Mapping Watershed Potential to Contribute 

Phosphorus from Geologic Materials to 

Receiving Streams, Southeastern United States

By Silvia Terziotti, Anne B. Hoos, Douglas A. Harned, and Ana Maria Garcia

U.S. Department of the Interior

U.S. Geological Survey

Originally published as Scientific Investigations Map 3102

Reformatted to 8 1/2" x 11" page size
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

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Suggested citation:

Terziotti, Silvia, Hoos, A.B., Harned, D.A., and Garcia, A.M, 2010, Mapping watershed potential to contribute phosphorus from geologic materials to receiving streams, southeastern United States: U.S. Geological Survey Scientific 

Investigations Map 3102, 1 sheet.
Contents

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

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

Mapping Watershed Potential to Contribute Phosphorus from Geologic Materials .........................2

Bed-Sediment Phosphorus Concentration Source Data ...............................................................2

Delineating Geologic Map Units ........................................................................................................4

Delineating Areas Affected by Phosphate Mining..........................................................................6

Characterization of Bed-Sediment Phosphorus Concentration Using Geologic Map Units.............7

Characterization of Contribution of Phosphorus to Streams from Geologic Materials 

Disturbed by Mining Activities.....................................................................................................................10

Using Regionalized Bed-Sediment Phosphorus Concentration in Watershed Models 

to Characterize Contribution of Phosphorus to Streams ........................................................10

Data Layer Products....................................................................................................................................12

Acknowledgments.......................................................................................................................................12

Selected References...................................................................................................................................12

Figures

1–2. Map showing—

1. Locations of stream sites sampled for bed-sediment phosphorus 

concentrations in the southeastern United States as part of the 

U.S. Geological Survey National Geochemical Survey.................................................3

2. Level IV ecoregions and geology in the southeastern United States.........................4

3. Images showing representations of the areal extent of mined land within 

a phosphate-mine operation.......................................................................................................6

4–6. Map showing—

4. Geologic map units for which mean value of bed-sediment phosphorus 

concentration was adjusted because of insufficient bed-sediment 

sampling sites ......................................................................................................................8

5. Mean values of bed-sediment phosphorus concentration within 

geologic map units. Bed-sediment samples collected from headwater 

streams draining relatively undisturbed areas, 1976–2006...........................................9

6. Areal extent of phosphate-mined land in the southeastern United States 

and geologic map units that overlap the mined land, with estimates of 

bed-sediment phosphorus contribution to streams.....................................................11

Tables

1. Basin drainage areas and percentages of agricultural land and developed land 

for National Geochemical Survey stream sites sampled for bed-sediment 

phosphorus content in the southeastern United States ........................................................2

2. Data sufficiency classification and methods for assigning average 

bed-sediment phosphorus concentrations to geologic mapping units...............................7

Abstract

As part of the southeastern United States SPARROW 

(SPAtially Referenced Regressions On Watershed attributes) 

water-quality model implementation, the U.S. Geological 

Survey created a dataset to characterize the contribution 

of phosphorus to streams from weathering and erosion of 

surficial geologic materials. SPARROW provides estimates 

of total nitrogen and phosphorus loads in surface waters from 

point and nonpoint sources. The characterization of the contribution of phosphorus from geologic materials is important to 

help separate the effects of natural or background sources of 

phosphorus from anthropogenic sources of phosphorus, such 

as municipal wastewater or agricultural practices. The potential of a watershed to contribute phosphorus from naturally 

occurring geologic materials to streams was characterized by 

using geochemical data from bed-sediment samples collected 

from first-order streams in relatively undisturbed watersheds 

as part of the multiyear U.S. Geological Survey National 

Geochemical Survey. The spatial pattern of bed-sediment 

phosphorus concentration is offered as a tool to represent the 

best available information at the regional scale. One issue may 

weaken the use of bed-sediment phosphorus concentration 

as a surrogate for the potential for geologic materials in the 

watershed to contribute to instream levels of phosphorus—an 

unknown part of the variability in bed-sediment phosphorus 

concentration may be due to the rates of net deposition and 

processing of phosphorus in the streambed rather than to variability in the potential of the watershed’s geologic materials to 

contribute phosphorus to the stream. Two additional datasets 

were created to represent the potential of a watershed to 

contribute phosphorus from geologic materials disturbed by 

mining activities from active mines and inactive mines.

Mapping Watershed Potential to Contribute 

Phosphorus from Geologic Materials to 

Receiving Streams, Southeastern United States

Silvia Terziotti, Anne B. Hoos, Douglas A. Harned, and Ana Maria Garcia

Introduction

The largest reservoir of phosphorus (P) in the environment is minerals in sedimentary rocks (Mueller and Helsel, 

1996). Although, in general, phosphorus compounds in rock 

minerals are relatively insoluble and do not readily move in 

runoff or groundwater, erosion of surface soil and rock may 

be a substantial source of suspended phosphorus in streams in 

some areas where sedimentary rock is exposed, where soils 

derived from sedimentary rock exist (Dillon and Kirchner, 

1975), or especially where deposits of phosphorus minerals 

are mined. Phosphate mineral deposits in consolidated 

material in the southeastern United States generally are of two 

types—phosphate nodules (oolites, or grains incorporated in 

sedimentary rocks) and accumulations in residuum derived 

from the weathering of bedrock (U.S. Geological Survey, 

1968). In addition, ore-grade deposits of phosphate minerals 

are present in formations of large areal extent in Florida and 

Tennessee (two areas where phosphate output is the largest in 

the Nation) and in formations of smaller areal extent in North 

Carolina and Alabama. Data describing phosphate mineral 

content of ore material are available for many mined areas; 

however, these data cannot be used to characterize the mass 

of P contributed from the watershed without additional 

information on density and distribution of the ore material 

within the host rock. Phosphorus concentrations in streambed 

sediment, however, may represent average phosphorus levels 

in the surficial geologic material of the contributing watershed 

and delivery of this material to the stream (Cannon and 

others, 2003). 

The U.S. Geological Survey (USGS) created a dataset 

to characterize the potential for a watershed to contribute 

phosphorus to streams from weathering and erosion of surficial 
geologic materials. The motivation for creating this dataset 

was to support calibration of a phosphorus-transport model 

(SPARROW—SPAtially Referenced Regressions On Watershed attributes) for streams in the southeastern United States. 

SPARROW (Schwarz and others, 2006) provides estimates 

of total nitrogen and phosphorus loads in streams from point 

and nonpoint sources, including background, or naturally 

occurring, sources. The data also may be useful for a variety of 

other purposes, including understanding regional background 

variations in surface-water phosphorus concentrations and 

developing nutrient criteria.

Mapping Watershed Potential 

to Contribute Phosphorus from 

Geologic Materials

In order to create a map of bed-sediment phosphorus 

concentration values that can be applied to the entire Southeast, 

values of phosphorus were compiled and summarized by geologic map units. The source data for phosphorus concentration, 

the method of deriving geologic map units, and the identification of phosphorus mine locations are described below.

Bed-Sediment Phosphorus Concentration 

Source Data

The USGS National Geochemical Survey (NGS) 

compiles data on geochemistry of stream-channel sediment 

(first-order streams) and soil in order to construct geochemical maps and refine estimates of baseline concentrations of 

chemical elements (U.S. Geological Survey, 2004). Phosphorus data are incorporated into the NGS data base primarily 

from the re-analysis of a subset of samples collected between 

1976 and 1980 for the National Uranium Resource Evaluation 

(NURE) Program (Averett, 1984) and analyses of new samples 

collected during 1997–1999 and 2004–2006 by the USGS 

and collaborating State agencies. In the Southeast, the NGS 

contains nine datasets based on NURE samples (Alabama I, 

Alabama II, Coastal 98, Coastal 99, Kentucky, South Carolina, 

2000, 2002, and 800) for a total of 6,842 samples, and nine 

State datasets (Alabama, Florida I, Florida II, Georgia, 

Mississippi I, Mississippi II, States 2006a, States 2006b, and 

Tennessee) for a total of 3,280 samples (U.S. Geological 

Survey, 2004). Thus, 10,122 bed-sediment samples were 

available across the southeastern U.S. for mapping bedsediment phosphorus concentration (BSPconc; fig. 1). 

Streams for bed-sediment sampling in the Southeast for 

the NURE program were selected randomly from topographic 

maps marked with grids that cover 10 to 30 square kilometers 

(km2

), with an average density of one site per 13 km2. 

If possible, no sites were selected that were closer than 

1.5 kilometer (km) to another site, and the area drained was 

no more than three times the area of the grid so that the 

maximum size of a basin draining to a site would be 90 km2

. 

The largest headwater stream within the grid was chosen if 

the site was accessible to field personnel (Bolivar, 1980), and 

an attempt to sample most streams draining from 8 to 26 km2

(3 to 10 square miles) was made (Ferguson and others, 

1976). Samples were collected upstream from roads, ponds 

or other disturbances that could have altered the sampling 

results, and samples were collected outside of populated areas 

(Information Systems Programs, Energy Resources Institute, 

1985) in order to accurately represent the local-scale variation 

in sediment mineral composition with surficial geology or 

other geographical characteristics. In the development of 

datasets for the NGS, NURE samples in most areas were 

selected randomly for re-analysis in order to achieve a density 

of approximately one sample per 289 km2; however, in 

densely sampled areas in Kentucky and Alabama, all of the 

NURE samples were re-analyzed. 

In order to verify that samples were in general from 

small, undisturbed basins, basin drainage area and watershed 

percentage of developed and agricultural area at each sampling 

site in the Southeast SPARROW study area (7,421 of the 

10,122 sampling sites) were examined using the National 

Hydrography Dataset Plus (NHDPlus, 2009). The NHDPlus 

maintains tables that include cumulative drainage area and 

basin characteristics derived from the 1992 National LandCover Data (NLCD; Vogelmann and others, 2001) for every 

stream segment. Because most samples were taken before 

1997, the 1992 land-cover data were most appropriate to use 

for evaluating anthropogenic effects.

Of the 7,421 sampling sites within the SPARROW 

study area, 5,560 could be assigned with confidence (within 

200 meters) to an individual stream segment on the 1:100,000-

scale NHDPlus. The size distribution of watersheds for these 

sites confirms that the sites drain small areas; half of the sites 

drain areas smaller than 9.1 km2

 and 90 percent of the sites 

drain areas smaller than 47 km2 (table 1). Presumably these 

values would be even lower had all 7,421 sampling sites 

been included in the analysis; the lack of association with 

Table 1. Basin drainage areas and percentages of agricultural 

land and developed land for National Geochemical Survey 

stream sites sampled for bed-sediment phosphorus content 

in the southeastern United States.

Percentile

Cumulative 

drainage 

area, in 

square 

kilometers

Agricultural 

land

Developed 

land

Forested 

land

Percentage in 1992

10th 2.2 0 0 2

25th 4.2 2.1 0 18.9

50th 9.1 12 0.1 37.9

75th 19.3 30.5 0.9 53.2

90th 46.6 50.3 5.1 67.2

Mean 56.4 19.2 2.4 36.9

2 Mapping Watershed Potential to Contribute Phosphorus from Geologic Materials to Receiving Streams, Southeastern U.S.
the 1:100,000-scale NHDPlus that caused 1,861 sites to be 

excluded from the analysis was, in most cases, due to location 

of the excluded site on a first-order stream too small for representation by 1:100,000 hydrography. The distribution of landcover characteristics of watersheds for the 5,560 sampling 

sites confirms that the majority of the sites represent basins 

with little urban development; that is, land cover is less than 

0.1-percent developed in half of the watersheds and less than 

5-percent developed in 90 percent of the watersheds. Although 

some of the sites are affected by agricultural activity (land 

cover is more than 50-percent agricultural for 10 percent of 

the sites), the majority of the sites are relatively undisturbed 

(less than 12-percent agricultural in half of the watersheds).

Based on the characteristics of the target stream 

population, several assumptions can be made about the bedsediment BSPconc from this dataset: BSPconc values from 

the majority of these sites represent conditions with minimal 

anthropogenic sources in the watershed, and consequently, the 

average values of BSPconc for different subregions indicate 

spatial variation in the potential of the upstream watershed to 

contribute P from naturally occurring geologic materials to the 

receiving stream channel. 

• BSPconc values represent the potential contribution 

of P from the local area adjacent to the sampling site 

(Bolivar, 1980). 

• BSPconc values represent streambed conditions in 

small headwater stream channels. 

All three of these assumptions are critical to the interpretation 

and application of the data in this report.

Bed-sediment sampling site

EXPLANATION

ATLANTIC

OCEAN

GULF OF MEXICO

Base modified from U.S. Geological Survey

National Elevation Dataset, 100-meter resolution

Mapping Watershed Potential to Contribute Phosphorus from Geologic Materials 3

Figure 1. Locations of stream sites sampled for bed-sediment phosphorus concentrations in the 

southeastern United States as part of the U.S. Geological Survey National Geochemical Survey.
Delineating Geologic Map Units

The BSPconc values were grouped into geologic map units 

(GMUs), defined by using regional-scale geologic features, 

and averaged to a single value for each GMU. The grouping 

and averaging accomplished two objectives: (1) creating a 

continuous surface of BSPconc throughout the Southeast 

regardless of sample-site density, and (2) minimizing the 

effect of BSPconc values that are affected by human activity. 

As previously discussed, the site-selection strategy for the 

bed-sediment data networks in NGS favor sites in relatively 

undisturbed areas; in practice, however, the BSPconc results 

at some sites may be affected by anthropogenic sources. 

Potential bias introduced by these sites was minimized by 

averaging values from many sites within a GMU. 

A

4 Mapping Watershed Potential to Contribute Phosphorus from Geologic Materials to Receiving Streams, Southeastern U.S.

Figure 2. (A) Level IV ecoregions (data from U.S. Environmental Protection Agency, 

Level IV Ecoregions, 1:250,000 scale, 2007) and (B) geology in the south-eastern United 

States (data from U.S. Geological Survey, Geologic map of the United States, 1:2,500,000 

scale, 1974. [Colors are used to illustrate the relative number of distinct classes, not to 

distinguish individual features].
The GMUs were created by delineating zones with 

similar lithochemistry based on geologic age (King and 

Beikman, 1974) and geomorphic setting (indicated by level 

IV ecoregions; U.S. Environmental Protection Agency, 2007). 

The digital maps of geologic age and level IV ecoregions 

were overlain and merged to create unique GMUs for each 

combination of geologic age and ecoregion. This combination 

creates units with similar geologic properties and accounts 

for regional variations reflected by ecoregions. Additional 

boundaries representing known phosphate-rich formations in 

Tennessee and Florida were incorporated to refine the GMU 

boundaries in these areas (Smith and Whitlatch, 1940, p. 8, 15; 

Clawges and Price, 1999; fig. 2).

B

Mapping Watershed Potential to Contribute Phosphorus from Geologic Materials 5

Figure 2. (A) Level IV ecoregions (data from U.S. Environmental Protection Agency, 

Level IV Ecoregions, 1:250,000 scale, 2007) and (B) geology in the south-eastern United 

States (data from U.S. Geological Survey, Geologic map of the United States, 1:2,500,000 

scale, 1974. [Colors are used to illustrate the relative number of distinct classes, not to 

distinguish individual features].—Continued
Delineating Areas Affected by Phosphate Mining

In this analysis, P derived from geologic material that 

has been extracted from a subsurface deposit and placed on 

the land surface was characterized separately from P derived 

from naturally occurring geologic materials in a watershed. 

The mapped location and description of phosphate mines from 

the Mineral Resources Data System (MRDS; U.S. Geological 

Survey, 2005b) were used to identify all possible areas where 

disturbed geologic materials may affect instream P concentration. The MRDS dataset contains 839 mine sites, active and 

inactive, in the southeastern United States. The determination 

of mine status is based on information from the active mines 

and mineral plants in the U.S. dataset (U.S. Geological Survey, 

2005a), which identifies 19 of the 839 phosphate mines sites 

as active mines. 

Only two active mine operations were identified outside 

of Florida, one in Mississippi and one in North Carolina. 

Boundaries of mined land for these operations were estimated 

by using aerial photography (fig. 3A). High-resolution imagery 

was used to identify the parcels and delineate the boundary for 

the mined areas. 

Because delineations of inactive mine operations were 

not available outside of west-central Florida, and because 

the land use at the inactive mine locations has been altered 

making it difficult to identify using current aerial photography, 

an estimate of the area associated with an inactive phosphate 

mine was based on active mining operations within a GMU. 

The mean area value of mined land (area classified as barren 

land in the NLCD (Homer and others, 2004) observed for a 

subset of mine operations within the GMU was assigned to 

inactive phosphate mine locations (fig. 3B). For example, 

inactive mine operations in central Tennessee were assigned a 

mined-land area of 0.1 km2

, whereas inactive mine operations 

in southwestern Florida were assigned a mined-land area 

of 4.3 km2. The areal extent was represented as a circular 

area centered over the coordinate of the point location (from 

MRDS), with radius sized to match the assigned area (for 

example, radii of 35 or 1,171 meters to correspond with 

assigned areas of 0.004 or 4.3 km2). 

For mining operations in west-central Florida, boundaries 

of mined land were estimated by using a combination of 

geospatial datasets describing location of land parcels and 

barren land cover (fig. 3C). The Mandatory Phosphate Mine 

Boundaries geospatial dataset from the Florida Department of 

Environmental Protection (2007) was used to delineate land 

parcels identified as inactive and active mining operations. 

The parcels from this layer were combined with the set of 

parcels of land disturbed by phosphate mining prior to 1975 

(Zellar-Williams, Inc., and Conservation Consultants, Inc., 

1980), some of which represent mines that are still active. 

Areas of actual mining activities within each of these parcels 

were identified and coded by using the areas classified as 

barren in the NLCD. For parcels identified by the Mandatory 

Phosphate Mine Boundaries not included in the ZellarWilliams parcels, only the sections that were classified as 

barren were considered to represent the areal extent of mined 

land. For parcels identified by the Zellar-Williams report, 

which explicitly delineate disturbed land, the entire parcel area 

(not just NLCD barren land) was used to represent the areal 

extent of mined land. 

A B C

6 Mapping Watershed Potential to Contribute Phosphorus from Geologic Materials to Receiving Streams, Southeastern U.S.

Figure 3. Representations of the areal extent of mined land within a phosphate-mine operation. (A) Based 

on aerial photography, active mine delineation in North Carolina (red lines indicate areal extent of mined land); (B) based 

on average size of barren land polygons within a geologic map unit, inactive mines in Tennessee (yellow lines represent 

the geologic map unit boundaries); and (C) based on land parcels and barren land cover, active and inactive mines in 

west-central Florida (red lines indicate areal extent of mined land).
Characterization of Bed-Sediment 

Phosphorus Concentration Using 

Geologic Map Units

By overlaying and merging digital maps of geologic age, 

level IV ecoregions, and boundaries of local formations, a 

set of 608 GMUs were delineated for the Southeast. GMUs 

smaller in area than 30 km2 were combined with an adjacent 

GMU to remove the artifact of numerous small units created 

along the edges of the intersecting map layers. The GMUs 

with fewer than three stream sites were considered to have 

insufficient data points to define a representative mean value 

and, therefore, were classified as “data poor.” The 278 GMUs 

in the data-poor classification compose only about 8 percent 

of the study area. An algorithm was developed to compute 

new values for the data-poor GMU by averaging the data-poor 

value with the mean BSPconc value from other GMUs with 

at least one of the following prioritized criteria: (1) GMUs 

within the same level III ecoregion and of the same geologic 

age as the data-poor GMU; (2) GMUs with the same geologic 

age (regardless of ecoregion) and adjacent to the data-poor 

GMU; or if these first two conditions fail, (3) using BSPconc 

values from all adjacent GMUs to calculate a mean BSPconc 

value for the data-poor GMU. A final test was conducted to 

determine if the new BSPconc value assigned to the data-poor 

GMU differed by more than 500 parts per million (ppm) from 

the original GMU BSPconc values. If this was the case, mean 

BSPconc values using all three conditions were calculated, 

and the BSPconc value closest to the original value was used. 

The areal extent and location of the data-poor GMUs are 

summarized in table 2 and displayed in figure 4, respectively.

The spatial pattern in BSPconc was evaluated with 

respect to the GMU boundaries using one-way ANOVA 

and the Tukey multiple-comparison test. The GMUs 

explained 56 percent of the total variation in BSPconc, and 

distributions between some GMUs were sufficiently different 

(at alpha = 0.1) to enable division into distinct groupings. 

Figure 5 displays the mean BSPconc from phosphorus values 

of bed-sediment samples collected in undisturbed, headwater 

streams within a GMU .

Table 2. Data sufficiency classification and methods for assigning average bed-sediment phosphorus concentrations to 

geologic mapping units.

[GMU, geologic map unit; BSPconc, bed-sediment phosphorus concentration; definition of data sufficiency classification: sufficient—sufficient number 

of sampling points (three or more) within the GMU to represent the average condition and no adjustments to mean value, poor—insufficient number of 

sampling points (less than three) to represent the average condition and adjustment to the original mean value was applied; percentages may not add up to 

100 because of rounding]

Data 

sufficiency 

classification

Number of 

GMUs in 

classification

Area, 

in square 

kilometers

Percentage 

of study 

area

Method for assigning an average 

BSPconc value for the GMU

Sufficient 329 993,908 91.7 Average the BSPconc of samples from within the GMU

Poor 244 85,652 7.9 Average the BSPconc of samples from within the GMU 

with average BSPconc from GMUs in same level III 

ecoregion that have the same geologic age

Poor 26 3,279 0.3 Average the BSPconc of samples from within the GMU 

with average BSPconc from adjacent GMUs with the 

same geologic age (regardless of ecoregion)

Poor 9 667 0.1 Average the BSPconc of samples from within the GMU 

with average BSPconc from adjacent GMUs (regardless 

of geologic age and ecoregion)

Total 608 1,083,506 100

Characterization of Bed-Sediment Phosphorus Concentration Using Geologic Map Units 7
Data-sufficiency classification

EXPLANATION

Poor (0 data points)

Poor (1–2 data points)

Sufficient, no adjustment

(3–805 data points)

8 Mapping Watershed Potential to Contribute Phosphorus from Geologic Materials to Receiving Streams, Southeastern U.S.

Figure 4. Geologic map units for which mean value of bed-sediment phosphorus 

concentration was adjusted because of insufficient bed-sediment sampling sites. 
90° 88° 86° 84° 82° 80° 78° 76°

40°

38°

36°

34°

32°

30°

28°

26°

Mean bed-sediment phosphorus 

 concentration, in parts per million, 

 by geologic map unit

EXPLANATION

Less than 200.0

200.1 to 300.0

300.1 to 400.0

400.1 to 500.0

500.1 to 700.0

700.1 to 900.0

900.1 to 18,150.0

VIRGINIA

WEST

VIRGINIA

OHIO

ILLINOIS

INDIANA

MISSOURI

ARKANSAS

LOUISIANA

KENTUCKY

TENNESSEE

NORTH CAROLINA

MISSISSIPPI

ALABAMA

GEORGIA

FLORIDA

SOUTH CAROLINA

0 100 200 300 KILOMETERS

0

Base modified from U.S. Geological Survey

National Atlas, 1:2,000,000 scale, 1972

50 100 150 MILES

A T L A N T I C

O C E A N

G U L F O F M E X I C O

Characterization of Bed-Sediment Phosphorus Concentration Using Geologic Map Units 9

Figure 5. Mean values of bed-sediment phosphorus concentration within geologic map units. Bed-sediment samples 

collected from headwater streams draining relatively undisturbed areas, 1976–2006.
Characterization of Contribution of 

Phosphorus to Streams from Geologic 

Materials Disturbed by Mining Activities

During phosphate mining activity, phosphate-rich mineral 

deposits are disturbed and can affect the concentration of P 

in nearby streams. Estimates of the potential of an upstream 

watershed to contribute P to streams from runoff from areas 

of disturbed geologic materials can be developed as a simple 

function of areal extent of mined land in the watershed, or as 

a combined function of areal extent of the mined land and the 

level of phosphate enrichment in the mined deposit. Following 

the second approach, a series of graded step functions was 

developed by assigning zero for areas mapped as undisturbed 

and by assigning a gradient of values in disturbed areas that 

reflects the varying levels of phosphate enrichment in mined 

deposits. A value for potential was assigned to each mined 

area in the Southeast by associating the estimated mined-area 

polygon with the overlapping GMUs. The method may 

assign a value to a mine based on bed-sediment sampling 

sites outside the estimated mined-area polygon (fig. 6A, B), 

depending on the location of the maximum BSPconc in the 

GMU relative to the polygon. 

The status of a mining operation was used to weight the 

value associated to a mine. An active mine is assigned the 

maximum value within the GMU that its areal extent overlaps, 

whereas an inactive mine is assigned a value midway between 

the maximum and mean value of the GMU that its areal extent 

overlaps. This is illustrated in figure 6C, for a GMU for which 

the maximum BSPconc for bed-sediment sampling sites is 

9,176 ppm and the average BSPconc is 708 ppm. The GMU 

contains mine polygons that fall in both active and inactive 

mining activity zones; active mines were assigned the value 

9,176 ppm (GMU maximum) and inactive mines were assigned 

the value of 4,942 ppm (average of 9,176 and 708 ppm). 

This approach makes two assumptions, both with 

unknown validity: (1) ore of similar phosphate-mineral 

content is extracted from mines within the same GMU, and 

(2) the BSPconc for at least one sample within the GMU 

reflects the influence of phosphate-mining activity. Estimates 

of BSPconc therefore, may be less reliable in areas with 

high density of mined land, as the density of geochemical 

survey data is insufficient to determine variable loading rates 

among individual mines or to differentiate contributions from 

mined-land from contributions from natural, undisturbed land 

in these areas.

Using Regionalized Bed-Sediment 

Phosphorus Concentration in 

Watershed Models to Characterize 

Contribution of Phosphorus to Streams 

Calibration and application of watershed models of 

instream transport and concentration of P require characterization of the potential of the upstream watershed to contribute P 

from naturally occurring geologic materials in the watershed to 

instream (water-column) P flux in the receiving stream. For an 

empirical watershed model such as SPARROW, characterizing 

potential contribution in absolute mass units is not necessary; 

rather, characterizing the relative variability, or gradient, of 

contribution across the region is sufficient. SPARROW’s 

regression analysis determines the appropriate functional 

relation between the explanatory variable (for example, 

BSPconc) and the mass contributed to instream P flux. 

The spatial distribution of BSPconc from the NGS 

dataset, which represents headwater streams draining 

relatively undisturbed watersheds, is useful for modeling the 

gradient in potential to contribute P to streambed sediments 

in headwater streams. The response variable of interest in 

watershed models, however, generally is contribution to 

instream flux rather than contribution to streambed sediments, 

and it is not clear that spatial distribution of BSPconc is useful 

in modeling instream P flux. Instream P flux and BSPconc 

in headwater streams are influenced by several of the same 

factors—variation in availability of P in geologic materials 

in the watershed and variation in basin rates of erosion and 

transport of P to the edge of the stream. BSPconc is influenced 

further by variation in the rates of net deposition and 

processing of P in the streambed (McDowell and Sharpley, 

2001; Walling and others, 2003). The spatial pattern of 

BSPconc, therefore, is useful as a surrogate for watershed 

contribution to instream P flux only under the assumptions 

of (1) equal rates across all headwater streams in the region, 

(2) net deposition from the water column, and (3) processing 

within the streambed sediment. Such assumptions are 

challenged by known variability among headwater streams in 

frequency of flushing, occurrence of low dissolved-oxygen 

conditions, and aquatic-plant processing. The spatial pattern 

of BSPconc is offered as a tool, representing the best 

available information at the regional scale, with the caveat 

that the surrogate relation is weakened by not accounting for 

variability in headwater streams in the rates of net deposition 

and processing of P in the streambed. 

10 Mapping Watershed Potential to Contribute Phosphorus from Geologic Materials to Receiving Streams, Southeastern U.S.
VIRGINIA

KENTUCKY

TENNESSEE

NORTH CAROLINA

MISSISSIPPI

ALABAMA

GEORGIA

SOUTH

CAROLINA

FLORIDA

950 9,176

1,890

4,330

28,000

Bed-sediment phosphorus concentration 

 in mined land, in parts per million

Geologic map unit that overlaps areal 

 extent of phosphate-mined land

Bed-sediment sampling site and 

 phosphorus concentration, in 

 parts per million—Site with 

 highest concentration value 

 within a geologic map unit

Phosphate mine operations 

 in Florida—Map B

Inactive

Active

Disburbed mine area

EXPLANATION

950

615.0 to 900.0

900.1 to 1,800.0

1,800.1 to 2,700.0

2,700.1 to 4,500.0

4,500.1 to 7,200.0

7,200.1 to 11,700.0

11,700.1 to 19,772.7

A

B C

Characterization of Contribution of Phosphorus to Streams from Geologic Materials Disturbed by Mining Activities 11

Figure 6. Areal extent of phosphate-mined land in 

the southeastern United States and geologic map 

units that overlap the mined land, with estimates of 

bed-sediment phosphorus contribution to streams, and 

(A) example in Tennessee showing the locations of 

phosphate-mined land relative to the locations of three 

geologic map units that overlap the phosphate-mined 

land, and the locations of three bed-sediment sampling 

sites from which the geologic map unit maximum 

concentration value was derived; (B) example in Florida 

showing parcels that contain inactive and active mine 

operations, with hachures indicating the disturbed mine 

areas within the parcels; and (C) example in Florida 

showing how mine operation status is used to weight 

the value associated to a phosphate-mined land area.
Data Layer Products

This study has resulted in three separate spatial data 

layers. “Pconc_background” characterizes the potential of a 

watershed to contribute P to streams from geologic materials 

that are undisturbed by mining or other human activities (fig. 5). 

“Pconc_active_mined” and “Pconc_inactive_mined” characterize 

the potential of a watershed to contribute P to streams from 

geologic materials that have been disturbed by mining activities 

(fig. 6). The data layers were each allocated to 8,404 individual 

watershed segments in the southeastern SPARROW study 

area for use as variables in the model to predict contribution 

of phosphorus to a stream from these two separate source 

categories. Data layers are available upon request from the 

USGS North Carolina Water Science Center (request form 

available at http://nc.water.usgs.gov/about/inforequests.html).

Acknowledgments

The authors gratefully acknowledge Dr. Jeffrey Grossman, USGS, for providing data and technical guidance 

throughout this study. The authors also thank Dr. Gerard 

McMahon and Michael Wieczorek of the USGS for their 

review of the manuscript. 

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Selected References 13
Terziotti and others—Mapping Watershed Potential to Contribute Phosphorus from Geologic Materials to Receiving Streams, Southeastern U.S.—Scientific Investigations Map 3102