Effects of Remedial Grouting on the

Ground-Water Flow System at

Red Rock Dam near Pella, Iowa

Water-Resources Investigations Report 00–4231

Prepared in cooperation with the

U.S. Army Corps of Engineers

Rock Island District

Rock Island, Illinois

U.S. Department of the Interior

U.S. Geological Survey
U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 00–4231

Effects of Remedial Grouting on

the Ground-Water Flow System at

Red Rock Dam near Pella, Iowa

By S. Mike Linhart and Bryan D. Schaap

Iowa City, Iowa

2001

Prepared in cooperation with the

U.S. ARMY CORPS OF ENGINEERS

ROCK ISLAND DISTRICT

ROCK ISLAND, ILLINOIS
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, 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. Geological Survey.

For additional information write to: Copies of this report can be purchased

U.S. Geological Survey

Information Services

Box 25286

Federal Center

Denver, CO 80225

from:

District Chief

U.S. Geological Survey

P.O. Box 1230

Iowa City, IA 52244
CONTENTS III

CONTENTS

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

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

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

Previous Studies........................................................................................................................................................... 2

Acknowledgments ....................................................................................................................................................... 4

Description of Study Area ..................................................................................................................................................... 4

Hydrogeology .............................................................................................................................................................. 4

Ground-Water Flow System Prior to Remedial Grouting............................................................................................ 6

Remedial Grouting Program.................................................................................................................................................. 6

Description of Observation Wells and Methods of Study ..................................................................................................... 8

Observation Wells........................................................................................................................................................ 8

Water-Level Data Collection and Analysis.................................................................................................................. 8

Water-Quality Data Collection and Analysis............................................................................................................... 10

Effects of Remedial Grouting on the Ground-Water Flow System ....................................................................................... 11

Flow Patterns ............................................................................................................................................................... 11

Relation Between Water-Level Changes in the Reservoir, Tailwater, and Observation Wells .................................... 14

Water Chemistry .......................................................................................................................................................... 25

Sulfate Concentrations....................................................................................................................................... 25

Chloride Concentrations .................................................................................................................................... 26

Stable Isotopes ................................................................................................................................................... 28

Hydrogen and Oxygen............................................................................................................................. 28

Sulfur ....................................................................................................................................................... 31

Summary and Conclusions .................................................................................................................................................... 33

Selected References ............................................................................................................................................................... 35

FIGURES

1. Map showing Red Rock Dam study area and location of observation wells.............................................................. 3

2. Hydrogeologic section through the northeastern part of Red Rock Dam; trace of section

shown in figure 1......................................................................................................................................................... 5

3. Graph showing amount of grout placed during remedial grouting at Red Rock Dam ............................................... 7

4. Map of potentiometric surface of the overburden, March 18, 1999 ........................................................................... 12

5. Map of potentiometric surface of the bedrock, March 18, 1999................................................................................. 13

6. Hydrograph showing water levels for Red Rock pool, tailwater, and selected bedrock

wells on the northeast side of Red Rock Dam ............................................................................................................ 16

7. Hydrograph showing water levels for Red Rock pool and selected bedrock wells on

the northeast and southwest sides of Red Rock Dam ................................................................................................. 19

8. Hydrograph showing water levels for Red Rock pool and bedrock wells R–92–2 and

R–92–2A on the northeast side of Red Rock Dam ..................................................................................................... 20

9. Graph showing correlation coefficients between water levels in selected bedrock wells,

well 30–O, pool, and tailwater elevations during pre-grout and post-grout periods on

the northeast side of the dam ...................................................................................................................................... 22

10. Graph showing correlation coefficients between water levels in selected bedrock wells,

pool, and tailwater elevations during pre-grout and post-grout periods on the southwest

side of the dam ............................................................................................................................................................ 24

11. Graph showing sulfate concentrations in the tailwater and selected observation wells

through time ................................................................................................................................................................ 26

12. Boxplot showing pre-grout and post-grout sulfate concentrations for wells 23–R

and 5–RB .................................................................................................................................................................... 27
IV CONTENTS

13. Graph showing chloride concentrations in the tailwater and selected observation

wells through time ...................................................................................................................................................... 28

14. Graph showing hydrogen isotope ratios for the tailwater and selected observation wells ......................................... 29

15. Graph showing oxygen isotope ratios for the tailwater and selected observation wells............................................. 30

16. Plot of oxygen-18/oxygen-16 relative to hydrogen-2/hydrogen-1 for stable isotope sampling events

at Red Rock Dam........................................................................................................................................................ 31

17. Graph showing sulfur isotope ratios for the tailwater and selected observation wells ............................................... 32

TABLES

1. Data pertaining to selected bedrock wells in the Red Rock Dam study area.......................................................... 9

2. Data pertaining to selected overburden wells in the Red Rock Dam study area..................................................... 10

3. Water levels for selected bedrock wells, Red Rock pool, and Red Rock tailwater on May 13, 1992,

and March 18, 1999, and the differences between the two days............................................................................. 15

4. Water levels for selected overburden wells, Red Rock pool, and Red Rock tailwater on May 13, 1992,

and March 18, 1999, and the differences between the two days............................................................................. 17

5. Spearman correlation coefficients for water levels in selected bedrock wells, well 30–O, Red Rock

pool, and Red Rock tailwater .................................................................................................................................. 21

CONVERSION FACTORS AND VERTICAL DATUM

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F = (1.8 × °C) + 32

Abbreviated water-quality units used in this report: Chemical concentration is given in milligrams per liter (mg/L) and micrograms per

liter (μg/L). Milligrams per liter is a unit expressing the concentration of chemical constituents in solution as weight (milligrams) of solute

per unit volume (liter) of water. Micrograms per liter is a unit expressing the concentration of chemical constituents in solution as weight

(micrograms) of solute per unit volume (liter) of water. For concentrations less than 7,000 mg/L, the numerical value is the same as for

concentrations in parts per million.

Per mil: A unit expressing the ratio of stable-isotopic abundances of an element in a sample to those of a standard material. Per mil units

are equivalent to parts per thousand. Stable-isotopic ratios are computed as follows (Kendall and Caldwell, 1998):

where

X is the heavier isotope and

R is the ratio of the heavier, less abundant stable isotope to the lighter, stable isotope in a sample or standard.

Multiply By To obtain

acre-foot (acre-ft) 1,233 cubic meter

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

square mile (mi2) 2.590 square kilometer

cubic foot (ft3) 0.02832 cubic meter

δX R(sample)

R(standard) ---------------------------- 1 –     = × 1 000 ,
CONTENTS V

The values for oxygen, hydrogen, and sulfur stable isotopic ratios discussed in this report are referenced to the following standard

material:

Sea level: In this report, “sea level” refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929)—a geodetic datum derived

from a general adjustment of the first-order level nets of both the United States and Canada, formerly called Sea Level Datum of 1929.

Water year: The 12-month period October 1 through September 30. The water year is designated by the calendar year in which it ends.

Thus, the year ending September 30, 1996, is called water year 1996.

Ratio (R) Standard identity and reference

hydrogen-2:hydrogen-1 Vienna Standard Mean Ocean Water (VSMOW)

oxygen-18:oxygen-16 Vienna Standard Mean Ocean Water (VSMOW)

sulfur-34:sulfur-32 Vienna Canyon Diablo Troilite (VCDT), defined by assigning a value of −0.3 per mil exactly to

IAEA–S–1 silver sulfide

δ
Abstract 1

Effects of Remedial Grouting on the Ground-Water

Flow System at Red Rock Dam near Pella, Iowa

By S. Mike Linhart and Bryan D. Schaap

Abstract

Previous studies have shown direct

evidence of underseepage at Red Rock Dam

on the Des Moines River near Pella, Iowa. Underseepage is thought to occur primarily on the

northeast side of the dam in the lower bedrock

of the St. Louis Limestone, which consists of

discontinuous basal evaporite beds and an overlying cavity zone. Because of concerns about the

integrity of the dam, the U.S. Army Corps of

Engineers initiated a remedial grouting program

in September 1991. To assess the effectiveness

of the remedial grouting program and to evaluate

methods for future assessments, a study was

conducted by the U.S. Geological Survey in

cooperation with the U.S. Army Corps of

Engineers.

Potentiometric surface maps of the overburden and bedrock indicate that the direction

of ground-water flow on the northeast side of the

dam has changed little from pre-grout to postgrout periods. A comparison of water levels,

between a pre-grout date and a post-grout date,

shows that water levels decreased but that the

decrease may be more attributable to changes in

dam operations than to remedial grouting. Waterlevel data for the same two dates indicate that

a more gradual potentiometric surface exists on

the northeast side of the dam than on the southwest side of the dam, which suggests that the

hydraulic connection between Lake Red Rock

and downgradient bedrock wells still is greater

on the northeast side of the dam than on the southwest side. Hydrographs for some wells on the

northeast side of the dam indicated a departure

from pre-grout trends at approximately the same

time grouting was initiated. To varying degrees,

hydrographs for the same wells then appear to

return to a trend similar to pre-grout years,

possibly as a result of new flow paths developing

over time after remedial grouting. Spearman

correlation coefficients computed for water levels

in wells, pool, and tailwater indicate that some

areas on the northeast side of the dam appear to be

less under the influence of changing pool elevations after grouting than before grouting. This

suggests that the hydraulic connection between

the Red Rock pool and some downgradient areas

has decreased.

Analysis of water samples collected from

selected wells on the northeast side of the dam

shows significant increases in sulfate concentrations beginning about the same time remedial

grouting was done upgradient from the wells,

possibly indicating that flow paths were cut off

to these wells, thereby reducing the amount of

mixing with fresh reservoir water. Observable

changes in chloride concentrations or trends as

a result of remedial grouting were not apparent.

Analysis results for hydrogen and oxygen stable

isotope samples collected since 1995 indicate

large seasonal fluctuations of isotope ratios in

the tailwater (assumed representative of the reservoir). Similar but more subdued fluctuations were

observed at some wells, but other wells appeared

to have little seasonal change. Stable sulfur

isotope results indicate the presence of distinct

water types between Lake Red Rock and in

ground water from downgradient bedrock wells.
2 Effects of Remedial Grouting on the Ground-Water Flow System at Red Rock Dam near Pella, Iowa

Sulfur isotope values from samples from a

bedrock well located upgradient from the grout

curtain indicate a mixture of pool and ground

water, whereas samples from downgradient overburden wells have values similar to the pool.

Samples from the bedrock wells downgradient

from the grout curtain have sulfur isotope values

similar to a value obtained from analysis of a

gypsum and anhydrite core sample.

Hydrographs, statistical analysis of waterlevel data, and water-chemistry data suggest that

underseepage on the northeast side of the dam

has been reduced but not completely eliminated.

Some areas appear to have been affected to a

greater degree and for a longer period of time than

other areas. Future monitoring of water levels,

water chemistry, and stable isotopes can aid in the

evaluation of the long-term effectiveness of remedial grouting.

INTRODUCTION

In 1991, a cooperative study by the

U.S. Geological Survey (USGS) and the U.S. Army

Corps of Engineers (COE) (Lucey, 1991) found direct

evidence of seepage through the bedrock foundation

of Red Rock Dam near Pella, Iowa (fig. 1), which

could have threatened the integrity of the dam. The

evidence for underseepage included correlation

between water levels and chloride concentrations

in Lake Red Rock with water levels and chloride

concentrations in bedrock observation wells downstream from the dam (Lucey, 1991). The COE

initiated a remedial grouting program at the dam

in September 1991. The purpose of the remedial

grouting program was to decrease the permeability of

the bedrock on the northeast side of the dam and limit

underseepage. Grouting was completed along approximately 3,000 feet of the axis of the dam, from 1991

through 1997 (fig. 1). A second study was conducted

in 1999 by the USGS in cooperation with the COE to

assess the effects of the remedial grouting on the

ground-water flow system. The study involved

comparisons of pre-grouting and post-grouting relations in water-level and water-chemistry data. Knowledge will be gained about the application of stable

isotope techniques that could be used to assess and

monitor ground-water flow systems in other regions

of the country.

Purpose and Scope

This report presents the results of the study to

evaluate the effects of the remedial grouting program

conducted by the COE from 1991 to 1997 on the

ground-water flow system at Red Rock Dam.

The report includes descriptions of (1) the

geology in the vicinity of Red Rock Dam; (2) the

ground-water flow system in the vicinity of Red

Rock Dam prior to the remedial grouting program;

and (3) the effects of the remedial grouting program

on the ground-water flow system. Changes in the time

correlation and absolute differences between water

levels, sulfate concentrations, and chloride concentrations in Lake Red Rock and the observation wells are

discussed. The effect of the remedial grouting program

on ground-water flow and chemistry (indicators of

underseepage) is described. Stable isotope (hydrogen,

oxygen, and sulfur) data from samples collected since

December 1995 at several observation wells in the

study area and from the Lake Red Rock tailwater are

evaluated as a possible tool for assessing underseepage

at Red Rock Dam in the future.

Previous Studies

There have been several studies investigating

underseepage at Red Rock Dam. The U.S. Army

Corps of Engineers (1970) detected higher water

levels in observation wells on the northeast side of

the dam than in wells on the southwest side. Large

calcium and sulfate concentrations were detected

below the dam in water flowing in the old river

channel and drainage ditches on the northeast side

of the valley, providing evidence of underseepage

through underlying bedrock. The effect on the municipal water supply of Pella as a result of the formation

of Lake Red Rock was reported by W.L. Steinhilber

(U.S. Geological Survey, written commun., 1971),

who cited increased concentrations of sulfate,

dissolved solids, and hardness as a result of the

upward movement of ground water from the St. Louis

Limestone into the alluvial aquifer. A U.S. Army

Corps of Engineers (1975) study found that specificconductance values were lower in observation wells

at higher pool (reservoir) elevations, indicating mixing

of reservoir water with ground water. Another

U.S. Army Corps of Engineers (1984) study found

further evidence of underseepage from temperature

data; a zone of colder water in glacial sands and upper
INTRODUCTION 3

Lake Red Rock

Des Moines River

(tailwater)

0 400 800 FEET

EXPLANATION

Well nest location and number

Single well location and number

Location of well sampled for

 isotopes and number

Cross section A—A’ (fig. 2)

Grout curtain

Road

Open water

IOWA

Des Moines

Pella

41¡22'30"

92¡59' 92¡59'30"

41¡22'

Figure 1. Red Rock Dam study area and location of observation wells.
4 Effects of Remedial Grouting on the Ground-Water Flow System at Red Rock Dam near Pella, Iowa

bedrock in the northeast abutment indicated a

possible flow path of colder reservoir water. During

very high reservoir levels in 1984 (U.S. Army Corps

of Engineers, 1984), mapping of the potentiometric

surface showed a more gradual gradient at the northeast side of the dam when compared to the southwest

side of the dam for both overburden and bedrock

zones, implying the existence of a hydraulic connection between Lake Red Rock and ground water at

the northeast side of the dam. Lucey (1991), using

chloride concentration data as a tracer, found peak

concentrations in both the overburden and bedrock

observation wells 1 to 4 months after peak chloride

concentrations occurred in Lake Red Rock, furthering

evidence of underseepage. In the same study, higher

correlation coefficients between pool-elevation

changes and water-level changes in observation wells

were found in data from the northeast abutment than

in areas closer to the tailwater (on the northeast side of

the dam) or on the southwest side of the dam. After

remedial grouting in 1991–94, potentiometric surfaces

appeared to be lower for both the overburden and

bedrock at the northeast side of the dam than during

pre-grout periods (B.D. Schaap, U.S. Geological

Survey, written commun., 1995). At one well, water

levels were determined to be less influenced by pool

elevations after the remedial grouting than water levels

before the grouting, and analysis of water quality

showed less correlation to reservoir concentrations

after grouting than before grouting (B.D. Schaap,

U.S. Geological Survey, written commun., 1995).

At a nested well site, water levels decreased, while

pool elevations increased or stayed the same, and the

water chemistry changed from a bicarbonate to sulfate

water type, indicating less mixing with reservoir water

(B.D. Schaap, U.S. Geological Survey, written

commun., 1995).

Acknowledgments

The authors thank Vern Greenwood of the

U.S. Army Corp of Engineers for providing waterlevel and water-chemistry data as well as technical

assistance. The authors also thank Donna Lutz of

Iowa State University, Department of Civil and

Construction Engineering, for collection of waterquality and stable isotope samples and responses to

inquiries regarding interpretations of isotope data.

DESCRIPTION OF STUDY AREA

Red Rock Dam is located on the Des Moines

River, 30 miles southeast of Des Moines, Iowa (fig. 1).

The study area is located in section 19 in township 76

north and range 18 west (T. 76 N., R. 18 W., sec. 19)

in Marion County and covers approximately 1 mi2

.

The crest of the dam extends 5,841 ft between the

bluffs in the Des Moines River valley and is 100 ft

high with an elevation at the top of the dam of 797 ft

above sea level. The land surface of the alluvial valley

ranges from 702 to 708 ft above sea level, and bluffs

rise to 850 ft above sea level along both sides of the

valley. The reservoir (Lake Red Rock) formed by

Red Rock Dam has a maximum storage capacity of

1,489,900 acre-ft at a full flood-control level of 780 ft

above sea level (May and others, 1999). The current

conservation pool level is 742 ft above sea level.

Hydrogeology

Bedrock units of the Des Moines River

valley within the study area include the Warsaw

Limestone of Mississippian age, which consists of

dolomitic shales and argillaceous shales. The Warsaw

Limestone is overlain by the St. Louis Limestone of

Mississippian age, which forms the bedrock surface

in the Des Moines River valley and consists of alternating beds of limestone, sandstone, dolomite, and

discontinuous basal evaporite beds. The alternating

beds of limestone, sandstone, and dolomite are

hereinafter referred to as the “upper bedrock” of

the St. Louis Limestone. The discontinuous basal

evaporite beds of the St. Louis Limestone consist

of gypsum and anhydrite beds, overlain by a cavity

zone. Gypsum is the predominant evaporite mineral

present in the evaporite beds within the study area.

The cavity zone formed from dissolution of evaporites

and resultant slumping of the overlying bedrock

and consists of numerous fractures and cavities. The

cavities are either open or are filled with clay, silt,

sand, and rock fragments. The discontinuous basal

evaporite beds and overlying cavity zone of the

St. Louis Limestone is hereinafter referred to as the

“lower bedrock.” Within the valley, the bedrock is

overlain by Quaternary-age alluvium and glacial

outwash deposits (Lucey, 1991).
DESCRIPTION OF STUDY AREA 5

In the valley bluffs, the St. Louis Limestone

is overlain by rocks of the Cherokee Group of

Pennsylvanian age, which consist primarily of

black shales with occasional interbedded siltstone,

sandstone, limestone, and coal. Overlying the

Pennsylvanian strata is Pleistocene pre-Illinoian

glacial till consisting mainly of clay with fine, clayey

sands and occasional fine- to medium-grained sand

layers. Along the center line of the dam, glacial till is

present primarily in the northeast bluff area (see fig. 1

for location of northeast and southwest bluff areas),

whereas dam fill material and Pennsylvanian-age

strata predominate in the southwest bluff area (Lucey,

1991). A layer of loess approximately 10 to 20 ft thick

overlies the glacial till on the bluffs (U.S. Army Corps

of Engineers, 1984). The unconsolidated glacial and

alluvial deposits and dam fill overlying the bedrock

units throughout the study area are hereinafter referred

to as the “overburden.”

Figure 2 shows elevations of the land surface,

bedrock surfaces, potentiometric surfaces, and

well-screen depths at the northeast side of the dam.

0 200 400 600 800 1,000 1,200 1,400 1,600 1,790

DISTANCE FROM A, IN FEET

600

850

650

700

750

800

ELEVATION, IN FEET ABOVE SEA LEVEL

EXPLANATION

Observation well

Screened interval

Grout curtain 

Land-surface elevation

Bedrock potentiometric surface, May 13, 1992

Bedrock potentiometric surface, March 18, 1999

23–R

21–O

21–UR

30–O

2–O

2–R

15–O

1–R

26–R

R–92–1

Bend in

section

South

A

North

A'

Grout

curtain

Well nests

Vertical exaggeration x 2.4

Datum is sea level

Overburden

Pennsylvanian-age 

strata

St. Louis Limestone

Warsaw Limestone

Top of dam 

at center line

Figure 2. Hydrogeologic section through the northeastern part of Red Rock Dam; trace of section shown in figure 1.
6 Effects of Remedial Grouting on the Ground-Water Flow System at Red Rock Dam near Pella, Iowa

Locations of wells and trace of section used to

construct section A–A' are shown in figure 1. The

overburden along this section ranges in thickness from

about 35 to 90 ft. The Pennsylvanian-age strata is as

much as 40 ft thick and the St. Louis Limestone ranges

from about 38 to 75 ft in thickness. Bedrock water

levels on two different dates ranged from about 710 to

701 ft above sea level. The grout curtain extends from

the bottom of the St. Louis Limestone to the top of the

bedrock surface.

The Warsaw Limestone is the lower confining

unit at the Red Rock Dam study area and has limited

permeability, which retards movement of ground

water. The St. Louis Limestone contains two waterbearing parts, referred to as the lower bedrock and

upper bedrock. Where overlying consolidated

units are absent, unconfined conditions are present

in the St. Louis Limestone. Where present, overlying

consolidated units produce confined conditions in the

St. Louis Limestone. The Pennsylvanian-age strata

are generally confining units and contain interspersed

sandstone water-bearing units, but the units are not

extensive. The overburden contains glacial and alluvial sands that are water-bearing units. Regional flow

in the St. Louis Limestone is generally in a southwesterly direction (Lucey, 1991).

Ground-Water Flow System Prior

to Remedial Grouting

Water levels measured on February 23, 1984,

July 13, 1984, and July 9, 1990, indicate that, in

general, the gradient of the potentiometric surface of

the overburden was from the topographic highs of the

valley bluffs toward the Des Moines River (Lucey,

1991). Potentiometric maps indicated that the direction of ground-water movement in the overburden at

the northeast side of the dam is approximately at a

45-degree angle (toward the south) to the axis of the

dam and the river regardless of pool levels (Lucey,

1991). Ground-water flow in the evaporite zone

of the St. Louis Limestone also was generally toward

the river. At lower pool elevations, ground-water flow

in the lower bedrock of the St. Louis Limestone was

more parallel to the axis of the dam, whereas at higher

pool elevations, ground-water flow had a slightly more

downstream direction. Potentiometric-surface elevations for nested well pairs indicate that ground-water

movement is in an upward direction in the river valley

where confining conditions exist in the evaporite beds

of the St. Louis Limestone. Ground-water movement

was found to be in a downward direction in the northeast bluff area as the St. Louis Limestone is recharged

from the overburden (Lucey, 1991).

Differences in hydraulic connectivity between

the reservoir and the ground water were evident from

potentiometric-surface gradients on the southwest and

northeast sides of the dam. A very steep gradient was

observed along the southwest side of the dam between

the reservoir and the downstream wells, indicating

restricted hydraulic connectivity. A more gradual

gradient was observed on the northeast side of the

dam, indicating the presence of underseepage from the

reservoir through bedrock to the wells downstream

from the dam axis. Correlations between water-level

changes in the reservoir and wells, along with observations of peak chloride concentrations in the reservoir

and wells, provided further evidence that, prior to

remedial grouting beginning in 1991, water was

flowing from the reservoir through the basal evaporite

zone and overlying cavity zone of the St. Louis

Limestone (Lucey, 1991).

Based on comparisons of observed changes

in water levels (during a period when the pool elevation increased 28.2 ft) between wells in the northeast

bluff area and wells located at a far enough distance

and in a completion interval high enough (completed

in shale bedrock of Pennsylvanian age) to reflect

water-table conditions above the zone of influence

of the reservoir, water-level increases in wells in

the northeast bluff area were thought to be primarily

a result of the hydraulic connection to Lake Red Rock

rather than from climatological effects (Lucey, 1991).

REMEDIAL GROUTING PROGRAM

The COE began the remedial grouting

program in September 1991. The grouting program

was intended to reduce the permeability of the

St. Louis Limestone at the northeast side of the dam

and to decrease underseepage through the evaporite

zone. Grouting was completed over a 3,000-ft length,

parallel (northwest of the center line of the dam) to the

axis of the dam. Grouting was completed in two stages

(stage I and stage II) (U.S. Army Corps of Engineers,

1998).
REMEDIAL GROUTING PROGRAM 7

Figure 3 shows the amount of grout required

at different locations along the grout curtain. The

grouting during the initial phase (1991 and 1992,

stage I) included stations 2500 to 3600 (distance

from a reference point, in feet; reference point not

shown in fig. 3). Hole spacing for the primary grouting

was on 20-ft centers with secondary holes placed

in between primary holes on 10-ft centers. Tertiary

holes at 5-ft centers and quaternary holes on 2.5-ft

centers also were placed in regions where large

volumes of grout were required (U.S. Army Corps

of Engineers, 1998).

Stage II grouting began in 1994 and continued

through 1997. This stage of the grouting included

stations 3550 to 5500. The primary holes were placed

on 10-ft centers, and secondary and tertiary holes

were placed on 5-ft and 2.5-ft centers, respectively.

Quaternary holes also were placed in regions where

large volumes of grout were required. Grout was

pumped into the holes until refusal. Multiple rows of

holes were grouted in regions of high grout volumes.

No grouting was done in 1993 because of high pool

elevations (U.S. Army Corps of Engineers, 1998).

In 1996 (fig. 3), significant amounts of grout

were required between stations 4400 and 5200; more

than 3,000 ft3

 of grout was placed at station 4760.

Between stations 4600 and 4900, large volumes of

grout were needed, even in some tertiary and quaternary holes, indicating the complexity and irregularity

of the geology. Many other stations, however, required

very little grout in tertiary and quaternary holes,

indicating closure of the grout curtain. Total volume

of grout placed during remedial grouting (stages I

and II) was approximately 77,650 ft3

. Most grout was

pumped at a 3-to-1 water-to-cement mix (U.S. Army

Corps of Engineers, 1998).

5,500 5,000 4,500 4,000 3,500 3,000 2,500

DISTANCE FROM REFERENCE POINT, IN FEET

0

3,500

500

1,000

1,500

2,000

2,500

3,000

AMOUNT OF GROUT PLACED, IN CUBIC FEET

Southwest Northeast

EXPLANATION

1991 (Red)

1992 (Blue)

1994 (Brown)

1995 (Yellow)

1996 (Green)

1997 (Purple)

Figure 3. Amount of grout placed during remedial grouting at Red Rock Dam.
8 Effects of Remedial Grouting on the Ground-Water Flow System at Red Rock Dam near Pella, Iowa

DESCRIPTION OF OBSERVATION WELLS

AND METHODS OF STUDY

Observation Wells

Currently (2000) there are about 85 observation

wells in the Red Rock Dam study area. The locations

of selected bedrock wells and overburden wells are

shown in figure 1. Selected wells are those for which

accurate location information is available. Wells designated “−O” are completed in the overburden materials,

either alluvium, glacial till, or dam fill material. Wells

completed in the evaporite zone or the cavity zone of

the St. Louis Limestone are designated by “−R” (lower

bedrock), “−RA” (evaporite zone), “−RB” (cavity

zone), or “−L” (lower bedrock). Wells designated “−U”

are completed in sandstone in the upper bedrock of the

St. Louis Limestone, and “−UR” designates wells

completed in limestone in the upper bedrock of the

St. Louis Limestone. Wells labeled “R−” generally

are completed in the lower bedrock of the St. Louis

Limestone. Exceptions to wells designated “R−”

include well R–88–3, which is completed in the

bedrock of the Cherokee Group in the bluff on the

northeast side of the dam, and well R–88–4, which

is completed in extremely fractured and weathered

limestone of the St. Louis Limestone in the former

Des Moines River channel. Observation well nests (for

example, 2–O, 2–R) were installed by nesting two separate pipe assemblies in a single well bore and isolating

the well screens at different depths (Lucey, 1991).

Tables 1 and 2 list observation well numbers,

station, distance offset from the center line on top of

the dam, elevations at the top of the wells, and elevations at the bottom of the well screens. Station

numbers indicate that station 3120 is 3,120 ft southwest (following the center line of the dam) of a reference point, station 0000, located in the bluff area to the

northeast of the dam. Direction of offset is designated

as either upstream (U/S) or downstream (D/S) from

the center line of the dam. Distances between wells

along the center line of the dam and offset distances

to wells may not necessarily agree between figure 1

and tables 1 and 2. Stations and offsets in tables 1

and 2 were obtained by the COE using conventional

surveying techniques, whereas well locations in

figure 1 were obtained by the USGS using a handheld

global positioning system (GPS). Elevations at the top

of the wells (tables 1 and 2) were obtained by the COE

using conventional surveying techniques.

Water-Level Data Collection and Analysis

Since Red Rock Dam was completed in

1969, the COE has collected water-level data from

observation wells. Water levels were measured at

62 observation wells from July 1988 to July 1990,

before remedial grouting began (Lucey, 1991). Since

that time, additional wells have been added to the

network and some wells have been removed from the

network. Currently (2000), there are about 85 observation wells in the water-level measuring network. Water

levels are measured monthly or bimonthly by COE

personnel. Water levels also are measured monthly at a

selected subset of the observation wells by Iowa State

University at the same time water-quality samples are

collected. Since 1969, elevation for Lake Red Rock

has been measured at a water-stage recorder (COE

station 05488100 Lake Red Rock near Pella, Iowa)

at the outlet works on the upstream side of the dam.

From 1968 to 1992, tailwater elevation was measured

at water-stage recorders at COE stations located at

various sites downstream from Red Rock Dam. Since

1992, tailwater elevation is measured at a water-stage

recorder (USGS station 05488110 Des Moines River

near Pella, Iowa) located 0.4 mi downstream from the

dam (May and others, 1999).

Potentiometric-surface maps are used to determine ground-water flow directions and gradients and to

evaluate any changes in flow direction that may result

from remedial grouting. Data used to construct potentiometric maps are from wells on the northeast side

of the dam for which accurate location information is

available. Wells completed in the upper bedrock of the

St. Louis Limestone were not used to construct potentiometric maps. Also, for wells with similar water levels

and with similar locations, only one of the water levels

was used in construction of the potentiometric map. In

addition, two points in time, a pre-grout day and a postgrout day, at similar pool elevations, were compared to

evaluate changes in relations between water levels in

selected observation wells, Lake Red Rock elevation,

and tailwater elevation. Some wells had not been drilled

prior to the pre-grout day; however, post-grout water

levels for these wells are presented in table 2 for reference to the potentiometric maps. Comparisons of water

levels and water-level changes through time, between

selected observation wells, tailwater, and the pool, were

made using time-series plots. Statistical analysis of

water-level data for selected bedrock wells using

Spearman correlation coefficients was used to detect

possible changes in the response of ground-water levels
DESCRIPTION OF OBSERVATION WELLS AND METHODS OF STUDY 9

Table 1. Data pertaining to selected bedrock wells in the Red Rock Dam study area

[D/S, downstream; U/S, upstream; shaded rows indicate wells located on the southwest side of dam]

Well number

(fig. 1)

Station

(distance from reference

point in northeast

bluff along center

line of dam)

(feet)1

1Station numbers indicate that station 3120 is 3,120 ft southwest (following the center line of the dam) of a reference point, station 0000, located in the

bluff area to the northeast of the dam.

Offset from center

 line of dam

(feet)

Elevation of

top of well

(feet above

sea level)

Elevation of

 bottom of

well screen

(feet above

sea level)

1–R 3120 85 D/S 784.40 642.60

2–R 3403 192 D/S 753.90 645.10

3–R 3802 230 D/S 730.10 643.40

4–R 4001 17 D/S 797.40 646.10

5–R 4279 229 D/S 729.30 645.40

5–RA 4264 229 D/S 733.40 651.10

5–RB 4264 229 D/S 732.30 659.20

6–R 4550 19 D/S 797.30 643.90

7–R 5040 19 D/S 796.90 648.10

8–R 5201 18 D/S 796.30 647.00

9–R 7247 237 D/S 723.90 640.60

10–R 7249 21 D/S 795.70 639.60

11–R 8001 22 D/S 796.60 633.00

12–R 8400 22 D/S 796.90 633.20

13–R 8700 23 D/S 796.90 633.50

14–R 8904 113 D/S 777.30 630.80

16–R 6000 230 D/S 724.60 639.60

18–R 4281 378 D/S 715.90 649.60

21–UR 3580 241 D/S 730.30 689.50

22–UR 3404 19 D/S 797.10 683.70

23–R 3817 371 D/S 718.20 645.70

26–R 2750 58 D/S 797.50 636.80

27–R 3250 375 D/S 774.90 642.60

83–L 8300 240 D/S 723.60 635.70

83–U 8300 240 D/S 723.90 654.00

84–L 8400 240 D/S 723.80 632.50

84–U 8400 240 D/S 723.70 652.00

85–L 8500 240 D/S 723.50 631.30

85–U 8500 240 D/S 723.80 656.20

R–88–3 2870 256 D/S 792.00 740.60

R–88–4 4924 503 D/S 701.90 652.80

R–88–5L 3786 230 D/S 731.90 639.70

R–88–5U 3786 230 D/S 732.00 650.40

R–88–6 4199 385 D/S 714.00 639.70

R–92–1 3092 200 U/S 780.80 649.20

R–92–2A 3440 25 U/S 797.40 647.00

R–92–2 3540 18 D/S 797.10 647.00

R–96–1 8686 289 D/S 751.00 641.80

R–96–2 8683 299 D/S 751.00 633.80
10 Effects of Remedial Grouting on the Ground-Water Flow System at Red Rock Dam near Pella, Iowa

to changes in pool and tailwater elevations between

pre-grout and post-grout periods. The Spearman rank

order correlation coefficient is a measure of the degree

to which two variables increase (or decrease) monotonically, even though the relation may not necessarily be

linear (Ott, 1993). Correlation coefficients closer to 1

indicate better correlation to, and possibly better

hydraulic connection with, the pool. Wells selected for

the Spearman statistical analysis were those which were

thought to be representative of the spatial distribution of

wells within the study area to enable comparisons

between the northeast and southwest sides of the dam.

Water-Quality Data Collection

and Analysis

Water-quality samples are collected monthly

from seven observation wells (5–RA, 5–RB, 23–R,

30–O, R–96–1, R–96–2, and 29–O–95), the tail

water, and the pool, by Iowa State University. Onsite measurements include specific conductance, pH,

temperature, and alkalinity. The wells are pumped, and

once the onsite measurements stabilize, samples are

collected. Samples are analyzed for major cations,

major anions, nitrate, total hardness, carbon dioxide,

Table 2. Data pertaining to selected overburden wells in the Red Rock Dam study area

[D/S, downstream; U/S, upstream; shaded rows indicate wells located on the southwest side of dam]

Well number

(fig. 1)

Station

(distance from point in

northeast bluff along

center line of dam)

(feet)1

1Station numbers indicate that station 3403 is 3,403 ft southwest (following the center line of the dam) of a reference point, station 0000, located in

the bluff area to the northeast of the dam.

Offset from center

line of dam

(feet)

Elevation of

top of well

(feet above

sea level)

Elevation of

bottom of

well screen

(feet above

sea level)

2–O 3403 192 D/S 753.90 698.90

3–O 3802 230 D/S 730.40 692.20

4–O 4001 17 D/S 797.50 685.80

5–O 4279 229 D/S 729.30 688.30

6–O 4550 19 D/S 797.40 677.00

7–O 5040 19 D/S 797.10 673.50

8–O 5201 18 D/S 796.50 673.70

9–O 7247 237 D/S 724.10 678.80

10–O 7249 21 D/S 795.90 679.80

11–O 8001 22 D/S 796.90 677.40

12–O 8400 22 D/S 797.20 672.70

13–O 8700 23 D/S 797.20 714.60

15–O 3251 108 D/S 775.60 726.50

16–O 6000 230 D/S 724.60 673.60

17–O 4600 319 D/S 715.90 693.70

18–O 4281 378 D/S 715.90 692.60

19–O 4051 261 D/S 728.90 686.60

20–O 3903 375 D/S 717.50 683.70

21–O 3580 241 D/S 730.40 702.30

22–O 3404 19 D/S 797.30 710.00

23–O 3717 371 D/S 717.70 690.30

25–O 4855 480 D/S 709.00 691.00

27–O 3250 375 D/S 775.00 741.50

28–O 3946 698 D/S 713.40 690.40

29–O–95 5099 355 D/S 706.30 676.30

30–O 3532 185 D/S 737.70 692.90

31–O 5368 359 D/S 704.80 677.50
EFFECTS OF REMEDIAL GROUTING ON THE GROUND-WATER FLOW SYSTEM 11

suspended solids, total solids, and dissolved silica.

Saturation indices are calculated for calcite, dolomite,

and gypsum. The Analytical Services Laboratory at the

Department of Civil and Construction Engineering of

Iowa State University provided the analytical services.

Time-series plots were used to compare concentrations of selected ions in water sampled from selected

observation wells and the tailwater (assumed to be

representative of water from Lake Red Rock). These

concentrations are assumed to reflect the mineralogy of

the adjacent geologic formations. Any changes in water

chemistry, excluding naturally occurring fluctuations,

from pre-grout periods to post-grout periods may indicate changes in flow paths or residence time and, therefore, a change in underseepage conditions.

Boxplots of water-quality data (sulfate concentrations) were used to illustrate differences between

pre- and post-grout years. Wilcoxon rank-sum

nonparametric statistical tests (Ott, 1993) were used

to evaluate the statistical significance of differences

between pre- and post-grout sulfate concentrations.

Values were generated by the tests to describe the

probability that observed differences between pre- and

post-grout periods occurred by chance. A probability

(p) value of 0.05 indicates a 95-percent confidence that

observed differences are not the result of chance

occurrence. Differences with probability values of

0.05 or less were considered significant.

Since December 1995, samples for stable

isotope analysis of oxygen, hydrogen, and sulfur

(

18O/16O, 2

H/1H, and 34S/32S) have been collected

from selected wells by Iowa State University. These

wells are the same ones used for the regular waterquality sampling with the exception of well 29–O–95

(fig. 1). Samples for isotope analysis also are collected

from the tailwater, which is assumed to be representative of water from Lake Red Rock. Samples are

collected bimonthly from wells 23–R and R–96–2

and the tailwater. The remaining wells are sampled

two or three times per year. In addition to the wells

sampled by Iowa State University, USGS personnel

collect samples for isotope analysis once a year from

wells 21–O, 21–UR, 26–R, R–88–3, and R–92–1.

These samples are analyzed by the USGS Stable

Isotope Laboratory in Reston, Virginia.

The use of natural stable isotopes and their relative abundance can be a useful tool for studying the

characteristics and origins of ground water (Gat and

Gonfiantini, 1981). The heavy isotopes of oxygen

(

18O) and hydrogen (2

H) and their relative abundance

can act as tracers for meteoric water (precipitation

derived from the atmosphere) and, therefore, be indicators of the degree to which reservoir water is mixing

with ground water. Because there are significant

sources of sulfur in the evaporite zone on the northeast

side of the dam, sulfur (34S) isotopes also can be

useful for determining the degree of mixing of reservoir water and ground water. Time-series plots of these

isotopes for selected wells and the tailwater are used to

show their relative abundance and to monitor changes.

As sampling for stable isotopes at Red Rock Dam has

been done only since December 1995, a comparison of

pre-grout and post-grout periods is not possible.

EFFECTS OF REMEDIAL GROUTING ON

THE GROUND-WATER FLOW SYSTEM

Flow Patterns

A potentiometric surface map of the overburden

for March 18, 1999, is shown in figure 4. The pool

elevation this day was 742.79 ft above sea level. In the

northeast bluff area, ground-water flow in the overburden is in a southeasterly direction. Farther to the

southwest along the axis of the dam, the flow through

the overburden is approximately parallel to the axis of

the dam, in a southwesterly direction. The potentiometric surface around well 21–O (fig. 4) is higher

than the surrounding wells (with the exception of

well 22–O). The nested well pair, 21–O and 21–UR,

had water levels of 706.6 ft and 705.5 ft, respectively,

on this day, indicating a downward component of flow

at this location. This pattern also is consistent with

historical data at this location, as the water level in

well 21–O is most often higher than in 21–UR and

the surrounding wells. The general flow direction

through the overburden in the northeast bluff area

appears very similar to that described by Lucey (1991)

for the overburden on February 23, 1984, at what then

was a conservation pool elevation of 731.1 ft and tailwater elevation of 697.4 ft. However, flow farther

southwest along the axis of the dam, closer to the

Des Moines River, is more similar to the flow direction

described by Lucey (1991) on July 13, 1984, when

the pool elevation was 773.4 ft and tailwater elevation

was 699.7 ft. Flow direction in this area of dam on

July 13, 1984, was more parallel to the axis of the

dam compared to February 23, 1984, when flow
12 Effects of Remedial Grouting on the Ground-Water Flow System at Red Rock Dam near Pella, Iowa

direction was more southerly away from the axis

of the dam. Similarly, the lower tailwater elevation

(690.72) on March 18, 1999 (fig. 4), compared to

February 23, 1984 (697.4 ft), resulting in a greater

head difference between the reservoir and the tailwater (52.07 ft on March 18, 1999, and 33.70 ft on

February 23, 1984) appears to result in a change

in flow direction in the overburden to a more southwesterly (parallel to the axis of the dam) direction in

this part of the dam site. It appears that there has been

little change in the general flow direction in the overburden since remedial grouting.

Lake Red Rock

706

710

704

702

700

698

696

694

692

708

Pontentiometric contour Shows elevation at

 which water level would have stood in tightly cased

 wells. Contour interval 2 feet. Datum is sea level

Overburden well location and number

Grout curtain

Open water

Generalized direction of ground-water flow

31 O

16 O

23 O 19 O

4 O

22 O

2 O

21 O 30 O

20 O

17 O

6 O

5 O

18 O

28 O

29 O 95

8 O 25 O

7 O

EXPLANATION

0 800 FEET 400

2 O

92o 92 58'15" o

58'45"

41o

22'15"

41o

22'45"

Figure 4. Potentiometric surface of the overburden, March 18, 1999.
EFFECTS OF REMEDIAL GROUTING ON THE GROUND-WATER FLOW SYSTEM 13

Ground-water flow in the bedrock (fig. 5)

closest to the northeast bluff is primarily in a southeasterly direction, with flow occurring in a southwesterly direction farther downstream from the dam

axis. The flow changes to a more southerly direction

as one moves toward the southwest along the dam

axis. This also is very similar to the flow patterns

described by Lucey (1991) on July 9, 1990 (at a pool

conservation level of 764.4 ft above sea level), on the

northeast side of the dam. Comparisons of potentiometric gradients for two days (February 23, 1984, and

July 9, 1990) indicated that when higher pool elevations exist, ground-water flow in the bedrock becomes

more parallel to the axis of the dam (primarily in the

Pontentiometric contour Shows elevation at

 which water level would have stood in tightly cased

 wells. Contour interval 2 feet. Datum is sea level

Bedrock well location and number

Grout curtain

Open water

Generalized direction of ground-water flow

2 R

EXPLANATION

0 800 FEET 400

Lake Red Rock 706 708

704

702

700

698

696

694

692

18 R

R 88 6

5 R

5 RA

23 R

R 88 5L

4 R

16 R

8 R

7 R

R 92 2A 2 R

27 R

1 R

26 R

R 92 1

R 92 2

41o

22'15"

41o

22'45"

92o

58'45" 92o

58'15"

Figure 5. Potentiometric surface of the bedrock, March 18, 1999.
14 Effects of Remedial Grouting on the Ground-Water Flow System at Red Rock Dam near Pella, Iowa

area to the southwest along the axis of the dam toward

the Des Moines River), as was the case within the

overburden. At lower pool elevations, ground-water

flow was more perpendicular to the dam axis (Lucey,

1991). The direction of ground-water flow in the

bedrock on March 18, 1999 (pool elevation 742.79 ft),

suggests flow patterns more consistent with higher

pool elevations, at least compared to flow patterns

described by Lucey (1991) on February 23, 1984,

when the pool elevation was 731.1 ft. Based on the

potentiometric surface on March 18, 1999, it appears

that the general direction of ground-water flow in the

bedrock has changed little since pre-grout periods. A

potentiometric high still exists in the northeast bluff

area, suggesting that seepage through the overburden

down into the bedrock and underseepage through

bedrock and the grout curtain may still be occurring,

at least to some degree.

Relation Between Water-Level Changes in

the Reservoir, Tailwater, and Observation

Wells

A comparison of water-level elevations

for selected bedrock wells for pre-grout and postgrout conditions (May 13, 1992, and March 18, 1999,

respectively) is given in table 3. These dates were

selected because of similar pool and tailwater elevations. May 13, 1992, was the earliest date at which the

pool elevation was near the conservation level of

742 ft above sea level to enable comparison to a postgrout day with a similar conservation pool elevation.

Grouting in 1992 began in late March at station 2935

and did not proceed very far along the axis of the

dam by May 13, 1992; therefore, hydrologic conditions represent a pre-grout date for the majority of

wells. Grouting occurred immediately upstream

from well 26–R in 1991 (fig. 3).

For most of the wells along the length of the

dam, water levels decreased between May 1992 and

March 1999 (table 3). Relatively large decreases

occurred in wells 16–R (–4.0 ft), 8–R (–3.7 ft), and

7–R (−3.3 ft). These three wells are closest to the

Des Moines River on the northeast side of the control

structure. Well 11–R on the southwest side of the

dam had a water-level decrease of –3.9 ft. Closer

to the northeast bluff area, water levels for some of

the bedrock wells increased between the two days.

This occurred for wells 1–R, 4–R, 21–UR, 26–R,

and 27–R. The closer to the northeast bluff, the

greater the increase in water levels, with the exception

of well 26–R. Increases of 3.8 ft (1–R), 1.4 ft (27–R),

and 0.6 ft (4–R) were larger than the increase in pool

elevation (0.39 ft) between the two days. There

were no increases in water levels on the southwest

side of the dam between the two days. Because

pool elevations in the months prior to May 13, 1992,

were lower (fig. 6) than pool elevations preceding

March 18, 1999, the increase in water levels at these

wells may indicate a response to the higher conservation pool elevations during the post-grout period.

These results indicate that there may still be a greater

hydraulic connection between the bedrock in the

vicinity of these wells and the reservoir compared to

other areas on the northeast side of the dam (that is,

farther to the southwest along the dam axis) and the

southwest side of the dam. Lucey (1991) observed that

these same wells (with the exception of 4–R) had

significant responses (increase in water levels) to

changing reservoir levels, indicating greater hydraulic

connectivity compared to other areas downgradient

from the dam. Median changes in water levels for the

two dates are very similar between the two sides of

the dam (northeast –2.25 ft and southwest –2.10 ft).

Most of the overburden wells along the length

of the dam showed a decrease in water levels between

the two dates (table 4). Most decreases were very

similar among these wells; however, well 30–O in

the northeast bluff area had the smallest decrease

of –0.3 ft. Overburden wells 9–O, 10–O, 11–O, and

12–O on the southwest side of the dam had somewhat

larger decreases than those on the northeast side of the

dam. The largest decreases on the northeast side of the

dam were in those wells closest to the river, (7–O,

–3.9 ft), (8–O, –3.7 ft) and, (16–O, –3.4 ft). The

larger decreases observed in these wells (compared

to those closer to the northeast bluff area) may be

related to their better hydraulic connection to the tailwater. Pre-grout tailwater elevations were higher for

several months prior to water-level measurements on

May 13, 1992, than for the preceding months before

water levels were measured on March 18, 1999 (see

tailwater elevations in fig. 6).

On the northeast side of the dam, the highest

water levels observed were in wells 22–O (711.3 ft),

21–O (706.6 ft), 2–O (706.5 ft), and 30–O (704.4 ft).

These wells are located in the northeast bluff area

and may indicate that ground-water flow in this

area is moving down through the overburden from
EFFECTS OF REMEDIAL GROUTING ON THE GROUND-WATER FLOW SYSTEM 15

the reservoir. Water levels in the overburden wells

on the southwest side of the dam (with the exception

of well 13–O, 715 ft) were lower than those measured

in overburden wells on the northeast side of the dam,

probably due to the absence of permeable glacial

material on the southwest side of the dam and the

presence of lower hydraulic conductivity dam fill

material (Lucey, 1991). Water levels in wells 21–O

and 22–O increased between May 13, 1992, and

March 18, 1999. This may be a result of higher conservation pool elevations preceding the post-grout day

compared to lower conservation pool elevations

Table 3. Water levels for selected bedrock wells, Red Rock pool, and Red Rock tailwater on May 13, 1992, and

March 18, 1999, and the differences between the two days

[--, no data; shaded rows are those wells located on the southwest side of the dam]

Well number or

surface-water body

May 13, 1992, water level

(feet above sea level)

March 18, 1999, water level

(feet above sea level)

Difference

(feet)

1–R 704.4 708.2 +3.8

2–R 704.0 703.0 –1.0

3–R 703.4 701.2 –2.2

4–R 704.9 705.5 +0.6

5–R 702.4 699.5 –2.9

5–RA 701. 9 699.3 –2.6

5–RB 701.9 699.3 –2.6

6–R No reading No reading --

7–R 699.7 696.4 –3.3

8–R 697.0 693.3 –3.7

9–R 692.1 690.9 –1.2

10–R 700.4 698.0 –2.4

11–R 703.6 699.7 –3.9

12–R 695.7 693.6 –2.1

13–R 696.5 693.9 –2.6

14–R 696.0 694.0 –2.0

16–R 696.3 692.3 –4.0

18–R 701.8 699.5 –2.3

21–UR 705.3 705.5 +0.2

22–UR 705.6 705.5 –0.1

23–R 703.1 701.2 –1.9

26–R 706.3 706.9 +0.6

27–R 704.4 705.8 +1.4

83–L 694.1 692.3 –1.8

83–U 694.1 692.7 –1.4

84–L 694.9 692.8 –2.1

84–U 694.7 693.6 –1.1

85–L 695.9 693.5 –2.4

85–U 695.9 693.7 –2.2

R–88–4 No reading 695.2 --

R–88–5L 703.4 701.0 –2.4

R–88–5U 703.2 701.1 –2.1

R–88–6 700.3 698.0 –2.3

R–92–1 No reading 709.8 --

R–92–2A No reading 708.2 --

R–92–2 No reading 704.2 --

Red Rock pool 742.40 742.79 +0.39

Red Rock tailwater 690.50 690.72 +0.22
16 Effects of Remedial Grouting on the Ground-Water Flow System at Red Rock Dam near Pella, Iowa

1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 680

790

690

700

710

720

730

740

750

760

770

780

WATER LEVEL, IN FEET ABOVE SEA LEVEL

Red Rock pool

Well R–92–1

Well 26–R

Well 23–R

Well 5–RB

Red Rock tailwater

preceding the pre-grout day (fig. 6). The effects on

water levels in the overburden, as a result of remedial

grouting, are probably minimal and may likely be

more related to dam operations and the geology

of the overburden material.

For the nested pairs of wells, almost all

of the water levels were higher in the bedrock

wells compared to the overburden wells for both the

pre-grout and post-grout days, indicating an upward

component of ground-water flow. Therefore, there was

little change in direction of the vertical component of

flow between the two days. Changes in the direction of

the vertical component of flow, from upward to downward, did occur between the two dates in the area

around the well nest 21–O and 21–UR. However,

when considering water levels in wells 21–O and

21–UR over time (data not shown), which includes

both pre-grout and post-grout periods, water levels

are typically higher in well 21–O, indicating that

the vertical component of ground-water flow in this

area is most often downward. The change in flow

direction is probably a result of the lower pool elevations in the months preceding May 13, 1992, when

the overburden in the vicinity of 21–O was not

being recharged from the reservoir. Water levels

in well nest 2–O and 2–R and well nest 22–O

and 22–UR indicate downward flow components on

March 18, 1999; however, water levels were not available on May 13, 1992, as 2–O was not measured and

22–O was dry. Based on the two comparison dates, it

Figure 6. Water levels for Red Rock pool, tailwater, and selected bedrock wells on the northeast side of Red Rock Dam.
EFFECTS OF REMEDIAL GROUTING ON THE GROUND-WATER FLOW SYSTEM 17

appears there has been little observable change in

the vertical component of ground-water flow on

either the southwest or the northwest side of the

dam. Wells in which measured water levels indicated

a downward flow component (with the exception

of well pair 13–O and 13–R) on March 18, 1999,

are wells that are located closest to the northeast

bluff area, possibly indicating, as was previously

mentioned, flow down through the overburden from

the reservoir. On May 13, 1992, on the southwest

side of the dam, there was only one well nest (9–O

and 9–R) where water levels indicated a downward

component of flow. Water levels measured in

wells 9–O and 9–R on March 18, 1999, then indicate

that the vertical component of flow in the vicinity of

these wells changed between the two dates from

downward to upward. While the direction of the

vertical component of flow remained much the same

at nested well sites for the two dates, three nested well

sites exhibited larger increases in head difference

between the bedrock and the overburden, indicating

an increased upward flow component. This occurred

at well nests 12–O, 12–R and 10–O, 10–R on the

southwest side of the dam with increased differences

of 2.4 ft and 1.6 ft, respectively. Nested well site 4–O

and 4–R on the northeast side of the dam had an

Table 4. Water levels for selected overburden wells, Red Rock pool, and Red Rock tailwater on May 13, 1992, and

March 18, 1999, and the differences between the two days

[--, no data; shaded rows are those wells located on the southwest side of the dam]

Well number or

surface-water body

May 13, 1992, water level

(feet above sea level)

March 18, 1999, water level

(feet above sea level)

Difference

(feet)

2–O No reading 706.5 --

3–O 702.9 700.8 –2.1

4–O 702.5 699.5 –3.0

5–O 701.4 698.5 –2.9

6–O 702.2 699.2 –3.0

7–O 696.3 692.4 –3.9

8–O 695.5 691.8 –3.7

9–O 693.8 690.4 –3.4

10–O 694.8 690.8 –4.0

11–O 694.4 689.9 –4.5

12–O 694.7 690.2 –4.5

13–O Dry 715.0 --

15–O Dry No reading --

16–O 694.3 690.9 –3.4

17–O 700.4 697.9 –2.5

18–O 701.2 698.8 –2.4

19–O 701.4 699.0 –2.4

20–O 702.7 700.6 –2.1

21–O Dry 706.6 --

22–O Dry 711.3 --

23–O 702.8 700.7 –2.1

25–O 698.3 695.9 –2.4

27–O Dry Dry --

28–O 701.7 699.0 –2.7

29–O–95 694.3 691.7 –2.6

30–O 704.7 704.4 –0.3

31–O 693.6 691.0 –2.6

Red Rock pool 742.40 742.79 +0.39

Red Rock tailwater 690.50 690.72 +0.22
18 Effects of Remedial Grouting on the Ground-Water Flow System at Red Rock Dam near Pella, Iowa

increased head difference of 3.6 ft, indicating an

increased upward component of flow at this site.

These increases in head difference at nested well sites

are largely due to the large decreases in water levels in

the overburden. Although not a nested well site, the

increased water level (3.8 ft) at bedrock well 1–R, in

an area of the dam where there is primarily a downward component of flow, might suggest an increase in

flow downward from the overburden into the bedrock

or an increase in flow through the grout curtain and

bedrock as result of continued hydraulic connectivity

to the reservoir. Overall, the majority of well nest sites

along the dam indicated an upward component of

flow, which is consistent with the description by Lucey

(1991), and there has been little change between the

two comparison dates. In addition, when comparing

the water levels for the two dates between the southwest side of the dam and the northeast side of the dam,

water levels remain somewhat higher in wells on the

northeast side of the dam, indicating that more gradual

gradients from the pool to the downstream wells probably still exist on the northeast side of the dam. This

may reflect a combination of flow from the reservoir

downward through the overburden into the bedrock

and continuing hydraulic connection between the pool

and the bedrock.

Some evidence for the effectiveness of the grout

curtain is the observable difference in water levels

between wells R–92–2A and R–92–2. Well R–92–2A

is upstream from the grout curtain and well R–92–2 is

downstream from the grout curtain, both on the northeast side of the dam (fig. 1). At the March 18, 1999,

pool elevation of 742.79 ft above sea level, the difference in water levels (R–92–2A, 708.2 ft and R–92–2,

704.2 ft) of 4.0 ft indicates a steep hydraulic gradient

within a relatively short distance (approximately 43 ft)

across the grout curtain. This same condition has been

observed repeatedly by the COE since the wells

were installed (V. Greenwood, U.S. Army Corps of

Engineers, oral commun., 2000). Because these wells

did not exist prior to remedial grouting, it is impossible to compare pre- and post-grout water levels

and determine the magnitude of the gradient between

these wells that might have existed prior to remedial

grouting.

Several water-level trends were observed in

selected bedrock wells for the period of record (fig. 6).

Hydrographs for wells 23–R and 5–RB indicate a

departure from pre-grout trends (as early as 1992)

when compared to hydrographs for Red Rock Lake

and wells R–92–1 (upstream from the grout curtain)

and 26–R. Water levels in 23–R and 5–RB also appear

to trend downward early in 1994 (the same year in

which grouting took place immediately upstream from

these wells), while water levels in R–92–1 and 26–R

continued to rise and the pool elevation remained

steady. There was then a continuing separation of

water levels through time between wells 23–R, 5–RB,

and well 26–R compared to pre-grout periods when

water levels in these wells tracked each other more

closely. This separation then appears to narrow somewhat (more so for well 23–R) starting in 1996. This

suggests that the grouting may have reduced the

hydraulic connection between the reservoir and areas

in the vicinity of these wells, especially initially. This

narrowing may be a result of the ground-water flow

system coming to an equilibrium after grouting or, in

the case of wells 23–R and 5–RB, the water levels in

the bedrock in this area of the dam responding much

slower (as a result of remedial grouting) to the

increase in the conservation pool elevation. This same

trend, to varying degrees, can be seen for water levels

in other bedrock wells on the northeast side of the

dam (figs. 7 and 8). In figure 7, hydrographs for

both wells 1–R and 2–R initially (1992) show a departure from pre-grout trends compared to well 26–R;

however, water levels in well 1–R then appear to again

track water levels in well 26–R (possibly as result of

new flow paths developing after high pool elevations

in 1993). The greater separation for the water level

in well 2–R may indicate that hydraulic connectivity

from the reservoir to the area in the vicinity of

well 2–R has been reduced to a greater degree

compared to the area in the vicinity of well 1–R.

The hydrograph for well 2–R then also shows a

slight narrowing in the late 1990’s when water levels

in well 2–R begin to track water levels observed in

wells 1–R and 26–R more closely. Well 13–R, located

on the southwest side of the dam, has much lower

water levels and very little increase over time in

response to the increased conservation pool elevation,

indicating that the hydraulic connectivity on the northeast side of the dam, from the reservoir to the bedrock

downgradient, is probably still greater than that on the

southwest side of the dam, although it appears that

remedial grouting has had some effect. The hydrographs (fig. 8) for wells R–92–2A (upstream from

grout curtain) and R–92–2 (downstream from grout

curtain) also show similar trends. Grouting between

these wells was done in 1992, with additional grouting
EFFECTS OF REMEDIAL GROUTING ON THE GROUND-WATER FLOW SYSTEM 19

being done 10 ft farther to the southwest (fig. 3) again

in 1994. These wells did not exist prior to initiation

of remedial grouting. There is greater separation

in water-level elevations early (1994), possibly in

response to remedial grouting, and then a narrowing as

water levels in R–92–2A trend downward and water

levels in R–92–2 trend upwards. Several other wells

(data not shown) on the northeast side of the dam,

3–R, 4–R, 21–UR, and 27–R, also show similar waterlevel trends of large initial responses to remedial

grouting and then varying degrees of narrowing

over time. The narrowing effect may be a result of

the ground-water flow system coming to an equilibrium as new flow paths develop in response to development of the grout curtain. Hydraulic gradients

across the grout curtain are evident (figs. 6 and 8),

with water levels in wells upgradient from the grout

curtain (R–92–1 and R–92–2A) ranging from approximately 10 to 4 ft higher than water levels in wells

downgradient from the grout curtain.

Pre- and post-grout Spearman correlation coefficients were calculated between water levels in selected

bedrock wells, pool elevations, and tailwater elevations

to determine if correlations between water levels have

changed. Pre-grout and post-grout periods were based

on grouting periods upgradient from the well location.

If a well location is such that it is near the middle of the

stations grouted during a given year, then that year was

considered the dividing line between pre-grout and

post-grout periods, with the grouting year removed,

to avoid any question as to whether grouting had any

effect on the well in that year. If the well is located

along the axis of the dam such that it is near the

boundary between different grouting years, then both

years were removed to separate pre-grout and postgrout periods. In some instances wherein the well

is located near the boundary of the 1992 and 1994

grouting years, a 3-year period divides pre- and postgrout years. No grouting was done in 1993 due to the

unusually high pool levels. Wells selected for analysis

1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 690

800

700

710

720

730

740

750

760

770

780

790

WATER LEVEL, IN FEET ABOVE SEA LEVEL

Red Rock pool

Well 1–R

Well 2–R

Well 13–R (southwest

 side of dam)

Well 26–R

Figure 7. Water levels for Red Rock pool and selected bedrock wells on the northeast and southwest sides of Red Rock Dam.
20 Effects of Remedial Grouting on the Ground-Water Flow System at Red Rock Dam near Pella, Iowa

include (1) wells located closest to and in the northeast

bluff area where underseepage is thought be greatest;

(2) wells that spatially represent hydrologic conditions

present in other areas of Red Rock Dam—namely, the

northeast side of the dam closer to the Des Moines

River, the area upgradient from the grout curtain,

and the southwest side of the dam where underseepage

is thought to be minimal; and (3) wells having accurate

location information. For comparison purposes, the

selected wells on the southwest side of the dam, where

no remedial grouting took place, were similarly divided

into pre-grout and post-grout periods by eliminating

1993 water-level data. This provides similar timeframes

for which pre- and post-grout comparisons between

water levels in wells on the southwest side of the dam

and the northeast side of the dam can be made. The

pre-grout and post-grout periods, along with the calculated Spearman correlation coefficients, are listed in

table 5.

Water levels in bedrock wells 1–R, 2–R, 3–R,

4–R, 5–RB, 23–R, 26–R, R–88–5L, and R–88–6, all

on the northeast side of the dam, had lower correlation

coefficients with pool elevations during post-grout

periods than during pre-grout periods (fig. 9). The

largest decrease was for well 4–R, from 0.93361

during pre-grout years to 0.58642 during post-grout

years. Well 16–R, located on the northeast side of the

dam closest to the tailwater, also had a lower correlation coefficient. Wells R–92–1, R–92–2, and R–92–2A

essentially had no pre-grout water levels as they were

constructed in 1992. Correlation coefficients for postgrout years for these wells were very similar and a

little higher than other northeast wells with the exception of well 1–R. Wells R–92–1 and R–92–2A are

upgradient from the grout curtain, whereas R–92–2

is downgradient from the grout curtain.

The water level in overburden well 30–O in the

northeast bluff area also had a lower post-grout correlation coefficient compared to the pre-grout period.

1993 1994 1995 1996 1997 1998 1999

700

790

710

720

730

740

750

760

770

780

WATER LEVEL, IN FEET ABOVE SEA LEVEL

Red Rock pool

Well R–92–2A

(upstream from grout curtain)

Well R–92–2

(downstream from grout curtain)

Figure 8. Water levels for Red Rock pool and bedrock wells R–92–2 and R–92–2A on the northeast side of Red Rock Dam.
EFFECTS OF REMEDIAL GROUTING ON THE GROUND-WATER FLOW SYSTEM 21

Table 5. Spearman correlation coefficients for water levels in selected bedrock wells, well 30

–O, Red Rock pool, and Red Rock tailwate

[--, no data; shaded rows indicate wells located on the southwest side of dam]

Well number Pre-grout dates Post-grout dates Red Rock pool pre-grout Red Rock pool post-grout Red Rock tailwate

pre-grout 

1–R 03/01/82 to 10/23/91 01/19/93 to 08/09/99 0.95396 0.76081 0.80005 2–R 03/01/82 to 04/09/91 01/19/95 to 08/09/99 0.93417 0.68411 0.84536 3–R 03/01/82 to 10/23/91 01/19/95 to 08/09/99 0.94634 0.66181 0.82198 4–R 03/01/82 to 11/22/93 01/19/95 to 08/09/99 0.93361 0.58642 0.82618 5–RB 04/17/86 to 11/22/93 01/19/95 to 08/09/99 0.95223 0.63585 0.89508 10–R 03/01/82 to 09/20/92 01/12/94 to 08/09/99 0.91857 0.83439 0.90889 11–R 03/01/82 to 11/23/92 01/12/94 to 08/09/99 0.93221 0.85094 0.89264 13–R 03/01/82 to 11/23/92 01/12/94 to 08/09/99 0.88175 0.79625 0.94272 16–R 03/01/82 to 09/20/92 01/12/94 to 07/14/99 0.85853 0.58125 0.93055 21–UR 03/01/82 to 10/23/91 01/19/95 to 08/09/99 0.89777 0.57081 0.79931 22–UR 03/02/83 to 10/23/91 01/19/93 to 08/09/99 0.94537 0.73020 0.82265 23–R 04/17/86 to 10/23/91 01/19/95 to 08/09/99 0.93922 0.64445 0.74371 26–R 03/01/82 to 11/05/90 01/19/93 to 08/09/99 0.92804 0.71051 0.82318 83–L 03/01/82 to 11/23/92 01/12/94 to 08/09/99 0.83865 0.74490 0.96554 83–U 03/01/82 to 11/23/92 01/12/94 to 08/09/99 0.83258 0.75113 0.96589 85–L 03/01/82 to 09/20/92 01/12/94 to 08/09/99 0.84966 0.79485 0.94985 85–U 03/01/82 to 11/23/92 01/12/94 to 08/09/99 0.84270 0.78730 0.95171 R–88–5L 07/27/88 to 11/22/93 01/19/95 to 08/09/99 0.91426 0.66009 0.89905 R–88–6 07/27/88 to 11/22/93 01/19/95 to 08/09/99 0.92315 0.63866 0.91885 R–92–1 Non-existing 11/23/92 to 08/09/99 -- 0.74540 -- R–92–2A Non-existing 01/19/95 to 08/09/99 -- 0.73032 -- R–92–2 Non-existing 01/19/95 to 08/09/99 -- 0.73795 -- 30–O 04/17/86 to 10/23/91 01/19/95 to 08/09/99 0.92513 0.68241 0.74974 Red Rock tailwater 03/01/82 to 11/23/92 01/12/94 to 07/14/99 0.73013 0.52302 -- 
22 Effects of Remedial Grouting on the Ground-Water Flow System at Red Rock Dam near Pella, Iowa

0.50

1.00

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

SPEARMAN CORRELATION COEFFICIENT

Pre-grout Post-grout

1–R

2–R3–R

4–R

5–RB

16–R

21–UR

22–UR

23–R

26–R

R–88–5L

R–88–6

R–92–1

R–92–2A

R–92–2

30–O

SITE Tailwater

EXPLANATION

0.50

1.00

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

POOL

TAILWATER

Figure 9. Correlation coefficients between water levels in selected bedrock wells, well 30–O, pool, and tailwater

elevations during pre-grout and post-grout periods on the northeast side of the dam.
EFFECTS OF REMEDIAL GROUTING ON THE GROUND-WATER FLOW SYSTEM 23

This well is in an area of the dam where the vertical

component of ground-water flow is thought to be

primarily downward (based on hydrographs for the

period of record for overburden wells located at well

nest sites in the same vicinity of well 30–O, data not

shown) through the overburden, and therefore, should

be less affected by remedial grouting. The lower correlation coefficient for well 30–O during post-grout

years might be a result of dam operations—that is,

smaller but more frequent rises in pool levels (fig. 6),

or possibly, the development of longer flow paths

to this well. If dam operations are at least partially

responsible for the lower correlation coefficient at

well 30–O during post-grout years, then it is possible

that the decreases in correlation observed for water

levels in bedrock wells on the northeast side of the

dam may also be due, at least in part, to dam operations. Other overburden wells (2–O, 15–O, 21–O, and

22–O) in the same vicinity as well 30–O, where there

is predominantly downward component of flow, were

often dry during pre-grout years before pool conservation levels were increased; therefore, reliable pre-grout

correlation coefficients for those wells could not be

calculated.

Water levels in selected wells (fig. 9), with the

exception of well 23–R, also had lower correlation

coefficients with the tailwater between pre-grout and

post-grout periods. Water levels in well 23–R had a

slightly higher post-grout correlation coefficient with

the tailwater. The correlation coefficient for the pool

and tailwater was lower for the post-grout period

compared to the pre-grout period, with 1993 acting as

the boundary. Therefore, it is possible that the lower

post-grout correlation coefficients between the water

levels in the wells and the tailwater may be due, at

least in part, to the greater fluctuations in outflow from

the dam control structure (fig. 6) beginning in 1994.

On the southwest side of the dam, correlation

coefficients between water levels in wells and pool

elevations (fig. 10) also were lower for post-grout

years compared to pre-grout years, although the

decreases were not as large as those occurring on the

northeast side of the dam. Correlation coefficients

between water levels in all selected wells (with the

exception of well 10–R) on the southwest side of the

dam and the tailwater also slightly decreased from pregrout to post-grout periods. Again, this may be related

to the greater fluctuations in pool elevations and

outflow from the dam control structure (fig. 6) beginning in 1994.

During pre-grout periods, water levels in

selected wells on the northeast side of the dam (fig. 9)

had higher correlation coefficients with pool elevations than with tailwater elevations, suggesting that

these wells were more under the influence of changing

pool levels than changing tailwater elevations. The

exception is the water level in well 16–R, which had

a higher correlation coefficient with the tailwater than

with the pool. This well is located closest to the

Des Moines River. Then, during post-grout years,

water levels in wells 2–R, 3–R, 4–R, 5–RB, 21–UR,

23–R, R–88–5L, and R–88–6 have higher correlation

coefficients with tailwater levels than with pool levels.

This suggests that these wells are now more under

the influence of the tailwater than the pool. For

wells 1–R, 22–UR, and 26–R, although correlation

coefficients were lower during post-grout years, their

relation between the pool and tailwater did not change

and appears still to be more under the influence of

changing pool elevations. The relation between water

levels in well 16–R, the pool, and the tailwater also

remained the same from pre-grout to post-grout years,

with a higher correlation coefficient with the tailwater

than with the pool.

Water levels in wells on the southwest side

of the dam (fig. 10) also had lower correlation

coefficients with pool levels for post-grout periods.

However, correlation coefficients with the tailwater remained higher when compared to the pool,

suggesting that changes in tailwater elevations still

have a greater influence on ground-water levels on that

side of the dam. This is in contrast to some of the

selected wells on the northeast side of the dam where

the relation between water levels in wells, pool, and

tailwater has changed. Of the selected wells, 10–R and

11–R are the only wells on the southwest side of the

dam where there was a change in the relation between

water levels in wells, pool, and tailwater. Wells 2–R,

3–R, 4–R, 5–RB, 16–R, 21–UR, 22–UR, 23–R, 26–R,

R–88–5L, and R–88–6 on the northeast side of the

dam had lower well/pool correlation coefficients

during post-grout years than did the well (83–L) on the

southwest side of the dam with the lowest well/pool

correlation coefficient. This differs from pre-grout

years when wells on the northeast side of the dam had

higher well/pool correlation coefficients than did wells

on the southwest side of the dam. This suggests that

grouting along the northeast side of the dam has

reduced the hydraulic connection relative to the southwest side of the dam where no remedial grouting was
24 Effects of Remedial Grouting on the Ground-Water Flow System at Red Rock Dam near Pella, Iowa

0.50

1.00

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

SPEARMAN CORRELATION COEFFICIENT

Pre-grout Post-grout

10–R

11–R

13–R

83–L

83–U

85–L

85–U

SITE Tailwater

EXPLANATION

0.50

1.00

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

POOL

TAILWATER

Figure 10. Correlation coefficients between water levels in selected bedrock wells, pool, and tailwater elevations during

pre-grout and post-grout periods on the southwest side of the dam.
EFFECTS OF REMEDIAL GROUTING ON THE GROUND-WATER FLOW SYSTEM 25

done. Although there is a hydraulic gradient across the

grout curtain from the upgradient well, R–92–2A, to

the downgradient well, R–92–2, correlation coefficients with the pool for water levels in these two wells

are very similar. This suggests that remedial grouting

may not have been as effective in this area of the

northeast side of the dam.

Comparison of the relative changes in correlation coefficients for pre- and post-grouting periods

between the northeast and southwest sides of the dam,

and the observed change in relations between water

levels in wells, pool, and tailwater on the northeast

side of the dam, suggests that remedial grouting has

had some effect in decreasing the overall hydraulic

connection between the reservoir and downgradient

wells. However, the wells in the extreme northeast

bluff area (1–R, 22–UR, R–92–2, and 26–R) had

well/pool correlation coefficients during the postgrout period similar to the upgradient wells (R–92–1

and R–92–2A) on the other side of the grout curtain,

suggesting that this area still has a greater hydraulic

connection. Grouting may have had some effect,

however, on the northeast side of the dam, as observable decreases in correlation coefficients are evident,

especially for wells 3–R, 4–R, 5–RB, 21–UR, 23–R,

R–88–5L, and R–88–6. The underseepage to the area

of well 23–R, thought to be the area with the greatest

hydraulic connection to the reservoir prior to remedial

grouting, based on chloride tracer data (Lucey, 1991),

appears to have been reduced as a result of remedial

grouting.

Water Chemistry

Sulfate Concentrations

Sulfate concentrations in samples collected

from selected wells and the Red Rock tailwater are

shown in figure 11 along with grouting periods. Tailwater concentrations are used because the monthly

sampling record is more complete than the pool

sampling record (due to ice formation during the

winter months on Lake Red Rock), and concentrations

of constituents of interest in same-day samples of the

pool and tailwater are similar. After the 1992 grouting

began, sulfate concentrations in well 23–R increased

while Red Rock tailwater concentrations remained

relatively stable. For example, increases in pool

elevations in 1990 and 1991 (fig. 6) were associated

with smaller sulfate concentrations at 490 and

428 milligrams per liter (mg/L), respectively, than in

1993, during even higher pool elevations when the

smallest sulfate concentration was 608 mg/L. The

sulfate concentration peak in 23–R during 1992

(1,007 mg/L) may have occurred because the primary

connection to the reservoir was cut off and residence

time increased compared to ground water that was

there prior to 1992. As other pathways developed

or the flow system equilibrated to the grout barrier,

sulfate concentrations decreased slightly and then

began to increase again in 1996, possibly as a result of

more grouting upgradient from 23–R during 1994 and

1995. When sulfate concentrations for well 23–R are

divided into pre-grout and post-grout periods (fig. 12),

as was done in calculating Spearman correlation

coefficients for water-level data, sulfate concentrations

are greater during the post-grout period (Wilcoxon

rank-sum test, p=0.0017). This suggests that there is

less flow through the grout curtain and mixing of

reservoir water and that ground-water residence time

at well 23–R may have increased, allowing water to

stay in contact with the gypsum for a longer period of

time. Decreased inflow and storage in Lake Red Rock

during post-grout years (fig. 6), and therefore less

mixing of reservoir and ground water, may also be a

factor in the greater sulfate concentrations observed

during post-grout years. It is unknown to what extent

the grout material is a contributor of sulfate to the

ground water. Sulfur trioxide (SO3) was the only

sulfur-bearing compound in the grout material and

constituted approximately 2.6 percent of the grout

material. Calcium oxide (CaO) and silicon dioxide

(SiO2) made up the largest proportions of the grout

material, approximately 63.1 percent and 20.5 percent,

respectively (L. Cook, Lafarge Corporation, written

commun., 1994).

Sulfate concentrations in samples collected

from well 5–RB prior to 1994 were similar to the Red

Rock tailwater sulfate concentrations (fig. 11). As the

grouting in 1994 proceeded, sulfate concentrations in

well 5–RB increased significantly while the Red Rock

tailwater sulfate concentrations remained stable. There

has, however, been a slight decrease in sulfate concentrations in well 5–RB since early 1998. When sulfate

concentrations are divided into pre-grout and postgrout periods (fig. 12), post-grout sulfate concentrations in well 5–RB are greater than the sulfate concentrations during the pre-grout period (p=0.0001). This

may indicate that flow from the reservoir to the cavity

zone or evaporite zone across the axis of the dam was
26 Effects of Remedial Grouting on the Ground-Water Flow System at Red Rock Dam near Pella, Iowa

cut off by the initial grouting and less Red Rock pool

water was mixing with water in the area of well 5–RB.

In turn, this may allow greater mixing of deeper waters

as an upward component of flow occurs from the

evaporite zone to the cavity zone. Sulfate concentrations for wells 5–RB and 23–R are similar to those

for wells R–96–1 and R–96–2 on the southwest side

of the dam. There is, however, less fluctuation in

sulfate concentrations for wells on the southwest side

of the dam, indicating that less hydraulic connectivity

probably still exists between Lake Red Rock and the

bedrock wells on the southwest side of the dam.

A peak concentration of 3,390 mg/L in

October 1988 was observed for well 5–RA (data

not shown). The large decrease in sulfate concentrations observed for well 5–RA beginning in late

1988 may have been due to the development of

a solution channel from adjacent strata, allowing

breakthrough from a more permeable zone and

consequently permitting mixing of water types

(Lucey, 1991). Sulfate concentrations then began to

increase again in 1994 and continued to increase

until 1998 when sulfate concentrations began

to decrease, following a similar pattern to

well 5–RB.

Chloride Concentrations

Chloride concentration data from tailwater

samples are used to represent reservoir water samples

because samples from the tailwater have been

collected on a consistent basis. It was not always

possible to collect samples from the reservoir due to

ice formation during winter months. In addition, chloride concentrations in same-day samples of the pool

and tailwater are similar. Chloride concentrations in

the tailwater vary seasonally, and peak concentrations

typically occur during late winter and early spring

(fig. 13). Sources of chloride are thought to be from

application of road salt upstream in the drainage basin

of the Des Moines River and possibly from application

of potash fertilizer to agricultural lands in the drainage

basin (Lucey, 1991). Prior to the initial grouting

in 1991, chloride concentrations in selected wells

1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 0

1,500

100

200

300

400

500

600

700

800

900

1,000

1,100

1,200

1,300

1,400

SULFATE CONCENTRATION, IN MILLIGRAMS PER LITER

Grouting dates

Well 5–RA

Well 23–R

Well 30–O

Well 5–RB

Well R–96–1

Well R–96–2

Red Rock tailwater

Figure 11. Sulfate concentrations in the tailwater and selected observation wells through time.
EFFECTS OF REMEDIAL GROUTING ON THE GROUND-WATER FLOW SYSTEM 27

appeared to be correlated to changes in reservoir chloride concentrations (fig. 13) and were thought to be

due to underseepage in the lower bedrock of the

St. Louis Limestone and not through downward

movement of water through the overburden because

potentiometric surfaces for nested well pairs indicated

an upward component of flow for the majority of wells

along the dam axis (Lucey, 1991). The vertical component of ground-water flow is similar for the post-grout

years, as was discussed earlier.

Throughout the grouting period, the variations

in chloride concentrations in the Red Rock tailwater

are evident. Chloride concentrations for selected wells

appear to vary to a greater degree prior to grouting,

especially during 1991. Then, chloride concentrations

remain relatively stable during periods of peak chloride concentrations in the tailwater throughout the

grouting period. This may indicate that the grout

curtain had some effect on underseepage to these wells

during the grouting period. However, in late 1997, a

large peak in chloride concentration in the Red Rock

tailwater was detected along with large concentrations

in all wells. It is unclear why this occurred, although a

new analytical method for chloride was implemented

at about the same time (D. Lutz, Iowa State University,

written commun., 1998). Those data are considered

0

1,200

100

200

300

400

500

600

700

800

900

1,000

1,100

SULFATE CONCENTRATION, IN MILLIGRAMS PER LITER

EXPLANATION

10th percentile

25th percentile

Median

75th percentile

90th percentile

10th and 90th percentiles

Data values outside the

Well 23–R

Pre-grout 04/14/87 to 11/11/91

Post-grout 01/19/95 to 09/09/99

10th percentile

25th percentile

Median

75th percentile

90th percentile

10th and 90th percentiles

Data values outside the

Well 5–RB

Pre-grout 04/14/87 to 12/02/93

Post-grout 01/19/95 to 09/09/99

Number of samples

Pre-grout Post-grout

(46)

(52)

(52)

(54)

(69)

Figure 12. Pre-grout and post-grout sulfate concentrations for wells 23–R and 5–RB.
28 Effects of Remedial Grouting on the Ground-Water Flow System at Red Rock Dam near Pella, Iowa

outliers, as chloride concentrations for previous and

subsequent samples are similar for all wells and tailwater. Large chloride concentrations also were

detected in late 1997 for wells R–96–1 and R–96–2

on the southwest side of the dam where all previous

hydrologic data indicate minimal underseepage. In

general, differences in trends for chloride concentrations between pre-grout and post-grout periods are

difficult to discern and, therefore, inconclusive.

Stable Isotopes

Hydrogen and Oxygen

The locations of the wells sampled for stable

isotopes are shown in figure 1. Figures 14 and 15

show the 2

H/1H and 18O/16O isotope ratios for the

wells and tailwater (assumed to be representative of

reservoir water) sampled by Iowa State University

from December 1995 to April 2000. Results are

reported in parts per thousand (per mil or δ2

H and

δ18O) and are defined as the relative enrichment or

impoverishment relative to the VSMOW (Vienna

Standard Mean Ocean Water), where δ2

H and δ18O

equal zero and are normalized on scales such that

the oxygen and hydrogen isotopic values of SLAP

(Standard Light Antarctic Precipitation) are –55.5 per

mil and –428 per mil, respectively. There is a 2-sigma

(2 standard deviations) uncertainty for oxygen and

hydrogen isotopic results of 0.2 per mil and 2 per mil,

respectively (Coplen, 1996). A positive δ value indicates that the sample contains more of the heavy

isotope than the standard, and a negative δ value indicates that the sample contains less of the heavy isotope

than the standard. For example, a δ2

H of –40 per mil

means that there are 40 parts per thousand or 4 percent

less 2

H in the sample relative to the standard (Kendall

and Caldwell, 1998). A δ2

H value of –40 per mil in a

sample contains more of the heavier isotope than does

a sample with a δ2

H value of –50 per mil, even though

both samples contain less 2H relative to the standard.

1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 0

50

5

10

15

20

25

30

35

40

45

CHLORIDE CONCENTRATION, IN MILLIGRAMS PER LITER

Grouting dates

Red Rock tailwater

Well 23–R

Well 30–O

Well 5–RA

Well 5–RB

Well R–96–1

Well R–96–2

Figure 13. Chloride concentrations in the tailwater and selected observation wells through time.
EFFECTS OF REMEDIAL GROUTING ON THE GROUND-WATER FLOW SYSTEM 29

In figure 14, δ2H for the tailwater varies over

time with the heavier isotopes becoming relatively

enriched in the summer months and relatively depleted

in the spring. A similar pattern is observed for δ18O

in the tailwater (fig. 15). This variability does not

appear to be due to fractionation caused by evaporation. Plots (fig. 16) of tailwater δ2

H and δ18O values

along the GWML (Global Water Meteoric Line) are

very close to the same slope as the GWML, indicating

little effect on isotopic composition due to evaporation. The seasonal variability in δ2

H values may

be due to the different sources of water for winter

and summer precipitation. The low-temperature,

high-latitude winter precipitation travels west to east

over the Rocky Mountains and is depleted in heavy

isotopes, whereas the high-temperature, low-latitude

precipitation from the Gulf of Mexico in the summer

is enriched in heavy isotopes (Simpkins, 1995). The

seasonal depletion of δ2

H (in the spring) for the tailwater seems to have trended toward enrichment since

1997. Simpkins (1995) also suggests that local

temperatures also may play a role in determining

isotopic composition in precipitation. In any case,

because of the large observable variability in the tailwater for both δ2

H and δ18O, if water from the Red

Rock pool is mixing with the ground water in downgradient areas, then similar observable trends in variability in the ground water might be indicative of a

hydraulic connection.

D J F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S O N D J F M A

1995 1996 1997 1998 1999 2000

–90

–30

–80

–70

–60

–50

–40

HYDROGEN-2 / HYDROGEN-1, IN PER MIL

740

770

745

750

755

760

765

ELEVATION, IN FEET ABOVE SEA LEVEL

EXPLANATION

Tailwater isotope ratios

23–R isotope ratios

30–O isotope ratios

5–RA isotope ratios

5–RB isotope ratios

R–96–1 isotope ratios

R–96–2 isotope ratios

Red Rock pool elevation

Figure 14. Hydrogen isotope ratios for the tailwater and selected observation wells.
30 Effects of Remedial Grouting on the Ground-Water Flow System at Red Rock Dam near Pella, Iowa

The δ2H and δ18O in samples from

wells R–96–1 and R–96–2 on the southwest side

of the dam, which are believed to be relatively

unaffected by underseepage, are relatively stable

compared to the tailwater and the other wells on the

northeast side of the dam (figs. 14 and 15). When

considering the variability of the analytical methods

for hydrogen and oxygen isotopes, the isotopic ratios

observed at most of the wells are essentially straight

lines, although they do vary somewhat more than at

wells R–96–1 and R–96–2, especially in the case of

oxygen isotopes. The exceptions are wells 5–RA

and 5–RB, which also are relatively stable over time.

There is evidence that the seasonal fluctuations seen

in precipitation are attenuated to a certain degree as

the water moves through the unsaturated zone into the

ground water (Simpkins, 1995). Hydrogen and oxygen

isotope ratios for samples from well 30–O, allowing

for a lag period, appear to show some seasonal variability (figs. 14 and 15). Precipitation and seepage

from Lake Red Rock through the overburden are probably contributing to the relatively greater variability

observed in samples from well 30–O. Beginning in

1998, less variability is observed at well 30–O, which

may be related to the decreased variability over time

observed in the tailwater, or possibly to decreased

seepage from the reservoir through the overburden.

Results for well 23–R also show some seasonal

D J F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S O N D J F M A

1995 1996 1997 1998 1999 2000

–15

–5

–14

–13

–12

–11

–10

–9

–8

–7

–6

OXYGEN-18 / OXYGEN-16, IN PER MIL

740

770

745

750

755

760

765

ELEVATION, IN FEET ABOVE SEA LEVEL

EXPLANATION

Tailwater isotope ratios

23–R isotope ratios

30–O isotope ratios

5–RA isotope ratios

5–RB isotope ratios

R–96–1 isotope ratios

R–96–2 isotope ratios

Red Rock pool elevation

Figure 15. Oxygen isotope ratios for the tailwater and selected observation wells.
EFFECTS OF REMEDIAL GROUTING ON THE GROUND-WATER FLOW SYSTEM 31

variability similar to well 30–O, although more

subdued (figs. 14 and 15). This well was in the area

of the dam thought to have the greatest amount of

underseepage prior to remedial grouting (Lucey,

1991). No relation is apparent between variations

in ground-water isotope ratios and changes in

Lake Red Rock elevation.

Sulfur

Figure 17 shows the relative abundance of

34S/32S isotope ratios for ground-water samples

collected from wells and the tailwater by Iowa State

University and the USGS. Results are reported as parts

per thousand (per mil or δ34S) relative to the standard

VCDT (Vienna Canyon Diablo Troilite), defined by

assigning a value of –0.3 per mil to IAEA-S-silver

sulfide (Coplen and Krouse, 1998). There is a 2-sigma

(2 standard deviations) uncertainty of 0.4 per mil

for δ34S. Positive and negative δ values correspond

to the enrichment or depletion, respectively, relative

to the standard as was discussed for the 2

H and

18O isotope ratios. A modern meteoric water value

of +5 per mil for δ34S (R. Carmody, U.S. Geological

Survey, oral commun., 1997) is plotted in figure 17

as a visual reference and for comparison to δ34S

values obtained from tailwater and ground-water

samples.

The 34S/32S isotope ratios are probably more

related to the composition of the bedrock (that is,

gypsum and anhydrite layers), whereas the 2H/1H

and 18O/16O isotopic ratios are more indicative of

sources of water in the system. Therefore, sulfur

isotopes may be a useful tool in distinguishing the

different water types and the degree of mixing that

may be occurring.

–14 –13 –12 –11 –10 –9 –8 –7 –6 –5 –4 –3 –2 –1 0

OXYGEN-18/OXYGEN-16, IN PER MIL

–100

10

–90

–80

–70

–60

–50

–40

–30

–20

–10

0

HYDROGEN-2/HYDROGEN-1, IN PER MIL

Global mean meteoric water lin e

Standard mean ocean water

Chicago meteoric water

Red Rock pool and tailwater

Well 23–R

Well 30–O

Well R–88–3

Well R–96–1

Figure 16. Plot of oxygen-18/oxygen-16 relative to hydrogen-2/hydrogen-1 for stable isotope sampling events at Red Rock Dam.
32 Effects of Remedial Grouting on the Ground-Water Flow System at Red Rock Dam near Pella, Iowa

The sulfur isotope data to date suggest that the

water samples from Lake Red Rock and the bedrock

wells are much different. Ground-water samples from

bedrock wells (5–RA, 5–RB, and 23–R) on the

northeast side of the dam have similar δ34S values

compared to samples collected from wells (R–96–1

and R–96–2) on the southwest side of the dam where

mixing of water from the reservoir with the ground

water is thought to be minimal. These values also are

very close to the δ34S value (16.7 per mil) determined

for a core sample of gypsum and anhydrite obtained

in 1994 from an exploratory hole, P–58–X, located

at station 3620 (approximately 200 ft southwest

of well R–92–2A) and 15 ft upstream from the

dam center line (unpublished data on file at the

U.S. Geological Survey, Iowa City, Iowa). The

δ34S values for samples collected from the overburden well 30–O on the northeast side of the dam

are very similar to those for the tailwater, which

supports interpretations of other hydrologic data

collected from well 30–O. Ground-water samples

collected from wells 21–O, 21–UR, and R–88–3

on the northeast side of the dam have enriched

δ34S values compared to the tailwater but are still

quite different than those samples collected from

bedrock wells thought to be in the area of greatest

D J F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S O N D J F M A

1995 1996 1997 1998 1999 2000

–10

20

–5

0

5

10

15

SULFUR-34/SULFUR-32, IN PER MIL

740

770

745

750

755

760

765

ELEVATION, IN FEET ABOVE SEA LEVEL

EXPLANATION

Red Rock pool elevation

Modern meteoric water

(sulfur-34/sulfur-32)

Tailwater isotope ratios

5–RA isotope ratios

5–RB isotope ratios

23–R isotope ratios

30–O isotope ratios

R–96–1 isotope ratios

R–96–2 isotope ratios

21–O isotope ratios

21–UR isotope ratios

R–88–3 isotope ratios

R–92–1 isotope ratios

Figure 17. Sulfur isotope ratios for the tailwater and selected observation wells.
SUMMARY AND CONCLUSIONS 33

underseepage. Well R–92–1 is located in the northeast

bluff area but is upgradient from the dam and grout

curtain. The δ34S values for samples collected from

this well suggest that there is mixing of water from the

reservoir with ground water that contains dissolved

sulfur from the evaporites. Similar values might be

expected in samples from other bedrock wells if a

greater mixing of reservoir water and ground water

was occurring. These results suggest that currently,

there appear to be distinct water types associated with

the tailwater (and overburden) and with the bedrock,

indicating that seepage, therefore mixing, through the

overburden is occurring to a greater degree than in the

bedrock, at least in the vicinity of the wells sampled

for stable isotopes. It is possible that this is a result of

remedial grouting but without pre-grout sulfur isotope

data, the relative degree of change is not known. Based

on δ34S values, the proportion of reservoir water in the

overburden and the bedrock appears to be relatively

constant over time since the initiation of stable isotope

data collection in 1997.

The sulfur isotope data collected so far

provide additional evidence that distinct water

types are present on the northeast side of the dam

in ground water in bedrock and in the tailwater

and overburden, and that mixing of fresh reservoir

water does not appear to be depleting the 34S/32S

ratios determined for water in the bedrock. A downward trend in δ34S values from samples collected

from bedrock wells would indicate an increase in the

amount of underseepage. This is still a relatively short

period of time for detecting changes, but additional

monitoring could aid an evaluation of what, if any,

changes are occurring. Because the δ34S values for

the tailwater (Lake Red Rock) are quite depleted

compared to the modern meteoric water value, as

well as ground water in bedrock, any increase in

mixing and therefore changes in isotopic composition

of water from the northeast bedrock wells, might

be readily detected using sulfur isotopes. It is possible

that the negative δ34S values in the tailwater, relative

to the modern meteoric water value of +5 per mil,

possibly may be from oxidized pyrite associated

with coal mining or burning upstream from Lake

Red Rock (R. Carmody, U.S. Geological Survey,

oral commun., 1997) or possibly from naturally

occurring sources such as evaporites in northcentral Iowa (D. Lutz, Iowa State University,

written commun., 1998).

SUMMARY AND CONCLUSIONS

Previous studies have shown direct evidence

of underseepage at Red Rock Dam located on the

Des Moines River near Pella, Iowa. Because

of concerns about the integrity of the dam, the

U.S. Army Corps of Engineers initiated a remedial

grouting program in September 1991 to decrease the

permeability and limit the underseepage in the underlying bedrock on the northeast side of the dam. Underseepage is thought to occur primarily on the northeast

side of the dam in the lower bedrock of the St. Louis

Limestone, which consists of discontinuous basal

evaporite beds and an overlying cavity zone. The

discontinuous basal evaporite beds are made up

primarily of gypsum and anhydrite. To assess the

effectiveness of the remedial grouting program and

to evaluate methods for future assessments, a study

was conducted by the U.S. Geological Survey in

cooperation with the U.S. Army Corps of Engineers.

Potentiometric-surface maps of the overburden

and bedrock indicate little change has occurred in

the general flow directions between pre-grout (Lucey,

1991) and post-grout periods. Flow in the overburden

is generally in a southeasterly direction in the northeast bluff area and, farther to the southwest along the

axis of the dam, flow is approximately parallel to the

axis of the dam in a southwesterly direction. Flow in

the bedrock is primarily in a south and southeasterly

direction in the extreme northeast bluff area and then

appears to be in a more southerly direction farther

along the axis of the dam toward the Des Moines

River. Changes in flow direction as a result of remedial

grouting are not apparent.

Water-level data for two dates (May 13, 1992,

and March 18, 1999) at similar pool and tailwater

elevations indicate that, at least for the two dates, the

water levels decreased in the vicinity of Red Rock

Dam. However, this may be more related to the hydrologic conditions (that is, dam operations) in Lake Red

Rock and the tailwater that preceded the comparison

dates than to any reduction in underseepage as a result

of remedial grouting. All overburden wells had a

decrease in water levels, with wells on the southwest

side of the dam showing a somewhat larger decrease.

Well 30–O had the smallest decrease of –0.3 ft. Most

of the bedrock wells also had decreased water levels

between the two dates. Some wells, however, had

increased water levels; the largest increase (3.8 ft)

occurred at well 1–R. These wells are all in or near

the northeast bluff area. Comparison of water levels
34 Effects of Remedial Grouting on the Ground-Water Flow System at Red Rock Dam near Pella, Iowa

between the southwest side of the dam (where underseepage in thought to be minimal) and the northeast

side of the dam on the post-grout date indicates that

more gradual potentiometric gradients from the pool

to the areas in the vicinity of the downstream wells

still exist on the northeast side of the dam (in particular the northeast bluff area), indicating a greater

hydraulic connection there, from flow downward

through the overburden into the underlying bedrock

and(or) from underseepage through the bedrock.

Water levels at well nest sites indicate that the vertical

component of ground-water flow is essentially

unchanged between the two dates with downward flow

occurring in the northeast bluff area and upward flow

occurring farther southwest along the axis of the dam

closer to the Des Moines River. Based on just waterlevel data between the two comparison dates, the

effects of remedial grouting are not apparent, but

water-level data do seem to indicate a continued difference between the southwest side of the dam and the

northeast side of the dam.

Hydrographs show that water levels in

wells 5–RB and 23–R showed a departure, after initiation of remedial grouting, from pre-grout trends when

these water levels tracked water levels for well 26–R

much more closely. Hydrographs for wells 1–R and

2–R also showed the same trend with a departure in

correlation with water levels in well 26–R from pregrout periods to post-grout periods. Similar trends were

observed for water levels in other wells. Hydrographs

for many of the wells on the northeast side of the dam

indicate what is probably an initial response to remedial

grouting as the hydraulic connection to the reservoir

was reduced. For some wells, over a period of time,

there was then a narrowing in water-level differences

until water levels again were tracking water levels in

well 26–R similar to pre-grout periods. This narrowing

may be the result of the ground-water flow system

coming to an equilibrium as new flow paths develop

after the initial response to remedial grouting. This was

particularly the case for wells 1–R, 4–R, and 27–R.

Narrowing of water-level differences occurred to a

lesser degree for wells 2–R, 3–R, 5–RB, and 23–R

as a greater water-level difference, compared to pregrout periods, was maintained over time, implying that

remedial grouting had a greater effect in reducing the

hydraulic connectivity between Lake Red Rock and

areas in the bedrock in the vicinity of these wells.

While it appears, based on hydrographs, remedial

grouting reduced underseepage to varying degrees

at different areas on the northeast side of the dam,

the hydraulic connection is still probably greater

on the northeast side of the dam as the hydrograph

for well 13–R (completed in evaporite material on

the southwest side of the dam) indicates, showing

lower water levels through time than wells on the

northeast side of the dam.

Statistical analysis of bedrock water levels,

pool levels, and tailwater elevations suggest that for

wells on the northeast side of the dam, the correlation

between ground-water levels and pool levels has

decreased from pre-grout periods to post-grout

periods, indicating some degree of reduced hydraulic

connectivity as a result of remedial grouting. Water

levels in many wells on the northeast side of the dam

now appear to be less affected by changes in pool

elevations. However, the observed lower correlation

coefficients during post-grout years may be due in part

to differences in dam operations and differences in

defining pre- and post-grout periods. Water-level data

and the hydrograph (fig. 7) of well 13–R plotted with

wells 1–R and 2–R still indicate a more gradual potentiometric gradient on the northeast side of the dam.

Although the statistical analysis indicates some

reduced hydraulic connectivity, the degree (based on

changes in the Spearman correlation coefficient from

pre-grout to post-grout years) of change and the

implied change in the influence of the tailwater

and pool upon ground-water levels in the bedrock

should be regarded with caution. Therefore, in general,

water-level data in conjunction with statistical analysis

suggest that underseepage to the areas of wells 2–R,

3–R, 5–RB, and 23–R appears to have been decreased

to a greater degree as a result of remedial grouting

than to the areas of wells 1–R, 4–R, 26–R, and

27–R, where remedial grouting probably had

lesser effect.

Sulfate data also suggest that the observed

changes in water chemistry at wells 5–RA, 5–RB, and

23–R may indicate less underseepage and, therefore,

less mixing of reservoir and ground water in the

vicinity of these wells. Significant increases in sulfate

concentrations for wells 5–RB and 23–R occurred at

the same time grouting occurred, which may indicate

flow paths from the reservoir were cut off to these

wells. Chloride concentrations (with the exception in

late 1997) have decreased at these same wells since

pre-grout years; however, the effect of remedial

grouting is not as apparent.
SELECTED REFERENCES 35

Although pre-grout and post-grout comparisons

were not possible, sulfur isotope data since 1995 show

distinct isotopic values for water from bedrock wells

compared to that from tailwater and overburden wells.

Samples collected from bedrock well R–92–1, located

upgradient from the grout curtain, had sulfur isotope

values indicating a greater mixture of reservoir and

ground water. This suggests a lesser degree of mixing

in the bedrock downgradient from the grout curtain,

where sulfur isotope ratios in samples collected from

those wells are more similar to those determined from

a gypsum and anhydrite core sample. This suggests

that the grout curtain restricts ground-water flow

from the reservoir to the St. Louis Limestone downgradient from the dam to some degree. Seasonal fluctuations in δ2

H and δ18O values for the tailwater are

large enough that some fluctuation also is observed

in water from some wells (30–O and 23–R), even

when one accounts for the analytical variability. Peak

enrichment of hydrogen and oxygen isotope ratios in

the tailwater have remained very similar from year

to year; however, peak enrichment observed at

wells 30–O and 23–R appears to have decreased

over time, especially for the oxygen isotopes. There

appears to be an overall dampening over time at these

wells, but it may be a result of the dampening also

observed for the tailwater.

Hydrographs, statistical analysis of water-level

data, and water-chemistry data suggest that underseepage on the northeast side of the dam has been

reduced as a result of remedial grouting, with some

areas affected to a greater degree and for a longer

period of time than other areas on the northeast side

of the dam. Comparisons between the southwest side

of the dam and the northeast side of the dam also indicate that there is still a greater hydraulic connection

between Lake Red Rock and downgradient bedrock

wells on the northeast side of the dam and that underseepage is probably still occurring there, but to a lesser

degree since completion of remedial grouting.

Continued measurement and evaluation of

water-level data along with isotope sampling that

possibly includes some of the newer wells (for

example, R–92–2A and the PX–97 series), located

upgradient from the dam, would provide further

insight into the long-term effects of remedial grouting

on underseepage along the northeast side of the dam.

Because there was no pre-grout isotope sampling,

inclusion of these existing additional wells on the

upgradient side of the dam for isotope sampling would

enable further comparisons between upgradient and

downgradient bedrock waters to assess the effectiveness of the grout curtain. Continuing monthly waterquality monitoring would add to the data base in

support of the hydraulic and isotope data. In addition,

the use of tracer tests could be used to determine

potential flow paths and travel times.

SELECTED REFERENCES

Coplen, T.B., 1996, Guidelines for the reporting of stable

hydrogen, carbon, and oxygen isotope-ratio data: Paleoceanography, v. 11, p. 369–370.

Coplen, T.B., and Krouse, H.R., 1998, Sulphur isotope data

consistency improved: Nature, v. 392, 32 p.

Gat, J.R., and Gonfiantini, R., 1981, Stable isotope

hydrology—Deuterium and oxygen-18 in the water

cycle: Vienna, International Atomic Energy Agency,

Technical Reports Series 210, 339 p.

Kendall, Carol, and Caldwell, E.A., 1998, Fundamentals

of isotope geochemistry, chapter 2 of Kendall,

Carol, and McDonnell, J.J., eds., Isotope tracers

in catchment hydrology: Amsterdam, Elsevier Science

B. V., p. 519–576, accessed March 26, 1999, from

URL http://wwwrcamnl.wr.usgs.gov/isoig/isopubs/

itchch2.html.

Lucey, K.J., 1991, Analysis of the ground-water flow

system, geochemistry, and underseepage in the

vicinity of the Red Rock Dam near Pella, Iowa: U.S.

Geological Survey Water-Resources Investigations

Report 91–4092, 67 p.

May, J.E., Gorman, J.G., Goodrich, R.D., Miller, V.E.,

Turco, M.J., and Linhart, S.M., 1999, Water resources

data—Iowa water year 1998: U.S. Geological Survey

Water-Data Report IA–98–1, 374 p.

Ott, R.L., 1993, An introduction to statistical methods

and data analysis: Belmont, Calif., Wadsworth, Inc.,

1051 p.

Simpkins, W.W., 1995, Isotopic composition of precipitation in central Iowa: Journal of Hydrology, v. 172,

p. 185–207.

U.S. Army Corps of Engineers, 1970, Red Rock Dam,

Des Moines River, Iowa, Investigation of underseepage, first year of operation: Rock Island, Ill.,

Rock Island District, Binders 1 and 2.

———1975, Red Rock Dam, Des Moines River, Iowa,

Investigation of underseepage, 1970–1974: Rock

Island, Ill., Rock Island District, Binders 1, 2, and 3.

———1984, Red Rock Dam, Des Moines River, Iowa,

Investigation of underseepage, 1980–1984: Rock

Island, Ill., Rock Island District, Binders 1 and 2.

———1998, Red Rock Dam, Des Moines River, Iowa,

Remedial grouting stage II—Foundation report: Rock

Island, Ill., Rock Island District, Binders 1 of 4.
Linhart, S.M., and Schaap, B.D.—Effects of Remedial Grouting on the Ground-Water Flow System at Red Rock Dam near Pella, Iowa USGS/WRIR 00–4231

Printedonrecycledpaper