U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2015–5112

Prepared in cooperation with Little Rock Air Force Base

Trace-Metal and Organic Constituent Concentrations in Bed 

Sediment at Big Base and Little Base Lakes, Little Rock Air 

Force Base, Arkansas—Comparisons to Sediment-Quality 

Guidelines and Indications for Timing of Exposure
 
Trace-Metal and Organic Constituent 

Concentrations in Bed Sediment at Big 

Base and Little Base Lakes, Little Rock 

Air Force Base, Arkansas—Comparisons 

to Sediment-Quality Guidelines and 

Indications for Timing of Exposure

By B.G. Justus, Phillip D. Hays, and Rheannon M. Hart

Prepared in cooperation with Little Rock Air Force Base

Scientific Investigations Report 2015–5112

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015

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Suggested citation:

Justus, B.G., Hays, P.D., and Hart, R.M., 2015, Trace-metal and organic constituent concentrations in bed sediment at 

Big Base and Little Base Lakes, Little Rock Air Force Base, Arkansas—Comparisons to sediment-quality guidelines 

and indications for timing of exposure: U.S. Geological Survey Scientific Investigations Report 2015–5112, 17 p., 

http://dx.doi.org/10.3133/sir20155112.

ISSN 2328-0328 (online)
iii

Contents

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

Introduction ....................................................................................................................................................1

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

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

Field and Laboratory Methods ....................................................................................................................2

Field Sampling ......................................................................................................................................2

Laboratory Analysis .............................................................................................................................4

Trace-Metal Comparisons ..................................................................................................................4

Sources and Methods Used to Determine Sediment-Quality Guidelines .........................4

Aging and Density Models ........................................................................................................6

Trace-Metal and Organic Constituent Concentrations in Bed Sediment ..........................6

Comparisons of Constituent Concentrations to Sediment-Quality Guidelines .................7

Sediment Deposition, Constituent Concentrations, and Indications for Timing 

of Exposure .....................................................................................................................7

Summary .......................................................................................................................................................13

References Cited .........................................................................................................................................13

Appendix 1. Concentrations of organic constituents in three surficial bed-sediment 

samples analyzed for a lake study conducted on Little Rock Air Force Base, 

Arkansas, May 2014 ...........................................................................................................15

Figure 

1. Map showing bed-sediment sampling sites on Big Base Lake and Little Base 

Lake, Little Rock Air Force Base, Arkansas, December 4, 2013, and May 8, 2014 ............3
iv

Tables

1. Approximate locations and depths for trace-metal samples collected in Big 

Base and Little Base Lakes on Little Rock Air Force Base, Arkansas, December 4, 

2013, and May 8, 2014 ..................................................................................................................4

2. Concentrations of 12 trace metals (in micrograms per kilogram) that were 

detected at high concentrations or are considered to be priority contaminants 

in 3 surficial bed-sediment samples collected in Big Base and Little Base Lakes 

on Little Rock Air Force Base, Arkansas, in May 2014 compared to results from 

October 2012 .................................................................................................................................5

3. Concentrations of selected contaminants in three surficial bed-sediment 

samples collected in Big Base and Little Base Lakes on Little Rock Air Force 

Base, Arkansas, compared to consensus-based threshold and probable effect 

concentrations (TECs and PECs) ...............................................................................................8

4. Mean probable effect concentration-quotients (PEC-Qs) for four contaminant 

groups and three surficial bed-sediment sampling locations in Big Base and 

Little Base Lakes on Little Rock Air Force Base, Arkansas, May 2013 ...............................9

5. Lead-210 (210Pb), radium-226 (226Ra), and cesium-137 (137Cs) isotope concentrations 

at various depths in a bed-sediment core sample collected from Big Base Lake 

on Little Rock Air Force Base, Arkansas, December 4, 2013 ..............................................10

6. Concentrations of selected trace metals detected at different depths in a bedsediment core sample from Big Base Lake (site NS6) on Little Rock Air Force 

Base, Arkansas, December 2013 ............................................................................................11
v

Conversion Factors

International System of Units to Inch/Pound

Multiply By To obtain

Length

millimeter (mm) 0.03937 inch (in.)

centimeter (cm) 0.3937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

kilometer (km) 0.5400 mile, nautical (nmi) 

meter (m) 1.094 yard (yd) 

Area

hectare (ha) 2.471 acre

hectare (ha) 0.003861 square mile (mi2

) 

Weight

gram (gm) 0.03527 ounce (oz)

Datum

Horizontal coordinate information is referenced to North American Datum of 1983 (NAD 83).

Supplemental Information

Concentrations of chemical constituents in sediment are given as micrograms per kilogram 

(μg/kg).
vi

Abbreviations

137Cs cesium-137

214Pb lead-214

226Ra radium-226

222Rn radon-222

As arsenic

Ba barium

Be beryllium

Ca calcium

Cd cadmium

CIC constant initial concentration

Co cobalt 

Cr chromium

CRS constant rate of supply

Cu copper

DDT dichlorodiphenyltrichloroethane

EPA U.S. Environmental Protection Agency

Hg mercury

K potassium

LRAFB Little Rock Air Force Base

Mg magnesium

Mn manganese

Mo molybdenum

Na sodium

Ni nickel

OC organochlorine pesticide

PAH polycyclic aromatic hydrocarbon

Pb lead

PCB polychlorinated biphenyl

PEC probable effect concentration

PEC-Q probable effect concentration-quotient

p,p’-DDE dichlorodiphenyldichloroethylene

Sb antimony

Se selenium

SQG sediment-quality guideline

TEC threshold effect concentration

USGS U.S. Geological Survey

V vanadium

Zn zinc
Trace-Metal and Organic Concentrations in Bed Sediment 

at Big Base and Little Base Lakes, Little Rock Air Force 

Base, Arkansas—Comparisons to Sediment-Quality 

Guidelines and Indications for Timing of Exposure

By B.G. Justus, Phillip D. Hays, and Rheannon M. Hart

Abstract 

This report compares concentrations for a wide range 

of inorganic and organic constituents in bed sediment from 

Big Base Lake and Little Base Lake, which are located 

on Little Rock Air Force Base, Arkansas, to sedimentquality guidelines. This report also compares trace-metal 

concentrations in a bed-sediment core sample to sediment age 

to determine when the highest concentrations of trace metals 

were deposited in Big Base Lake. 

Trace-metal results often were higher than background 

concentrations in the surrounding Pulaski County area, and 

concentrations of arsenic, cadmium, cobalt, copper, lead, 

manganese, mercury, nickel, and zinc at one or more of three 

study sites were higher than median concentrations for a study 

involving 98 urban streams in seven metropolitan areas of the 

United States. Concentrations for most polycyclic aromatic 

hydrocarbons, polychlorinated biphenyls, and organochlorine 

pesticides in all three bed-sediment samples were less 

than the laboratory reporting limit or were detected at low 

concentrations. 

Some contaminants were detected at concentrations that 

are potentially toxic to sediment-dwelling biota; however, in 

general, the analyses suggest that the risk of sediment toxicity 

may be relatively low. Threshold effect concentrations were 

exceeded for 14 constituents—arsenic, copper, lead, nickel, 

and zinc, five polycyclic aromatic hydrocarbons compounds, 

chlordane, and all three dichlorodiphenyltrichloroethane 

(DDT) congeners—which suggests potential toxicity to 

some sediment-dwelling biota. Only two constituents had 

concentrations that exceeded published probable effect 

concentrations—arsenic (at the deepest site in Big Base Lake, 

NS6) and p,p’-dichlorodiphenyldichloroethylene (p,p’-DDE; 

at both sites in Big Base Lake, NS5 and NS6). 

Regarding highest concentrations and associated 

timing of exposure, trace metals analyzed in the sediment 

core seem to indicate three fairly distinct exposure patterns. 

For 11 trace metals that had the highest concentration 

measured in the shallowest and most recently deposited 

sediment, the most likely explanation is recent exposure by 

anthropogenic activities. Most of the 11 trace metals with 

highest concentrations in shallow sediment are relatively 

innocuous; however, arsenic, copper, selenium, and zinc 

are among the U.S. Environmental Protection Agency’s 

126 priority pollutants. For three trace metals (cadmium, 

lead, and mercury), for which concentrations were highest 

in sediments that were 16–20 centimeters down the core, it 

is likely that a source associated with those contaminants 

during the period when those sediments were deposited, was 

reduced or eliminated. The eight remaining trace metals, for 

which concentrations were highest in sediments that were 

just below the prereservoir surface, likely had sources that 

were eliminated soon after lake construction or occurred at 

relatively high background concentrations in soils in the area 

around Little Rock Air Force Base.

Introduction

The U.S. Geological Survey (USGS) first analyzed trace 

metals in soil and bed-sediment samples collected at Little 

Rock Air Force Base (LRAFB) in the late 1980s and early 

1990s as part of a broad screening of inorganic and organic 

constituents in soils and groundwater (Freiwald and others, 

1991). In 2005, a study was conducted by the USGS to 

evaluate water quality in the Big Base and Little Base Lakes 

located on LRAFB (Justus, 2005). 

This is the third report provided to LRAFB by the USGS 

in a 4-year period. The first two reports were administrative 

reports that addressed trace-metal concentrations, primarily in 

bed-sediment samples, from Big Base and Little Base Lakes. 

In 2011, bed-sediment and fish-tissue sampling was conducted 

at Big Base and Little Base Lakes, and the analytical results 

were provided to LRAFB. Results from the bed-sediment 

sampling in 2011 indicated that trace-metal concentrations 

at LRAFB were higher than expected background 

concentrations. 

In 2012, additional bed-sediment samples were collected 

near inflows to Big Base and Little Base Lakes to determine 

if trace-metal concentrations could be linked to a particular 
2 Trace-Metal and Organic Concentrations in Bed Sediment at Big Base and Little Base Lakes, Little Rock Air Force Base

area in the watershed. Concentrations for some trace metals in 

bed-sediment samples collected in 2012 exceeded sedimentquality guidelines (SQGs; MacDonald and others, 2000) that 

are protective of sediment-dwelling biota; however, tracemetal concentrations could not be linked to specific areas in 

the watershed. Later in 2012, more bed-sediment samples 

were collected from a range of lake depths. Results from that 

study indicated that trace-metal concentrations were higher in 

deeper parts of Big Base Lake compared to Little Base Lake 

and shallower parts of Big Base Lake. 

In December 2013, as part of this study, more bedsediment sampling was conducted at two sites previously 

sampled by the USGS in deep areas of Big Base Lake. At the 

deeper of those two sites, the USGS collected sediments with 

a core sampler in addition to surficial sediments (defined as 

bed-sediment samples collected with a grab sampler from the 

top 2.5 centimeters [cm] of substrate). Samples throughout 

the core were analyzed for trace-metal concentrations to 

determine distribution as a function of depth. Sediment ages 

were determined to facilitate determinations for the most 

likely time of input for the trace metals. Because of concerns 

about the overall sediment toxicity, bed-sediment samples also 

were analyzed for organic compounds (including polycyclic 

aromatic hydrocarbons [PAHs], polychlorinated biphenyls 

[PCBs], and organochlorine pesticides [OCs]) in addition to 

trace metals. As a consequence of discrepancies associated 

with some laboratory analyses, it was necessary to repeat 

the surficial sediment sampling in May 2014, which was 

previously completed in December 2013, and a surficial 

sediment sample also was collected at Little Base Lake. 

Purpose and Scope 

The purpose of this report is to compare concentrations 

for a wide range of inorganic and organic constituents in 

surficial sediment at two sampling locations in Big Base Lake 

and at one location in Little Base Lake to SQGs. This report 

also examines trace-metal concentrations for a bed-sediment 

core sample in relation to sediment age to determine when the 

highest concentrations of trace metals were deposited in Big 

Base Lake. The samples were collected in December 2013 and 

May 2014. 

Study Area Description 

More detailed descriptions of the study area can be found 

in an earlier USGS report (Justus, 2005). Big Base Lake and 

Little Base Lake are connected beneath Arnold Drive by a 

culvert, and Big Base Lake has a much larger surface area and 

is much deeper than Little Base Lake (fig. 1). Under normal 

(rainfall) conditions, Big Base Lake has a surface area of 

approximately 15.8 hectares, whereas the surface area of Little 

Base Lake is approximately 1.2 hectares. Big Base Lake was 

constructed from 1954 to 1956 as dirt was removed during 

the construction phase of adjacent residential areas (James 

Popham, Little Rock Air Force Base, written commun., June 

2014). 

Field and Laboratory Methods

Field Sampling 

On December 4, 2013, the USGS collected two bedsediment samples for trace-metal analyses at two sites (NS5 

and NS6) on Big Base Lake that had previously been sampled 

(fig. 1, table 1). Both samples were collected from the water/

sediment interface using a Young grab sampler. The Young 

grab sampler was cleaned and samples were processed 

according to USGS protocols (Shelton and Capel, 1994; 

Justus, 2013). The top 2.5 cm of the surficial sediment from 

three grabs was composited into a prewashed glass bowl. After 

being thoroughly mixed, bed sediment was transferred into 

appropriate sample containers with a Teflon spoon. 

A bed-sediment core sample also was collected on 

December 4, 2013, at NS6, which was the deeper of the two 

sampling sites. The bed-sediment core sample was collected 

using a percussion gravity corer with a diameter of 89 

millimeters (mm). The depth of Big Base Lake at NS6 was 

approximately 6.5 meters. The corer was gently lowered from 

an anchored boat until it reached the substrate, and a slide 

hammer was used to drive the sampling tube into the bed 

sediment to refusal, which is the depth below the bottom of 

the lake at the time of construction. The corer and the sample 

material were gently lifted up through the water column and 

back onto the boat, and the bottom of the tube was quickly 

capped to prevent sample loss. Approximately 20 cm of water 

above the sediment surface in the collected core was carefully 

poured off. 

Sediment samples from the core were processed in a 

manner to provide information about when trace metals were 

deposited and whether concentrations increased or decreased 

through time (with depth from the top of the sediment core). 

The core was sectioned at 1-cm intervals from top to bottom, 

and sections were split to make two subsamples—one 

subsample for trace-metal analysis and one subsample for 

sediment-density and age models. A half of each 1-cm section 

was placed into a glass sample jar with a Teflon spoon for 

eventual trace-metals analysis. The remaining individual 

section halves were each placed into a plastic zip-top bag, 

labeled by depth, and used later for developing the sedimentdensity and age-dating models. Trace-metal samples submitted 

to the laboratory were typically a composite of five sequential 

subsamples; however, only 4 cm of core were available for 

the last sample. Samples were collected from approximately 

74 cm of core, with 15 composite samples retained for tracemetal analysis. Initially, 16 samples that were composites 

of 4–6 cm were evaluated for the sediment-density and 

age-dating model (which involved analysis of isotopes of 

lead-210 [210Pb], radium-226 [226Ra] and cesium-137 [137Cs]). 
S5

NS7

NS5

NS4

NS3

NS6

NS1

LITTLE

BASE

LAKE

BIG

BASE

LAKE

ARNOLD DRIVE

Picnic

area

Picnic

area

0

0 0.25 MILE

0.25 KILOMETER

34°54'

34°53'45"34°53'30"

92°10'15" 92°10' 92°09'45" 

EXPLANATION

Base from U.S. Geological Survey digital data,1:100,000 Horizontal coordinate information is referenced to North American Datum of 1983 (NAD 83)

 

Little Rock Air Force Base

SECOND STREET HARRIS ROAD NS2 NS6 NS5 • • Little Rock Air Force Base 107

•

•

ARKANSAS

 Sediment sampling site and 

sample location identifier

NS5

Figure 1. Bed-sediment sampling sites on Big Base Lake and Little Base Lake, Little Rock Air Force Base, Arkansas, December 4, 2013, and May 8, 2
4 Trace-Metal and Organic Concentrations in Bed Sediment at Big Base and Little Base Lakes, Little Rock Air Force Base

Based on results of the sediment-density model, sample 

material remaining after sediment-density measurements 

was eventually analyzed from other depths for the age-dating 

model. As a result of some quality-assurance issues at the 

laboratory (described below), the two sites on Big Base Lake 

were resampled on May 8, 2014, and an additional bedsediment sample also was collected from a site on Little Base 

Lake (NS2) at that time (fig. 1, table 1).

Laboratory Analysis

Trace metals from the December 2013 bed-sediment 

samples were analyzed using the same methods that had been 

used for previous studies conducted by the USGS on LRAFB; 

however, analyses were conducted by a different laboratory 

than in previous years. The U.S. Environmental Protection 

Agency (EPA) Solid Waste Method 6010B was used for all 

trace-metals but mercury, which was analyzed using method 

7174A (U.S. Environmental Protection Agency, 2002). The 

USGS National Water Quality Laboratory conducted analyses 

for PAHs, PCBs, and OCs. The PAH samples were analyzed 

using schedule 5506 (Zaugg and others, 2006) and PCBs and 

OCs were analyzed using a custom analysis (laboratory code 

8093; U.S. Geological Survey, 2015). Isotope analysis of 

the core sample was conducted at the University of Florida, 

Land Use and Environmental Change Institute, Radioisotope 

Laboratory for the 137Cs, 210Pb, and 226Ra isotopes. Sediment 

data were stored in the USGS National Water Information 

System water-quality database (http://waterdata.usgs.gov/

nwis/qw/).

Quality-assurance samples collected in December 2013 

indicated the trace-metal analyses for the bed-sediment 

samples and the core sample were compromised at the 

laboratory; consequently, a second set of analyses was 

necessary. Bed-sediment samples collected at the water/

sediment interface had to be resampled; fortunately, some 

sediment from all samples taken throughout the core had 

been archived and was available for the second analysis. As 

another quality-assurance step, trace-metal analyses for the 

samples collected in May 2014 were analyzed by two contract 

laboratories—Test America, the laboratory that had conducted 

previous analysis for the ongoing LRAFB project, and RTI, a 

new contract laboratory that analyzed the samples collected 

in December 2013. Unfortunately, even though material 

remained from the original core, those subsamples were not 

analyzed a second time for trace metals until June 2014, and 

holding times were exceeded for all trace-metal constituents. 

Trace-Metal Comparisons 

Trace-metal concentrations in Big Base and Little Base 

Lakes were compared to those of bed-sediment samples 

analyzed across different spatial and temporal scales. Results 

were provided along with results of previous samples 

collected at the two lakes in 2012, background concentrations 

for soils in the surrounding Pulaski County area (U.S. 

Geological Survey, 2004), concentrations measured in 198 

stream sites across the United States (Gilliom and others, 

1998), concentrations measured in 98 urban streams in the 

United States (Moran and others, 2012), and threshold effect 

concentrations (MacDonald and others, 2000) (table 2). 

Concentrations measured in 98 urban streams and threshold 

effect concentrations typically had the highest median tracemetal concentrations of the four reference concentrations, and 

direct comparisons were made only between 2014 samples and 

these two selected references (table 2). 

Sources and Methods Used to Determine 

Sediment-Quality Guidelines

Consensus-based SQGs such as threshold effect 

concentrations (TEC; below which adverse effects are not 

expected to occur), probable effect concentrations (PEC; 

above which adverse effects are expected to occur more often 

than not), and guidelines for probable effect concentration 

quotients (PEC-Qs; the combined toxicity of different 

chemical constituent groups) that were used in this study to 

evaluate sediment quality were determined by MacDonald and 

others (2000) by matching sediment chemistry and toxicity 

data. The PEC values for multiple constituents were combined 

to calculate mean PEC-Qs for each of the four chemical 

constituent groups—trace-metals, PAHs, (total) PCBs, and 

OCs. The PEC-Q values were calculated by dividing the 

Table 1. Approximate locations and depths for trace-metal samples collected in Big Base and Little Base Lakes on Little Rock Air 

Force Base, Arkansas, December 4, 2013, and May 8, 2014.

Lake

Sample location 

identifier 

(see fig. 1)

Latitude Longitude Depth

(meter) Dates sampled Types of bed-sediment samples collected

Little Base NS2 34o53ʹ47ʺ 92o09ʹ44ʺ 1.0 5/8/2014 Surficial 

Big Base 1

NS5 34o53ʹ34ʺ 92o09ʹ41ʺ 3.0 12/4/2013; 5/8/2014 Surficial and core

Big Base 1

NS6 34o53ʹ45ʺ 92o10ʹ06ʺ 6.5 12/4/2013; 5/8/2014 Surficial and core

1

Global Positioning System battery failure. Locations estimated from map coordinates from Google Earth.
Table 2. Concentrations of 12 trace metals (in micrograms per kilogram) that were detected at high concentrations or are considered to be priority cbed-sediment samples collected in Big Base and Little Base Lakes on Little Rock Air Force Base, Arkansas, in May 2014 compared to results from Oct[Data also are compared to four criteria: (1) median concentrations measured in 23 bed-sediment samples collected in Pulaski County, Arkansas (which includes LRAFB), for a (U.S. Geological Survey, 2004); (2) median values for a national study of 198 stream sampling sites from across the United States (Gilliom and others, 1998); (3) median concenurban streams in seven metropolitan areas of the U.S. (Moran and others, 2012); and (4) Threshold Effect Concentrations (TECs, MacDonald and others, 2000). Red font color imedian concentration measured at 98 urban stream sampling sites; bold font indicates values that exceeded TECs; bold italic font indicates values that exceeded the median con

stream sampling sites and TECs; NA, not determined; J, value was less than the reporting limit but greater than or equal to the method detection level; B, constituent concentratiand in the sample; H, sample holding time exceeded; --, reference concentration not available]

Sample location 

identifier (fig. 1) 

and sampling year

Lake Percent moisture Aluminum Arsenic Cadmium Chromium Cobalt Copper Lead Manganese Mercury N

NS2 2014 Little Base NA 12,000 6.8 0.22J 16.0 17.0 15.0 29.0 1,100B 0.82H 1NS5 2014 Big Base 82 21,000 8.3J 0.53J 26.0B 21.0 28.0 68.0 2,100 0.10H 2NS6 2014 Big Base 86 29,000 44.0 0.80J 37.0B 25.0 47.0 84.0 1,800 0.11H 3NS2 2012 Little Base 75 15,000 6.3J 0.38JB 18.0 16.0 17.0 34.0 960B 0.068 NS5 2012 Big Base 78 24,000 8.5J 0.39JB 28.0 21.0 30.0 69.0 930B 0.090 2NS6 2012 Big Base 81 27,000 8.5J 0.61JB 31.0 23.0 36.0 79.0 1,700B 0.110 2

Reference (median) concentrations

Pulaski County, Arkansas1 29,700 6.35 -- 33.0 7.0 8.0 20.5 339 0.032 

198 stream sites across the United States -- 6.35 0.40 62.0 -- 26.0 24.3 -- 0.060 2

98 urban stream samples across the 

United States 

58,000 8.60 0.48 65.3 12.7 38.7 35.2 852 0.089 2

Threshold Effect Concentration -- 9.79 0.99 43.4 -- 31.6 35.8 -- 0.182 2

1Data were accessed from U.S. Geological Survey at http://mrdata.usgs.gov/geochem/county.php?place=f05119&el=Hg&rf=south-central.
6 Trace-Metal and Organic Concentrations in Bed Sediment at Big Base and Little Base Lakes, Little Rock Air Force Base

concentration for the constituent of interest at a site by the 

PEC value assigned to that constituent by MacDonald and 

others (2000). A total mean PEC-Q was calculated for all 23 

constituents measured, which indicated the overall toxicity of 

the four chemical constituent groups (trace-metal, PAH, PCB, 

and OC) in bed sediment by site. 

Aging and Density Models

Sediment-age data were compared to trace-metal 

analyses for the core sample to estimate the year when the 

highest concentrations of several trace metals were deposited. 

Sediment densities were determined for 1-cm interval 

subsamples from the core, and a sediment-density model—

entailing detailed core density measurements and empirical 

estimation of sedimentation rates—was developed at the 

University of Arkansas Stable Isotope Laboratory. Sediment 

density was used to determine the base of the lake sediment 

and whether the sediment had been disturbed in the past. Any 

sediment disturbance or mixing would invalidate some of the 

assumptions applied for development of the age-dating model. 

Using sediment density, the accumulated mass through the 

core was calculated, the accumulation was compared to age, 

and a sedimentation rate was determined. A linear relation 

between excess 210Pb and accumulated mass was expected for 

undisturbed sediment. 

A sediment deposition chronology was determined by 

measuring 210Pb, 226Ra, and 137Cs (throughout the core) at the 

University of Florida, Land Use and Environmental Change 

Institute, Radioisotope Laboratory. Sixteen sample depths 

were counted for 210Pb and 137Cs. Five to 15 grams of sediment 

were sealed in petri dishes and aged for approximately 

30 days to enable 226Ra to equilibrate with daughters lead-214 

(214Pb) and bismuth-214. After aging, samples in the sealed 

dishes were counted on a gamma spectrometer (HyperpureGe 

detector) to determine 210Pb, 226Ra, and 137Cs activities. 

Sediment dating used the 210Pb uranium-thorium series 

disequilibrium method. This method is based on the process 

of radon-222 (222Rn) gas that escapes from sediments to the 

atmosphere where it then decays to 210Pb. The 210Pb attaches to 

particles and is deposited on land or water and can be used to 

date sediments as much as 150 years in age. This atmospheric 

210Pb is not the only source of 210Pb in sediments. Most 

sediments contain uranium and 226Ra, which also produce 210Pb. This in situ 210Pb is termed supported 210Pb. The amount 

of supported 210Pb is related to the amount of 214Pb (determined 

by gamma spectroscopy). The difference between total 210Pb 

and 214Pb gives the unsupported 210Pb used for dating. 

The sediment deposition chronology was calculated 

using the constant rate of supply (CRS) model (Appleby and 

Oldfield, 1978; Oldfield and Appleby, 1984) and checked 

for accuracy using the constant initial concentration (CIC) 

model (Robbins, 1978). Age errors for the CRS model were 

propagated using first-order approximations and calculated 

according to Binford (1990). Because the reservoir was only 

a little more than 50 years old at the time of core collection 

and reservoir sediments had accumulated for less than 

six half-lives of 210Pb, total integrated, unsupported 210Pb 

(megabecquerel) that should have accumulated at the core 

site could not be directly calculated. Instead, the amount of 

expected, unsupported 210Pb was calculated by stipulating that 

sediment had been accumulating before the date of reservoir 

construction. This calculation approach can add error but is 

the standard approach used for sediments that are near the 

younger end of the sediment 210Pb dating range (Robbins, 

1978; Oldfield and Appleby, 1984). Final results were feasible 

and reasonable solutions with good comparability using 

CRS and CIC models, indicating effective application of the 

method.

Trace-Metal and Organic Constituent 

Concentrations in Bed Sediment 

Some trace-metal concentrations in surficial sediment 

samples were comparable to results from recent sampling 

efforts at Big and Little Base Lakes; however, other 

concentrations were higher than previous results (table 2). 

Trace-metal concentrations measured in bed sediment in 

the Big Base Lake NS5 and Little Base Lake NS2 samples 

in May 2014 were comparable to concentrations measured 

at those two sites in October 2012 (table 2). Trace-metal 

concentrations measured in bed sediment in the Big Base Lake 

NS6 sample in May 2014, however, were higher for 11 of 12 

trace metals of concern compared to concentrations measured 

in October 2012. Higher concentrations in 2014 were likely 

the result of small-scale spatial variability that is commonly 

associated with trace-metal analysis (Rice, 1999; Justus, 2013) 

rather than temporal changes in concentration during the 

19-month period between samples. 

Trace-metal concentrations were higher than localand national-scale background concentrations. On a local 

scale, trace-metal concentrations collected in 2014 at the 

three sampling sites often were higher than background 

concentrations for soils in the surrounding Pulaski County 

area. This was true for arsenic (As), cobalt (Co), copper 

(Cu), lead (Pb), manganese (Mn), mercury (Hg), nickel (Ni), 

and zinc (Zn) (table 2; U.S. Geological Survey, 2004). On 

a national scale, concentrations of As, cadmium (Cd), Co, 

Cu, Pb, Mn, Hg, Ni, and Zn at one or more of the three sites 

sampled in May 2014 were higher than median concentrations 

for a study involving 98 urban streams in seven metropolitan 

areas of the United States (table 2; Moran and others, 2012). 

Concentrations for most PAH, PCB, and OC pesticide 

constituents in all three bed-sediment samples were less 

than the laboratory reporting limit or were detected at low 

concentrations (app. 1). Unlike trace metals, substantially 

higher concentrations were not observed at one site compared 

to the remaining two sites. 
Field and Laboratory Methods 7

Comparisons of Constituent Concentrations to 

Sediment-Quality Guidelines 

Available SQGs suggest that some contaminants 

were detected at concentrations that are potentially toxic 

to sediment-dwelling biota. Detected concentrations were 

above TECs for 14 of the 23 trace-metal, PAH, PCB, and 

OC constituents evaluated. Trace-metal concentrations of 

As, Cu, Pb, Ni, and Zn exceeded established TECs, as did 

five PAH compounds (benz[a]anthracene, benzo[a]pyrene, 

chrysene, fluoranthene, and pyrene), chlordane, and all three 

DDT congeners (table 3). Trace metal TECs were exceeded 

most frequently and to the greatest degree at the deepest site 

in Big Base Lake, NS6. The PAH TECs were only exceeded 

at NS2, whereas OC TECs were exceeded at all sites. Only 

two constituents had concentrations that exceeded published 

PECs—As (NS6) and the p,p’-DDE (NS5 and NS6, table 3).

The consensus-based SQGs also can provide a basis for 

predicting sediment toxicity when sediments contain mixtures 

of contaminants. MacDonald and others (2000) reported 

that based on various freshwater toxicity analyses, mean 

PEC-Qs greater than 0.5 (calculated in their case using the 

PECs for eight trace metals, the PEC for total PAHs [rather 

than the PECs for individual PAHs], the PEC for PCBs, and 

the PEC for p,p’-DDE), were toxic to sediment-dwelling 

organisms about 85 percent of the time. Of the 12 mean 

PEC-Qs calculated for the LRAFB samples (for trace metals, 

PAHs, PCBs, and OC pesticides at each of the three sites), 

only 1 equaled 0.50 (a level that is toxic to sediment-dwelling 

organisms about 85 percent of the time)—the mean tracemetal PEC-Q calculated for the Big Base Lake NS6 sample 

(table 4). The overall mean PEC-Qs for the 23 trace metal, 

PAH, PCB, and OC pesticide constituents evaluated (for 

example, 0.30 at NS6, table 4) suggest that the risk of toxicity 

may be relatively low. 

Sediment Deposition, Constituent 

Concentrations, and Indications for Timing of 

Exposure

Results of the density model and sediment chronology in 

Big Base Lake at NS6 indicate that lake-sediment deposition 

was initiated at a depth between 23 and 28 cm (table 5). 

However, sediment at a depth of 28 cm is estimated to have 

been deposited in approximately 1947, which would have 

been prior to construction of the reservoir (circa 1954). 

Sediment at a depth of 23 cm is estimated to have been 

deposited in approximately 1968. 

Regarding the highest concentrations and associated 

timing of exposure, trace metals analyzed in the bed-sediment 

core sample seem to indicate three fairly distinct exposure 

patterns. For 11 trace metals—antimony (Sb), As, barium 

(Ba), calcium (Ca), Cu, iron (Fe), Mn, molybdenum (Mo), 

selenium (Se), sodium (Na), and Zn—concentrations 

were highest in sediments nearest to the water/sediment 

interface (table 6). For three trace metals—Cd, Pb, and Hg—

concentrations were highest in sediments that were 16–20 cm 

deep. For eight trace metals—Al, beryllium (Be), chromium 

(Cr), Co, magnesium (Mg), Ni, potassium (K), and vanadium 

(V)—concentrations were highest in sediments that were just 

below the prereservoir surface at the time of construction or at 

a current sediment depth of about 30 cm (table 6). 

For the 11 trace metals that had the highest concentration 

measured in the shallowest and most recently deposited 

sediment, the most likely explanation is recent exposure 

to anthropogenic activities (such as general urbanization, 

vehicular wear and tear, and parking lot runoff; Justus, 2013). 

Most of the 11 trace metals with the highest concentrations 

in shallow sediment are relatively innocuous; however, As, 

Cu, Se, and Zn are listed by the EPA as being in the top 126 

priority pollutants (U.S. Environmental Protection Agency, 

2014). 

For Cd, Pb, and Hg, three trace metals for which 

concentrations were highest in sediments that were 16–20 

cm down the bed-sediment core, it is likely that a source 

associated with those priority contaminants (and that period) 

was reduced or eliminated. This pattern has been noted in 

numerous other studies (Mahler and others, 2006) and likely 

reflects a strong, atmospherically derived, anthropogenic 

input that occurred through the 1970s. For example, the 

discontinued use of leaded fuels in the 1970s has resulted 

in decreases in anthropogenic lead from 30 to 79 percent 

(Mahler and others, 2006), whereas lower concentrations 

of Hg and other trace metals during that same period may 

be associated with industrial advances in the reduction of 

air emissions. Higher concentrations of some constituents 

in the surficial sample from site NS6 compared to median 

concentrations from the 98 urban stream samples could be 

related more to differences in the physical properties of lake 

sediment compared to stream sediment than to sources of 

contamination. For instance, given the same source exposure, 

the higher the sand:silt ratio, the lower the trace metal 

concentration that might be expected.

Two scenarios could apply to the remaining eight trace 

metals that had the highest concentrations near the bottom of 

the lake at the time of construction. It is possible that sources 

for some trace metals were eliminated after lake construction; 

however, concentrations in soils sampled in the Pulaski 

County area also suggest that background concentrations in 

soils in the LRAFB area can be relatively high (table 2).
8 Trace-Metal and Organic Concentrations in Bed Sediment at Big Base and Little Base Lakes, Little Rock Air Force Base

Table 3. Concentrations of selected contaminants in three surficial bed-sediment samples collected in Big Base and Little Base Lakes 

on Little Rock Air Force Base, Arkansas, compared to consensus-based threshold and probable effect concentrations (TECs and PECs).

[Bold values indicate threshold effect concentration (TEC) values were exceeded]

Chemical constituent TEC1 PEC1

Sampling lake and site

Little Base

NS2

Big Base

NS5

Big Base

NS6

Metals, in milligrams per kilograms

Arsenic 9.79 33.00 6.80 8.30 44.00

Cadmium 0.99 4.98 0.22 0.53 0.80

Chromium 43.40 111.00 16.00 26.00 37.00

Copper 31.60 149.00 15.00 28.00 47.00

Lead 35.80 128.00 29.00 68.00 84.00

Mercury 0.18 1.06 0.08 0.10 0.11

Nickel 22.70 48.60 16.00 24.00 33.00

Zinc 121.00 459.00 110.00 130.00 180.00

Polycyclic aromatic hydrocarbons (PAHs), in micrograms per kilograms

Anthracene 57.2 845 33 6.14 18

Fluorene 77.4 536 50 2

44.5 60

Naphthalene 176 561 40 2

44.5 41

Phenanthrene 204 1,170 160 31 84

Benz[a]anthracene 108 1,050 2

160 21.2 57

Benzo[a]pyrene 150 1,450 2

280 244.5 110

Chrysene 166 1,290 2

360 60 150

Fluoranthene 423 2,230 520 83.2 240

Pyrene 195 1,520 420 61 190

Total Polycyclic aromatic hydrocarbons 1,610 22,800 1,223 263 950

Polychlorinated biphenyls (PCBs), in micrograms per kilograms

Total polychlorinated biphenyls 59.8 676 0.0 25.5 30.2

Organochlorine pesticides (OCs), in micrograms per kilograms

Chlordane (including cis-Chlordane and cis-Nonachlor) 3.24 17.6 1.278 4.66 7.74

Dieldrin 1.9 61.8 1.03 1.32 1.78

p,p'-dichlorodiphenyldichloroethane (DDD) 4.88 28 2.63 2

6 26

p,p'-dichlorodiphenyldichloroethylene (DDE) 3.16 31.3 9.13 33.7 33.1

p,p'-dichlorodiphenyltrichloroethane (DDT) 4.16 62.9 2

10 212 223

Total DDD, DDE, and DDTs 5.28 572 21.76 51.7 62.1

1

MacDonald and others, 2000.

2

Values were below the detection limit and estimated by the laboratory but were considered to be actual values in the calculation of sediment-quality 

guidelines for these PAH, DDD, and DDT constituents.
Field and Laboratory Methods 9

Table 4. Mean probable effect concentration-quotients (PEC-Qs) for four contaminant groups and three surficial bed-sediment 

sampling locations in Big Base and Little Base Lakes on Little Rock Air Force Base, Arkansas, May 2013. 

[0.50, mean PEC-Qs greater than 0.50 are toxic to sediment-dwelling organisms about 85 percent of the time; NA, not applicable]

Chemical constituent PEC1

Sampling lake and 

site concentration Sampling lake and site PEC-Q

Little Base

NS2

Big Base

NS5

Big Base

NS6

Little Base

NS2

Big Base

NS5

Big Base

NS6

Metals, in milligrams per kilograms

Arsenic 33.00 6.80 8.30 44.00 0.21 0.25 1.33

Cadmium 4.98 0.22 0.53 0.80 0.04 0.11 0.16

Chromium 111.00 16.00 26.00 37.00 0.14 0.23 0.33

Copper 149.00 15.00 28.00 47.00 0.10 0.19 0.32

Lead 128.00 29.00 68.00 84.00 0.23 0.53 0.66

Mercury 1.06 0.08 0.10 0.11 0.08 0.09 0.10

Nickel 48.60 16.00 24.00 33.00 0.33 0.49 0.68

Zinc 459.00 110.00 130.00 180.00 0.24 0.28 0.39

Mean metal PEC-Q NA NA NA NA 0.17 0.27 0.50

Polycyclic aromatic hydrocarbons (PAHs), in micrograms per kilograms

Anthracene 845 33 6.14 18 0.04 0.01 0.02

Fluorene 536 50 44.5 60 0.09 0.08 0.11

Naphthalene 561 40 44.5 41 0.07 0.08 0.07

Phenanthrene 1,170 160 31 84 0.14 0.03 0.07

Benz[a]anthracene 1,050 160 21.2 57 0.15 0.02 0.05

Benzo[a]pyrene 1,450 280 44.5 110 0.19 0.03 0.08

Chrysene 1,290 360 60 150 0.28 0.05 0.12

Fluoranthene 2,230 520 83.2 240 0.23 0.04 0.11

Pyrene 1,520 420 61 190 0.28 0.04 0.13

Mean PAH PEC-Q NA NA NA NA 0.16 0.04 0.08

Total polychlorinated biphenyls (PCBs), in micrograms per kilogram

Mean total PCB PEC-Q 676 0.0 25.5 30.2 0.00 0.04 0.04

Organochlorine pesticides (OCs), in micrograms per kilograms

Chlordane 17.6 1.278 4.66 7.74 0.07 0.26 0.44

Dieldrin 61.8 1.03 1.32 1.78 0.02 0.02 0.03

p,p'-dichlorodiphenyldichloroethane (DDD) 28 2.63 6 6 0.09 0.21 0.21

p,p'-dichlorodiphenyldichloroethylene (DDE) 31.3 9.13 33.7 33.1 0.29 1.08 1.06

p,p'-dichlorodiphenyltrichloroethane (DDT) 62.9 10 12 23 0.16 0.19 0.37

Mean OC PEC-Q NA NA NA NA 0.13 0.35 0.42

Total mean PEC-Q for the 23 constituents 

(listed above)

NA NA NA NA 0.15 0.19 0.30

1

MacDonald and others, 2000.
10 Trace-Metal and Organic Concentrations in Bed Sediment at Big Base and Little Base Lakes, Little Rock Air Force Base

Table 5. Lead-210 (210Pb), radium-226 (226Ra), and cesium-137 (137Cs) isotope concentrations at various depths in a bed-sediment core 

sample collected from Big Base Lake on Little Rock Air Force Base, Arkansas, December 4, 2013.

[cm, centimeter; dpm/g, disintegrations per minute per gram; MDA, minimum detection activity; 210Pb, 226Ra, and 137Cs, are isotopes of lead, radium, and cesium; 

gray line indicates the approximate time of lake construction]

Approximate 

sample depth 

(cm)

Density 

corrected 

210Pb activity 

(dpm/g)

210Pb MDA 

(dpm/g)

Mean 226Ra 

activity 

(dpm/g)

Mean 226Ra 

MDA 

(dpm/g)

137Cs activity 

(dpm/g)

137Cs MDA 

(dpm/g)

Approximate 

deposition 

date

4–6 16.804 0.482 4.250 0.433 1.460 0.046 2008.5

10–11 15.621 0.417 3.899 0.374 1.102 0.040 2000.2

14–15 10.060 0.373 4.171 0.334 2.141 0.036 1992.5

19–20 6.203 0.269 4.208 0.242 4.088 0.026 1977.7

22–23 6.001 0.256 4.303 0.230 4.629 0.025 1968.2

28–29 4.630 0.262 4.287 0.236 5.766 0.025 1946.8

34–35 3.709 0.234 4.529 0.210 5.399 0.022 1922.7

38–39 3.984 0.237 4.488 0.213 4.452 0.023 1904.6

39–40 2.866 0.192 4.037 0.173 2.028 0.018 1899.3

44–45 2.444 0.110 3.198 0.100 0.455 0.010 1865.6

49–50 2.247 0.112 3.199 0.101 0.334 0.010 1833.1

54–55 1.848 0.109 3.018 0.099 0.325 0.010 1802.1

59–60 2.654 0.106 3.507 0.096 0.262 0.009 1773.2

65–66 2.061 0.104 3.096 0.095 0.234 0.009 1737.5

69–70 1.573 0.106 2.868 0.096 0.337 0.009 1713.9

71–72 2.377 0.128 3.606 0.116 0.306 0.011 1702.4
Table 6. Concentrations of selected trace metals detected at different depths in a bed-sediment core sample from Big Base Lake (site NS6) on Little Arkansas, December 2013. [All results are in milligrams per kilogram except for mercury which is reported in micrograms per kilogram; bold font indicates highest constituent concentration detected; cm, than the reporting limit but greater than or equal to the method detection level; B, constituent concentrations were detected in the blank and in the sample; ND, not detected; labofirst analysis necessitated a rerun and all constituents were analyzed past the recommented holding time]

Sample 

depth 

(cm)

Approximate 

sediment 

age

Aluminum Antimony Arsenic Barium Beryllium Cadmium Calcium Chromium Cobalt Coppe

0–5 2010 20,000 0.67J 29 220B 1.1 0.15J 1,100 25 17 33 

6–10 2002 13,000 ND 22 120B 0.67 0.062J 600 16 11 24 

11–15 1992 18,000 0.47J 21 160B 0.93 0.19J 680 22 16 21 

16–20 1978 21,000 0.56J 15 190J 1.1 0.26J 690 24 20 20 

21–25 1966 23,000 0.4J 14 190B 1.2 0.24J 720 24 22 20 

26–30 1948 26,000 0.47J 12 190B 1.2 0.23J 760 26 22 20 

31–35 1926 21,000 ND 11 170B 1.1 0.17J 680 22 20 17 

36–40 1900 21,000 ND 9.9 160B 1.1 0.1J 680 21 20 16 

41–45 1865 17,000 ND 6.9 120B 0.83 ND 680 18 13 13 

46–50 1834 13,000 ND 6.2 82B 0.62 ND 700 16 12 10 

51–55 1802 12,000 ND 6.2 79B 0.58 ND 620 15 11 9.

56–60 1774 14,000 ND 6.6 97B 0.7 ND 690 17 12 11 

61–65 1740 12,000 ND 6.3 85B 0.66 ND 610 15 10 9.

66–70 1714 13,000 ND 5.7 96B 0.072 ND 770 14 13 9.

71–75 1700 16,000 ND 6.6 130B 0.86 ND 770 17 16 12 
Table 6. Concentrations of selected trace metals detected at different depths in a bed-sediment core sample from Big Base Lake (site NS6) on Little Arkansas, December 2013.—Continued[All results are in milligrams per kilogram except for mercury which is reported in micrograms per kilogram; bold font indicates highest constituent concentration detected; cm, than the reporting limit but greater than or equal to the method detection level; B, constituent concentrations were detected in the blank and in the sample; ND, not detected; labofirst analysis necessitated a rerun and all constituents were analyzed past the recommented holding time]

Sample 

depth 

(cm)

Approximate 

sediment 

age

Magnesium Manganese Mercury Molybdenum Nickel Potassium Selenium Silver Sodium 

0–5 2010 1,300B 1,100B 100 1.4J 22 1,200 0.95J ND 140J 

6–10 2002 890B 540B 100 0.8J 14 810 ND ND 80J 

11–15 1992 1,100B 650B 100 1.2J 20 1,100 ND ND 84J 

16–20 1978 1,300B 560B 110 0.76J 22 1,200 ND ND 82J 

21–25 1966 1,300B 650B 99 0.86J 23 1,200 ND ND 84J 

26–30 1948 1,400B 630B 86 0.69J 24 1,400 ND ND 91J 

31–35 1926 1,200B 550B 77 0.65J 20 1,100 ND ND 73J 

36–40 1900 1,200B 440B 73 0.87J 20 1,100 ND ND 73J 

41–45 1865 990B 700B 56 0.44J 16 880 ND ND 60J 

46–50 1834 760B 860B 39 0.31J 12 680 ND ND ND 

51–55 1802 720B 570B 39 0.3J 11 650 ND ND ND 

56–60 1774 830B 520B 49 0.36J 13 750 ND ND 58J 

61–65 1740 690B 400B 49 0.29J 12 630 ND ND ND 

66–70 1714 720B 1,000B 51 0.23 12 650 ND ND ND 

71–75 1700 890B 920B 68 0.4J 15 790 ND ND 62J 
References Cited 13

Summary

This report compares concentrations for a wide range of 

inorganic and organic constituents in surficial bed sediment at 

two sampling locations in Big Base Lake and at one location 

in Little Base Lake, which are located on Little Rock Air 

Force Base, Arkansas, to sediment-quality guidelines. This 

report also compares trace-metal concentrations in a bedsediment core sample to sediment age to determine when the 

highest concentrations of trace metals were deposited in Big 

Base Lake.

Trace-metal concentrations were higher than background 

concentrations at the local and national scale. On a local 

scale, trace-metal concentrations from samples collected in 

2014 often were higher than background concentrations for 

the surrounding Pulaski County area. On a national scale, 

concentrations of arsenic, cadmium, cobalt, copper, lead, 

manganese, mercury, nickel, and zinc at one or more of the 

three sites sampled in May 2014 were higher than median 

concentrations for a study involving 98 urban streams in 

seven metropolitan areas of the United States. Concentrations 

for most polycyclic aromatic hydrocarbons (PAHs), 

polychlorinated biphenyls (PCBs), and organochlorine (OC) 

pesticide constituents in all three bed-sediment samples were 

less than the laboratory reporting limit or were detected at low 

concentrations. 

Some contaminants were detected at concentrations that 

are potentially toxic to sediment-dwelling biota; however, 

in general, the analyses suggest that the risk of sediment 

toxicity may be relatively low. Detected concentrations 

were above threshold effect concentrations (TECs) for 

14 of the 23 trace-metal, PAH, PCB, and OC constituents 

evaluated. Trace-metal concentrations for arsenic, copper, 

lead, nickel, and zinc exceeded established TECs, as did 

five PAH compounds (benz[a]anthracene, benzo[a]pyrene, 

chrysene, fluoranthene, and pyrene), chlordane, and all 

three dichlorodiphenyltrichloroethane (DDT) congeners. 

Trace metal TECs were exceeded most frequently and to 

the greatest degree at the deepest site in Big Base Lake, 

NS6. The PAH TECs were only exceeded at the site in 

Little Base Lake, but all sites had OC TECs that were 

exceeded. Only two constituents had concentrations that 

exceeded published probable effect concentrations (PECs)—

arsenic (at the deepest site in Big Base Lake, NS6) and 

dichlorodiphenyldichloroethylene (p,p’-DDE, at both sites in 

Big Base Lake, NS5 and NS6). Of the 12 mean probable effect 

concentration-quotients (PEC-Qs) calculated (for trace metals, 

PAHs, PCBs, and OC pesticides at each of the three sites), 

only the sample from Big Base Lake, NS6 exceeded 0.50. The 

overall mean PEC-Qs for the 23 trace metal, PAH, PCB, and 

OC pesticide constituents evaluated, suggest that the risk of 

toxicity may be relatively low.

Regarding the highest concentrations and associated 

timing of exposure, trace metals analyzed in the bedsediment core sample seem to indicate three fairly distinct 

exposure patterns. For the 11 trace metals that had the 

highest concentration measured in the shallowest and most 

recently deposited sediment, the most likely explanation is 

recent exposure to anthropogenic activities (such as general 

urbanization, vehicular wear and tear, parking lot runoff). 

Most of the 11 trace metals with the highest concentrations 

in shallow sediment are relatively innocuous; however, 

arsenic, copper, selenium, and zinc are listed by the U.S. 

Environmental Protection Agency as being in the top 126 

priority pollutants. 

For three trace metals for which concentrations were 

highest in sediments that were 16–20 cm down the bedsediment core, it is likely that a source associated with those 

contaminants (and that period), was reduced or eliminated. 

The discontinued use of leaded fuels in the 1970s has resulted 

in decreases in anthropogenic lead from 30 to 79 percent, 

whereas lower concentrations of mercury and other trace 

metals during that same period may be associated with 

industrial advances in the reduction of air emissions. 

Two scenarios could apply to the remaining eight trace 

metals that had the highest concentrations near the bottom of 

the lake at the time of construction. It is possible that sources 

for some trace metals were eliminated after lake construction 

or it is possible that background concentrations in soils in the 

LRAFB area are relatively high. 

References Cited

Appleby, P.G., and Oldfield, F., 1978, The calculation 

of lead-210 dates assuming a constant rate of supply of 

unsupported 210Pb to the sediment: Catena, v. 5, no. 1, 

p. 1–8.

Binford, M.W., 1990, Calculation and uncertainty analysis of 210Pb dates for PIRLA project lake sediment cores: Journal 

of Paleolimnology, v. 3, p. 253–267.

Freiwald, D.A., Petersen, J.C., and Kilpatrick, J.M., 1991, 

Installation restoration program, stage 1, Little Rock Air 

Force Base: U.S. Geological Survey Technical Report, 

variously paginated. 

Gilliom, R.J., Mueller, D.K., and Nowell, L.H., 1998, Methods 

for comparing water-quality conditions among National 

Water-Quality Assessment study units, 1992–1995: U.S. 

Geological Survey Open-File Report 97–589, 60 p.

Justus, B.G., 2005, Water-quality, phytoplankton, and trophicstatus characteristics of Big Base and Little Base Lakes, 

Little Rock Air Force Base, Arkansas, 2003–2004: U.S. 

Geological Survey Scientific Investigations Report 2005–

5068, 37 p.

MacDonald, D.D., Ingersoll, C.G., Berger, T.A., 2000, 

Development and evaluation of consensus-based sediment 

quality guidelines for freshwater ecosystems: Archives 

of Environmental Contamination and Toxicology, v. 39, 

p. 20–31.
14 Trace-Metal and Organic Concentrations in Bed Sediment at Big Base and Little Base Lakes, Little Rock Air Force Base

Mahler, B.J., Van Metre, P.C., and Callender, E., 2006, Trends 

in metals in urban and reference lake sediments across the 

United States, 1970 to 2001: Environmental Toxicology and 

Chemistry, v. 25, p. 1698–1709.

Moran, P.W., Calhoun, D.L., Nowell, L.H., Kemble, N.E., 

Ingersoll, C.G., Hladik, M.L., Kuivila, K.M., Falcone, 

J.A., and Gilliom, R.J., 2012, Contaminants in stream 

sediments from seven U.S. metropolitan areas—Data 

summary of a National Pilot Study: U.S. Geological 

Survey Scientific Investigations Report 2011–5092, 

66 p.

Oldfield, F., and Appleby, P.G., 1984, Empirical testing of 210Pb-dating models for lake sediments, in Haworth, E.Y., 

and Lund, J.W.C., eds., Lake sediments and environmental 

history: Leicester University Press, p. 93–124.

Rice, K.C., 1999, Trace-element concentrations in streambed 

sediment across the conterminous United States: 

Environmental Science and Technology, v. 33, no. 15, 

p. 2499–2504.

Robbins, J.A., 1978, Geochemical and geophysical 

applications of radioactive lead isotopes, in Nriago, J.P., 

ed., The biogeochemistry of lead in the environment: North 

Holland, Amsterdam, Elsevier, p. 285–393.

Shelton, L.R., and Capel, P.D., 1994, Guidelines for collecting 

and processing samples of stream bed sediment for analysis 

of trace elements and organic contaminants for the National 

Water-Quality Assessment program: U.S. Geological Survey 

Open-File Report 94–458, 20 p. 

U.S. Environmental Protection Agency, 2002, Test methods for 

evaluating solid waste physical/chemical methods SW-846, 

accessed January 7, 2015, at http://nepis.epa.gov/Adobe/

PDF/500024PF.pdf.

U.S. Environmental Protection Agency, 2014, Priority 

pollutants, accessed April 29, 2015, at http://water.epa.gov/

scitech/methods/cwa/pollutants.cfm.

U.S. Geological Survey, 2004, Mineral resources on-line 

spatial data, accessed March 19, 2015, at http://mrdata.usgs.

gov/geochem/county.php?place=f05119&el=Hg&rf=southcentral. 

U.S. Geological Survey, 2015, NWQL catalog, accessed March 

19, 2015, at http://wwwnwql.cr.usgs.gov/USGS/catalog/

index.cfm?a=bs&sa=l&sap=8093&uid=.

Zaugg, S.D., Burkhardt, M.R., Burbank, T.L., Olsen, M.C., 

Iverson, J.L., and Schroeder, M.P., 2006, Determination of 

semivolatile organic compounds and polycyclic aromatic 

hydrocarbons in solids by gas chromatography/mass 

spectrometry: U.S. Geological Survey Techniques and 

Methods, book 5, chap. B3, 44 p.
Appendix 1 15

Appendix 1. Concentrations of organic constituents in three surficial bed-sediment samples analyzed for a lake 

study conducted on Little Rock Air Force Base, Arkansas, May 2014. 

[Results are in micrograms per kilogram; <, less than the specified laboratory reporting limit; bold values were estimated at the 

laboratory; M, missing data; all results were used as absolute values when calculating probable effect concentrations]

Organic group or constituent

Sampling lake and site

Little Base 

NS2

Big Base 

NS5

Big Base 

NS6

Polycyclic aromatic hydrocarbons

1,2,4-Trichlorobenzene <50 <44.5 <60

1,2-Dimethylnaphthalene <50 <44.5 <60

1,6-Dimethylnaphthalene <50 <44.5 29

1-Methyl-9H-fluorene <50 <44.5 <60

1-Methylphenanthrene 14 <44.5 12

1-Methylpyrene <50 <44.5 <60

2,3,6-Trimethylnaphthalene <50 <44.5 <60

2,6-Dimethylnaphthalene 105 49.3 411

2-Ethylnaphthalene <50 <44.5 <60

2-Methylanthracene <50 <44.5 <60

4H-Cyclopenta[def]phenanthrene 30 7.814 21

9H-Fluorene <50 <44.5 <60

Acenaphthene <50 <44.5 M 

Acenaphthylene <50 <44.5 <60

Anthracene 33 6.14 18

Benzo[a]anthracene 160 21.2 57

Benzo[a]pyrene 280 <44.5 110

Benzo[b]fluoranthene 600 101 260

Benzo[e]pyrene 328 51.9 147

Benzo[ghi]perylene 320 <44.5 <120

Benzo[k]fluoranthene 200 32.6 80

Chrysene 360 60 150

Dibenzo[a,h]anthracene <50 <44.5 <60

Dibenzothiophene <50 <44.5 <60

Fluoranthene 520 83.2 240

Indeno[1,2,3-cd]pyrene 280 <44.5 120

Naphthalene 40 <44.5 41

Octachlorostyrene <2.5 <3.0 <3.0

Pentabromotoluene <2.5 <3.0 <3.0

Perylene 446 474 1,180

Phenanthrene 160 31 84

Pyrene 420 61 190

Appendix 1
16 Trace-Metal and Organic Concentrations in Bed Sediment at Big Base and Little Base Lakes, Little Rock Air Force Base

Organic group or constituent

Sampling lake and site

Little Base 

NS2

Big Base 

NS5

Big Base 

NS6

Miscellaneous organic constituents

1,2-Bis(2,4,6-tribromophenoxy)ethane 4.62 3.58 3.95

9,10-Anthraquinone 100 21.4 65

BDE congener 100 0.35 0.365 0.464

BDE congener 138 <2.5 <3.0 <3.0

BDE congener 153 0.103 0.3 0.3

BDE congener 154 0.122 0.188 0.272

BDE congener 183 0.25 0.3 0.35

BDE congener 47 2.58 1.73 1.89

BDE congener 66 <2.5 <3.0 <3.0

BDE congener 71 <2.5 <3.0 <3.0

BDE congener 85 0.28 0.224 0.283

BDE congener 99 1.11 0.901 1.18

Bis(2-ethylhexyl) phthalate 1,200 86.1 480

Bis(hexachlorocyclopentadieno) cyclooctane 0.198 0.307 0.378

Diethyl phthalate <50 <44.5 <60

Phenanthridine <50 <44.5 <60

Methyl triclosan M <18 <18

Triclosan 3.62 6.07 6.14

Polychlorinated biphenyls

PCB congener 101 <2.5 <3.0 1.2

PCB congener 110 <2.5 <3.0 7.85

PCB congener 118 0.342 1.09 0.956

PCB congener 138 0.25 0.6 2.05

PCB congener 146 0.107 0.268 0.284

PCB congener 149 <2.5 0.705 0.766

PCB congener 151 <0.25 0.078 0.111

PCB congener 170 0.202 0.415 0.442

PCB congener 174 <0.1 0.108 0.17

PCB congener 177 0.0849 0.194 0.238

PCB congener 180 0.15 0.487 0.516

PCB congener 183 <0.25 0.0752 0.0353

PCB congener 187 0.0488 0.236 0.296

PCB congener 194 0.0729 0.145 0.178

PCB congener 206 0.0435 0.119 0.142

PCB congener 49 <5.0 <6.0 <6.0

PCB congener 52 <2.5 <3.0 <3.0

PCB congener 70 <5.0 <6.0 <6.0

Appendix 1. Concentrations of organic constituents in three surficial bed-sediment samples analyzed for a lake 

study conducted on Little Rock Air Force Base, Arkansas, May 2014.—Continued

[Results are in micrograms per kilogram; <, less than the specified laboratory reporting limit; bold values were estimated at the 

laboratory; M, missing data; all results were used as absolute values when calculating probable effect concentrations]
Appendix 1 17

Organic group or constituent

Sampling lake and site

Little Base 

NS2

Big Base 

NS5

Big Base 

NS6

Organochlorine pesticides

alpha-Endosulfan <0.5 <0.6 <0.6

Benfluralin <0.5 <0.6 <0.6

beta-Endosulfan <0.5 <0.6 <0.6

Carbazole 66 10.4 39.9

Chlorpyrifos 0.108 <0.6 0.393

cis-Chlordane 0.844 2.48 3.84

cis-Nonachlor 0.434 2.18 3.9

Cyfluthrin <0.5 <0.6 <0.6

DCPA <0.5 <0.6 <0.6

Desulfinylfipronil 0.17 0.388 0.339

Dieldrin 1.03 1.32 1.78

Endosulfan sulfate <1 <1.2 <1.2

Fipronil sulfide 0.228 <0.3 0.329

Fipronil <0.25 <0.3 <0.3

lambda-Cyhalothrin <0.5 <0.6 <0.6

Oxychlordane <2.5 <3.0 <3.0

Oxyfluorfen <10 <12 <12

p,pʹ-DDD 2.63 <6.0 <6.0

p,pʹ-DDE 9.13 33.7 33.1

p,pʹ-DDT <10 <12 <23

Pendimethalin <2.5 <3.0 <3.0

Pentachloroanisole <50 <44.5 <60

Tefluthrin <1.25 <1.5 <1.5

Tetradifon M <0.6 <0.6

trans-Chlordane 0.474 1.37 2.24

trans-Nonachlor 0.555 1.4 2.28

Trifluralin <0.5 <0.6 <0.6

Hexachlorobenzene <50 <44.5 <60

Pentachloronitrobenzene <50 <44.5 <60

Appendix 1. Concentrations of organic constituents in three surficial bed-sediment samples analyzed for a lake 

study conducted on Little Rock Air Force Base, Arkansas, May 2014.—Continued

[Results are in micrograms per kilogram; <, less than the specified laboratory reporting limit; bold values were estimated at the 

laboratory; M, missing data; all results were used as absolute values when calculating probable effect concentrations]

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Justus and others—Trace-Metal and Organic Constituent Concentrations in Bed Sediment at Big Base and Little Base Lakes, Little Rock Air Force Base—SIR 2015–5112

ISSN 2328-0328 (online)

http://dx.doi.org/10.3133/sir20155112