Research

The Urban Environmental Gradient:

Anthropogenic Influences on the

Spatial and Temporal Distributions

of Lead and Zinc in Sediments

EDWARD CALLENDER*

U.S. Geological Survey, National Center MS 432,

12201 Sunrise Valley Drive, Reston, Virginia 20192

KAREN C. RICE

U.S. Geological Survey, P.O. Box B,

Charlottesville, Virginia 22903

Urban settings are a focal point for environmental

contamination due to emissions from industrial and

municipal activities and the widespread use of motor

vehicles. As part of the National Water-Quality Assessment

Program of the U.S. Geological Survey, streambedsediment and dated reservoir-sediment samples were

collected from the Chattahoochee River Basin and analyzed

for total lead (Pb) and zinc (Zn) concentrations. The

sampling transect extends from northern Georgia, through

Atlanta, to the Gulf of Mexico and reflects a steep

gradient in population density from nearly 1000 people/

km2 in the Atlanta Metropolitan Area to fewer than 50 people/

km2 in rural areas of southern Georgia and northern

Florida. Correlations among population density, traffic

density, and total and anthropogenic Pb and Zn concentrations

indicate that population density is strongly related to

traffic density and is a predictor of Pb and Zn concentrations

in the environment derived from anthropogenic activities.

Differences in the distributions of total Pb and Zn

concentrations along the urban-suburban-rural gradient

from Atlanta to the Florida Panhandle are related to

temporal and spatial processes. That is, with the removal

of leaded gasoline starting in the late 1970s, peak Pb

concentrations have decreased to the present. Conversely,

increased vehicular usage has kept Zn concentrations

elevated in runoff from population centers, which is reflected

in the continued enrichment of Zn in aquatic sediments.

Sediments from rural areas also contain elevated

concentrations of Zn, possibly in response to substantial

power plant emissions for the region, as well as vehicular

traffic.

Introduction

Many of the common anthropogenic pollution problems are

focused in urban settings. Besides point-source inputs to

the environment, such as industrial stack emissions and

effluents, there are municipal-wastewater and solid-waste

incinerator inputs. In addition, there is an input of contaminants related to the process of urbanization, termed

“street dust” (1). While these are all significant sources of

urban environmental contamination, recent environmental

legislation has attempted to reduce the severity and amount

of the industrial inputs. A population shift from urban and

rural settings to the suburban environment, however, has

caused sources of contaminants to aquatic systems to be

contributed by larger geographic areas. Suburbia does not

contribute much by way of industrial pollution, but it does

serve as a source of contaminants to the environment from

household and yard wastes, storm drains, construction

activities and materials, and traffic.

Lead (Pb) and zinc (Zn) have been used by humans for

a variety of purposes throughout the 19th and 20th century.

Point-source inputs of Pb and Zn to aquatic systems (streams,

lakes, and reservoirs) include industrial effluents, municipalwastewater effluents, and stack emissions from smelting

operations and fossil-fuel combustion (2-4). From 1950 to

the 1970s, automobile use increased in response to economic

and population growth, and the predominant source of Pb

became automobile exhaust emission of tetraethyl Pb (5).

Population growth has continued to the present (6), while

Pb concentrations have declined due to the removal of leaded

gasoline (7). Environmental Zn concentrations, however, may

have remained elevated because of the lack of specific

regulatory actions regarding Zn.

Today, most of the new anthropogenic Pb and Zn

additions to the environment are derived from material

sources. Lead is used in paper, plastics, and ceramics (1).

Zinc is used in most commercial metal products (e.g., brass,

bronze, castings, galvanized metal) and is added during the

manufacture of automobile tires in the form of zinc oxide

(ZnO), as an accelerator in the vulcanization process (8). In

addition, Zn is a common contaminant in agricultural and

food wastes. Both Pb and Zn have significant emissions from

coal fly ash, although the quantity of Zn emissions is nearly

double that of Pb (9). Fossil fuel combustion is the main

contributor to worldwide anthropogenic emissions of Zn (10).

The National Water-Quality Assessment (NAWQA) Program of the U.S. Geological Survey is designed to describe

the quality of the Nation’s water resources and to provide

a sound understanding of the natural and human factors

that affect the quality of these resources (11). The NAWQA

Program, which began in 1991, consists of 59 study units,

defined as study areas that comprise large river basins and/

or aquifer systems. Study units are spatially distributed across

the conterminous United States. In general, the NAWQA

Program is targeting the predominantly residential and

commercial land-use settings and de-emphasizing drainage

basins with abundant point sources and large areas of

industrial activity.

The focus of this paper is analysis of streambed-sediment

and reservoir-sediment-core data collected from the Apalachicola-Chattahoochee-Flint (ACF) River basin NAWQA study

unit in Georgia, Alabama, and Florida (Figure 1). Lead and

Zn concentrations in streambed sediment collected by the

study unit are integrated and interpreted with Pb and Zn

concentrations in age-dated reservoir-sediment cores also

collected by the NAWQA program, allowing interpretation

of both spatial and temporal patterns. The spatial record of

streambed-sediment samples collected during 1992-1994

extends along a longitudinal gradient (the Chattahoochee

River), from rural sites north of Atlanta, through Metropolitan

Atlanta, and southward to rural areas in the Florida Panhandle

(Figure 1). The spatial and temporal record of reservoir-

* Corresponding author phone: (703)648-5826; fax: (703)648-5832;

e-mail:eccallen@usgs.gov.

232 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 2, 2000 10.1021/es990380s Not subject to U.S. copyright. Publ. 2000 Am. Chem.Soc.

Published on Web 12/08/1999
sediment cores collected from five reservoirs located along

this gradient extends over a period of 25 years, from 1970 to

1995.

The ACF River basin comprises 52 840 km2 and extends

from the Blue Ridge Physiographic Province in the north,

through the Piedmont Province, to the Coastal Plain Province

in the southern half of the basin (Figure 1). Five major

hydroelectric reservoirs exist on the Chattahoochee River:

Lake Sidney Lanier, West Point Lake, Lake Harding, Walter

F. George Reservoir, and Lake Seminole (Figure 1). These

five reservoirs account for nearly 100% of the surfacestorage capacity in the ACF River basin. Population density

is highest in the Upper Chattahoochee basin (835 people/

km2), which includes Metropolitan Atlanta, and decreases

south of Atlanta to 90 people/km2 around Lake Harding and

fewer than 20 people/km2 in the Lower Chattahoochee basin

(12).

Objectives of this paper are to (1) document concentrations of Pb and Zn in streambed-sediment and reservoirsediment-core samples; (2) show how the concentrations

vary over the urban-rural gradient and are related to

anthropogenic activities; and (3) demonstrate the differences

in Pb and Zn concentrations as related to anthropogenic

influences over both space and time.

Experimental Section

Streambed-sediment sampling sites were selected to be

representative of upstream land-use activities, such as

urbanization in Atlanta and Columbus, GA; surface-water

impoundments; and agricultural activities in the Coastal Plain

province (Figure 1; (13)). Samples were collected in depositional zones by wading along a cross-section of the stream.

The upper 2-3 cm of sediment in five to 10 depositional

zones was sampled and composited. Compositing of samples

within and between depositional zones was assumed to

smooth any local-scale variability and provide a sample with

average concentrations of trace elements at each site (14).

Samples were collected in the fall of one of three successive

years using a hand-held core sampler or a Teflon spoon,

scoop, or spatula, depending on local conditions. Sediments

were sieved through a 63-μm nylon cloth, held in a plastic

frame, into a glass jar. Sediment-sampling and sieving tools

were cleaned between sample sites by soaking in 0.2%

phosphate-free detergent, rinsing with deionized water,

rinsing with a 5%, high-purity, nitric-acid solution, and rinsing

multiple times with deionized water.

Reservoir-sediment cores were collected using a Benthos

gravity corer fitted with a 3.05-m long, 6.3-cm diameter,

plastic-lined barrel (15). The cores were extruded vertically

and subsampled for metals and radionuclides. The sampling

tools were rinsed in ambient lake water, soaked in 0.2%

phosphate-free detergent, and rinsed again in ambient lake

water between subsamples. All subsamples were chilled until

shipped to the laboratory. Subsamples of the core were not

sieved due to the small amount (e10 g) of material available.

Sediment cores were collected near the reservoir dam, where

silts and clays accumulate. Generally, these lacustrine

sediments are characterized by uniform, fine-textured muds

(15) and are of comparable grain size to the sieved streambed

sediments.

In the laboratory, streambed-sediment samples were

air-dried and ground to a fine powder. Reservoir-sedimentcore subsamples were frozen, freeze-dried, and ground

to a fine powder. Elemental concentrations were determined

on concentrated-acid digests (nitric-hydrofluoric) using

either inductively coupled plasma-atomic emission spectrometry (ICP/AES) [streambed-sediment samples; (16)] or

graphite furnace atomic absorption spectrometry (GF/AAS)

[reservoir-sediment-core samples; (17)]. Quality assurance

was provided by determining the elemental concentrations

of strong-acid digests for duplicate samples and a variety of

soil, lake, and marine reference samples (16). Six streambedsediment sites were sampled twice on the same day. Replicate

analyses for Pb and Zn varied by ( 25% and ( 20%,

respectively.

For age dating, activities of 137Cs were measured by

counting freeze-dried samples of reservoir-sediment cores

in a fixed geometry with a high-resolution, intrinsic germanium detector γ spectrometer (18). Four dates can be

identified in cores from reservoirs that were constructed

before 1952: the reservoir impoundment date, the first

occurrence of detectable 137Cs in 1953, the peak 137Cs

concentration in 1963-1964, and the date of collection of

the sediment core. Based on these four dates, mean

sedimentation rates were calculated for intervening time

intervals, which were then used to assign approximate

deposition dates to samples from each core.

FIGURE 1. Location of study area, physiographic provinces,

streambed-sediment sampling sites, and reservoir-coring sites.

VOL. 34, NO. 2, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 233
Generally, metal concentrations that have been corrected

for their background contribution correlate better with

measures of human activity or impact (population density,

traffic volume, land use, toxic release inventory, etc.) than

uncorrected metal concentrations. In this study, background

refers to metal concentrations of soil samples collected from

undeveloped areas in the streambed-sediment samplingsite basins and the reservoir watersheds. Deeply weathered

soils were sampled from the two major physiographic

provinces in the study area: the Piedmont and the Coastal

Plain. Eight and nine soil samples were collected from the

Piedmont and Coastal Plain Provinces, respectively. The

chemical composition of the background soils in the two

provinces are substantially different (see Figure 2A) and

bracket concentrations in the upper continental crust (20

ug/g Pb and 71 ug/g Zn; (19)). For reservoir-core sediments,

the baseline concentration is obtained from the bottom of

the core, which usually represents minimal anthropogenic

activity during the early stages of reservoir development.

Anthropogenic metal concentrations are calculated for

sediment samples by subtracting the mean background or

the baseline metal concentration from the total metal

concentration.

Vehicular traffic data were obtained from the Georgia

Department of Transportation (written communication,

1998) for counties that border the five reservoirs and for the

Metropolitan Atlanta area. For each county, all sections of

roads with average annual daily traffic greater than the 75th

percentile for the county were tabulated. For those highly

traveled road sections, the total average annual daily traffic

was divided by the total number of miles of road to yield the

traffic density, in number of vehicles per mile. Traffic density

FIGURE 2. River kilometer upstream of Apalachicola Bay versus (A) total Pb and Zn concentrations in streambed sediment collected

1992-1994, reservoir sediment for the time interval 1990-1995, Pb and Zn background concentrations in streambed-sediment samples,

and Pb and Zn baseline concentrations in reservoir-core samples and (B) population density of basins containing sampling sites. Population

density was calculated from the 1990 census data and the watershed area.

234 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 2, 2000
was calculated for the year 1995, and results were correlated

with metal concentrations and population density.

The cultural enrichment factor (CEF) is used to indicate

whether the entry of metal contaminants into a water body

has increased in relation to an earlier period baseline or

background value (20). As used here, the CEF is defined as

the ratio of normalized (to aluminum) concentration of

sediment at any depth to the normalized (to aluminum)

background or baseline concentration. CEFs that are about

1 indicate no significant anthropogenic enrichment, whereas

CEFs greater than 2 suggest anthropogenic contribution.

Results and Discussion

Sedimentary Pb and Zn concentrations (i.e., the metal

concentration associated with solid sediment) in streambed

sediment collected during 1992-1994 and in reservoirsediment-core subsamples that correspond to the 137Cs -dated

time period 1990-1995 have somewhat different spatial

patterns (Figure 2a). Lead concentrations are elevated (65-

90 μg/g) in the Atlanta Metropolitan area but decrease to

near the South Atlantic Coastal Plain background concentration of 18 μg/g (21) in the Lower Chattahoochee River

Basin, some 300 km downstream of Atlanta. The Zn concentrations are proportionally more elevated (390 μg/g) than

Pb in the Atlanta area and are substantially higher (90-170

μg/g) than the South Atlantic Coastal Plain background

concentration of 35 μg/g (21) in the Lower Chattahoochee

River Basin. The Pb concentrations do not decrease as sharply

as the Zn concentrations in the urban-affected area from

Atlanta to Lake Harding. Downstream of Lake Harding, the

Pb concentration decreases from 65 to 20 μg/g, whereas Zn

decreases by a smaller percentage, from 200 to 100 μg/g

(Figure 2a). The slight increases in both Pb and Zn around

Walter F. George Reservoir may be due to a smaller urban

source, such as Columbus, GA and/or Phenix City, AL. We

have no explanation for the increases in both Pb and Zn

around Lake Seminole (Figure 2a). Both increases are

substantially greater than the range of Pb and Zn concentrations in replicate samples.

One explanation for the different spatial patterns in Pb

and Zn for the time period 1990-1995 is that the elevated

Pb concentrations (above a background value of about 20

μg/g) originated in the Metropolitan Atlanta area, probably

as street dust and urban soils with elevated Pb concentrations

derived from previous use of leaded products (gasoline, paint

(22, 23)). In contrast, there may be perpetual sources of Zn,

i.e., motor-vehicle tire wear (24) and fossil fuel combusion

(10). Because motor vehicles abound everywhere, there is a

continual terrestrial source of Zn that could cause concentrations in sediments to be elevated, even in rural areas,

above the background value, which is about 80 μg/g in the

Piedmont Province and 20 μg/g in the Coastal Plain Province.

For the streambed-sediment samples, total Pb and total

Zn concentrations are strongly correlated with the population

density of each basin (Table 1). The anthropogenic Zn

concentrations vs population density also are strongly

correlated, but the substantial correlation between anthropogenic Pb and population density is not significant (Table

1). Because of these correlations, the total metal concentrations, rather than the anthropogenic concentrations, are used

to illustrate the urban gradient distribution (Figure 2a).

We hypothesized that the best indicator of anthropogenic

activity affecting the concentration of those trace metals (Pb

and Zn) that are related to motor-vehicle use would be some

measure of traffic density in the basin where the streambedsediment site or reservoir is located. Traffic-density data were

obtained only for the reservoir-sediment core data set (for

counties upstream of the reservoirs), because it was difficult

to acquire traffic-density data for specific watersheds where

streambed-sediment samples were collected. Metal-concentration and traffic-density correlations, however, are not

significant (Table 1) for this small data set of five reservoirs

and the Atlanta Metropolitan area. Lead and Zn concentrations are well correlated with population density (R ) 0.86

and 0.93, respectively) for the reservoir-sediment core data

set, although the correlation of Pb and population density

is not significant (Table 1). The small size of the reservoirsediment data set limits the degrees of freedom for the

correlations. The metal correlations (except anthropogenic

Pb) with population density in the larger streambed-sediment

data set, however, are significant. Population density is

strongly correlated with traffic density (Table 1) for the

reservoir-sediment-core data set, and thus, we consider

population density (Figure 2b) as an approximation for the

quantity of automobiles that affects the watershed upstream

of the sampling sites. One possible reason that anthropogenic

Pb is not strongly correlated with traffic and population

density is that by the mid-90s the anthropogenic Pb signal

is not directly related to leaded gasoline use that has been

phased out for at least 10-15 years (25).

Population-density data for streambed-sediment sites

along the main stem of the Chattahoochee River indicate

that Metropolitan Atlanta is the main anthropogenic source

of Pb and Zn (Figure 2). While the decrease in population

density away from the Metropolitan Atlanta area is sharp

and the decline relatively smooth, the pattern for total Zn is

TABLE 1. Pearson Correlation Coefficients (r Values) for Total and Anthropogenic Metal Concentrations and Population

Density for Streambed-Sediment and Reservoir-Core Samples

and for Total and Anthropogenic Metal Concentrations and

Traffic Density for Reservoir-Core Samplesa

TotPb AntPb TotZn AntZn Popdn

Streambed-Sediment Samples (n ) 11)

TotPb 1.00

AntPb 0.87 (0.01) 1.00

TotZn 0.76 (0.10) 0.56 (1.00) 1.00

AntZn 0.70 (0.25) 0.69 (0.30) 0.93 (0.00) 1.00

Popdn 0.73 (0.15) 0.63 (0.58) 0.93 (0.00) 0.92 (0.00)

Reservoir-Core Samples (n ) 6)

TotPb 1.00

AntPb 0.79 (0.92) 1.00

TotZn 0.94 (0.09) 0.84 (0.55) 1.00

AntZn 0.82 (0.68) 0.92 (0.13) 0.95 (0.05) 1.00

Popdn 0.86 (0.44) 0.73 (1.00) 0.93 (0.12) 0.87 (0.38) 1.00

Trfdn 0.64 (1.00) 0.53 (1.00) 0.80 (0.81) 0.77 (1.00) 0.93 (0.10)

a Level of significance shown in parentheses; TotPb, total lead

concentration; AntPb, anthropogenic lead concentration; TotZn, total

zinc concentration; AntZn, anthropogenic zinc concentration; Popdn,

population density in the sampled basin; Trfdn, traffic density in the

county upstream of the reservoir.

FIGURE 3. River kilometer upstream of Apalachicola Bay versus

the zinc cultural enrichmentfactor (Zn CEF) for Chattahoochee River

streambed-sediment and reservoir-coring sites.

VOL. 34, NO. 2, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 235
somewhat more irregular than that for total Pb (Figure 2a).

It should be remembered that population density is an

integrated value for the basin that contains the streambedsediment sampling site, whereas metal concentrations for

specific sampling sites are not as well integrated and are

subject to more variability in space and time.

Another process that may affect the spatial distribution

of metal concentrations is the sediment-trapping effect that

occurs along a series of reservoirs down river. This does

happen to some degree in the Chattahoochee River system.

For instance, Westpoint Lake was built in 1975 to reduce

pollution loads from Atlanta that were being transported

downstream to Lake Harding. A slight decrease in sedimentary Zn occurs from Westpoint Lake to Lake Harding (Figure

2a). Trapping of sedimentary metals along this series of

reservoirs, however, is not the only process affecting the metal

concentrations along the urban-suburban-rural gradient.

Streambed-sediment samples in the Metropolitan Atlanta

area exhibit an average Zn CEF of 5, whereas streambedsediment sites and 1990-1995 dated reservoir sediments

downstream of Atlanta have CEF values that range from 1.9

to 3.4, with a mean of 2.5 (Figure 3). This pattern suggests

that dilution of contaminated sediment by the addition of

uncontaminated earth materials may also be responsible for

the attenuation of values away from the main contaminant

source.

Like the spatial patterns, the temporal patterns of

sedimentary Pb (Figure 4a) and sedimentary Zn (Figure 4b)

concentrations in reservoir-sediment-core subsamples that

are dated for four time intervals (1975-1980, 1980-1985,

1985-1990, 1990-1995) are somewhat different. Peak concentrations of Pb (140 μg/g) occur in Westpoint Lake

sediments dated 1975-1980. In successive time intervals,

the peak Pb concentrations decrease to about 65 μg/g in

sediments dated 1990-1995 (Figure 4a). Westpoint Lake is

the first reservoir on the Chattahoochee River downstream

FIGURE 4. River kilometer upstream of Apalachicola Bay versus concentrations of (A) Pb and (B) Zn in dated reservoir cores from five

sites along the Chattahoochee River.

236 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 2, 2000
of Atlanta and thus intercepts the contaminant inputs from

the Metropolitan Atlanta area. Sedimentary Pb concentrations in urban- and suburban-reservoir sediments nationwide

peaked in the mid-1970s in response to maximum usage of

leaded gasoline (7). After that, concentrations declined rapidly

due to the removal of lead from gasoline (25). Atlanta has a

very high population density relative to other areas in the

Chattahoochee River basin (Figure 2b). Sediments in Westpoint Lake appear to have been most impacted by human

activities, primarily vehicular traffic, in the Metropolitan

Atlanta area. The decline in Pb concentrations in the sediment

core from Westpoint Lake with time clearly documents the

reduced burdens of Pb released to the environment by the

removal of lead from gasoline. The farther downstream from

the major contaminant source, the smaller the peak Pb

concentration in the 1970s and the smaller the decreases in

concentrations as time approaches the present (Figure 4a).

There is a prominent spatial peak in Zn in the 1975-1980

dated samples from Westpoint Lake downstream of Atlanta,

similar to that of Pb (Figure 4b). Unlike Pb, however, the

younger samples (1980-1990) have identically high concentrations of Zn (240 μg/g). Only the most recent set of

samples (1990-1995) has an average Zn concentration (215

μg/g) that is slightly reduced from the maximum concentration present in the 1980-1990 interval. The temporal

distribution of sedimentary Zn concentrations in dated

reservoir cores downstream of Atlanta suggests that there is

a significant source of this metal that persists to the present.

A large number of coal-fired power plants are located

throughout Georgia and Alabama (26). Total Zn concentrations and population density are strongly correlated, and

population density and traffic density are strongly correlated

(Table 1). These strong positive correlations suggest that

automobiles as well as coal combustion may be significant

terrestrial sources of Zn to sediments.

Automobile tires contain large quantities of ZnO that is

used as an accelerator in vulcanization (8). Using an average

Zn concentration of 0.73 wt %, a tire size of 65 cm in diameter

and 15 cm wide and assuming that 1 cm of rubber is worn

off in 32 000 km of driving, Christensen and Guinn (8)

calculate that 3.0 mg of Zn are released to the road surface

and the immediately adjacent areas. Thus, runoff from street,

road, and highway surfaces can contribute significant

quantities of Zn to the aquatic environment. Corrosion of

galvanized metal and tire wear are frequently cited sources

of Zn in urban runoff (23). Wiesner et al. (24) note that there

is a direct correlation between particulate Zn concentrations

in urban runoff from various cities around the world and

fuel consumption (one measure of traffic density). They (24)

suggest that this relationship indicates that tire wear may be

the principal source of particulate Zn in urban runoff and

that much of the global variability in particulate Zn concentrations might be explained by motor-vehicle use.

Highway runoff is a major contributor to urban-suburban

impoundments that are designed to catch stormwater runoff

and trap sediments and sorbed contaminants (27).

Census data (6) indicate that the U.S. population has

grown 29% during the period 1970 to 1995. National trafficvolume data (28) show that vehicle miles have increased

124% for the same time period. Thus, there is an everincreasing supply of particulate Zn to pollute our waterways.

Even though modern water-pollution control measures have

reduced the loads of many contaminants to aquatic resources,

they appear to have barely kept pace with Zn inputs that

have been, in part, contributed by ever-increasing traffic

volume. Also, it is unknown whether current use of Zn in the

rubber manufacturing process is equivalent to past usage.

In this paper we showed that total Pb and Zn concentrations in sediments correlate equally well with population

density and traffic density as do anthropogenic concentrations, eliminating the need for data manipulation of total

metal concentrations. The facts that streambed sediment is

sieved to less than 63-μm grain size and that reservoir

sediments are generally in the silt-clay grain-size class are

major factors contributing to this relationship (29). The

differences in the spatial and temporal distributions of metal

concentrations along the urban-suburban gradient in the

Atlanta Metropolitan area are related to anthropogenic

activities such as the use and subsequent removal of leaded

gasoline and the wear of rubber tires on motor vehicles.

Elevated Zn concentrations in rural settings may be related

to fossil-fuel combustion, notably from power plants that

dominate the energy-producing landscape in Georgia and

Alabama (26) as well as rubber tire wear.

Acknowledgments

This research was supported by the USGS NAWQA Program.

Thanks goes to Evelyn Hopkins for providing Figure 1 and

Peter C. Van Meter, Michelle I. Hornberger, and three

anonymous reviewers for their helpful comments. Any use

of trade, product, or firm names is for descriptive purposes

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

Geological Survey.

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Received for review April 5, 1999. Revised manuscript received October 21, 1999. Accepted October 21, 1999.

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