U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2010–5176

National Water-Quality Assessment Program

Contributions of Phosphorus from Groundwater to 

Streams in the Piedmont, Blue Ridge, and Valley and Ridge 

Physiographic Provinces, Eastern United States
Cover. Photo showing area near U.S. Geological Survey surface-water site 01620500 North River near Stokesville, VA. Photo by U.S. 

Geological Survey.
Contributions of Phosphorus from 

Groundwater to Streams in the Piedmont, 

Blue Ridge, and Valley and Ridge 

Physiographic Provinces, Eastern United 

States

By Judith M. Denver, Charles A. Cravotta III, Scott W. Ator, and Bruce D. Lindsey

National Water-Quality Assessment Program

Scientific Investigations Report 2010–5176

U.S. Department of the Interior

U.S. Geological Survey
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:

Denver, J.M., Cravotta, C.A., III, Ator, S.W., and Lindsey, B.D., 2010, Contributions of phosphorus from groundwater to 

streams in the Piedmont, Blue Ridge, and Valley and Ridge Physiographic Provinces, Eastern United States: U.S. Geological Survey Scientific Investigations Report 2010–5176, 38 p.

ISBN 978-1-4113-3044-3
iii

Foreword

The U.S. Geological Survey (USGS) is committed to providing the Nation with reliable scientific information that helps to 

enhance and protect the overall quality of life and that facilitates effective management of water, biological, energy, and 

mineral resources (http://www.usgs.gov/). Information on the Nation’s water resources is critical to ensuring long-term 

availability of water that is safe for drinking and recreation and is suitable for industry, irrigation, and fish and wildlife. 

Population growth and increasing demands for water make the availability of that water, measured in terms of quantity and 

quality, even more essential to the long-term sustainability of our communities and ecosystems.

The USGS implemented the National Water-Quality Assessment (NAWQA) Program in 1991 to support national, 

regional, State, and local information needs and decisions related to water-quality management and policy 

(http://water.usgs.gov/nawqa). The NAWQA Program is designed to answer: What is the quality of our Nation’s streams 

and ground water? How are conditions changing over time? How do natural features and human activities affect the quality 

of streams and ground water, and where are those effects most pronounced? By combining information on water chemistry, 

physical characteristics, stream habitat, and aquatic life, the NAWQA Program aims to provide science-based insights for 

current and emerging water issues and priorities. From 1991 to 2001, the NAWQA Program completed interdisciplinary 

assessments and established a baseline understanding of water-quality conditions in 51 of the Nation’s river basins and 

aquifers, referred to as Study Units (http://water.usgs.gov/nawqa/studies/study_units.html).

In the second decade of the Program (2001–2012), a major focus is on regional assessments of water-quality conditions and 

trends. These regional assessments are based on major river basins and principal aquifers, which encompass larger regions 

of the country than the Study Units. Regional assessments extend the findings in the Study Units by filling critical gaps in 

characterizing the quality of surface water and ground water, and by determining water-quality status and trends at sites 

that have been consistently monitored for more than a decade. In addition, the regional assessments continue to build an 

understanding of how natural features and human activities affect water quality. Many of the regional assessments employ 

modeling and other scientific tools, developed on the basis of data collected at individual sites, to help extend knowledge 

of water quality to unmonitored, yet comparable areas within the regions. The models thereby enhance the value of our 

existing data and our understanding of the hydrologic system. In addition, the models are useful in evaluating various 

resource-management scenarios and in predicting how our actions, such as reducing or managing nonpoint and point 

sources of contamination, land conversion, and altering flow and (or) pumping regimes, are likely to affect water conditions 

within a region.

Other activities planned during the second decade include continuing national syntheses of information on pesticides, 

volatile organic compounds (VOCs), nutrients, trace elements, and aquatic ecology; and continuing national topical studies 

on the fate of agricultural chemicals, effects of urbanization on stream ecosystems, bioaccumulation of mercury in stream 

ecosystems, effects of nutrient enrichment on stream ecosystems, and transport of contaminants to public-supply wells.

The USGS aims to disseminate credible, timely, and relevant science information to address practical and effective waterresource management and strategies that protect and restore water quality. We hope this NAWQA publication will provide 

you with insights and information to meet your needs, and will foster increased citizen awareness and involvement in the 

protection and restoration of our Nation’s waters. 

The USGS recognizes that a national assessment by a single program cannot address all water-resource issues of interest. 

External coordination at all levels is critical for cost-effective management, regulation, and conservation of our Nation’s 

water resources. The NAWQA Program, therefore, depends on advice and information from other agencies—Federal, State, 

regional, interstate, Tribal, and local—as well as nongovernmental organizations, industry, academia, and other stakeholder groups. Your assistance and suggestions are greatly appreciated.

 William H. Werkheiser

USGS Associate Director for Water
iv

Contents

Foreword ........................................................................................................................................................iii

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

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

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

Phosphorus Geochemistry..................................................................................................................4

Description of Study Area ...................................................................................................................5

Physiography and Geology ........................................................................................................5

Climate and Hydrology................................................................................................................5

Land Use and Anthropogenic Phosphorus Sources..............................................................6

Methods...........................................................................................................................................................6

Data Compilation ..................................................................................................................................6

Graphical and Statistical Analyses .................................................................................................10

Geochemical Modeling......................................................................................................................10

Contributions of Phosphorus from Groundwater to Streams...............................................................11

Phosphorus in Streams and Groundwater.....................................................................................11

Sources, Fate, and Transport of Phosphorus ................................................................................16

Phosphorus from Natural Sources .........................................................................................20

Phosphorus from Human Sources..........................................................................................21

Fate and Transport of Phosphorus .........................................................................................21

Summary and Implications.........................................................................................................................32

Acknowledgments.......................................................................................................................................34

Selected References...................................................................................................................................34
v

Figures

1–3. Maps showing—

1. The Piedmont, Blue Ridge, and Valley and Ridge Physiographic Provinces and 

principal aquifers.................................................................................................................3

2. Generalized land use and land cover in the Piedmont, Blue Ridge, and Valley 

and Ridge Physiographic Provinces.................................................................................7

3. The distribution of groundwater and surface-water sampling networks in the 

Piedmont, Blue Ridge, and Valley and Ridge Physiographic Provinces ....................9

4. Boxplots showing base-flow indices for different bedrock types in the Piedmont, 

Blue Ridge, and Valley and Ridge Physiographic Provinces...............................................11

5. Probability plots showing the distribution of dissolved concentrations of (A)

phosphate and (B) nitrate in groundwater (left) and streams (right) for crystalline,

siliciclastic, and carbonate bedrock of the Piedmont, Blue Ridge, and Valley and

Ridge study area .........................................................................................................................12

6. Boxplots showing concentrations of dissolved phosphate in groundwater and

surface water by network for different bedrock types of the Piedmont, Blue Ridge,

and Valley and Ridge study area..............................................................................................13

7. Graphs showing seasonal trends in base-flow concentrations of phosphorus in

three streams draining watersheds of varying hydrogeologic and land-use

characteristics ............................................................................................................................14

8. Piper diagrams showing major-ion composition of groundwater and stream

base-flow samples from different bedrock settings of the Piedmont, Blue Ridge,

and Valley and Ridge study area..............................................................................................15

9–14. Boxplots showing—

9. Comparison of (A) dissolved oxygen and (B) pH of groundwater and surface 

water by network for different bedrock types of the Piedmont, Blue Ridge, 

and Valley and Ridge study area.....................................................................................17

10. Comparison of (A) specific conductance, and (B) concentrations of dissolved 

silica in groundwater and surface water by network for different bedrock 

types of the Piedmont, Blue Ridge, and Valley and Ridge study area ......................18

11. Comparison of (A) chloride and (B) nitrate in groundwater and surface water 

by network for different bedrock types of the Piedmont, Blue Ridge, 

and Valley and Ridge study area. ...................................................................................19

12. Saturation indices for calcite, dolomite, and fluorite in groundwater and surface 

water by network for different bedrock types of the Piedmont, Blue Ridge, 

and Valley and Ridge study area.....................................................................................24

13. Saturation indices for apatite in groundwater and surface water by network 

for different bedrock types of the Piedmont, Blue Ridge, and Valley and Ridge 

study area ...........................................................................................................................25

14. Saturation indices for ferrihydrite, strengite, and vivianite in groundwater and 

surface water by network for different bedrock types of the Piedmont, 

Blue Ridge, and Valley and Ridge study area ...............................................................26

15. Graph showing equilibrium fractions of initial concentrations of phosphate that 

may be dissolved or adsorbed on a finite amount of hydrous ferric oxide at 

25 degrees Celsius as a function of pH...................................................................................32
vi

Tables

1. Nutrient criteria for concentrations of phosphorus and nitrate in streams and

reservoirs in Ecoregions IX and XI as recommended by the U.S. Environmental

Protection Agency........................................................................................................................2

2. Description of groundwater and surface-water networks in the Piedmont, Blue

Ridge, and Valley and Ridge Physiographic Provinces..........................................................8

3. Median concentrations of dissolved phosphate and associated water-quality

constituents in groundwater and surface water from selected networks with

natural or nearly natural water quality, or with water quality impacted by human

activities in the Piedmont, Blue Ridge, and Valley and Ridge study area.........................20

4. Spearman’s rank correlation coefficients for dissolved phosphate, major chemical

constituents, and land use for 609 groundwater samples and 358 surface-water

samples from the Piedmont, Blue Ridge, and Valley and Ridge study area .....................22

5. Compositions of rainwater and background groundwater from crystalline,

siliciclastic, and carbonate aquifers plus paired groundwater and surface water 

from selected monitoring networks in these settings ..........................................................23

6. Saturation index values computed with PHREEQC for compositions of

solutions used in inverse mass-transfer models...................................................................27

7. Mass-transfer estimates to obtain median background groundwater quality by 

reactions with evaporated rainfall in crystalline, siliciclastic, and carbonate 

bedrock settings .........................................................................................................................28

8. Mass-transfer estimates that produce median surface-water quality by reactions 

with median groundwater quality in selected networks......................................................30

9. Estimated fraction of phosphate concentration in median surface water that 

originated from groundwater in selected networks .............................................................31
vii

Conversion Factors and Datum

Multiply By To obtain

Length

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

Area

square meter (m2) 10.76 square foot (ft2) 

square kilometer (km2) 0.3861 square mile (mi2)

Flow rate

millimeter per year (mm/yr) 0.03937 inch per year (in/yr) 

Mass

gram (g) 0.03527 ounce, avoirdupois (oz)

kilogram (kg) 2.205 pound, avoirdupois (lb)

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

°F=(1.8×°C)+32

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 

(NGVD 29).

Altitude, as used in this report, refers to distance above the vertical datum.

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).
viii

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Contributions of Phosphorus from Groundwater to 

Streams in the Piedmont, Blue Ridge, and Valley and Ridge 

Physiographic Provinces, Eastern United States

By Judith M. Denver, Charles A. Cravotta III, Scott W. Ator, and Bruce D. Lindsey

Abstract

Phosphorus from natural and human sources is likely to 

be discharged from groundwater to streams in certain geochemical environments. Water-quality data collected from 

1991 through 2007 in paired networks of groundwater and 

streams in different hydrogeologic and land-use settings of the 

Piedmont, Blue Ridge, and Valley and Ridge Physiographic 

Provinces in the eastern United States were compiled and 

analyzed to evaluate the sources, fate, and transport of phosphorus. The median concentrations of phosphate in groundwater from the crystalline and siliciclastic bedrock settings 

(0.017 and 0.020 milligrams per liter, respectively) generally were greater than the median for the carbonate setting 

(less than 0.01 milligrams per liter). In contrast, the median 

concentrations of dissolved phosphate in stream base flow 

from the crystalline and siliciclastic bedrock settings (0.010 

and 0.014 milligrams per liter, respectively) were less than the 

median concentration for base-flow samples from the carbonate setting (0.020 milligrams per liter). Concentrations of 

phosphorus in many of the stream base-flow and groundwater 

samples exceeded ecological criteria for streams in the region.

Mineral dissolution was identified as the dominant source 

of phosphorus in the groundwater and stream base flow draining crystalline or siliciclastic bedrock in the study area. Low 

concentrations of dissolved phosphorus in groundwater from 

carbonate bedrock result from the precipitation of minerals 

and (or) from sorption to mineral surfaces along groundwater 

flow paths. Phosphorus concentrations are commonly elevated 

in stream base flow in areas underlain by carbonate bedrock, 

however, presumably derived from in-stream sources or from 

upland anthropogenic sources and transported along short, 

shallow groundwater flow paths.

Dissolved phosphate concentrations in groundwater were 

correlated positively with concentrations of silica and sodium, 

and negatively with alkalinity and concentrations of calcium, 

magnesium, chloride, nitrate, sulfate, iron, and aluminum. 

These associations can result from the dissolution of alkali 

feldspars containing phosphorus; the precipitation of apatite; 

the precipitation of calcite, iron hydroxide, and aluminum 

hydroxide with associated sorption of phosphate ions; and 

the potential for release of phosphate from iron-hydroxide 

and other iron minerals under reducing conditions. Anthropogenic sources of phosphate such as fertilizer and manure and 

processes such as biological uptake, evapotranspiration, and 

dilution also affect phosphorus concentrations. The phosphate 

concentrations in surface water were not correlated with 

the silica concentration, but were positively correlated with 

concentrations of major cations and anions, including chloride 

and nitrate, which could indicate anthropogenic sources and 

effects of evapotranspiration on surface-water quality. Mixing 

of older, mineralized groundwater with younger, less mineralized, but contaminated groundwater was identified as a critical 

factor affecting the quality of stream base flow. In-stream 

processing of nutrients by biological processes also likely 

increases the phosphorus concentration in surface waters. 

Potential geologic contributions of phosphorus to 

groundwater and streams may be an important watershedmanagement consideration in certain hydrogeologic and 

geochemical environments. Geochemical controls effectively 

limit phosphorus transport through groundwater to streams 

in areas underlain by carbonate rocks; however, in crystalline 

and siliciclastic settings, phosphorus from mineral or human 

sources may be effectively transported by groundwater and 

contribute a substantial fraction to base-flow stream loads.

Introduction

Phosphorus is the limiting nutrient controlling biological 

productivity in most freshwater environments. Concentrations 

of dissolved phosphorus in freshwaters tend to be maintained 

at low levels (less than 0.1 mg/L, or milligrams per liter) 

because of the rapid uptake of phosphorus by algae and plants 

and its tendency to combine with calcium, iron, and aluminum as sparingly soluble “metal-phosphate” minerals and (or) 

to sorb on surfaces of other common minerals (Hem, 1985; 

Stumm and Morgan, 1996; Drever, 1997; Filippelli, 2008; 
2 Contributions of Phosphorus from Groundwater to Streams, Eastern United States

Oelkers and Valsami-Jones, 2008). Consequently, most of the 

phosphorus transported to and within streams is associated 

with suspended sediment, particularly during and following 

storm runoff events (Pionke and others, 1999; Litke, 1999; 

Bowes and others, 2005). Transport of dissolved phosphorus 

in groundwater as base flow to streams and lakes generally is 

presumed to be minor in comparison to overland transport. 

Nevertheless, even very small amounts of phosphorus transported by groundwater to surface waters may have ecological 

impacts (Kang and others, 2005).

Effective nutrient management of a watershed requires 

quantitative information on possible phosphorus sources 

and transport processes. Natural phosphorus sources include 

wastes and skeletal remains from fish, water fowl, and other 

animals; decay of foliage and leaf litter; and weathering of certain minerals in rocks and soils (Litke, 1999; Filippelli, 2008). 

Although phosphorus occurs naturally in such biological and 

geological substances, anthropogenic sources, such as fertilizers, animal manure, and sewage, typically dominate in waters 

draining agricultural and urban areas. 

Elevated concentrations of phosphorus in streams, lakes, 

and reservoirs can promote the rapid growth of algae, resulting in eutrophication (Wetzel, 2001). To minimize nutrient 

enrichment and eutrophication of freshwaters, the U.S. Environmental Protection Agency (USEPA) (1986) recommended 

national nutrient water-quality criteria for total phosphorus 

of 0.025 mg/L in reservoirs, 0.05 mg/L in streams directly 

discharging to reservoirs, and 0.1 mg/L in streams not directly 

discharging to reservoirs. In consideration of regional variability in geology, climate, soils, and other natural conditions that 

may affect phosphorus concentrations, the USEPA expanded 

the strategy for adapting these criteria on the basis of 14 level 

III ecoregions (U.S. Environmental Protection Agency, 2000a, 

2000b). Proposed criteria for streams in Ecoregion IX, which 

includes the Piedmont Physiographic Province, and Ecoregion 

XI, which includes the Blue Ridge and Valley and Ridge Provinces, are lower than those proposed for the Nation in 1986 

(fig. 1, table 1).

Groundwater discharge provides a large amount of water 

to streams, reservoirs, and lakes (particularly in the eastern 

United States), and, therefore, may contribute dissolved 

phosphorus to surface waters. Such contributions may be 

particularly critical during periods of diminished precipitation when groundwater discharge sustains streamflows and 

lake levels. Overall, concentrations of phosphorus in shallow 

groundwater in selected parts of the Nation from 1991 through 

2001 were generally less than 0.01 mg/L and not significantly 

different among undeveloped, agricultural, and urban settings 

(Nolan and Hitt, 2003). Concentrations of dissolved phosphorus in groundwater in selected areas of the eastern Piedmont 

frequently exceeded regional criteria, however, especially in 

crystalline and siliciclastic rocks, and were comparable to 

estimated total phosphorus concentrations in undeveloped 

headwater streams of the area (Smith and others, 2003). Contributions from natural phosphorus sources to surface waters 

may be an important consideration in nutrient-management 

strategies for the Piedmont, Blue Ridge, and Valley and Ridge 

Physiographic Provinces given the potential for concentrations 

of dissolved phosphorus in groundwater to exceed 0.01 mg/L 

in certain areas (Sharpley and others, 1994, 2003; Abrams and 

Jarrell, 1995). Groundwater in contact with phosphate minerals, such as apatite, has been suggested as a possible natural 

source of high phosphorus concentrations in some streams 

of the North Carolina Coastal Plain (Spruill and others, 

1998; Fuhrer and others, 1999). The dissolution of feldspar 

containing traces of phosphorus is another possible source 

(Manning, 2008). Thus, total phosphorus concentrations in 

certain streams, lakes, and reservoirs may equal or exceed the 

recommended Federal nutrient criteria in parts of Ecoregions 

IX and XI because of the contribution of dissolved minerals in 

groundwater and geochemical conditions that allow elevated 

concentrations of phosphorus to persist in groundwater. To 

address this issue, the U.S. Geological Survey (USGS), as 

part of the National Water-Quality Assessment (NAWQA) 

Program, assembled and analyzed data on water chemistry; 

and the geologic and land-use characteristics of the Piedmont, 

Blue Ridge, and Valley and Ridge Physiographic Provinces to 

study sources of phosphorus to groundwater and the potential 

for phosphorus transport from groundwater to surface water in 

this region. 

Purpose and Scope

Potential contributions of phosphorus from natural and 

anthropogenic sources to groundwater and from groundwater 

to streams in the Piedmont, Blue Ridge, and Valley and Ridge 

Physiographic Provinces in the eastern United States (fig. 1) 

are described and discussed in this report. Water-quality data 

collected from 1991 through 2007 from more than 600 wells 

and more than 350 streams during base-flow conditions in 

various hydrogeologic and land-use settings from northern 

Alabama to New Jersey are presented. Graphical and statistical techniques are used to compare phosphorus concentrations 

Table 1. Nutrient criteria for concentrations of phosphorus 

and nitrate in streams and reservoirs in Ecoregions IX and XI 

as recommended by the U.S. Environmental Protection Agency 

(2000a, 2000b).

[mg/L, milligrams per liter]

Nutrient Physiographic Province 

(Ecoregion)

Stream 

criterion

(mg/L)

Reservoir 

criterion

(mg/L)

Phosphorus Piedmont (IX) 0.037 0.02

Blue Ridge/Valley and Ridge (XI) 0.01 0.008

Nitrate Piedmont (IX) 0.69 0.36

Blue Ridge/Valley and Ridge (XI) 0.31 0.46
Introduction 3

Figure 1. The Piedmont, Blue Ridge, and Valley and Ridge Physiographic Provinces and principal aquifers.

85° 80° 75°

40°

35°

MICHIGAN

OHIO

KENTUCKY

TENNESSEE

NORTH CAROLINA

SOUTH CAROLINA

GEORGIA

VIRGINIA

WEST VIRGINIA

NEW JERSEY

PENNSYLVANIA

NEW YORK

DELAWARE

MARYLAND

INDIANA

ALABAMA

CANADA

Base from U.S. Bureau of the Census, 1990, 1:500,000 to 1:5,000,000 and 

Statistics Canada digital data, 2006

Albers Equal-Area Conic projection: Standard parallels 29°30' N and 45°30' N,

Central meridian 96°00' W, Latitude of origin 23°00' N

0 50 100 KILOMETERS

0 50 100 MILES

National Elevation Data from U.S. Geological Survey, 1999

Principal Aquifers from U.S. Geological Survey, 2003

Va l l e y a n d R i d g e

B l u e R i d g e

P i e d m o n t

EXPLANATION

PRINCIPAL AQUIFER NAMES

STATE BOUNDARY

PHYSIOGRAPHIC BOUNDARY

Early Mesozoic basin aquifers

Piedmont and Blue Ridge carbonate-rock aquifers

Piedmont and Blue Ridge crystalline-rock aquifers

Valley and Ridge siliciclastic-rock aquifers

Valley and Ridge carbonate-rock aquifers
4 Contributions of Phosphorus from Groundwater to Streams, Eastern United States

in groundwater and streams among different hydrogeologic 

and land-use settings and to proposed ecoregion nutrient criteria. Interactions between groundwater and streams in various 

settings inferred through comparison of major-ion chemistry 

are described. Geochemical models are used to identify and 

evaluate potential contributions from various sources of phosphorus in different settings, and geochemical and hydrologic 

processes affecting phosphorus fate and transport through 

groundwater to streams. Implications of observations and 

conclusions for regional nutrient management and proposed 

criteria are discussed.

Phosphorus Geochemistry

Phosphorus is a minor element in rock, soil, and water, 

but it is essential for the growth of microorganisms, plants, 

and animals (Rudnick and Gao, 2003; Krauskopf, 1979; 

Hem, 1985). In rocks and water near the surface of the Earth, 

phosphorus occurs primarily as the anion PO4

3-, which is 

referred to as orthophosphate or phosphate (Hem, 1985). 

The most abundant phosphate mineral is apatite, a calciumphosphate compound with varying amounts of fluorine, 

chlorine, and (or) hydroxide (Ca5(PO4)3(F,Cl,OH)), that occurs 

widely as an accessory in all types of rocks (Dietrich and 

Skinner, 1979; Oelkers and Valsami-Jones, 2008). In igneous 

and metamorphic (crystalline) rocks, the main phosphatebearing mineral is fluorapatite, whereas in sedimentary 

(siliciclastic and carbonate) rocks, phosphorous is typically 

associated with detrital fluorapatite and authigenic carbonatefluorapatite (Ca9.316Na0.36Mg0.144(PO4

)4.8(CO3)1.2F2.48) (Guidry 

and MacKenzie, 2003; Oelkers and Valsami-Jones, 2008). 

Monazite, a compound of rare-earth elements and phosphate, 

is rare, but can be found in high-grade metamorphic and 

granitic intrusions in parts of the eastern United States (Feiss 

and others, 1991). About 25 percent of United States monazite 

production is from the Carolina Slate Belt in the Piedmont 

Physiographic Province, which extends from southern Virginia 

to northern Georgia (Feiss and others, 1991). 

Although apatite is a common accessory mineral in many 

types of rocks, the assumption that apatite is the dominant 

“natural” source of dissolved phosphorus in groundwater 

may be incorrect. Feldspars are widely distributed and are the 

dominant mineral (40–50 percent) in igneous, metamorphic, 

and associated siliciclastic sedimentary rocks that constitute 

the continental crust of the Earth (Dietrich and Skinner, 1979; 

Krauskopf, 1979). Due to their abundance and their potential 

to contain trace amounts of phosphorus through the Berlinite 

substitution, where phosphorus and aluminum can replace 

silicon in the mineral lattice (2Si4+ = Al3+ + P5+), feldspars can 

account for 50 to 90 percent of the estimated crustal P2O5 content (Manning, 2008). Feldspars in granite can contain up to 

1 percent P2O5 in this form, but more commonly they contain 

0.2 to 0.3 percent (Kontak and others, 1996). Accordingly, the 

formula for alkali feldspar containing 0.3 percent P2O5 can be 

written as (Na,K)Al1.011Si2.978P0.011O8. 

Whether dissolved phosphorus has originated from 

natural biological or anthropogenic sources, or the weathering of rocks, its transport may be affected by the presence 

of certain dissolved elements or its contact with common 

minerals, plants, and microorganisms. Dissolved calcium, 

aluminum, and iron can combine with phosphate to form 

sparingly soluble phosphate minerals such as apatite, variscite 

(AlPO4

.

2H2O), or strengite (FePO4

.

2H2O), for example, which 

may limit phosphate availability in water (McBride, 1994; 

Oelkers and Valsami-Jones, 2008). In anoxic environments, 

the ferrous phosphate mineral vivianite (Fe3(PO4)2

.

8H2O) 

also may limit dissolved phosphorus concentrations (Drever, 

1997). Furthermore, concentrations of dissolved phosphate 

may be limited by adsorption to surfaces of other solids such 

as calcium carbonate, aluminum oxides and silicates, and 

hydrous iron and manganese oxides (Kuo and Lotse, 1972; 

Borggaard, 1983; Parfitt, 1989; Borggaard and others, 1990; 

Geelhoed and others, 1997; Maine and others, 2007; Filippelli, 2008). Adsorption or coprecipitation of phosphate with 

calcium carbonate could account for the active removal of dissolved phosphate from groundwater in carbonate aquifers, for 

example (Jarvie and others, 2005), and from sewage effluent 

in constructed wetlands (Maine and others, 2007).

Adsorption-desorption processes and biological uptakedecay processes tend to occur at faster rates than mineral 

dissolution-precipitation processes; they also tend to sequester 

phosphorus as more labile forms (Bennett and others, 2001; 

Filippelli, 2008). Hydrous ferric oxides, such as goethite 

(FeOOH) and ferrihydrite (Fe(OH)3), that commonly form 

in well-drained soils and other oxidizing environments, for 

example, can be important as sinks or sources of dissolved 

phosphate in aquatic environments. At typical pH (5–8) of natural water, dissolved phosphate species (H2

PO4

-

, HPO4

2-) tend 

to be strongly adsorbed by hydrous ferric oxides, with sorption 

increasing as pH decreases (Dzombak and Morel, 1990; Geelhoed and others, 1997). Such phases commonly constitute the 

main, long-term storage pool for soil phosphorus (Filippelli, 

2008). Accordingly, a large fraction of phosphorus transported 

by runoff may be associated with iron oxide coatings on soil 

particles (Eghball and others, 1996; Pionke and others, 1999). 

The sorbed phosphate may be displaced by other ions such as 

silica and sulfate, however (Obihara and Russell, 1972; Parfitt, 

1989; Geelhoed and others, 1997), and, if subjected to reducing conditions, the oxidized iron compounds may dissolve, 

releasing phosphorus (Borggaard, 1983; Ehrlich, 1990). The 

reductive dissolution of ferric oxides typically is coupled with 

the oxidation of organic compounds after supplies of dissolved 

oxygen, nitrate, and nitrite and manganese oxides have been 

depleted (Stumm and Morgan, 1996; Drever, 1997; McMahon 

and Chapelle, 2008). Consequently, dissolved phosphorus, 

iron, and manganese concentrations may be elevated in anoxic 

aquatic environments that lack dissolved nitrate and nitrite, 

and dissolved phosphorus concentrations may be diminished 

through sorption and coprecipitation reactions in oxidizing aquatic environments with abundant iron and aluminum 

oxides and (or) calcium carbonates. 
Introduction 5

Phosphorus cycling in the aquatic environment is 

complex. Once phosphorus enters the aquatic environment, 

concentrations are largely controlled by plant uptake and degradation and by geochemical reactions (Drever, 1997). During 

photosynthesis, plants will grow until the available phosphorus or nitrate is depleted, at which point photosynthesis ceases 

and respiration and decay begin. Naturally occurring bacteria 

degrade plant matter, releasing phosphorus and other chemical constituents back to the aquatic system. Under natural 

conditions, the cycle of biological uptake and decay generally 

maintains a nutrient balance. Anthropogenic inputs of nutrients and processes such as bacterial denitrification can disrupt 

the balance, however—bacterial denitrification can lead to 

the accumulation of excess phosphate and the deposition of 

inorganic phosphate compounds such as carbonate fluorapatite 

(Ehrlich, 1990). 

Description of Study Area

The Piedmont, Blue Ridge, and Valley and Ridge Physiographic Provinces (Fenneman and Johnson, 1946) study 

area includes 368,000 km2 (square kilometers) in parts of 11 

states and the District of Columbia and extends from northern 

Alabama to New Jersey in the eastern United States (fig. 1). 

Land characteristics, geology, hydrology, and water quality 

in the area are described in detail in Hollyday and Hileman 

(1996), Rutledge and Mesko (1996), Briel (1997), Trapp and 

Horn (1997), Swain and others (2004), and Lindsey and others 

(2006), and are summarized below.

Physiography and Geology

The study area is underlain by consolidated sedimentary 

(siliciclastic or carbonate) and crystalline (igneous or metamorphic) bedrock and includes areas of steep topography and 

considerable relief. Unconsolidated regolith (weathered rock) 

overlies the bedrock in most areas, and is typically thinner 

in uplands than in adjacent valleys. Topographic features 

(mountains and valleys) are generally aligned from southwest 

to northeast (Fenneman and Johnson, 1946). Land-surface 

altitudes range from around 90 m (meters) in Piedmont Lowlands to over 1,800 m in the highest peaks of the Blue Ridge 

Mountains.

The Valley and Ridge Province (fig. 1) is underlain by 

folded and faulted, unmetamorphosed sedimentary bedrock. 

Siliclastic rocks (shale, sandstone, arkose, greywacke, and 

conglomerate) dominate in uplands, whereas carbonate rocks 

(limestone and dolomite) dominate in valleys (Swain and 

others, 1991). Karst terrain with associated caves, sinkholes, 

and other solution features is common in the carbonate valleys 

(Hollyday and Hileman, 1996). The rocks of the Valley and 

Ridge are structurally deformed by folding and thrust faulting, 

which are important controls on the landforms and the orientation and abundance of fractures (Woodward, 1985).

The Blue Ridge Province (fig. 1) is an area of high relief 

that is underlain by crystalline and siliciclastic bedrock. 

Crystalline metamorphic and igneous rocks dominate northward from Virginia, and metamorphic and sedimentary rocks 

dominate southward from North Carolina (Mesko and others, 

1999). Metamorphic rocks are generally more abundant than 

igneous rocks in the crystalline terrains. Sedimentary rocks are 

predominantly sandstone, shale, and conglomerate and include 

minor coal in North Carolina and Virginia. As in the Valley 

and Ridge, the rocks of the Blue Ridge are faulted, folded, and 

fractured.

Crystalline metamorphic and igneous rocks underlie 

92 percent of the Piedmont Province (fig. 1). The crystalline 

rocks generally are resistant to erosion and form hilly terrain 

of moderate relief (Lindsey and others, 2006). Siliciclastic 

rocks underlie approximately 7 percent of the Piedmont and 

are present from northern North Carolina to New Jersey. 

Siliciclastic rocks in the Piedmont are primarily restricted to 

fault-bound basins in the surrounding crystalline-rock lowlands. The basins contain sequences of Triassic and Jurassic 

sandstone and siltstone that can exceed a total thickness of 

6,000 m (Bain and Brown, 1981). Carbonate rocks, including 

limestone, dolomite, and marble, also are present in the Piedmont of Maryland and Pennsylvania. Marble also is present in 

other parts of the Piedmont, but is a minor rock type compared 

to the igneous and other metamorphic rocks (Overstreet and 

Bell, 1965). 

Climate and Hydrology

Climate in the study area is humid and primarily temperate to subtropical (Swain and others, 1991). Average annual 

precipitation (from 1951 through 1980) ranged from 1,020 to 

1,270 mm/yr (millimeters per year) in most of the Piedmont 

and Blue Ridge Provinces to over 1,500 mm/yr in the southern 

part of the Blue Ridge in North and South Carolina, Tennessee, and Georgia (Swain and others, 1991). 

Groundwater flow in the study area occurs primarily in 

fractures, solution cavities, and other secondary openings in 

the generally impermeable bedrock, and in intergranular and 

secondary voids in the overlying regolith. Local and regional 

groundwater-flow systems are defined by topography and 

associated hydraulic gradients, and the influence of geologic 

structure and mineral weathering on the formation of secondary voids in the rocks. The regolith serves as a reservoir for 

storage of recently recharged water and supplies recharge 

to the fractures in the underlying crystalline and siliciclastic 

bedrock and contributes to stream base flow (Heath, 1980). In 

contrast with the crystalline and siliciclastic bedrock, fractures 

and other secondary openings are commonly enlarged by 

dissolution of the carbonate bedrock, and groundwater flow 

can be rapid (Lindsey and others, 2006). Due to the extensive 

secondary porosity, carbonate aquifers typically provide much 

greater well yields than crystalline or siliciclastic aquifers. 
6 Contributions of Phosphorus from Groundwater to Streams, Eastern United States

Groundwater typically flows along relatively short flow 

paths from recharge areas in uplands to discharge areas in 

local streams or springs, although the direction of flow is 

controlled by the orientation of fractures and other secondary 

openings in the rock. Groundwater is an important component 

of flow to streams in the Piedmont, Blue Ridge, and Valley 

and Ridge Physiographic Provinces. Previous studies in this 

region calculated the proportion of total streamflow derived 

from groundwater, referred to as the “base-flow index,” to be 

greater than 50 percent in most areas (Bachman and others, 

1998; Rutledge and Mesko, 1996). 

The overall relations among major bedrock type, geohydrology, and water quality in the study area are consistent with 

different water-quality characteristics for major rock types 

described by Drever (1997). Specifically, in crystalline-rock 

terrains, slow weathering of silicate minerals tends to produce 

natural groundwater with low concentrations of dissolved 

solids, alkalinity, and hardness. In contrast, in the siliciclasticrock and carbonate-rock terrains, because of faster weathering 

of carbonates and associated minerals, the water commonly 

has greater concentrations of dissolved solids, alkalinity, and 

hardness under natural conditions than water in the adjacent 

crystalline rocks (Risser and Siwiec, 1996)

Land Use and Anthropogenic Phosphorus 

Sources

Land cover in the study area is primarily forest, particularly in areas of steep topography and along relatively inaccessible ridge tops (fig. 2). Agricultural and urban land uses are 

present primarily in the valleys in the Valley and Ridge and in 

flatter areas of the Piedmont. Many of these areas are underlain by carbonate rocks, and local groundwater is therefore 

particularly vulnerable to contamination from human activities (Lindsey and others, 2009). Areas of intensive agriculture 

include the Great Valley in the northern Valley and Ridge and 

the northern Piedmont. Densely populated urban areas include 

those around Atlanta, Washington, D.C., Philadelphia, and 

New York City. 

Although phosphorus may be derived from a variety of 

sources, anthropogenic sources such as inorganic fertilizer, 

manure, and sewage generally dominate in agricultural and 

urban areas. In the Potomac River Basin, the wider Chesapeake Bay watershed, and the Albemarle and Pamlico Sound 

watersheds, most of the phosphorus in surface water is derived 

from fertilizer and manure; point sources such as sewage 

effluent are relatively minor contributors (Blomquist and 

others, 1996; Spruill and others, 1998; Alexander and others, 

2008). Additional anthropogenic sources of phosphorus can 

be important locally, such as industrial discharges to the South 

Fork Catawba River in South Carolina (Hughes and others, 

2000), and combined and sanitary sewer effluent in urban 

and suburban areas (Frick and others, 1998; Hampson and 

others, 2000).

Phosphorus from fertilizer and manure can be removed 

from soil water by plant uptake, adsorption, and mineral 

precipitation processes (McBride, 1994; Drever, 1997; Litke, 

1999). Thus, phosphorus unused by crops tends to be retained 

in the upper soil horizons in association with minerals or biomass (Sharpley and others, 2003). Where fertilizer or manure 

is repeatedly applied to agricultural fields, disposal sites, and 

urban lawns, however, the phosphorus retention capacity of 

the soil may be exceeded (Eghball and others, 1996; Beauchemin and others, 1996, 1998). Consequently, runoff and soil 

water that infiltrates to groundwater may contain elevated 

concentrations of dissolved phosphate (Sharpley and others, 

1994, 2003). 

Methods

Groundwater and stream-quality data collected as part of 

the USGS NAWQA Program (Gilliom and others, 1995) from 

1991 through 2007 were compiled to support a regional interpretation of water quality (particularly phosphorus concentrations) in the study area. The NAWQA design is discussed in 

Gilliom and others (1995) and Lapham and others (1995); 

protocols for collection of water-quality data are presented in 

Shelton (1994) and Koterba and others (1995). 

Data Compilation 

Groundwater data were collected from 609 bedrock wells 

in 18 networks that were designed to document and explain 

the occurrence and distribution of selected chemical compounds in groundwater in particular settings (Gilliom and others, 2006) (table 2, fig. 3). A variety of crystalline, siliciclastic, 

and carbonate bedrock types with associated lithologies were 

sampled by use of these wells. Stream base-flow water-quality 

data were collected and compiled from 358 streams in 18 

networks in the study area, which are commonly co-located 

with groundwater networks. Data include analyses from one 

round of sampling in each network. The base-flow stream-synoptic surveys (table 2) were designed to assess the quality of 

water in small streams of particular settings during base-flow 

conditions when groundwater supplies most of the streamflow. Most of these data were collected from the late spring 

through the late summer. Although the groundwater quality 

and stream-synoptic survey data were collected over a period 

of 17 years (from 1991 through 2007), the data were assumed 

to be comparable for the purpose of evaluating spatial patterns 

in water quality for this study. Additionally, routine (typically 

monthly) water-quality data collected over multiyear periods 

at 40 streams in the study area were compiled for evaluation of seasonal or temporal patterns in stream base flow in 

selected settings. Selected data from these compilations are 

presented in this report; all data compiled for this report are 
Methods 7

85° 80° 75°

40°

35°

MICHIGAN

OHIO

KENTUCKY

TENNESSEE

NORTH CAROLINA

SOUTH CAROLINA

GEORGIA

VIRGINIA

WEST VIRGINIA

NEW JERSEY

PENNSYLVANIA

NEW YORK

DELAWARE

MARYLAND

INDIANA

ALABAMA

CANADA

Base from U.S. Bureau of the Census, 1990, 1:500,000 to 1:5,000,000 and 

Statistics Canada digital data, 2006

Albers Equal-Area Conic projection: Standard parallels 29°30' N and 45°30' N,

Central meridian 96°00' W, Latitude of origin 23°00' N

0 50 100 KILOMETERS

0 50 100 MILES

Land cover digital data from Nakagaki and others, 2007

Va l l e y a n d R i d g e

B l u e R i d g e

P i e d m o n t

EXPLANATION

LAND COVER

STATE BOUNDARY

PHYSIOGRAPHIC BOUNDARY

Agriculture

Urban

Forest

Other

Figure 2. Generalized land use and land cover in the Piedmont, Blue Ridge, and Valley and Ridge Physiographic Provinces.
8 Contributions of Phosphorus from Groundwater to Streams, Eastern United States

accessible on the World Wide Web at http://infotrek.er.usgs.

gov/traverse/f?p=NAWQA:HOME:0. 

Most samples were analyzed for selected nutrients 

(including phosphorus and nitrogen compounds), major 

ions, various trace elements (including iron, manganese, and 

aluminum), and dissolved organic carbon (DOC), as well as 

temperature, specific conductance (SC), pH, and concentrations of dissolved oxygen (DO). Nutrient, major-ion, and DOC 

analyses were conducted at the USGS National Water-Quality 

Laboratory (NWQL) in Denver, Colorado (Patton and Truitt, 1992; Brenton and Arnett, 1993; Fishman, 1993; Werner 

and others, 1996). The water temperature, SC, pH, and DO 

concentrations were measured in the field immediately prior to 

sample collection. Bicarbonate concentrations were calculated 

on the basis of alkalinity titrations conducted in the field. 

Water-quality data were screened to maximize their 

utility for evaluating regional patterns in phosphorus concentrations and possible water-quality relations between 

groundwater and streams in the study area. Water-quality data 

from streams draining watersheds containing known sewage 

outfalls or other point sources were omitted, as phosphorus 

loads from such sources often greatly exceed those from 

natural sources and (or) groundwater discharge. Water-quality 

samples collected from streams during periods of known or 

likely overland runoff (such as following precipitation) also 

were omitted. Total and dissolved concentrations of phosphorus were determined for surface-water samples. Because only 

dissolved concentrations of phosphorus and other constituents occur in groundwater, however, analyses for dissolved 

phosphate (also referred to as orthophosphate) were used for 

Table 2. Description of groundwater and surface-water networks in the Piedmont, Blue Ridge, and Valley and Ridge Physiographic 

Provinces.

[Network code abbreviations: P=Piedmont, V=Valley and Ridge, B=Blue Ridge, X=crystalline, S=siliciclastic; C=carbonate, third letter in network code 

indicates location; F=forest; “-” indicates no samples collected; predominant land-use designations: AG = predominantly agricultural (greater than 50 percent 

agricultural and less than 5 percent urban), URB = predominantly urban (greater than 25 percent urban and less than 25 percent agricultural), UND = predominantly undeveloped (less than 25 percent agricultural and less than 5 percent urban), MIX = mixed land use (any sampling site not meeting the criteria for AG, 

URB, or UND)]

Network 

code River Basin/Location

Predominant Land Use

Principal Aquifer/ Physiographic Province Groundwater Surface water 

Crystalline Bedrock 

PXD Delaware/Northern New Jersey URB MIX Piedmont and Blue Ridge crystalline rock

PXL Lower Susquehanna AG AG Piedmont and Blue Ridge crystalline rock

PXP Potomac MIX MIX Piedmont and Blue Ridge crystalline rock

PXB Albemarle-Pamlico MIX - Piedmont and Blue Ridge crystalline rock

PXS Santee UND MIX Piedmont and Blue Ridge crystalline rock

PXA Apalachicola-Chattahoochee-Flint URB MIX Piedmont and Blue Ridge crystalline rock

BXK Kanawha-New MIX UND Piedmont and Blue Ridge crystalline rock

Siliciclastic Bedrock 

PSD Delaware MIX MIX Early Mesozoic basin (Piedmont)

PSP Potomac AG AG Early Mesozoic basin (Piedmont)

VSD Delaware MIX MIX Valley and Ridge siliciclastic rock

VSL Lower Susquehanna AG - Valley and Ridge siliciclastic rock

VSLF Lower Susquehanna - UND Valley and Ridge siliciclastic rock

VSP Potomac AG UND Valley and Ridge siliciclastic rock

VST Upper Tennessee - UND Valley and Ridge siliciclastic rock

Carbonate Bedrock 

PCL Lower Susquehanna AG AG Piedmont carbonate-rock

VCLA Lower Susquehanna (Appalachian Section) AG AG Valley and Ridge carbonate-rock

VCL Lower Susquehanna (Great Valley Section) AG AG Valley and Ridge carbonate-rock

VCLU Lower Susquehanna (Urban) URB MIX Valley and Ridge carbonate-rock

VCP Potomac AG MIX Valley and Ridge carbonate-rock

VCT Upper Tennessee MIX MIX Valley and Ridge carbonate-rock
Methods 9

Figure 3. The distribution of groundwater and surface-water sampling networks in the Piedmont, Blue Ridge, and Valley and 

Ridge Physiographic Provinces.

PXA

PXL

PXP

PXS

PXB

PSD

PSP

PCL

VCL and VCLU

VCLA

VCP

VSD

VSP

VSL

VSLF PXD

BXK VCT

VST

85° 80° 75°

40°

35°

MICHIGAN

OHIO

KENTUCKY

TENNESSEE

NORTH CAROLINA

SOUTH CAROLINA

GEORGIA

VIRGINIA

WEST VIRGINIA

NEW JERSEY

PENNSYLVANIA

NEW YORK

DELAWARE MARYLAND

INDIANA

ALABAMA

CANADA

Base from U.S. Bureau of the Census, 1990, 1:500,000 to 1:5,000,000 and 

Statistics Canada digital data, 2006

Albers Equal-Area Conic projection: Standard parallels 29°30' N and 45°30' N,

Central meridian 96°00' W, Latitude of origin 23°00' N

0 50 100 KILOMETERS

0 50 100 MILES

Principal Aquifers from U.S. Geological Survey, 2003

PXD1

PXL

PXP

PXB2

PXS

PXA

BXK

PSD

PSP

PCL

EXPLANATION

STUDY AREA: PROVINCE, BEDROCK 

TYPE, AND LOCATION—See table 2 

for description of abbreviations

1 Areas with surface-water samples only

2 Areas with groundwater samples only

3 Urban area is indistinguishable at this scale,

 vcl and vclu have same shading

VSD

VSL2

VSLF1

VSP

VST1

VCLA

VCL

VCLU3

VCP

VCT
10 Contributions of Phosphorus from Groundwater to Streams, Eastern United States

all comparisons of phosphorus concentrations between surface 

water and groundwater. 

Ancillary geospatial data were compiled to describe the 

physical characteristics of the watersheds draining to sampled 

streams and the potential contributing areas within 500 m of 

sampled wells. Each surface-water and groundwater sampling 

site was classified on the basis of the bedrock type (crystalline, siliciclastic, or carbonate) and physiographic setting 

(table 2). Land-cover data were compiled and used to compute 

the percentage of four major land uses (wetland, forested, 

agricultural, and urban) within 500 m of sampled wells and 

within watersheds of sampled streams. Generalized land cover 

was modified from the National Land Cover Database 1992 

(NLCD 1992) using historical land-use and land-cover data 

(Price and others, 2007). 

Graphical and Statistical Analyses 

Various graphical and statistical techniques were used 

to compare water quality in different geologic settings, to 

estimate natural and anthropogenic sources of phosphate, and 

to identify possible factors affecting the transport of phosphate 

from groundwater to streams in the study area. In general, 

nonparametric, rank-based statistical approaches were used 

to accommodate non-normally distributed and censored data 

(Helsel and Hirsch, 2002). Data for individual continuous 

variables (such as chemical concentrations) were censored to 

a common level and censored values were set to a lower common level before ranks were computed for use in statistical 

tests. Relations between continuous variables were evaluated 

with scatter plots and correlation coefficients (Spearman’s 

rho, Kendall’s tau); distributions of continuous variables were 

compared among different groups (such as geologic settings) 

using boxplots and rank-sum or rank-transform analysis-ofvariance (ANOVA) (Helsel and Hirsch, 2002). 

Groundwater and stream interactions within and among 

different geologic settings were evaluated using graphical 

techniques. Considering bedrock and redox classifications, 

boxplots and probability graphs were used to compare univariate distributions of the water-quality data (Helsel and Hirsch, 

2002). The redox class was determined on the basis of concentrations of DO, nitrate, manganese, iron, and sulfate (McMahon and Chapelle, 2008). Trilinear diagrams (Drever, 1997) 

were used to illustrate dominant geochemical conditions in 

streams and groundwater in different settings and infer likely 

interactions between regional groundwater and stream base 

flow. The relative contribution of base flow to total streamflow 

over multiyear periods and during collection of individual 

samples at continuously monitored streams was evaluated 

by hydrograph separation with the PART computer program 

(Rutledge, 1998). The relative importance of groundwater discharge and overland runoff to the transport of selected chemical compounds to continuously monitored streams in different settings was inferred through evaluations of the relation 

between concentrations and flow condition (stream discharge). 

Geochemical Modeling

Geochemical models were developed to explain relations among concentrations of dissolved phosphate and other 

constituents in groundwater and stream water. Aqueous 

speciation computations with WATEQ4F (Ball and Nordstrom, 1991) and mass-transfer computations with PHREEQC 

(Parkhurst and Appelo, 1999) using the WATEQ4F database 

were used to test the following hypotheses: (1) evaporation 

of typical rainfall and dissolution of minerals can produce 

observed concentrations of phosphate and other trace constituents in “natural” (background) groundwater; (2) mineralsolubility processes can explain the range of concentrations 

of phosphate in groundwater and surface water in different 

geologic settings; and (3) even if they are undersaturated 

with phosphate minerals, adsorption of phosphate by hydrous 

ferric oxide, aluminum oxide, and (or) calcium carbonate 

minerals could be an important control on dissolved phosphate concentrations.

Inverse modeling with PHREEQC was used to evaluate mass-transfer processes that could feasibly produce the 

observed concentrations of constituents in the groundwater 

and surface-water samples (Parkhurst and Appelo, 1999; 

Nordstrom, 2003). The first set of models simulated reactions 

between partially evaporated rainfall (concentrated by a factor 

of 3) and ubiquitous mineral phases that could produce background groundwater quality for each geologic setting (crystalline, siliciclastic, or carbonate bedrock). The background 

quality was estimated using the medians for samples with 

measurable DO (greater than 0.5 mg/L), but concentrations 

of nitrate less than 1.0 mg/L and chloride less than 10 mg/L. 

Censored concentrations were set at half the detection limit 

for aqueous speciation and mass-transfer calculations. 

A second set of inverse models was developed to evaluate possible anthropogenic sources of phosphate and associated contaminants (NO3

, Cl, Na, and K) in groundwater and 

streams of the study area. Paired groundwater- and surfacewater-quality datasets that indicated primarily natural conditions or human impacts on water quality in specific land-use 

and geologic settings (networks) were selected for these 

models. The median water quality for all samples in a specific 

network was used to estimate the composition of the groundwater (initial solution) and the surface water (final solution). 

Censored concentrations were set at half the detection limit. 

The generalized mineral assemblage for a particular 

bedrock setting, based on lithogeochemical classifications, 

and the computed saturation index (SI) values for various 

minerals were used to constrain the mineral reactants and 

products in the inverse models (Nordstrom, 2003). If the 

initial and final solutions were undersaturated (SI less than 0) 

with respect to the mineral phase, that phase was specified to 

dissolve (reactant). Likewise, if the initial and final solutions 

were supersaturated (SI greater than 0) with respect to a mineral, that phase was specified to precipitate (product) or was 

excluded from the mass-transfer model. 
Contributions of Phosphorus from Groundwater to Streams 11

Adsorption and desorption of phosphate on hydrous ferric 

oxide-coated surfaces were evaluated using a diffuse doublelayer modeling approach with PHREEQC (Parkhurst and 

Appelo, 1999) and surface complexation data from Dzombak 

and Morel (1990). The hydrous ferric oxide was specified as a 

0.09 grams per kilogram solution, with a specific surface area 

of 600 cubic meters per gram consisting of 5 x 10-6 moles of 

strong binding sites and 2 x 10-4 moles of weak binding sites. 

Aqueous speciation and adsorption distribution for a range 

of pH values were computed for initial phosphate concentrations of 0.001 mol/kg (moles per kilogram, or 31 mg/L) and 

0.1 mol/kg (3,100 mg/L) in a 10-mol/kg sodium chloride 

background matrix. Plots were created to summarize the 

percentage of phosphate distributed between the solution and 

sorbent as a function of pH. 

Contributions of Phosphorus from 

Groundwater to Streams

Phosphorus from natural and human sources may be 

discharged from groundwater to streams in certain geochemical environments. Thus, some of the phosphorus in streams 

during base flow, when most water is discharging groundwater, is potentially derived from groundwater. Base flow is 

an important component of streamflow throughout the study 

area, comprising more than 50 percent of the total annual 

streamflow in most settings (fig. 4). The highest base-flow 

indices (which indicate the percentage of water in a stream 

that is from groundwater) are in the Valley and Ridge Carbonate setting and the lowest are in the Piedmont Siliciclastic 

setting. Estimated base-flow indices for streams included in 

this study are similar to, although somewhat higher than, those 

from previous studies in this area (Bachman and others, 1998; 

Rutledge and Mesko, 1996). 

Phosphorus in Streams and Groundwater

Concentrations of dissolved phosphate in the study area 

varied by rock type, but commonly were higher in streams 

than in groundwater (fig. 5a). Dissolved phosphate concentrations in the 609 groundwater samples ranged from less than 

0.01 to 0.54 mg/L, with a median of 0.010 mg/L; dissolved 

phosphate concentrations during base flow in 358 streams 

ranged from less than 0.01 to 1.23 mg/L, with a median of 

0.016 mg/L. Median concentrations of dissolved phosphate in 

groundwater from crystalline and siliciclastic rocks (0.017 and 

0.020 mg/L, respectively) were greater than median concentrations in groundwater from carbonate rocks (<0.01 mg/L) 

(fig. 5a). In contrast, the median concentrations of dissolved 

phosphate in streams draining crystalline and siliciclastic bedrock (0.010 and 0.014 mg/L, respectively) were less than those 

in streams draining carbonate bedrock (0.020 mg/L) (fig. 5a). 

The reported concentrations of dissolved phosphate in 

streams during base flow were commonly higher than recommended ecoregion nutrient criteria (table 1, fig. 6). This finding is important as these criteria are based on total phosphorus 

Figure 4. Base-flow indices for different bedrock types in the Piedmont, Blue Ridge, and Valley and Ridge Physiographic Provinces.

10

20

30

40

50

60

70

80

90

100 16 14 59 38 60 55 14 38 9

Valley and Ridge

Carbonate

Valley and Ridge

Siliciclastic

Piedmont

Crystalline

Blue Ridge

Crystalline

Piedmont

Siliciclastic

National Water-Quality

Assessment Program 

stream sampling site 

during study period

Open box = data from Rutledge and Mesko, 1996

Gray box = data from Bachman and others, 1998

5 5 8 4

74

90th percentile

75th percentile

Number of samples

Median

25th percentile

10th percentile

EXPLANATION

0

BASE-FLOW INDEX, IN PERCENT
12 Contributions of Phosphorus from Groundwater to Streams, Eastern United States

Figure 5. Distribution of dissolved concentrations of (A) phosphate and (B) nitrate in groundwater (left) and 

streams (right) for crystalline, siliciclastic, and carbonate bedrock of the Piedmont, Blue Ridge, and Valley and 

Ridge study area.

0.01

0.1

1

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

2

Crystalline

Siliciclastic

Carbonate

All samples

PHOSPHATE CONCENTRATION, IN MILLIGRAMS PER LITER AS P NITRATE CONCENTRATION, IN MILLIGRAMS PER LITER AS N

GROUNDWATER SETTING

Crystalline

Siliciclastic

Carbonate

All samples

SURFACE-WATER SETTING

0.00.10.20.30.40.50.60.70.80.9

FREQUENCY AT WHICH CONCENTRATION

WAS EXCEEDED

0.01

0.1

1

10

0.02

0.04

0.06

0.08

0.2

0.4

0.6

0.8

2

4

6

8

20

30

40

Crystalline

Siliciclastic

Carbonate

All samples

GROUNDWATER SETTING

0.00.10.20.30.40.50.60.70.80.9

FREQUENCY AT WHICH CONCENTRATION

WAS EXCEEDED

Crystalline

Siliciclastic

Carbonate

All samples

SURFACE-WATER SETTING

A.

B.

1.01.0
Contributions of Phosphorus from Groundwater to Streams 13

concentrations, which include concentrations of both dissolved 

and particulate phosphorus. In the Piedmont, the phosphorus 

criterion of 0.037 mg/L was exceeded at some of the streams 

in each network and at more than half of the streams in the 

PSP and PCL networks (fig. 6; see table 2 for identification 

and description of networks). Median dissolved phosphate 

concentrations in four carbonate and two siliciclastic synoptic stream networks exceeded the 0.01-mg/L criterion in the 

Valley and Ridge (fig. 6). Phosphorus concentrations during 

base flow may vary seasonally in streams and are often higher 

during the summer (fig. 7), when most synoptic samples were 

collected for this study.

Although the recommended ecoregion nutrient criteria 

apply specifically to streams, concentrations of dissolved 

phosphate greater than regional criteria in groundwater are 

noteworthy as a potential source of phosphorus to streams. 

Dissolved phosphate was consistently present in groundwater from the crystalline and siliciclastic aquifers, but seldom 

greater than 0.01 mg/L in carbonate aquifers (fig. 6). In the 

Piedmont, the median phosphate concentration in groundwater exceeded the criterion only in the PXS network (table 2); 

the lower Blue Ridge and Valley and Ridge criterion was 

exceeded by the median concentrations of all three Valley and 

Ridge siliciclastic networks and in the crystalline Blue Ridge 

network (fig. 6).

Although phosphate concentrations commonly differed 

between groundwater and surface water in a given bedrock 

setting or sampling network (figs. 5a and 6), there were similarities in the major-ion characteristics of the groundwater and 

surface water in each setting (fig. 8). The differences in majorion composition among the three major bedrock types resulted 

from variable contributions from predominant minerals, such 

as carbonates in the carbonate setting and silicates and sulfides 

in the other settings. Although the similarity in major-ion proportions for the groundwater and surface water of a particular setting indicates common water-rock interactions in that 

setting, additional processes such as dilution and evaporation 

(atmospheric exchange) plus contributions from anthropogenic 

sources affect the concentrations of dissolved ions and associated water-quality characteristics such as specific conductance, 

pH, and DO. An increase in the proportion of nitrate and 

chloride is apparent for waters impacted by human activities in 

all three settings (fig. 8). This increase was observed in surface 

water more commonly than in groundwater.

Figure 6. Concentrations of dissolved phosphate in groundwater and surface water by network for different bedrock 

types of the Piedmont, Blue Ridge, and Valley and Ridge study area.

 .001 

 .01 

 .1 

 1 

 10 

PHOSPHATE CONCENTRATION,

IN MILLIGRAMS PER LITER AS P

LABORATORY REPORTING LIMIT = 0.01

CRYSTALLINE BEDROCK SILICICLASTIC BEDROCK CARBONATE BEDROCK

EXPLANATION

25th percentile

U.S. Environmental Protection Agency Ecoregion IX criterion (0.037 milligrams per liter)

U.S. Environmental Protection Agency Ecoregion XI criterion (0.01 milligrams per liter)

Median

75th percentile

Data value less than or equal to 1.5 times the interquartile

range outside the quartile

Outlier data value less than or equal to 3 and more than

1.5 times the interquartile range outside the quartile

Outlier data value more than 3 times the interquartile range

outside the quartile

PXD_SW

PXL_GW

PXL_SW

PXP_GW

PXP_SW

PXB_GW

PXS_GW

PXS_SW

PXA_GW

PXA_SW

BXK_GW

BXK_SW

PSD_GW

PSD_SW

PSP_GW

PSP_SW

VSD_GW

VSD_SW

VSL_GW

VSL_SW

VSP_GW

VSP_SW

VST_SW

PCL_GW

PCL_SW

VCLA_GW

VCLA_SW

VCL_GW

VCL_SW

VCLU_GW

VCLU_SW

VCT_GW

VCT_SW
14 Contributions of Phosphorus from Groundwater to Streams, Eastern United States

Figure 7. Seasonal trends in base-flow concentrations of phosphorus in three streams draining watersheds 

of varying hydrogeologic and land-use characteristics.

0.01

0.001

0.001

0.01

0.01

0.1

0.1

0.1

1

1

0 180 360

DAY OF THE YEAR (1993–2005)

PHOSPHORUS OR ORTHOPHOSPHATE CONCENTRATION DURING 

LOW-FLOW CONDITIONS, IN MILLIGRAMS PER LITER

Detected

Detected

Not detected

Not detected

LITTLE NESHAMINY CREEK

BOBS CREEK

MUDDY CREEK

Mixed urban and agricultural 

watershed, Piedmont siliciclastic 

setting

Forested watershed, Valley and 

Ridge siliciclastic setting

Agricultural watershed, Valley and 

Ridge carbonate setting

Phosphorus

Orthophosphate

EXPLANATION
Contributions of Phosphorus from Groundwater to Streams 15

Figure 8. Major-ion composition of groundwater and stream base-flow samples from different bedrock settings of the Piedmont, Blue 

Ridge, and Valley and Ridge study area.

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Crystalline

bedrock 

(Groundwater)

Crystalline

bedrock

(Surface water)

Siliciclastic

bedrock

( Groundwater)

Siliciclastic

bedrock

(Surface water)

Carbonate 

bedrock

(Groundwater)

Carbonate

bedrock

(Surface water)

MAGNESIUM

PERCENT

PERCENT

PERCENT

SULFATE, CHLORIDE PLUS NITRATE

CALCIUM PLUS MAGNESIUM

SULFATE

CALCIUM CHLORIDE PLUS NITRATE

SODIUM PLUS POTASSIUM

CARBONATE PLUS BICARBONATE

MAGNESIUM

PERCENT

PERCENT

PERCENT

SULFATE, CHLORIDE PLUS NITRATE

CALCIUM PLUS MAGNESIUM

SULFATE

CALCIUM CHLORIDE PLUS NITRATE

SODIUM PLUS POTASSIUM

CARBONATE PLUS BICARBONATE

MAGNESIUM

PERCENT

PERCENT

PERCENT

SULFATE, CHLORIDE PLUS NITRATE

CALCIUM PLUS MAGNESIUM

SULFATE

CALCIUM CHLORIDE PLUS NITRATE

SODIUM PLUS POTASSIUM

CARBONATE PLUS BICARBONATE

MAGNESIUM

PERCENT

PERCENT

PERCENT

SULFATE, CHLORIDE PLUS NITRATE

CALCIUM PLUS MAGNESIUM

SULFATE

CALCIUM CHLORIDE PLUS NITRATE

SODIUM PLUS POTASSIUM

CARBONATE PLUS BICARBONATE

MAGNESIUM

PERCENT

PERCENT

PERCENT

SULFATE, CHLORIDE PLUS NITRATE

CALCIUM PLUS MAGNESIUM

SULFATE

CALCIUM CHLORIDE PLUS NITRATE

SODIUM PLUS POTASSIUM

CARBONATE PLUS BICARBONATE

MAGNESIUM

PERCENT

PERCENT

PERCENT

SULFATE, CHLORIDE PLUS NITRATE

CALCIUM PLUS MAGNESIUM

SULFATE

CALCIUM CHLORIDE PLUS NITRATE

SODIUM PLUS POTASSIUM

CARBONATE PLUS BICARBONATE

EXPLANATION

Concentration of chloride is 

greater than 10 milligrams per 

liter and (or) concentration of 

nitrate as N is greater than 

1.0 milligram per liter

Concentration of chloride is 

less than or equal to 10 milligrams 

per liter and concentration of 

nitrate as N is less than or equal 

to 1.0 milligram per liter

Rainwater
16 Contributions of Phosphorus from Groundwater to Streams, Eastern United States

Sources, Fate, and Transport of Phosphorus 

Identification of the source(s) of phosphorus in water 

samples is complicated by multiple sources and hydrological, geochemical, and biological processes affecting phosphorus fate and transport. Groundwater sampled in a given 

network may not represent all of the water being discharged 

to streams, for example. In areas of carbonate bedrock, dissolved phosphate exceeded the detection limit of 0.01 mg/L 

in 75 percent of the surface-water samples but was detected 

in only 25 percent of the groundwater samples (fig. 5a). The 

greater frequency of detection of phosphorus in surface water 

could result from in-stream biological cycling of nutrients, as 

well as shallow or near-stream groundwater sources that are 

not represented by groundwater sampling. Greater phosphorus concentrations in relatively shallow groundwater than in 

deeper groundwater have been reported in areas of carbonate (Galeone and others, 2006) and siliciclastic (Lindsey and 

others, 2003) aquifers in the study area. Although shallow 

groundwater sources could be an important component of the 

stream base flow in some settings, most of the groundwater 

data available for this analysis represent deeper sources used 

for domestic supplies. 

DO concentrations in groundwater generally were less 

than those in associated surface water in each of the three 

bedrock settings (fig. 9a). Generally, concentrations of DO 

were lower in groundwater from the crystalline and siliciclastic bedrock aquifers than in groundwater from the carbonate aquifer, implying groundwater was not completely 

isolated from the atmosphere in the carbonate setting or that 

DO persists because of a lack of available electron donors to 

support its reduction in the flow system. After groundwater 

enters the stream environment, atmospheric exchange results 

in the loss of CO2 and gain of O2 . The concentrations of 

these dissolved gases also can be influenced by photosynthesis by aquatic plants. As the concentration of dissolved CO2

decreases, the pH tends to increase. Consequently, the pH 

of surface water typically was 0.5 to 1 unit higher than that 

of the associated groundwater (fig. 9b). Such changes in DO 

and pH could affect phosphate mobility through processes 

involving dissolution-precipitation, oxidation-reduction, and 

adsorption-desorption reactions. 

Information on the concentrations of various chemical 

constituents associated with mineral dissolution or a particular 

land use can help to resolve the sources and causes of variability in phosphorus concentrations in associated samples. 

Differences in the median specific conductance (an indicator of ionic strength) and dissolved silica (an indicator of 

groundwater age) between groundwater and surface water 

from paired networks indicate that other (unsampled) groundwater is contributing to stream base flow. Samples from many 

stream networks have lower median specific conductance and 

median concentrations of silica than samples from the paired 

groundwater network, which could indicate the stream is also 

receiving a substantial portion of flow from groundwater that is 

younger and more dilute (fig. 10, table 3). Conversely, specific 

conductance that is substantially higher in stream base flow 

than in sampled groundwater could indicate greater amounts of 

anthropogenic chemicals in shallow (unsampled) groundwater 

than in deeper (sampled) groundwater sources and (or) loss 

of surface water to evapotranspiration. For processes such as 

dilution and evapotranspiration, the specific conductance will 

decrease or increase, respectively, but the proportions of major 

ions will remain constant. In contrast, mineral dissolution and 

precipitation processes can change both the specific conductance and the proportions of constituents involved in such 

reactions. 

The potential for human impacts on water chemistry 

also may be indicated by the presence of certain chemicals, in 

addition to phosphorus, that are associated with a particular 

land use and that are not commonly mobilized by mineral dissolution or desorption processes. Elevated concentrations of 

chloride and nitrate, in particular, can be indicators of human 

impacts. Both of these solutes are relatively mobile in groundwater and surface water, although nitrate may be reduced in 

areas with low DO. Elevated concentrations of chloride are 

associated with agricultural applications of “potash” (KCl) and 

manure and with discharges of sewage effluent, although the 

highest concentrations are associated with the application of 

road deicing salts such as sodium chloride (NaCl) and, more 

recently, calcium chloride (CaCl2) in the northern part of this 

region. Chloride concentrations greater than 10 mg/L are substantially greater than background levels in atmospheric precipitation. Nevertheless, concentrations greater than 10 mg/L 

are common in many areas (fig. 11a). 

Natural levels of nitrate in streams from rainfall and plant 

and animal sources are generally less than 1 mg/L in this region 

(Peters and Bonelli, 1982; Puckett, 1994; Holloway and others, 

1998). Elevated concentrations of nitrate are most commonly 

associated with agriculture and turf (lawns, golf courses) in 

a watershed, and also with discharges from septic systems or 

sewage-treatment plants. The recommended nutrient criteria for 

nitrate in the Piedmont (0.69 mg/L) and Blue Ridge and Valley 

and Ridge (0.31 mg/L) were commonly exceeded in streams 

and groundwater in this study (table 1, figs. 5b and 11b). 

Although most groundwater is well-oxygenated (92 percent of 

609 samples had DO concentrations greater than 0.5 mg/L), 

reducing conditions affect nitrate concentrations in groundwater in parts of this region, especially in groundwater in areas 

with siliciclastic rocks, where DO concentrations are frequently 

less than 0.5 mg/L (fig. 9a). Under such anoxic conditions, 

reduction of nitrate to gaseous nitrogen by bacterial denitrification can decrease nitrate concentrations to values that can not 

be detected (McMahon and Chapelle, 2008). 
Contributions of Phosphorus from Groundwater to Streams 17

Figure 9. Comparison of (A) dissolved oxygen and (B) pH of groundwater and surface water by network for different bedrock 

types of the Piedmont, Blue Ridge, and Valley and Ridge study area.

 0 

 2 

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 12 

 14 

 16 

DISSOLVED OXYGEN CONCENTRATION, 

IN MILLIGRAMS PER LITER

 4 

 4.5 

 5 

 5.5 

 6 

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 7 

 7.5 

 8 

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 9 

 9.5 

 10 

A.

B.

pH, IN STANDARD UNITS

CRYSTALLINE BEDROCK SILICICLASTIC BEDROCK CARBONATE BEDROCK

EXPLANATION

25th percentile

Median

75th percentile

Data value less than or equal to 1.5 times the interquartile

range outside the quartile

Outlier data value less than or equal to 3 and more than

1.5 times the interquartile range outside the quartile

Outlier data value more than 3 times the interquartile range

outside the quartile

PXD_SW

PXL_GW

PXL_SW

PXP_GW

PXP_SW

PXB_GW

PXS_GW

PXS_SW

PXA_GW

PXA_SW

BXK_GW

BXK_SW

PSD_GW

PSD_SW

PSP_GW

PSP_SW

VSD_GW

VSD_SW

VSL_GW

VSL_SW

VSP_GW

VSP_SW

VST_SW

PCL_GW

PCL_SW

VCLA_GW

VCLA_SW

VCL_GW

VCL_SW

VCLU_GW

VCLU_SW

VCT_GW

VCT_SW
18 Contributions of Phosphorus from Groundwater to Streams, Eastern United States

Figure 10. Comparison of (A) specific conductance, and (B) concentrations of dissolved silica in groundwater and surface water 

by network for different bedrock types of the Piedmont, Blue Ridge, and Valley and Ridge study area.

 1 

 10 

 100 

 1,000 

 10,000 

SPECIFIC CONDUCTANCE, IN 

MICROSIEMENS PER CENTIMETER 

AT 25 DEGREES CELSIUS

 1 

 10 

 100 

A.

B.

SILICA CONCENTRATION,

IN MILLIGRAMS PER LITER

CRYSTALLINE BEDROCK SILICICLASTIC BEDROCK CARBONATE BEDROCK

EXPLANATION

25th percentile

Median

75th percentile

Data value less than or equal to 1.5 times the interquartile

range outside the quartile

Outlier data value less than or equal to 3 and more than

1.5 times the interquartile range outside the quartile

Outlier data value more than 3 times the interquartile range

outside the quartile

PXD_SW

PXL_GW

PXL_SW

PXP_GW

PXP_SW

PXB_GW

PXS_GW

PXS_SW

PXA_GW

PXA_SW

BXK_GW

BXK_SW

PSD_GW

PSD_SW

PSP_GW

PSP_SW

VSD_GW

VSD_SW

VSL_GW

VSL_SW

VSP_GW

VSP_SW

VST_SW

PCL_GW

PCL_SW

VCLA_GW

VCLA_SW

VCL_GW

VCL_SW

VCLU_GW

VCLU_SW

VCT_GW

VCT_SW
Contributions of Phosphorus from Groundwater to Streams 19

Figure 11. Comparison of (A) chloride and (B) nitrate in groundwater and surface water by network for different bedrock 

types of the Piedmont, Blue Ridge, and Valley and Ridge study area. 

 .01 

 .1 

 1 

 10 

 100 

 .1 

 1 

 10 

 100 

 1,000 

CHLORIDE CONCENTRATION,

IN MILLIGRAMS PER LITER

A.

B.

NITRATE CONCENTRATION,

IN MILLIGRAMS PER LITER AS N

CRYSTALLINE BEDROCK SILICICLASTIC BEDROCK CARBONATE BEDROCK

EXPLANATION

25th percentile

Median

75th percentile

Data value less than or equal to 1.5 times the interquartile

range outside the quartile

Outlier data value less than or equal to 3 and more than

1.5 times the interquartile range outside the quartile

Outlier data value more than 3 times the interquartile range

outside the quartile

U.S. Environmental Protection Agency Ecoregion IX criterion (0.69 milligrams per liter)

U.S. Environmental Protection Agency Ecoregion XI criterion (0.31 milligrams per liter)

PXD_SW

PXL_GW

PXL_SW

PXP_GW

PXP_SW

PXB_GW

PXS_GW

PXS_SW

PXA_GW

PXA_SW

BXK_GW

BXK_SW

PSD_GW

PSD_SW

PSP_GW

PSP_SW

VSD_GW

VSD_SW

VSL_GW

VSL_SW

VSP_GW

VSP_SW

VST_SW

PCL_GW

PCL_SW

VCLA_GW

VCLA_SW

VCL_GW

VCL_SW

VCLU_GW

VCLU_SW

VCT_GW

VCT_SW
20 Contributions of Phosphorus from Groundwater to Streams, Eastern United States

Phosphorus from Natural Sources

To identify possible natural sources of phosphorus 

in groundwater, it is necessary to examine waters that are 

minimally impacted by human activities. Natural or nearly 

natural background conditions (NO3-N less than 1 mg/L 

where the concentration of DO is greater than 0.5 mg/L and 

(or) that of Cl is less than 10 mg/L) were indicated for 62 of 

246 (26.4 percent) groundwater samples from the crystalline bedrock setting, 24 of 154 (15.6 percent) samples from 

the siliciclastic bedrock setting, and 26 of 209 (12.4 percent) 

samples from the carbonate bedrock setting. For the natural groundwaters, the concentration of dissolved phosphate 

ranged from less than 0.01 to 0.285 mg/L, with medians of 

0.029, 0.018, and less than 0.01 mg/L for the crystalline, siliciclastic, and carbonate bedrock settings, respectively. For the 

surface-water samples, natural conditions were indicated for 

17 of 136 (12.5 percent) samples from the crystalline bedrock 

setting, 35 of 98 (35.7 percent) samples from the siliciclastic 

bedrock setting, and 7 of 124 (5.6 percent) samples from the 

carbonate bedrock setting. For these natural surface waters, the 

concentration of dissolved phosphate ranged from less than 

0.01 to 0.074 mg/L, with medians of 0.01 mg/L for the carbonate bedrock setting and less than 0.01 mg/L for the crystalline 

and siliciclastic bedrock settings.

Natural or nearly natural water chemistry was indicated 

in both groundwater and surface water from two of the paired 

networks in the crystalline region (BXK and PXS; table 2) and 

two in the siliciclastic region (VSP and VSL) (table 3, fig 11). 

In addition, data from one carbonate network, VCT, represents water with concentrations of nitrate and chloride that 

were only slightly greater than the presumed natural levels. 

Elevated concentrations of dissolved phosphate in groundwater from these networks would be primarily associated with 

natural mineral sources, and could potentially be sources of 

phosphorus to surface water. In three of the networks, feldspar dissolution as a source of phosphorus could be indicated by dissolved phosphate concentrations in groundwater 

that are positively correlated with concentrations of silica 

(PXS: tau=0.53, p<0.0001; BXK: tau=0.34, p=0.0118; VSL: 

tau=0.31, p=0.0347). In the two Valley and Ridge siliciclastic 

networks, VSP and VSL, reducing conditions (median DO 

concentration less than 0.5 mg/L) are exhibited in many of 

the groundwater samples and are associated with elevated 

concentrations of phosphorus (fig. 9a). In addition, in the VSP 

network, elevated concentrations of dissolved iron (median 

440 micrograms per liter) and negligible concentrations of 

dissolved nitrate indicate strongly reducing conditions. Concentrations of phosphorus in groundwater from the carbonate 

network with nearly natural conditions, VCT, were very low, 

as they were in all of the carbonate networks. 

Median concentrations of dissolved phosphate were 

generally higher in groundwater than in surface water in three 

of the crystalline and siliciclastic networks with inferred 

natural water chemistry, and were similar in the groundwater and surface water of the BXK network (table 3, fig. 6). 

Median concentrations of dissolved phosphate in groundwater 

exceeded the 0.037-mg/L criterion for total phosphorus in 

Piedmont streams (Ecoregion IX) in the PXS network and 

the 0.01-mg/L criterion for Blue Ridge and Valley and Ridge 

streams (Ecoregion XI) in the BXK, VSP, and VSL networks. 

The median dissolved phosphate concentrations were less than 

Table 3. Median concentrations of dissolved phosphate and associated water-quality constituents in groundwater and surface water 

from selected networks with natural or nearly natural water quality, or with water quality impacted by human activities in the Piedmont, 

Blue Ridge, and Valley and Ridge study area. 

[Location of networks shown in figure 3; networks described in table 2; medians are shown as groundwater / surface water; mg/L, milligrams per liter; μS/cm, 

microsiemens per centimeter at 25 degrees Celsius; <, less than]

Constituent Phosphate 

(PO4-P, mg/L)

Nitrate 

(NO3-N, mg/L)

Chloride 

(Cl, mg/L)

Silica 

(SiO2, mg/L)

Specific conductance

(SC, μS/cm)

Networks with Natural Water Quality

BXK 0.013 / 0.012 0.42 / 0.49 2.1 / 2.7 19.3 / 12.5 110 / 63

PXS 0.055 / <0.01 0.70 / 0.29 2.3 / 2.9 30.8 / 11.0 198 / 51

VSP 0.02 / <0.01 <0.05 / 0.42 3.2 / 2.7 18 / 7.7 340 / 122

VSL 0.03 / <0.01 0.13 / 0.28 2.3 / 1.5 18 / 5.7 165 / 77

VCT <0.01 / 0.16 1.04 / 1.27 2.4 / 5.08 8.5 / 6.8 370 / 375

Networks with Human Impacts on Water Quality

PXL <0.01 / <0.01 6.6 / 3.8 8.6 / 12 12 / 5.4 148 / 153

PSD 0.033 / 0.027 2.5 / 1.4 15.5 / 25.8 21.2 / 11.5 401 / 305

PCL <0.01 / 0.07 11 / 8.2 18.5 / 23 9.4 / 7.4 654 / 626
Contributions of Phosphorus from Groundwater to Streams 21

0.01 mg/L in the associated Valley and Ridge stream networks. 

In the VCT network, phosphorus concentrations were greater 

in surface water than in groundwater; concentrations of nitrate 

were slightly greater than 1 mg/L, and chloride concentrations 

were elevated in surface water relative to groundwater, indicating human impacts on groundwater from shallower sources 

that were not sampled were potentially affecting phosphorus 

concentrations in surface water.

With the exception of specific conductance in the VCT 

network, specific conductance and concentrations of silica 

were lower in surface water than in groundwater from the 

networks representing predominantly natural conditions 

(table 3, fig. 10). Dilution with younger (unsampled) groundwater with a lower concentration of phosphorus or biotic

uptake may produce the lower concentration of phosphorus in 

stream water. 

Phosphorus can be added to that contributed from 

groundwater by the microbial decomposition of terrestrial 

plant material that enters most streams. Such in-stream 

production of phosphorus from leaves and associated organic 

matter is indicated by concentrations of phosphate during base 

flow that are commonly highest during warmer months with 

longer days when the growth and decay of algae and other 

aquatic plants are likely to be fueling in-stream cycling of 

phosphorus (fig. 7). In addition, the mineralization of phosphorous sequestered in streambank and streambed sediments 

is generally greater in warmer than colder months (Surridge 

and others, 2007). This seasonal biological cycling of phosphorus can enrich phosphate in streams relative to the amount 

delivered by groundwater or overland sources (Manny and 

Wetzel, 1973; Kaplan and Bott, 1982; Carlton and Wetzel, 

1988). As the majority of base-flow synoptic samples for this 

study were collected during the growing season, in-stream 

biological cycling of nutrients is a potential factor affecting 

observed phosphorus concentrations in surface water.

Phosphorus from Human Sources

Elevated dissolved phosphate concentrations in streams 

are commonly associated with human sources. In most 

networks, for example, elevated phosphorus is associated 

with elevated concentrations of nitrate and agricultural land 

use (tables 2 and 3, fig. 11b). An anthropogenic groundwater 

source for phosphorus is indicated in PSD by similar concentrations of dissolved phosphate in groundwater and surface 

water and a positive correlation between dissolved phosphate 

and nitrate in groundwater and surface water (groundwater: 

tau = .39, p = <.0001; streams: tau = .43, p = .0029). In PCL, 

where agricultural land use is dominant and concentrations 

of both chloride and nitrate are elevated in groundwater and 

surface water, the median dissolved phosphate concentration 

was less than the reporting limit in groundwater and higher in 

surface water, as is commonly seen in other paired networks 

in a carbonate setting (table 3, figs. 6 and 11). The elevated 

concentrations in streams could indicate a shallow groundwater source for phosphorus associated with anthropogenic 

sources, or biological cycling of phosphorus transported in 

overland runoff or sequestered in near-stream sediments. 

Concentrations of dissolved phosphate are commonly low in 

surface water from the crystalline networks even where nitrate 

concentrations in groundwater and surface water are elevated, 

as, for example, in the PXL network, where the median dissolved phosphate concentrations were below 0.01 mg/L in 

both groundwater and surface water (table 3, figs. 6 and 11b). 

Fate and Transport of Phosphorus 

Examination of overall water chemistry and land use 

indicates that concentrations of dissolved phosphate sometimes may be attributed to mineral dissolution in groundwater, 

particularly in siliciclastic and crystalline bedrock settings, 

and to anthropogenic inputs in surface water, particularly in 

carbonate bedrock settings. Considering all 609 groundwater 

samples from the crystalline (N = 246), siliciclastic (N = 154), 

and carbonate (N = 209) aquifers as a single dataset, Spearman’s rank correlation coefficients (rho) generally indicated 

that the concentration of dissolved phosphate was correlated 

(p <0.001) positively with temperature and concentrations of 

silica and sodium, and negatively with alkalinity and concentrations of calcium, magnesium, chloride, nitrate, sulfate, iron, 

and aluminum (table 4). Additionally, the phosphate concentration in groundwater was positively correlated with the 

percentages of wetland and forested land uses and negatively 

correlated with the percentage of agricultural land use in the 

vicinity of sampled wells. In contrast, the phosphate concentration in the 358 surface-water samples was not correlated 

with the concentration of silica, but was positively correlated 

with concentrations of all other major cations and anions, 

including chloride and nitrate, which could indicate anthropogenic sources and effects of evapotranspiration on streamwater quality; and with concentrations of calcium, magnesium, 

and bicarbonate, which could indicate dissolution of carbonate 

minerals (table 4). Phosphate concentration in surface water 

was positively correlated with the percentage of agricultural 

and urban land uses, negatively correlated with the percentage 

of forested land use, and not correlated with the percentage of 

wetland land use in the watershed. 

Geochemical models were developed to evaluate mineral 

weathering and other hydrochemical processes that could control concentrations of phosphorus in groundwater and surface 

water. Background, or natural, quality for groundwater was 

estimated using the medians for samples that contained measurable DO (greater than 0.5 mg/L) and relatively low concentrations of nitrate (less than 1.0 mg/L) and chloride (less 

than 10 mg/L) and literature values for rainfall composition 

(Peters and Bonelli, 1982; Hem, 1985) (table 5). Although DO 

is reduced in some groundwater, it was greater than 0.5 mg/L 

in 92 percent of the samples and assumed to be present in all 

initial recharge; for these reasons, it was used in the definition 

of natural groundwater. The medians for paired groundwater 

and surface-water-quality data that had been identified as 

natural or affected by human influences (tables 3 and 5) were 
22 Contributions of Phosphorus from Groundwater to Streams, Eastern United States

Additionally, the groundwater and surface-water samples 

from all settings generally were undersaturated with respect to 

primary aluminosilicate minerals such as feldspars (anorthite, 

albite, adularia), amphiboles and pyroxenes (diopside), and 

micas (chlorite, illite), but were saturated or supersaturated 

with associated secondary minerals (kaolinite, gibbsite, quartz) 

(table 6). Thus, negative SI values indicate that dissolution 

of alkali feldspar, hydroxyapatite, and (or) fluorapatite could 

provide a source of phosphate, whereas positive SI values 

indicate that precipitation of various other phosphate minerals 

(carbonate-fluorapatite, strengite, variscite) could impose an 

upper limit on phosphate concentrations. Additionally, positive SI values for calcite and (or) oxides of iron, aluminum, 

and manganese indicate that these minerals could be feasible 

sorbents of phosphate. 

Inverse models were developed with PHREEQC to 

evaluate mass transfer of possible sources and sinks of 

phosphate and other solutes in the groundwater and surface 

water. In addition to specific minerals, oxygen, carbon dioxide, and organic matter (C106H263O110N16P) were included as 

possible reactants for each bedrock setting (table 7). Instead 

of the general formula CH2O, the formula for organic matter with a molar C:N:P ratio of 106:16:1 (Redfield, 1934; 

Drever, 1997) was used to simulate nutrient release by decay 

of vegetation (reactant) or uptake by photosynthesis (product). Furthermore, phosphorus equivalent to 0.3 percent P2

O5

(Manning, 2008) was included in the formulas for Na-feldspar 

(NaAl1.011Si2.978P0.011O8) and K-feldspar (NaAl1.011Si2.978P0.011O8). 

The first set of inverse models simulated reactions of 

rainfall (concentrated by a factor of 3 to represent evaporation 

prior to recharge) with a generalized mineral assemblage that 

could produce the background water quality for each of the 

three bedrock types (table 7). To produce background groundwater for the crystalline bedrock setting, the principal reactants 

were diopside, anorthite, Na-feldspar, K-feldspar, biotite, and 

chlorite, plus fluorapatite, fluorite, pyrite, and halite; carbonate 

minerals were excluded. In contrast, for the carbonate bedrock 

setting, calcite, dolomite, mangano-siderite, and gypsum, plus 

fluorite and illite, were principal reactants. For the siliciclastic 

setting, a combination of these mineral reactants with intermediate mass-transfer values was indicated. 

One or more minerals were indicated as a possible sink 

for phosphorus, which could be released in excess of the 

groundwater concentration by various reactants. For siliciclastic and carbonate groundwaters, carbonate-fluorapatite 

(FCO3Apatite) was indicated as a possible product, whereas 

for crystalline groundwater, strengite and variscite were 

indicated as possible products. Although strengite and variscite commonly were undersaturated (table 6, fig. 14), these 

phases were used as proxies for adsorption of phosphate by 

hydrous ferric and aluminum oxides. Other phases that were 

plausible products in the background groundwater-quality 

models include ferrihydrite (Fe(OH)3(a)), gibbsite, and 

quartz (table 7). 

The second set of inverse models simulated reactions 

with groundwater, minerals, and various anthropogenic 

Table 4. Spearman’s rank correlation coefficients for dissolved 

phosphate, major chemical constituents, and land use for 609 

groundwater samples and 358 surface-water samples from the 

Piedmont, Blue Ridge, and Valley and Ridge study area.

[Coefficient values multiplied by 100 and rounded; values shown are 

significant at p<0.0001; “--” indicates correlation was insignificant; 

n.a., not applicable] 

Constituent or Property Groundwater Surface Water

Specific Conductance -32 44

Dissolved Oxygen -- --

pH -- 23

Dissolved Organic Carbon -16 --

Alkalinity -29 31

Calcium -30 30

Magnesium -30 38

Sodium 18 43

Potassium -14 53

Iron -- --

Manganese -- --

Aluminum -24 n.a.

Silica 53 --

Chloride -17 45

Sulfate -23 41

Fluoride -- 21

Bromide -15 --

Nitrate -18 42

Ammonia 27

Land-Use Percentage Groundwater Surface Water

Wetland 21 --

Forested 29 -33

Urban -- 20

Agricultural -22 32

evaluated to indicate varying effects of natural processes and 

anthropogenic contaminants on phosphorus fate and transport. 

The SI values for major and accessory minerals, computed for individual samples (figs. 12–14) and using the 

median values for background and human-influenced water 

quality (table 6), were interpreted to indicate thermodynamically feasible mineral reactants and products in mass-transfer 

models. Groundwaters and surface waters from the crystalline 

setting typically were undersaturated with apatite (hydroxyapatite, fluorapatite, carbonate-fluorapatite) and other phosphate minerals (variscite, strengite, vivianite, MnHPO4

), 

whereas the waters from the carbonate and siliciclastic settings 

frequently were supersaturated with respect to carbonatefluorapatite, strengite, and (or) MnHPO4

 (table 6; figs. 12–14). 
Contributions of Phosphorus from Groundwater to Streams 23

Table 5. Compositions of rainwater and background groundwater from crystalline (CRY), siliciclastic (SIL), and carbonate (CAR) aquifers plus paired groundwater and surface water from selected monitoring networks in these settings. [Units are milligrams per liter unless otherwise indicated; italics indicate estimated concentrations for constituent medians that were less than laboratory reporting limits; solution name with suffix “_GW” is median groundwater and “_SW” is median surface water; paired groundwater-to-surface-water models involve monitoring networks identified in tables 2 and 3, row shading indicates network rock type: gray indicates crystalline, blue indicates carbonate, no shading indicates siliciclastic; °C, degrees Celsius] 

Solution 

Identification

Temperature, °C

pH

Oxygen

Alkalinity (CaCO

3

)

Calcium (Ca)

Magnesium (Mg)

Sodium (Na)

Potassium (K)

Silicon (Si)

Sulfate (SO

4

)

Chloride (Cl)

Fluoride (F)

Nitrate (NO

3

-N)

Phosphate (PO

4

-P)

Iron (Fe)

Manganese (Mn)

Aluminum (Al)

Rainwater to “background” groundwater models

RAINWATER 15.0 4.9 10.0 0.1 0.03 0.02 0.3 0.1 0.016 0.6 0.5 0.014 0.05 0.005 0.0001 0.0001 0.0001CRY_GW 16.1 6.2 6.6 33.5 7.0 2.0 5.7 1.1 11.8 0.8 1.8 0.05 0.22 0.029 0.003 0.002 0.002SIL_GW 12.4 7.4 6.2 57.6 19.5 4.7 4.1 0.6 5.0 5.3 1.7 0.05 0.21 0.018 0.003 0.001 0.001CAR_GW 14.3 7.0 6.9 156.0 34.9 14.1 0.9 0.9 3.8 2.4 1.4 0.05 0.36 0.005 0.003 0.001 0.001

Paired groundwater to surface-water models

BXK_GW 13.0 6.3 5.4 34.3 10.2 1.5 4.3 1.5 9.0 3.4 2.1 0.05 0.42 0.013 0.005 0.001 0.003PXS_GW 19.0 6.0 7.0 31.0 7.0 2.3 6.8 1.5 14.4 0.7 2.3 0.05 0.70 0.055 0.005 0.002 0.001VSP_GW 13.5 7.3 0.6 125.0 29.5 12.5 8.7 0.9 8.4 25.0 3.2 0.05 0.05 0.015 0.395 0.115 0.001VSL_GW 12.3 7.2 0.6 59.4 16.0 5.6 6.3 0.6 8.4 8.0 2.3 0.10 0.13 0.030 0.018 0.046 0.001PSD_GW 13.4 7.5 4.4 135.0 45.8 13.8 11.6 1.0 9.1 19.9 15.2 0.05 2.5 0.032 0.005 0.001 0.001VCT_GW 14.4 7.1 6.3 172.0 46.4 17.3 1.2 1.3 4.0 3.9 2.4 0.05 1.04 0.005 0.001 0.001 0.001PCL_GW 13.1 7.3 6.0 210.5 74.5 24.5 5.7 2.0 4.4 37.5 18.5 0.05 11.0 0.005 0.003 0.001 0.001BXK_SW 13.5 7.2 10.8 16.1 4.1 1.6 3.7 1.5 5.8 4.9 2.7 0.05 0.49 0.012 0.068 0.021 0.001PXS_SW 16.5 6.4 8.6 13.0 3.1 1.6 2.5 2.5 5.1 1.6 2.8 0.05 0.29 0.005 0.116 0.016 0.001VSP_SW 19.8 7.7 8.2 36.0 14.0 3.5 2.5 1.9 3.6 10.2 2.7 0.05 0.43 0.005 0.029 0.004 0.001VSLF_SW 22.3 7.2 8.2 23.0 8.3 2.5 1.5 1.0 2.7 6.4 1.5 0.05 0.28 0.010 0.062 0.009 0.001PSD_SW 19.8 7.7 9.9 63.4 25.4 9.1 15.5 2.2 5.4 24.0 25.8 0.05 1.35 0.027 0.028 0.020 0.001VCT_SW 20.7 8.3 9.3 160.0 54.4 16.7 2.5 2.3 3.2 6.6 5.1 0.14 1.3 0.016 0.003 0.009 0.001PCL_SW 20.0 8.1 8.9 174.0 59.0 14.0 9.6 4.0 3.5 28.0 23.0 0.10 8.2 0.070 0.009 0.022 0.001
24 Contributions of Phosphorus from Groundwater to Streams, Eastern United States

Figure 12. Saturation indices (SI) for calcite, dolomite, and fluorite in groundwater and surface water by network for 

different bedrock types of the Piedmont, Blue Ridge, and Valley and Ridge study area. SI, FLUORITE 

 -7 

 -6 

 -5 

 -4 

 -3 

 -2 

 -1 

 0 

SI, CALCITE 

 -7 

 -6 

 -5 

 -4 

 -3 

 -2 

 -1 

 0 

 1 

SI, DOLOMITE 

 -14 

 -12 

 -10 

 -8 

 -6 

 -4 

 -2 

 0 

 2 

EXPLANATION

25th percentile

Median

75th percentile

Data value less than or equal to 1.5 times the interquartile

range outside the quartile

Outlier data value less than or equal to 3 and more than

1.5 times the interquartile range outside the quartile

Outlier data value more than 3 times the interquartile range

outside the quartile

CRYSTALLINE BEDROCK SILICICLASTIC BEDROCK CARBONATE BEDROCK

A.

B.

C.

PXD_SW

PXL_GW

PXL_SW

PXP_GW

PXP_SW

PXB_GW

PXS_GW

PXS_SW

PXA_GW

PXA_SW

BXK_GW

BXK_SW

PSD_GW

PSD_SW

PSP_GW

PSP_SW

VSD_GW

VSD_SW

VSL_GW

VSL_SW

VSP_GW

VSP_SW

VST_SW

PCL_GW

PCL_SW

VCLA_GW

VCLA_SW

VCL_GW

VCL_SW

VCLU_GW

VCLU_SW

VCT_GW

VCT_SW
Contributions of Phosphorus from Groundwater to Streams 25

Figure 13. Saturation indices (SI) for apatite in groundwater and surface water by network for different bedrock types of 

the Piedmont, Blue Ridge, and Valley and Ridge study area.

 -48 

 -40 

 -32 

 -24 

 -16 

 -8 

 0 

 8 

 16 

 -32 

 -28 

 -24 

 -20 

 -16 

 -12 

 -8 

 -4 

 0 

 4 

SI, FLUORAPATITE

 -36 

 -30 

 -24 

 -18 

 -12 

 -6 

 0 

 6 

SI, HYDROXYAPATITE SI, CARBONATE-FLUORAPATITE

CRYSTALLINE BEDROCK SILICICLASTIC BEDROCK CARBONATE BEDROCK

EXPLANATION

25th percentile

Median

75th percentile

Data value less than or equal to 1.5 times the interquartile

range outside the quartile

Outlier data value less than or equal to 3 and more than

1.5 times the interquartile range outside the quartile

Outlier data value more than 3 times the interquartile range

outside the quartile

A.

B.

C.

PXD_SW

PXL_GW

PXL_SW

PXP_GW

PXP_SW

PXB_GW

PXS_GW

PXS_SW

PXA_GW

PXA_SW

BXK_GW

BXK_SW

PSD_GW

PSD_SW

PSP_GW

PSP_SW

VSD_GW

VSD_SW

VSL_GW

VSL_SW

VSP_GW

VSP_SW

VST_SW

PCL_GW

PCL_SW

VCLA_GW

VCLA_SW

VCL_GW

VCL_SW

VCLU_GW

VCLU_SW

VCT_GW

VCT_SW
26 Contributions of Phosphorus from Groundwater to Streams, Eastern United States

Figure 14. Saturation indices (SI) for ferrihydrite, strengite, and vivianite in groundwater and surface water by network 

for different bedrock types of the Piedmont, Blue Ridge, and Valley and Ridge study area.

 -20 

 -16 

 -12 

 -8 

 -4 

 0 

 -5 

 -4 

 -3 

 -2 

 -1 

 0 

 1 

 2 

 3 

SI, FERRIHYDRITE

 -6 

 -5 

 -4 

 -3 

 -2 

 -1 

 0 

 1 

SI, STRENGITE SI, VIVIANITE

EXPLANATION

25th percentile

Median

75th percentile

Data value less than or equal to 1.5 times the interquartile

range outside the quartile

Outlier data value less than or equal to 3 and more than

1.5 times the interquartile range outside the quartile

Outlier data value more than 3 times the interquartile range

outside the quartile

CRYSTALLINE BEDROCK SILICICLASTIC BEDROCK CARBONATE BEDROCK

A.

B.

C.

PXD_SW

PXL_GW

PXL_SW

PXP_GW

PXP_SW

PXB_GW

PXS_GW

PXS_SW

PXA_GW

PXA_SW

BXK_GW

BXK_SW

PSD_GW

PSD_SW

PSP_GW

PSP_SW

VSD_GW

VSD_SW

VSL_GW

VSL_SW

VSP_GW

VSP_SW

VST_SW

PCL_GW

PCL_SW

VCLA_GW

VCLA_SW

VCL_GW

VCL_SW

VCLU_GW

VCLU_SW

VCT_GW

VCT_SW
Contributions of Phosphorus from Groundwater to Streams 27

Table 6. Saturation index (SI) values computed with PHREEQC2 for compositions of solutions used in inverse mass-transfer models.[Negative value indicates undersaturation, positive value (red) indicates saturated; solution name with suffix “_GW” is median groundwater and “_SW” is median surface water; input solution compositions indicated in table 5; formula for phases with “*” in parentheses indicated in table 7; yellow shading indicates phase contains phosphorus; row shading in paired groundwater to surface-water models indicates network rock type: gray indicates crystalline, blue indicates carbonate, no shading indicates siliciclastic] 

Solution

Identification

Adularia (KAlSi3O8)

Albite (NaAlSi3O8)

Anorthite (*)

Calcite (*)

Chlorite14A (*)

Diopside (*)

Dolomite (*)

FCO3Apatite (*)

Fe(OH)3(a)

Fluorapatite (*)

Fluorite (*)

Gibbsite (*)

Gypsum (*)

Halite (*)

Hydroxyapatite (Ca5(PO4)

3

OH)

Illite (*)

Kaolinite (Al2Si2O5(OH)4)

MnHPO4

Pyrolusite (*)

Quartz (*)

Siderite (FeCO3)

Strengite (*)

Variscite (*)

Vivianite (Fe3(PO4)

2

:8H2O)

Rainwater to “background” groundwater models

RAIN -17.6 -19.6 -26.5 -8.1 -63.0 -26.5 -16.1 -52.1 -2.7 -32.4 -7.7 -3.5 -7.6 -11.3 -36.0 -23.4 -11.0 -4.9 1.0 -2.4 -15.3 -2.8 -4.7 -44.8RAIN3X -11.6 -13.6 -16.9 -6.8 -42.2 -20.2 -13.6 -32.6 -1.1 -21.6 -6.3 -0.5 -6.7 -10.4 -24.6 -13.3 -4.1 -2.9 3.6 -2.0 -15.2 -1.9 -2.3 -41.3CRY_GW -2.6 -4.4 -7.8 -2.9 -23.8 -11.4 -6.0 -9.7 -0.0 -10.7 -4.3 0.6 -4.4 -9.5 -13.4 -1.6 2.8 -1.8 4.6 0.4 -12.2 -0.7 -1.1 -37.5SIL_GW -3.0 -4.7 -6.4 -1.0 -12.2 -6.7 -2.5 5.0 1.0 -3.2 -3.9 0.3 -3.2 -9.7 -4.9 -2.2 1.6 -1.2 7.1 0.1 -12.5 -1.6 -3.4 -39.1CAR_GW -3.3 -5.8 -6.6 -0.7 -13.2 -7.7 -1.6 1.2 0.5 -5.5 -3.7 0.6 -3.4 -10.4 -7.5 -2.3 1.8 -2.4 6.0 -0.1 -12.0 -2.1 -3.1 -40.1

Paired groundwater to surface-water models

BXK_GW -2.3 -4.4 -7.2 -2.6 -23.7 -11.4 -5.9 -8.6 0.5 -10.4 -4.1 0.9 -3.6 -9.6 -13.2 -1.0 3.3 -2.3 4.5 0.3 -12.0 -0.7 -1.3 -37.7PXS_GW -3.0 -4.8 -8.7 -3.0 -24.9 -11.5 -6.2 -10.0 -0.1 -10.5 -4.4 0.1 -4.5 -9.3 -13.3 -2.6 1.9 -1.6 4.3 0.5 -11.9 -0.2 -1.2 -36.2VSP_GW -2.2 -3.7 -5.9 -0.6 -10.3 -6.1 -1.5 5.3 3.0 -3.3 -3.8 0.4 -2.4 -9.1 -5.1 -1.3 2.1 0.3 8.1 0.3 -9.7 0.4 -3.3 -32.0VSL_GW -2.3 -3.8 -6.1 -1.3 -12.8 -7.1 -2.9 4.6 1.7 -3.5 -3.3 0.5 -3.1 -9.4 -5.7 -1.1 2.4 0.3 7.6 0.3 -11.3 -0.4 -2.6 -35.1PSD_GW -2.0 -3.4 -5.6 -0.2 -8.3 -4.9 -0.8 10.0 1.2 -0.8 -3.6 0.2 -2.4 -8.3 -2.3 -1.2 1.9 -1.5 6.8 0.4 -11.9 -1.4 -3.5 -38.2VCT_GW -3.2 -5.7 -6.4 -0.5 -12.3 -7.2 -1.3 2.6 0.3 -4.8 -3.6 0.5 -3.1 -10.1 -6.8 -2.2 1.7 -2.4 6.0 -0.0 -12.3 -2.5 -3.3 -41.1PCL_GW -2.7 -4.8 -6.1 -0.1 -10.0 -6.1 -0.5 6.1 0.9 -3.3 -3.4 0.4 -2.0 -8.5 -5.1 -1.9 1.6 -2.4 6.4 0.0 -11.9 -2.2 -3.9 -40.3BXK_SW -2.5 -4.6 -7.0 -2.4 -15.4 -8.3 -4.9 -3.7 2.2 -7.5 -4.5 0.4 -3.8 -9.5 -9.3 -1.7 1.9 -0.3 7.9 0.1 -11.6 -0.2 -3.1 -35.0PXS_SW -3.1 -5.6 -8.2 -3.3 -22.5 -11.5 -6.7 -13.0 1.8 -12.2 -4.6 0.6 -4.4 -9.7 -14.8 -2.3 2.2 -1.0 6.1 0.0 -11.0 0.4 -1.8 -33.6VSP_SW -3.5 -5.8 -7.1 -1.0 -10.0 -5.7 -2.3 2.3 1.6 -4.2 -4.1 -0.3 -3.0 -9.7 -5.3 -3.7 -0.1 -1.4 7.9 -0.2 -11.8 -1.9 -4.9 -37.8VSL_SW -4.4 -6.6 -7.6 -1.9 -14.6 -8.0 -3.9 -2.3 1.7 -6.1 -4.3 0.1 -3.4 -10.2 -7.7 -4.2 0.3 -0.9 7.3 -0.3 -11.2 -0.6 -3.3 -34.8PSD_SW -2.9 -4.5 -6.5 -0.5 -7.2 -4.5 -1.1 8.4 1.6 -1.0 -3.9 -0.4 -2.5 -7.9 -2.1 -3.1 0.1 -0.1 8.7 0.0 -11.6 -1.3 -4.4 -36.7VCT_SW -3.7 -6.0 -6.7 0.8 -1.8 -2.0 1.4 14.4 0.6 1.8 -2.7 -1.0 -2.8 -9.5 0.9 -4.7 -1.7 -1.3 9.0 -0.2 -12.8 -3.8 -6.5 -42.6PCL_SW -3.3 -5.3 -6.6 0.7 -3.8 -2.8 1.0 16.6 1.1 3.1 -3.0 -0.8 -2.2 -8.2 2.2 -4.1 -1.1 -0.0 9.2 -0.2 -12.0 -2.3 -5.2 -39.0

2Parkhurst and Appelo, 1999
28 Contributions of Phosphorus from Groundwater to Streams, Eastern United States Table 7. Mass-transfer estimates (minimum/maximum) to obtain median background groundwater quality (final solution) by reactions with evaporated rainfall (initial solution) in crystalline, siliciclastic, and carbonate bedrock settings. [PHREEQCa used to compute mass transfer given input-solution compositions in table 5 and saturation indices in table 6; values are in millimoles; positive value indicates phase added to solution; negative value (red) indicates phase removed from solution; “.” indicates phase excluded from model; yellow shading indicates phase contains phosphorus; CRY, crystalline, SIL, siliciclastic; CAR, carbonate] Phase name in PHREEQC Composition used in Mass-Transfer Models CRY50_GW SIL50_GW CAR50_GW Diopside CaMgSi

O2 6 0 / 0.079 . .

Anorthite CaAl2Si

O2 8 0.082 / 0.170 . .

Na-Feldspar NaAl1.011Si2.978

P0.011

O8 0.188 / 0.200 0.139 / 0.139 .

K-Feldspar KAl1.011Si2.978

P0.011

O8 0 / 0.022 0 / 0.008 .

Biotite KMg AlSi

3 O3 10(OH)2 0 / 0.022 . .

Chlorite14A Mg Al

5 2Si

O3 10(OH)8 0 / 0.016 0 / 0.035 .

Illite

K0.6Mg0.25Al2.3Si3.5

O10(OH)2 . 0 / 0.014 0.028 / 0.029

Calcite CaCO3 . 0.265 / 0.453 0.318 / 0.352Dolomite CaMg(CO3

)2 . 0 / 0.174 0.573 / 0.573

Mn-Siderite Fe0.95Mn0.05CO3 . 0 / 0.932 0 / 2.163Fluorite CaF2 . 0.001 / 0.003 0 / 0.005Gypsum CaSO4.2

H

O2 . . 0.022 / 0.022

Pyrite FeS2 0.003 / 0.003 0.026 / 0.026 .Pyrolusite MnO2 . -0.047 / 0 -0.108 / 0Quartz SiO2 -0.840 / -0.767 -0.484 / -0.356 -0.038 / -0.033Fluorapatite Ca5(PO4

)3

F 0 / 0.0004 . .

FCO3Apatite Ca9.316Na0.36Mg0.144(PO4

)4.8(CO3

)1.2F2.48 . -0.0021 / 0 -0.0043 / -0.0001

Fe(OH)3(a) Fe(OH)3 -0.003 / 0 -0.911 / -0.026 -2.055 / 0Gibbsite Al(OH)3 -0.584 / -0.385 -0.241 / -0.149 -0.066 / -0.063Strengite FePO4.2

H

O2 -0.003 / 0 . .

Variscite AlPO4.2

H

O2 -0.004 / 0 . .

Halite NaCl 0.009 / 0.009 . .Sylvite KCl . . .NH4H2PO4 NH

H4 2PO4 . . .

Urea NH CONH 2 2 . . .Organic matter

C106

H263

O110

N16P 0.001 / 0.001 0 / 0.206 0 / 0.455

CO2(g) CO2 1.722 / 1.754 0 / 0.932 0 / 2.060O2(g)

O2 . 0 / 0.984 0 / 2.262

N2(g)

N2 -0.008 / -0.006 -0.069 / 0 -0.161 / 0

H2O(g)

H

O2 . . .

aParkhurst and Appelo, 1999
Contributions of Phosphorus from Groundwater to Streams 29

compounds plus evaporation or dilution to produce the median 

base-flow stream-water quality in various geologic settings 

(table 8). Phases added to represent anthropogenic sources 

included urea, ammonium phosphate, and sylvite, which are 

common constituents in commercial fertilizer (Teply and 

others, 1980). To simulate evaporation and dilution processes in the groundwater-to-surface-water models, moles of 

water (H2O) were subtracted (product) or added (reactant) 

as an additional phase (table 8). As 1 kg of solution contains 

55.5 moles of H2O, the fraction of the initial groundwater 

solution was decreased or increased accordingly to maintain a 

constant 1-kg mass of solution. Dilution of groundwater was 

indicated if the concentrations of chloride, sodium, and other 

major solutes in the groundwater were greater than those in 

surface water. Thus, dilution could represent addition of a 

younger source of groundwater not represented by sampling 

for this study. Evaporation and (or) dissolution of mineral 

sources or addition of anthropogenic sources was indicated if 

the concentrations of chloride, sodium, and other major solutes 

in the groundwater were less than those in surface water. 

Generally, the major-ion composition of the initial 

groundwater solutions for the groundwater-to-surface-water 

models was similar to that for background (natural) groundwater in a particular bedrock setting and, to a lesser extent, to 

that for the associated surface water (table 5). Consequently, 

the same principal mineral reactants and products but smaller 

mole transfers were indicated in the groundwater to surfacewater models (table 8) than in the rainwater-to-backgroundgroundwater models (table 7). 

For the crystalline bedrock setting, inverse models 

relating paired groundwater and surface-water quality were 

developed for the BXK and PXS networks, which had 

been described as representing relatively natural conditions 

(table 3). The groundwater in these networks contained 

appreciable dissolved phosphate (medians were 0.013 and 

0.055 mg/L, respectively) (table 5). Generally, the median 

concentrations of phosphate in surface water for both networks 

were less than or equal to the concentration in the groundwater (table 5), indicating that groundwater could be a principal 

source of dissolved phosphate in the base flow. In addition, the 

median concentration of silica and median specific conductance in surface water were less than those in groundwater, 

indicating that dilution could attenuate solute concentrations 

in these two settings (table 3). With dilution of the groundwater, the inverse models indicated that minor additions of 

phosphate by dissolution of fluorapatite and (or) Na-feldspar 

plus the addition of potassium and chloride by the dissolution 

of sylvite could produce the surface-water composition (table 

8). Excess phosphate was balanced by the precipitation of 

strengite or the production of organic matter (table 8). As the 

production of organic matter also removed nitrogen, the final 

nitrogen mole balance was achieved by the addition of urea. 

The mole transfers indicated for sylvite and urea (anthropogenic sources) were small, but comparable in magnitude to the 

transfers for minerals needed to balance major solutes. Therefore, the inverse models indicated that the concentration of 

phosphate in the groundwater could account for all or nearly 

all of the dissolved phosphate in the surface water for the 

PXS network, with minor contributions from minerals (little 

or no mass transfer); for the BXK network, the concentration 

of phosphate in groundwater was estimated to supply about 

half of the dissolved phosphate in the surface water, with the 

remainder contributed by the dissolution of feldspar and fluorapatite (table 9). 

For networks in the siliciclastic setting (VSP, VSL, and 

PSD) and the carbonate setting (VCT and PCL), the concentrations of major ions, including calcium, magnesium, and 

silica, and alkalinity in the surface water were less than those 

in the groundwater (tables 3 and 5). Thus, as for the crystalline setting, dilution was indicated for the inverse models 

relating paired groundwater and surface-water quality in the 

siliciclastic and carbonate settings (table 8). For networks in 

the siliciclastic setting, the concentrations of phosphate in 

surface water were less than those in the associated groundwater, indicating that groundwater could be a principal source 

of dissolved phosphate in surface water. For the networks 

in the carbonate setting, the concentrations of phosphate in 

surface water were greater than those in groundwater, however, indicating that sources of phosphate in addition to the 

dissolved concentration in groundwater would be needed to 

achieve the concentrations in surface water. After dilution, the 

concentrations of chloride, nitrate, sodium, and (or) potassium in groundwater were less than those in surface water for 

the siliciclastic and carbonate networks. To maintain the mole 

balances for these constituents, the inverse models involved 

contributions from halite, sylvite, ammonium phosphate, and 

(or) urea, even for the networks that previously had been identified as having natural water quality (VSP, VSL, and VCT) 

(table 3). Hence, these mass transfers may be accounting for 

chemicals released into surface water by the decomposition 

of plant material. The networks with human impacts on water 

quality (PSD, PCL) (table 3) could be distinguished from 

the other pairs mainly by greater additions of halite (table 8). 

In the models for networks in siliciclastic settings, a small 

amount of phosphate was added by the dissolution of fluorapatite, whereas in the models for networks in carbonate settings, 

greater amounts of phosphate were added by the dissolution 

of ammonium phosphate fertilizer. To maintain mole balances, 

excess phosphate was removed by the precipitation of strengite and carbonate-fluorapatite or by the production of organic 

matter (table 8). Thus, for the siliciclastic bedrock setting, the 

models indicate that groundwater supplied 70 to 90 percent 

of the dissolved phosphate in the surface water (table 9). For 

the carbonate setting, however, the groundwater potentially 

supplied only 5 percent of the dissolved phosphate to surface 

water in the PCL network, which has substantial anthropogenic inputs, and about 30 percent of the dissolved phosphate 

to surface water in the VCT network, which represented nearly 

natural groundwater quality (table 9). 

Alkali feldspar containing 0.3 percent P2O5 ((Na, K)

Al1.011Si2.978P0.011O8), was indicated as a possible source of dissolved phosphate in the crystalline and siliciclastic bedrock 
30 Contributions of Phosphorus from Groundwater to Streams, Eastern United States Table 8. Mass-transfer estimates (minimum/maximum) that produce median surface-water quality (final solution) by reactions with median groundwater quality (initial solution) in selected networks. [Location of networks shown in figure 3; networks described in table 2; PHREEQC used to compute mass transfer given solution compositions in table 5, saturation indices in table 6, and phase compositions in table 7; values are in millimoles, except H O, which is in moles; positive value indicates phase added to solution; negative value (

2 red) indicates phase removed from solution; “.” indicates phase excluded 

from model; yellow shading indicates phase contains phosphorus; column shading indicates network rock type: gray indicates crystalline, blue indicates carbonate, no shading indicates siliciclastic]

BXK PXS VSP VSL PSD VCT PCL

GW Fraction 0.426 / 0.461 0.365 / 0.366 0.284 / 0.301 0.281 / 0.281 0.600 / 0.614 0.985 / 1 0.672 / 0.69Diopside . . . . . . .Anorthite . . . . . . .Na-Feldspar 0.037 / 0.080 . 0 / 0.007 . . . .K-Feldspar . . 0 / 0.006 . . . .Biotite 0 / 0.013 . . . . . .Chlorite14A 0 / 0.009 0.005 / 0.005 . . . . .Illite -0.041 / -0.024 . 0 / 0.005 . 0 / 0.068 0 / 0.033 .Calcite . . 0 / 0.071 . . . 0.212 / 0.327Dolomite . . . 0.055 / 0.055 . . .Mn-Siderite . . 0 / 0.072 . 0.007 / 0.007 0.003 / 0.003 0.008 / 0.008Fluorite . . . . . 0.002 / 0.012 0.002 / 0.002Gypsum . . . . . 0.028 / 0.035 .Pyrite 0.017 / 0.018 0.008 / 0.008 0.005 / 0.016 0.022 / 0.022 0.052 / 0.063 . .Pyrolusite . . -0.004 / -0.001 . . . .Quartz -0.102 / 0 . . . -0.247 / 0 -0.128 / 0 .Fluorapatite 0.001 / 0.001 0.002 / 0.002 0.002 / 0.077 0.001 / 0.001 0.001 / 0.001 . .FCO3Apatite . . -0.030 / 0 . . -0.008 / 0 .Fe(OH)3(a) -0.017 / -0.016 -0.006 / 0 . -0.021 / -0.021 -0.069 / -0.059 -0.003 / -0.003 -0.007 / -0.007Gibbsite . -0.010 / -0.010 -0.012 / -0.006 . -0.156 / 0 -0.076 / 0 .Strengite . -0.006 / 0 -0.085 / -0.007 . . . .Variscite . . . . . . .Halite . . . . 0.398 / 0.404 0.056 / 0.062 0.247 / 0.251Sylvite 0.035 / 0.036 0.052 / 0.052 0.038 / 0.042 0.021 / 0.021 0 / 0.040 0 / 0.026 0.067 / 0.068NH4H2PO4 . . . . . 0 / 0.038 0.002 / 0.003Urea 0.038 / 0.039 0.002 / 0.272 0.013 / 0.015 0.472 / 0.474 . 0 / 1.927 .Organic matter -0.004 / -0.003 -0.005 / 0 . -0.004 / -0.004 -0.002 / -0.002 -0.026 / 0 -0.001 / 0CO2(g) . -0.271 / 0 . . . -0.861 / 0 .O2(g) . 0 / 0.224 0.321 / 0.386 0.577 / 0.578 0.133 / 0.196 0 / 0.136 0 / 0.159N2(g) . -0.228 / 0 . -0.432 / -0.431 . -1.727 / 0 .H2O(g) 29.93 / 31.86 35.19 / 35.26 38.8 / 39.72 39.89 / 39.99 21.45 / 22.2 0 / 0.86 17.2 / 18.21
Contributions of Phosphorus from Groundwater to Streams 31

Table 9. Estimated fraction of phosphate concentration in median surface water (final solution) that originated from groundwater 

(initial solution) in selected networks.

[Network locations shown in figure 3; networks described in table 2; PHREEQCa used to compute groundwater fraction and mass-transfer estimates in table 7; 

kg, kilogram; <, less than; >, greater than; mg/L, milligrams per liter; median concentration less than laboratory reporting limit of 0.01 mg/L were estimated 

at 0.005 mg/L and are shown in italics; row shading indicates network rock type: gray indicates crystalline, blue indicates carbonate, no shading indicates 

siliciclastic]

Network

Fraction of 1 kg initial 

groundwater solution in 1 kg 

final surface-water solution 

(values <1 indicate dilution; 

values >1 indicate evaporation) 

Median phosphate 

concentration 

(mg/L)

Estimated phosphate 

concentration in groundwater 

after dilution or evaporation 

to produce the fraction in 

surface water

Estimated percentage of 

phosphate in surface water 

that was derived from 

groundwater

Minimum Maximum Surface water Groundwater Minimum Maximum Minimum Maximum

BXK 0.426 0.461 0.012 0.013 0.0055 0.0060 46 50

PXS 0.365 0.366 0.005 0.055 0.0201 0.0201 100 100

PSD 0.600 0.614 0.027 0.032 0.0192 0.0196 71 73

VSP 0.284 0.301 0.005 0.015 0.0043 0.0045 85 90

VSL 0.281 0.281 0.010 0.030 0.0084 0.0084 84 84

PCL 0.672 0.690 0.070 0.005 0.0034 0.0035 <5 5

VCT 0.985 1.000 0.016 0.005 0.0049 0.0050 <31 31

a

Parkhurst and Appelo, 1999.

settings. Congruent dissolution of this phase by pure water 

would produce a Si/P mass ratio of 299:1. In comparison, 

the median ratios of dissolved concentrations of silica and 

phosphate (SiO2-Si/PO4-P) for groundwater and surface water 

in this study were 402 and 234, respectively. Smaller values 

for the Si/P ratio in surface water than in groundwater may 

result from the addition of phosphate to surface water from 

enriched natural sources (apatite, organic matter) or anthropogenic sources (fertilizer, manure, sewage) and (or) the attenuation of silica transport (precipitation of quartz or kaolinite). In 

addition, both silica and phosphorus may be taken up by plants 

in surface water, which may change this ratio. Higher values 

for Si/P in groundwater may result from the attenuation of 

phosphate transport (adsorption, precipitation, uptake) and (or) 

from lower concentrations of phosphate in feldspar than are 

assumed in the models. Although not precisely the stoichiometry considered in the mass-transfer models, the median Si/P 

ratio of 402 for the groundwater samples is consistent with 

the dissolution of nutrient-enriched feldspar with 0.22 percent 

P2O5 content ((Na,Al)1.008Si2.984P0.008O8), which is within the 

reported range of 0.2 to 0.3 percent (Kontak and others, 1996; 

Manning, 2008). Such nutrient-enriched feldspar may weather 

more rapidly than other silicate minerals because of selective 

dissolution of nutrient-bearing minerals by microorganisms 

and plants (Bennett and others, 2001; Manning, 2008), particularly in areas of groundwater recharge and discharge where 

plant roots may access groundwater. 

Although carbonate-fluorapatite was considered a potential sink for phosphate in groundwater, kinetic barriers to the 

precipitation of this mineral may be indicated by its supersaturation (fig. 13). The precipitation-dissolution kinetics of 

apatite and feldspar generally occur at slow rates compared to 

typical groundwater velocities and compared to adsorptiondesorption and biological uptake-decay processes. Consequently, precipitation-dissolution of minerals with favorable 

kinetics, such as calcite and amorphous iron hydroxide, and 

their sorption/release of phosphate ions could be the primary 

mechanisms that control phosphate concentrations in the 

groundwater systems. Nevertheless, despite the slow kinetics of feldspar dissolution, phosphate concentrations could 

become elevated in certain crystalline and siliciclastic rock 

settings where a solution remains undersaturated with calcite 

and apatite and where conditions are strongly reducing (ferric 

iron reduction). As the stability of apatite minerals and calcite 

generally would not be affected by DO whereas the stability 

of iron minerals would be affected by redox conditions, the 

relative importance of solubility control by apatite and (or) 

sorption by calcite or ferric iron minerals likely would not 

vary with redox conditions. 

The inverse models emphasized mineral precipitation 

as a mechanism for removal of phosphate from solution and 

did not simulate adsorption processes as possible controls 

on transport. Nevertheless, sorption and coprecipitation of 

phosphate with carbonate or iron oxide could be the primary 

mechanisms for removal of phosphate rather than precipitation 

of a pure phase such as apatite or strengite (Golterman, 1973; 

Jarvie and others, 2005; Maine and others, 2007). Several 

mass-transfer models indicated precipitation of strengite or 
32 Contributions of Phosphorus from Groundwater to Streams, Eastern United States

variscite (tables 7 and 8). Although typically undersaturated 

(table 6), these ferric iron and aluminum phosphate phases 

were included in the models as proxies for phosphate adsorption by the oxides of ferric iron and aluminum. Adsorption 

processes could be particularly important in surface-water 

environments because of the tendency for pH to increase and 

mineral precipitation (for example, CaCO3, Fe(OH) 3) to take 

place after groundwater that enters the stream begins to equilibrate with the atmosphere (loss of CO2

 and gain of O2):

Ca2+ + 2 HCO3

-

 = CaCO3 + H2O + CO2(g).

Fe2+ + 1/4 O2(g) + 2 1/2 H2O = Fe(OH)3 + 2 H+ .

Consequently, the pH and SI values for calcite and 

Fe(OH)3(a) of surface water generally were greater than those 

of groundwater in the same network (table 5, figs. 12 and 14). 

Such freshly precipitated solids generally consist of fine particles with relatively large surface areas that may be suspended 

or form coatings on bed material. 

Using PHREEQC with adsorption coefficients from 

Dzombak and Morel (1990), the potential for adsorption 

of phosphate by Fe(OH)3(a) was evaluated (fig. 15). With 

increased pH from 6 to 9, the adsorbed fraction of orthophosphate decreases from more than 90 percent to less than 20 percent. Hence, adsorption and desorption in response to changes 

in pH within the stream environment could be an important 

factor affecting phosphorus concentrations. Although silica 

and sulfate are less strongly sorbed than phosphorus, elevated 

concentrations of these major ions have been shown to influence phosphorus sorption (Geelhoed and others, 1997; Caraco 

and others, 1989). Under anoxic conditions, however, reductive dissolution of the ferric hydroxide could occur and result 

in the release of phosphate to solution. 

Summary and Implications

Various natural and anthropogenic sources contribute 

phosphorus to groundwater and surface water in the Piedmont, 

Blue Ridge, and Valley and Ridge Physiographic Provinces. 

As groundwater is an important source of water to streams and 

provides more than 50 percent of annual flow in most settings, 

phosphorus dissolved in groundwater is likely to contribute to 

the nutrient load in streams. To determine settings where this 

may occur, the U.S. Geological Survey compiled and evaluated water-quality data for a total of 358 stream base-flow 

samples and 609 groundwater samples that were collected 

from 1991 through 2007 from paired networks of groundwater 

and surface-water sites in different hydrogeologic and land-use 

settings. The dissolved phosphate concentrations for the stream 

base-flow samples ranged from less than 0.01 to 1.23 milligrams per liter, with a median of 0.016 milligrams per liter; 

those for the groundwater samples ranged from less than 0.01 

to 0.54 milligrams per liter, with a median of 0.010 milligrams 

per liter. The phosphate concentrations in groundwater from 

the crystalline and siliciclastic bedrock settings (medians of 

0.017 and 0.020 milligrams per liter, respectively) generally 

were greater than those in groundwater from the carbonate 

bedrock setting (median of less than 0.01 milligrams per liter). 

In contrast, the median concentrations of dissolved phosphate 

in surface water from the crystalline and siliciclastic bedrock 

settings (0.010 and 0.014 milligrams per liter, respectively) 

were less than the median in surface-water samples from the 

carbonate setting (0.020 milligrams per liter).

The reported concentrations of phosphorus in streams 

during base flow were commonly higher than ecoregion nutrient criteria of 0.037 milligrams per liter for the Piedmont and 

0.01 milligrams per liter for the Blue Ridge and Valley and 

Figure 15. Equilibrium fractions of initial concentrations of phosphate (0.1 

and 0.001 moles per kilogram) that may be dissolved or adsorbed on a finite 

amount of hydrous ferric oxide at 25 degrees Celsius as a function of pH.

0

10

20

30

40

50

60

70

80

90

100

100

90

80

70

60

50

40

30

20

10

0

pH

PERCENT DISSOLVED

PERCENT ADSORBED

5 6 7 8 9 10

PHOSPHATE CONCENTRATION, 

IN MOLES PER KILOGRAM

0.1

0.001
Summary and Implications 33

Ridge Physiographic Provinces. In the Piedmont, the criterion 

was exceeded at some sites in each of the eight surface-water 

networks, with the median concentrations for two networks 

exceeding the criterion. The Blue Ridge and Valley and Ridge 

criterion also was exceeded at some sites in each of the nine 

surface-water networks, with the medians exceeding the criterion in four carbonate and two siliciclastic networks. Although 

the recommended ecoregion criteria do not apply to groundwater, the 0.037-milligram-per-liter Piedmont criterion was 

exceeded by the median concentration of dissolved phosphate 

in one Piedmont crystalline groundwater network, and the 

0.01-milligram-per-liter criterion was exceeded by median 

concentrations in all three siliciclastic networks in the Valley 

and Ridge and in the crystalline Blue Ridge network. These 

data indicate that groundwater from the crystalline and siliciclastic settings could be an important source of phosphorus to 

streams, especially during base-flow conditions.

Identifying chemical indicators of sources of phosphorus 

in water samples is important in determining whether natural 

or human sources are impacting water chemistry. Groundwater that had low but measurable concentrations of dissolved 

nitrate (less than or equal to 1.0 milligram per liter) and chloride (less than or equal to 10 milligrams per liter) was considered to indicate natural phosphate concentrations. For the 

samples representing “natural” groundwaters, the median concentration of dissolved phosphate was 0.029, 0.018, and less 

than 0.01 milligram per liter for the crystalline, siliciclastic, 

and carbonate bedrock settings, respectively. Most of the natural groundwaters were associated with forested areas and wetlands where minerals in soil and rock are the principal sources 

or sinks of phosphorus and other solutes. Phosphate minerals 

such as apatite and aluminosilicate minerals such as feldspar 

could be sources in crystalline and siliciclastic rock settings, 

for example, whereas apatite could be a source or sink of dissolved phosphate in carbonate rock settings. In the siliciclastic 

setting, in particular, phosphorus could be desorbed from iron 

hydroxides where groundwater is under reducing conditions. 

The potential presence of such minerals generally was inferred 

by the lithology of the bedrock in the study area and by the 

computed mineral saturation indices. In other samples, possible anthropogenic sources of phosphorus were indicated by 

the presence of elevated concentrations of nitrate (greater than 

1 milligram per liter) or chloride (greater than 10 milligrams 

per liter) in the groundwater or surface water. These solutes 

are not commonly acquired from natural geologic sources, but 

typically are associated with animal manure, fertilizer, sewage, 

and road deicing salts used in agricultural and urban areas. 

Mixing of deep, mineralized groundwater with shallow, 

less mineralized, but contaminated groundwater was identified 

as a critical factor affecting stream base-flow water quality. 

In particular, samples from many surface-water networks had 

lower median specific conductance (an indicator of solute 

concentrations) and median concentrations of silica (an indicator of groundwater age) than samples from the paired groundwater networks, which could indicate the stream is receiving 

a substantial portion of flow from water that is younger and 

more dilute than that in the part of the aquifer sampled. In 

the carbonate setting, the concentrations of phosphate in 

surface water were much greater than those in groundwater; 

however, in the crystalline and siliciclastic bedrock settings, 

the concentrations of phosphate in surface water commonly 

were less than or equal to those in groundwater. The elevated 

concentrations of chloride and nitrate and the persistence of 

phosphate in surface water indicate that near-surface sources 

and (or) transport pathways could be important, in particular 

in the carbonate setting, in addition to the groundwater sources 

of phosphorus, which are more important in the crystalline and 

siliciclastic settings. 

Geochemical models were used to infer potential contributions of phosphate from the dissolution of minerals and to 

evaluate hydrochemical processes that could affect transport 

of phosphate from groundwater to surface water in different 

settings. Initial models demonstrated that dissolved phosphate 

and associated solutes in natural groundwater can be derived 

by the interaction of rainwater with the generalized mineral 

assemblage for each bedrock setting. Considering the positive 

correlations of phosphate, sodium, and silica in the groundwater, alkali feldspar containing phosphorus as an impurity 

((Na,K)Al1.011Si2.978P0.011O8) was indicated as a possible source 

of dissolved phosphate in the crystalline and siliciclastic 

bedrock settings. The addition of phosphate from other natural 

sources (apatite, organic matter) or anthropogenic sources (fertilizer, manure, sewage) was also considered in the models. 

The generalized geochemical mass-transfer models relating groundwater and stream base-flow surface-water quality 

in the crystalline and siliciclastic bedrock settings indicate that 

groundwater, which acquired phosphate by mineral dissolution, could be a major source of phosphate in surface water 

(estimates range from 46 to 100 percent). Additional sources, 

such as ammonium phosphate in fertilizers, must be considered in the carbonate bedrock setting, where 5 to 30 percent 

of the phosphorus in surface water was estimated to be from 

groundwater (mineral) sources. Because dilution was indicated 

as a critical process for most of the paired groundwater and 

surface-water models, the direct contribution of phosphate 

from groundwater to surface water and the importance of other 

sources or attenuation processes were difficult to demonstrate. 

The models assumed that median values for pH and solute 

concentrations represented steady-state compositions of the 

two end-member solutions. The compositions of surface water 

and recharge could vary seasonally, however, depending 

on the extent of evaporation and inputs from anthropogenic 

sources such as road deicing salt, fertilizer, manure, and sewage, as well as in-stream processing of nutrients. This recharge 

may travel along shallow flow paths, eventually mixing with 

deeper, more mineralized groundwater to produce the stream 

base flow. 

Although the geochemical mass-transfer models could 

be interpreted to indicate major processes along a flow path 

(recharge to groundwater to surface water), spatial and temporal variables were not considered. Additional work would 

be needed to document interactions with specific minerals 
34 Contributions of Phosphorus from Groundwater to Streams, Eastern United States

containing phosphorus, the effects of pH on sorption processes, contributions from shallow and deep water sources, 

and the temporal variability of hydrological processes (evaporation and dilution) and biological processes (microbial release 

and plant uptake of phosphorus). Groundwater flow-path studies in selected settings may be helpful in identifying spatial 

and temporal variability in the compositions of groundwater 

and base flow and verifying the existence and compositions 

of relevant solid phases that could affect phosphorus concentrations and transport. Watershed studies in selected settings 

that utilize continuous streamflow data coupled with periodic 

water-quality measurements at the outlet of the watershed also 

may be useful in developing phosphorus and other chemical 

budgets. For such studies, the chemical inputs and outputs can 

be computed to infer contributions from mineral and anthropogenic sources, and potential rates of transport. Data obtained 

from watershed studies may be applicable for the development 

of nutrient management plans. 

Determining potential geologic contributions of phosphate may be an important component of managing phosphorus sources to meet nutrient criteria for surface-water bodies, 

particularly in crystalline and siliciclastic bedrock settings in 

the eastern United States, where conditions could be favorable 

for transport of phosphorus by groundwater. An understanding 

of geologic sources of phosphate, the relation between total 

phosphorus load and base-flow phosphorus load in streams, 

and the fate of phosphorus in groundwater discharge areas 

would aid in effective resource management. In discharge 

areas, the deeper groundwater represented by sampling in 

this study may interact with shallow groundwater and surface 

water, and geochemical conditions are likely to vary because 

of evapotranspiration, plant uptake, and microbial processes. 

An understanding of the potential for the release of phosphorus from natural sources and the processes affecting its 

transport would be needed for resource managers to determine 

whether phosphorus loading from groundwater is sufficiently 

large to warrant additional guidelines for in-stream phosphorus criteria. 

Acknowledgments

This report is a product of the National Water-Quality 

Assessment (NAWQA) Program of the U.S. Geological 

Survey (USGS). The authors wish to especially acknowledge 

William F. Falls of the USGS for his assistance in the early 

stages of report planning and preparation. Joseph L. Domalgalski and Anthony J. Tesoriero of the USGS are thanked for 

their reviews.

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For additional information, write to:

Director, U.S. Geological Survey

Maryland-Delaware-D.C. Water Science Center

5522 Research Park Drive

Baltimore, MD 21228

http://md.usgs.gov/

For additional information about NAWQA studies in the Piedmont and Valley and Ridge aquifers contact:

Bruce D. Lindsey

U.S. Geological Survey

215 Limekiln Road

New Cumberland, PA 17070

http://water.usgs.gov/nawqa/

Document prepared by the West Trenton Publishing Service Center
Denver and others—Contributions of Phosphorus from Groundwater to Streams, Eastern United States—Scientific Investigations Report 2010–5176

Printed on recycled paper

Early Mesozoic basin aquifers

Piedmont and Blue Ridge carbonate-rock aquifers

Piedmont and Blue Ridge crystalline-rock aquifers

Valley and Ridge siliciclastic-rock aquifers

Valley and Ridge carbonate-rock aquifers