................................................................................................................ 5 

2. Grain-size distribution and cation-exchange capacity of three sediment samples from boreholes 

for observation wells CHW-559 and CHW-576.................................................................................................. 14 

3. Average background concentrations, standard deviations, and detection limits for selected 

constituents in ground water from 24 observation wells outside the contamination plume............................... 28 

4. Ground-water compositions used for strontium-sorption experiments............................................................... 28 

5. Representative chemical analyses of water from observation wells near the middle, edge, and 

outside of the contamination plume.................................................................................................................... 29 

6. Correlation coefficients resulting from regression analyses for selected water-quality variables 

and well clusters at the study site........................................................................................................................ 48 

7. Concentrations of combined major cations (calcium, magnesium, and sodium), and of strontium 

adsorbed by sediment samples .................................................................................................. .................... .... .. 51 

8. Equilibrium distribution coefficients for strontium and calculated strontium-90 concentrations 

adsorbed by sediment samples ............................................................................................................................ 52 

9. Selected transport constituents for the study site, Wood River Junction, Rhode Island, and the 

Chalk River site, Ontario, Canada........................................................................................................................ 53 

10. Qualitative characterization of lithology and hydraulic conductivity................................................................. 63 

11. Measured and calculated potentiometric heads for the steady-state flow simulations........................................ 64 

12. Final parameter values for areal and vertical simulations................................................................................... 65 

13. Calculated nitrate cleanout times for selected parameter values used in the transport model ............................ 77 

14. Calculated strontium-90 cleanout times for selected parameter values used in the transport model.................. 81 

VI Contents 
CONVERSION FACTORS, ABBREVIATED WATER-QUALITY UNITS, AND VERTICAL DATUM 

CONVERSION FACTORS 

Multiply 

acre 

cubic foot per day (ft3/d) 

cubic foot per second (ft3/s) 

cubic foot per second per square mile [(ft3/s)/mi2] 

foot (ft) 

foot per day (ft/d) 

foot per mile (ft/mi) 

foot per year (ft/yr) 

gallon (gal) 

gallon per day (gal/d) 

gallon per minute (gal/min) 

inch (in.) 

inch per year (in/yr) 

mile (mi) 

million gallons per day (Mgal/d) 

ounce per cubic inch (oz/in3) 

square foot (ft2) 

square foot per day (ft2/d) 

square mile (mi2) 

degree Fahrenheit ("F) 

RADIOCHEMICAL CONCENTRATION 

picocurie per gram (pCi/g) 

picocurie per liter (pCi/L) 

RADIOCHEMICAL ACTIVITY 

RADIOLOGICAL DOSE 

SORPTION FACTOR 

curie (Ci) 

millirem per hour (mrernlhr) 

millirem per year (mrem/yr) 

gallon per ounce (gal/oz) 

by 

0.4047 

.02832 

.02832 

.01093 

.3048 

.3048 

.1894 

.3048 

3.785 

.003785 

.003785 

25.4 

25.4 

1.609 

.04381 

1.730 

.09290 

.09290 

2.590 

·c = 519 x CF-32) 

3.7 x w-2 

3.7 x w-2 

3.7 X 1010 

.01 

.01 

.1335 

133.5 

To obtain 

hectare 

cubic meter per day 

cubic meter per second 

cubic meter per second per square kilometer 

meter 

meter per day 

meter per kilometer 

meter per year 

liter 

cubic meter per day 

cubic meter per minute 

millimeter 

millimeter per year 

kilometer 

cubic meter per second 

gram per cubic centimeter 

square meter 

square meter per day 

square kilometer 

degree Celsius ("C) 

becquerel per gram 

becquerel per liter 

disintegration per second 

millisievert per hour 

millisievert per year 

liter per gram 

milliliter per gram 

Contents VII 
VIII Contents 

CHEMICAL CONCENTRATION 

mg/L x F1 = meq/L 

mg/L x F2 = mmoi/L 

Bicarbonate (HCOj) 

Calcium (Ca++) 

Chloride ccn 

Iron (Fe) 

Magnesium (Mg++) 

Silica (Si02) 

Sodium (Na+) 

Strontium (Sr++) 

Sulfate (S04) 

Zinc (Zn++) 

ABBREVIATED WATER-QUALITY UNITS 

microgram per liter (Jlg/L) 

microgram per gram (Jlg/g) 

microsiemen per centimeter at 25"C (JlS/cm) 

milliequivalent per gram (meq/g) 

F1 

0.01639 

.04990 

.02821 

.08229 

.04350 

.02283 

.02082 

.03059 

milliequivalent per 100 grams (meq/100 g) 

milliequivalent per liter (meq/L) 

milliequivalent per milliliter (meq/mL) 

milligram per liter (mg/L) 

milligram per I 00 grams (mg/100 g) 

milliliter per gram (mUg) 

VERTICAL DATUM 

F2 

0.01639 

.02495 

.02821 

.01791 

.04114 

.01664 

.04350 

.01141 

.01041 

.01530 

Sea level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929-a 

geodetic datum derived from a general adjustment of the first-order level nets of the United States 

and Canada, formerly called Sea Level Datum of 1929. 
Ground-Water Flow and Contaminant Transport at a 

Radioactive-Materials Processing Site, Wood River 

Junction, Rhode Island 

By Barbara J. Ryan and Kenneth L. Kipp, Jr. 

Abstract 

Liquid wastes from an enriched-uranium 

cold-scrap recovery plant at Wood River Junction, 

Rhode Island, were discharged to the environment 

through evaporation ponds and trenches from 

1966 through 1980. Leakage from the ponds and 

trenches resulted in a plume of contaminated 

ground water extending northwestward to the 

Pawcatuck River through a highly permeable sand 

and gravel aquifer of glacial origin. 

Electrical conductivities, determined by 

electromagnetic methods, in combination with 

water-quality data from about 150 observation 

wells, indicated that the plume was about 

2,300 feet long and 300 feet wide during 1981-84, 

and that it was restricted to the upper 80 feet 

of saturated thickness, where sediments consist 

of medium to coarse sand and gravel. No 

contamination was detected in the fine sands 

and silts that underlie the coarser materials. 

Piezometric-head and water-quality data from 

wells screened at multiple depths on both sides of 

the river indicate that contaminants discharge to 

the Pawcatuck River and to a swampy area west of 

the river. Dilution precludes detection of 

contaminants once they have entered the river. 

Concentrations of gross beta emitters in 

contaminated ground water ranged from 5 to 

1,600 picocuries per liter. Specific conductance 

of the contaminated water ranged from 150 to 

5,400 microsiemens per centimeter at 25 degrees 

Celsius. Strontium-90, technetium-99, boron, 

calcium, nitrate, potassium, and total strontium 

exceeded background concentrations by an order 

of magnitude in most of the plume. Zones of 

concentrated contaminants at both ends of 

the plume were separated by a zone of less 

contaminated water. Decreasing concentrations 

of the constituents over the time period of the 

study indicate cleanout of the aquifer. Laboratory 

tests of exchangeable cations indicated little 

capacity for uptake on the medium to coarse sands 

and gravels within the aquifer. Sorption of 

radionuclides has, however, occurred on riverbed 

sediments and on organic material in the swampy 

area adjacent to the river. 

Modeling of ground-water flow and solute 

transport was done to (1) simulate numerically, at 

a field scale, the transport of strontium-90, which 

interacts with the sediments in the aquifer and is 

affected by concentrations of other constituents in 

the contaminated ground water; (2) evaluate the 

fate of the contamination plume with regard to 

several chemical and radiochemical species of 

interest; and (3) indicate to what extent the 

ground-water resources of this area could be 

developed without producing water with 

unacceptable levels of contaminants. 

Times for transporting strontium-90 out of the 

ground-water system ranging from 8 to 18 years 

were calculated by a simple one-dimensional 

solute-transport model. Future monitoring of the 

plume would be necessary to assess the accuracy 

of the simulations. Ground-water quality in the 

Wood River Junction area will be affected by 

the contamination plume but probably not enough 

to prevent development of water supplies if 

pumping rates are limited to about 0.25 million 

gallons per day. The extensive data set obtained 

during this study can be used to support additional 

numerical-modeling efforts. 

Abstract 1 
INTRODUCTION 

Liquid wastes containing radionuclides and other 

chemical solutes from an enriched uranium cold scrap 

(materials containing low levels of radioactivity) 

recovery plant have leaked from polyethylene- and 

polyvinyl chloride-lined ponds and trenches into a 

highly permeable sand and gravel aquifer in southern 

Rhode Island. During 1981-84, the resultant plume of 

contaminated ground water extended about 2,300 ft 

from the ponds and trenches and discharged to the 

Pawcatuck River and contiguous swamp. 

In April 1981, the U.S. Geological Survey (USGS) 

began a 3 1 /2-year study of this ground-water 

contamination near Wood River Junction, R.I. (fig. 1) 

as part of the Low-Level Radioactive Waste Program. 

The objectives of the study were to (1) identify 

chemical and radiochemical constituents in the plume; 

(2) assess the interaction of solutes with the aquifer 

materials; (3) determine the location, movement, and 

fate of constituents in the plume; and (4) estimate the 

effects of ground-water contamination on future 

ground-water development adjacent to the site. Results 

and data from this study may be of use to other 

investigators researching radionuclide transport, 

particularly in similar hydrogeologic environments. 

Contaminated ground water at this site moves 

through a highly permeable glacial outwash aquifer. 

The possibility that future water-supply we11s might 

become contaminated as a result of migration of 

·contaminated water from the site was a concern to local 

water managers. 

Purpose and Scope 

This paper describes methodologies used during the 

study, geohydrologic conditions at the site, source and 

extent of ground-water contamination, and groundwater-flow and solute-transport modeling that was 

done to accomplish the stated objectives. This work is 

based primarily on data contained in a report by Ryan 

and others (1985). Interpretations and conclusions 

presented here will be of use to policymakers 

concerned with ground-water development in southern 

Rhode Island and with the disposal of low-level 

radioactive wastes in similar areas nationwide. 

Description of Study Area 

The plant site is in the lower Pawcatuck River 

Basin, about 2 mi east of the junction of the Pawcatuck 

and Wood Rivers. It is in a relatively flat grassland area 

that was cleared for agriculture in the late 1800's. 

Potatoes were grown in the field directly north of the 

plant until1976; potatoes and turf are still grown in the 

Ellis Flats area immediately northwest of Meadow 

Brook Pond (fig. 1). Wooded areas consisting 

primarily of pines border the farmland. The Pawcatuck 

River, flowing from the east, and the Wood River, 

flowing from the north, dissect this flat land. Much of 

the land around the rivers is wooded swamps and 

wetlands primarily containing oak, maple, and 

hornbeam. 

Irregular hills and depressions that range in altitude 

from 50 to 200 ft above sea level surround the broad 

flat plain. Scrub oak and other deciduous varieties of 

trees replace the pine trees common in the flatter areas. 

The closest permanent long-term precipitation 

recording station is maintained by the National 

Weather Service at Kingston, R.I. (about 9 mi northeast 

of the study area). Average annual precipitation during 

1889 through 1962 was 48.39 in.; the minimum was 

31.75 in. in 1943; and the maximum was 71.22 in. in 

1898. About 30 in. of snow falls annually (Allen and 

others, 1966, p. 17). A rain gage was installed at the 

study site and generally was operational from August 

through December 1981, April through December 

1982, and July through the first week of November 

1983. Data are presented in Ryan and others (1985, 

p. 295-296). Mean monthly temperature for 1943 

through 1983 at Kingston ranged from 19.8°F in 

January 1981 to 74.0oF in July 1955 and averaged 

49.1 °F. 

Acknowledgments 

The authors thank the property owner and 

employees of the plant for their cooperation and 

assistance during the 3 I /2 years of study at the site. 

Philip B. Duran and Susan Goldman, Boston 

University, Boston, Mass., did the electromagnetic 

conductivity survey and Lizanne A von of Brown 

University, Providence, R.I., did the gravity survey. 

2 Ground-Water Flow and Contaminant Transport at a Radioactive-Materials Processing Site, Wood River Junction, Rhode Island 
Pawcatuck 

River Basin 

Line of geohydrologic section 

- ____..,___ -

_- !!_1dian Cedar S'f!_mp 

0 1,000 

0 300 

2,000 FEET 

600 METERS 

Figure 1. Locations of study area and geohydrologic section A-A', Wood River Junction, Rhode Island. 

Introduction 3 
SITE HISTORY AND INVESTIGATIONS 

During 1964-80, a radioactive-materials processing 

plant (fig. 2) on a 1,114-acre site at Wood River 

Junction was operated to recover enriched uranium 

from cold scrap. Acid digestion with hydrofluoric and 

nitric acids and organic separation with tributyl 

phosphate and kerosene were used to extract uranium 

from test-reactor fuel rods, from machinery used in 

manufacturing fuel rods, and from miscellaneous 

laboratory and hospital equipment. Solid wastes from 

the process were shipped offsite, and lime-neutralized 

liquid wastes were initially discharged to the 

Pawcatuck River through a drainpipe and later (1966 

through 1973) to uncovered "evaporation" ponds and 

&renches. The depth of the ponds and trenches ranged 

from 3 to 15 ft below land surface; the bottoms of the 

ponds were 9 to 13 ft above the water table, and the 

bottoms of the trenches were 1 to 3 ft above the water 

table. 

In southern Rhode Island, however, average annual 

precipitation is much greater than average annual 

evaporation. For example, precipitation at the National 

Weather Station at Kingston averaged 46.06 in/yr 

from 1950 through 1970, but estimated annual freewater-surface evaporation for the same period was 

only 29 in/yr (National Oceanic and Atmospheric 

Administration, 1982). Because of this fact and 

because sediments beneath the ponds and trenches are 

highly permeable, much of the liquid waste discharged 

to the ponds and trenches did not evaporate but instead 

percolated into unconsolidated deposits beneath the 

site. 

Beginning in 1966, liquid wastes were discharged 

to a pond that was about 5,000 ft2 in area and 6 ft 

deep. Overflow problems due to precipitation and 

disposal-flow rates that ranged from 360 to 1,400 galld 

led to periodic construction of additional ponds and 

trenches. In 1968, a second pond (8,400 ft2 in area and 

4 ft deep) was used as a replacement for the original 

pond. In 1971 , a new pond was constructed in the 

same area as the original pond. A series of trenches 

were built to replace the first and second ponds in 

1977, and they were used until 1979. The liquidwaste-disposal ponds and trenches encompassed about 

Not to scale 

Figure 2. Detailed sketch of the radioactive-materials processing site, Wood River Junction, Rhode Island, 1979. 

4 Ground-Water Flow and Contaminant Transport at a Radioactive-Materials Processing Site, Wood River Junction, Rhode Island 
25,000 ft2. This 25,000-square-foot area is the source 

of the contaminated liquid that percolated to the water 

table. Finally, in 1979, a covered tank with a double 

polyethylene liner was constructed 50 ft north of the 

original pond area (fig. 2). The covered tank was 

1 00 ft in diameter and 12 ft high, and the bottom of the 

tank was 4ft below land surface. The tank could hold 

as much as 500,000 gal of liquid waste during 

evaporation and concentration processing. A 150 by 

150 ft catchment basin lined with Hypalon 1 was built 

around the tank. The above dates and sequence of 

events represent the best judgment of the 

investigators; information was derived from published 

and unpublished sources of data that were not entirely 

consistent. The actual source fluxes are not definable 

because data on chemical composition and physical 

properties of the liquid wastes are incomplete and 

concentrations of chemical and radiochemical 

constituents in waste discharges changed with time. 

Chemicals used in the recovery or waste-treatment 

processes are shown in table I. 

Pumpage from a water-supply well drilled for the 

plant in 1963 may have been as much as 40,000 to 

50,000 gal/d during peak operations, but probably 

averaged 20,000 gal/d. Pumpage averaged 15,000 gal/d 

in 1975 and ranged from 6,000 to 10,000 gal/d since 

1980. 

Although primarily non irradiated ( <1 00 mrem/hr) 

fuel elements were processed during 1964-80, slightly 

irradiated fuel elements from zero-power critical 

assemblies in test reactors were processed during 

1967-80. This could account for the strontium-90 

and technetium-99 that were measured in the 

contaminated ground water. The source for the 

technetium may have been an uranium-molybdenum 

alloy that was processed after October 1978. Heavy 

metals including cadmium, lead, silver, and zinc were 

found in the waste sludge on the bottoms of the ponds 

and trenches. 

Processing at the plant ended in August 1980. 

Decommissioning of the plant began in 1980 and was 

completed in 1990. Materi'd from the bottom of the 

ponds and trenches and several feet of sediment from 

1 Use of brand names in this report is for identification 

purposes only and does not constitute endorsement by the U.S. 

Geological Survey. 

Table 1. Chemicals used in recovery or waste-treatment 

processes at the study site, Wood River Junction, Rhode 

Island 

Aluminum nitrate 

Ammonia 

Boron salts 

Cadmium salts 

Calcium carbonate 

Calcium hydroxide 

Chlorothene1 

Hydrofluoric acid 

Kerosene 

1

Trade name. 

Mercury 

Nitric acid 

Oxalic acid 

Potassium hydroxide 

Sodium carbonate 

Sodium hydroxide 

Strontium carbonate 

Tributyl phosphate 

below the ponds and trenches were removed and 

combined with a cementlike mixture and shipped 

offsite for burial during 1981-83. 

Previous Investigations 

Ground-water samples collected by the company 

from seven well points on the plant site in 1972 and 

1973 indicated above-background concentrations of 

nitrate, gross alpha emitters, and gross beta emitters. 

From 1974 through 1977, the Rhode Island Water 

Resources Board drilled about 20 test holes on the 

plant property to obtain lithologic and water-quality 

data for evaluating areas for future ground-water 

development. Water-quality data obtained as part of 

the Water Resources Board investigation indicated 

ground water of high specific conductance 

(5,500 J..LS/cm), high nitrate concentration 

(225 mg/L), and significant concentrations of gross 

alpha (43 pCi/L) and gross beta (489 pCi/L) emitters 

1,100 ft from the source area (Dickerman and Silva, 

1980, p. 177-178). 

Shortly thereafter, the company installed 

10 observation wells between the plant and the river 

that ranged in depth from about 50 to 80ft. Water- · 

quality data obtained by the company from one of 

these wells (CHW -446) indicated ground water of 

unusually high specific conductance (14,500 J..LS/cm), 

unusually high nitrate concentration (2,200 mg/L), 

and significant concentrations of gross beta emitters 

( 1,518 pCi/L) 200ft from the source area (Dickerman 

and Silva, 1980, p. 177-178). 

Site History and Investigations 5 
Results of resistivity surveys in 1975 and 1977 by 

David Huntley, University of Connecticut, and Daniel 

Urish, University of Rhode Island indicated a plume of 

high specific conductance in ground water between the 

plant site and the Pawcatuck River. Adjacent to the 

source area, the water of highest specific conductance 

was estimated to be 40 ft below land surface (David 

Huntley, University of Connecticut, written commun., 

1981). Urish (1981) and Frohlich and Kelly (1985) 

correlated resistivity surveys with hydraulic properties 

of the aquifer. Urish (1981) found quantitative 

correlation was imprecise because of inherent variation 

in aquifer characteristics. An aerial radiological survey 

was conducted by the U.S. Department of Energy in 

1979 (EG&G, Energy Measurements Group, 1979). 

Surface-soil contamination was evident from this 

survey; however, no indication of ground-water 

contamination was derived from the data. The known 

length of the contamination plume at the beginning of 

the present study ( 1981) was about 1 ,200 ft from the 

source area; the width of the plume was unknown. 

Current Investigation 

The field and laboratory techniques used for the 

current investigation are described here. 

Field Techniques 

Geophysical techniques and well drilling were used 

from April 1981 through February 1984 to define the 

hydrogeologic system and the extent of the 

contamination plume. These techniques included 

seismic-refraction and gravity surveys to determine 

depth to bedrock; geophysical well logging (neutron, 

gamma gamma, natural gamma, and dual density) to 

determine relative lithologic differences within a given 

well and from well to well; and electromagnetic 

conductivity (Duran and Haeni, 1982; Barlow and 

Ryan, 1985) and surface-resistivity surveys to locate 

areas within the aquifer containing water of high 

specific conductance. Data from the electromagneticconductivity surveys were highly correlated with 

ground-water contamination, and these data were used 

to locate observation wells. Data from the geophysical 

logs were particularly useful in locating the boundary 

between the coarse sands and gravels and the fine sands 

and silt at depth. Data from all the geophysical surveys 

are shown in Ryan and others (1985, p. 274-294). 

Gravity measurements obtained from November 

1983 through March 1984 by A von ( 1984) covered an 

area 7,000 by 5,000 ft. These measurements were used 

to help define the topography of the bedrock surface 

near the site. Two northeast-southwest traverses, 

approximately perpendicular to the suspected axis of 

the bedrock valley, spanned the width of the valley 

from the till boundary on the east to approximately the 

till boundary on the west. For control, gravity 

measurements also were made at selected observation 

wells that were drilled to bedrock or refusal. Standard 

corrections were applied to the data, and a computer 

program based on indirect methods of interpretation 

(Talwani and others, 1959) was used to estimate the 

depth of the bedrock valley. Results indicated that the 

vertical relief of the bedrock surface ranges from 70 ft 

above sea level to 280 ft below sea level and that the 

asymmetrical, V -shaped bedrock valley trends north 

15° west and decreases in depth to the south. 

Additional data are needed to verify the hypothesis that 

glacial scouring resulted in a depression in the bedrock 

surface in the northern part of the valley. 

About 150 observation wells (figs. 3 and 4) ranging 

in depth from 10 to 230ft were installed during six 

drilling phases by use of hollow-stem-auger, mudrotary, or drive-and-wash methods. Most of the wells 

were installed by use of the hollow-stem auger. Mud 

rotary was used with only partial success where largediameter wells (>6 in.) were needed for geophysical 

logging. The drive-and-wash method was used in the 

swamp because that type of drilling rig was mounted 

on belt-type tracks, making swamp access possible. 

Wells were not drilled in the lagoon area because of the 

risks associated with personnel and equipment 

contamination; as a result, the source area was 

excluded from the investigation. The U.S. Nuclear 

Regulatory Commission and Oak Ridge Associated 

Universities did test drilling and sediment sampling in 

the source area in 1983 and 1984. 

In Rhode Island, each well or test hole inventoried 

by the USGS is assigned a local number on the basis of 

the town in which it is located. The town name is 

indicated by a two-letter code followed by a one-letter 

descriptor code. The town code for Charlestown is 

6 Ground-Water Flow and Contaminant Transport at a Radioactive-Materials Processing Site, Wood River Junction, Rhode Island 
"CH" and the town code for Richmond is "RI." The 

third letter in the local number indicates whether it is a 

well, "W," or a boring, "B." Numbers that follow this 

three-letter code show the chronological sequence of 

well construction and/or inventorying. Wells drilled 

specifically for this study have local well numbers 

higher than 469 and 636 for Charlestown and 

Richmond, respectively. Any local number with either 

a "CHW" prefix and a numerical value of "469" or 

lower, or an "RIW" prefix and a numerical value of 

"636" or lower, represents a well inventoried before 

April 1981. 

Numerous split-spoon samples were collected 

during drilling to define lithologic layers accurately. 

Ten split-spoon samples were collected for sediment 

analyses of cation-exchange capacities, microscopic 

mineralogic descriptions, porosity tests, and sieve 

analyses. Lithologic logs from about 175 wells and 

results of the physical properties analyses of sediment 

samples are presented in Ryan and others ( 1985, 

p. 33-66). 

Most wells were constructed of 1-1/4 inch- or 

1-1/2 inch-diameter2 flexible polyethylene or rigid 

polyvinyl chloride (PVC) plastic pipe. Two wells were 

constructed with 5-inch-diameter rigid PVC for 

geophysical logging, and two wells were constructed 

with 1-1 /4-inch-diameter galvanized steel for 

continuous water-level recording. Five monitoring 

wells were installed with 3/8-inch-diameter PVC in 

each of two holes in the swamp for water-quality 

analyses (water levels were difficult to measure in 

these wells). Screened intervals or well points ranged 

from 2 to 10ft in length, and slot sizes were either #1 0 

(0.01 0 in.) or #12 (0.012 in.). The first drilling phases 

involved installation of shallow (depths less than 30ft) 

observation wells to determine water-table 

configuration. Later phases were devoted to the 

installation of wells ranging in depth from 10 to 100 ft 

to locate the contamination plume horizontally and 

vertically. As wells were drilled, they were added to the 

observation network, which consisted of wells for 

monthly water-level measurements and approximately 

bimonthly water-quality sampling. Procedures for 

water-quality sampling were developed by Ryan and 

Healy ( 1991) to minimize cross contamination among 

2Well diameters mentioned in this report are nominal inside 

diameters. 

wells and to maximize sampling efficiency. Some 

surface-water sampling was done to determine 

contaminant concentration in the Pawcatuck River 

using a peristaltic pump to obtain samples at selected 

depths along cross sections of the river. 

A hydraulic potentiometric manometer 

(differential-head piezometer) (fig. 5) designed by 

Winter and others ( 1988) was used to measure 

differences in head between the free surface and 

ground water at some point in the aquifer. A 

differential manometer was attached to a flexible 

plastic tube extending to the river, and another piece of 

tubing was connected to a 3-inch-long well screen at 

the end of aT -bar that could be inserted as deep as 3 ft 

below the land surface. A hand pump was connected to 

a vacuum bottle so that water was pulled up into the 

manometer. 

Differential-head measurements were made on 

three separate occasions (October 1981, April 1982, 

and January 1983) at data stations shown in figure 6. 

At most locations, an adequate hydraulic seal was 

obtained between the piezometer tube and the 

sediments. At some locations, however, coarse gravels 

or extremely tight material that did not collapse around 

the piezometer point prevented an adequate seal. At a 

few locations, fine sediments and organic material 

clogged the well screen. Head differences were 

measured either between the free surface in the 

Pawcatuck River and ground water beneath the 

riverbed or between standing water in the wetland and 

ground water 1 to 3 ft deep below the wetland. Head 

measurements were almost always higher in the 

riverbed than in the river, indicating that ground-water 

flow was upwards into the river. Hydrostatic 

equilibrium was measured at six locations and higher 

heads in the river than in the riverbed were measured at 

four locations on the Richmond side of the river in 

April 1982. 

The differential head probe also was used to collect 

ground-water samples from the riverbed. These data 

were used to supplement qualitative interpretations of 

the contamination plume. Specific conductance of 

water in the river ranged from 55 to 79 J.1Sicm, whereas 

specific conductance of water in the riverbed ranged 

from 39 to 1 ,230 J.lS/cm. 

Site History and Investigations 7 
71° 41' 35' 

41°26'25" 

Dam 

~649 

~638 

~650 

655~ 

672-677 ~ 

~504 

~474 

~503 

~569 

642-645 

~505 

~479 

~481 

41°25' 45" 

Figure 3. Location of observation wells with local well numbers, Wood River Junction, Rhode Island. 

8 Ground-Water Flow and Contaminant Transport at a Radioactive-Materials Processing Site, Wood River Junction, Rhode Island 
429 

%. 

502 

475 ~ 

450 

~ 

536~ 565 559 ~~ 

Figure 3. -Continued. 

500~ 

501 ~ 

482~ 

A 

511 

~ ~510 

~ 

512 

A' LINE OF 

71° 40'30" 

--- GEOHYDROLOGIC SECTION 

~ 501 OBSERVATION WELL 

-X- FENCE 

0 

I 

0 

527 ~ 

~99 528~ 

A' 

Radioactivematerials 

processing site 

516~ 

500 FEET 

I I 

150 METERS 

Site History and Investigations 9 
-

0 

C) 

0 

c 

:s 

c. 

~ 

i 

., 

0 

~ 

m 

:I 

Q. 

0 

0 

:I 

S' 

3 

:;· 

m 

a 

~ ... 

m 

:I 

U'l 

"C 

0 

:J. 

~ 

m 

:::0 

m 

c. 

0 

m 

2. <" (D 

~ 

~ (D 

... 

iii" 

iii 

"'0 

0 

0 

(D 

U'l 

U'l 

:i" 

IQ 

~ 

JD 

=E 

0 

0 

c. 

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2. o· 

:? 

:::0 

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0 

Q. 

(D 

u; 

iii" 

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Q. 

A 

FEET 

90 

~ 60 

30 --1 

SEA ~ LEVEL 

I 

-30 

-60 

-90 

;..... ;..... 

~ ~ 

;::::.. .:: 

rQ 0::: 

~ 

(...) 

I661P I 656 

1668 

I662P I657 

I670 I663P I658 

-671w~ I664P 

I659 

I665P 

I660 

"' 

bend in 

section 

:.... 

~ 

;::::.. 

rQ 

o•ur ~ I 489 

641r 941 

I517 

I495 

642I I518 

I576 

I496 

643I I519 

I I525 

1498 

645I 

1493 

I520P 

I 521P 

I522P 

I548P 

5o9:rr I44o 

523P I557 

lagoon area 

land surface 

water table v 538 . __ 1 553 I531 

I 542 480 I 508:.: I 

I539 555 

I543P 535 534 532 

577I 

I I 449 540 533 

I 1446 I544P 

537Pli 561 

564P 

563 I I545P I541P 

II 

A' 

FEET 

I572 

I573 

562P COARSE SAND AND GRAVEL 

FINE SAND AND SILT 
en 

i 

::z:: 

iii" 

0 .., 

'< 

I» 

::::J 

Q. 

5' 

< 

CD 

~ 

<0' 

~ 

o· 

:J 

en 

.... 

.... 

-90 

-120 

FINEISAND AND SILT 

-150 

-180 

-210 

-240 

-270 

s46PI 

_illL 

EXPLANATION 

SCREENED INTERVAL AND WELL 

NUMBER--Well number with the 

letter P indicates well that was 

projected less than 20 feet to this 

line of geohydrologic section 

WETLAND 

0 200 400 600 800 1,000 FEET 

0 100 200 300 METERS 

VERTICAL SCALE GREATLY EXAGGERATED 

-300 ~~~~~~~~~~~~~--~~~~--~~~~~~~~~~~~~~~~~~~ 

Figure 4. Locations of screened intervals for observation wells, Wood River Junction, Rhode Island. Configuration of bedrock modified from 

Gonthier and others (1974). Line of geohydrologic section shown in figure 1. 
Tube A 

Probe 

~ Clamp 

Valve 

" 

Tube C 

I 

-- Manometer 

board 

Tube B 

TubeD 

~ 

Vacuum 

bottle 

\ 

Hand 

pump 

To surface water 

Figure 5. Hydraulic potentiomanometer (differential-head piezometer). (Modified from 

Winter and others, 1988, fig. 3.) 

Laboratory Techniques 

Cation-sorption properties of the sediments were 

determined by laboratory measurements. The three 

sediment samples used for laboratory study were collected with a split-spoon sampler from three locations 

(CHW-550, CHW-576, and CHW-559), which included 

contaminated and uncontaminated zones of the aquifer. 

Grain-size distribution and cation-exchange capacity 

for three of these samples from two boreholes are listed 

in table 2. The sediment was predominantly sand size. 

The cation-exchange capacity was low, 0.18 to 

0.30 meq/1 00 g, as determined by extraction with 1 N 

ammonium acetate at pH 7 (Page, 1982, p. 160). These 

exchange capacities are lower than those reported by 

Bell and Shearin (1967) for Bridgehampton soils (0.5 to 

1.9 mg/1 00 g), which are present at the site. Standard 

petrographic techniques and X-ray diffraction were 

used to determine the mineral constituents of the sediment (Manheim and others, 1984). About 90 to 95 percent of the sediment consisted of quartz, feldspars, and 

amphiboles and 5 to 10 percent consisted of vermiculite, biotite, chlorite, and muscovite. Average mineralgrain specific gravity was 2.65, as determined by use of 

a Beckman air pycnometer and helium as a flushing gas 

(Manheim and others, 1984). 

Column experiments were done to determine the 

ion-exchange parameters of strontium-90 with the sediments. Uncontaminated ground water from the site was 

used to dilute a synthetic contaminated ground water 

with the same major-ion composition as in the most 

contaminated ground water. Dilutions of 20: I, I 0: I, 

5:1, 1: I, 0:1 (uncontaminated: contaminated) ground 

water were used to simulate solute-concentration 

ranges in the aquifer. Several pore volumes of these 

ground-water solutions were passed through columns 

packed with sediment. Excess solution then was 

drained from the column; the sediment was removed 

and adsorbed cations were extracted with l N ammonium acetate at pH 7. This solution was used to extract 

adsorbed strontium-90 from sediment sample 576; the 

sample was analyzed according to methods in Thatcher 

and others ( 1977). This procedure allows measurement 

of adsorbed ions in situations where the quantity 

removed from solution is too low to be detected by conventional batch experiments. Cations were determined 

with an inductively coupled atomic-emission spectrophotometer. A detailed description of this methodology 

can be found in Kipp and others ( 1986). Results of the 

laboratory analysis are discussed in the section of this 

report on "Strontium-Ion Exchange." 

12 Ground-Water Flow and Contaminant Transport at a Radioactive-Materials Processing Site, Wood River Junction, Rhode Island 
Area shown 

in figure 6 

Radioactivematerials 

LOCATION MAP 

EXPLANATION 

TRAIL 

--- TOWN BOUNDARY 

Charlestown 

• so DIFFERENTIAL-HEAD PIEZOMETRIC 

OAT A-COLLECTION SITE AND 

NUMBER 

WETLAND 

-&c.---"*'----- -- ~-

-----~---*---

~----"*"---__..,__ 

-~-------_._ 

________ ___ _ _..._ __ 

0 

I 

0 

_.,___-l&c__--i.I.L_ 

_ __ _._ __....._ _______ _ 

100 

I I 

25 

I 

200 FEET 

I 

50 METERS 

Figure 6. Location of differential-head piezometric data-collection sites, Wood River Junction, Rhode Island. 

Site History and Investigations 13 
Table 2. Grain-size distribution and cation-exchange capacity of three sediment samples from boreholes for 

observation wells CHW-559 and CHW-576, Wood River Junction, Rhode Island 

[Cation-exchange capacity: Detennined as the sum of exchangeable cations (Page, 1982, p. 160). mm, millime.ter; meq/1 00 g, milliequivalent per 

100 grams.<, actual value is less than value shown] 

Sediment- Percentage of grain-size distribution Cation-exchange 

capacity in (meq/1 00 g 

for the <2 mm fraction) 

sample Pebble Gravel Sand Silt and clay 

identifier (4-64 mm) (2-4 mm) (0.06-2 mm) (< 0.06 mm) 

CHW-559-3 1 

CHW-559-41 

CHW-5762 

7.0 

.42 

0 

2.1 

.9 

0 

1

Collected from uncontaminated part of aquifer. 2Collected from partially contaminated part of aquifer. 

HYDROGEOLOGIC SETTING 

The hydrogeology of the study area is determined 

by the geology, which in tum determines both aquifer 

characteristics and to a large extent aquifer boundaries. 

These components and the resulting ground-waterflow regime are discussed in this section. 

Geology 

The study area is underlain by the Hope Valley 

Alaskite Gneiss, a metamorphic rock unit of late 

Proterozoic age. The gneiss formed from an igneous 

rock unit that underwent one and possibly two episodes 

of metamorphism (Moore, 1959). Bedrock crops out 

east, northeast, west, and southwest of the study area. 

Unconsolidated glacial deposits of Pleistocene age 

(0.0 1 to 1.6 million years old) were deposited on top of 

the bedrock and in the bedrock valley previously 

described. Till deposits (poorly sorted clays, silts, 

sands, gravels, and boulders) form a thin (less than 

20-foot thick) mantle over the bedrock (LaSala and 

Hahn, 1960) and are exposed at land surface east of the 

plant site (fig. 7). 

Outwash deposits (well-sorted, silts, sands, and 

gravels) were deposited in the bedrock valley (figs. 4 

and 8) and range in thickness from 0 to 300 ft. The 

outwash deposits consist of predominantly medium to 

coarse sands and gravels to about 100 ft below land 

surface and mostly fine sands and silts below this 

depth. Slow-moving glacial meltwater flowing into a 

lake behind a terminal moraine (till containing some 

stratified deposits) about 3 mi south of the study area 

apparently resulted in deposition of fine-grained 

sediments at depth. 

84.3 

84.5 

99.8 

6.8 

14.3 

.2 

0.18 

.25 

.30 

The fine sands and silts are cohesive in places; 

however, few clay-sized particles were found. Claysized particles constituted 2.9 and 3.1 percent of two 

split-spoon samples from the fine sand and silt deposit. 

Clay-sized particles constituted 0.12 to 7.6 percent of 

seven split-spoon samples from the coarse sand and 

gravel deposit; average content was 1 .5 percent and 

median content was 0.38 percent. In two locations (one 

about 100 ft south of Meadow Brook Pond and one 

between the plant and river at CHW-569), where test 

holes were deeper than 150 ft, a zone of coarse sand 

5- to 15-foot thick was found between the fine silt and 

sand and the bedrock surface; no other holes were 

drilled deep enough to find this coarse-sand zone. The 

bedrock was reached at a depth of 230ft at CHW-569 

between the plant and the river. 

The mineralogy of the outwash deposits is predominately quartz, feldspars, and amphiboles; dark minerals (biotite and hornblende) are generally more abundant in the fine sediments (Manheim and others, 1984). 

Extensive radiochemical analyses were done on two 

sediment samples from wells CHW-470 and CHW-516 

upgradient from the plant area (Ryan and others, 1985, 

p. 269-270). Detected constituents included potassium40, lead-21 0, radium-226, radium-228, thorium-228, 

uranium-235, and uranium-238. 

Aquifer Characteristics and Boundaries 

Hydraulic conductivities of outwash deposits at the 

plant, estimated from lithologic logs of 104 wells and 

8 test holes, range from 30 to 380 ft/d; the median is 

124 ft/d. These estimates were based on thicknessweighted averages of horizontal hydraulic conductivity. 

Unless otherwise noted, the term hydraulic 

conductivity as used in this report will refer to 

horizontal hydraulic conductivity. Gonthier and others 

(1974, pis. 2 and 4) also used lithologic logs to derive 

14 Ground-Water Flow and Contaminant Transport at a Radioactive-Materials Processing Site, Wood River Junction, Rhode Island 
C/) 

0> 

c 

S2 

~ 

1-

::::: 

Q) 

C/) 

c 

C1l 

0> 

~ 

~ 

z 

Narragansett Trail 

____ _._ - _......_----

EXPLANATION 

t:·::.::::>::::-.·:j 

I I 

~ 

TILL 

OUTWASH 

BEDROCK OUTCROP 

Indian Cedar Swamp _______ _......__ 

_ _.__ ~------

0 

I 

0 

1,000 

I I 

300 

B B' LINE OF GEOHYDROLOGIC SECTION 

FENCE 

WETLAND 

2,000 FEET I 

I 

600 METERS 

Figure 7. Generalized surficial geology of the study area and location of geohydrologic section 8-8', Wood 

River Junction, Rhode Island. 

Hydrogeologic Setting 15 
B 

FEET 

100 

50 

'""" C) 

~ 

i:X:l 

$ 

~ <::1 

~ 

~ 

I 

81 

FEET 

SEA COARSE SAND AND GRAVEL 

LEVEL 

-50 

FINE SAND AND SILT 

-100 

-150 

-200 

EXPLANATION 

----- CONTACT --Dashed where 

approximately located 

0 500 1 ,000 FEET 

0 100 200 300 METERS 

VERTICAL SCALE GREATLY 

EXAGGERATED 

Figure 8. Generalized geohydrologic section B-8', Wood River Junction, Rhode Island. 

Configuration of bedrock modified from Gonthier and others (1974). 

an estimate of 180 ft/d for hydraulic conductivity near 

the plant site. Hydraulic conductivities determined 

from analyses of three aquifer tests done by the USGS 

within a 1-mile radius of the site (fig. 2) range from 

195 to 335 ft/d (Dickerman and others, 1990). 

Hydraulic conductivities determined from analysis of 

an aquifer test at the plant supply well was estimated 

as 125 ft/d (Gonthier and others, 1974). Hydraulic 

conductivity of till in the study area is estimated to 

average about 1 ft/d, based on that of the till in the 

nearby upper Pawcatuck River Basin (Allen and 

others, 1966, p. 9). Hydraulic conductivity of the fine 

sands and silts at depth probably is somewhere 

between that of the tills and of outwash deposits (1 to 

380 ft/d). Fractures in the bedrock also yield water to 

wells, but generally only enough for domestic supplies 

(5 gal/min or less) (Allen, 1953, p. 26). 

Ground-water-flow velocities between the plant 

and the river, ranging from about 2.0 to 2.7 ft/d, were 

calculated from a water-table gradient of 28 ft/mi, 

from an estimated aquifer porosity of 0.38 obtained by 

averaging porosity values from six sediment samples 

(Ryan and others, 1985, p. 66) and from hydraulicconductivity estimates that ranged from 140 to 

190ft/d. 

Unconsolidated deposits near the junction of the 

Pawcatuck and Wood Rivers comprise the most 

extensive accumulation of sediments that form the 

principal aquifer in the lower Pawcatuck River valley 

(Gonthier and others, 1974, p. 7). The aquifer is about 

8 mi long and ranges from 2,000 to 8,000 ft in width; 

most of the aquifer extends north, northwest, and west 

of the plant site. Swamp and till deposits form the 

southern and eastern limits of the aquifer, respectively 

(fig. 7). The aquifer is unconfined, and the bedrock 

surface forms the lower boundary (fig. 9). Saturated 

thickness of the unconsolidated deposits ranges from 

0 to 300 ft, although thickness of the coarse sands and 

gravels is fairly uniform across the site. Ground water 

occurs in the bedrock, but quantities and flow rates 

are negligible compared to the more porous and 

permeable sands and gravels of the aquifer. 

16 Ground-Water Flow and Contaminant Transport at a Radioactive-Materials Processing Site, Wood River Junction, Rhode Island 
41 ° 25'30" 

---~--

- - - - - ____...__ - - - - - - -~-- c --\L_ 

f --~==~=~~~~ Indian Cedar Swamp 

_----loi,_ -------~- _ _l&t...._ _____ _ 

EXPLANATION 

-50-- BEDROCK CONTOUR -- Shows 

altitude of bedrock surface. Dashed 

where approximately located. Contour 

interval 50 feet. Datum is sea level 

•--•--• FENCE 

WETLAND 

0 

0 

1,000 

300 

2,000 FEET 

600 METERS 

Figure 9. Altitude of bedrock surface, Wood River Junction, Rhode Island. (Modified from Gonthier and 

others, 197 4, pl. 1.) 

Hydrogeologic Setting 17 
lP©>~®IFO~O@U'iJil®~tro© ~!UJ!Jil[61©®~ (611JilcdJ CGltr©>!UliFOcdl= 

Weal~@[( [F~@W 

The water table slopes westward from the plant site 

towards the Pawcatuck River at an average gradient of 

0.0053 (28 ft/mi) (fig. 10). Generally, ground-water 

flow in the aquifer is from the lateral boundaries and till 

upland areas toward the Pawcatuck River. The pond 

behind the dam on Meadow Brook provides increased 

seepage to the aquifer and results in a steeper watertable gradient between the pond and the river than 

likely was present before the dam was built. 

Ground water discharges to the Pawcatuck River, 

which is the major surface-water drainage from the 

study area. The potentiometric contours (fig. 11) and 

data from the differential-head piezometer survey 

indicate upward vertical flow to the Pawcatuck River 

and to the contiguous swampy area west of the river. In 

observation wells RIW -640 through RIW -645, which 

are near the river, the range of measured head 

differences between the deepest well (RIW-645, 

screened from 95 to 100 ft below land surface) and the 

shallowest well (RIW -640, screened from 7 to 1 0 ft 

below !and surface) was 0.44 ft at the lowest measured 

water levels (September 1983) and 1.06 ft at the 

highest measured water levels (February 1982) 

(fig. 12). JBoth head differences indicate upward 

vertical flow from well RIW -645 to well RIW -640. 

Seasonal fluctuations in water levels are evident. 

f'or the purposes of this study, however, the groundwater-flow system was assumed to be in a steady 

periodic state. There was no evidence that the system 

was evolving toward a flow-field configuration 

substantially different than that reflected by the 

average conditions. Therefore, the average of 

somewhat more than one year of data adequately 

represents average flow conditions that affect 

contaminant transport. However, the data set spanning 

1983 dloes not allow the effects of atypical precipitation 

years to be addressed. 

Seasonal fluctuations in the water table are greatest 

in we Us near the till upland areas and least in wells near 

the Pawcatuck River. The hydrograph from well 

CHW-510 near the till boundary (fig. 13) shows a 

16-foot range in water-table altitude from November 

1981 toN ovember 1983, whereas the hydrograph from 

wen RKW -641 near the river shows a 5-foot range in 

water-table altitude for about the same period. 

The hydrograph for well CHW-538, midway between 

the river and the till boundary, shows a range of about 

6.5 ft in water-table altitude. 

The controlling potential of the river on adjacent 

ground-water levels also can be seen in figure 14. 

River stage at a USGS streamflow-gaging station 

(about 0.5 mi upstream from the site) and at a location 

near well CHW-567 and water-level data from well 

CHW-567 were collected from December 1-9, 1982. 

The degree of interconnection between the river and the 

well is shown in figure 14. The muting effect on 

ground-water fluctuations can be observed about 

midway through the record (125 hours). 

Seasonal fluctuations in high and low water levels in 

the river and wells near the river are affected by runoff 

characteristics of the basin; indicated by the lag times 

between storms and changes in water levels. In upland 

areas near the till boundary, high and low water-level 

fluctuations in wells are more likely to be affected by 

precipitation and the related runoff from till; indicated 

by the shorter lag times between storms and rises in 

water levels (fig. 15). The effects of precipitation in 

June 1982, and April and November 1983 differ considerably among water levels in the river and in wells 

CHW-538 and RIW-641. Water-level rises at wells in 

the upland areas lagged about I month behind these 

storms, whereas water levels at wells near the river may 

or may not respond strongly to a given storm. The wells 

near the river respond to river stage as shown earlier. 

Areal and vertical representations of ground-water 

potentiometric heads are shown for a low-flow period, 

December 1982 (figs. 16 and 17), and for a high-flow 

period, April 1983 (figs. 18 and 19). Ground-water 

gradients are steeper at high flow, April 1983, than at 

low flow, December 1982. Generally, horizontal 

ground-water-flow direction does not change 

substantially in response to high and low flows. Some 

minor changes in flow direction were observed 

southwest of the plant area and south of the 

contamination plume. Presumably, the 

evapotranspiration from Indian Cedar Swamp (fig. 1 0) 

was higher in April 1983 than in December 1982 and 

resulted in some diversion of flow to the southwest of 

the plant. Downward vertical gradients increased in 

April 1983 (fig. 19) near the plant area, whereas 

upward vertical gradients increased in December 1982 

(fig. 17) below the first intersection of the river. 
---- ___....__ - ____.___----

---~-­

__...__ - - -- - -

. ~==~=~1~~~ Indian Cedar Swamp _______ .......\k___ 

-----"*'----- --loiL...-------

EXPLANATION 

f:'·\_· .. ~:·-:·::::] TILL 

I OUTWASH 

A A' LINE OF GEOHYDROLOGIC SECTION 

- 50 -- WATER-TABLE CONTOUR -- Shows 

altitude of water table. Dashed where 

approximately located. Contour interval 

2 feet. Datum is sea level 

•-•-• FENCE 

~ OBSERVATION WELL 

WETLAND 

0 

0 

1,000 

300 

2,000 FEET 

600 METERS 

Figure 1 0. Configuration of the water table based on average measured water levels in observation wells, 

Wood River Junction, Rhode Island, January through December 1983. 

Hydrogeologic Setting 19 
N 

C) 

G) 

0 

c 

:I 

Q. 

r 

... 

., 

0 

~ 

C» 

:I 

Q. 

0 

0 

:I 

S' 

3 

:; 

C» 

a 

-t 

; 

:I 

(I) 

"8 

~ 

! 

C» 

::D 

C» 

Q. 

0 

C» 

!1. 

<:-

CD 

3: 

C» 

~ iii" 

u; 

"'CI 

0 

n 

CD 

(I) 

(I) 

:; 

CQ 

en 

j 

:e 

0 

0 

Q. 

:::D 

:c· 

9! 

c.. 

c 

:I 

!1. 

a· 

? 

:II 

::T 

0 

Q. 

CD 

u; 

iii" 

:I 

Q. 

A 

FEET 

90 

60 

30 

SEA 

-30 

-60 

..... - • ..,.. ... __ _ 

FINE SAND AND SILT 

---·-------~-----

bend in 

section 

I 

I 

!... 

<:u 

;::.. 

iQ 

'"t3 

,2! 

1:::$ 

~ 

~ 

I 

I 

I 

I 

pI \ 

PI l 

PI 

\ 

;I .LII 

-------~--- ----• ------------~-5~0-0------~---------1-,0-0_0_F_E_E~T~ 

0 100 200 300 METERS 

VERTICAL SCALE GREATLY EXAGGERATED 

lagoon area 

land surface 

water table \7 

.ri I I I l 1 00 

'<:1-

COARSE PI I I I 

SAND AND 

GRAVEL 

p 

PI I I 

I 

- 46 --

!P 

il1lL 

measured water level, in feet above sea level. 

Dashed where approximately located. Contour 

interval, in feet, is variable 

SCREENED INTERVAL--The letter P indicates well that 

was projected more than 20 feet but less than 1 00 feet 

to this line of geohydrologic section 

A' 

FEET 

-90~~~~~--------~------------~~------~~--------------~~~~--~~--~~--~~--~ 

Figure 11. Configuration of average ground-water potentiometric head based on average measured water levels in observation wells, Wood River Junction, 

Rhode Island, January through December 1983. 
47 47 

RIW-640 RIW-643 

46 screened 46 screened 

interval interval 

7-10 feet 55-60 feet 

45 45 

44 44 

43 43 

42 42 

_J 41 41 

w 

> 1981 1982 1983 1981 1982 1983 

w 

_J 

<( 

w 47 48 

(/) 

w RIW-641 RIW-644 

> 46 screened 47 screened 

0 interval interval Ill 

<( 17-20 feet 75-80 feet 

Iw 45 46 

w 

LL 

z 44 45 

_j 

w 

> w 43 44 _J 

a: 

w 

~ 42 43 

~ 

LL 

0 41 42 

w 

Cl 1981 1982 1983 1981 1982 1983 

::::> 

Ii= 47 48 _J 

<( RIW-642 RIW-645 

46 screened 47 screened 

interval interval 

37-40 feet 95-100 feet 

45 46 

44 45 

43 44 

42 43 

41 42 

1981 1982 1983 1981 1982 1983 

Figure 12. Altitude of potentiometric surface in selected observation wells near the Pawcatuck River screened at different 

depths, Wood River Junction, Rhode Island, September 1981 through December 1983. Screened interval is in feet below 

land-surface datum. 

Hydrogeologic Setting 21 
...J 

w 72 > 

w 

...J 70 <( 

w 68 (J) 

w 66 > 

0 64 co 

<( 

62 1-

w 

w 60 u. 

~ 58 

...~- w 56 > 

w 54 ...J 

a: 

w 52 1-

<( 50 ~ 

u. 48 

0 

w 46 0 

::> 44 1-

~ 42 <( 

1981 1982 1983 

Figure 13. Seasonal fluctuations of water levels in 

observation well CHW-510 near the till boundary, 

observation well CHW-538 near the radioactive-materialsprocessing plant, and observation well RIW-641 near the 

Pawcatuck River, Wood River Junction, Rhode Island, 

October 1981 through December 1983. 

Again, increased evapotranspiration in April may 

have affected the flow directions below the river and 

the adjacent swamp. Lastly, hydro graphs from the 

vertical cluster of wells near the river (fig. 12) indicate 

no substantial differences among well responses to 

seasonal fluctuations; all responses are similar to that 

of the river, as shown in figure 15. Response is muted 

somewhat with depth. 

The ground-water system is recharged by 

infiltration of precipitation (rainfall or snowmelt), 

overland runoff, and ground-water flow from adjacent 

till-covered bedrock areas; however, evidence of 

overland runoff was not found during this 

investigation. Assuming ground-water outflow is a 

conservative estimate of the amount of natural 

recharge, Mazzaferro and others ( 1978, p. 45) relate 

ground-water outflow to the percentage of stratified 

drift in a drainage basin. This relation is described by 

the empirical equation 

Y = 35 +0.6X, (1) 

u. 41 0 

w 

0 

::> 

1-

~ 40 

<( 0 50 

RIVER STAGE AT STREAMFLOWGAGING STATION 

100 

RIVER STAGE NEAR 

WELL CHW-567 

150 200 

TIME, IN HOURS 

Figure 14. Altitude of water levels in the Pawcatuck River 

near streamflow-gaging station and observation well 

CHW-567, and in observation well CHW-567 during 

215 hours, December 1-9, 1982, Wood River Junction, 

Rhode Island. 

where 

Y is ground-water outflow as a percentage of 

total runoff, and 

X is percentage of total basin area underlain 

by stratified drift. 

For this study area, X= I 00 percent, therefore, 

250 

Y = 95 percent. On the basis of an annual average 

runoff of26.80 in. (Ryan and others, 1985, p. 132-174), 

long-term average annual recharge to the aquifer was 

estimated to be 25.5 in/yr during 1943-83 for the area 

upstream from the USGS gaging station on the 

Pawcatuck River at Wood River Junction. This 

estimate is 54 percent of the average annual 

precipitation for the same period. 

Discharge from the aquifer occurs through groundwater runoff and outflow--evaporation (primarily 

where the water table is within 5 ft of the land surface) 

(Bear, 1979, p. 58) and transpiration (where the water 

table intersects the root zone). Surface-water 

measurements of the Pawcatuck River at the 

streamflow-gaging station and at Kings Factory Road 

were done at low-flow conditions to obtain a 

measurement of aquifer discharge. No discernible 

differences were measured in the approximately 

1.4-mile-long reach of the river. 

22 Ground-Water Flow and Contaminant Transport at a Radioactive-Materials Processing Site, Wood River Junction, Rhode Island 
14 

(f) 12 w 

I 

() 

~ 10 

z 

0 8 

~ a: 6 

0 

w 

a: 

a... 4 

2 

llilml PLANT SITE 

KINGSTON 

1981 1982 1983 

~ 54 ~~~~~~~~~~~~~~~~~~~~~~ 

> 

~ WELL CHW-538 

LtJ (in upland area) (f) 

w 6 52 CD 

<t: 

1-

w 

w 

u.. 

~ 50 _j 

w 

> w 

...J 

a: 

w 

~ 48 

3: 

u.. 

0 

w 

0 

::J 

1-5 46 ~~~~~~~~~~~~~~~~~~~~~~ 

<( 1981 1982 1983 

~ 48 ~~~~~~~~~~~~~~~~~~~~~~ 

> 

w 

...J 

<( 

~ 47 

w 

> 

0 

CD 

<( 46 1-

w 

w 

u.. 

~ 45 w-

<.9 

<( 

1-

~ 44 w 

> 

a: 

~ 43 

w 

0 

::J 

1-

~ 2~~~~~~~~~~~~~~~-L~~-L~~~ 

...J 

<t: 1981 1982 1983 

~ 47 ~~~~~~~~~~~~~~~~~~~~~~ 

> 

~ WELL RIW-641 <( 

~ 46 (near river) 

w 

> 0 

CD 

<( 45 1-

w 

w 

u.. 

~ 44 ...Jw 

> w 

...J 

a: 43 w 

~ 

3: 

~ 42 

w 

0 

::J 

~ 41 ~~~~~~~~~~~~~~~-L~~~~~~ ...J 

<( 1981 1982 1983 

Figure 15. Relation between precipitation, stage of the Pawcatuck River, and water levels in observation wells CHW-538 in 

upland area near the radioactive-materials processing plant and RIW-641 near the Pawcatuck River, Wood River Junction, 

Rhode Island, January 1981 through December 1983. 

Hydrogeologic Setting 23 
EXPLANATION 

--50 --- WATER-TABLE CONTOUR--Shows 

altitude of water table. Dashed 

where approximately located. Contour 

interval 2 feet. Datum is sea level 

x--x--x FENCE 

~ OBSERVATION WELL 

_;ilu_ WETLAND 

0 

I 

0 

1,000 FEET 

I I I I I 

I I 

300 METERS 

Figure 16. Altitude and configuration of the water table during a low-flow period, Wood River Junction, Rhode Island, 

December 1982. 

24 Ground-Water Flow and Contaminant Transport at a Radioactive-Materials Processing Site, Wood River Junction, Rhode Island 
X 

'< c. 

... 

0 

<C 

(II 

0 

0 

<C 

c:;· 

en 

(II 

= :r 

<C 

N 

CJ1 

A 

FEET 

90 

60 

30 

SEA 

-30 

I 

I 

I I 

I 

FINE SAND AND SILT 

I 

I 

I 

I 

bend in 

section 

I 

I 

I 

I 

1... 

~ 

;:>. 

~ 

I 

lagoon area 

p~ 

I 1 .JI I ~ I 

COARSE ( P \ I 1I I I SAND AND 

GRAVEL 

p 

pi } ~ PI r; p 

Ip 

I 

--45 - POTENTIOMETRIC CONTOUR--Shows altitude of 

-60 -J I measured water level, in feet above sea level. 

0 500 1 ,000 FEET 

0 100 200 300 METERS 

VERTICAL SCALE GREATLY EXAGGERATED 

!P 

~ 

Contour interval, in feet, is variable 

SCREENED INTERVAL--The letter P indicates well that 

was projected more than 20 feet but less than 100 feet 

to this line of geohydrologic section 

WETLAND 

I 

I 

A' 

FEET 

-90~~~----------------------------~------------------------~~~~--~~--~~--~~--~~ 

Figure 17. Altitude of ground-water potentiometric head during a low-flow period, Wood River Junction, Rhode Island, December 1982. 
EXPLANATION 

--50 --- WATER-TABLE CONTOUR--Shows 

altitude of water table. Dashed 

where approximately located. Contour 

interval 2 feet. Datum is sea level 

x--x--x FENCE 

~ OBSERVATION WELL 

-->IlL._ WETLAND 

0 

I I I 

0 

\ 

\ 

\ 

\ 

\ 

\ 

I 

I 

~ ~ / 

' , . 

' ' ' 

1,000 FEET 

I I 

I 

300 METERS 

I 

Figure 18. Altitude and configuration of the water table during a high-flow period, Wood River Junction, Rhode 

Island, April 1983. 

26 Ground-Water Flow and Contaminant Transport at a Radioactive-Materials Processing Site, Wood River Junction, Rhode Island 
:r 

~ ., 

0 

cg 

0 

0 

CQ 

c:;· 

(J) 

CD 

:;-= 

CQ 

N 

...., 

A 

FEET 

90 -, bend in 

section 

I .... ~ .... ~ .:: ~ 

;::.. .:: ~ Q:.:; Q:.:; 

60 --i ~ ~ ~ ...::.c u ~ \..) 

.2! 

!;;j 

\..) 

~ 

d: :Ioiii.! I Ill 

0 

I 1/I 30 ----J l 

I I 

I I I 

I I I I I 

I 

SEA I - ~ I I I I 

I 

I /I 

I 

I I I 

I 48 

-30 

FINE SAND AND SILT 

i i 

I 

-60 

0 500 1 ,000 FEET I I I I I I I I 

I I I 0 100 200 300 METERS 

- 48 -

Jp 

~ 

land surface 

lagoon area 

p 

.ri 

POTENTIOMETRIC CONTOUR--Shows altitude of 

measured water level, in feet above sea level. 

Contour interval, in feet, is variable 

SCREENED INTERVAL--The letter P indicates well that 

was projected more than 20 feet but less than 100 feet 

to this line of geohydrologic section 

I 

I 

A' 

FEET 

r 

VERTICAL SCALE GREATLY EXAGGERATED WETLAND 

-90 X'\ I II 

/y 7 y y y 7 y 

~ 

Figure 19. AMude of ground-water potentiometric head during a high-flow period, Wood River Junction, Rhode Island, April1983 . 
GROUND-WATER QUALITY 

The contamination plume is superimposed on the 

background water quality at the site. A major part of 

this study was the strontium component of this plume 

and determination of its ion-exchange characteristics. 

Background Water Quality 

Uncontaminated ground water and surface water in 

the study area generally met U.S. Environmental 

Protection Agency National primary drinking water 

regulations (U.S. Environmental Protection Agency, 

1990). Specific conductance, an indication of the total 

concentration of dissolved minerals in the water, 

generally was less than 100 flS/cm. The principal 

cations-sodium, calcium, potassium, and 

magnesium-were present in concentrations of 

14 mg!L or less. The principal anions-sulfate, 

chloride, and nitrate-were present in concentrations 

of 20 mg/L or less. The results of some statistical 

analyses for average background concentrations in 

Table 3. Average background concentrations, standard 

deviations, and detection limits for selected constituents in 

ground water from 24 observation wells outside the 

contamination plume, Wood River Junction, Rhode Island 

[J.lg/L, microgram per liter; mg/L, milligram per liter; pCi/L, 

picocurie per liter; J.lS/cm, microsiemen per centimeter at 

25 degrees Celsius; ND, not determined. <, actual value is less 

than value shown] 

Average 

Constituent back- Standard Detection 

or property ground con- deviation limit 

centration 

Specific conductance 102 61 ND 

(J.lS/cm) 

Calcium (mg!L) 2.6 .52 ND 

Potassium (mg/L) 1.9 2 0.1 

Nitrate (mg/L) .7 .78 0.1 

Boron (J.lg/L) <10 ND 10-20 

Strontium-90 (pCi/L) 1

.55 .83 0.4 

Gross beta (pCi/L) 4.8 .99 0.8-0.9 

1 Assymetric confidence interval of 0.08 to 1.9 pCi/L based on 

31 measurements over the study area. 

ground water of selected constituents from 24 

observation wells outside the plume are listed in 

table 3. 

Naturally occurring total strontium was detected at 

very low concentrations (on the order of micrograms 

per liter) in water samples from three wells (table 4). 

Some naturally occurring radionuclides, such as 

potassium-40, radium-226, radium-228, and 

strontium-90, also were detected; concentrations 

generally were less than 3 pCi/L. Except for the 

relatively high iron and manganese concentrations, 

the chemical analysis shown in table 5 for an 

observation well outside the contamination plume 

(well CHW-485) can be considered representative of 

background water quality. 

Table 4. Ground-water compositions used for strontiumsorption experiments, Wood River Junction, Rhode Island 

[Constituents in milligrams per liter unless otherwise noted. 

Uncontaminated ground water collected from same interval as sediment 

samples CHW-559-3 and CHW-559-4. Partially contaminated ground 

water collected from same interval as sediment sample CHW-576. 

Ground water from most contaminated part of plume, CHW -550. f.lS/cm, 

microsiemen per centimeter at 25 degrees Celsius; pCi/L, picocurie per 

liter. <,actual value is less than value shown] 

Partially Most 

Constituent Uncontam- contam- contamor property inated inated, inated, 

CHW-559 CHW-576 CHW-550 

8-24-83 8-24-83 8-24-83 

Specific conductance 53 880 5,000 

(J.lS/cm) 

pH (standard units) 5.6 5.3 4.6 

Calcium 3.0 150 770 

Magnesium .73 5.0 26 

Sodium 3.6 8.6 27 

Potassium 1.5 4.0 21 

Bicarbonate 9 4 7 

Sulfate 13 14 9 

Chloride 6.0 9.6 25 

Nitrogen, nitrate 

plus nitrite .18 120 580 

Barium <.02 .38 .78 

Strontium, total .02 .30 1.6 

Strontium-90 (pCi/L) .55 46 250 

28 Ground-Water Flow and Contaminant Transport at a Radioactive-Materials Processing Site, Wood River Junction, Rhode Island 
Table 5. Representative chemical analyses of water from observation wells near the middle, edge, and outside of the 

contamination plume, Wood River Junction, Rhode Island 

[Concentrations in milligrams per liter unless otherwise noted; ~giL. microgram per liter; ~S/cm, microsiemen per centimeter at 25 degrees Celsius; pCi/L, 

picocurie per liter; ·c. degrees Celsius; ND, not determined.<, actual value is less than value shown] 

Constituent or property 

CHW-519 

Observation well 

in middle of plume 

2-17-82 

Specific conductance (J.lS/cm) .................. .. pH (standard units) ................................... .. Temperature, water ("C) ........................... .. Hardness (as CaC03) ................................. . 

Calcium ...................................................... . 

Magnesium ................................................ . Sodium ....................................................... . Potassium ................................................... . Alkalinity (as CaC03) ................................ . 

Sulfate ........................................................ . 

Chloride ..................................................... . Fluoride ..................................................... .. Silica .......................................................... . Nitrogen, nitrite plus nitrate (as N) ............ . Phosphorus, orthophosphate (as P) 

Boron (J.lg/L) .............................................. . Cadmium (J.lg/L) ........................................ . Copper (J.lg/L) ............................................ . Iron (J.lg/L) ................................................ .. Lead (J.lg/L) ................................................ . 

Manganese (J.lg/L) ...................................... . Nickel (J.lg/L) ............................................. . Strontium-90 (pCi/L) ................................ .. Strontium, total .......................................... . Zinc (J.lg/L) ................................................ . 

Contamination Plume 

4,260 

5.6 

12.0 

1,900 

720 

23 

25 

21 

7 

50 

180 

<.1 

<.1 

580 

<.01 

230 

1 

4 

250 

5 

600 

14 

222 

ND 

50 

The chemical characteristics of the contamination 

plume will be described followed by a discussion of its 

location and configuration at the study site. 

Chemical Characteristics 

Representative chemical analyses of ground water 

from observation wells near the middle and edge of the 

contamination plume, as well as outside of the plume, 

are listed in table 5. Data for the wells near the middle 

and the edge of the plume, however, are not necessarily 

the maximum or minimum concentrations in the plume. 

Chemical and radiochemical constituents in the 

contaminated water included nitrate (5 to 730 mg!L), 

CHW-518 

Observation well on 

edge of plume 

2-3-82 

376 

5.7 

11.5 

130 

50 

1.5 

7.8 

3.4 

3 

14 

9.2 

<.1 

11 

37 

<.01 

50 

2 

2 

20 

6 

67 

1 

6.7 

ND 

11 

CHW-485 

Observation well 

outside of plume 

2-23-81 

77 

5.6 

10.5 

16 

4.1 

1.4 

4.4 

2.5 

9 

14 

5.0 

<.1 

6.9 

.18 

<.01 

<10 

<1 

5 

310 

I 

1,600 

2 

2.9 

ND 

16 

boron (20 to 400 J.lg/L), potassium (3 to 25 mg!L), 

strontium-90 (4 to 290 pCi!L) and technetium-99 (75 to 

1 ,350 pCi/L). Technetium-99 and strontium-90 were 

detected in water from observation wells at distances of 

1,500 and 2,000 ft from the plant, respectively. Ryan 

and others (1985, p. 230-231) present additional data 

on concentrations of selected radionuclides, including 

cesium-I37, cobalt-60, potassium-40, radium-226, 

radium-228, and tritium. Concentrations of gross beta 

emitters range from 5 to I ,600 pCi!L. Laboratory 

detection limits for each of the commonly measured 

constituents are as follows: nitrate, 0.1 mg!L; boron, 

20 J..Lg/L; potassium, O.I mg/L; strontium, I 0 Jlg/L; 

gross alpha, 0.4 Jlg/L as uranium; and gross beta, 

0.4 pCi/L as strontium-90. The two constituents that 

Ground-Water Quality 29 
exceeded the maximum contaminant level (MCL) of 

the U.S. Environmental Protection Agency National 

primary drinking-water regulations (U.S. 

Environmental Protection Agency, 1990) were nitrate 

and strontium-90. The MCL for nitrate is 10 mg!L. The 

MCL for beta particles and photon radioactivity is 

4 mrem/yr; this dose would be produced by an average 

annual strontium-90 concentration of 8 pCi/L. Future 

reference to a drinking-water criterion of 8 pCi/L for 

strontium-90 is based on the relation to the MCL for 

beta particles and photon radioactivity. 

Because of the expense of the analytical procedure, 

only ten water samples (from observation wells 

CHW-429, CHW-440, CHW-443, CHW-446, 

CHW-449, CHW-480, and a well (T-2) inside the plant 

(not assigned a local well number) were analyzed (in 

March and June of 1982) for technetium-99 (one by 

the USGS and nine by Oak Ridge Associated 

Universities). Values ranged from less than 20 to 

1,380 pCi/L. Little is known of the redox chemistry of 

technetium. In the presence of dissolved oxygen, 

technetium will be present in the+ 7 oxidation state and 

occur as the soluble pertectonate (Tco4-) anion. In 

reducing environments, Tc4+ is the stable oxidation 

state and may complex with organic matter, precipitate 

as technetium oxide (Tc02), or precipitate as 

technetium sulfide (TcS2) in the presence of hydrogen 

sulfide (H2S). In the ten samples analyzed, strontium90 accounts for 1 0 to 30 percent of the gross beta 

activity; the remainder is attributed to technetium-99. 

The sums of strontium-90 and technetium-99 can 

exceed the gross beta activity level for a given sample 

most likely because the separation and counting 

efficiency for individual radionuclide measurements is 

greater than that of the gross-beta-counting apparatus. 

Specific conductance of the ground water ranged 

from 150 to 5,400 JlS/cm. Dissolved-solids 

concentrations were as high as 3,500 mg/L; these 

elevated concentrations interfered with the detection of 

alpha emitters. Temperature of the ground water was 

measured with each sample and ranged from 7.0 to 

15.0°C. The differences are attributed to seasonal 

variations in rainfall and river temperature rather than 

to the presence or absence of contaminated water. 

Location and Configuration 

The three-dimensional plume of contaminated 

ground water at the study site is depicted in figure 20. 

This block diagram displays the three-dimensional 

aspects of the plume that are not observable in the twodimensional maps or hydrologic sections. Areal views 

of the plume (figs. 21-25) were constructed by projecting the maximum concentration, regardless of depth, to 

the plane of the figure. Similarly, sectional views of 

the plume (figs. 26-31) were constructed with maximum concentration data from wells within 20 ft of the 

line of section or with maximum concentration data 

from wells within 1 00 ft of the line of section which 

were projected to the section. Figure 27, a section without vertical exaggeration, was drawn to show the actual 

sectional perspective of the plume. The threedimensional aspects of the plume are difficult to 

portray on a two-dimensional map or cross section. 

Additionally, the low density of the sampling points 

further limits determination and depiction of the 

detailed structure of the plume. 

Three time periods for nitrate and two time periods 

for strontium-90 were presented to illustrate short- and 

long-term trends in the contamination-plume 

configuration. Strontium-90 data for the April-May 

1983 time period were not illustrated because they did 

not add significantly to the understanding of the 

contamination system. 

The plume of contaminated ground water, primarily 

resulting from advective transport of contaminants, 

extends from the source area northwestward about 

I ,500ft to the Pawcatuck River and then southwestward about 800 ft in a downstream direction through 

the swampy area west of the river, for a total distance 

of2,300 ft (figs. 21-25). The unsaturated zone between 

the source area and water table also was assumed to be 

contaminated. As stated earlier, however, sampling of 

the unsaturated and saturated zones in the source area 

was not done. Therefore, figures 26-31 show only 

contamination in the saturated zone. Extrapolation of 

field observations showed that contamination enters 

the saturated-flow zone under the ponds and trenches 

and begins to descend as it is transported horizontally 

(figs. 26-31 ). The contamination continues to descend 

from this location to the Pawcatuck River. A significant fraction of the contaminated ground water rises 

and enters the river at this point, but the rest moves 

30 Ground-Water Flow and Contaminant Transport at a Radioactive-Materials Processing Site, Wood River Junction, Rhode Island 
Power Station 

Sand and Gravel 

Sand and 

Gravel 

EXPLANATION 

- 8 - LINE OF EQUAL STRONTIUM-90 CONCENTRATION--

Interval, in picocuries per liter, is variable 

Not to Scale 

Figure 20. Generalized view of contaminated ground water at the radioactive-materials processing site, Wood River Junction, 

Rhode Island. 

downstream, rising gradually to enter the river at the 

second or third intersection (figs. 26-31 ). The dashed 

lines at the leading edge of the plume under the swamp 

(figs. 26-31) were based on data from later drilling 

phases and from the results of the electromagnetic 

survey (fig. 32). Dilution precludes detection of 

contaminants once they have entered the Pawcatuck 

River, which has a long term ( 40-year) average 

discharge of 193 ft3 Is. 

The plume is about 300ft wide and is confined to the 

upper 80ft of saturated thickness (fig. 26-31 ), where 

sediments consist of medium to coarse sand and gravel. 

The width of the plume generally is constant in space 

and with time. Lateral dispersion of the contaminants 

appears to be offset by the convergence of the flow field 

toward the river (fig. 10). 

The top of the contamination plume is depressed 

below the water table (figs. 26-31) except beneath the 

source area, and its depth increases as distance from 

the source area increases. The maximum depth of the 

plume is 80 ft below land surface-1 ,400 to I ,500 ft 

from the source area. Beneath the discharge area in the 

river and adjacent swamp, the plume rises to the land 

surface. The thickness of the plume moving along the 

flow direction remains fairly constant at about 40 ft. 

The absence of contaminants in the fine-grained 

silts and sands at depth may be a result of very low 

advective transport into this zone. Additional data 

obtained from the interface between the fine- and 

coarse-grained deposits would have been required to 

detect or even estimate migration of contaminants due 

to diffusion similar to that reported by Gillham and 

others (1984). 

Ground-Water Quality 31 
71° 41'20" 71° 41 ' 

41°26'10" .--------,----------""-----------.---------~-------, 

EXPLANATION 

Radioactivematerials 

processing 

site 

0 1,000 FEET 

--10--- LINE OF EQUAL NITRATE CONCENTRATION--

Dashed where approximately located. Interval, in 

milligrams per liter, is variable. Lines of equal 

concentration are based on maximum 

concentration at each location, regardless of depth 

I 

0 

I 

I 

300 METERS 

x--x--x FENCE 

~ OBSERVATION WELL 

_.llli.L_ WETLAND 

Figure 21. Areal distribution of nitrate concentrations in ground water at the radioactive-materials processing 

site, Wood River Junction, Rhode Island, September and October 1982. 

Historically, the fields north of the plant and west of 

the river, as well as a chemical plant west of the river 

between the railroad and Route 92, had been suggested 

as possible sources of the increased nitrate concentrations in wells near the river. Results of water-quality 

sampling in and around the areas mentioned above 

eliminated these areas as sources of contamination for 

the plume described herein. 

Specific gravity of three samples of contaminated 

ground water from observation wells CHW -440, 

CHW-449, and CHW-480 collected in July 1981 

ranged from 1.000 to 1.001 at 25°C (D. Urish, 

University of Rhode Island, written commun., 1982). 

The slight density difference between the most 

contaminated ground-water samples (specific conductance of 3,500 J.l.Sicm and specific gravity of 1.001) 

and the least of the contaminated ground-water 

samples (specific conductance of 1,190 J.l.Sicm and 

specific gravity of 1.000) may be sufficient to cause 

some downward movement of contamination. The 

ratio between the driving force due to density differences and the driving force due to potentiometric head 

gradients ranged from 0.1 to 0.25 in different parts of 

the plume. Ratios were lower in areas with greater 

vertical movement (downward or upward), and ratios 

32 Ground-Water Flow and Contaminant Transport at a Radioactive-Materials Processing Site, Wood River Junction, Rhode Island 
71° 41'20" 71° 41' 

41°26'10" .---------.---------.---.-..------------..,.....-----------.---------, 

_11< 

EXPLANATION 

Radioactivematerials 

processing 

site 

0 1,000 FEET 

--50--- LINE OF EQUAL STRONTIUM-90 CONCENTRATION--

Dashed where approximately located. Interval, in 

picocuries per liter, is variable. Lines of equal 

concentration are based on maximum 

I 

0 

I 

I 

300 METERS 

concentration at each location, regardless of depth 

x--x--x FENCE 

~~< OBSERVATION WELL 

_.l.lliL._ WETLAND 

Figure 22. Areal distribution of strontium-90 concentrations in ground water at the radioactive-materials 

processing site, Wood River Junction, Rhode Island, September and October 1982. 

were higher in areas of predominantly horizontal movement. Therefore, freshwater recharge on top of the 

plume was assumed to be primarily responsible for 

increased plume depth with increased distance from the 

source area. 

Seasonal variations in hydrologic conditions may 

affect dimensions and depth of the plume, particularly in 

the swamp adjacent to the river. Above-normal 

precipitation in spring 1983 depressed the contamination plume below the water table in the swamp (fig. 29). 

Previous and subsequent chemical analyses from wells 

RIW -661 and RIW -666, located in the swamp, 

indicated a flushing of contaminants due to this 

precipitation and a reemergence of the contaminants 

once the slug of freshwater moved out of the swamp. 

These concentration changes, however, could not be 

specifically correlated with river stage because the river 

stage represents change in pressure head, not a bulk 

displacement of ground water under the swamp. 

Two zones of concentrated contaminants separated 

by a zone of less contaminated water were observedone near the plant and one near the river. The zone near 

the Pawcatuck River resulted from infiltration of 

Ground-Water Quality 33 
71° 41' 20" 71° 41' 

41°26' 10" r--------.--------~----r----------....-------------.---------, 

EXPLANATION 

Radioactivematerials 

processing 

site 

0 1,000 FEET 

--10--- LINE OF EQUAL NITRATE CONCENTRATION--

Dashed where approximately located. Interval, in 

milligrams per liter, is variable. Lines of equal 

concentration are based on maximum 

concentration at each location, regardless of depth 

I 

0 

I 

I 

300 METERS 

x--x--x FENCE 

11\ OBSERVATION WELL 

-~ WETLAND 

Figure 23. Areal distribution of nitrate concentrations in ground water at the radioactive-materials processing 

site, Wood River Junction, Rhode Island, April and May 1983. 

contaminants while the plant was processing radioactive material during 1964-80. The zone near the source 

area probably resulted from flushing of additional 

contaminants from the unsaturated zone as a result of 

precipitation while the sediment below the ponds and 

trenches was being excavated for site-decommissioning during 1981-82. Concentrations of contaminants 

in this zone near the source area diminished with time. 

In October 1982, the nitrate concentration in this zone 

was 740 mg!L but concentrations decreased to 

99 mg!L by January 1984. Nitrate and strontium-90 

concentrations in the plume in October 1982 (figs. 21 

and 22) indicate that the zone of maximum nitrate 

concentration near the plant may have been ahead of 

the zone of maximum strontium-90 concentration. 

Therefore, retardation of strontium-90 may have been 

occurring near the plant. Seasonal variations in these 

zones of concentrated contaminants were minor. 

34 Ground-Water Flow and Contaminant Transport at a Radioactive-Materials Processing Site, Wood River Junction, Rhode Island 
EXPLANATION 

Radioactivematerials 

processing 

site 

0 1,000 FEET 

--10--- LINE OF EQUAL NITRATE CONCENTRATION--

Dashed where approximately located. Interval, in 

milligrams per liter, is variable. Lines of equal 

concentration are based on maximum 

concentration at each location, regardless of depth 

lf-.L.__.____._I ,--1 -'------'--'--.--1 ..__I --'-~1 

0 300 METERS 

x--x--x FENCE 

~ OBSERVATION WELL 

-~ WETLAND 

Figure 24. Areal distribution of nitrate concentrations in ground water at the radioactive-materials processing 

site, Wood River Junction, Rhode Island, January 1984. 

Additional maps and sections (not included in this 

report) were drawn for concentrations of boron and 

potassium; the overall structure of these two plumes 

was similar in space and time to that of the nitrate and 

strontium-90 plumes. Although Sykes and others 

( 1982) characterized potassium transport by use of a 

linear equilibrium isotherm of 3.9xl o-4 L/g with a 

retardation factor of 2. 7, no retardation of potassium 

was observed during this study. 

In September and October of 1982, the leading edge 

of the nitrate plume (fig. 21) was farther west than the 

leading edge of the strontium-90 plume (fig. 22). This 

observation was substantiated by nitrate and strontium90 data collected from wells RIW -661 through RIW666 in October and November 1983. Specificconductance data from the differential piezometer 

surveys in April 1982 and February 1983 confirmed the 

longitudinal ~xtent of contaminated ground water. 

Ground-Water Quality 35 
71 ° 41' 20" 71 ° 41' 

41 ° 26'10" .-------,.-------~-or-.-----------,-----------.--------, 

EXPLANATION 

Radioactivematerials 

processing 

site 

0 1,000 FEET 

-- 8 --- LINE OF EQUAL STRONTIUM-90 CONCENTRATION--

Dashed where approximately located. Interval, in 

picocuries per liter, is variable. Lines of equal 

concentration are based on maximum 

I 

0 

I 

I 

300 METERS 

concentration at each location, regardless of depth 

X--X--X FENCE 

"!< OBSERVATION WELL 

_.ill!L_ WETLAND 

Figure 25. Areal distribution of strontium-90 concentrations in ground water at the radioactive-materials 

processing site, Wood River Junction, Rhode Island, January 1984. 

Precipitation and contamination levels do not seem 

to be correlated (fig. 33), except for the major storm in 

June 1982. Gross beta concentrations in June 1982 for 

observation well CHW -538 corresponded to the peak 

rainfall, but peak gross beta concentration for 

observation well RIW -641 lagged the storm by 

1 month. Similarly, water levels in these wells and 

concentrations of contaminants were not correlated. 

Concentrations of gross beta emitters decreased at 

observation wells CHW -538 and RIW -641 after June 

and July 1982, respectively. The relatively high grossbeta concentrations measured in observation well 

RIW-641, compared to those in observation well 

CHW-538, are attributed to the fact that well RIW-641 

penetrates the zone of concentrated contaminants near 

the river, whereas observation well CHW -538 does not 

penetrate the zone of concentrated contamination near 

the plant. 

36 Ground-Water Flow and Contaminant Transport at a Radioactive-Materials Processing Site, Wood River Junction, Rhode Island 
G') 

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0 500 1 ,000 FEET I I I I I I I I 0 100 200 300 METERS 

VERTICAL SCALE GREATLY EXAGGERATED 

lagoon area 

land surface 

water table v _ 

-10--

I p 

illlL 

I 

p 

COARSE 

SAND AND 

GRAVEL 

Dashed where approximately located. Interval, in 

milligrams per liter, is variable. Lines of equal 

concentration are based on the maximum 

concentration at each location, regardless of lateral 

position 

SCREENED INTERVAL--The letter P indicates well 

that was projected more than 20 feet but less than 

100 feet to this line of geohydrologic section 

I 

I 

A' 

FEET 

-90 ~~~----------------------------~~--------------------~~~~--~~--~~~ ~~~ 

Figure 26. Vertical distribution of nitrate concentrations in ground water at the radioactive-materials processing site, Wood River Junction, Rhode Island, 

September and October 1982. 
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0 500 1,000 FEET I I I I I I I 0 100 200 300 METERS 

VERTICAL SCALE GREATLY EXAGGERATED 

-90 .. 

-so--

!P 

~ 

COARSE 

SAND AND 

GRAVEL 

Dashed where approximately located. Interval, 

in milligrams per liter, is variable. Lines of equal 

concentration are based on the maximum 

concentration at each location, regardless of 

lateral position 

SCREENED INTERVAL--The letter P indicates well 

that was projected more than 20 feet but less than 

1 00 feet to this line of geohydrologic section 

I 

A' 

FEET 

Figure 29. Vertical distribution of nitrate concentrations in ground water at the radioactive-materials processing site, Wood River Junction, Rhode Island, 

April and May 1983. 
C) 

~ 

0 

c:: 

:s 

a. 

A 

FEET 

90 ---, 

60 l ~ 

30 ---1 l 

I 

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SEA I 

-30 

-60 

bend in 

section 

I lagoon area "- "- ~ ~ 

;::.. ;::.. land surface 

ct: ....... ..:.c ..:.c '-l '-l 

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I 

watertable\]I rg ---

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~~50~ y; -1 COARSE 

~-- PI 

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SAND AND 

....... ""'- GRAVEL 

-..,.,, I 

---·-<I-·----·-- FINE SAND AND SILT 

I 

I 

0 500 1 ,000 FEET 

I I I I I I 0 100 200 300 METERS 

VERTICAL SCALE GREATLY EXAGGERATED 

-50-- LINE OF EQUAL NITRATE CONCENTRATION--

Interval, in milligrams per liter, is variable. Lines 

!P 

.lllJ.L. 

of equal concentration are based on the maximum 

concentration at each location, regardless of 

lateral position 

SCREENED INTERVAL--The letter P indicates well 

that was projected more than 20 feet but less than 

100 feet to this line of geohydrologic section 

WETLAND 

A' 

FEET 

I 

I 

~ !i -90~~~--------------------~------ ------~--------------~~~~--~~--~~~~~~~ 

0 

~ Figure 30. Vertical distribution of nitrate concentrations in ground water at the radioactive-materials processing site, Wood River Junction, Rhode Island, 

~ January 1984 . 

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FINE SAND AND SILT --1-

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0 500 1,000 FEET 

I I I I I 0 100 200 300 METERS 

VERTICAL SCALE GREATLY EXAGGERATED 

I 

lagoon area 

land surface 

water table V _ p 

-t---,- p I J' 

8 

-50--

!P 

~ 

I 

I I / 

I / 

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I COARSE 

SAND AND 

GRAVEL 

Dashed where approximately located. Interval, in 

picocuries per liter, is variable. Lines of equal 

concentration are based on the maximum 

concentration at each location, regardless of 

lateral position 

SCREENED INTERVAL--The letter P indicates well 

that was projected more than 20 feet but less than 

100 feet to this line of geohydrologic section 

I 

I 

A' 

FEET 

-90 ~~~------------~--------------~--------~----------~--~--~~--~~--~~--~~--~ 

Figure 31. Vertical distribution of strontium-90 concentrations in ground water at the radioactive-materials processing site, Wood River Junction, Rhode Island, 

January 1984. 
71° 41'20" 71° 41' 

41°26'10" .-----------.--------.--'<""'"T""-------r--,----------.-------, 

EXPLANATION 

--6 -- LINE OF EQUAL ELECTROMAGNETIC 

CONDUCTANCE--Interval, in millisiemens 

per meter, is variable 

Radioactivematerials 

processing 

site 

0 

I 

0 

1,000 FEET 

I 

I 

300 METERS 

------·-150·------ LINE OF EQUAL SPECIFIC CONDUCTANCE--

Interval, in microsiemens per centimeter 

at 25 degrees Celsius, is variable 

·-·-·-·- POWER LINE 

x--x--x FENCE 

~ OBSERVATION WELL 

ELECTROMAGNETIC STATION 

WETLAND 

Figure 32. Areal distribution of electromagnetic and specific conductance at the radioactive-materials 

processing site, Wood River Junction, Rhode Island, April 1982 (from Barlow and Ryan, 1985). 

A comparison of gross beta concentrations with 

depth below land surface at wells CHW-534 and 

CHW-538 to CHW-541 (fig. 36), which are clustered at 

one location near the plant, indicates that the weB that 

was the most contaminated varied with time among the 

three shallowest weBs (CHW -534, CHW -538, and 

CHW-539). The highest concentration of gross beta 

emitters was in a sample from observation well 

CHW -534 in April of 1982; from about June 1982 to 

January 1984, samples from the two shallowest wells 

(CHW -538 and CHW -539) were the most 

contaminated. Concentrations of gross beta emitters 

in water from the deeper wells (CHW -540 and 

CHW -541) decreased with time, except for the 

anomalously high concentration in water from well 

CHW-541 inApril1982. 

The second highest concentration peak for the 

shallowest wells occurred in December 1982, 

whereas the second highest concentration peak for 

the next two deeper wells (CHW-534 and CHW-540) 

occurred 2 months later (February 1983). 

Ground-Water Quality 43 
350 16 

WELL CHW-538 

300 14 

12 

cc 

250 

w 

I- 10 ::J 

cc 200 

w 

0... 8 

(/) 

w 150 

cc 6 ::::> 

() 

0 100 

() 4 

0... 

z 

50 (/) 

z 

2 w 

0 I 

() 

I- z ~ 0 0 

cc A M J J A F M A M J J A s 0 N D I J z 

I- 1982 1983 1984 z z 

w 0 () 

z 1,000 16 

I0 ~ 

() I-

~ WELL RIW-641 14 0... 

I- 800 

() 

w w 

co 12 cc 

(/) 0... 

(/) 

0 600 10 

cc 

(!J 8 

400 6 

4 

200 

2 

0 A 0 

M J J A s 0 F M A M J J A s 0 N D I J 

1982 1983 1984 

RAIN GAGE AT STUDY AREA 

RAIN GAGE AT KINGSTON, RHODE ISLAND 

GROSS BETA CONCENTRATION 

Figure 33. Relation between precipitation and concentration of gross beta emitters in observation wells CHW-538 

near the radioactive-materials processing plant and RIW-641 near the Pawcatuck River, Wood River Junction, Rhode 

Island, April 1982 through January 1984. 

44 Ground-Water Flow and Contaminant Transport at a Radioactive-Materials Processing Site, Wood River Junction, Rhode Island 
71° 41'20" 

41°26'10" ,...------r-------~ -.--------.--...------------r------, 

EXPLANATION 

--6-- LINE OF EQUAL ELECTROMAGNETIC 

CONDUCTANCE--Interval, in millisiemens 

per meter, is variable 

Radioactivematerials 

processing 

site 

0 

I 

0 

1,000 FEET 

I 

I 

300 METERS 

--------150------- LINE OF EQUAL SPECIFIC CONDUCTANCE--

Interval, in microsiemens per centimeter 

at 25 degrees Celsius, is variable 

·-·-·-·- POWER LINE 

x-x--x FENCE 

't9_ OBSERVATION WELL 

ELECTROMAGNETIC STATION 

_.llilt_ WETLAND 

Figure 32. Areal distribution of electromagnetic and specific conductance at the radioactive-materials 

processing site, Wood River Junction, Rhode Island, April 1982 (from Barlow and Ryan, 1985). 

A comparison of gross beta concentrations with 

depth below land surface at wells CHW -534 and 

CHW-538 to CHW-541 (fig. 36), which are clustered at 

one location near the plant, indicates that the well that 

was the most contaminated varied with time among the 

three shallowest wells (CHW -534, CHW -538, and 

CHW-539). The highest concentration of gross beta 

emitters was in a sample from observation well 

CHW -534 in April of 1982; from about June 1982 to 

January 1984, samples from the two shallowest wells 

(CHW -538 and CHW -539) were the most 

contaminated. Concentrations of gross beta emitters 

in water from the deeper wells (CHW -540 and 

CHW -541) decreased with time, except for the 

anomalously high concentration in water from well 

CHW -541 in April 1982. 

The second highest concentration peak for the 

shallowest wells occurred in December 1982, 

whereas the second highest concentration peak for 

the next two deeper wells (CHW -534 and CHW -540) 

occurred 2 months later (February 1983). 

Ground-Water Quality 43 
350 16 

WELL CHW-538 

300 14 

12 

a: 

250 

w 

t- 10 ::J 

a: 200 

w 

a.. 8 

C/) 

w 150 

a: 6 ::::> 

() 

0 100 

() 4 

a.. 

z 

50 C/) 

z 2 w 

0 I 

() 

t- z ~ 0 0 

a: A M J J A F M A M J J A s 0 N D I J z 

tz 1982 1983 1984 z w 0 () 

z 1,000 16 

t0 ~ 

() t-

~ WELL RIW-641 14 a.. 

t- 800 

() 

w w 

co 12 a: 

C/) a.. 

C/) 

0 600 10 

a: 

(!) 8 

400 6 

4 

200 

2 

0 A 0 

M J J A s 0 N DIJ F M A M J J A s 0 N DIJ 

1982 1983 1984 

RAIN GAGE AT STUDY AREA 

RAIN GAGE AT KINGSTON, RHODE ISLAND 

GROSS BETA CONCENTRATION 

Figure 33. Relation between precipitation and concentration of gross beta emitters in observation wells CHW-538 

near the radioactive-materials processing plant and RIW-641 near the Pawcatuck River, Wood River Junction, Rhode 

Island, April 1982 through January 1984. 

44 Ground-Water Flow and Contaminant Transport at a Radioactive-Materials Processing Site, Wood River Junction, Rhode Island 
1-

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u. 

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0 

10 

1-

UJ 20 UJ 

u. 

~ 

ui 

(.) 

<( 

u. 

a: 

:J 

en 

Cl 

z 

~ 

~ 

0 

...J 

UJ 

CD 

I 

1-

a.. 

UJ 

Cl 

0 

10 

1-

UJ 20 UJ 

u. 

~ 

ui 30 (.) 

<( 

u. 

a: 

:J 40 en 

Cl 

z 

~ 50 

~ 

0 

...J 

UJ 60 CD 

I 

1-

a.. 

UJ 70 Cl 

sand, medium to coarse; some pebbles 

CHW-517 well screen 

sand, medium to coarse; gray 

sand, fine to medium, some very fine, some coarse; tan 

sand, fine to medium, some very fine, some coarse, some cobbles; tan 

sand, coarse; gravel 

CHW-518 well screen 

sand, medium to coarse; darker tan 

CHW-519 well screen 

sand, coarse to very coarse; some gravel 

sand, medium, some fine; some gravel, coarse; dark minerals present 

sand, medium to coarse; some fine; some gravel, coarse 

sand, medium, some fine; some gravel 

sand, medium, but proportion of fine increasing 

sand, fine, some medium 

CHW-525 well screen 

a: 

UJ 

1-

::::i 

a: 

UJ 

a.. 

en 

UJ 

(( 

:J 

(.) 

0 

(.) 

0: 

~ 

z 

0 

i= 

<( 

a: 

1-

z 

UJ 

(.) 

z 

0 

(.) 

~ UJ 

CD 

en 

en 

0 

a: 

C) 

40 

35 

30 

25 

15 

5 

0 

80 

0 

850 

800 

750 

700 

650 

600 

550 

450 

CHW-517 

screened 

interval 

47-50 feet 

CHW-518 

screened 

interval 

58-61 feet 

CHW-519 

screened 

interval 

78-81 feet 

CHW-525 

screened 

interval 

91.5-94 feet 

20LL~~-LLL~~LL~~~LL~~-LLL~~~ 

1981 1982 1983 11984 

Figure 34. Relation between depth below land surface and concentration of gross beta emitters in selected wells near the 

Pawcatuck River, Wood River Junction, Rhode Island, June 1981 through January 1984. (Screened interval is in feet below 

land surface.) 

Ground-Water Quality 45 
RIW-656-659 SEDIMENT DESCRIPTION 160 0 

sand, medium; some gravel, coarse 140 RIW-656 

1- no return; black water; peat screened w 

w 10 RIW-656 well 120 interval screen lL 7-10 feet 

~ sand, medium to coarse; dark minerals present 100 

ui 20 RIW-657 well screen () 80 

~ a: 

::::> 30 60 

en RIW-658 well screen 0 40 z 

j 40 20 

:5: sand, medium to coarse; some fine 

0 RIW-659 well 0 screen _J 

w 50 co 

I 

1- 400 a. 

w 60 0 RIW-657 350 

screened 

70 300 interval 

a: 17-20 feet 

w 250 1-

::::i 200 Figure 35. Relation between depth below land surface and a: 

w 

concentration of gross beta emitters in selected wells in the a. 150 

swamp at the radioactive-materials processing site, Wood en 

w 

River Junction, Rhode Island, January 1983 through a: 100 

::::> 

January 1984. (Screened interval is in feet below land () 50 0 

surface.) () 

0.:: 0 

~ 

These peaks probably are a result of additional z 0 400 i= contaminants being flushed from the unsaturated zone <( 

350 RIW-658 a: 

when sediment was removed in the lagoon area during 1- screened z 

w 300 interval 

1981-82. Concentration of gross beta emitters decreased () 

z 27-30 feet 

in the three deepest wells subsequent to February 1983. 0 250 () 

Concentrations increased slightly for the shallowest two <( 200 1-

w 

wells in the last quarter of 1984. Differences among each co 150 en 

en 

of the graphs in figure 36 also could be explained by 0 100 a: 

heterogeneities in aquifer materials and faster cleanout (!) 

50 

times for zones containing coarse sands and gravels. 0 

The above observations for the clusters of wells near the 

river, near the swamp, and near the plant (figs. 34-36) 80 

illustrate the difficulty of interpreting observation-well- RIW-659 

response data. Confounding factors-such as flow-field screened 

60 interval 

variations, source-strength variations, and apparent 42-45 feet 

variations caused by sampling and analytical or reporting 

errors-precluded the use of time-series analysis to 

identify specific responses with their causes. 

Regression analyses were done on paired sets of 

water-quality variables for various conditions, including 

ranges of concentrations and groups of wells. The analyses 0 

were based on data collected through February 1983. 1983 11984 

46 Ground-Water Flow and Contaminant Transport at a Radioactive-Materials Processing Site, Wood River Junction, Rhode Island 
1-

w 

w 

LL 

~ 

ui 

(.) 

~ a: 

::J 

CJ) 

0 

z 

:5 

s: 

g 

w 

CD 

I 

1-

a.. 

w 

0 

a: 

w 

1-

:J 

a: 

w 

a.. 

CJ) 

w 

~ 

::J 

(.) 

0 

(.) 

a: 

~ 

z 

0 

~ a: 

1-

z 

w 

(.) 

10 

20 

30 

40 

50 

60 

70 

80 

300 

250 

200 

150 

sand, fine, some medium; some pebbles 

sand, medium, some coarse; gravel; pebbles 

CHW-538 well screen 

CHW-539 well screen 

sand, very coarse; gravel; pebbles 

CHW-534 well screen 

CHW-540 well screen 

CHW-541 well screen 

screened 

interval 

22-25 feet 

a: 

w 

1-

:J 

a: 

w 

a.. 

CJ) 

w 

~ 

::J 

(.) 

0 

(.) 

a: 

~ 

z 

0 

~ a: 

1-

z 

w 

(.) 

z 

0 

(.) 

~ w 

CD 

CJ) 

CJ) 

0 

a: 

(.!) 

CHW-534 

screened 

interval 

42-45 feet 

screened 

interval 

52-55 feet 

CHW-541 

screened 

interval 

72-75 feet 

z 

0 

(.) 

o~-L~~~~~~~-L~~-L~~~~~~ 

~ CD 

CJ) 

CJ) 

0 

a: 

(.!) 

screened 

interval 

32-35 feet 

o~-L~~~-L~~~~~~~~~~~-L~~ 

1982 1983 11984 

1982 1983 

Figure 36. Relation between depth below land surface and concentration of gross beta emitters in selected wells near 

the radioactive-materials processing plant, Wood River Junction, Rhode Island, January 1982 through January 1984. 

(Screened interval is in feet below land surface.) 

11984 

Ground-Water Quality 47 
One group of wells consisted· of all wells in and 

adjacent to the plume. The other two groups 

consisted of wells in and adjacent to the plume near 

the plant and near the river. The group of all wells 

contained more wells than the sum of the plant and 

river well groups. Although strontium-90 is not 

present in sufficient concentrations to provide a 

measurable contribution to specific conductance, 

we know that strontium-90, gross beta, nitrate, and 

high specific conductance are associated with the 

contaminated ground water at this site. A strong 

correlation between two water-quality variables 

might provide an opportunity for reducing the 

number of water-quality analyses necessary for the 

study. 

Generally, the correlations were highest in data 

for wells near the river rather than in data for wells 

near the plant, and correlations for these groups 

were higher than for the group of all wells (table 6). 

From the regressions, the measure of correlation 

was the coefficient of determination, ,2. For wells 

near the river, the correlation (?=0.96) was highest 

between specific conductance and nitrate plus 

nitrite. Strontium-90 and gross beta also were 

highly correlated (?=0.91) for this group of wells 

(fig. 37A). For wells near the plant, the correlation 

(?=0.90) was highest between specific conductance 

and strontium-90 (fig. 37 B). For all wells, the 

correlation (?=0.86) was highest between specific 

conductance and nitrate plus nitrite (fig. 37C). As a 

result of these regression analyses, we were able to 

reduce the frequency of the more costly strontium90 analyses by using the less costly gross beta and 

nitrate analyses as an indicator of contamination. 

Sediment samples were collected and analyzed 

to determine the presence of sorbed chemical and 

radiochemical constituents. Locations of sampling 

points and concentrations of these constituents are 

shown in Ryan and others (1985, p. 269-270). 

Generally, naturally occurring radionuclides 

(including potassium-40, lead-210, radium-226, 

radium-228, thorium-228, uranium-235, and 

uranium-238) were detected at background 

concentrations. Background concentrations of 

gross alpha and gross beta emitters tended to be 

higher in the fine sand and silt deposits than in the 

coarse sand and gravel deposits. Concentrations of 

gross beta emitters and gross alpha emitters (2.5 and 

6.7 pCi/g, respectively) were as high as four and ten 

times those of background, respectively, at well 

Table 6. Correlation coefficients resulting from regression 

analyses for selected water-quality variables and well 

clusters at the study site, Wood River Junction, Rhode 

Island 

[Because the highest nitrite concentration in ground water at the site was 

only 0.16 mg/L, it is assumed that nitrate plus nitrite consists almost 

entirely of nitrate] 

Specific Specific 

Well cluster conductance conductance Stronti u m-90 

locations and and and 

nitrate plus strontium-90 gross beta 

nitrite 

All wells 0.86 0.61 0.59 

Wells near .86 .90 .88 

plant 

Wel1s near .96 .94 .91 

river 

CHW-516 (an upgradient well near the till boundary). 

Sediments from this well were sampled in medium to 

coarse sands 60 to 65 ft below the land surface just 

above till or bedrock. Gross alpha concentration at 

well CHW-470 was 0.69 pCi/g. 

Sediment samples from one location in the swamp 

and two from the bed of the Pawcatuck River near the 

plume of contaminated ground water yielded 

concentrations of gross alpha emitters that ranged from 

5 to 14 times that of background and gross beta 

emitters that ranged from 4 to 22 times that of 

background. Concentrations of gross alpha emitters 

ranged from 4.09 to 10.7 J..Lg/g and gross beta emitters 

ranged from 2.56 to 13.8 pCi/g. 

Sorbed strontium-90 was detected in two samples 

from the riverbed sediments on the west side of the 

river. These two samples and one from where the 

ground water was contaminated are the only sediment 

samples where strontium-90 was measured. Although 

20x1 o-2 pCi/g was the maximum strontium-90 

concentration measured, our laboratory work showed 

that the amount of strontium-90 expected to be sorbed 

on the sediment was less than 4x1 o-2 pCi/g. Because 

this expected concentration was near the detection 

limit (3.3xl o-2 pCi/g) of the analytical capability for 

strontium-90, only a few strontium-90 analyses were 

made. Measured and calculated strontium-90 

concentrations sorbed on the sediments are discussed 

further in the section of this report on "Strontium-Ion 

Exchange." 

48 Ground-Water Flow and Contaminant Transport at a Radioactive-Materials Processing Site, Wood River Junction, Rhode Island 
300 I I I I 6 ffi 6,000 Iw 

A 0 ~ c 0 

0 i= 

z 250- 0 0 - ffi 5,000 0 0 

(..) 0 

0 0 a: 0 0 0 

~ffi 00 w 0 

a: I- a.. 0 0 

I- :::J 200 f- - ~ 4,000 Za: Ww w 

Oa.. ~ 8800 0 6(/) w 

~ 3,000 0 0 (..)!:!:! 150 f- - 0 Oc:POo0o 00 0 0 oa: a: 0 0 0 

0 0&,0 #': en::> 0 0 0 (..) 

~ 000 0 '(..) 

~0 ~ 0 0 0 

~(..) z 0 0 0 

~a: 100~ - ;:J 2,000 Oo 0 0 ($) 0 

oz 0o 8 go 

0 0 

(..) a:- z 000 I- <( 8o 0 (/) 0 I- 0 

50 f- - g 1,000 0 80 0 oo 0 00 0 Cl 0 z 0 oo 0 

0~~ I I I I I (..) 

(..) 0 

0 200 400 600 800 1,000 1,200 u::: 0 200 400 600 800 

u 

CONCENTRATION OF GROSS BETA, IN PICOCURIES PER LITER w NITRATE CONCENTRATION, IN MILLIGRAMS PER LITER a.. 

(/) 

130 

B 0 

Figure 37. Relation between (A) concentrations of strontium-90 

z and gross beta emitters in wells near the Pawcatuck River, (B) 

0 0 strontium-90 concentrations and specific conductance in wells 

t=a: 0 <(W near the radioactive-materials processing plant, and (C) specific a: I1-:::J conductance and nitrate concentrations in all wells, Wood River Za: Ww Junction, Rhode Island. Oa.. 0 ZCI) 

Ow 0 (..)_ 

oa: was over) and submitted for multielement analysis by en::> co 0 '(..) :::!:o 

~(..) 8 

I-- proton-induced X-ray emission. Preliminary analysis za.. 

oz 0 00 a:- of the data indicated that the trees growing in the 0 0 00 I-

(/) 

swamp over the discharge area of contaminated 

ground water were enriched in strontium in the growth 

1,000 2,000 3,000 4,000 5,000 6

•

000 rings corresponding to the years 1973-75 (D.A. 

SPECIFIC CONDUCTANCE, IN MICROSIEMENS PER CENTIMETER 

Additional radionuclide data from a sample 

collected in the swamp by Oak Ridge Associated 

Universities indicated 2.76 pCi/g of uranium-235, 

22.0 pCi/g of uranium-238, and 6.86 pCi/g of cesium137 had sorbed to sediments (Linda Sowell, Oak Ridge 

Associated Universities, written commun., 1984). A 

cesium-137 concentration of 2.67 pCi/g detected in 

vegetation collected from the same sampling location at 

the same time indicates that bioaccumulation could be 

occurring at the site in areas where the root zone 

intersects contaminated ground water. Other studies 

have shown that ground-water contamination can be 

reflected in the chemistry of tree growth rings 

(Vroblesky and Yanosky, 1990; Vroblesky and others, 

1992; Yanosky and Vroblesky, 1992). Sample cores of 

several oak trees in the swamp were collected in 

August, September, and December 1988 (after the study 

Vroblesky, U.S. Geological Survey, written commun., 

July 1990). 

Strontium-len Exchange 

The aqueous chemistry of strontium is similar to 

that of the other alkaline-earth elements. Generally, 

adsorption of strontium by sediments increases with 

pH (McHenry, 1958) and decreases as the 

concentrations of other alkaline-earth elements 

increase (Chittenden, 1983). Adsorbed strontium can 

be removed from soil by leaching with calcium 

solution (Lagerwerff and Kemper, 1975). Formation of 

aqueous complexes such as SrC03°, SrAC03+, and 

SrS04° increases the mobility of strontium. Because of 

their small charge density (charge/ionic size), these 

complexes can be expected to sorb and (or) exchange 

much less readily than s~+. 

Ground-Water Quality 49 
The composition of ground water associated 

with the three sediment samples analyzed is listed in 

table 4. Dissolved-solids concentrations in the 

uncontaminated ground water, as inferred from the 

specific-conductance measurements, were low. 

Calcium and nitrate were the major ions in the 

contamination plume. Background strontium-90 

concentration was 0.55 pCi/L, with an asymmetric 

confidence interval of 0.08-1.9 pCi/L based on 31 

measurements-these numbers were caJculated taking 

into account the truncation effect of the detection limit 

for strontium-90 (0.4 pCi!L). Total strontium 

concentration was 0.02 mg!L in the uncontaminated 

sample. (In all subsequent discussions, total strontium 

will be referred to as "strontium," and strontium-90 

will be noted accordingly.) The highest measured 

concentration of strontium-90 was 250 pCi/L, with a 

confidence interval of 225-275 pCi/L, with an 

associated strontium concentration of 1.6 mg/L. 

Composition of a ground-water sample collected from 

a zone of partial contamination also is shown in table 4. 

The concentration ratios among the major ions 

and of strontium between the most contaminated 

and partially contaminated ground-water samples 

were virtually constant (5.1 to 5.3) with the exception 

of sodium. This indicated a characteristic proportion 

of the major ions and strontium in contaminated 

ground water. The ratios of the concentrations of the 

major ions and of strontium, between the most 

contaminated and uncontaminated ground-water 

samples, however, ranged from 2.4 to 50, indicating a 

different chemical-ratio signature in the natural system. 

Sediment and water-quality analyses from sampling 

locations between the plant and the river indicate little 

sorption of chemical and radiochemical constituents on 

aquifer materials. Cation-exchange capacities from 

five additional split-spoon samples collected at 

different locations ranged from 0.1 to 4.2 meq/100 g, 

with a median value of 0.5 meq/1 00 g (Ryan and Kipp, 

1985). In the swamp, however, reducing conditions 

may promote additional solute interaction with 

sediments or organic material once the plume rises to 

hmdl surface. 

Laboratory experiments with ground-water 

dilutions of 20:1, I 0:1, 5:1, 1:1, 0:1 (uncontaminated:contaminated) were used to simulate soluteconcentration ranges in the aquifer. Results indicated 

that Htde strontium was adsorbed by the sediment 

(table 7). Strontium accounted for about 0.2 percent of 

tlhe exchangeable cations on the sediments. Calcium 

accounted! for about 85 percent of the exchangeable 

cations; combined magnesium and sodium accounted 

for about 10 percent. Potassium was not measured on 

every sample because of analytical difficulties; 

however, several analyses indicated that potassium 

accounted for as much as 5 percent of the exchangeable 

cations. For the laboratory experiments, however, 

potassium was eliminated from the set of exchangeable 

cations studied. 

For the contaminant-transport modeling work 

discussed later, a simplified binary-exchange 

mechanism was adopted to describe the strontium 

interaction with the sediments. In support of this 

model, selectivity coefficients (table 7) were calculated 

for the exchange of strontium with calcium plus 

magnesium plus sodium, according to Rubin and 

James ( 1973), Schweich and Sardin ( 1981 ), and Grove 

(1984): 

Sr w

1

Src

1

Ca + Mg + Na 

Kca+Mg+Na = -----=--, 

c

1

Srw

1

Ca + Mg + Na 

(2) 

where 

I d I 

Csr an cCa+Mg+Na 

I d I 

W s r an w Ca + M g + N a 

are the concentrations of 

strontium and calcium 

plus magnesium plus 

sodium in the fluid phase 

(meq/L); and 

are the concentrations of 

strontium and calcium 

plus magnesium plus 

sodium in the sorbed 

phase on the porous 

matrix (meq/g). 

As shown in table 7, selectivity coefficients range 

from 1.5 to 2.2; the mean is 1.8. The ratio of adsorbed 

calcium plus magnesium plus sodium to adsorbed 

strontium averaged 500:1. This high ratio can be 

attributed to the low concentration of strontium in 

solution. The ratio of calcium plus magnesium plus 

sodium to strontium in solution ranged from 800:1 

for uncontaminated ground water to 1,000:1 for 

contaminated ground water. Although the ratios of the 

concentrations of calcium, magnesium, and sodium to 

each other were constant in the laboratory solutions for 

each set of solutions used for the sediment samples, the 

ratios were not constant in the field. To illustrate this, 

the ratio of calcium to magnesium to sodium in 

uncontaminated ground water was 4.1 to 1.0 to 4.9, and 

in the most contaminated ground water was 30.0 to 1.0 

to 1.0 (calculated from data in table 4). The range of 

selectivity coefficients in table 7 indicated a low 
Table 7. Concentrations of combined major cations (calcium, magnesium, and sodium), and of strontium adsorbed by 

sediment samples, Wood River Junction, Rhode Island 

[meq/100 g, milliequivalent per 100 grams; meq/mL, milliequivalent per milliliter; Ca, calcium; Mg, magnesium; Na, sodium; Sr, strontium] 

Sediment- Ratio of Adsorbed concentration Solution concentration Selectivity 

sample uncontaminated (meq/100 g) (meqlml) coefficient, 

identifier to contaminated Ca+Mg+Na ground water 

CHW-559-3 Uncontaminated 0.15 

20:1 .16 

10: I .17 

5:1 .18 

1:1 .19 

Contaminated .19 

CHW-559-4 Uncontaminated .26 

20:1 .24 

10:1 .25 

1:1 .27 

Contaminated .23 

CHW-576 20:1 .30 

10:1 .33 

5.4: I .28 

1:1 .30 

Contaminated .31 

sensitivity ofthe selectivity coefficient to changes in the 

calcium plus magnesium plus sodium ratio. Therefore, 

selectivity coefficients are not expected to vary greatly 

over the site. 

The selectivity coefficient (table 7) was virtually 

constant for each sediment sample for all ratios of 

uncontaminated to contaminated ground water. Differences in selectivity coefficient and adsorbed strontium 

concentration between sediment samples resulted from 

different cation-exchange properties (table 2). Calculated cation-exchange capacities on the three samples 

used in this study ranged from 0.18 to 0.30 meq/1 00 g, 

with a mean of0.24 meq/100 g, whereas Ryan and Kipp 

( 1985) reported cation-exchange capacities that ranged 

from 0.1 to 4.2 meq/1 00 g, with a median of 0.5 

meq/100 g based on the five sediment samples previously mentioned. Although sand-sized particles (0.06 to 

2 mm) predominated in each sample, the samples were 

obtained from different horizontal and vertical locations 

within the aquifer. 

The interaction of a solute with sediment is 

commonly characterized by the distribution coefficient, 

Kd (Freeze and Cherry, 1979, p. 403). Distribution 

coefficients for strontium (table 8) were calculated from 

the data in table 7 by use of the following equation 

(Freeze and Cherry, 1979): 

Sr 

3.9xl04 

4.2xl0-4 

3.4xl04 

3.5xl04 

3.7xl04 

3.5xl04 

4.8xl04 

5.0x104 

4.5xl04 

4.7xl04 

3.7xl04 

6.4xto-4 

6.9x104 

4.8xl04 

5.9x104 

6.2xl04 

where 

w'sr 

c'sr 

Ca+Mg+Na Sr 

3.7xi04 4.5xi0-7 

2.7xi0-3 3.2xto-6 

4.8xto-3 5.6xi0-6 

9.2xi0-3 9.3xi0-6 

2.2xi0-2 2.2xi0-5 

4.2xw-2 4.lxi0-5 

3.7xi04 4.5xw-1 

2.7xi0-3 3.2xto-6 

4.8xto-3 5.6xto-6 

2.2xto-2 2.2xi0-5 

4.2xi0-2 4.Ixto-5 

3.7x104 4.5xi0-7 

2.7x1o-3 3.2xi0-6 

4.8xi0-3 5.6x1o-6 

2.2x1o-2 2.2xi0-5 

4.2xi0-2 4.1xi0-5 

w'sr 

Kd=-,-, 

c Sr 

KSr Ca+Mg+Na 

2.1 

2.2 

1.7 

1.9 

1.9 

1.9 

1.5 

1.6 

1.5 

1.7 

1.6 

1.8 

1.8 

1.5 

2.0 

2.0 

(3) 

is the concentration in the sorbed phase on 

the porous matrix (meq/g); and 

is the concentration of strontium in the fluid 

phase (meq!L). 

The range of strontium distribution coefficients 

listed in table 8 was a function of the concentration of 

strontium and the other cations in solution. The 

amount of strontium adsorbed was virtually constant 

for each sediment sample (table 7); however, the 

concentration of strontium and the other cations in 

solution increased as the percentage of contaminated 

water in the sample increased. As a result, strontium 

distribution coefficients decreased. 

The strong dependence of the distribution coefficient on the total cation concentration in the ground 

water indicated competition between strontium and 

other cations for exchange sites. The greatest proportion of strontium was sorbed from the uncontaminated 

ground water where the concentrations of the other 

cations were the lowest. These data indicate that the 

Ground-Water Quality 51 
Table 8. Equilibrium distribution coefficients for strontium and calculated strontium-90 concentrations adsorbed by sediment 

samples, Wood River Junction, Rhode Island 

[Strontium distribution coefficient calculated from data in table 7 by use of equation 3 from this report (Freeze and Cherry, 1979). mUg, miililiter per gram; 

pCi/L, picocurie per liter; pCilg, picocurie per gram] 

Strontium Calculated 

Sedimentsample 

Identifier 

Ratio of 

uncontaminated to distribution Measured strontium-90 in strontium-90 

contaminated coefficient, solution adsorbed 

ground water 

CHW-559-3 Uncontaminated 

20:I 

IO:I 

5:I 

I: I 

Contaminated 

CHW-559-4 Uncontaminated 

20:1 

10:1 

1:1 

Contaminated 

CHW-576 20:I 

10:I 

5.4:1 

I: I 

Contaminated 

1 Measured value for strontium-90. 

use of a constant distribution coefficient to characterize the interaction of strontium-90 with these outwash 

sands is not realistic. This result is consistent with 

Reardon's ( 1981) discussion of equilibrium distribution coefficients and their applicability to contaminant 

migration. 

Most of the experimental results (tables 7 and 8) 

were obtained for strontium. Corresponding 

concentrations for strontium-90 in the solution phase 

also were measured and are shown in table 8. The 

concentration of strontium-90 adsorbed on the 

sediment samples can be calculated from equation 3, 

and results are listed in table 8. 

Concentrations of strontium-90 adsorbed by the 

sediments were, for the most part, less than 

33x1 o-3 pCi/g, the detection limit of the radiochemical 

procedure for the obtainable sample size. Thus, direct 

measurement of the amount of strontium-90 adsorbed 

was not practical. The one exception was a sediment 

sample at CHW-576, at a dilution ratio of 5.4:1. Both 

solution and sorbed concentrations of strontium-90 

Klr (pCill) by sediment 

(mUg) {pCilg) 

8.5 0.55 4.7x1o-3 

1.3 I2 I6xl0·3 

.61 25 15x10·3 

.3~ 50 I9x10·3 

.17 I25 2Ixl0·3 

.085 250 21x10·3 

10 .55 5.7xl0·3 

1.5 12 18xl0·3 

.81 25 29xl0·3 

.22 125 28xl0·3 

.090 250 22x10·3 

2.0 12 24xl0·3 

1.2 25 30xJo-3 

.69 (1.71) 46 32( 1

33)x I o-3 

.27 125 34xl0·3 

.15 250 38xl0·3 

were measured on a core collected from a partly 

contaminated zone of the aquifer. The strontium 

distribution coefficient (Kdsr) from the strontium-90 

measurements agreed closely with the value obtained 

from strontium measurements. 

A strontium-90 contamination plume at Chalk 

River, Ontario, Canada, has been extensively studied 

by Jackson and Inch (1980), Patterson and Spoel 

( 1981 ), and Pickens and others ( 1981 ). Patterson 

and Spoel ( 1981) determined distribution coefficients 

for strontium (2 to 36 mL/g) in the laboratory on 

sediments from a site at Chalk River, Ontario, Canada, 

with clean quartz sand sediments that are quite similar 

to those at this site. These findings correlated with 

the results of Jackson and Inch (1980, 1983), who 

observed that most (about 80 percent) of the 

strontium-90 could be reversibly desorbed by 

0.1 M SrC12 solution. Jackson and Inch (1980, 1983) 

assumed that the exchangeable fraction was sorbed by 

vermiculite and feldspar. The remaining 20 percent of 

the sorbed fraction of strontium-90 was associated 

with extractable iron, aluminum, and manganese. 

52 Ground-Water Flow and Contaminant Transport at a Radioactive-Materials Processing Site, Wood River Junction, Rhode Island 
They concluded that strontium-90 was specifically 

adsorbed to oxides of the metals. Patterson and Spoel 

(1981) also measured an increase in the strontium 

distribution coefficient with a decrease in 

exchangeable calcium. 

The geology of the study area is similar to that of 

the Chalk River site; thus, one would expect that the 

strontium-90 solute/sediment interaction would be 

similar. This was not the case, however, because the 

corresponding concentrations of the other cations in 

the contamination plumes were substantially different. 

Selected transport constituents for the study site and 

the Chalk River site are presented in table 9. Cationexchange capacities measured by Jackson and Inch 

( 1980) at the Chalk River site ranged from 0.3 to 1.8 

meq/1 00 g. The selectivity coefficient values for the 

study site, calculated from the experimental data in 

table 3, ranged from 1.5 to 2.4 and were similar to the 

range of 1.5 to 2.1 calculated for the Chalk River site 

by Patterson and Spoel ( 1981 ). At the Chalk River site, 

calcium accounted for about 75 percent of the 

exchangeable cations, compared to 85 percent at the 

study site. 

Values of the distribution coefficient for strontium 

ranged from 2.8 to 20 mL/g for contaminated 

sediments and surrounding ground water at the Chalk 

River site (Jackson and Inch, 1980; Patterson and 

Spoel, 1981 ; Seguin, 1982). The range at the study site 

(shown in table 8), was 0.085 to I 0 mL/g, and the mean 

was 1.7 mL/g. This mean is somewhat lower than the 

means of 10 mL/g (Patterson and Spoel, 1981) and 4.9 

mL/g (Pickens and others, 1981) at the Chalk River 

site. Data from this study confirmed the effect of an 

increase in strontium distribution coefficient with a 

decrease in exchangeable calcium observed by 

Patterson and Spoel (1981) at the Chalk River site. 

Calcium concentrations in ground water were as 

much as two orders of magnitude greater than those at 

the Chalk River site; magnesium, sodium, and 

potassium concentrations were about an order of 

magnitude greater. As a result, competition for ionexchange sites in study-area sediments is greater, and 

this could account for the smaller amount of strontium90 interaction with the sediments at this site relative to 

the Chalk River site. These experimental results were 

used to develop a one-dimensional model of strontium 

transport, which is discussed later in the report. 

Table 9. Selected transport constituents for the study site, 

Wood River Junction, Rhode Island, and the Chalk River 

site, Ontario, Canada 

[meq/100 g, milliequivalent per 100 grams; mUg, milliliter per gram] 

Constituent Range at 

or property study site 

Cation-exchange 0.1-4.2 

capacity (meq/1 00 g) 

Selectivity coefficient, Ksr 1.5-2.4 

Calcium, as percentage of 85 

exchangeable cation 

Strontium distribution 0.085-10 

coefficient, K}r (mUg) 

1

From Jackson and Inch ( 1980). 2From Patterson and Spoel ( 1981 ). 

Range at 

Chalk River 

site 

1

0.3-1.8 

21.5-2.1 

75 

2.8-20 

NUMERICAL SIMULATION OF GROUNDWATER FLOW AND CONTAMINANT 

TRANSPORT 

Numerical simulation was used as a tool to aid in 

the understanding of the hydrologic environment and 

contaminant transport at the site. It also was used to 

address questions that pertain to effects of groundwater development. The primary goals of the groundwater-flow and solute-transport modeling were to (1) 

simulate numerically, at a field scale, the transport of 

strontium-90, which interacts with the sediments in the 

aquifer and is affected by concentrations of other 

constituents in the contaminated ground water; (2) use 

the model to evaluate the fate of the contamination 

plume with regard to several chemical and 

radiochemical species of interest; and (3) use the 

model to indicate to what extent the ground-water 

resources of this region could be developed without 

encountering unacceptable levels of contamination. 

Results from previously described laboratory studies 

were used during model development to determine 

mechanisms for solute-sediment interaction. 

Approach and Limitations 

The potentiometric surface maps and maps 

showing the location of the contamination plume 

indicate that the flow field has a highly threedimensional structure. This three-dimensional 

structure can be only approximated by areal and 

Numerical Simulation of Ground-Water Flow and Contaminant Transport 53 
cross-sectional simulations of ground-water flow. 

Whereas the study of the flow field with regard to 

water-quantity balance probably can be treated 

adequately by two-dimensional models, realistic 

representation of contaminant transport commonly 

requires three-dimensional models. However, data 

requirements for full three-dimensional simulations of 

the entire region made such simulations infeasible for 

this study. Computer-storage and execution-speed 

limitations at the time of this study also limited the 

degree of realism that could be attempted in threedimensional flow and transport simulations for this 

site. Therefore, 2 two-dimensional flow simulations 

(areal and cross-sectional) were done, and a onedimensional solute-transport simulation also was done. 

A first estimate of time required for contamination 

to move naturally out of the aquifer and into the 

Pawcatuck River once plant operations had ceased was · 

obtained from the two-dimensional models by 

calculation of the traveltime of a particle from the 

source area to the river at the farthest downstream point 

where contamination was detected. This estimate 

provided an advective transport time that was realistic 

if dispersive transport and solute-sediment interaction 

could be neglected. Dispersive transport effects can 

lengthen the amount of time required for natural 

cleanout of the aquifer; thus the advective traveltimes 

represent a lower limit for that time estimate. The 

laboratory studies discussed previously indicated very 

little interaction between the strontium-90 and the 

sediments. As previously stated, it is believed that there 

is no interaction between the nitrate or the technetium99 and the aquifer sediments. 

A numerical simulator is a simplified representation 

of the actual system. The lack of complete knowledge 

of parameter distributions (such as hydraulic 

conductivity and specific yield) and of boundarycondition flow rates (such as rates of infiltration and 

evapotranspiration) means that no simulation will 

reproduce exactly the field measurements of 

potentiometric heads. With regard to solute transport, 

discrepancies between model results and field 

conditions can be even greater because the 

uncertainties and lack of data in the ground-water-flow 

model are incorporated into the solute-transport model 

as uncertainties in the interstitial velocity field, which 

controls the advective transport rate and direction and 

the dispersion coefficient distribution. The other 

transport parameters for which little or no site-specific 

data are available are the longitudinal and transverse 

dispersivities, the effective porosity, and parameters 

characterizing the solute interaction with the porous 

media. The initial- and boundary-condition 

distributions for the solutes, which include information 

on the chemical and radiochemical source infiltration 

rates, also are not completely known. 

Therefore, any numerical simulation of groundwater flow and solute transport is necessarily limited in 

its ability to realistically portray hydrologic 

interactions in the field. If, however, a sufficient 

amount of parameter information is available, 

numerical simulation can be useful in quantifying 

mechanisms and relations in the region under study and 

in testing hypotheses about them. Such was the role of 

the numerical simulation in this study. 

Simulation of Ground-Water Flow 

The first step for simulating ground-water flow for 

the study area was to describe our conceptual model of 

the hydrologic system. Inflow and outflow fluxes of 

the system were identified. Water enters the system 

from recharge of precipitation, ground-water inflow 

from surrounding areas (till upland area, for example) 

and surface-water inflow (fig. 38). Water leaves the 

system by evapotranspiration from the swamp, 

evaporation from the river, ground-water outflow to 

surrounding areas, surface-water outflow, and 

discharge from the plant supply welL There is an 

exchange of water from the ground-water system to 

and from the surface-water system by leakage through 

the river bottom. We assumed that no flow took place 

across the aquifer bottom boundary. Two different 

configurations were specified for the lower boundary. 

Theory and Assumptions 

The numerical simulation of ground-water flow for 

this study site consisted of the computation of 

numerical solutions to the equation of saturated 

ground-water flow. For an assumed isothermal 

situation of constant fluid density, this equation was a 

combination of Darcy's equation of flow in porous 

media and the equation of conservation of volumetric 

flow (Bear, 1972). 

54 Ground-Water Flow and Contaminant Transport at a Radioactive-Materials Processing Site, Wood River Junction, Rhode Island 
The general equation was reduced from three to 

two dimensions and applied to both areal and crosssectional simulation regions. The areal equation was 

obtained by integrating over the vertical direction 

from the bottom of the aquifer to the water table or 

free surface. The cross-section equation was obtained 

by assuming that the gradients perpendicular to the 

cross-section plane were zero. 

A ground-water-flow simulation code developed 

by Cooley (1974) was selected for the twodimensional flow simulations. Certain modifications 

were made to include unconfined-aquifer modeling, 

river-aquifer interaction, comparison of calculated 

and measured heads, and some graphical output. 

Suface-water 

outflow 

Recharge 

of 

precipitation 

These modifications are detailed and the expanded 

input-data file structure is contained in Appendix I (on 

the enclosed diskette). 

This flow simulator is based on finite-difference 

representation of the partial differential equation 

derived by the subdomain collocation method 

described by Cooley (1974). The grid of node points 

can have variable node spacing, and a pointdistributed or lattice-centered grid is used as opposed 

to a cell-centered grid. Aquifer properties are specified 

by zones that are subregions bounded by the nodes. 

Assembly of the nodal equations is by an integrated 

subdomain method whereby the influence of each 

zone that pertains to a given node is summed into the 

coefficients for the equation for that node. This 

Evaporation 

Suface-water 

inflow 

inflow 

Discharge from 

plant supply well 

Not to scale 

Figure 38. Conceptual model of the hydrologic-flow system, Wood River Junction, Rhode Island. 

Ground-water 

inflow from till 

Numerical Simulation of Ground-Water Flow and Contaminant Transport 55 
resembles a finite-element method for definition of 

aquifer properties and equation assembly. Distributed 

recharge, leaky aquifer boundaries, point- or linesegment specified sources and (or) sinks, and riveraquifer leakage boundaries are available features. 

Specified-head and no-flow boundary conditions can 

be selected. Specified-flux boundary conditions are 

included in the line segment source and (or) sink terms. 

The selected version of the code makes use of the 

strongly implicit method of Stone ( 1968) for solution 

of the equations. The convergence tolerance for this 

iterative equation solver was set at 1o-4 ft. 

For calibration purposes, steady-state simulations 

were made in an attempt to represent average flow 

conditions for 1 year (1983) of measured data. A 1-

year average of measured potentiometric head was 

done for several reasons. Although seasonal 

fluctuations were observed, long-term average flow 

conditions were assumed to be primarily responsible 

for the observed contamination configuration. 

Calendar year 1983 represented the most complete year 

of spatial data; however, water levels and precipitation 

were somewhat higher in 1983 than the long-term 

average for the area (52 versus 48 in/yr). Data from 

RIW -231, a long-term monitoring well about 2 mi 

northwest of the site, were used to place hydrologic 

conditions in 1983 in context with hydrologic 

conditions back to 1955. The 10 years up to 1983 was 

a period of gradually rising water levels (on the order 

of 3 ft), with the exception of 1981 when the well was 

dry for the entire calendar year. The measured water 

level in May 1983 was the highest water level of record 

but similar water levels were measured in 1984. After 

the steady-state simulations, transient-flow simulations 

for a 20-year period were done to investigate the effects 

of development of a proposed water supply. 

Areal-Flow Model 

The areal ground-water-flow model was based on 

unconfined or free-surface flow; no confined 

conditions were found in the study area. The flow 

equation was averaged over the aquifer thickness (from 

the bottom of the aquifer to the free surface) and nearly 

horizontal flow and isotropic permeability were 

assumed. The following flow equation (Bear, 1972; 

Cooley, 1974) is obtained for a cartesian coordinate 

system: 

a ( ah) ah 

ax. K(h-b)ax. +L+l+Q = Sy at (4) l l 

i = 1, 2, 3 with summation convention, 

where 

x; are coordinate directions (L), 

K is isotropic hydraulic conductivity (Lff), 

h is hydraulic potentiometric head averaged 

over the aquifer thickness (L), 

b is aquifer bottom elevation (L), 

L is leakage from an adjacent aquifer or river 

(L3/L2T), 

I is infiltration rate (L3/L 2 T), 

Q is point or line source (L3/L2T), 

S.v is specific yield, and 

t is time (T). 

The mesh was constructed by use of a pointdistributed grid with unequal node spacing. This 

spacing ranged from I 00 to 250ft, with the finest 

divisions in the area of the contamination plume 

(fig. 39). 

The right-hand side of the equation is set to zero 

for steady-state simulations. Appropriate boundary 

conditions can include specified hydraulic potential, 

specified fluid flux, precipitation infiltration flux, 

and leakage from an adjacent aquifer, river, or lake. 

Initial conditions consist of the potentiometric head 

distribution at the beginning of the simulation for a 

transient simulation or any reasonable potentiometric 

distribution for a steady-state simulation. 

The boundary conditions for the areal flow model 

(fig. 39) were specified as follows: along segment A-B, 

specified head; along segment B-C, specified flux~ 

along segment C-D, specified head, and along segment 

D-A, specified head. Boundary B-C approximates the 

till outcrop above land surface so the specified-flux 

distribution along this boundary was estimated by 

determining the surface-water drainage area for each 

boundary segment up to the crest of the hills and then 

by assuming a net recharge per unit surface area to 

compute an average flow rate for the year. The actual 

values associated with the various boundary conditions 

were assigned by various methods. The specified-head 

boundary conditions were discretized on a nodal basis 

by interpolation from the average January through 

56 Ground-Water Flow and Contaminant Transport at a Radioactive-Materials Processing Site, Wood River Junction, Rhode Island 
December 1983 areal water-table map (fig. 1 0) and the 

corresponding cross-sectional potentiometric head 

map (fig. II). 

In the first series of simulations, the bedrock surface 

represented the bottom boundary of the modeled 

region. In later simulations, on the basis of the 

contamination configuration, the bottom boundary for 

the aquifer was assumed to be the interface between the 

coarse sands and gravels and the finer sands and silts 

(fig. 40); original data came from drilling logs. In most 

areas of the modeled region, the location of the sandsilt boundary could be determined with more 

confidence than the location of the bedrock surface. 

Although, the change in the bottom boundary from the 

A SPECIFIED-HEAD BOUNDARY B 

>- a: 

<!: 

0 

z 

:::::> 

0 

en 

0 

<!: 

w 

I 

6 

w 

LL 

(.) 

w 

a.. 

CJ) 

\ \ Mead ~wBr ok\ 1\ ~mul 

v 

( v / 

\ \ 

4 l! 

&)\ 

~(( 

I ~r::, }) 

\;) (( 

~ Jl 

f 

N 

. . ~ ~ 

'~" ~ ); ~v 

v 

D 

0 

0 200 

;lf 

~ v fj 

(~~ 

1\ ~~ '?<-

~ I~ ~ 

"~ K ~ 0> 

"- L". 

,\ 

1---' v r-t-t-t-'--1'---

/'I-:: t2 ~v -- / 1~ ~· A~ ~v L2!_ 

v 

SPECIFIED-HEAD BOUNDARY c 

1,000 2,000 FEET 

400 600 METERS 

EXPLANATION 

LEAKY BOUNDARY (Pawcatuck River, 

Meadow Brook, and Meadow Brook Pond) 

A A' LINE OF GEOHYDROLOGIC SECTION 

t--------i FOR CROSS-SECTION SIMULATION 

@ PUMPED WELL 

* PLANT SUPPLY WELL 

- LAGOON AREA 

>- a: 

<!: 

0 

z 

:::::> 

0 

en 

X 

:::::> 

_j 

LL 

6 

w 

LL 

(.) 

w 

a.. 

CJ) 

Figure 39. Grid and boundary conditions for the areal-flow model, Wood River Junction, Rhode Island. 

Numerical Simulation of Ground-Water Flow and Contaminant Transport 57 
bedrock surface to the sand/silt interface had no 

apparent effect on simulation results. In all simulations, 

the bottom boundary was assumed to be a no-flow 

boundary. 

The Pawcatuck River, Meadow Brook, and 

Meadow Brook Pond were represented as leaky 

boundaries. The river and brook locations were a 

sequence of line segments joining node points. The 

leakage rate was assumed to be proportional to the 

difference in head between the river and the aquifer 

under the river. The riverbed cross-sectional perimeter 

length was assumed to be 50 ft. The corresponding 

\ .... .\ ' tV ..... ........ ', 

MeaCJ owBrt ok\ ' ...... I", 

\ ond '!"... 

... ... ~~v J //. -r-...... (x~ ~r--..... ( r-... ..... ..... ~. r-... 

---\ - \ \~ ~ 

-r- ~ ~t .. '"?<- [) 

~ f - -" t ,.,I 

---- - Dr- ~ • /fl v -- 2 5- ~( 

1"~ -~"-r- ~~ ~,...-- ............ • I ~6 '~ 

perimeter for Meadow Brook was 10 ft. The river 

lengths in each cell of the finite-difference grid were 

estimated by scaling. The river and pond 

potentiometirc heads were determined by stage 

measurements at the gaging station upstream on the 

Pawcatuck River, at four downstream locations near 

the plant, at one location on Meadow Brook Pond, and 

at one location on Meadow Brook. 

The remaining parameter values were established 

during the parameter determination step of the model 

construction. They included the net precipitation 

recharge flux, the riverbed and lakebed hydraulic 

/ \ / \ / 

~ / I 

I I II 

v 

1/ 1/ 

v ..,.... ...... ~ 

I 1// I) 

// vv / I --- ,v ~--; I v ~-"'"'" 

1.- 2' J 

/ ~-"" L L / / I hs .. » -- 111ii 1-1-::f-" / " / I I~ (( I ~JJ 

\ 1/ 

\ (/ 

# 

~ \ 

\ 

\ 

~ ~- ff 

); ~< / 

............ 

v 

0 

II 

0 200 

/7 

? 

--

1,000 

I 

./ ,.. • ,.v 

/ / 

_,..... .JIIV. rI ,.- ::::::~-. v 

( ;I "' 

,.,~ 

.1---:;;_/ I-f-1-

--r-1- '~ ·"' 

I 

400 

-

• 

_ ...... 

• 

1// 

........ ~--' v 

/ 

vv 

vv 

2,000 FEET 

I 

I 

.,. ...... v 

/y 

// 

600 METERS 

41 

v 

v 

v 

• 

..,.... 1.4 / / v~"" / / ' _,., 

- v / / / / t / 1/ / / 

// // /v • /v / 

1--"'v 

// 

,/ / / / 

/ / 

// 

\)v / 

~/ / 

/ I / 

v o< /,; "' I V'v v" "' "' 

I / / v / / 

(_ ..,.....,. "' .) •,v- _ / ...... 

"' 

/ / / 

,."' / ,.1--- / 

·" / 

...... / 

.... r--

~--~ .... I 

EXPLANATION 

- 25-- SUBSURFACE CONTOUR-- Shows altitude 

of sand/silt boundary. Dashed where 

appoximately located. Contour interval 25 feet. 

Datum is sea level 

e LOCATION OF TEST HOLE OR DATA POINT 

USED TO CONSTRUCT THIS MAP 

Figure 40. Altitude and configuration of aquifer bottom for areal-flow model, Wood River Junction, Rhode Island. 

58 Ground-Water Flow and Contaminant Transport at a Radioactive-Materials Processing Site, Wood River Junction, Rhode Island 
conductivities and thicknesses, the lateral recharge flux 

through the till boundary for the areal-flow model, and 

the hydraulic-conductivity distribution. 

The water-supply well at the plant was represented 

as a point sink. Pumpage from the water-supply well 

(fig. 39) was estimated to range from 1,100 to 

5,200 ft3/d; 2,000 ft3/d was assumed to be the average 

for 1983. This water-supply well was outside the 

simulation region for the cross-sectional-flow model. 

The disposal ponds were not simulated because they 

were not in operation during 1983; the plant was in the 

process of being decommissioned. Precipitation 

recharge across the water-table boundary was 

represented as an areally distributed flux over the 

simulation region. 

Cross-Sectional-Flow Model 

The purpose of the cross-sectional-flow model was 

to establish the interactions of the potentiometric head, 

the river boundary condition, and the net recharge from 

precipitation that contribute to the plume pattern. The 

two-dimensional cross-sectional ground-water-flow 

model was developed from the following general 

steady-flow equation with the assumption that all 

gradients in they-direction (perpendicular to the cross 

section) are negligible, that is (Bear, 1972; Cooley, 

1974): 

V!fVh = 0, 

where 

K is the hydraulic-conductivity tensor (LIT), 

and 

h is the potentiometric head (L). 

(5) 

The assumption of no lateral gradients or lateral flow 

was based on the assumption that the maximum 

observed contamination followed a stream tube 

aproximated by line A-A' (fig. 39). This assumption, 

however, is violated in areas near the river where 

ground-water flow converges. The preclusion of lateral 

inflow in the simulated region will cause the computed 

velocity field to be different than the actual velocity 

field. The mesh was constructed with node spacing in the 

horizontal direction that ranged from 75 to 230ft and in 

the vertical direction that ranged from 3 to 30ft (fig. 41). 

Boundary conditions can include specified head, 

and (or) specified fluid flux, including no flux. Riverleakage boundaries are treated as specified-head 

boundaries. Initial conditions for a steady-state 

simulation consist of any reasonable distribution of 

potentiometric head. 

The water-table or free-surface boundary condition 

is difficult to calculate for this model. In fact, two 

boundary conditions apply because the location of the 

free surface is unknown before equation 5 is solved. 

The first condition is that of atmospheric pressure 

(taken to be zero); thus, h = z (where z is the coordinate 

in the upward vertical direction) on the free surface. 

The second condition comes from the derivative 

following the motion of a particle on the free surface 

(Bear, 1972). It states that there is fluid continuity at a 

free surface for steady-state flow. The resulting 

nonlinear kinematic boundary condition on the free 

surface (Bear, 1972) is 

Kx(~~r + Kl~~r + R- (K, + R) ~~ = 0, (6) 

where 

K,K 

X Z 

are the hydraulic conductivities in the x 

and z directions (LIT), and 

R is the recharge flux in the vertical direction 

(L3/L2T). 

For purposes of these steady-state simulations, the 

nonlinear terms were neglected; thus, equation 6 

simplifies to 

ah -(K+R)-a +R=O. 

z z (7) 

The free-surface boundary was assumed to be the 

calculated potentiometric head at the relevant nodes. A 

more sophisticated but computationally expensive 

approach to treatment of the free-surface boundary 

condition would have been to adjust the grid iteratively 

so that the kinematic and the pressure boundary 

conditions would be satisfied when the correct freesurface location was found. The gain in accuracy of 

this latter approach did not warrant the additional 

computational requirements. 

Along segment 1-2, the free-surface parts were 

treated as specified-recharge-flux boundaries in view of 

equation 7, and the river parts were treated as specifiedhead boundaries as is appropriate between an open body 

of water and an aquifer. Unlike the areal-flow model in 

which the river was treated as a leaky boundary, the 

head in the river forms a boundary condition for the 

head in the aquifer. Along segment 2-3, a specifiedhead boundary was selected on the basis of extrapolation of the nearest measured potentials. Segment 2-3 is 

not a physical boundary in the system; and head 

measurements seemed more reliable than flu~ estimates. 

Numerical Simulation of Ground-Water Flow and Contaminant Transport 59 
en 

0 

C) 

~ 

0 

c 

:s 

Q. 

~ m 

iD 

~ 

"T1 

0 1 ~ 

m 

:s 

Q. 

0 

0 

a 

m 

3 

:;-

m 

a 

-t 

; 

:s 

(I) 

"tJ 

0 

~ 

~ 

m 

:D 

m 

Q. 

o· 

m 

0 

~ (I) 

3:: 

m 

iD 

~· 

iii 

"'0 

0 

0 

(I) 

(I) 

(I) 

:;· 

cc 

en 

j 

~ 0 

0 

Q. 

:D 

;c· 4 ~ 

c.. 

c 

:s 

2-

0 

w:s 

:D 

:::r 

0 

Q. 

(I) 

iii 

iii" 

:s 

Q. 

A bend in A' 

section 

:... :... :... ~ 

;::. ~ ~ 

Q2 .2; ::::. lagoon area 

~ Q2 ~ ~ ~ 

land surface ~ 

~ 

~ ~ 

t; E1 E1 ~ ( ~ s l:::! 

S: ~ ~ 

\j S: S: S: 

Q., l:::! \j 

Q., Q., ~ .,.--, l 

water ao1e v -I- I--- r ... 2 rll rll ill \ I I _,__ -I- ... J 

1--.L ~ -I- - \-,. - \: - \ I r II T .L 

I ] 1 I I rl h ::s:Ll .L 

T 

... 

I I I I .L 

T TT 1 T 

T T T I CfOARSE I [ T EAND AI\ D 

~ ~ I I .L C RAVEL ~ 

I I I I 

T 

I LI 

~ I 

I I ~II I 

T I - no; ilii + \ -l T v I I I 

I. .. k:: t> I 

I ... 

I ~~>>~ FINES AND 

~~ 1-2 SPECIFIED-FLUX BOUNDARY AT WATER TABLE 

F.NE SAIJD AND SILT BED80CK AND~ ILT 

SPECIFIED-HEAD BOUNDARY AT RIVER I 

k?~r<K 2-3 SPECIFIED-HEAD BOUNDARY 

r---...-H 3-4 SPECIFIED-FLUX BOUNDARY 

I ~ KX 

4-1 .i "' 

SPECIFIED-HEAD BOUNDARY 

v ~t>~ v -- MODEL ZONE BOUNDARIES 

' 

I WELL SCREEN INTERVAL 

~ WETLAND /k /10<~1" )< ~ v 3 

BEDROCK 

0 100 200 300 METERS 

VERTICAL SCALE GREATLY EXAGGERATED 

Figure 41. Grid and boundary conditions for the cross-sectional-flow model, Wood River Junction, Rhode Island. (Location of section A-A' shown in 

figure 39.) 
The bottom of the cross-section simulation region 

(segment 3-4) was assumed to be the bedrock surface 

over parts of the region and a horizontal line drawn 

through the fine sands and silt for the remainder of the 

boundary. The areas of the model containing the 

bedrock (the southeastern third of the region, as shown 

in fig. 41) were excluded from the simulation. Along 

segment 3-4, a specified-flux boundary was eventually 

selected. Initially, a specified-head boundary was 

assigned by means of projection of nearest measured 

heads. Fluxes across this boundary were calculated 

from a simulation. These fluxes then were assigned to 

nodes along this boundary to establish the final 

boundary condition. Along segment 4-1, a specifiedpotentiometric head boundary was selected for the 

same reasons as along segment 2-3. 

The actual values associated with the various 

boundary conditions were assigned by various 

methods. The specified potentiometric head boundary 

conditions were discretized on a nodal basis by 

interpolation from the average areal water-table map 

for January through December 1983 (fig. I 0) and the 

corresponding cross-section potentiometric head map 

(fig. 11 ). The river potentiometric heads were 

determined by stage measurements at the streamflowgaging station upstream on the Pawcatuck River, and at 

four downstream locations near the plant. The watersupply well at the plant was outside the simulation 

region for the cross-sectional-flow model. 

The other parameter values were established during 

the parameter-determination step of the model 

construction. They included the net-precipitationrecharge flux and the aquifer hydraulic-conductivity 

distribution. 

Parameter Estimation 

The first step in parameter determination was to 

estimate ranges for the various parameters that were to 

be adjusted to their optimum values. Because of time 

constraints, however, no numerical parameter 

optimization calculation was done. 

For the areal-flow model, the river- and lakeleakage boundaries were characterized by bed thicknesses from 0.5 to 3ft and hydraulic conductivities of 

0.1 to 70ft/d. This combination resulted in a leakance 

factor that ranged from 0.033 to 140 d- 1

. The netprecipitation-recharge flux was estimated to range 

from 26 in/yr through the sand and gravel (based on 

one-half of the 1983 annual precipitation) to 2 in/yr in 

the swampy area, once increased evapotranspiration 

was taken into account. The recharge flux through the 

till boundary (segment B-C in fig. 39) was estimated 

by scaling the area from the surface-water drainage 

divide to each boundary cell. The net-precipitation 

recharge was assumed to be 10 in/yr [0.74 (ft3

/s)/mi2]. 

An alternative estimate was made by assuming an 

equal-flux distribution of a total recharge flow of 0.17 

(ft3/s)/mi2 (2.3 in/yr) as given by Gonthier and others 

(1974). 

For the cross-sectional model, the recharge flux 

varied spatially and was adjusted to obtain a close 

agreement between the calculated and measured freesurface location. The net precipitation was estimated 

to range from 2 to 26 in. 

Estimates of hydraulic conductivity for the arealflow model (fig. 42) were based on a value from an 

aquifer test at the plant supply well, and on qualitative 

estimates from 104 wells and 8 test holes. Aquifer-test 

data from the water-supply well at the plant indicated 

an estimated transmissivity of 12,500 ft2/d and a 

corresponding hydraulic conductivity of 125 ft/d 

(Gonthier and others, 1974). The qualitative estimates 

were based on thickness weighted averages of 

hydraulic conductivity derived from a characterization 

of lithology and hydraulic conductivity (D.C. 

Dickerman, U.S. Geological Survey, written commun., 

1982) given in table I 0. Use of table 10 with our 

lithologic data provided a range of hydraulic 

conductivities from 30 to 380 ft/d at the 112 borehole 

locations. The hydraulic conductivity chosen for each 

zone in the areal-flow model ranged from 75 to 

150 ft/d. Each cell in the model was assigned the value 

shown in figure 42. 

For the cross-sectional-flow model, a two-layer 

hydraulic-conductivity distribution was used. 

Hydraulic conductivity in the upper layer, which 

corresponded to the coarse sands and gravels, was 

estimated to be 100 ft/d; in the lower layer, which 

corresponded to the finer sands and silts, hydraulic 

conductivity was estimated to be 10ft/d. Although a 

value for hydraulic conductivity of about 50 ft/d for 

fine sands and silts was reported for this area by 

Rosenshein and others ( 1968), a range of values of 4 to 

20 ft/d was selected from table 10. An anisotropy of 

10: I was assumed in the upper sand and gravel layer; 

the lower fine sand and silt layer was assumed to be 

isotropic due to lack of information. 

Numerical Simulation of Ground-Water Flow and Contaminant Transport 61 
0 

I I 

0 200 

1,000 

I I 

400 

2,000 FEET 

I 

I 

600 METERS 

EXPLANATION 

HYDRAULIC CONDUCTIVITY, 

IN FEET PER DAY 

[=::J 75 

rn 1oo 

a.1! 125 

- 150 

- 100 - LINE OF EQUAL HORIZONTAL 

HYDRAULIC CONDUCTIVITY--

Interval is 50 feet per day 

Figure 42. Areal distribution of horizontal hydraulic conductivity for the areal-flow model, Wood River Junction, 

Rhode Island_ 

The objective function for the determination of the 

optimum aquifer parameters was the root mean 

square difference (RMSD) between computed and 

measured hydraulic potentials at 52 locations for the 

areal-flow model and at 46 locations for the crosssectional model. Comparisons were made between 

the average measured hydraulic potential in a well, 

and the simulated hydraulic potential of the cell 

containing that well. The measured potential values 

were taken to be the average values during 1983 and 

ranged from a low of about 43 ft to a high of 64 ft. 

Observation well number, corresponding node 

number, measured hydraulic potential, calculated 

hydraulic potential, and difference between calculated and measured hydraulic potential are listed in 

table 11. 

62 Ground-Water Flow and Contaminant Transport at a Radioactive-Materials Processing Site, Wood River Junction, Rhode Island 
Table 1 0. Qualitative characterization of lithology and 

hydraulic conductivity, Wood River Junction, Rhode Island 

[Source: D.C. Dickennan, U.S. Geological Survey, written commun., 1982; 

modified from Allen and others, 1963; Rosenshein and others, 1968; and 

Dickennan, 1984) 

Hydraulic conLog material ductivity 

(feet per day} 

Gravel ............................................................. . 470 

Gravel, sandy .................................................. . 335 

Sand, gravelly ................................................. . 200 

Sand and gravel ............................................... . 200 

Sand, very coarse ............................................ . 160 

Sand, coarse to very coarse ............................. . 150 

Sand, medium to very coarse .......................... . 135 

Sand, coarse .................................................... . 135 

Sand, medium to coarse .................................. . 120 

Sand, medium ................................................. . 105 

Sand, fine to very coarse ................................. . 105 

Sand, fine to coarse ......................................... . 95 

Sand, very fine to very coarse ......................... . 90 

Sand, fine to medium ..................................... .. 80 

Sand, very fine to medium .............................. . 65 

Sand, fine ........................................................ . 55 

Sand, very fine to fine ..................................... . 35 

Sand, silty to fine ............................................ . 33 

Sand, very fine ................................................ . 20 

Sand, silty ....................................................... . 15 

Sand, very fine to silt ...................................... . 15 

Sand, silt to silty ............................................. . 9 

Silt ................................................................... . 4 

Clay and silt .................................................... . 2 

Clay, till (hardpan) .......................................... . .I 

For the areal ground-water-flow model, hydraulic 

conductivity, river-leakage factors, net-precipitationrecharge fluxes, and till-boundary-recharge fluxes were 

adjusted to minimize the objective function. A 

minimum RMSD of 1.42 ft and a maximum deviation 

of -3.01 ft was achieved with the following parameter 

values: net-precipitation recharge of 26 in/yr, except 

2 in/yr in the swamp area (fig. 7); till boundary flow of 

4,500 ft3/d, which was distributed by surface catchment 

area (this represents about 20 percent of the annual1983 

precipitation); hydraulic-conductivity distribution 

shown in figure 42; and. a river-leakage factor of 

7.5 d- 1

. Additional statistics computed include the mean 

difference of0.25 ft and the mean absolute difference of 

1.18 ft. The input and output data files for the final 

simulation are contained in Appendix 2 on the enclosed 

diskette. 

For the cross-sectional-flow model, a RMSD 

of 1.19 ft and a maximum deviation of 3.91 ft over a 

range of about 13 ft, was achieved with the following 

parameter values: net precipitation recharge of 26 in/yr, 

except 2 in/yr in the swamp area; horizontal hydraulic 

conductivity of 100ft/din the upper layer and 10 ft/d 

in the lower layer; and vertical hydraulic conductivity 

of I 0 ft/d in both layers. These final parameter values 

for the areal and vertical simulations are shown in 

table 12. Additional statistics computed include a 

mean hydraulic potential difference of 0.08 ft and a 

mean absolute difference of 0.87 ft. The input and 

output data files for the final simulation are contained 

in Appendix 3 on the enclosed diskette. 

Discussion of Simulation Results 

Areal-Flow Model 

The areal-flow model simulated steady-state watertable altitudes representing the average water-table 

altitudes for January through December 1983 (fig. 1 0). 

The following observations are based on a comparison 

of the simulated water-table altitudes with the measured 

water-table altitudes (fig. 43). The region of the model 

from the eastern boundary to an area east of the plant 

site shows measured water levels to be higher than 

simulated water levels. A range of values for hydraulic 

conductivity and flux across the till boundary was tried 

in the region of the eastern boundary to raise heads in 

this area and to shift the direction of flow to be more 

nearly perpendicular to that boundary. An S-shaped 

curve in the 52-, 54-, and 56-foot contours also was not 

reproduced. The measured gradient in the region of the 

S-shaped curve is greater than the simulated gradient. 

The high measured gradient may be a result of the 

bedrock high beneath the lagoon area (fig. 4). The areal 

simulation did not contain this feature as the lower 

boundary was the sand/silt interface. 

Gradients from the plant to the river were 

reproduced fairly well, with the exception of the 52-foot 

contour directly southwest of the plant. The shape of 

the water table in the southwestern quadrant of the 

model also was reproduced fairly well. Simulated 

Numerical Simulation of Ground-Water Flow and Contaminant Transport 63 
Table 11. Measured and calculated potentiometric heads for the steady-state flow simulations, Wood River Junction, 

Rhode Island 

[Measured head: Mean of potentiometric heads measured during 1983] 

Well 

No. 

Node 

No. 

Head, 

measured 

(feet) 

Head, calculated 

(feet) 

Areal simulation 

CHW-451 390 

CHW-465 423 

CHW-473 180 

CHW-474 486 

CHW-475 491 

CHW-476 275 

CHW-477 376 

CHW-478 375 

CHW-479 193 

CHW-480 314 

CHW-481 200 

CHW-482 606 

CHW-483 355 

CHW-484 137 

CHW-486 55 

CHW-487 84 

CHW-488 265 

CHW-489 339 

CHW-492 303 

CHW-493 307 

CHW-499 356 

CHW-500 426 

CHW-501 498 

CHW-502 381 

CHW-503 448 

CHW-504 520 

CHW-505 270 

CHW-506 152 

CHW-507 42 

CHW-510 646 

CHW-511 645 

CHW-513 632 

CHW-515 592 

CHW-516 395 

CHW-526 342 

54.57 

50.08 

59.75 

46.13 

47.12 

49.85 

45.52 

44.73 

47.26 

49.62 

50.22 

54.28 

56.18 

53.02 

51.08 

50.32 

44.37 

44.94 

44.75 

46.32 

57.43 

54.85 

51.10 

46.65 

45.57 

45.78 

47.45 

44.02 

49.21 

63.96 

63.69 

48.30 

46.12 

58.86 

45.97 

54.26 

51.79 

57.21 

47.30 

49.37 

50.22 

45.91 

45.10 

45.73 

51.82 

49.34 

53.85 

54.81 

54.12 

51.67 

49.46 

43.94 

45.07 

45.19 

47.88 

55.53 

54.12 

53.79 

48.59 

46.18 

46.86 

47.52 

44.52 

48.11 

62.07 

60.68 

48.33 

47.18 

57.13 

47.28 

Head, calculated 

minus 

measured 

(feet) 

-0.31 

1.71 

-2.54 

1.17 

2.25 

.37 

.39 

.37 

-1.53 

2.20 

- .88 

- .43 

-1.37 

1.10 

.59 

-.86 

- .43 

.13 

.44 

1.56 

-1.90 

-.73 

2.69 

1.94 

.61 

1.08 

.07 

.50 

-1.10 

-1.89 

-3.01 

.03 

1.06 

-1.73 

1.31 

Well 

No. 

Node 

No. 

Head, 

measured 

(feet) 

Head, calculated 

(feet) 

Areal simulation-Continued 

CHW-527 431 

CHW-529 352 

CHW-530 318 

CHW-531 280 

CHW-542 347 

CHW-552 242 

CHW-553 243 

CHW-554 169 

CHW-564 310 

CHW-568 174 

RIW-637 477 

RIW-638 185 

RIW-639 472 

RIW-640 337 

RIW-648 326 

RIW-649 364 

RIW-650 330 

mean 0.25 ft 

variance 2.02 ft 

59.20 

52.45 

56.00 

52.56 

47.37 

50.79 

50.76 

51.23 

47.04 

53.53 

44.96 

43.99 

47.27 

44.14 

45.44 

46.10 

45.22 

standard deviation 1.42 ft 

mean absolute 1.18 ft 

57.23 

53.00 

54.40 

53.55 

49.84 

52.66 

53.25 

53.10 

49.39 

54.37 

44.76 

43.92 

48.85 

44.10 

43.95 

46.41 

45.64 

Vertical simulation 

CHW-440 101 

CHW-446 149 

CHW-493 211 

CHW-494 186 

CHW-495 142 

CHW-497 76 

CHW-498 54 

CHW-517 164 

CHW-518 120 

CHW-519 96 

CHW-520 166 

CHW-521 144 

CHW-522 122 

45.83 

50.55 

46.32 

44.64 

44.96 

45.37 

45.89 

44.75 

44.91 

44.63 

45.79 

45.84 

45.87 

46.29 

52.49 

46.46 

44.07 

44.25 

44.50 

44.55 

44.18 

44.35 

43.97 

45.60 

45.60 

45.60 

Head, calculated 

minus 

measured 

(feet) 

-1.97 

.55 

-1.60 

.99 

2.47 

1.87 

2.49 

1.87 

2.35 

.84 

-.20 

-.07 

1.58 

-.04 

-1.49 

.31 

.42 

0.46 

1.94 

.14 

-.57 

-.71 

-.87 

-1.34 

-.57 

-.54 

-.66 

-.19 

-.24 

-.27 

64 Ground-Water Flow and Contaminant Transport at a Radioactive-Materials Processing Site, Wood River Junction, Rhode Island 
Table 11. Measured and calculated potentiometric heads for Table 12. Final parameter values for areal and vertical 

the steady-state flow simulations, Wood River Junction, Rhode simulations, Wood River Junction, Rhode Island 

Island-Continued 

Head, cal- [in/yr, inch per year; ft3/d, cubic foot per day; ft/d, foot per day; 

d- 1

, per day] 

Well Node Head, Head, cal- culated 

No. No. measured culated minus Parameters Final values 

(feet) (feet) measured Areal model (feet) 

Net precipitation recharge ............... 26 inlyr (2 in/yr in swamp) 

Vertical simulation-Continued Till boundary flow distributed by 

surface catchment area ................. 4,500 ft3/d 

CHW-523 100 45.74 45.60 -0.14 Hydraulic conductivity 

CHW-526 78 45.97 45.61 -.36 (input by zones) ........................... 75-150 ft/d 

CHW-532 172 50.94 53.73 2.79 River leakage ................................. 7.5 d- 1 

CHW-539 193 50.02 52.52 2.50 Cross-sectional model 

CHW-541 127 48.56 52.47 3.91 Net precipitation recharge .............. 26 in/yr (2 in/yr in swamp) 

CHW-542 213 47.37 49.04 1.67 Hydraulic conductivity 

CHW-543 169 47.42 49.02 1.60 Upper layer: 

CHW-544 147 47.34 49.01 1.67 Horizontal ............................ 100 ft/d 

CHW-546 103 47.28 48.93 1.65 Vertical ................................. 10 ft/d 

CHW-561 125 47.45 48.97 1.52 

Lower layer: 

Horizontal ............................ 10 ft/d 

CHW-562 102 46.91 47.37 .46 Vertical ................................. 10 ft/d 

CHW-564 124 47.04 47.47 .43 

CHW-571 285 55.90 56.83 .93 

CHW-573 153 56.70 56.82 .12 gradients in the lagoon area are lower than measured 

CHW-575 65 56.81 56.81 .00 gradients. Simulated gradients along the northern part 

RIW-641 185 44.05 43.98 -.07 of the Pawcatuck River are steeper than measured. 

RIW-642 119 44.05 44.16 .11 Simulated water levels near Meadow Brook Pond, 

RIW-643 97 44.31 44.23 -.08 which are significantly higher ( 4 ft) than the measured 

RIW-644 75 45.72 44.28 -1.44 water levels, indicate that leakance through the lakebed 

RIW-645 31 45.72 44.42 -1.30 may have been artificially high or that hydraulic 

RIW-656 183 43.92 43.54 -.38 conductivity of the sediments in the area southeast of 

RIW-657 161 44.00 43.58 -.42 the pond may have been too low. The convergence of 

RIW-658 139 44.00 43.60 -.40 ground-water flow from the east and the west sides of 

the Pawcatuck River indicate a stagnation plane along RIW-659 117 44.05 43.65 -.40 the axis of the river. Similar stagnation points were 

RIW-660 74 44.64 44.00 -.64 found by Winter ( 1976) in his two-dimensional 

RIW-661 181 43.81 42.98 -.83 simulations of ground-water-flow fields interacting 

RIW-662 159 43.88 43.03 -.85 with lakes. Downstream flow under the river, however, 

RIW-663 137 43.91 43.07 -.84 prevents true stagnation. 

RIW-664 115 43.89 43.16 -.73 

RIW-665 93 44.03 43.22 -.81 For the areal~ flow model, a calculation of the water 

RIW-668 178 43.66 42.70 -.96 balance resulted in an imbalance of 9.68 ft3/d, (see 

RIW-669 156 43.61 42.82 -.79 "Water Balance Summary" in Appendix 2). Inflow 

RIW-670 134 43.81 42.89 -.92 from the specified-head boundaries was 8.37xl o5 ft3

/d; 

outflow across the specified-head boundaries was 

mean 0.08 ft 1.06xl 06 ft3

/d. Inflow from specified-flux boundaries 

variance 1.39 ft (till boundary) was 1.03xi04 ft3

/d, and outflow from 

standard deviation 1.19 ft line or point sinks (the water-supply well) was 

mean absolute 0.87 ft 2,000 ft3

/d. Inflow from recharge was 5.22x104 ft3/d. 

Numerical Simulation of Ground-Water Flow and Contaminant Transport 65 
\ \ 

Meac pwBrr ok \ \ ~ond 

J 

( /v 

\ \ 

lise ... .............. 

,. .... 

. \ ···1:: ~)]' •,, <:l ....... ... ... .. .. ...... - _..;;; ~ 

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600 METERS 

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EXPLANATION 

- LAGOON AREA 

·······50 ······· SIMULATED WATER-TABLE CONTOUR--

Shows altitude of simulated water 

table. Contour interval 2 feet. Datum is 

sea level 

- 50 -- WATER-TABLE CONTOUR--Shows altitude 

of measured water table. Dashed where 

approximately located. Contour interval 

2 feet. Datum is sea level 

Figure 43. Configuration of simulated water table and measured water table based on average measured water 

levels in observation wells, Wood River Junction, Rhode Island, January through December 1983. 

Leakage into the aquifer was 8.14x104 ft3/d. Most of 

the ground water enters the modeled region through 

the northeast section and leaves through the southwest 

section. 

The RMSD of 1.42 ft was about 6 percent of the 

overall water-level spatial variation for the simulated 

region. The seven largest differences between 

measured and simulated ground-water potentials were 

not associated with any particular hydrologic feature 

with the exception of two which were located in an 

area of high gradient of hydraulic conductivity. The 

six smallest differences were mostly near the 

Pawcatuck River and adjacent swamp. 

Leakage rates were computed for each river node. 

The Pawcatuck River is a gaining stream, and the 

ratio of the gain to the total flow rate is about 

3 percent. This gain is too small to be measured over 

the approximately 1-mile-long reach of river in the 

66 GroundaWater Flow and Contaminant Transport at a Radioactive-Materials Processing Site, Wood River Junction, Rhode Island 
study area; estimated measurement accuracy was ±1 0 

percent. As previously mentioned, the average flow 

rate of the river was 193 ft3

/s during 1983. Differential 

piezometer surveys done in October 1981 and April 

1983, however, confirm that it is a gaining stream over 

this reach (Ryan and others, 1985, p. 177-183). 

Although no rigorous sensitivity analyses were 

done after the final parameter values were determined, 

some observations regarding sensitivity can be made 

based on model responses during the parameterestimation procedure. During the search for the 

optimum parameters, selected parameters were 

changed at different times in different orders. 

Therefore, the sensitivity analyses discussed below are 

based on pairs of sequential simulations where only 

one parameter was varied. 

For the areal-flow model, sensitivity analyses were 

done for horizontal-hydraulic conductivity, recharge, 

flux from the till boundary, river leakage, aquifer 

bottom location and pumpage rate. A 30-percent 

decrease in hydraulic conductivity along the eastern till 

boundary (from 150 to 100ft/d) resulted in a 2-percent 

decrease in the RMSD, with the same set of seven wells 

mentioned above exhibiting the largest differences 

between the simulated and measured hydraulic 

potentials. 

Recharge was varied areally, and ranged from 2 to 

28 in/yr over the swamp and area adjacent to the river 

and from 20 to 28 in/yr elsewhere. The maximum 

change of RMSD as a result of variations in recharge 

was a 40-percent decrease when the recharge on cells 

traversed by the river was increased from 12 to 18 in/yr 

and the recharge elsewhere also was increased from 20 

to 28 in/yr. The overall configuration of the water table 

was not changed significantly except along the 

southern part of the till boundary where water levels 

rose on the order of 0.5 ft. A decrease of 2 in/yr over 

the entire simulation region resulted in no change in the 

RMSD and no substantial change in the water-table 

configuration, however the mean difference between 

simulated and measured hydraulic potentials decreased 

by one-third (0.25 to 0.16 ft). 

Flux off the till boundary also was varied in 

quantity and in spatial distribution. The RMSD was 

largest ( 11.8 ft) when the flux was calculated from a 

vector analysis of head differences at a triangle of 

wells. The RMSD was lowest (1.4 ft) when the flux 

from the till was 20 percent of the 1983 precipitation. 

The maximum change of RMSD as a result of 

variations in flux from the till boundary was an 

82-percent decrease when the flux based on vector 

analysis was changed to a flux based on the drainage 

area. Heads along the till boundary ranged from 4 to 60 

ft higher when the flux from vector analysis was used. 

Varying river leakance from 0.033 to 140 d- 1 

resulted in a 30-percent decrease in the RMSD and 

significant changes in the water-table configuration. 

The wide range of variation in river leakance reflects 

the uncertainty associated with this parameter. 

Thickness of the riverbed sediments was estimated to 

range from 0.5 to 3 ft, and composition of the 

sediments ranged from fine sands, silts and muck to 

coarse sands and gravels. With the low leakance 

factor, the presence of the river is barely reflected in the 

water table. Whereas the high leakance factor resulted 

in a simulated water-table configuration that closely 

matched the measured configuration within 1 ,000 ft of 

the river. 

Varying the geometric configuration of the bottom 

boundary of the aquifer from the bedrock surface to the 

sand silt boundary reduced RMSD by 13 percent. 

Maximum differences between the simulated and 

. measured potentiometric heads decreased by 1 to 2 ft, 

with the same set of seven wells exhibiting the largest 

differences. This implies the fine sands and silts do not 

contribute significantly to the ground-water-flow 

system in the study area. 

Results were insensitive to the presence or the 

absence of the water-sup~ly well, which withdrew 

water at a rate of 2,000 ft /d (I 0 gal/min). This seems 

reasonable as the recharge flux in the system is 

25 times larger than the discharge rate of the well. 

Cross-Sectional-Flow Model 

Simulated and measured gradients in the area from 

the plant to the river were similar (fig. 44), although the 

simulated gradient was consistently steeper. With the 

exception of an S-shaped curve in the potentials near 

the plant, hydrostatic equilibrium generally exists from 

the plant to the river, and vertically upward flow 

appears beneath the river and swamp. The measured 

vertical upward flow beneath the river and adjacent 

swamp was reproduced by the simulation, with the 

exception of some perturbations that appear only on 

the simulated cross section. The actual flow system 

has three-dimensional aspects; however, the simulation 

represents only two-dimensional ground-water 

flow. The simulated potentiometric head field in the 

region beneath the swamp and river was affected by 

the boundary conditions of specified flux at the water 

table and specified head at the river nodes. 

Numerical Simulation of Ground-Water Flow and Contaminant Transport 67 
: 

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J j I ; E water level. Dashed where approximately located. f __ E-- Contour interval, in feet, is variable. Datum is sea level. 

- - - -: : ••••••••••• 46 ••••••••••• SIMULATED POTENTIOMETRIC CONTOUR··Shows altitude $ : of simulated potentiometric head. Contour interval, in feet, 

$ is variable. Datum is sea level 

j : I WELL SCREEN INTERVAL 

j ~ .ll1J.L WETLAND 

A' 

' 

j t j !V_LIVIVIV 1/ v 1'-. v 

\ 0 BEDROCK 500 1,000 FEET I I I I 

I 0 100 200 300 METERS 

VERTICAL SCALE GREATLY EXAGGERATED 

Figure 44. Configuration of simulated and average measured ground-water potentiometric head, Wood River Junction, Rhode Island, January 

through December 1983. 
Therefore, the water could only leave the system along 

the 1-2 boundary at the river nodes. An alternative 

method of modeling this part of the region would have 

been to approximate the river by a continuous straight 

line extending from the first intersection of the river to 

the left-hand boundary of the model; this method 

would have represented all ground-water that 

discharges into the swamp and river as discharging into 

the river alone. The equipotentiometric head lines near 

the bedrock high intersect the bedrock at right angles to 

indicate the no-flow conditions at the bedrock 

boundary; vertical exaggeration of the illustration 

distorts this view. 

Along the left-hand boundary, the simulated 

hydraulic potentials show flow into the region. This is 

not realistic, because the plane of the cross section is 

oriented down the Pawcatuck River valley, so outflow 

would be expected along this boundary. Unrealistic 

specification of the potentiometric head distribution 

along this boundary is the cause of this inflow. 

Potentiometric heads in the fine sand and silt were 

estimated at this boundary location. Convergence of 

flow from regions normal to the cross section exist but 

are not represented in the two-dimensional crosssectional model. 

The water balance showed a flow imbalance of 

0.0198 ft3/d (see "Water Balance Summary" in 

Appendix 3). Inflow from specified-head boundaries 

was 456 ft3

/d; outflow from specified-head boundaries 

was 480 ft3

/d. Inflow from specified-flux boundaries 

was 23.6 ft3/d. 

The RMSD error of 1 .19 ft was about 7 percent of 

the overall potentiometric head spatial variation for the 

simulated region. Qualitatively, the simulated groundwater potentiometric heads matched fairly well with 

the measured ground-water potentiometric heads, 

except in the fine sand and silt under the swamp, where 

boundary-condition data were not available. The four 

largest differences between the measured and 

simulated ground-water potentials were underneath the 

lagoon area and above the bedrock high where there is 

a zone of relatively low hydraulic conductivity. Three 

of the five smallest differences were underneath the 

first intersection of the Pawcatuck River. The threedimensional aspects of the flow system preclude the 

two-dimensional cross-sectional model from matching 

the flow system precisely. 

Sensitivity analyses were based on pairs of 

sequential simulations where only one parameter was 

varied. Sensitivity analyses that were done included 

varying horizontal and vertical hydraulic conductivity, 

recharge flux, and bottom boundary conditions. 

Reducing the vertical hydraulic conductivity by a 

factor of I 0 (from 150 to 15ft/d) in the sand resulted in 

a reduced RMSD of 12 percent and increased groundwater potentiometric heads by 30 ft beneath the river 

and adjacent swamp by 0.25 ft. Doubling the vertical 

hydraulic conductivity in the sand from 15 to 30 ft/d 

produced no change in the RMSD. 

Recharge was adjusted from 2 to 28 in/yr in the 

swamp and from 20 to 28 in/yr elsewhere. Generally, 

the variations in recharge had no effect on the RMSD 

or the overall configuration of the ground-water 

potentiometric heads. 

Changes in the RMSD (an increase of 130 percent) 

were most significant when the bottom boundary 

conditions were changed. Bottom boundary conditions 

that were used along the left most two-thirds of the 

modeled region (fig. 41) included: no flow, specified 

potential, and specified flux. The change in the 

configuration of ground-water potentiometric heads 

was most significant when the bottom boundary was 

changed from no flow to specified potentiometric 

heads, which resulted in upward flow to the river and 

adjacent swamp. A change from specified 

potentiometric heads to specified flux along the bottom 

boundary resulted in a more uniform and lower 

gradient configuration of ground-water potentiometric 

heads beneath the river and swamp. This model was, 

therefore, most sensitive to the type and location of 

boundary conditions rather than to variations in 

hydraulic conductivity or recharge. The greatest 

uncertainty in boundary-condition potentiometric 

heads and flux existed along this bottom boundary. 

A calculation was made to check the degree to 

which the free-surface linearized boundary condition 

was satisfied. The ratio of recharge to vertical 

hydraulic conductivity was calculated to be less than 

4x1 o-4

; therefore, treatment of recharge as a specified 

flux resulted in a sufficiently good approximation. 

For the areal-flow- and cross-sectional-flow models, 

the calculated Darcy velocity field (Appendix 2) was 

used to obtain traveltimes from points under the 

disposal ponds to the Pawcatuck River according to 

Numerical Simulation of Ground-Water Flow and Contaminant Transport 69 
(8) 

where: 

t2 - t1 is the traveltime from point 1 to point 2 (T), 

cr is the porosity (dimensionless), 

N is the number of path length increments 

from points 1 to 2, 

Llxi is the segment of path length in the 

x-direction (L), and 

qxi is the Darcy velocity in the x-direction 

(LIT). 

Equation 8 is the scalar parametric equation in the 

x-direction obtained from the vector equation relating 

particle displacement to velocity. 

A flow path was sketched down the core of 

the contamination plume in the areal representation 

shown in figure 45. The reciprocal Darcy velocity 

x-component was then summed over increments of 

path length and multiplied by the effective porosity to 

obtain the traveltimes. For each path segment, the 

Darcy velocity used was that of the nearest internodal 

face, which is where the velocity vectors are calculated 

by the simulator. A weighted interpolation between 

the closest velocity vectors would have been more 

accurate, but such a refinement did not seem warranted 

in view of the uncertainties in the distribution of porosity and hydraulic conductivity. Traveltimes of 2.5 to 

2.9 years from the disposal ponds to the river (A to B) 

and 14 to 17 years from the disposal ponds to the 

downstream discharge area (A to C) were obtained, 

with the assumption that effective porosity ranged 

from 0.33 to 0.39 (Ryan and others 1985, p. 66). 

The traveltime calculation was sensitive to the 

path length, porosity, and interpolation technique. The 

disproportionately long traveltime to the downstream 

discharge area was caused by the low calculated velocities under the river and swamp area. These traveltimes are based on the vertically averaged velocity 

field computed by the areal-flow model. Because the 

simulated water-table gradient was slightly lower than 

the measured gradient in the area from the plant to the 

river, these traveltime calculations may be artificially 

long for this flow path. The traveltime estimates under 

the river and swamp area should be somewhat more 

realistic because the simulated water-table gradient 

more closely matches the measured gradient. 

The traveltime from the lagoon area to the river 

seems somewhat lengthy compared to the range of 1.6 

to 2.2 years obtained from the estimated average 

ground-water interstitial flow velocity of 1.9 to 2.6 ft/d. 

This estimated average velocity of ground-water flow 

was based on the average hydraulic gradient between 

the plant site and the river. Attempts to determine traveltimes from concentration breakthroughs at selected 

wells presumed to be along the same flow path were 

unsuccessful. Contributing factors included the 

inability to correlate concentration peaks in one well 

with those in another and the inherent difficulty in 

identifying wells along the same flow path. 

As was done in the areal-flow model, traveltimes 

for the cross-sectional-flow model also were calculated 

from under the lagoon area to the river (A' to A) for 

three selected flow paths (fig. 46). The contamination 

cross sections (figs. 26-31) were used to select one 

flow path near the top of the plume, one at the core of 

maximum contamination, and one near the bottom of 

the plume. Traveltimes for each flow path were calculated only to a point at or underneath the first river 

crossing because confidence in the calculated groundwater potentiometric heads underneath the swamp area 

was low because of lateral flow convergence and 

uncertainty in boundary condition values. On the basis 

of porosities of 0.33 to 0.39, the resultant traveltimes 

ranged from 3.2 to 3.8 years, 5.9 to 6.9 years, and 12 to 

14 years for the top, middle, and bottom flow paths, 

respectively. The values for the longest flow path were 

the least realistic, however, because the twodimensional cross-sectional-flow model probably is 

not accurate under the Pawcatuck River and swamp 

area. Differences in traveltimes between the areal and 

cross-sectional models primarily are due to the differences in velocity fields and path lengths. The areal 

model computes a vertically averaged velocity field, 

whereas the cross-sectional model accounts for vertical 

velocity variation but neglects flow perpendicular to 

the cross section. 

The traveltimes for the shallowest flow path 

(fig. 46) are somewhat longer than those calculated 

from the areal-flow model for the same end points 

(plant to river) (3.2 to 3.8 years compared with 2.5 to 

2.9 years). The differences can be explained by the 

increased distance covered for the flow path in the 

cross-sectional-flow model compared to the flow path 

in the areal-flow model and differences in the velocity 

fields. 

Proposed Ground-Water Development 

Although at the time of this study there were no 

current plans for ground-water development in this 

area, the extensive aquifer in the Pawcatuck River 

Valley has historically been considered for future 

development. With this in mind, the areal-flow model 

70 Ground-Water Flow and Contaminant Transport at a Radioactive-Materials Processing Site, Wood River Junction, Rhode Island 
\ 

Mew 

1\ 

v 

( 

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(( 

1 ... c)) 

I :i} {( 

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f 

rt 

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f 

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0 

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// 

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----=:: ~ 7'?-

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2,000 FEET 

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600 METERS 

-- +- ~ 

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A 

EXPLANATION 

LAGOON AREA 

FLOW PATH OF CONTAMINATION 

ALONG PLUME AXIS 

TRAVEL TIMES 

A-8 2.5-2.9 YEARS 

A-C 14 - 17 YEARS 

Figure 45. Generalized path of ground-water flow and calculated traveltimes along the axis of maximum 

contamination in the plume, Wood River Junction, Rhode Island. 

was used to evaluate the effects of potential groundwater development in the area northwest of the contamination plume-an area where deposits are thick 

and are some distance from the contamination. The 

two locations selected for this evaluation (two hypothetical pumped wells) are shown in figure 39. Each 

hypothetical well was pumped individually for the 

simulations. The individual pumping rates ranged 

from 0.25 to 1.0 Mgal/d. Recharge was kept constant 

throughout the simulations but varied areally as 

previously described. Steady-state and transient 

simulations were done. Aquifer properties from the 

steady-state calibration were used. 

For pumping rates greater than 0.25 Mgalld at 

some well locations (fig. 47) and 1 Mgalld at others 

(fig. 48), the steady-state simulations showed that the 

draw down cone (defined as draw down greater than 

0.1 ft) extended to the area that contained the contamination plume as it existed in 1983. Pumping rates of 

0.5 and 1 Mgal/d resulted in the aquifer going dry at 

pumped-weB locations west of the Pawcatuck River. 

Numerical Simulation of Ground-Water Flow and Contaminant Transport 71 
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land surface lagoon area 

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water ~a01e VT----= -=..L =----:. -=+- -I ~ f-- ~ -; --ro~. ---±=_ =.r- ..;.:..;.;....--r--- -r- - r 7T J LV I I I 

/ r II .H I I I Lr_ 

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--I y- I.L 

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" TRAVEL TIMES WERE CALCULATED 

- -E - FLOW PATH OF CONTAMINATION ALONG WHICH 

TRAVEL TIMES WERE NOT CALCULATED 

I WELL SCREEN INTERVAL 

~ WETLAND 

A' 

~ _'-L_ 

\ 0 500 1,000 FEET 

BEDROCK I I I I I I I I I I I 0 100 200 300 METERS 

VERTICAL SCALE GREATLY EXAGGERATED 

Figure 46. Generalized paths of ground-water flow and calculated traveltimes at the top, middle, and bottom of contamination plume, Wood River 

Junction, Rhode Island. 
The water-table altitude after I ,000 days of pumping 

for a transient simulation is shown (fig. 49) for 

pumping at 1 Mgal/d between the Pawcatuck River 

and Meadow Brook Pond. Steady-state conditions, 

however, were reached after 500 days of pumping, 

which is well within the anticipated operating life of a 

pumping center. Meadow Brook Pond was deleted 

from the transient simulation to create a worst-case 

scenario. Only some of the water withdrawn by the 

pumped wells will be contaminated. Because the 

( 

1\ 

1\ 

----

maximum measured strontium-90 concentration 

(290 pCi/L) exceeds the drinking-water criterion of 

8 pCi!L3 (U.S. Environmental Protection Agency, 

1990) by a factor of 36, and the maximum measured 

nitrate concentration (730 mg!L as N) exceeds 

the drinking-water standard of 10 mg/L as N 

(U.S. Environmental Protection Agency, 1990) by a 

factor of 73 in parts of the plume, some of the water 

3 Average annual concentration of strontium-90 assumed to 

produce a total body (or organ) dose of 4 millirem/yr is 8 pCi/L. 

V II 

1\ 17 rr 

1\ 

J l 

1/ I 

I 

-....... I! J 

0 1,000 

I I 

I I I 

0 200 400 

lA I V 

,.--- """"' I L 

v 

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1\ 

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EXPLANATION 2,000 FEET 

I 

I - LAGOON AREA 

600 METERS - 50 - SIMULATED WATER-TABLE CONTOUR--

Shows altitude of simulated water table. 

Contour interval 2 feet. Datum is sea level 

PUMPED WELL 

Figure 47. Configuration of simulated water table under steady-state conditions for a projected pumping rate of 

0.25 million gallons per day west of the Pawcatuck River, Wood River Junction, Rhode Island. 

Numerical Simulation of Ground-Water Flow and Contaminant Transport 73 
\ I ~ .L~ - r-... 

/fteaa pwBr£ ok '~ ........ - ond ,...., ............ ~ / ~~ 

/VI ~ 111 ~( 

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0 1,000 2,000 FEET 

0 200 400 600 METERS 

\ 

\ 

\ 

I 

I 

'1:> 

/ 

/ 

1 ..... / 

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I 

I; 

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v I 

If I 

I 

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1/ 

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EXPLANATION 

- LAGOON AREA 

- 50 - SIMULATED WATER-TABLE CONTOUR--

Shows altitude of simulated water table. 

Contour interval 2 feet. Datum is sea level 

PUMPED WELL 

Figure 48. Configuration of simulated water table under steady-state conditions for a projected pumping rate of 

1 million gallons per day between the Pawcatuck River and Meadow Brook Pond, Wood River Junction, Rhode 

Island. 

withdrawn by the pumped wells will be contaminated. 

Limitations of the two-dimensional model with regard 

to the three-dimensional structure of the plume precluded further quantification of concentrations of contaminants in the water-supply wells. Later discussion 

of the solute-transport model shows aquifer cleanout 

times to be on the order of decades. Therefore, 

delayed development of this ground-water resource 

would reduce potential adverse effects from remaining 

contaminated ground water. 

Thus, ground-water development in the Wood 

River Junction area would be affected by the 

contamination plume but probably not enough to 

preclude development. If pumping rates in new wells 

were limited to about 0.25 Mgalld, excessive intake 

of contaminated ground water could be avoided. The 

restrictive assumptions of the two-dimensional arealflow model must be kept in mind when interpreting 

these results. 

74 Ground-Water Flow and Contaminant Transport at a Radioactive-Materials Processing Site, Wood River Junction, Rhode Island 
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--f- f---48 l./~ r\ / 1/ 1\ - 46 j-..... f--.. ....... r-.. r-.. ~~~ \ ""' ~ "'~ ~ ~· 

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2,000 FEET 

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1\ 

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1/ 

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l/v /v 

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v 

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1\ 

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f\ 

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EXPLANATION 

- LAGOON AREA 

- 50 - SIMULATED WATER-TABLE CONTOUR--

Shows altitude of simulated water table. 

Contour interval 2 feet. Datum is sea level 

PUMPED WELL 

Figure 49. Configuration of simulated water table for a projected pumping rate of 1 million gallons per day 

after 1 ,000 days of pumping between the Pawcatuck River and Meadow Brook Pond, Wood River Junction, 

Rhode Island. 

Simulation of Contaminant Transport 

The two contaminant species selected for 

numerical simulation of transport were nitrate and 

strontium-90. Nitrate was considered to be a 

noninteracting or conservative solute. Strontium-90 

was considered an interacting or nonconservative 

solute because it was seen to sorb or exchange slightly 

on the sediments. 

Theory and Assumptions 

The one-dimensional solute-transport model 

included the following transport mechanisms: 

advection, dispersion, sorption or ion exchange, and 

radioactive decay. The model was based on the 

following assumptions: ( 1) one-dimensional flow and 

solute transport; and (2) constant and uniform pore 

velocity, dispersion coefficient, and sorption 

parameters. Continuity of solute flux determined the 

Numerical Simulation of Ground-Water Flow and Contaminant Transport 75 
boundary conditions. The inlet-boundary condition 

was taken to be a decaying source, initially at 

background concentration. The initial condition was 

assumed to be a uniform solute distribution over the 

flow-path length for all simulations except two in 

which a stepwise variable distribution was used. 

The solute-transport equation for the fluid phase, in 

dimensionless form, is 

The transport equation for the sorbed phase, in 

dimensionless form, is 

dw 

a-= -S-a A w dt ' 

where 

e is e"le0 " (dimensionless fluid-phase 

concentration), 

t is (v/L (dimensionless time), 

y is DlvL (dimensionless dispersion 

coefficient), 

x is x" /L (dimensionless distance down the 

flow path), 

A is A"Uv (dimensionless radioactive-decay 

rate), 

S is S" Uv (dimensionless solute-transfer rate 

from sorbed to fluid phase), 

woP 

a is e' £ (dimensionless sorption factor 

relating fluid and sorbed 

concentrations), and 

w is w"lw0 ", (dimensionless sorbed-phase 

concentration). 

The dimensional variables and parameters are as 

follows: 

e" is fluid-phase concentration (AIL), 

C0 " is fluid-phase scaling concentration (AIL), 

( is time (T), 

v is interstitial velocity (Lff), 

L is flow-path length (L), 

D" is dispersion coefficient (L 2tr), 

x' is distance down the flow path (L), 

A is decay constant (T- 1 ), 

S' is solute-transfer rate from porous medium 

to fluid phase (A/T), 

p is bulk density of porous medium (MIL 3), 

(9) 

(10) 

£ is porosity (dimensionless), 

w" is sorbed-phase concentration (AIM), 

w0 " is sorbed-phase scaling concentration 

(AIM), and 

A is a suitable measure of the amount of 

species of interest (mass, activity, 

equivalents). 

Equations 9 and 10 are combined to eliminate 

the transfer rate, S, giving 

d

2

e de 

= y----Ae-aAw. 

dx2 dx (11) 

Nitrate Transport 

The transport equation for the nitrate concentration 

is obtained by removing the sorption and radioactivedecay terms from equation 11. Thus, 

de d

2

e de 

dt = ydx2- dx' 

The boundary conditions are, at x = 0, 

and, at x = 1 

de 

dx = O, 

where 

e is the dimensionless nitrate-ion 

concentration. 

The initial condition, at t = 0, is 

e = ei (x) , 

where 

(12) 

(13) 

(14) 

(15) 

ei is the dimensionless initial distribution of 

nitrate-ion concentration. 

A finite-difference algorithm was used to solve 

equations 9-15 numerically, with central differencing 

in space and time. One hundred nodes in the spatial 

domain were used with a time step of 0.001 

dimensionless time units. This avoided numerical 

oscillations from spatial or temporal discretization. 

Average background nitrate concentration was 

0.67 mg/L. The nitrate concentration used as a scaling 

concentration was 500 mg/L. Flow-path length for this 

one-dimensional analysis was taken to be 2,000 ft 

(fig. 46). Interstitial velocity was assumed to be 1 ft/d. 

76 Ground-Water Flow and Contaminant Transport at a Radioactive-Materials Processing Site, Wood River Junction, Rhode Island 
The longitudinal dispersivity value was estimated to 

range from 40 to 200 ft on the basis of the scale of the 

flow-path length (Lallemand-Barres and Peaudecerf, 

1978). This gave a range of 0.02 to 0.1 for the 

dimensionless dispersion coefficient. 

A uniform nitrate concentration of 500 mg!L was 

used as the initial condition. As a result, the amount of 

nitrate along the flow path was overestimated with 

regard to the measured nitrate concentration as shown 

in figure 26. 

The time required for the one-dimensional flow 

path column to become decontaminated (cleanout 

time) was taken to be that at which no calculated nitrate 

concentration exceeded the drinking-water MCL of 

1 0 mg!L as N (U.S. Environmental Protection Agency, 

1990). In dimensionless form, this translates to 

c <0.02. Cleanout times were calculated in 

dimensionless time (column volumes) and then 

converted to years. 

Results of the calculations are summarized in 

table 13 and figure 50. The times necessary for nitrate 

in the contaminated ground water to meet the drinkingwater MCL along the entire flow path ranged from 5.5 

to 12 years. The minimum value resulted from the 

unrealistic assumption of only advective transport with 

no dispersion. The maximum value was based upon a 

larger dispersion coefficient. Reducing the dispersion 

parameter increases the concentration gradient at the 

rear of the plume as it is transported out of the system. 

Thus, concentrations less than the drinking-water MCL 

are achieved more quickly. 

z 1 Oa: I -w l-ID 2 ::s~ =>:::> 

~z 3 

en 

I I I I I 

0 5 

I 

Table 13. Calculated nitrate cleanout times for selected 

parameter values used in the transport model, Wood River 

Junction, Rhode Island 

Simulation 

No. 

2 

3 

Dis persion 

(dimensionless) 

0 

.02 

.1 

Strontium-90 Transport 

Time to cleanout ( cx0.02) 

(dimen- t' 

sionless) (years) 

5.5 

1.5 8.0 

2.1 12 

The concept of a constant equilibrium-distribution 

coefficient commonly has been used to characterize 

strontium solute-sediment interaction. Distributioncoefficient values were compiled by Borg and others 

( 1976) and Isherwood (1981 ). 

Results of this investigation indicated that a 

constant equilibrium-distribution coefficient would not 

realistically characterize the strontium interaction with 

the outwash, sand and gravel sediments found in the 

study area. The approach taken was to develop a 

simplified, mass-action relation for ion-exchange of 

strontium-90 to account for the presence of the other 

major cations in the contamination plume. The 

parameters were determined from laboratory 

experiments, and this relation was then used in the 

formulation of a strontium-transport model. 

Calcium, magnesium, sodium, potassium, and 

strontium are the major cations in the contamination 

plume that undergo ion exchange with strontium-90 

and with each other on the sediments. Of these, calcium 

is the dominant cation, constituting about 90 percent of 

the equivalents in the cation mixture. The term 

I 

I 

I I I I I I I I 

10 15 

TIME TO CLEANOUT, IN YEARS 

Figure 50. Calculated nitrate-cleanout times at the radioactive-materials processing site, 

Wood River Junction, Rhode Island. 

Numerical Simulation of Ground-Water Flow and Contaminant Transport 77 
strontium refers to all strontium that is not strontium90. This term is virtually equal to the total strontium in 

the system because the concentration of strontium-90 is 

extremely small. As previously mentioned, potassium 

was dropped from the set of exchangeable cations 

because of analytical difficulties and its limited 

proportion in the system. 

The conceptual model developed for strontium-90 

interaction with the sediments was a simplified ionexchange relation rather than a complex, multispecies, 

reactive-solute transport model. Strontium-90 

transport simulation was based on the assumption of 

binary, divalent-ion exchange described previously, 

with the other component consisting of all the other 

exchanging cations. This simplification eliminates 

need to determine the parameters and solve the 

transport equations for the five remaining 

exchangeable cation species known to be present at this 

site. Simulation of solute transport with a multiplicity 

of chemical reactions and interactions and with 

concentration distributions of several species 

computed concurrently has been demonstrated with the 

CHEMTRN numerical model of Miller and Benson 

( 1983). Concentration-profile development for binary 

and tertiary ion-exchange systems with constant and 

variable total-ion concentration was shown by 

Valocchi and others ( 1981 ). The modeling approach 

taken in the present work is similar to their binary case 

with variable total-ion concentration; however, the 

additional mechanism of radioactive decay is included 

here, and the boundary conditions are different. All of 

these models account for competition among different 

cations for ion-exchange sites. 

As the first step, the five ionic components were 

reduced to three by considering calcium, magnesium, 

and sodium, as a single chemical component referred to 

as "C." This simplification is valid provided that the 

concentration ratios of Ca:Mg:Na are constant and 

uniform throughout the ground-water system. As 

previously discussed these ratios were, in fact, only 

approximately constant and uniform (computed from 

data in table 3). The simplification is also based on the 

assumption that the exchange of monovalent Na for 

divalent strontium is equivalent to that of exchanging 

divalent Ca and Mg for divalent strontium when 

expressed as chemical equivalents. The experimental 

determination of the exchange selectivity coefficients 

included sodium in the mixture of ions, and thus, 

included effects of monovalent and divalent ion 

exchange. The simplification was still considered 

acceptable because component C could be regarded as 

only calcium to a good approximation. In fact, the ratio 

of calcium to the other ions in contaminated ground 

water ranges from 6:1 to 33:1. The simplified tertiary 

system consists of ion exchange of ( 1) component C 

with strontium-90, (2) component C with strontium, 

and (3) strontium with strontium-90. The ratio of 

strontium to component C in the contaminated ground 

water is about 0.001:1. The exchange of strontium with 

strontium-90 was neglected because it would be much 

less than the amount of strontium-90 exchanged with 

component C. For example, 250 pCi/L of strontium-90 

is equivalent to a concentration of 1.77 Jlg/L. The two 

remaining ion-exchange reactions were assumed to be 

chemically equivalent. In fact, the component C 

exchange with strontium was used to determine the 

parameters for the exchange of component C with 

strontium-90. 

The second step was to combine the strontium with 

component C to form component B. This reduced the 

ion-exchange system to a binary system, component A 

being strontium-90. This further simplification is valid 

if the ratio of strontium to component C is assumed to 

be constant and uniform throughout the aquifer; again, 

constancy and uniformity were only approximately 

met. 

Divalent-ion exchange of two species can be 

described by the mass-action equation 

A+++ BX ¢:::)AX+ B++ , 

where 

(16) 

A and B represent the ion species in solution, 

and 

AX and BX represent the ion species sorbed 

onto the porous matrix at the 

stationary divalent anionic sites, X. 

At equilibrium, a selectivity coefficient can be 

defined with respect to the concentrations (Rubin and 

James, 1973; Schweich and Sardin, 1981; and Grove, 

1984) as given in equation 2. It is often assumed that 

. the selectivity coefficient is a constant. Valocchi and 

others (1981) indicated, however, that the selectivity 

coefficient can be variable in natural soil systems. If n 

exchangeable ions are considered, there is an 

equilibrium equation for each pair, and n-1 are 

78 Ground-Water Flow and Contaminant Transport at a Radioactive-Materials Processing Site, Wood River Junction, Rhode Island 
independent. For two ions, an isotherm curve relates 

the ionic concentration in the fluid phase to that on the 

porous medium, at a given temperature and normality 

(equivalents per liter) of the fluid phase (Schweich and 

Sardin, 1981). If the normality of the solution or the 

exchange capacity of the porous medium are not 

constant, or if more than two ions are considered, a 

single isotherm curve does not exist. This was the case 

at the study site. 

For a binary, divalent, ion-exchange mechanism 

with strontium-90 as the species of interest, the 

equilibrium sorption isotherm is (Valocchi and others, 

1981, Grove, 1984) 

~Qc'A w' = --------~----

A c' T + ( K ~ - 1 ) c'A ' 

where 

w A is the strontium-90 sorbed concentration 

(meq/g); 

(17) 

K~ is the selectivity coefficient (dimensionless) 

defined in equation 2; 

Q is the cation-exchange capacity (meq/g); 

c 'A is the strontium-90 fluid concentration 

(meq/L); and 

c 'r is the total-ion concentration (meq/L). 

Because the concentration of strontium-90 was 

much less than the concentration of each of the other 

cations, the isotherm equation 17 can be simplified to 

with slope 

w' A 

dw' A 

de' A 

= ~Q 

c' T 

(18) 

(19) 

Equation 19 is the ion-exchange relation that was 

used in the formulation of the strontium-transport 

model. It is based on the assumptions that the 

selectivity coefficient, total exchange capacity, 

Ca:Mg:Na concentration ratios, and Sr:Ca 

concentration ratio are all uniform and constant in the 

ground-water system. 

Equation 18 becomes a linear-equilibrium isotherm, 

if~' Q, and c' T are constants, that is, 

(20) 

where Kct is the distribution coefficient (mL/g) and 

the numerical factor converts liters to milliliters. The 

distribution coefficient can also be defined as the ratio 

of sorbed concentration to solution concentration under 

a given set of measurement conditions (Schweich and 

Sardin, 1981); that is, 

(21) 

Equation 20 relates the distribution coefficient to 

binary ion-exchange parameters provided that the 

concentration of the exchanging species of interest is 

low. In this case, Kd represents the slope of the 

exchange isotherm and is a constant only if the 

isotherm is linear and if the total-ion concentration is 

constant. Reardon ( 1981) describes why constant 

equilibrium distribution coefficients cannot properly 

describe reversible sorption reactions in ground-waterflow systems that are in a state of dynamic change. He 

shows an example where strontium-90 varies over an 

order of magnitude with time as the concentrations of 

major ions change. The simplified model of this work 

simulates the same effect. 

The transport equation for strontium-90 is obtained 

by converting equation 20 into dimensionless form and 

using the relation 

aw ac aw acT 

= ---+----- ac at acT at . 

By combining equations 11 and 22 and the 

dimensionless form of equation 20, the following 

equation is obtained: 

2 

(22) 

a cA acA ( <1> ) = y - - - - A. 1 + _!!_ c (23) 

a 2 ax C A' X T 

where the dimensionless ion-exchange parameter is 

tho = e~Q 'Y <" ' ' c. C To 

(24) 

Numerical Simulation of Ground-Water Flow and Contaminant Transport 79 
where 

cr is the dimensionless total-ion concentration, 

c A is the dimensionless strontium-90 

concentration, and 

c 'ro is the total-ion scaling concentration 

(meq/L). 

The boundary conditions are, at x = 0 

de A -'At 

-"{ dX + CA = CAbe 

and, atx = 1, 

where 

cAb is the dimensionless initial background 

concentration of strontium-90. 

The initial condition is at t = 0 is 

where 

(25) 

(26) 

(27) 

cAi is the dimensionless initial concentration 

distribution of strontium-90 (not necessarily 

background). 

Changes in the total-ion concentration resulting 

from radioactive decay were neglected because of the 

small amount of strontium-90 in the system. Because 

all of the concentrations are expressed in equivalents, 

there is no change of total-ion concentration from other 

exchanges that take place. The transport equation for 

the total-ion concentration is the same as equation 11 

with the associated boundary and initial conditions. 

Results from the laboratory measurements were 

used to establish parameter ranges for the sediment 

interaction of strontium-90. Background strontium-90 

concentration was 0.55 pCi/L (table 3), and 

background total-ion concentration was 0.45 meq!L. 

Cation-exchange capacity ranged from 0.18 to 

0.30 meq/g. The maximum strontium-90 

concentration measured was used as a scaling 

concentration; that is, c' =250 pCi/L. The associated 

total-ion concentration used as the scaling 

concentration for c' was 42.5 meq/L. The ionexchange selectivity coefficient was taken to be the 

average experimental value: ~ = 2. The half-life of 

strontium-90 is 28.5 years. Previously reported data 

(Ryan and others, 1985 and Ryan and Kipp, 1985) were 

used to determine the other hydrologic parameters for 

the transport simulations. Bulk-density and porosity 

measurements of p/E ranged from 3 to 5.5 g/cm3. 

Flow-path length for this one-dimensional analysis 

was assumed to be 2,000 ft (fig. 46; core or middle 

path), and interstitial velocity was assumed to be 

1.0 ft/d. Laboratory and field measurements led to the 

following ranges for the various parameters previously 

defined: 

'Y = 0.02 to 0.1 

'A = 0.1333 

CAb = 0.0022 

Crb = 0.011 and 

<l>o = 0.254 to 0.751. 

A uniform strontium-90 concentration of 250 pCi/L 

was used as the initial condition for most of the 

simulations. For two of the simulations (run numbers 

12 and 13) the initial condition was a distributed 

concentration down the flow path. Initial concentration 

was set at 200 pCi/L for the first 5 percent of the path 

length, 50 pCi/L for the next 75 percent of the path 

length, and 200 pCi/L for the final 20 percent of the 

path length. This scheme approximates the measured 

concentration distribution along the axis of the 

contamination plume, as shown in figure 28. 

The time required for the one-dimensional flowpath column to become decontaminated ( cleanout 

time) was taken to be that at which no calculated 

strontium-90 concentration exceeded the drinking 

water criterion of 8 pCi/L. In dimensionless form, this 

translates to cA<0.032. The cleanout time was 

calculated in dimensionless time (column volumes) 

and then converted to years. Cleanout times were 

calculated for various combinations of dispersion and 

ion-exchange parameters, with and without radioactive 

decay. The later cases (without radioactive decay) 

were run for illustrative purposes only; strontium-90 

will continue to decay as it moves through the system. 

Calculated strontium-90 cleanout times are 

summarized in table 14 and figure 51. The cleanout 

times necessary for the strontium-90 ground-water 

contamination to meet the drinking-water criterion 

along the entire flow path ranged from 5.5 to 18 years. 

The minimum value resulted from the unrealistic 

assumption of only advective transport with no 

dispersion, no ion exchange, and no radioactive decay. 

The maximum value was based upon the greater 

so Ground-Water Flow and Contaminant Transport at a Radioactive-Materials Processing Site, Wood River Junction, Rhode Island 
I 

2 

cr: 

w 3 

Ill 

:2 

::J 4 z 

z 

0 5 i= 

::s 

::J 6 :2 

Ci5 I 

7 

8,10 Constant I 

exchange 

9,11 Constant 

I exchange 

I I I I I I I I I l l l l I I I I I I 

0 5 10 15 

TIME TO CLEAN OUT, IN YEARS 

Figure 51. Calculated strontium-90 cleanout times at the radioactive-materials processing site, Wood River Junction, 

Rhode Island. 

20 

Table 14. Calculated strontium-90 cleanout times for selected parameter values used in the transport model, Wood River 

Junction, Rhode Island 

Radioactive Time to clean out ( cx0.032) 

Simulation Dispersion Sorption decay No. (dimensionless) {dimensionless) t' 

(dimensionless) (dimensionless) (years) 

1 0 0 0 1.0 5.5 

2 .02 0 .133 1.4 7.7 

3 .02 0 0 1.4 7.7 

4 .02 .254 .133 1.5 8.2 

5 .02 .751 .133 1.5 8.2 

6 .1 0 .133 1.9 10 

7 .1 0 0 2.0 11 

8 .1 .254 .133 2.0 11 

9 .1 .751 .133 2.0 11 

1

to .1 .254 .133 2.4 13 

•n .1 .751 .133 3.2 18 

212 .1 .751 .133 1.6 8.8 

213 .I .254 .133 1.6 8.8 

1

Constant linear equilibrium distribution coefficient. 

2Three-step initial condition distribution. 

Numerical Simulation of Ground-Water Flow and Contaminant Transport 81 
sorption-parameter value and a constant, linear, ionexchange distribution coefficient. For constant 

SOlr]ption, cleanout time was estimated to be 13 to 18 

years, whereas the binary, ion-exchange calculation 

yielded results of 8.2 to 11 years. The time necessary 

for radioactive decay alone to reduce strontium-90 

from its initial uniform concentration to the drinkingwater criterion was about 141 years. The more 

realistic, three-step initial-concentration distribution 

with the maximum concentration at 200 pCi/L, the 

higher dispersion coefficient (0.1) and the same range 

of sorption parameters as before (0.54 to 0.75I ), 

yielded a cleanout time of 8.8 years. 

The range of cleanout times was 5.5 to II years 

when no solute-sediment interaction mechanisms were 

included. The minimum time, however, is based on the 

assumption of no dispersion, which is unrealistic. 

Inclusion of dispersion and radioactive decay resulted 

in a range of 7. 7 to 10 years, with dispersion being the 

more controlling parameter. Although the low cationexchange-capacity values that were measured and the 

presence of high calcium concentrations initially 

indicated that essentially no strontium-sediment 

interaction was occurring, the constant linear-isotherm 

calculations showed a significant retardation of the 

transport rate by the slight exchange that took place at 

the high total-ion concentrations. Radioactive decay 

under the binary-exchange mechanism had only a 

minor effect because cleanout times were 11 years or 

ness, only 40 percent of the half-life for strontium-90. 

The similarity of the clean out times for strontium90 with those for the nitrate ion is somewhat 

coincidental; although the advective and dispersive 

mechanisms dominate the transport rates in both cases, 

the cleanout times are strongly dependent on the initial 

concentration distribution selected relative to the 

drinking-water criteria. 

CHeanout times were insensitive to the range of 

binary-exchange sorption parameters estimated to 

apply to this site but were somewhat sensitive to the 

range of dispersion parameters. The reasons for this 

can be deduced from an understanding of the transport 

rates of the total-ion concentration and the strontium90 and their binary-exchange interaction under the 

conditions assumed for this simulation study. To 

iHustrate, concentration profiles in space at selected 

times for the strontium-90 concentration and the totalion concentration in the ground water and the 

strontium-90 concentration sorbed on the porous 

medium are shown in figure 52 for simulation run 9 

(parameters are given in table 14 ). All variables are in 

dimensionless form. The concentration profiles 

(fig. 52) are shown at dimensionless time values of0.2, 

0.7, and 2.0. Initial-condition profiles are all at a 

dimensionless concentration of 1.0, and the 

dimensionless concentration corresponding to the 

drinking-water criterion for strontium-90 is shown as a 

dashed line at the value of 0.032. The different scaling 

concentrations cause the curves for strontium-90 and 

total-ion concentration to lie close together; in absolute 

units, the strontium-90 curves would be difficult to 

portray on the same graph as the total-ion 

concentration curves because they would be 

indistinguishable from the x-axis. 

The decrease of exchanged strontium-90 with time 

resulted from the small value of the exchange 

parameter and indicated that the concentration of 

strontium-90 in the fluid phase decreased faster than 

the exchange parameter increased at each point along 

the flow path. This behavior was not substantially 

affected by radioactive decay. 

For the case of a constant equilibrium distribution 

coefficient, the fluid-phase and sorbed-phase 

strontium-90 concentration curves would be coincident 

when expressed in dimensionless form. The 

synchronized movement of the sorbed- and fluid-phase 

solute is a characteristic of the constant-equilibriumdistribution-coefficient sorption mechanism. If a 

constant equilibrium distribution coefficient is 

assumed, the cleanout times are much more sensitive to 

the exchange or effective sorption parameter because 

the changes in the total-ion concentration do not affect 

the strontium exchange rate in this case. 

During the numerical simulations involving binary 

ion exchange, most of the strontium moved with the 

plume of high total-ion concentration. At a given point 

along the flow path, the strontium-90 concentration 

and the total-ion concentration in the flowing phase 

and the strontium-90 concentration in the sorbed phase 

all decreased with time. The ratio of sorbed to flowingphase strontium-90 concentration, however, increased 

with time as the decreasing total-ion concentration 

caused the ion exchange of strontium-90 to increase. 

The effective distribution coefficient changed by 

almost two orders of magnitude. The increased amount 

of strontium-90 that was sorbed on the sediments 

disappeared primarily by radioactive decay as shown in 

figure 52 by the horizontal parts of the sorbed 

strontium-90 curves that drop with time. The lower 

~2 Grrcii.!IU1JdloW!Jil~rr IF~ow SlU1ldl CoU1l~atmiU1latll1~ Trrsunsport m~ m Rmdlozcihte-Mm~erlmls !Processing Slie, Wood River Junction, Rhode lslmnd 
1.0 

z 

0 

~ <( 0.8 

0:: 

Iz - w 

() 

z 0.6 0 

() 

CJ) 

CJ) 

w 

_J 

z 0.4 

0 

CJ) 

z 

w 

~ 

0 0.2 

0.2 0.4 0.6 

STRONTIUM-90 IN GROUND WATER 

TOTAL ION IN GROUND WATER 

STRONTIUM-90 ON POROUS MEDIUM 

STRONTIUM-90 DRINKING WATER 

CRITERION 

0.8 1.0 

DIMENSIONLESS DISTANCE ALONG FLOW PATH 

Figure 52. Dimensionless strontium-90 and total ion concentration profiles for selected dimensionless 

times at the radioactive-materials processing site, Wood River Junction, Rhode Island. 

sorbed concentrations near the beginning of the flow 

path are caused by the leaching of strontium-90 that 

occurs as a result of the uncontaminated ground water 

entering the system. The relatively small amount of 

ion exchange that occurs on these clay-free quartz and 

feldspar sands prevents the total.-ion concentration 

plume from moving completely ahead of the high 

strontium-90 contamination plume in the fashion of 

separation chromatography. The cleanout-time 

calculation is not sensitive to the range of exchange 

parameter employed because the reduction in total-ion 

concentration with time at each point along the flow 

path dominates the exchange calculation for strontium90. This reduction of total-ion concentration increases 

the effective sorption parameter by about two orders of 

magnitude, and the concentration-reduction rate is 

independent of the exchange parameter. 

Strontium-90 concentrations in ground water 

decreased slightly faster for the lower value of the 

sorption parameter, but the time to cleanout was 

reduced by less than 0.5 year. The time for the totalion concentration plume to reach background value 

was 4.2 dimensionless time units, whereas the 

strontium-90 concentration in the ground water 

decreased to drinking-water criterion by 2.0 

dimensionless time units. 

The effects of reducing the dispersion parameter 

from 0.1 to 0.02 were a quicker reduction of the totalion concentration to background level (2.1 instead of 

4.2 dimensionless-time units) and a faster cleanout of 

the strontium-90 (1.5 instead of2.0 dimensionless-time 

units). With reduced dispersion, the effective sorption 

parameter changed from the lower (0.254) to the higher 

value (0.751) more abruptly as the high total-ion 

concentration plume moved out of the system, and 

more of the strontium-90 moved out with it. Note that 

the solute dispersive fluxes were in the opposite 

direction of ground-water flow. 

Transport simulations made with a constant linearequilibrium sorption parameter for strontium-90 

showed that fluid- and sorbed-phase concentrations 

decreased with time at a given point along the flow path 

but with a constant concentration ratio as required by 

this mechanism. Strontium-90 concentrations in the 

ground water were higher than the corresponding ones 

Numerical Simulation of Ground-Water Flow and Contaminant Transport 83 
for the binary-exchange mechanism until the time 

when, under the binary-exchange mechanism, the 

strontium started leaching from the porous medium 

lbacl!c into the flowing ground water. The shorter 

decmout times calculated by the binary-exchange 

mechanism were caused by the increased ion exchange 

relative to the constant-parameter equilibrium-sorption 

mechMism, which caused more strontium-90 to be 

sorbed! onto the porous medium. The resulting decline 

in strontium-90 concentration in the ground water 

enalblledl the drinking-water criterion to be achieved all 

a.nong the flow path more quickly; however, the time 

necessary to reduce both flowing- and sorbed-phase 

stronth.llm-90 concentrations to background was longer 

under the binary-exchange mechanism. Thus, 

increased! sorption provides a beneficial reduction in 

peal!c solute concentration in the ground water while not. 

increasing the sorbed concentration above the initial 

condition. 

Uncertainties in the dispersive and sorptive 

transport parameters and their effect on the ca1culated 

strontium-90 deanout times were discussed above. 

Variations in assumed interstitial ground-water 

vellocity and! flow path length, however, will have a 

major effect on calculated! cleanout times. U the 

interstitnan velocity or flow-path length are changed 

from the assumed values of 1 ft/d and 2,000 ft, 

respectiveny, the simulations must be recalculated 

because the dimensionless decay rate and the 

dimensionless dispersion coefficient will change. 

Three-dimensionan flow and transport effects were 

exdudledl from this analysis, as were variations in 

interstitia] ground-water velocity along the flow path. 

These uncertainties and! limitations must be kept in 

mind! when interpreting the calculated cleanout times at 

this site. 

The day-free quartz and feldspar sands at this site 

have nnune potential for strontium-90 sorption 

compared! to sediments with strongly sorbing clay 

mineranogy. The high calcium, magnesium, and 

sodlium concentrations in the contamination plume 

monopoHze the few sorption or ion-exchange sites that 

are present; however, the ion exchange that does take 

pHace retards the transport rate of strontium-90 through 

the system and! reduces the peak concentrations in the 

ground! water. 

This experimental work and the previous work of 

others have shown that a constant equilibriumdistribution coefficient is inadequate to characterize 

the sorption of strontium-90 onto sandy outwash 

sediments. By including the dependence of the 

strontium-90 ion exchange on the total-ion 

concentration in the ground water, numerical 

simulations show that the contamination plume of 

total-ion concentration moves slightly ahead of the 

strontium-90 plume. Reduction in concentration of 

competing ions causes an increase in strontium-90 

exchange, which reduces the strontium-90 

concentration in the ground water. This enables 

the drinking-water criterion for strontium-90 to be 

reached more quickly than if equilibrium sorption 

were occurring under conditions of constant total-ion 

concentration. The cleanout times necessary for 

ground-water flow to reduce the strontium-90 

concentration to the drinking-water criterion, 

calculated by a one-dimensional transport model with 

binary-ion exchange and total-ion-concentration 

dependence, were about two-thirds of the clean out time 

calculated when a constant equilibrium-distribution 

coefficient was used. 

The transport model simulations indicated that it 

would take on the order of one or two decades for 

natural ground-water flow and radioactive decay to 

reduce the strontium-90 contamination to the drinkingwater criterion at this site; however, the restrictive 

assumptions on which these simulations are based 

must be kept in mind. In many contaminated groundwater systems, this type of model, with binary-ion 

exchange and parameter dependence on the total-ion 

concentration, simulates solute transport more realistically than constant parameter exchange models. The 

assumption that ion exchange is described by a single 

isotherm is avoided; thus, the model is suitable under 

conditions of variable total-ion concentration that are 

usually found at ground-water contamination sites. 

The assumptions of one-dimensional and uniformtransport parameters for the model formulation, 

however, restrict the realism of the model. No field 

data are available for quantitative judgment of the 

accuracy of this solute transport model. The results 

should be viewed qualitatively rather than quantitatively; future monitoring of the plume would be necessary to assess the accuracy of these cleanout-time 

forecasts and to validate the model. 
SUMMARY AND CONCLUSIONS 

Liquid wastes from an enriched uranium cold-scrap 

recovery plant in southern Rhode Island have leaked 

into a highly permeable sand and gravel aquifer. The 

resultant plume of contamination extends 2,300 ft from 

the source area (evaporation ponds and trenches) to the 

aquifer's discharge area in the Pawcatuck River and 

swampy area west of the river. Dilution, however, 

precludes detection of contaminants once they have 

entered the river. Freshwater recharge commonly 

keeps the contaminants depressed below the water 

table on the plant side of the river, and during seasonal 

high flow conditions, below the water table in the 

swamp. Electromagnetic conductivity and differential 

piezometer surveys were successfully used to estimate 

and locate the extent of contamination, but the highly 

three-dimensional structure of the plume is difficult to 

portray and model. 

Chemical and radiochemical constituents in the 

plume include boron, calcium, nitrate, potassium, 

strontium-90, and technetium-99. Decreasing 

concentrations of these constituents over the time 

period of the study indicate flushing or clean out of the 

aquifer. Unconsolidated deposits that form the aquifer 

contain few clay-sized particles, and little sorption has 

been detected. Although some strontium-90 sorption 

was measured, the strontium-90 ion exchange is 

directly affected by competition with calcium. In the 

swamp, reducing conditions may promote observable 

solute interaction with sediments or organic material. 

Ion exchange retards the transport rate of strontium90 through the system and reduces the peak 

concentrations in the ground water. The simple onedimensional solute transport model for strontium-90 

calculated cleanout times of 5.5 to 18 years. Future 

monitoring of the plume would be necessary to assess 

the accuracy of cleanout-time forecasts. 

The areal- and cross-sectional-flow models 

matched observed conditions with root mean square 

differences in potentiometric head at 1.42 ft and 

1.19 ft, respectively. Throughout different subareas, 

the simulated areal water levels compared fairly well 

to the observed levels. The models were used to 

evaluate several pumping scenarios to determine the 

effect of the contamination plume on water-supply 

development. 

Ground-water development in the Wood River 

Junction area will be affected by the contamination 

plume at the site but probably not enough to prevent 

development. Simulation results indicate that pumping 

rates would have to be limited to about 0.25 MgaVd to 

avoid excessive intake of contaminated ground water. 

The restrictive assumptions of two-dimensional 

models must be kept in mind when interpreting the 

results, especially those pertaining to potential for 

ground-water development in the area. 

The primary conclusions drawn from this work are 

related to the objectives of the study and are 

highlighted with bullets. The secondary conclusions 

are more closely related to lessons learned during the 

investigation and, as a result, are related to field 

techniques or study approach. 

Objective I: Identify chemical and radiochemical 

constituents in the plume. 

• The constituents of most concern from a public 

health standpoint were nitrate and strontium-90. 

The maximum nitrate concentration exceeded 

the MCL ( 10 mg/L as N) by 73 times, and the 

maximum strontium-90 concentration exceeded 

the drinking water criterion (8 pCi/L) by 36 

times. 

Objective 2: Determine solute interaction with aquifer 

materials. 

• The clay-free quartz and feldspar sands at the site 

have little potential for sorption, however, heterogeneities in the sediments observed particularly 

in the swamp and river, increased the potential 

for sorption on sediments in these locations. 

• The strontium-90 ion exchange, however, is 

directly affected by competition with calcium. 

In the swamp, reducing conditions may promote 

detectable solute interaction with sediments or 

organic material. 

Objective 3: Determine location, movement, and fate 

of constituents in the plume. 

• The structure of the plume of contaminated 

ground water was three-dimensional and considerably more complex than anticipated. For 

example, the contaminated water discharged not 

only to the Pawcatuck River (the primary sink 

for the basin) but also to an adjacent swamp. 

Summary and Conclusions 85 
• Seasonal fluctuations of recharge affected movement of the contaminated water. Freshwater 

recharge depressed the plume of contaminated 

ground water below the water table. Vertical displacement of the contaminated water was easily 

observed seasonally in wells near the swamp and 

the river. 

• Seasonal fluctuations in the horizontal structure 

of the plume were not as easily discernible, perhaps because the sampling network was less 

dense in the horizontal plane than in the vertical 

plane. 

• The transport-chemistry experiments showed 

that a constant equilibrium-distribution 

coefficient was inadequate to characterize the 

sorption of strontium-90 onto the sandy outwash 

sediments. The cleanout times necessary for 

ground-water flow to reduce the strontium-90 

concentration to the drinking-water criterion, 

calculated by a one-dimensional transport model 

with binary-ion exchange and total-ion concentration dependence, were about two-thirds of 

those calculated when the more approximate 

approach of a constant equilibrium-distribution 

coefficient was used. 

• The simplified transport model used at the site 

indicated that it will be on the order of one or 

two decades for natural ground-water flow and 

radioactive decay to reduce the strontium-90 

concentration to the drinking-water criterion. 

• The time period of this study precluded detection 

of a leading edge of contaminated ground water 

at the surface-water discharge points (river and 

swamp) or in wells, preventing the measurement 

of observed traveltimes to compare with I 

calculated values. 

• Eventually, most of the solutes in the contaminated ground water will discharge to the Pawcatuck River and adjacent swamp. The exception 

will be those constituents that have irreversibly 

sorbed onto the sediments (none identified) or 

those that have been permanently incorporated 

into the biomass. There is evidence obtained by 

others subsequent to this study that strontium 

enrichment has occurred in selected growth rings 

of oak trees in the swamp. 

Objective 4: Estimate the effects of ground-water 

contamination on future ground-water development 

adjacent to the site. 

• According to simulation results using a twodimensional areal-flow model developed for the 

site, ground-water development in the Wood 

River Junction area would be influenced by the 

contaminated ground water but probably not 

enough to preclude development. If pumping 

rates of new water-supply wells were limited 

to about 0.25 Mgalld, excessive intake of 

contaminated ground water could be avoided. 

Secondary conclusions of the investigation are as follows: 

• Of the numerous field techniques used at the site 

to define the geologic and hydrologic environment, three were most effective in guiding the 

research: an electromagnetic conductivity 

meter, a differential-head piezometer, and 

borehole geophysical logging. 

• A screened auger flight for simultaneous drilling 

and sampling would have accelerated the process 

of locating contaminated ground water .. 

• It was not possible to determine if the boundary 

between the coarse sands and gravels and the fine 

sands and silts was an effective barrier to 

ground-water contamination. The lower hydraulic conductivities of the finer materials indicates 

that contamination in these materials is spread 

only by diffusion and by some invasion by 

vertical flow from the more conductive zone. 

• The plume of contaminated water has a threedimensional configuration; therefore, it would 

have been appropriate to model the flow and 

transport in three dimensions. However, at the 

time of this study, computational costs and limitations of the data set precluded development of 

a fully three-dimensional model. Instead, an 

areal and a cross-sectional two-dimensional flow 

model and a one-dimensional transport model 

were developed. 

• The data set that resulted from this study would 

support additional flow and transport modeling. 

• Additional monitoring would be required to 

validate the cleanout times calculated by the 

modeling. 

86 Ground-Water Flow and Contaminant Transport at a Radioactive-Materials Processing Site, Wood River Junction, Rhode Island 
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Selected References 89 


ISBN 0-607-86634-9 

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