Hydraulic Characteristics and Nutrient 

Transport and Transformation Beneath a 

Rapid Infiltration Basin, Reedy Creek 

Improvement District, Orange County, Florida

by D. M. Sumner and L. A. Bradner

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 95-4281

Prepared in cooperation with the

Reedy Creek Improvement District

Tallahassee, Florida

1996
U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Gordon P. Eaton, Director

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

purchased from:

District Chief U.S. Geological Survey

U.S. Geological Survey Earth Science Information Center

Suite 3015 Open-File Reports Section

227 North Bronough Street P.O. Box 25286, MS 517

Tallahassee, Florida 32301 Denver, CO 80225-0425
Contents III

CONTENTS

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

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

Purpose and Scope....................................................................................................................................................... 3

Previous Studies .......................................................................................................................................................... 3

Hydrologic Setting ............................................................................................................................................................ 3

Hydrogeology .............................................................................................................................................................. 5

Surficial Aquifer System ................................................................................................................................... 5

Intermediate Confining Unit.............................................................................................................................. 6

Floridan Aquifer System ................................................................................................................................... 6

Water Budget Components.......................................................................................................................................... 6

Wastewater Treatment and Disposal at Reedy Creek Improvement District .................................................................... 6

Hydraulic Characteristics of a Rapid Infiltration Basin .................................................................................................... 8

Hydraulic Response to Reclaimed-Water Disposal at a Rapid Infiltration Basin ....................................................... 8

Moisture Characteristic Curves ................................................................................................................................... 17

Numerical Model of Flow System .................................................................................................................................... 19

Boundary and Initial Conditions.................................................................................................................................. 20

Model Layering ........................................................................................................................................................... 21

Model Calibration........................................................................................................................................................ 21

Application of the Model............................................................................................................................................. 29

Nutrient Transport and Transformation Beneath a Rapid Infiltration Basin..................................................................... 33

Biogeochemistry of Nitrogen ...................................................................................................................................... 35

Nitrogen Transport and Transformation Beneath Basin 50......................................................................................... 37

Biogeochemistry of Phosphorus.................................................................................................................................. 41

Phosphorus Transport and Transformation Beneath Basin 50 .................................................................................... 43

Summary and Conclusions................................................................................................................................................ 45

Selected References........................................................................................................................................................... 48

FIGURES

1. Map showing location of Reedy Creek Improvement District rapid infiltration basins, Orange County, Florida . 4

2. Graph showing daily mean discharge of Whittenhorse Creek near Vineland, Florida, and monthly precipitation

at Orlando, Florida................................................................................................................................................... 7

3. Photograph showing reclaimed water application to rapid infiltration basin.......................................................... 9

4. Photograph showing rapid infiltration basins.......................................................................................................... 9

5. Map showing location of wells in vicinity of basin 50 ........................................................................................... 10

6-8. Graphs showing:

6. Textural description and identification of model layers at basin 50 ................................................................ 11

7. Tensiometric measurements of matric potential at basin 50 during the time interval 5:40 a.m. 

February 28, 1992, to 5:25 a.m. February 29, 1992......................................................................................... 13

8. Tensiometric measurements of total head at basin 50 during the time interval 5:45 a.m. 

February 28, 1992, to 5:25 a.m. February 29, 1992......................................................................................... 13

9. Maps showing water table in the vicinity of basin 50 during and following the FebruaryMarch 1992 loading event ....................................................................................................................................... 15

10. Hydrographs of wells within basin 50 during February-March 1992 loading event .............................................. 17

11. Graphs showing lab-derived moisture characteristic curves for 2- and 10-foot depth soil samples from basin 50 18

12. Diagram showing boundary conditions for radial flow model................................................................................ 19

13. Diagram showing VS2D model grid ....................................................................................................................... 20
IV Contents

FIGURES--Continued

14-38. Graphs showing:

14. Comparison of simulated and observed infiltration rate at basin 50 during 

February-March 1992 loading event ............................................................................................................... 22

15. Comparison of simulated and observed basin 50 cumulative infiltration for the February-March 

1992 loading event .......................................................................................................................................... 23

16. Model-simulated matric potential at several depths within basin 50 .............................................................. 24

17. Model-simulated total head at several depths within basin 50........................................................................ 24

18. Comparison of simulated and measured head deviations from background water-level trend....................... 25

19. Model-estimated saturation profile at center of basin 50 during initial 3 days of February-March 

1992 basin operation........................................................................................................................................ 30

20. Model-estimated ground-water mounding distribution after conclusion of last day of 

February-March 1992 loading event ............................................................................................................... 31

21. Model-estimated head distribution along radial cross-section after conclusion of last day of 

basin 50 loading during February-March 19922 loading event ...................................................................... 32

22. Simulated basin infiltration rate under 20-hour wet/4-hour dry loading cycle for a range of rises 

in water-table altitude above a base water-table depth of 37 feet at basin 50 ................................................. 33

23. Model-simulated infiltration rate at basin 50 under various basin-ponding depths ........................................ 34

24. Simulated head response to 40-day continuous basin loading at basin 50 with 96-foot 

radius ponding ................................................................................................................................................. 35

25. Profiles of organic nitrogen concentration at basin 50 during February-March 1992 loading event.............. 37

26. Profiles of organic nitrogen concentration at basin 50 during May 1992 loading event................................. 37

27. Near-surface nitrate concentrations at basin 50 during May 1992 loading event ........................................... 38

28. Near-surface nitrate concentrations at basin 50 during May 1992 loading event ........................................... 39

29. Profiles of ammonia concentration (fine fraction) at basin 50 during May 1992 loading event..................... 39

30. Profiles of nitrate concentration at basin 50 during February-March 1992 loading event.............................. 40

31. Profiles of nitrate concentration at basin 50 during May 1992 loading event................................................. 40

32. Comparison of nitrogen and phosphorus concentrations in three point-sampler clusters at basin 50 

during initial basin loading on February 25, 1992 .......................................................................................... 42

33. Distribution of soil orthophosphate at various pH values ............................................................................... 42

34. Profiles of total phosphorus concentration at basin 50 during February-March 1992 loading event ............. 43

35. Profiles of total phosphorus concentrations at basin 50 during May 1992 loading event............................... 43

36. Near-surface phosphorus concentration at basin 50 during May 1992 loading event..................................... 44

37. Profiles of fine- and coarse-fraction orthophosphate at basin 50 during May loading event.......................... 45

38. Profiles of fine- and coarse-fraction organic phosphorus at basin 50 during May 1992 loading event.......... 46

TABLES

1. Textural data for five soil samples obtained by split-spoon sampling beneath basin 50........................................ 5

2. Mineralogy of silt and clay fractions of samples obtained by split-spoon sampling beneath basin 50.................. 5

3. Results of chemical extractions of “free” oxides of iron and aluminum from samples obtained by 

split-spoon sampling beneath basin 50................................................................................................................... 5

4. Typical Reedy Creek Improvement District wastewater treatment plant reclaimed water quality prior 

to treatment plant expansion in 1993...................................................................................................................... 8

5. Station identification numbers and descriptions of data-collection sites in the vicinity of basin 50...................... 11

6. Volumetric moisture content in vicinity of basin 50 tensiometer site during early stages of soil 

drainage................................................................................................................................................................... 14

7. Summary of calibration-inferred hydraulic parameters for radial-flow model of subsurface flow in 

the vicinity of basin 50 ........................................................................................................................................... 21
Contents V

CONVERSION FACTORS, VERTICAL DATUM, ABBREVIATED WATERQUALITY UNITS, AND ABBREVIATIONS

Sea level: In this report “sea level” refers to the National Geodetic Vertical Datum of 1929--a geodetic datum derived from a general 

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

Altitude, as used in this report, refers to distance above or below sea level.

Transmissivity: The standard unit for transmissivity is cubic foot per day per square foot times foot of aquifer thickness [(ft3/d)/ft2

]ft. 

In this report, the mathematically reduced form, foot squared per day (ft2/d), is used for convenience.

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 25 °C).

Leakance: The standard unit for leakance is foot per day per foot of confining unit thickness [(ft/d)/ft]. In this report, the mathematically 

reduced form, per day (d-1), is used for convenience.

Matric potential: The standard unit for matric potential is foot-pounds per pound [(ft-lb)/lb]. In this report, the mathematically reduced 

form, feet (ft), is used for convenience.

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

Moisture content: The standard unit for moisture content is cubic foot per cubic foot [(ft3/ft3)]. In this report, the mathematically reduced 

form, dimensionless, is used for convenience.

ADDITIONAL ABBREVIATIONS 

DNRA = dissimilatory nitrate reduction to ammonium 

ft/ft = feet per foot

g = gram

μm = micrometer

PVC = polyvinylchloride 

RIBs = rapid infiltration basins

RCID = Reedy Creek Improvement District

USGS = United States Geological Survey

WWTP = Wastewater treatment plant

Multiply By To obtain 

Length

inch (in.) 2.54 centimeter 

foot (ft) 0.3048 meter 

foot per mile (ft/mi) 0.1894 meter per kilometer

mile (mi) 1.609 kilometer 

Area

acre 0.4047 hectare 

square mile (mi2) 2.590 square kilometer

Volume

million gallons (Mgal) 3,785 cubic meters 

Flow

foot per day (ft/d) 0.3048 meter per day

cubic foot per second (ft3

/s) 0.02832 cubic meter per second 

million gallons per day (Mgal/d) 0.04381 cubic meter per second 

inch per year (in/yr) 2.54 centimeter per year 

Transmissivity

foot squared per day (ft2

/d) 0.0929 meter squared per day 

Leakance

foot per day per foot [(ft/d)/ft] 1.0 meter per day per meter

Matric potential

feet of water (ft) 2.988 kilopascal 

feet of water (ft) 0.02988 bar
SYMBOLS

VI Symbols List

Roman

C(ψ) specific moisture capacity as a function of 

matric potential, defined as the slope of the 

moisture characteristic curve, [L-1]

h total hydraulic head (z-ψ), [L]

K(ψ) hydraulic conductivity as a function of 

matric potential, [L/T]

Kr(ψ) radial hydraulic conductivity as a function 

of matric potential, [L/T]

Kz(ψ) vertical hydraulic conductivity as a function 

of matric potential, [L/T]

Ks saturated hydraulic conductivity, [L/T]

(Ks)r radial saturated hydraulic conductivity, 

[L/T]

(Ks)z vertical saturated hydraulic conductivity, 

[L/T]

m 1 - 1/n, [dimensionless]

n empirical van Genuchten coefficient 

descriptive of the slope of the moisturecharacteristic curve between air entry and 

residual moisture content, [dimensionless]

r radial coordinate, [L]

s liquid saturation, [L3/L3]

Ss specific storage, [L-1]

t time, [T]

Vr radial component of pore-water velocity, 

[L/T]

Vz vertical component of pore-water velocity, 

[L/T]

z vertical coordinate, positive upwards, [L]

Greek

α empirical van Genuchten coefficient, the 

reciprocal of which defines the matric 

potential at which liquid saturation is 0.5, 

[L-1]

φ porosity, [L3

/L3]

θ(ψ) volumetric moisture content as a function of 

matric potential, defined as the moisture 

characteristic curve, [L3

/L3]

θr residual volumetric moisture content, 

[L3/L3]

θs saturation volumetric moisture content, 

[L3/L3]

ψ matric potential, [L]
Abstract 1

Hydraulic Characteristics and Nutrient Transport 

and Transformation Beneath a Rapid Infiltration 

Basin, Reedy Creek Improvement District, 

Orange County, Florida

by D.M. Sumner and L.A. Bradner

Abstract

The Reedy Creek Improvement District 

disposes of about 7.5 million gallons per day 

(1992) of reclaimed water through 85 1-acre rapid 

infiltration basins within a 1,000-acre area of 

sandy soils in Orange County, Florida. The U.S. 

Geological Survey conducted field experiments in 

1992 at an individual basin to examine and better 

understand the hydraulic characteristics and 

nutrient transport and transformation of reclaimed 

water beneath a rapid infiltration basin. At the 

time, concentrations of total nitrogen and total 

phosphorus in reclaimed water were about 3 and 

0.25 milligrams per liter, respectively. 

A two-dimensional, radial, 

unsaturated/saturated numerical flow model was 

applied to describe the flow system beneath a 

rapid infiltration basin under current and 

hypothetical basin loading scenarios and to 

estimate the hydraulic properties of the soil and 

sediment beneath a basin. The thicknesses of the 

unsaturated and saturated parts of the surficial 

aquifer system at the basin investigated were 

about 37 and 52 feet, respectively. The model 

successfully replicated the field-monitored 

infiltration rate (about 5.5 feet per day during the 

daily flooding periods of about 17 hours) and 

ground-water mounding response during basin 

operation. Horizontal and vertical hydraulic 

conductivity of the saturated part of the surficial 

aquifer system were estimated to be 150 and 

45 feet per day, respectively. The field-saturated 

vertical hydraulic conductivity of the shallow soil, 

estimated to be about 5.1 feet per day, was 

considered to have been less than the fullsaturation value because of the effects of air 

entrapment. Specific yield of the surficial aquifer 

was estimated to be 0.41.

The upper 20 feet of the basin subsurface 

profile probably served as a system control on 

infiltration because of the relatively low fieldsaturated, vertical hydraulic conductivity of the 

sediments within this layer. The flow model 

indicates that, in the vicinity of the basin, flow in 

the deeper, saturated zone was relatively slow 

compared to the more vigorous flow in the 

shallow saturated zone. The large radial 

component of flow below the water table in the 

vicinity of the basin implies that reclaimed water 

moves preferentially in the shallow part of the 

saturated zone upon reaching the water table. 

Therefore, there may be some vertical 

stratification in the saturated zone, with recently 

infiltrated water overlying ambient water. The 

infiltration capacity at the basin would be 

unaffected by a small (less than 10 feet) increase 

in background water-table altitude, because the 

water table would remain below the system 

control on infiltration. However, water-table rises 

of 15 and 20 feet were estimated to reduce the 

infiltration capacity of the basin by 8 and 

25 percent, respectively. Model simulations 

indicate that increasing ponded depth within the 

basin from 4 to 12 inches and from 4 to 24 inches 
2 Hydraulic Characteristics and Nutrient Transport, Reedy Creek Improvement District, Orange County, Florida

would increase basin infiltration capacity by less 

than 6 and 11 percent, respectively. A loading 

strategy at the basin that relies on long, 

uninterrupted flooding was shown to offer the 

possibility of inducing a more anaerobic 

environment conducive to denitrification while 

maintaining reclaimed-water disposal capacity.

Field measurements indicated that 

transient, elevated concentrations or “spikes” of 

nitrate (as high as 33 milligrams per liter as 

nitrogen) occurred at the leading edge of the 

infiltrating water and in the shallow saturated 

zone following a prolonged basin rest period. This 

phenomenon probably is the result of 

mineralization and nitrification of organic 

nitrogen retained within the subsurface during 

earlier basin loading events. The organic nitrogen 

was retained in the shallow soil (due to 

adsorption/straining) and the shallow saturated 

zone (due to deposition under slacking pore-water 

velocity). The magnitude of the nitrate spikes 

appears to be influenced by the scheduling of 

basin loading, with short flooding and resting 

periods being most favorable to minimization of 

nitrate spikes. Removal of nitrogen by 

denitrification from the percolating reclaimed 

water is minimal in the vicinity of the basin, 

probably because of the lack of reducing 

conditions and a relative paucity of organic 

carbon substrates. It is speculated that longer 

flooding periods could induce reducing 

conditions that would be favorable for nitrogen 

removal from the system, but probably would 

lead to more pronounced nitrate spiking.

Phosphorus concentrations were decreased 

by about 90 percent from concentrations in 

reclaimed water after moving through the upper 

15 feet of the soil profile. This most likely was a 

result of adsorption onto abundant iron and 

aluminum hydrous oxyhydroxide coatings on 

sand grains. However, some phosphorus (coarse 

fraction, organic) passes through the shallow soil 

and accumulates below the water table under 

slacking pore-water velocity. This phosphorus is 

immobilized by adsorption, fixation, or 

precipitation reactions during basin rest periods.

INTRODUCTION

Disposal of reclaimed water (treated wastewater) 

by means of rapid infiltration basins (RIBs) has become 

increasingly popular as an alternative to the traditional 

practice of disposal by discharging to surface waters. 

RIBs offer the possibility of cost-effective tertiary treatment through percolation of the reclaimed water within 

the biologically and chemically active soil environment. 

Additionally, the environmental impacts, principally 

eutrophication of downstream surface-water bodies, 

associated with the discharge of reclaimed water to surface waters are mediated with land-based disposal.

The Reedy Creek Improvement District (RCID), a 

State (Florida) chartered organization in southwestern 

Orange and northwestern Osceola Counties that includes 

the Walt Disney World complex, is an area of about 

38 mi2

. In compliance with directives of the Florida 

Department of Environmental Protection, RCID, in 1990, 

curtailed discharges of reclaimed water into a natural 

wetland treatment system and diverted most of the flow 

of reclaimed water to RIBs. In 1992, RCID disposed of 

95 percent of the reclaimed water generated from its 

wastewater treatment plant through 85 1-acre RIBs 

within a 1,000-acre site constructed on sandy soils of the 

Lake Wales ridge. This site previously was used for citrus 

cultivation. The relatively high permeability of the soils 

of this site allows for “rapid” infiltration of applied 

reclaimed water. Reclaimed-water application rates in 

1992 were about 7.5 Mgal/d. The concentrations of total 

nitrogen (as N) and total phosphorus (as P) in the 

reclaimed water in 1992 were about 3 mg/L and 

0.25 mg/L, respectively.

The operation of RIBs in the United States and 

elsewhere has demonstrated the ability of the soil 

environment to reduce nutrient loads in reclaimed 

water. Nitrification/denitrification reactions, with suitable cycling of aerobic and anaerobic conditions and 

organic carbon availability, serve to produce gaseous 

nitrogen compounds which can be lost to the atmosphere. Adsorption reactions can reduce concentrations of phosphorus in the soil water provided that an 

appropriate substrate is available–such as the oxyhydroxides of iron and aluminum commonly present as 

coatings on sand grains within the vadose zone of 

much of central Florida’s upland soils.

The U. S. Geological Survey (USGS), in cooperation with the RCID, performed this investigation 

from October 1990 through September 1994 in an 

effort to better understand the movement and chemical 

transformation of reclaimed water applied to RIBs.
Hydrologic Setting 3

Purpose and Scope

This report presents the results of a study to 

define (1) the flow system within the unsaturated and 

saturated zones beneath a RIB, and (2) nutrient transport and transformation within reclaimed water percolating beneath a RIB. The report describes the results 

of field and laboratory investigations and numerical 

flow modeling.

The investigation focused on a detailed analysis 

of a single basin (RCID basin 50) during 1992. Waterquality data was collected along a vertical profile during basin operation. Descriptions of nutrient transport 

and transformation consistent with the water chemistry and the inferred flow system were developed. The 

nature of the flow system was evaluated through 

development of an unsaturated/saturated numerical 

flow model calibrated to approximately replicate the 

field-observed hydraulic features of the system. Laboratory analyses of the unsaturated hydraulic properties 

of basin soils were used as an aid to model calibration. 

The calibrated flow model was applied to hypothetical 

basin operation scenarios (higher water table, 

extended flooding periods, and increased ponded 

depth) to evaluate system performance. 

Previous Studies

The first summary appraisal of the hydrologic 

conditions within RCID was presented by Putnam 

(1975), in which the effects of the development of 

Walt Disney World during the period 1966-73 were 

evaluated. German (1986) completed an updated 

appraisal for the period 1966-80. These reports 

provide an overview of the climatological conditions, 

ground- and surface-water quality, stream and lake 

characterization, the hydrogeologic setting, and the 

nature of ground-water flow within RCID. Hampson 

(1993) described the hydrology and quality of Reedy 

Creek during the period 1986-89.

An evaluation of the effect of spray irrigation of 

reclaimed water on the water quality of the surficial 

aquifer system at RCID was reported by German 

(1990). Nutrient migration and transformation were 

described based on the results of water-quality monitoring within the saturated zone of the surficial aquifer 

system. Nitrification/denitrification and phosphorus 

immobilization reactions were postulated to explain 

some of the observed water-quality data.

A report prepared by the engineering consulting 

firm CH2M Hill (1989) summarized a pre-construction 

assessment of the suitability of the 1,000-acre RCID 

RIBs site for land-based reclaimed-water disposal. The 

nature of the surficial sediments was extensively examined by means of numerous soil borings, aquifer tests, 

geophysical logging, and piezocone sounding tests. An 

areally two-dimensional, saturated ground-water flow 

model, constructed on the basis of the collected data, 

indicated that total capacity of the 85 basins was greater 

than 10 Mgal/d and possibly as much as 20 Mgal/d.

Other sites of disposal of reclaimed water in 

RIBs include the Boulder, Colo., system (Carlson and 

others, 1982), the Water Conserv II facility in Orange 

County, Fla. (Camp, Dresser, and McKee, Inc., 1983), 

the 23rd Avenue rapid infiltration project in Phoenix, 

Ariz. (Bouwer and Rice, 1984), and the Dan Region 

Project in Israel (Idelovitch and Michail, 1984).

The results of a regional numerical flow model 

of the Floridan aquifer system in east-central Florida, 

including RCID, were presented by Tibbals (1990). 

Flow characteristics of the Floridan aquifer system 

were discussed and quantified in that report.

The Soil Conservation Service soil survey of 

Orange County (Doolittle and Schellentrager, 1989) 

mapped and described the shallow soils of the surficial 

sediments within the RCID 1,000-acre reclaimedwater disposal facility.

HYDROLOGIC SETTING

The RCID RIB disposal facility is located within 

the Lake Wales Ridge physiographic district, a region 

characterized by sandy, droughty soils, hilly karstic 

topography, and relatively high altitudes. The wetlands 

adjacent to the facility are within the Osceola Plain, a 

low-lying, low-relief area of poorly drained soils (White, 

1970, plate 1-B). Altitudes within the disposal site range 

from 100 ft to over 140 ft above sea level, whereas the 

adjacent wetlands are at an altitude of about 100 ft above 

sea level (fig. 1). 

Whittenhorse Creek drains 12.4 mi2

, including 

Bear Bay and part of the area encompassing the disposal 

facility. Mean flow over the period 1968-94 is 4.0 ft3/s as 

recorded by a gage at Hartzog Road (fig. 1). The annual 

mean flow ranged from 12.8 ft3

/s in water year 1970 to 

0.0 ft3/s in water year 1981. Whittenhorse Creek empties 

into the RCID perimeter canal which ultimately discharges to Reedy Creek. The area east of the disposal 

facility is drained by the perimeter and C-4 canals.
4 Hydraulic Characteristics and Nutrient Transport, Reedy Creek Improvement District, Orange County, Florida

Figure 1. Location of Reedy Creek Improvement District rapid infiltration basins, Orange County, Florida.

110

120 110

130

130

110 120

110

120

130

140

110

100

110

120

110

120

130

140

100

110 110

120

130

140 100

100

110

120 130

130

110 100 130

Bear Bay

Reedy

Lake

545

Hartzog Road

120

Whittenhorse

Creek

LAKE

WETLAND

STREAM/CANAL

100

C-4

EXPLANATION

RAPID INFILTRATION BASIN

0 500 1000 1500 2000 FEET

81o 38’ 81o 37’

28o 25’

28o 23’

0 100 200 300 400 500 600 METERS

STREAM GAGING STATION

110

Perimeter canal

Basin 50

ELEVATION OF LAND SURFACE CONTOUR --

Datum is sea level. Contour interval 10 feet.

Hachures indicate depressions.
Hydrologic Setting 5

Hydrogeology

The hydrogeologic profile in the vicinity of the 

RCID reclaimed-water disposal facility consists of 

(from land surface down): the surficial aquifer system, 

the intermediate confining unit, the Floridan aquifer 

system, the sub-Floridan confining unit, and basement 

rocks. Hydraulic head in the surficial system generally 

is at or above that of the Floridan aquifer system, defining the surficial system as a source of recharge to the 

Floridan aquifer system. This situation prevailed before 

RCID basin loading began in 1990 (Tibbals, 1990), but 

the recharge rates probably were lower than at present.

Surficial Aquifer System

The surficial aquifer system consists of unsaturated and saturated zones, with the position of the water 

table (representing the upper boundary of the saturated 

zone) dependent on the recent history of basin loading, 

rainfall, evapotranspiration, and leakage to adjacent 

wetlands and to the Floridan aquifer system. The water 

table prior to development of basin loading probably 

was relatively flat because topographic control of watertable altitude is slight in the study area (A.M. O’Reilly, 

USGS, oral commun., 1994). With basin loading, the 

water table reflects the recent loading history. Water that 

enters the surficial aquifer system moves laterally to 

discharge at adjacent wetlands or flows downward 

through the intermediate confining unit to the Floridan 

aquifer system. 

The thickness of the unsaturated zone varies from 

0 ft in the wetland areas to over 40 ft in the upland 

areas. The thickness of the saturated zone is typically 40 

to 50 ft, but can be greater near sinkhole breaches in the 

intermediate confining unit or near loaded basins. The 

surficial sediments are largely composed of fine-tomedium-grained sand, with minor amounts of silt and 

clay--primarily kaolinite (tables 1 and 2). Iron and aluminum oxyhydroxides occur as coatings on sand grains 

and are abundant in the oxidizing environment of the 

unsaturated zone (table 3). Lenticular strata, several feet 

thick with clay and/or silt content as much as 40 percent 

and an areal extent of several thousand feet, are present 

in the surficial sediments (CH2M Hill, 1989). These 

strata may allow for the development of a perched water 

table in some areas. Near-surface trenching revealed 

that even sandy soils contain lenticular lenses of darkercolored, finer-textured (but still sandy) material a few 

feet in areal extent embedded in the sand matrix. These 

lenses probably are capable of imparting an anisotropic 

nature to subsurface hydraulic properties.

Table 1. Textural data for five soil samples obtained by 

split-spoon sampling beneath basin 50

[Source: W.G. Harris, University of Florida, Soil and Water Science 

Department, written commun., 1993. Soil classification based on Unified 

Soil Classification System (American Society for Testing Materials, 1985)]

Soil classification Depth

(feet) Sand Silt(percent) Clay

Poorly-graded sand 3 96.0 1.2 2.8

Clayey sand 11 84.4 2.5 13.1

Poorly-graded sand 

 with clay

19 91.5 2.4 6.1

Poorly-graded sand 39 96.8 2.2 1.0

Poorly-graded sand 63 96.6 2.1 1.4

Poorly-graded sand 

 with clay

79 89.0 2.2 8.8

Table 2. Mineralogy of silt and clay fractions of samples 

obtained by split-spoon sampling beneath basin 50

[(Source: W.G. Harris, University of Florida, Soil and Water Science 

Department, written commun., 1993.) X-ray diffraction identification of 

minerals based on diagnostic d-spacing under specified conditions of cation saturation, glycerol solvation, and temperature (Whittig and Allardice, 

1986). Minerals are reported in the order of their estimated relative abundance based on relative x-ray diffraction peaks. Ka, kaolinite; HIV, 

hydroxy-interlayered vermiculite; Gi, gibbsite; Ha, halloysite; Qz, quartz; 

Sm, smectite; Mi, mica. =, similar amounts; >, more abundant than; >>, 

much more abundant than. Thermogravimetry analysis performed on clay 

fraction (Tan and others, 1986). --, negligible quantity]

Depth

(feet)

X-ray diffraction Thermogravimetry

Kaolinite Gibbsite

Silt Clay (percent of clay

fraction)

3 Qz>Ka=Gi Ka>HIV>Gi>Qz 49 20

11 Qz>Ka>Gi Ka>Gi>HIV 53 22

19 Qz=Ka Ka>>Qz 91 --

39 Qz>Ka Ka>Qz>Ha 85 --

63 Qz>Ka Ka>Qz>Gi>Ha 74 7

79 Qz=Ka>>Mi Ka>>Sm=Qz 83 --

Table 3. Results of chemical extractions of “free” oxides of 

iron and aluminum from samples obtained by split-spoon 

sampling beneath basin 50

[(Source: W.G. Harris, University of Florida, Soil and Water Science Department, written commun., 1993). Concentrations were determined by citratedithionite extraction (Olson and Ellis, 1982). μg/g, micrograms of element 

per gram of dry soil]

Depth

(feet)

Iron

(μg/g)

Aluminum

(μg/g)

3 1630 920

11 7950 1700

19 550 250

39 680 270

63 250 220

79 30 130
6 Hydraulic Characteristics and Nutrient Transport, Reedy Creek Improvement District, Orange County, Florida

Intermediate Confining Unit

The intermediate confining unit consists of the 

marine, phosphatic sands, silts, and clays of the Hawthorn Formation. This unit is less permeable than the 

adjacent surficial and Floridan aquifer systems and 

impedes the downward flow of water. The thickness of 

the intermediate confining unit over the RCID disposal 

facility generally ranges from 20 to 60 ft. However, the 

unit is absent in some areas of sinkhole breaches, leaving the surficial sands in direct contact with the Floridan 

aquifer system. The leakance (flow per unit area per 

unit head difference across the confining unit) of the 

intermediate confining unit in the vicinity of the RCID 

reclaimed-water disposal facility has been estimated in 

a recent investigation to be about 0.0003 d-1 (L. C. Murray, USGS, oral commun., 1994). Tibbals (1990) estimated a similar value of leakance for this area.

Floridan Aquifer System

The Floridan aquifer system is a thick sequence 

of carbonate strata that supplies most of the potable 

water in Florida. The thickness of this unit in the vicinity of the RCID reclaimed-water disposal facility is 

about 2,300 ft. The Floridan aquifer system is underlain by the sub-Floridan confining unit—low-permeability gypsiferous and anhydritic carbonate beds. The 

Floridan aquifer system is divided into two more permeable units—the Upper and Lower Floridan aquifers, 

which are separated by a zone of lower permeability, 

the middle semiconfining unit. The Upper Floridan 

aquifer directly underlies the intermediate confining 

unit and receives water leaking from the surficial aquifer system. The transmissivity of the Upper Floridan 

aquifer (estimated at 80,000 ft2

/d in the study area by 

L. C. Murray, USGS, oral commun., 1994) is more 

than an order of magnitude higher than that of the surficial aquifer. For this reason, little local mounding due 

to basin loading is expected in the Upper Floridan 

aquifer or underlying units. The hydraulic head within 

the Upper Floridan aquifer in the vicinity of the 

reclaimed-water disposal facility decreases to the 

northeast with a gradient of about 4 ft/mi (Sumner and 

others, 1992).

Water Budget Components

Principal components of the water budget of the 

surficial aquifer system include infiltration of 

reclaimed water, rainfall, evapotranspiration, runoff, 

and leakage to the underlying aquifer. Average annual 

rainfall over the study area is about 50 in., with 55 percent of this total occurring during June, July, August, 

and September (Tibbals, 1990). Evapotranspiration 

ranges from a potential rate of 48 in/yr to a rate of 

about 30 in/yr as a function of depth to the water table. 

Evapotranspiration also exhibits a strong seasonal 

component, with maximum rates in the wetter and 

warmer summer months. Leakage from the surficial 

aquifer system to the Upper Floridan aquifer was on 

the order of 4 to 9 in/yr prior to the initiation of basin 

loading (Tibbals, 1990). In 1992, the rate of reclaimedwater infiltration was 7.5 Mgal/d (about 100 in/yr over 

the 1,000-acre disposal facility). The increase in surficial aquifer head as a result of reclaimed-water disposal is estimated to increase leakage to the Upper 

Floridan aquifer 1.3 (in/yr)/ft of increase in surficial 

aquifer head (based on an intermediate confining unit 

leakance of 0.0003 d-1). Runoff rates from the Whittenhorse Creek drainage basin (including water that 

enters the surficial aquifer system and then moves laterally to discharge in wetland areas) averaged about 4 

in/yr prior to reclaimed-water disposal in the area. Following the commencement of basin loading in 1991, 

the ground-water contribution to runoff has probably 

increased, as suggested by a comparison of the preand post-basin loading discharge record of Whittenhorse Creek (fig. 2). The relatively low frequency of 

zero-flow conditions in Whittenhorse Creek following 

commencement of basin loading is better explained by 

basin loading than by the rainfall record. 

WASTEWATER TREATMENT AND 

DISPOSAL AT REEDY CREEK 

IMPROVEMENT DISTRICT

Wastewater generated within RCID was (prior 

to 1993) subjected to secondary treatment (activated 

sludge with denitrification process) at a wastewater 

treatment plant (WWTP) operated by Reedy Creek 

Energy Services, Inc. to produce reclaimed water. The 

WWTP was expanded in 1993 to include a five-stage, 

modified Bardenpho system to enhance nitrogen and 

phosphorus removal (Harkness and Otta, 1994). Typical RCID WWTP reclaimed-water quality prior to 

implementation of the plant expansion is summarized 

in table 4. Reclaimed water is stored in two 5-Mgal 

concrete ground-storage tanks prior to routing to RIBs.
Wastewater Treatment and Disposal at Reedy Creek Improvement District 7

Reclaimed water is pumped from storage tanks 

at the WWTP approximately 2.5 mi through a 48-in. 

diameter, reinforced concrete pipe to the 1,000-acre 

RIB disposal site and then routed to the center of 

selected basins where the reclaimed water spills over a 

vertical section of pipe into a RIB (figs. 3 and 4). 

Basins are approximately 1 acre in surface area with 

side slopes consisting of stabilized soil cement overlain with a geotextile liner (McKim, 1993). Basins are 

Figure 2. Daily mean discharge of Whittenhorse Creek near Vineland, Florida, and monthly precipitation 

at Orlando, Florida.

1970 1980 1990 1975 1985

20

15

10

5

0

CALENDAR YEAR

0

10

20

30

40

50

60

70

DISCHARGE, IN CUBIC FEET PER SECOND

WHITTENHORSE CREEK

MONTHLY PRECIPITATION,

IN INCHES

Commencement basin loading

ORLANDO AIRPORT RAINFALL

constructed in native soils--fine-grained sand of the 

Candler Series (Doolittle and Schellentrager, 1989). 

The soil profile is undisturbed at depths greater than 

about 1 to 1.5 ft. The upper profile of a basin is tilled 

between extended uses to disrupt the surface-filter 

cake formed by deposition of fine materials and algal 

growth. The tillage operation incorporates the surfacefilter cake within the upper profile, perhaps leading to 

long-term degradation of basin infiltration capacity.
8 Hydraulic Characteristics and Nutrient Transport, Reedy Creek Improvement District, Orange County, Florida

Reclaimed water usually is applied to eight 

selected basins during an approximately 15- to 20-hour 

period, with a rest period of about 4 to 9 hours between 

applications. Generally, daily loading is continued in 

this fashion at a given set of basins for 1 week, followed 

by 4 weeks of basin rest during which the reclaimed 

water is routed to other sets of basins. More extended 

loading and resting periods were applied to the basins 

prior to and during the February-March 1992 hydraulic 

test discussed in this report. Basin infiltration capacity 

ranges from 0.5 to 2.0 Mgal/d, with an average of about 

1.25 Mgal/d (McKim, 1993). Infiltration capacity for 

most basins is such that loading rates are not sufficient 

to cover the entire basin bottom. Rather, the ponded 

area assumes an irregular shape in conformance with 

basin topography.

Table 4. Typical Reedy Creek Improvement District 

wastewater treatment plant reclaimed water quality prior to 

treatment plant expansion in 1993 (Kohl and Bedford, 1993)

[All values expressed in milligrams per liter unless otherwise noted. °C, 

degrees Celsius; μS./cm at 25 °C, microsiemens per centimeter at 

25 degrees Celsius; NTU, Nephelometric turbidity units; Pt-Co Units, Platinum cobalt units; <, less than] 

Parameter Design

concentration Range

Alkalinity (as CaCO3) 69 29 - 125

Bicarbonate 84 52 - 150

Carbon dioxide 7.7 6.6 - 36

Calcium 30 25 - 35

Magnesium 7.6 5 - 10

Sodium 57 45 - 80

Chloride 110 60 - 155

Dissolved solids 

 (at 180 °C)

347 288 - 400

Specific conductance

 (μS/cm at 25 °C)

510 480 - 550

Turbidity (NTU) 3.5 0.5 - 5.0

Color (Pt-Co Units) 7.5 5 - 25

pH (units) 7.26 5.9 - 8.17

Temperature (°C) 26 20 - 28

Aluminum 0.075 <0.05 - 0.24

Iron (total) 0.46 0.16 - 0.72

Ammonia (as N) 0.5 <0.03 - 35.5

Nitrate (as N) 6.0 <0.02 - 9.17

Nitrite (as N) 0.05 <0.02 - 0.67

Total Kjeldahl nitrogen 

 (as N)

3.0 0.04 - 18.0

Orthophosphate (as P) 0.13 <0.01 - 1.11

Total phosphorus (as P) 0.27 0.07 - 2.15

Total suspended solids 7.0 1.0 - 12.0

Total organic carbon 7.2 <1.0 - 21.6

HYDRAULIC CHARACTERISTICS OF A 

RAPID INFILTRATION BASIN

The hydraulic characteristics of RIB disposal of 

reclaimed water were evaluated from field and laboratory data obtained in conjunction with a field experiment conducted at RCID basin 50 (figs. 1 and 5) in 

February-March 1992. The results of this experiment 

were used to define soil and aquifer hydraulic parameters. A subsurface flow model was used to simulate the 

system response to the field-experiment conditions 

(includes infiltration rates, rate and direction of subsurface movement of reclaimed water, ground-water 

mounding), as well as for hypothetical scenarios of 

basin operation.

Hydraulic Response to Reclaimed-Water 

Disposal at a Rapid Infiltration Basin

Reclaimed water was applied to basin 50 in 

a controlled and metered fashion beginning at 

10:20 a.m. on February 25, 1992, and ending at 

10:00 p.m. on March 10, 1992. Similar to normal 

basin operation, nightly basin rest periods several 

hours in length were maintained, during which the 

basin floor dried within the initial 1.5 hours. Berms 

were constructed to restrict the ponding area to a 

circular shape to produce radial symmetry. Pond 

radius was maintained at 66 ft during the initial 

6 days of the experiment and was expanded to 96 ft 

for the following 8 days, after which the basin was 

dry during the remainder of the experiment. Initial 

daily flooding of the bermed area usually required 1 

to 2 hours with a fully open flow valve. After fullbasin flooding, the flow rate was decreased incrementally throughout the day to maintain a nearly 

constant depth of ponding (averaging 4 in.). 

Metered flow rate allowed for estimation of basin 

infiltration rate. To monitor the transient nature of 

ground-water mounding and recession beneath the 

basin (fig. 5 and table 5), wells in the vicinity of 

basin 50 were periodically measured before and 

during basin operation and for a 2-week period after 

operation ceased. Inside the basin, where vertical 

head gradients were expected to be most pronounced, wells have relatively short screen lengths 

(2 ft) to define the vertical head distribution beneath 

the basin. Outside the basin, where vertical head 

gradients were expected to be relatively small, wells 

have longer screen lengths (10 ft).
Hydraulic Characteristics of a Rapid Infiltration Basin 9

Figure 3. Reclaimed water application to rapid infiltration basin.

Figure 4. Rapid infiltration basins.
10 Hydraulic Characteristics and Nutrient Transport, Reedy Creek Improvement District, Orange County, Florida

Reclaimed water was applied to basin 50 during 

the daily loading cycle at the rate of about 0.3 and 

0.9 Mgal/d for the initial 6 days and final 9 days, respectively. A total of almost 10 Mgal of reclaimed water was 

applied over the 15-day loading period of the field experiment. The average basin-infiltration rate (flow rate per 

unit area) was about 5.5 ft/d, both before and after the 

flooded area was increased on March 2, 1992. This lack 

Figure 5. Location of wells in vicinity of basin 50.

1

2-7

8-9

11 10

12

13

14

15

17

18

19

21-26

27-32

33-38

20

16

50

48

49

51

54 53 52

Hartzog Road

N

16

EXPLANATION

SURFICIAL AQUIFER WELL -- Location and reference number (see table 5)

POINT SAMPLERS -- Location and reference number (see table 5)

CIRCULAR FLOODED AREAS OF FEBRUARY-MARCH 1992

FIELD EXPERIMENT -- 66 and 96 feet radius

21

0 50 100 150 METERS

0 100 200 300 400 500 FEET

50 RAPID INFILTRATION BASIN -- Delineation and basin number

of flux response to changes in ponded area indicates 

that lateral components of flow in the unsaturated zone 

are small relative to the magnitude of vertical flow. 

Relatively finer-textured material present at the site at 

depths from 7 to 20 ft (fig. 6, and represented by 11-ft 

depth sample in table 1) apparently is not of sufficiently low hydraulic conductivity to produce perching adequate to induce significant lateral flow.
Hydraulic Characteristics of a Rapid Infiltration Basin 11

Table 5. Station identification numbers and descriptions of 

data-collection sites in the vicinity of basin 50 

[Reference numbers are shown in figure 3. --, no data]

Station number Reference

number

Depth

(feet)

Screen 

length

(feet)

Reclaimed water

282358081371700 -- -- --

Surficial aquifer wells

282358081371601 1 62 10

282358081371707 2 25 2

282358081371708 3 35 2

282358081371709 4 45 2

282358081371710 5 60 2

282358081371711 6 75 2

282358081371712 7 89 2

282358081372201 8 35 2

282358081372202 9 48 2

282358081371801 10 60 10

282358081371802 11 60 10

282357081371701 12 56 10

282356081371701 13 55 10

282358081371501 14 60 10

282358081372001 15 48 10

282359081371701 16 56 10

282400081371701 17 55 10

282359081371801 18 60 10

282352081371601 19 45 10

Upper Floridan aquifer well

282402081371701 20 406 open 

hole

Point samplers

282358081371701 21 1 0.25

282358081371702 22 3 0.25

282358081371703 23 5 0.25

282358081371704 24 7 0.25

282358081371705 25 10 0.25

282358081371706 26 15 0.25

282358081371713 27 1 0.25

282358081371714 28 3 0.25

282358081371715 29 5 0.25

282358081371716 30 7 0.25

282358081371717 31 9 0.25

282358081371718 32 10 0.25

282358081371719 33 1 0.25

282358081371720 34 3 0.25

282358081371721 35 5 0.25

282358081371722 36 7 0.25

282358081371723 37 9 0.25

282358081371724 38 12 0.25

If the ground-water mound remains below this layer of 

fine-textured material, the layer probably serves as the 

system control on basin infiltration. This observation 

appears at odds with the textural differences between 

the fine-textured layer and the overlying coarser material, but probably is explained by the field-observed air 

entrapment in the coarser soils. Air entrapment in soils 

can result in significant reduction in hydraulic conductivity because air-filling occurs preferentially in the 

larger pores. Additionally, the constant infiltration rate 

under a variable flooded area indicates that spatial 

variability in near-basin soil hydraulic properties probably is not extreme or is small-scale.

Point samplers, intended primarily for waterquality sampling in the upper 15 ft, served a secondary 

role as binary pressure sensors. The ability to produce 

water from a point sampler is indicative of positive 

pressure conditions. Inability to produce water is 

Figure 6. Textural description and identification of model 

layers at basin 50. Note: Particle size data determined from 

2-foot length split-spoon samples obtained during drilling of 

89-foot depth intra-basin well. z = depth of more complete 

textural (table 1) and mineralogical analysis (table 2).

0 0.5 0.1 0.2 0.3 0.4

PARTICLE FRACTION BY WEIGHT SMALLER

0

90

10

20

30

40

50

60

70

80

DEPTH, IN FEET BELOW LAND SURFACE

LAYER 1

LAYER 2

LAYER 3

 THAN 0.075 MILLIMETER
12 Hydraulic Characteristics and Nutrient Transport, Reedy Creek Improvement District, Orange County, Florida

indicative of soil-water tension. Positive water pressures were noted in the upper 15 ft of the profile following basin flooding, as evidenced by the ability of 

the point samplers to produce water during this period. 

However, positive pressures were never noted in the 

25-ft depth well, indicating that the base of the system 

control on infiltration lies between 15 and 25 ft and 

probably occurs at about 20 ft based on textural considerations (fig. 6). If the ground-water mound 

remains below this layer of fine-textured material, the 

surface material (0-20 ft) probably serves as the system control on basin infiltration. If the ground-water 

mound is above the base of this layer, the ability of the 

aquifer to laterally dissipate the infiltrating water 

becomes the control on basin infiltration. 

Surface clogging-induced reduction of basin 

infiltration rate has been a recurring problem in many 

instances of recharge basin operation (Bouwer and 

others, 1974; Jones and others, 1974; Schuh and 

Shaver, 1988). However, the constant infiltration 

capacity at basin 50 during the field experiment indicates that soil clogging probably is a minor factor, possibly due to the relatively low suspended-sediment 

load in the reclaimed water and a maintenance schedule involving basin disking after each 1- or 2-week 

period of use. An additional factor that could have prevented soil clogging is detachment of algal growth 

noticed during the experiment. Rice (1974) showed 

through laboratory experiments that oxygen bubbles 

produced by algae can cause the algal mat to become 

buoyant and break loose from the soil surface, disrupting formation of a surface-clogging layer.

Observations during the experiment indicated 

that air entrapment within the soil during infiltration is 

a prominent feature of the system. Mechanical disturbance of flooded soil resulted in the release of significant numbers of gas bubbles. Air trapped within the 

soil can cause large reductions in hydraulic conductivity, and therefore infiltration rate, from that of a fully 

saturated soil. The operation schedule (1992) allowed 

for nightly basin rest periods, permitting reentry of air 

and preventing the gradual disappearance of entrapped 

air through dissolution. Additionally, a mobile gaseous 

phase exists in the soil, particularly during the initial 

stages of basin loading. Gas bubbles were seen exiting 

the subsurface at several points in the basin. 

Tensiometric measurements of soil-water matric 

potential were made in basin 50 over a 24-hour basin 

flooding and drying period on February 28-29, 1992. 

Soil-water matric potential reflects the energy status of 

the soil water resulting from both pressure and adsorptive forces. In this report, matric potential is reported 

on an energy-per-unit weight basis (units of length). 

Some soil physicists prefer to use the term “matric 

potential” only in the case of unsaturated conditions 

(soil-water pressure less than atmospheric pressure) 

and to use the term “pressure potential” in the case of 

saturated conditions (soil-water pressure greater than 

atmospheric pressure). In this report, the suggestion of 

Hillel (1980, p. 143-144) to universally apply the term 

“matric potential” was adopted. In this convention, 

matric potential assumes positive values under unsaturated conditions and negative values under saturated 

conditions.

Water wells are only capable of measuring negative matric potential (positive water pressure). Tensiometers can measure both positive (at values less than 

about 32 ft) and negative values of matric potential 

(Hillel, 1980, p.156-158). Tensiometers consist of a 

water-filled tube that is allowed to equilibrate with the 

energy status of soil water across a ceramic cup. 

Manometer- or transducer-assisted measurement of 

tube water pressure allows an inference of soil-water 

matric potential based on hydrostatic principles.

The tensiometers were placed within RCID 

basin 50 at 1-ft intervals to a depth of 4 ft with duplicate tensiometers at each depth. The tensiometers 

were arrayed symmetrically along a 3-ft diameter circle with equal-depth tensiometers on opposite sides of 

the circle. The tensiometers were constructed of 

1/2-in. diameter PVC pipe. A sight tube was connected with epoxy to the upper end of the pipe. A Soilmoisture Equipment Corporation model 655 1 bar, 

high-flow ceramic cup was connected with epoxy to 

the lower end of the pipe. A rubber stopper, perforated 

with 1/8-in. purge and pressure-monitoring tubes, was 

placed in the sight tube. Holes were hand-augered with 

a 4-in. diameter auger. The tensiometers were filled 

with water and placed with the ceramic cup at the 

desired depths. The holes were backfilled with silica 

flour around the ceramic cup, with native soil to within 

1 ft of land surface, and with a bentonite cap to land 

surface. Water levels within the tensiometers were 

monitored using the sight tubes and then maintained as 

necessary. Tensiometer pressure was monitored with a 

mercury manometer. Average matric potential and 

total head for one 24-hour period at the four monitored 

depths are shown in figures 7 and 8, respectively.
Hydraulic Characteristics of a Rapid Infiltration Basin 13

Figure 7. Tensiometric measurements of 

matric potential at basin 50 during the time 

interval 5:40 a.m. February 28, 1992, to 

5:25 a.m. February 29, 1992. Note: Plotted 

values reflect average of two measurements 

obtained from two tensiometers at each of 

the monitored depths, laterally separated by 

3 feet. Curve annotation indicates depth of 

tensiometer, in feet. Ponded depth during 

flooded period was about 0.33 foot. Basin 

50 loading began at time = 0 hours. Positive 

and negative values of matric potential 

correspond to soil-water tension and positive 

pressure conditions, respectively. -2.5

1.5

-2.0

-1.5

-1.0

-0.5

0

0.5

1.0

MATRIC POTENTIAL, IN FEET OF WATER

0 24 5 10 15 20

TIME, IN HOURS

3

2

1

4

Tensiometers flooded

Loading ends

Ponding dissipated; drying begins

Figure 8. Tensiometric measurements of 

total head at basin 50 during the time interval 

5:45 a.m. February 28, 1992, to 5:25 a.m. 

February 29, 1992. Note: Plotted values 

reflect average of two measurements 

obtained from two tensiometers at each of 

the monitored depths, laterally separated by 

3 feet. Curve annotation indicates depth of 

tensiometer, in feet. Ponded depth during 

flooded period was about 0.33 foot. Basin 50 

loading began at time = 0 hours. Positive and 

negative values of matric potential 

correspond to soil-water tension and positive 

pressure conditions, respectively.

-6

1

-5

-4

-3

-2

-1

0

TOTAL HEAD, IN FEET OF WATER

0 24 5 10 15 20

TIME, IN HOURS

1

2

3

4

Tensiometers flooded

Loading ends Ponding dissipated; drying begins
14 Hydraulic Characteristics and Nutrient Transport, Reedy Creek Improvement District, Orange County, Florida

Following an 11-hour basin rest period, matric 

potential was about 1 ft in the three deeper monitoring 

locations and about 1.3 ft at the 1-ft depth (time from 0 

to 0.7 hour, fig. 7). Upon basin flooding, matric potential was reduced as the wetting front progressed downward. The unexpectedly fast response by the 

tensiometers at all depths indicates the likelihood of a 

transient, pneumatic effect due to compression of the 

soil gases ahead of the infiltration front. Ponded depth 

was maintained at about 4 in. A few hours after field 

saturation was achieved, matric potential below a 

depth of 2 ft was relatively constant and the vertical 

hydraulic gradient below a depth of 2 ft was approximately 1 ft/ft, indicative of an infiltration rate close to 

the field-saturated hydraulic conductivity of the 2- to 

4-ft depth interval. Above a depth of 2 ft, the hydraulic 

gradient following saturation was less than 1 ft/ft. 

Pond head was only slightly greater than the measured 

head at a depth of 1 ft, indicating that little head loss 

occurs over the upper foot of soil. The measured head, 

at a depth of 2 ft, was only about 0.5 ft less than the 

pond head. These effects reflect the presence of relatively high-permeability soil in the upper 1.5 ft of the 

profile. Structural changes in the soil caused by earthmoving during basin construction and by disking 

between basin operations are the cause of the high permeability of these soils. Drainage occurred rapidly 

over the monitored profile for 2 to 3 hours following 

the end of flooding, followed by a period of much 

slower drainage, during which matric potential 

returned to values observed prior to flooding. Under 

unsaturated conditions, little difference existed 

between duplicate tensiometers at equal depth; however, differences in matric potential of as much as 

0.3 ft existed during saturated conditions. These differences probably reflect small-scale variations in 

hydraulic properties, the effects of which were amplified during saturated (relatively high flow) conditions. 

Sampling of the soil profile with a Soilmoisture 

Equipment Corporation Model 0200 core sampler was 

completed at six depths for gravimetric determination 

of moisture content. Two cores, laterally separated by 

4 to 5 ft, were taken at each depth. The moisture 

content of the soil profile during periods of basin 

flooding is of particular interest and was expected to 

be less than porosity based on field-observed air 

entrapment. Intact core removal is difficult under nearsaturated conditions, but is practical when the soil 

water is under tension (positive matric potential). 

However, if core samples are taken after matric potential exceeds air entry values, significant desaturation 

will have occurred. Sampling was timed in an attempt 

to procure cores in low tension environments. To this 

end, sampling progressed downward with the drainage 

front over a 2-hour period, sampling a given depth 

when intact core removal was possible. Acknowledging that some slight soil desaturation might have taken 

place before core removal, the resulting estimates of 

field-saturated moisture content represent a lower 

bound (table 6). Volumetric moisture content ranged 

from 0.21 to 0.31, compared to a porosity range of 

0.43 to 0.46 for the soil samples, indicating the possibility of significant air entrapment.

Water-level measurements, made throughout the 

period of basin loading and for 2 weeks thereafter, 

were made in 19 surficial aquifer wells of 2-in. diameter and 1 Upper Floridan well (fig. 5 and table 5). Prior 

to basin loading, the water table was relatively flat 

(fig. 9) and about 37 ft below the basin floor. A 

ground-water mound began forming beneath the basin 

about 1.5 days after basin loading commenced 

(fig. 10). The mound was asymmetric, probably due to 

heterogeneity in aquifer hydraulic properties (fig. 9). 

Maximum mound height was about 7 ft in the 35-ft 

depth well. The water table remained below the 25-ft 

depth well throughout the basin-loading event. Diurnal 

variations in mound height resulting from the daily 

basin loading and resting cycles were pronounced near 

the basin (fig. 10) and were evident as far as 195 ft 

from the basin center. Large vertical hydraulic gradients were apparent in the upper saturated zone in the 

vicinity of the basin. However, in the deeper saturated 

zone (below about 75 ft depth), vertical head gradients 

were small, indicating a relatively minor amount of 

vertical flow in this zone. Thus, the predominant 

movement of infiltrated water seemed to be lateral 

after the water entered the shallow saturated zone, 

rather than vertical through the lower saturated zone 

and the intermediate confining unit, and into the Upper 

Floridan aquifer. Upon infiltration reaching the water 

table, pore-water velocity is expected to have declined 

substantially as head gradients declined and the crosssectional pore space available for flow increased. 

Table 6. Volumetric moisture content in vicinity of basin 50 

tensiometer site during early stages of soil drainage

Core sample 

number

Sample depth (feet)

1 2 3 4 56

Location 1 0.21 0.24 0.27 0.26 0.25 0.27

Location 2 0.20 0.25 0.28 0.31 0.24 0.23
Hydraulic Characteristics of a Rapid Infiltration Basin 15

Water levels in wells within the basin began to decline 

about 4 hours after the end of basin loading on 

March 10. The mound dissipated and the water table 

again became flat within 2 weeks after the end of basin 

loading (fig. 9). 

The potentiometric surface of the Upper Floridan 

aquifer remained at a nearly constant level during the 

field experiment—about 100 ft above sea level—as 

Figure 9. Water table in the vicinity of basin 50 during and following the 

February-March 1992 loading event. Note: Some water-level measurements 

are not honored by contour lines due to relatively large vertical gradients 

immediately beneath the basin. Elevation of the basin floor is about 140.5 feet.

February 24, 1992

EXPLANATION

103

50

48

49

51

54 53 52

Hartzog Road

N

0 50 100 150 meters

0 100 200 300 400 500 feet

CIRCULAR FLOODED AREAS OF FEBRUARY-MARCH 1992

FIELD EXPERIMENT -- 66 and 96 feet radius

50 RAPID INFILTRATION BASIN -- Delineation and basin number

WATER TABLE ALTITUDE CONTOUR --

Contour interval varies

WELL SITE -- Number is altitude of water level in feet above sea level

103.5

103.25

103.38

103.35

103.34

103.33 103.39

103.40

103.24

103.32

103.37103.35

103.59

103.29

103.17

103.59

indicated by water-level measurements made in the 

Upper Floridan aquifer well 20 (fig. 5 and table 5), 

about 500 ft from the center of basin 50. The effect of 

basin loading on the potentiometric surface of the 

Upper Floridan aquifer is thought to be minimal, even 

directly beneath the basin, due to the relatively high 

transmissivity of this aquifer—about 80,000 ft2

/d (L.C. 

Murray, USGS, oral commun., 1994) in this area.
16 Hydraulic Characteristics and Nutrient Transport, Reedy Creek Improvement District, Orange County, Florida

Figure 9. Water table in the vicinity of basin 50 during and following the February-March 1992 loading event. Note: Some 

water-level measurements are not honored by contour lines due to relatively large vertical gradients immediately beneath 

the basin. Elevation of the basin floor is about 140.5 feet--Continued.

104

103

March 2, 1992

50

48

49

51

54 53 52

N

0 50 100 150 meters

0 100 200 300 400 500 feet

102.19

103.37

103.38

103.61103.89 104.23

104.20 103.69

103.79

103.67

105.39

104.97

104.42

104.44

103.81

103.36

Hartzog Road

106

104

103

March 11, 1992

50

48

49

51

54 53 52

N

Hartzog Road

0 50 100 150 meters

0 100 200 300 400 500 feet

108.18

107.81

107.31

106.54 105.22 106.65

105.77

105.54 105.62 106.42

106.75 105.84 105.02

105.62

104.77

104.12

104.10

102.35

105

103

March 16, 1992

50

48

49

51

54 53 52

N

0 50 100 150 meters

0 100 200 300 400 500 feet

Hartzog Road

103.25

103.5

103.75

104.14

104.05

103.88

103.93

102.45

103.54

103.74

103.86 103.70

103.81

103.94

104.01

104.19 104.00 103.89

103.77

103.74

104

March 24, 1992

50

48

49

51

54 53 52

N

0 50 100 150 meters

0 100 200 300 400 500 feet

Hartzog Road

103

102.75

102.5

102.25

102.89

102.85

102.76

102.78 102.97

102.52

102.67 102.55

102.72

102.63

102.15

103.08

103.07

103.03102.96 103.03

103.02
Hydraulic Characteristics of a Rapid Infiltration Basin 17

soil moisture-characteristic data was limited to the wet 

range (0 to 2.5 ft-water matric potential) prevalent during RIB operation. The soil moisture-characteristic 

data reflect desorption (drainage) conditions only. 

Hysteresis (Hillel, 1980, p. 152-155) can produce variation between desorption and sorption (wetting) soil 

moisture-characteristic properties; however, this effect 

was not evaluated.

A parameterized form of the moisture-characteristic curves was developed by van Genuchten (1980). 

Unlike saturated-flow settings in which hydraulic conductivity at a given location is constant (assuming 

unchanging fluid viscosity and density), unsaturated 

hydraulic conductivity values change with the saturation of the soil as the cross-sectional area available for 

flow changes. Likewise, the storage characteristics of 

the system are not defined by a saturation-invariant 

value. The van Genuchten parameterization accommodates the description of both moisture-characteristic 

and hydraulic conductivity curves (as well as specific 

capacity, the slope of the moisture-characteristic 

curve).

The functional forms of hydraulic conductivity, 

specific moisture capacity, and volumetric moisture 

content were parameterized in the manner of 

van Genuchten (1980).

Moisture Characteristic Curves

Laboratory determination of soil-moisture characteristic data was made for two soil samples from 

basin 50 (fig. 11). The samples were collected at 

depths of 2 and 10 ft and were intended to be representative of the undisturbed, near-surface soil (poorly 

graded sand) and the relatively fine-textured layer 

from about 7 to 20 ft (clayey sand), respectively. The 

degree to which spatial variability affects the representativeness of these samples was not examined. The 

1.18-in. length, 2.125-in. diameter cylindrical samples 

were extracted with a Soilmoisture Equipment Corporation Model 0200 core sampler. The samples were 

placed in a Soilmoisture Equipment Corporation 

Model 1400 Tempe cell and, within a few minutes, saturated from the bottom up. The relatively fast saturation was intended to simulate, in a gross fashion, the 

manner of field saturation during basin operation. 

With this method of saturation, air entrapment is 

expected. The Tempe cell was attached by a tube to a 

buret, forming a hanging water column. The samples 

were drained over about a week’s time, incrementally 

increasing the imposed suction, and monitoring the 

water released from the sample. The determination of 

3 35

4 45

5 60

6 75

7 89

Note: Hydrographs arranged 

sequentially from most shallow

well at top to deepest well on

bottom. Hydrographs for 75-

and 89-foot wells are almost 

(figure 5; table 5)

coincident. 

 Well reference number Depth, in feet

Figure 10. Hydrographs of wells within basin 50 during February-March 1992 loading event. 

Note: Elevation of basin floor is about 140.5 feet. The 35-foot well screen was above the water 

table before March 4.

110

109

108

107

106

105

104

103

102

WATER LEVEL, IN FEET ABOVE SEA LEVEL

slope = -0.06 foot per day

24 1 10 20

FEBRUARY MARCH 1992

27

 Note: February 24 head estimated based on nearby water level measurements End of basin loading Basin loaded
18 Hydraulic Characteristics and Nutrient Transport, Reedy Creek Improvement District, Orange County, Florida

α is empirical van Genuchten coefficient, the 

reciprocal of which defines the matric potential at which effective saturation is 0.5, [L-1];

 n is empirical van Genuchten coefficient 

descriptive of the slope of the moisture 

characteristic curve between air entry and 

residual moisture content, [dimensionless];

 m is 1 - 1/n, [dimensionless];

 θr is residual volumetric moisture content, 

[L3/L3]; 

 θs is saturation volumetric moisture content, 

[L3

/L3]; and

ψ is matric potential, [L].

Differences were detected in the moisture-characteristic data of the two samples. The more gradual 

desaturation of the clayey sand was symptomatic of 

the relatively fine texture of this sample. The poorly 

graded sand exhibited rapid desaturation beyond air 

entry. The residual moisture content of the clayey sand 

(0.13) also was significantly higher than that of the 

poorly graded sand (0.06), which was expected due to 

the textural differences between the two samples. For 

both samples, saturation moisture content was less 

than porosity, indicating air entrapment. However, the 

poorly graded sand exhibited a greater volumetric air 

content (0.13) than the clayey sand (0.05) during lab 

“saturation.”

Hydraulic conductivity:

Specific moisture capacity:

Volumetric moisture content:

where 

Ks is saturated hydraulic conductivity, [L/T];

K( ) ψ KS

1 ( ) αψ n – 1 1 ( ) αψ n ( ) + –m –     2

1 αψn + m/2 = ---------------------------------------------------------------------------------, ifψ > 0

K( ) ψ KS = , ifψ ≤ 0

C( ) ψ

∂θ

∂ψ-------

mnα θs θr ( ) – ( ) αψ n – 1

1 ( ) αψ n ( ) + m + 1 = = –--------------------------------------------------------------, if ψ > 0

C( ) ψ = 0 if , ψ ≤ 0

Figure 11. Lab-derived moisture characteristic curves for 2- and 10-foot depth soil samples from basin 50. Data fit to 

van Genuchten formulation (1980) with Powell’s algorithm (Press and others, 1989). Positive values of matric potential 

correspond to soil-water tension conditions--Continued.

0 0.5 1.0 1.5 2.0 2.5

MATRIC POTENTIAL, IN FEET OF WATER

VOLUMETRIC MOISTURE CONTENT0.5

0.4

0.3

0.2

0.1

0

0 0.5 1.0 1.5 2.0 2.5

MATRIC POTENTIAL, IN FEET OF WATER

0.5

0.4

0.3

0.2

0.1

0

Basin 50: 2-foot depth - poorly-graded sand

saturation moisture content, θs = 0.28

residual moisture content, θr = 0.06

van Genuchten parameter, α = 1.13 per foot

van Genuchten parameter, n = 7.4

porosity, φ = 0.41

Basin 50: 10-foot depth - clayey sand

saturation moisture content, θs = 0.34

residual moisture content, θr = 0.13

van Genuchten parameter, α = 1.08 per foot van Genuchten parameter, n = 3.9

porosity, φ = 0.39

θ ψ() θs θr ( ) – 1

1 ( ) αψ n +

------------------------     m θr = + , ifψ > 0

θ ψ() θs = , ifψ > 0
Hydraulic Characteristics of a Rapid Infiltration Basin 19

NUMERICAL MODEL OF FLOW SYSTEM

The model chosen for subsurface flow simulation was the numerical code VS2D (Lappala and others, 1987). VS2D is a finite-difference model capable 

of simulating both unsaturated and saturated transientflow conditions in a two-dimensional lateral (either 

radial or cartesian) and vertical (cartesian) setting. The 

model assumes that only a single, isothermal liquid 

phase is mobile and that any gaseous phase is at a uniform, constant pressure. The state (dependent) variable 

of the flow system is matric potential. The spatial and 

temporal distribution of matric potential is determined 

by solution of the unsaturated/saturated flow equation, 

under appropriate boundary conditions. The flow 

equation is solved numerically in VS2D through application of the equation to discrete points in space and 

time and simultaneous solution of the resulting set of 

equations. In radial coordinates, the unsaturated/saturated, two-dimensional flow equation assumes the following form:

where

 z is vertical coordinate, positive 

upwards, [L]; 

 r is radial coordinate, [L];

 t is time, [T];

ψ is matric potential, [L];

Figure 12. Boundary conditions for radial flow model.

Saturated zone

Unsaturated zone

Specified pressure over flooded basin area during basin loading

No-flow during basin rest periods

No-flow outside basin

No-flow

Specified head

No-flow

Basin centerline

sS ( ) s – C( ) ψ ∂ψ

∂t -------= ∂∂z

----- KZ( ) ψ ∂ψ

∂z ------- 1 –         +

∂

∂r

----- Kr( ) ψ ∂ψ

∂r -------     +

Kr( ) ψ

r -------------- ∂ψ

∂r -------

Kr(ψ), Kz(ψ) are hydraulic conductivity as a function 

of matric potential for radial and vertical directions, respectively, [L/T];

 C(ψ) is specific moisture capacity as a function of matric potential, defined as the 

slope of the moisture characteristic 

curve, [L-1];

 θ(ψ) is volumetric moisture content as a 

function of matric potential, defined as 

the moisturecharacteristic curve, 

[L3

/L3];

 Ss is specific storage, [L-1]; and

 s is liquid saturation, [L3

/L3].

The above equation is a modified version of the 

Richards’ equation (Richards, 1931) for unsaturated 

soil-water flow. The modifications extend the equation 

to include saturated flow and a radial setting.

The subsurface domain simulated was a radial 

slice from the basin center line and included the unsaturated and saturated parts of the surficial aquifer system (fig. 12). Spatial discretization (fig. 13) was finest 

(as fine as 0.5 ft vertical by 3 ft radial) in areas of 

relatively steep hydraulic gradients (directly beneath 

the basin and particularly in the unsaturated zone). The 

model grid is assumed to be aligned with respect to the 

principal directions of hydraulic conductivity. Because 

anisotropy in hydraulic conductivity is usually associated with horizontally-oriented bedding planes, this 

assumption is probably met with the gravity-aligned 

model grid (fig. 13) used in this study.
20 Hydraulic Characteristics and Nutrient Transport, Reedy Creek Improvement District, Orange County, Florida

Boundary and Initial Conditions

The basin center line was assumed to serve as a 

no-flow boundary condition based on radial symmetry 

(fig. 12). A specified head boundary was maintained at a 

radial distance of 1,000 ft. At this distance the fieldobserved response to basin loading was negligible. A 

constant value of head was specified for all model layers 

at this boundary. Several ambient fluxes of the flow system were not directly simulated, such as rainfall recharge, 

evapotranspiration, and leakage between the surficial 

aquifer system and the Upper Floridan aquifer. At the 

space and time scales of interest to this investigation (in 

the vicinity of basin 50 over a few weeks’ time), these 

fluxes were small relative to basin infiltration. Rainfall 

was less than 5 in. during the February-March 1992 field 

experiment (NOAA, 1992a; NOAA, 1992b). A potential 

evapotranspiration estimate of 48 in/yr (Tibbals, 1990) 

suggests that less than 4 in. of water evapotranspired during the field experiment. Leakage to the Upper Floridan 

aquifer during the field experiment was estimated to be 

less than 1 in., based on an estimated leakance value of 

0.0003 d-1 for the intermediate confining unit (L. C. Murray, USGS, oral commun., 1994) and the observed surfiFigure 13. VS2D model grid. Note 10:1 vertical-to-radical scale distortion.

0 100 200 300 400 500 600 700 800 900 1000

RADIAL DISTANCE FROM BASIN CENTER, IN FEET

DEPTH, IN FEET BELOW BASIN BOTTOM

0

10

20

30

40

50

60

70

80

90

cial/Upper Floridan aquifer head differential of about 7 ft 

(maximum). 

 Land surface was assumed to be a no-flow 

boundary outside the basin area. Within the basin area, 

a no-flow boundary was applied during rest periods 

and a specified pressure head equal to the average 

pond depth (4 in.) was applied during loading periods. 

The ponded area was specified within the model to 

expand linearly over the observed period of time 

required for inundation of the basin bermed area. 

Because of the expected relative insignificance of 

leakage to the Upper Floridan aquifer–at the space and 

time scales of interest–a no-flow boundary was 

assumed at the base of the surficial system.

Initial conditions within the saturated zone of 

the model were defined by an equilibrium head distribution below a free-water surface at a depth of 37.5 ft 

from the basin floor. Tensiometric data (fig. 7) 

indicates that the unsaturated zone had not drained to 

an equilibrium head distribution. For this reason, the 

equilibrium head distribution within the unsaturated 

zone was continued only to a height of 1.5 ft above the 

free-water surface. Above this height, matric potential 

was prescribed to be 1.5 ft.
Hydraulic Characteristics of a Rapid Infiltration Basin 21

Model Layering

The vertical distribution of hydraulic properties 

(model layering) was developed through a combination of textural analysis and model calibration. The 

surficial system was divided into three distinct layers 

(fig. 6). The relatively fine-textured (table 1), iron- and 

aluminum-rich (table 3) zone between a depth of about 

7 to 20 ft was identified as a distinct layer 

(layer 2, fig. 6). This impeding layer seems to have 

lateral continuity, based on drilling observations near 

basin 50, and a probable surface expression several 

hundred feet from the basin. Soil texture above and 

below the impeding layer are similar. Air entrapment, 

which was expected (and was observed) above the 

impeding layer, was not expected below the impeding 

layer because of the expected unsaturated status of a 

coarser layer underlying a finer layer. For this reason, 

the sediments above the impeding layer (layer 1, 

fig. 6) are distinguished from the sediments below the 

impeding layer (layer 3, fig. 6). The lower part of layer 

3 (below a depth of 70 ft) is finer and more kaoliniterich than the upper part of that layer. 

Model Calibration

Data from field and lab experiments were used 

to calibrate the numerical flow model. These data 

included measured infiltration rate, field-measured 

hydraulic head in the surficial aquifer system, fieldmeasured transit time for the moisture front, laboratory-derived soil moisture-characteristic curves, fieldobserved soil/aquifer textural patterns, field-observed 

tensiometric data from shallow soil, and literaturederived estimates of subsurface hydraulic properties.

The numerical flow model simulated the flow system as affected by the loading of a single basin (RCID 

basin 50). However, about 6.7 Mgal of reclaimed water 

was applied to a nearby basin (RCID basin 52, fig. 5) 

during the 1-week period immediately preceding the 

field experiment. Other basins more distant from basin 

50 also were loaded during the experiment. Therefore, 

the effects of the multiple basin loadings were filtered 

from ground-water level observations so that the measurements reflected only the effects of basin 50 loading. 

This filtering was accomplished by estimating a “background” ground-water level trend prevalent during the 

field experiment. This trend, estimated based on water 

levels during a 2-day period prior to the loading of basin 

50, indicated a water-level decline rate of 0.06 ft/d 

(fig. 10). Error in estimation of the background groundwater level trend has a progressively greater effect with 

increasing time. For this reason, primary emphasis was 

placed on obtaining acceptable model replication of the 

observed growth, rather than decay, of the ground-water 

mound beneath basin 50.

Based on the calibration criteria, a model was 

developed which approximately replicated the 

observed measurements of the field experiment. The 

hydraulic parameters that reflect this calibration are 

summarized in table 7. Model calibration was simplified to some extent by the lack of feedback from the 

deep, saturated zone to the shallow unsaturated zone. 

Therefore, estimation of the shallow hydraulic parameters was attempted with little regard for the properties 

of the deeper zone. A successful calibration of the 

shallow zone provided the proper infiltration flux to 

the saturated zone, whereupon the deeper hydraulic 

properties were estimated based on replication of 

ground-water mound growth and dissipation.

Model calibration indicated that the ability of 

basin 50 to accept water primarily is a function of (Ks)z

of the shallow soils. As mentioned earlier, the lack of a 

reduction in basin infiltration rate with an increase in 

ponded area during the February-March 1992 field 

experiment indicates that lateral flow components in 

the unsaturated zone probably are relatively small in 

magnitude, which implies that (Ks)z is vertically fairly 

uniform above the base (estimated to be at a depth of 

20) of the system control. That is, the field-saturated 

hydraulic conductivity of layer 2 is approximately 

equal to the field-saturated hydraulic conductivity of 

layer 1, precluding perching over layer 2 and the resulting lateral flow. Based on the above rationale, both 

layers 1 and 2 were assigned a value of 5.1 ft/d for 

(Ks)z. Estimation of (Ks)r for the shallow layers was not 

possible, given the relatively minor amount of radial 

flow in this zone and the resulting lack of model sensitivity to this parameter (a value of 5.1 ft/d was used in 

model simulations). Observed infiltration data were 

Table 7. Summary of calibration-inferred hydraulic 

parameters for radial-flow model of subsurface flow in the 

vicinity of basin 50

[(Ks)r, radial saturated hydraulic conductivity; (Ks)z, vertical saturated 

hydraulic conductivity; θs, saturation volumetric moisture content; θr, 

residual volumetric moisture content; α, van Genuchten coefficient; n, 

van Genuchten coefficient; --, parameter not reliably estimated by model 

owing to a lack of sensitivity; ft/d, feet per day; ft3/ft3, cubic feet per cubic 

foot]. 

Layer (Ks)r 

(ft/d)

(Ks)z

(ft/d)

θs (ft3/ft3) θr 

(ft3/ft3)

α 

(ft-1)

n

(dimensionless)

1 -- 5.1 0.28 0.06 1.1 7

2 -- 5.1 .35 .13 1.1 4

3 150 45 .45 .04 1.1 7
22 Hydraulic Characteristics and Nutrient Transport, Reedy Creek Improvement District, Orange County, Florida

replicated reasonably well by the model (figs. 14 and 

15). Although only the daily average field-observed 

infiltration rate was determined, model-simulated infiltration indicated that a relatively high infiltration rate 

should occur during basin filling when the gradient in 

matric potential is relatively large. Later, the flow is 

thought to have been driven almost exclusively by 

gravity and the model indicates that the flux decreased 

to a lower and time-invariant value.

Some features of the tensiometric data (figs. 7 

and 8) were replicated through trial-and-error manipulation of the van Genuchten parameters for layer 1. As 

with the observed data, simulated values of matric 

potential were reduced after a few hours of fast drainage to about 1.0 ft, after which followed a period of 

slow drainage (figs. 7 and 16). This effect probably 

was due to the dramatic, sharp change in moisture content, and therefore hydraulic conductivity, in sandy 

soils undergoing drainage. An appreciable vertical 

head gradient existed during the period of slow drainage (figs. 8 and 17); however, the relatively low values 

Figure 14. Comparison of simulated and observed infiltration rate at basin 50 during FebruaryMarch 1992 loading event. Note: Observed flux shown is daily mean value. Unrecorded basin 

flow interruptions suspected to have occurred on March 4, 1994.

SIMULATED INFILTRATION RATE

OBSERVED INFILTRATION RATE

24 1 11

10

9

8

7

6

5

4

3

2

1

0

INFILTRATION RATE, IN FEET PER DAY

FEBRUARY MARCH 1992 

25 26 27 28 29 2 3 4 5 6 7 8 9 10

EXPLANATION

of hydraulic conductivity at the ambient matric potential limited flow. The comparison of model results and 

field observations during the initial stages of basin 

flooding were complicated by the suspected pneumatic response in tensiometer data during this period. 

The model (VS2D) is based on an assumption of a 

constant-pressure soil gas and is therefore incapable of 

simulating air pressurization caused by an advancing 

wetting front. During ponded conditions, observed 

values of matric potential were about 1.5 ft lower 

(pressure was 1.5 ft higher) than simulated values. 

This effect probably was caused by the lack of inclusion in the model of a high hydraulic conductivity 

upper layer (about 1 to 1.5 ft thick) representing the 

mechanically disturbed soil. The disturbed layer has 

little effect on system infiltration, which is governed 

by the hydraulic gradient in the underlying relatively 

low-permeability soil. The vertical hydraulic gradient 

during ponding was approximately one, below a depth 

of 2 ft, in both observed data and simulated values 

(figs. 8 and 17). The van Genuchten parameters 
Hydraulic Characteristics of a Rapid Infiltration Basin 23

summarized in table 7 provided model results reasonably consistent with laboratory-derived moisture-characteristic data (fig. 11), field-derived estimates of 

saturated moisture content (table 6), literature values 

for similar soils (Lappala and others, 1987, table 1), 

and some aspects of observed tensiometric data.

Although there were no direct measurements of 

the hydraulic status of layer 2, indirect measurements 

(infiltration rate, water-table response), textural definition, and the moisture-characteristic curve obtained for 

a sample at 10-ft depth (fig. 11), were used to infer the 

hydraulic properties of this layer. The van Genuchten 

parameters - α and n - for layer 3 cannot be reliably 

estimated because of a lack of model sensitivity to 

these parameters, but were inferred based on textural 

similarities with layer 1. Model simulations that 

attempted to acknowledge the textural differences 

within layer 3 (fig. 6) were unsuccessful in reproducing the observed response. Specifically, the observed 

low vertical hydraulic gradient in the lower zone of the 

Figure 15. Comparison of simulated and observed basin 50 cumulative infiltration for the FebruaryMarch 1992 loading event. Note: Observed cumulative infiltration shown is based on daily mean flux 

values. Unrecorded basin flow interruptions suspected to have occurred on March 4, 1992.

SIMULATED CUMULATIVE INFILTRATION

OBSERVED CUMULATIVE INFILTRATION

10

9

8

7

6

5

4

3

2

1

0

EXPLANATION

24 1 11

FEBRUARY MARCH 1992 

25 26 27 28 29 2 3 4 5 6 7 8 9 10

surficial aquifer system during basin loading (fig. 10) 

was not reproduced with a model division of layer 3. 

The most successful simulations in this respect 

assumed homogeneous properties within layer 3. This 

phenomenon might be indicative of (1) little hydraulic 

difference between the two zones, (2) vertical heterogeneity that is not laterally extensive, or (3) inaccuracies in model assumptions.

An additional appraisal of model reliability that 

incorporates the entire unsaturated-zone profile 

involved a comparison of observed and simulated 

transit time for the initial moisture front. For a given 

infiltration rate, this transit time is largely a function of 

specific yield. A large specific yield implies a large 

amount of pore space for the infiltrating water to fill 

and therefore, a relatively slow-moving moisture 

front. The model produced a transit time from land 

surface to water table reasonably close to that inferred 

from hydrographs of wells within basin 50 (fig. 18).
24 Hydraulic Characteristics and Nutrient Transport, Reedy Creek Improvement District, Orange County, Florida

Figure 16. Model-simulated matric potential at several depths within basin 50. Note: Curve 

annotation refers to depth in feet. Positive and negative values of matric potential correspond to 

soil-water tension and positive pressure conditions, respectively.

TIME, IN DAYS

MATRIC POTENTIAL, IN FEET OF WATER

25

10

4

3

2

1

0123

2

1.5

1

0.5

0

-0.5

Figure 17. Model-simulated total head at several depths within basin 50. Note: Curve 

annotation refers to depth in feet. Datum is land surface.

TIME, IN DAYS

TOTAL HEAD, IN FEET OF WATER

4

2

1

3

0123

1

0

-1

-2

-3

-4

-5

-6
Hydraulic Characteristics of a Rapid Infiltration Basin 25

 

Figure 18. Comparison of simulated and measured head deviations from background waterlevel trend. Note: Background water level trend assumed to be -0.060 foot per day. Well locations 

shown in figure 5.

SIMULATED

MEASURED

Well 4 - radial distance of 17 feet; 45-foot depth

Basin loaded

24 1 10 20

FEBRUARY

8

7

6

5

4

3

2

1

0

HEAD ABOVE BACKGROUND TREND, IN FEET

30

End of basin

loading

MARCH 1992

Figure 18. Comparison of simulated and measured head deviations from background water-level 

trend. Note: Background water level trend assumed to be -0.060 foot per day. Well locations 

shown in figure--Continued.

SIMULATED

MEASURED

Well 5 - radial distance of 20 feet; 60-foot depth

basin loaded

8

7

6

5

4

3

2

1

0

HEAD ABOVE BACKGROUND TREND, IN FEET

24 1 10 20

FEBRUARY

30

end of basin

loading

MARCH 1992
26 Hydraulic Characteristics and Nutrient Transport, Reedy Creek Improvement District, Orange County, Florida

Figure 18. Comparison of simulated and measured head deviations from background waterlevel trend. Note: Background water level trend assumed to be -0.060 foot per day. Well 

locations shown in figure 5--Continued.

SIMULATED

MEASURED

Well 6 - radial distance of 24 feet; 75-foot depth

basin loaded

8

7

6

5

4

3

2

1

0

HEAD ABOVE BACKGROUND TREND, IN FEET

24 1 10 20

FEBRUARY

30

end of basin

loading

MARCH 1992

Figure 18. Comparison of simulated and measured head deviations from background waterlevel trend. Note: Background water level trend assumed to be -0.060 foot per day. Well 

locations shown in figure 5--Continued.

SIMULATED

MEASURED

Well 7 - radial distance of 29 feet; 89-foot depth

8

7

6

5

4

3

2

1

0

HEAD ABOVE BACKGROUND TREND, IN FEET

24 1 10 20 30 basin loaded end of basin loading

FEBRUARY MARCH 1992
Hydraulic Characteristics of a Rapid Infiltration Basin 27

Figure 18. Comparison of simulated and measured head deviations from background waterlevel trend. Note: Background water level trend assumed to be -0.060 foot per day. Well 

locations shown in figure 5--Continued.

SIMULATED

MEASURED

Well 1 - radial distance of 125 feet; 55-foot depth

8

7

6

5

4

3

2

1

0

HEAD ABOVE BACKGROUND TREND, IN FEET

24 1 10 20 30 basin loaded

FEBRUARY

end of basin

loading

MARCH 1992

Figure 18. Comparison of simulated and measured head deviations from background waterlevel trend. Note: Background water level trend assumed to be -0.060 foot per day. Well 

locations shown in figure 5--Continued.

SIMULATED

 -- Well 10

 -- Well 12

 -- Well 16

- radial distance of 150 feet; 55-foot depth

basin loaded

8

7

6

5

4

3

2

1

0

HEAD ABOVE BACKGROUND TREND, IN FEET

MEASURED

24 1 10 20

FEBRUARY

30

end of basin

loading

MARCH 1992
28 Hydraulic Characteristics and Nutrient Transport, Reedy Creek Improvement District, Orange County, Florida

 

SIMULATED

- radial distance of 195 feet; 55-foot depth

basin loaded

8

7

6

5

4

3

2

1

0

HEAD ABOVE BACKGROUND TREND, IN FEET

 -- Well 11

 -- Well 14

 -- Well 18

MEASURED

24 1 10 20

FEBRUARY

30

end of basin

loading

MARCH 1992

Figure 18. Comparison of simulated and measured head deviations from background waterlevel trend. Note: Background water level trend assumed to be -0.060 foot per day. Well 

locations shown in figure 5--Continued.

SIMULATED

 - radial distance of 240 feet; 55-foot depth

basin loaded

8

7

6

5

4

3

2

1

0

HEAD ABOVE BACKGROUND TREND, IN FEET

 -- Well 13

 -- Well 17 MEASURED

24 1 10 20

FEBRUARY

30

end of

basin

loading

MARCH 1992

Figure 18. Comparison of simulated and measured head deviations from background 

water-level trend. Note: Background water level trend assumed to be -0.060 foot per day. 

Well locations shown in figure 5--Continued.
Hydraulic Characteristics of a Rapid Infiltration Basin 29

The ability of the model to replicate the fieldobserved ground-water mounding beneath and in the 

vicinity of the RIB was predominantly governed by 

the following hydraulic parameters (all within 

layer 3): (Ks)r, (Ks)z, θs, and θr

. Replication of the 

observed water-level response to basin loading was 

reasonably good (fig. 18). The disparity between (Ks)r

and (Ks)z in layer 3 implies significant anisotropy (3.3 

to 1), although noticeably less pronounced than that 

estimated for the surficial aquifer system (39 to 1) in 

an area about 25 mi northeast of the study area (Bush, 

1979). The estimated value of (Ks)r of 150 ft/d is near 

the high end of the range (25 to 160 ft/d) estimated by 

slug tests in ten wells throughout the 1,000-acre RCID 

disposal facility (CH2M Hill, 1989). Thus, this value 

(150 ft/d) may not be representative of areas that are 

distant from basin 50. Model sensitivity to (Ks)r was 

evaluated by changing the value of this parameter. 

Changing (Ks)r to values of 100 and 200 ft/d resulted 

in changes in maximum ground-water mound height 

of about +2 and -1 ft, respectively, from that of the calibrated model. The van Genuchten parametrization of 

unsaturated hydraulic variables is not a function of θs

and θr individually, but of their difference, which was 

estimated to be 0.41 for layer 3. This difference is an 

upper bound on aquifer specific yield and represents 

the value of specific yield under conditions of full 

gravity drainage of the subsurface profile. The specific 

yield is less than this difference in cases in whichprofile drainage to residual moisture content is not complete. Because the sands of the surficial aquifer drain 

relatively quickly to values of moisture content close 

to residual moisture content, the value of (θs - θr) of 

0.41 is a reasonably good estimate of specific yield. 

This value of specific yield is indicative of a high 

porosity, low moisture-retention sediment with little 

air entrapment. The model did not exhibit sensitivity 

to Ss, precluding estimation of this parameter.

Application of the Model

The calibrated model incorporates a physicsbased framework of the subsurface flow system. This 

understanding allows for an inference regarding the 

nature of the unmeasured parts of the system. Additionally, the model can be used to estimate system 

response to operational strategies other than that 

employed during the February-March 1992 field 

experiment.

The model-simulated saturation of the basin 

center-line profile for the initial 3 days of basin operation is shown in figure 19. The initial moisture front 

moved through the unsaturated zone and reached the 

water table in about 1.5 days. The effects of basin rest 

periods between daily loading events are exhibited in 

diurnal fluctuations in the water table and substantial 

drainage of the subsurface profile, particularly within 

the upper 10 ft. Following initial wetting, the fine-textured layer between 7 and 20 ft deep remains relatively 

close to full field-saturation. The sediments underlying 

the fine-textured layer never obtained saturation, 

which is to be expected with a fine-overlying-coarse 

sequence of sediments.

The simulated distribution of total head (relative 

to the initial water-table elevation) at the conclusion of 

the last day of basin flooding during February-March 

1992 is shown in figure 20. The head gradient was relatively steep and predominantly vertical above the 

water table, but decreased sharply in magnitude below 

the water table. The hydraulic gradient became 

progressively more radial below the water table with 

increasing distance from the basin centerline. 

Additionally, the magnitude of the hydraulic gradient in 

the vicinity of the basin was greater in the shallow saturated zone than in the deep saturated zone. The effects 

of this head distribution on the subsurface porewater velocity were estimated using VS2DT (Healy, 

1990), which is a companion program to VS2D. 

Based on Darcy’s law, VS2DT determines the components of pore-water velocity in both radial and vertical 

directions:

where

vr is radial component of pore-water velocity, 

[L/T];

vz is the vertical component of pore-water 

velocity, [L/T];

h is total hydraulic head (z - ψ), [L].

The direction and magnitude of pore-water 

velocity were calculated from the component velocities based on the principles of vector geometry.

The model-estimated velocity distribution after 

conclusion of the last day of basin 50 flooding during 

the February-March 1992 field experiment is shown in 

vr

Kr( ) ψ

θ ψ( ) -------------- dh

dr = -----

vz

Kz( ) ψ

θ ψ( ) -------------- dh

dz = -----
30 Hydraulic Characteristics and Nutrient Transport, Reedy Creek Improvement District, Orange County, Florida

Figure 19. Model-estimated saturation profile at center of basin 50 during initial 3 days of 

February-March 1992 basin operation. Note: Saturation defined in this figure as volumetric 

moisture content divided by field-saturated moisture content.

EXPLANATION

Saturation < .20

.20 < Saturation < .40

.40 < Saturation < .60

.60 < Saturation < .80

Saturation > .80

0

10

20

30

40

DEPTH, IN FEET BELOW LAND SURFACE

0 10 20 30 40 60 70 50

TIME SINCE INITIAL BASIN LOADING, IN HOURS

Flood

Water table

Rest Flood Rest Flood Rest Flood
Hydraulic Characteristics of a Rapid Infiltration Basin 31

figure 21. Pore-water velocity above the water tabledirectly beneath the basin was determined to have been 

relatively high–about 20 ft/d–and predominantly vertical in direction. As infiltrating water reached the water 

table, pore-water velocity was estimated to have 

changed markedly in both magnitude (decreased to 

less than 10 ft/d) and direction (substantial radial component, particularly with increasing depth and radial 

distance). The model indicated that the magnitude of 

pore-water velocity and the relative importance of vertical flow decreased with depth in the saturated zone. 

Flow was estimated to have been radial at distances 

greater than about 125 ft from the basin centerline 

(about 30 ft beyond the edge of the flooded area) and 

to have decreased in magnitude with radial distance 

from the basin.

The large radial component of flow below the 

water table (fig. 21) implies that reclaimed water 

moves preferentially in the shallow saturated zone 

after reaching the water table. Therefore, there exists 

the possibility of some vertical stratification of water 

within the saturated zone, with recently-infiltrated 

reclaimed water overlying ambient water. With 

increasing distance from the basin, little advective 

mixing of the upper and lower parts of the saturated 

zone occurs because of the predominantly radial flow. 

However, dispersion transverse to the radial flow 

direction could contribute to a reduction in vertical 

stratification.

Figure 20. Model-estimated ground-water mounding distribution after conclusion of last day of February-March 

1992 loading event (347.6 hours after initiation of basin 50 loading). Note: Curve annotation indicates head 

change, in feet-water, above initial water table elevation.

 

RADIAL DISTANCE FROM BASIN CENTERLINE, IN FEET

0 50 100 150 200

0

10

20

40

50

60

70

80

90

3

4

5

6

30

water table

DEPTH, IN FEET BELOW LAND SURFACE

7

An areal, two-dimensional, saturated flow 

model (CH2M Hill, 1989) has indicated that the watertable altitude at the RCID RIB facility could rise 

substantially following an increase in reclaimed-water 

disposal. The water table is expected to rise to a height 

of as much as 124 ft above sea level in the upland 

areas for a disposal rate of 20 Mgal/d. This rise in the 

background water level at basin 50 represents about a 

21 ft increase from the water level prior to the 

February-March 1992 field experiment. An increase in 

background water-level altitude can impact basin infiltration capacity because of the effect this increase 

would have on the altitude of the superimposed 

ground-water mound formed beneath the basin during 

loading. If the mound remains beneath the impeding 

layer (the base of fine-textured sediments at a depth of 

20 ft beneath basin 50 which is thought to act as the 

system control), the hydraulic gradient within the shallow sediments–and therefore basin infiltration–would 

be unaffected. However, if the background water-table 

altitude is sufficiently high that the ground-water 

mound intersects the system control, a “back pressure” 

is transmitted to the surface, reducing the hydraulic 

gradient, and therefore the basin infiltration rate. 

The effect of a rise in water table on infiltration 

rates at basin 50 was estimated by the calibrated 

model. A week-long basin loading, with a daily pattern of 20 hours wet/4 hours dry, was tested using the 
32 Hydraulic Characteristics and Nutrient Transport, Reedy Creek Improvement District, Orange County, Florida

flow model. For a relatively small rise in water-table 

altitude (less than 10 ft), the infiltration rate was unaffected (fig. 22). However, for greater increases in 

water-table altitude, the effect on the infiltration rate 

could be substantial. Rises in water-table altitude of 15 

and 20 ft at basin 50 produced reductions in infiltration capacity of 8 and 25 percent, respectively. In conditions of ground-water mound development above 

the base of the system control, enhanced lateral flow 

within this layer would be expected. An additional 

sensitivity analysis was performed because the lateral 

hydraulic conductivity derived from model calibration 

was not strongly defined due to relatively little model 

sensitivity to this parameter for the deep water table 

conditions of the calibration period. Lateral hydraulic 

conductivity of the system control was changed in the 

model from a value reflecting an isotropic assumption 

(5.1 ft/d) to a value reflecting a 3.3 to 1 anisotropy 

(17 ft/d), similar to that estimated for the deeper layer 

at basin 50. Basin infiltration was relatively insensitive (less than 3 percent) to this change in lateral 

Figure 21. Model-estimated velocity distribution along radial cross-section after conclusion of last day of basin 50 

flooding during February-March 1992 loading event (347.6 hours after initiation of loading event)

EXPLANATION

Arrow direction indicates the direction of water flow.

Arrow length is linearly proportional to pore water velocity.

velocity = 20 feet per day

velocity = 10 feet per day

1 foot per day < velocity < 5 feet per day

velocity < 1 foot per day

RADIAL DISTANCE FROM BASIN CENTERLINE, IN FEET

0

10

20

30

40

50

60

70

80

90

0 50 100 150 200

water table

DEPTH, IN FEET BELOW LAND SURFACE

hydraulic conductivity of the system control for deep 

water-table conditions. Rises in water-table altitude of 

15 and 20 ft at basin 50 produced reductions in infiltration capacity of 7 and 22 percent, respectively, in 

model simulations incorporating an anisotropic system 

control layer.

Manipulation of basin ponded depth is a possible management practice to increase infiltration capacity. The relative magnitudes of ponded depth, surface 

system control thickness, and matric potential at the 

base of the system control determine the effectiveness 

of changes in ponded depth on infiltration rate 

(Sumner and others, 1991, p. 6). If ponded depth is 

large relative to the sum of the thickness of the surface 

system control and the value of basal matric potential, 

the infiltration response to a change in ponded depth 

will approach a 1:1 proportionality (for example, doubling ponded depth will double infiltration rate). If 

ponded depth is small in relation to this sum, the infiltration response to a change in ponded depth will be 

negligible. The system control at basin 50 is made up 
Nutrient Transport and Transformation Beneath a Rapid Infiltration Basin 33

of layers 1 and 2 (relatively low permeability layers) 

and has a thickness of about 20 ft. The matric potential 

at the base of the system control is about 1 ft. Ponded 

depth was maintained at about 4 in. during basin operation, but basin geometry would allow use of a ponded 

depth of 2 ft. The possible values of ponded depths 

available to a manager are small relative to the sum of 

the thickness of the system control and basal matric 

potential, implying that infiltration response to a 

change in ponded depth will be small. To investigate 

the magnitude of the infiltration response, model simulations under varying ponded depths were made. 

Simulations indicate that increasing ponded depth 

from 4 to 12 in. and from 4 to 24 in. will increase basin 

infiltration capacity by 6 and 11 percent, respectively 

(fig. 23). Because of the lack of the model’s inclusion of 

the high-permeability, disturbed soil in the upper 1 to 

1.5 ft of the subsurface profile, these values of percentage increase in infiltration capacity under increasing 

ponded depth represent upper bounds. These simulations assume that any potential enhancement of biological clogging of basin soils due to increased water 

detention time and any change in the size of entrapped 

air bubbles due to pressure increases are negligible.

Figure 22. Simulated basin infiltration rate under 20-hour wet/4-hour dry loading cycle for a 

range of rises in water-table altitude above a base water-table depth of 37 feet at basin 50. 

Note: Curve annotation refers to rise in water table elevation in feet. Isotropic conditions are 

assumed in upper 20 feet of subsurface profile.

TIME, IN DAYS

INFILTRATION, IN FEET PER DAY

<10

10

9

8

7

6

5

4

3

2

1

0

01234567

15

20

Basin-loading strategies that rely on long, uninterrupted flooding, rather than daily wet/dry cycles, 

offer operational simplicity and the possibility of 

inducing a more anaerobic environment conducive to 

denitrification reactions. Model simulations indicated 

that basin 50 infiltration capacity would not be diminished even after 40 days of continuous loading 

(assuming a pre-loading water-table depth of 37.5 ft). 

Ground-water mounding at the 45-ft depth would be 

about 10 ft after this period of loading (fig. 24). This 

simulation was based on an assumption that the lack of 

basin disking/drying and the dissolution of entrapped 

air during this extended loading period would have a 

negligible effect on basin infiltration.

NUTRIENT TRANSPORT AND 

TRANSFORMATION BENEATH A RAPID 

INFILTRATION BASIN

Nutrient transport and transformation beneath 

RIBs used for disposal of reclaimed water was evaluated based on two field experiments conducted at 
34 Hydraulic Characteristics and Nutrient Transport, Reedy Creek Improvement District, Orange County, Florida

basin 50 in February-March and May 1992. Nitrogen 

and phosphorus are two nutrients of particular environmental concern because of their potential for producing eutrophic conditions in surface water. In the 

upper profile (15 ft and above), samples were obtained 

by KVA Analytical Systems miniature shield-point 

samplers. The samplers consist of a 1/2-in. diameter 

slotted aluminum point with a 1/8-in. diameter 

polypropylene tube to land surface. The sampler was 

driven into place by a 7/8-in. diameter shaft and the 

annular space was filled with sand and a bentonite cap. 

These samplers depend on positive pressure for operation, limiting sampling to periods of basin flooding. 

Sampling in the upper profile was of both “tracer” and 

“snapshot” varieties. Tracer-type sampling followed 

the infiltrating water through the profile, with sampling beginning as infiltrating water first reached a 

given point sampler and sampling progressing downward with time at depths of 1, 3, 5, 7, 10, and 15 ft 

(table 5). Some compositing of infiltrating and ambient water is expected in these samples. Snapshot-type 

sampling involved sampling all point samplers at 

approximately the same time. Spatial variability in 

nutrient concentrations was evaluated through three 

Figure 23. Model-simulated infiltration rate at basin 50 under various basin-ponding depths. 

Note: Curve annotation refers to basin ponding depth in inches.

TIME, IN DAYS

INFILTRATION, IN FEET PER DAY

01234567

10

9

8

7

6

5

4

3

2

1

0

24

12

4

sets of point samplers, about 80 ft from each other, 

within the basin. In the lower profile (below 15 ft), 

samples were obtained from 2-in. wells at depths of 35 

(when saturated), 45, 60, 75, and 89 ft within approximately 2-hour windows. Thus, upper profile samples 

were obtained through both tracer and snapshot techniques, whereas lower profile samples provide waterquality snapshots.

Water samples were obtained from wells and 

point samplers using standard USGS collection 

methods (Brown and others, 1970). The point samplers, installed above the water table, were sampled 

with a peristaltic pump. Initially, wells were purged 

with a submersible pump. Samples were collected 

after clearing three casing volumes of water from the 

well. Because of pump failure following 1 week of 

sampling during the February-March 1992 field experiment, samples were thereafter obtained by a bailer. 

The pump failure was associated with high concentrations of suspended solids that developed in purge 

water during basin loading, as pumped or bailed water 

became orange-colored and milky turbid. Between 

flooding events, water pumped or bailed from basin 

wells generally was clear, indicative of few mobile, 
Nutrient Transport and Transformation Beneath a Rapid Infiltration Basin 35

suspended solids. Bailing can cause overestimation of 

colloidal/particulate matter and slow pumping is the 

preferred method of sample extraction (Backus and 

others, 1993). The high concentrations of suspended 

solids in the ground-water samples probably resulted 

from the high velocity of the infiltrating water and the 

resulting particulate leaching from the unsaturated 

zone. Ground-water turbidity resulting from artificial 

recharge also was noted by Nightingale and Bianchi 

(1977).

Water samples were mixed in a sample-splitting 

churn after recovery from wells, split to sample bottles, and immediately preserved. Water samples for the 

dissolved or “fine-fraction” analyses were filtered 

through a 0.45-micron filter and immediately 

preserved. Unfiltered samples also were collected and 

immediately preserved for use in quantifying the 

“coarse fraction” of a given constituent. This differentiation of a constituent into fine and coarse fractions 

was based on the expected differences in the transport 

characteristics of the two fractions. Water samples 

were analyzed using USGS methods described by 

Wershaw and others (1983) and Fishman and 

Friedman (1985). Quality assurance included collecFigure 24. Simulated head response to 40-day continuous basin loading at basin 50 with 

96-foot radius ponding. Note: Curve annotation refers to intra-basin observation well depth, in 

feet. Datum is land surface.

TIME, IN DAYS

TOTAL HEAD, IN FEET WATER

35

45

60 75

89

0 5 10 15 20 25 30 35 40

-25

-30

-35

-40

tion of split and duplicate samples. Water-quality data 

for this investigation were published in a USGS waterdata report (USGS, 1992). Organic phosphorus concentrations were estimated from the difference 

between the concentrations of total and orthophosphate phosphorus.

Biogeochemistry of Nitrogen

The principal forms of nitrogen in the subsurface environment are nitrate, organic nitrogen, nitrite, 

ammonia, ammonium, and gaseous forms of nitrogen 

(elemental nitrogen, nitric oxide, and nitrous oxide). 

Organic nitrogen can exist in either particulate (subject 

to settling), colloidal (remains suspended indefinitely), 

or dissolved forms. The size ranges of colloids and 

particulates overlap - particulates are larger than about 

0.8 μm, whereas colloids are smaller than about 10 μm 

(Stumm, 1977).

Previous investigators have described several 

reactions and mechanisms important in the transport 

and transformation of nitrogen in the subsurface 

environment. A brief description of some of the more 

important mechanisms follows.
36 Hydraulic Characteristics and Nutrient Transport, Reedy Creek Improvement District, Orange County, Florida

Mineralization (Jansson and Persson, 1982; 

Brady, 1984) involves the breakdown of organic forms 

of nitrogen through enzymatic hydrolysis by a variety of 

soil organisms, both aerobic and anaerobic. The product 

of this reaction is ultimately ammonium, as exemplified 

by the hydrolysis of an amino compound below:

R-NH2 + HOH › R-OH + NH3 + energy

2NH3 + H2CO3 › (NH4)2CO3 ÷ 2NH4

+ + 

CO3

-2

Nitrification (Schmidt, 1982; Brady, 1984) is the 

enzymatic oxidation of ammonium to nitrate. The 

reaction consists of two principal steps, both of which 

are facilitated by autotrophic bacteria. The relatively 

slow Nitrosomonas-facilitated conversion of ammonium to nitrite is followed by a relatively fast Nitrobacter-facilitated conversion of nitrite to nitrate. 

Because of the relative speed of the two reactions, 

nitrite does not accumulate.

2NH4

+ + 3O2 › 2NO2-

 + 2H20 + 4H+ + energy

2NO2

-

 + O2 › 2NO3

-

 + energy

As an oxidation process, nitrification is strongly 

affected by oxygen availability. Nitrification rates are 

temperature dependent, obtaining optimal values 

between 27 and 32 oC.

Denitrification (Firestone, 1982; Brady, 1984; 

Korom, 1992) is the enzymatic reduction of nitrate to 

nitrite and gaseous nitrogen compounds, which may 

escape to the atmosphere, removing nitrate from subsurface water. 

NO3

-

 › NO2

-

 › NO › N2O › N2

Denitrification is a dissimilatory process in that 

the reduced nitrogen is not incorporated into microbial 

biomass and energy is produced for cell growth and 

maintenance. A variety of bacteria are effective at 

facilitating nitrate reduction, particularly species of 

Pseudomonas. All denitrifiers are facultative anaerobes, which, although preferring elemental oxygen as 

an electron acceptor, will use nitrogen as the electron 

acceptor under conditions of inadequate aeration. The 

extent of denitrification is influenced by the quantity 

and type of electron donor available. 

Heterotrophic denitrification requires a carbon 

source to serve as an electron donor, as exemplified 

below, showing an arbitrary organic compound, C, as 

the carbon source:

5C + 4NO3

-

 + 2H2O › 2N2 + 4HCO3

-

 + CO2

Because of the wide range of carbon oxidation 

states in organic compounds, a single stoichiometric 

carbon/nitrogen ratio appropriate for denitrification 

does not exist. Additionally, other microbial reactions 

generally compete with denitrification for available 

carbon. However, Leach (1980) has reported that 

approximately 2 g of wastewater carbon are required 

to denitrify 1 g of nitrate nitrogen.

Autotrophic denitrification relies on a non-carbon electron donor, such as reduced sulfur, iron, or 

manganese. This type of reaction is exemplified by the 

oxidation of ferrous disulfide, facilitated by the bacteria Thiobacillus denitrificans:

5FeS2 + 14NO3

-

 + 4H+ › 7N2 + 10SO4

-2 + 5Fe+2

+ 2H2O

Whether heterotrophic or autotrophic, denitrification requires anaerobic conditions, suitable bacterial 

populations, and ample amounts of electron donors to 

proceed. Denitrification is known to occur in anaerobic microsites of otherwise aerobic media (Rice and 

Gilbert, 1978).

Dissimilatory nitrate reduction to ammonium 

(DNRA) (Tiedje and others, 1981; Korom, 1992) is a 

nitrogen-conserving process, in contrast to denitrification which results in a loss of nitrogen from the system. 

Similar to denitrification, DNRA requires anaerobic 

conditions. DNRA can compete with denitrification for 

nitrate; DNRA is favored over denitrification in a 

nitrate-limiting environment and disfavored in a carbonlimiting environment. The ammonium produced by 

DNRA is susceptible to reconversion to nitrate through 

nitrification under oxygenated conditions.

Several mechanisms can serve to retard the 

transport of nitrogen species, including adsorption, 

colloid attachment, straining, flushing, and sedimentation. Adsorption involves the electrostatic attachment 

of an ion or colloid onto a soil or colloidal surface 

(Brady, 1984). The positively charged ammonium ion 

can participate in cation exchange-adsorption reactions with the predominantly negatively charged surfaces of clay or organic material. Colloids (clay or 
Nutrient Transport and Transformation Beneath a Rapid Infiltration Basin 37

organic) can attach to soil surfaces based on the 

interplay between London-van der Waals forces 

(attractive), electrostatic double layer forces (attractive/repulsive), ionic strength (high ionic strength 

favors colloid attachment), and soil water velocity 

(shear forces associated with fluid movement can 

detach colloids) (McDowell-Boyer, 1992). Colloids 

mobilized by relatively high flow rates or flushing 

with low ionic-strength water can serve to transport 

adsorbed ions, a process known as colloid-facilitated 

transport (Huling, 1989). Straining is a process in 

which particles or flocculations of colloids are lodged 

within smaller pores. Particulate forms of nitrogen 

(non-dissolved organic nitrogen or particulateadsorbed ammonium) are particularly susceptible to 

this process. Similar to colloidal forms, particulate 

forms of nitrogen are subject to the flushing action of 

high velocity pore water. Sedimentation of particulate 

forms of nitrogen can occur due to density effects, particularly in microsites of slack water (Ives, 1970).

Nitrogen Transport and Transformation 

Beneath Basin 50

Total organic nitrogen concentrations generally 

decreased by roughly 50 percent from concentrations in 

reclaimed water in the upper 15 ft of the soil profile 

(figs. 25, 26, and 27). This effect probably results from 

colloid attachment to soil surfaces for the largely colloidal, finer fraction of organic nitrogen; and from straining, sedimentation, and colloid attachment for the 

mixed colloidal/particulate coarser fraction. Bennet and 

Leach (1983) report similar observations of organic 

nitrogen removal. Mineralization reactions, which are 

relatively slow-acting, probably are not a dominant 

mechanism behind the rapid reduction in organic nitrogen concentrations in the upper 15 ft of the soil profile.

Snapshot-type samples were collected several 

times during the first two days of the May 1992 flooding 

event. Organic nitrogen concentrations in the upper 15 ft 

exhibited several time-variant features: (1) a large 

decrease in concentration from the start of flooding to 

the end of flooding each day and (2) a rejuvenation of 

Figure 25. Profiles of organic nitrogen concentration at 

basin 50 during February-March 1992 loading event. Note: 

Tracer-type sampling used in upper 15 feet of profile. 

Snapshot-type sampling used in lower profile.

0

90

10

20

30

40

50

60

70

80

DEPTH, IN FEET BELOW LAND SURFACE

0.2 1.4 0.2 0.4 0.6 0.8 1.0 1.2 1.4

TOTAL ORGANIC NITROGEN AS N,

IN MILLIGRAMS PER LITER

2/25/92

3/3/92

3/10/92

measurement

pre-loading

water table

2/24/92

Following cycle of:

3 weeks loading 

20 weeks dry

Figure 26. Profiles of organic nitrogen concentration at 

basin 50 during May 1992 loading event. Note: Tracer-type 

sampling used in upper 15 feet of profile. Snapshot-type 

sampling used in lower profile.

measurement

pre-loading

water table

5/18/92

Following cycle of:

1 week loading 

4 weeks dry

ORGANIC NITROGEN AS N,

 IN MILLIGRAMS PER LITER

DEPTH, IN FEET BELOW LAND SURFACE

90

0

10

20

30

40

50

60

70

80

0 0.2 0.4 0.6 0.8 1.0

fine fraction

5/19/92

coarse fraction

5/19/92

fine fraction

5/26/92

coarse fraction

5/26/92
38 Hydraulic Characteristics and Nutrient Transport, Reedy Creek Improvement District, Orange County, Florida

relatively high concentration at the beginning of the 

second day, although at a lower concentration than at the 

beginning of the first day (fig. 27). These effects probably were the result of several factors: (1) straining effectiveness increased during basin loading due to 

accumulation of particulates in pore spaces, (2) a 

decrease in pore-water velocity during a daily flooding 

period as matric potential gradients became smaller, 

leading to an increase in sedimentation, straining, and 

colloid attachment, and (3) mineralization processes 

occurring between basin-loading events degraded straining capacity. The latter probably was a minor process 

between daily loading events, because nitrate concentrations in the upper 15 ft were not elevated on May 20, 

1992, after only a several-hour rest period following 

basin operation (fig. 28). However, based on elevated 

nitrate concentrations in water samples on May 19, 

1992, the longer rest periods (several weeks) between 

basin rotation allowed for significant mineralization 

(followed by nitrification) (fig. 28). On May 19, 1992, 

the first day of basin flooding following an extended rest 

period, organic nitrogen concentrations at a depth of 3 ft 

(fig. 27) were substantially higher than concentrations in 

reclaimed water and were slightly higher than concentrations in reclaimed water on the second day of flooding. 

This effect was probably caused by decomposition processes during the extended rest period, which contributed to creation of a more mobile form of organic 

Figure 27. Near-surface nitrate concentrations at basin 50 during May 1992 loading event.

2400 0400 0800 1200 1600 2000 2400 0400 0800 1200 1600 2000 2400

2

10

4

6

8

TOTAL NITRATE NITROGEN AS N,

IN MILLIGRAMS PER LITER

EXPLANATION

3-ft well

7-ft well

15-ft well

Reclaimed water

May 19, 1992 May 20, 1992

Loading

period

Loading

period

nitrogen that was subject to flushing upon basin reflooding. By the eighth day of basin operation (May 26, 

1992), the total organic nitrogen concentration in the soil 

water (sample collected at a depth of 3 ft after initial 

daily flooding) was 0.62 mg/L, slightly less than the 

0.78 mg/L concentration in reclaimed water.

Ammonium concentrations decreased by more 

than 50 percent in the upper 3 ft of the soil profile 

(fig. 29), probably the result of ammonium adsorption 

onto the soil clay fraction as suggested by other investigators (Lance and Whisler, 1972; Bouwer, 1974). Of 

the clay minerals, vermiculite has the greatest capacity 

to adsorb ammonium (Brady, 1984) and occurs in the 

soil of the RIB under investigation (table 2). Elevated 

concentrations of ammonia in the lower part of the 

surficial aquifer system were perhaps indicative of dissimilatory nitrate reduction to ammonia in this relatively anoxic zone. 

Transient nitrate “spikes” (constituent concentrations in infiltrating water that are higher than concentrations in applied reclaimed water) occurred in the 

upper 15 ft of the soil profile (figs. 28, 30, and 31) and 

sometimes exceeded 30 mg/L as N. This condition 

was caused by organic nitrogen retention during basin 

loading, followed by mineralization and nitrification 

in the pore water during basin rest. Additionally, 

ammonium adsorbed during loading could have been 

nitrified during rest periods, contributing to nitrate 
Nutrient Transport and Transformation Beneath a Rapid Infiltration Basin 39

spiking. The intervals between basin loading were 

ideal for both mineralization and nitrification reactions 

to proceed because the soils drained and became aerated. Nitrate, generated by mineralization and nitrification, was relatively immobile during these rest 

periods because the pore water was relatively static 

after draining to field capacity. However, upon basin 

re-flooding, the nitrate was mobilized, moving essentially as a conservative ion with the infiltrating 

reclaimed water, thereby generating a nitrate spike. 

The nitrate peak in the upper 15 ft of the soil profile 

was short-lived (a few hours) (fig. 28) and was subject 

to the dissipating action of dispersion and dilution 

with further downward and lateral movement of the 

infiltrating water. Nitrate spiking commonly has been 

observed during the operation of RIBs (Lance and 

Whisler, 1972; Bouwer and others, 1974; Carlson and 

Linstedt, 1982; Leach and Enfield, 1983; Diab and 

Shilo, 1988). Snapshot-type sampling of the saturated 

zone was made prior to basin flooding. The elevated 

concentrations of nitrate observed in the shallow saturated zone on the first day of both the February-March 

1992 and May 1992 events were not due to downward 

movement of the nitrate spike in the upper 15 ft of the 

profile. An explanation for the observed nitrate concentrations within the saturated zone is discussed later 

in this report.

Figure 28. Near-surface organic nitrogen concentrations at basin 50 during May 1992 loading 

event.

2400 0400 0800 1200 1600 2000 2400 0400 0800 1200 1600 2000 2400

May 19, 1992 May 20, 1992

0.2

1.0

0.4

0.6

0.8

TOTAL ORGANIC NITROGEN AS N,

IN MILLIGRAMS PER LITER

EXPLANATION

3-foot well

7-foot well

15-foot well

Reclaimed water

Loading

period

Loading

period

Figure 29. Profiles of ammonia concentration (fine fraction) 

at basin 50 during May 1992 loading event. Note: Coarse 

fraction concentrations of ammonia are negligible. Tracertype sampling used in upper 15 feets of profile. Snapshottype sampling used in lower profile.

0

90

10

20

30

40

50

60

70

80

DEPTH, IN FEET BELOW LAND SURFACE

0 0.15 0.05 0.10

AMMONIA (FINE FRACTION) AS N, IN MG/L

May 19

May 26

measurement

pre-loading

water table

5/18/92

Following cycle of:

1 week loading 

4 weeks dry
40 Hydraulic Characteristics and Nutrient Transport, Reedy Creek Improvement District, Orange County, Florida

The magnitude of the nitrate spike within the 

upper profile depended on the length of basin loading 

and resting. The long loading and resting periods (3 

and 20 weeks, respectively) before the February 25, 

1992, loading event produced nitrate spikes as high as 

33 mg/L as N. However, the short loading and resting 

periods (1 and 4 weeks, respectively) of May 1992 

produced nitrate spiking only as high as 9 mg/L as N. 

Longer flooding periods allowed more accumulation 

of organic nitrogen and ammonium, whereas the 

longer resting periods provided more time for the processes of mineralization and nitrification to proceed, 

producing a nitrate product. 

Following a rest period of several weeks, nitrate 

concentrations in the shallow saturated zone were elevated above concentrations in reclaimed water 

(figs. 30 and 31). Nitrate concentrations (prior to the 

February-March 1992 loading event) were as high as 

11 mg/L as N in the 45-ft well. This nitrate could have 

formed a second nitrate spike which was radially 

flushed away from the basin upon basin flooding. This 

effect probably was the result of mineralization and 

nitrification of organic nitrogen which accumulated in 

the saturated zone, particularly near the water table, 

during the previous loading event. Because the porewater velocity lessened as the infiltrating water 

reached the water table (fig. 21), the part of the particulate and colloidal organic nitrogen that passed 

through the unsaturated zone was prone to retention in 

the shallow saturated zone. Shallow saturated-zone 

organic nitrogen concentrations therefore increased 

over the course of basin loading (figs. 25 and 26). The 

effect of the reduction in pore-water velocity in the 

vicinity of the water table had a greater impact on the 

transport of the coarse fraction of organic nitrogen 

than on that of the fine fraction. Coarse-fraction 

organic nitrogen was preferentially retained in the 

shallow saturated zone disproportionate to the relative 

concentration of the coarse fraction in the reclaimed 

water (fig. 26). The retained organic nitrogen was subject to mineralization and nitrification in the relatively 

shallow saturated zone during basin rest periods. The 

aerobic conditions which enhance these processes 

likely exist in the shallow saturated zone. Smith and 

Figure 30. Profiles of nitrate concentration at basin 50 

during February-March 1992 loading event. Note: Tracertype sampling used in upper 15 feet of profile. Snapshottype sampling used in lower profile.

0 35 5 10 15 20 25 30

TOTAL NITRATE NITROGEN AS N, 

IN MILLIGRAMS PER LITER

0

90

10

20

30

40

50

60

70

80

DEPTH, IN FEET BELOW LAND SURFACE

2/25/92

3/3/92

3/10/92

measurement

pre-loading

water table

2/24/92

Following cycle of:

3 weeks loading 

20 weeks dry

Figure 31. Profiles of nitrate concentration at basin 50 

during May 1992 loading event. Note: Tracer-type sampling 

used in upper 15 feet of profile. Snapshot-type sampling 

used in lower profile.

0 35 5 10 15 20 25 30

TOTAL NITRATE NITROGEN AS N,

IN MILLIGRAMS PER LITER

0

90

10

20

30

40

50

60

70

80

5/19/92

5/26/92

DEPTH, IN FEET BELOW LAND SURFACE

measurement

pre-loading

water table

5/18/92

Following cycle of:

1 week loading 

4 weeks dry
Nutrient Transport and Transformation Beneath a Rapid Infiltration Basin 41

Duff (1988) observed dissolved oxygen concentrations 

greater than 0.6 mg/L down to a depth of about 8 ft 

below the water table in a shallow aquifer that was 

subject to reclaimed-water disposal from overlying 

infiltration beds. The relatively low concentrations of 

soil-solution organic nitrogen measured at the end of a 

flooding event in the shallow saturated zone (figs. 25 

and 26) did not fully account for the concentrations of 

nitrate measured in this zone following a rest period 

(figs. 30 and 31). This discrepancy could result from 

the fact that soil solution samples are only an indicator 

of the amount of organic nitrogen within the solid and 

liquid matrix. Most of the organic nitrogen present 

probably was attached to the solid phase and was not 

directly measured with a soil-solution sample.

After passage of the nitrate spike, nitrate concentrations within the unsaturated zone were roughly 

equal to those of the applied reclaimed water, indicating little denitrification within this zone. This lack of 

significant nitrate renovation within the unsaturated 

zone probably was due to the schedule of daily basin 

cycling through wet and dry periods (typically 

17 hours flooded and 7 hours dry). This schedule 

probably allows the soil to be reaerated daily, hindering the creation of a reducing environment needed for 

denitrification. Additionally, the method of reclaimed 

water application to RCID basins (fig. 3) likely provided substantial aeration to the applied water. Bouwer 

and others (1974) noted that negligible nitrate removal 

occurred if short, frequent flooding periods (2 days 

flooded and 5 to 10 days dry) were used; however, 

they discovered that longer periods of flooding and 

drying (10 days flooded and 2 weeks dry) produced 

overall nitrogen removal of about 30 percent. It is 

likely that longer flooding periods at the RCID RIBs 

also would lead to greater nitrate removal beneath the 

basins, particularly in the upper 15 ft of the subsurface 

where conditions probably alternated between saturated and unsaturated under the imposed loading 

schedule (fig. 19). However, longer flooding cycles 

also could lead to a greater accumulation of organic 

nitrogen and ammonium in the soil profile, possibly 

producing more pronounced nitrate spikes. Thus, 

basin management requires judgement regarding the 

importance of maximizing nitrogen removal as 

opposed to minimization of nitrogen spikes.

An additional factor that could limit denitrification under reducing conditions is the availability of 

electron donors, generally carbon. The carbon-tonitrate ratio in the applied reclaimed water averaged 

about 1.7. However, presence of substantial organic 

nitrogen, which is subject to conversion to nitrate, 

implies an “effective” carbon-to-nitrate ratio of 1.3. 

Both of these values are below the empirical minimum 

ratio of 2 suggested by Leach (1980) for complete 

denitrification. Lance and Whisler (1976) achieved 

nitrogen removal of 90 percent in column studies in 

which the applied reclaimed water was amended with 

dextrose to provide a carbon-to-nitrate ratio of 6. 

Sludge also has been shown to be an effective carbon 

source to stimulate denitrification (Lance, 1986). The 

low carbon-to-nitrate ratio of the reclaimed water also 

was indicative of a system preference for denitrification rather than dissimilatory nitrate reduction to 

ammonium.

Spatial variability in soil properties can lead to 

corresponding variability in nutrient concentrations. 

Variation in the three point-sampler clusters are shown 

in figure 32. Although the general form of the profiles 

were similar, variability was evident.

Biogeochemistry of Phosphorus

The principal forms of phosphorus in the subsurface environment are orthophosphate species (primarily H2PO4

-

 and HPO4

-2 in a pH range of 4 to 10), 

organic phosphorus, and condensed forms of phosphate. Organic phosphorus can exist in particulate, 

colloidal, or dissolved forms.

Mineralization reactions rely on microbes to 

decompose organic forms of phosphorus in a manner 

similar to that of organic nitrogen mineralization 

(Brady, 1984). Inorganic orthophosphate species are 

the products of these reactions.

The nitrogen-retarding mechanisms mentioned 

above--adsorption, colloid attachment, straining, and 

sedimentation--also are effective in retarding the transport of phosphorus. Additionally, pH-dependent precipitation reactions are important to phosphorus 

chemistry (fig. 33) (Brady, 1984). 

Above a pH of 6, orthophosphate precipitates 

from the soil solution as calcium phosphates (Brady, 

1984), and becomes progressively less soluble with 

increasing pH and time. Silicate minerals, such as 

kaolinite, have been shown to be effective in the fixation of orthophosphate in moderately acid conditions, 

although the actual mechanism is not completely 

understood. Chemical precipitation of orthophosphate 

by iron, aluminum, and manganese ions can occur in 

low pH (less than about 5.4) conditions as exemplified 

for the case of aluminum ion below:

Al+3 + H2PO4

-

 + 2H2O ÷ 2H+ + Al(OH)2H2PO4

(insoluble) 
42 Hydraulic Characteristics and Nutrient Transport, Reedy Creek Improvement District, Orange County, Florida

Adsorption of orthophosphate onto oxyhydroxides of iron, aluminum, and manganese can occur 

below a pH of 7.8. Similarly, organic phosphorus is 

retained by adsorption through orthophosphate groups 

to oxyhydroxides (Bohn and others, 1985, p. 194). 

Orthophosphate adsorption is exemplified below in 

which an orthophosphate ion undergoes anion 

exchange with a hydroxide ion:

Figure 32. Comparison of nitrogen and phosphorus concentrations in three point-sampler clusters at basin 50 

during initial basin flooding on February 25, 1992. Note: Tracer-type sampling used.

0 1.2 0.2 0.4 0.6 0.8 1.0

TOTAL PHOSPHORUS AS P,

IN MILLIGRAMS PER LITER

0

16

2

4

6

8

10

12

14

DEPTH, IN FEET BELOW LAND SURFACE

0 35 5 10 15 20 25 30

TOTAL NITRATE NITROGEN AS N,

IN MILLIGRAMS PER LITER

0

16

2

4

6

8

10

12

14

0.4 0.6 0.8 1.0 1.2 1.4

TOTAL ORGANIC NITROGEN AS N,

IN MILLIGRAMS PER LITER

0

16

2

4

6

8

10

12

14

cluster 1

cluster 2

cluster 3

measurement

Figure 33. Distribution of soil orthophosphate at various pH values (modified from Brady, 1984).

4 5 67 8

0

50

100

DISTRIBUTION, IN PERCENT

SOIL pH

Calcium phosphates

Soil solution

Adsorption by hydrous oxides 

of Fe, Al, and Mn

Precipitation

with Fe, Al, and Mn

Silicate

 reaction

Al(OH)3 + H2PO4

- ÷ Al(OH)2H2PO4 (insoluble) + 

OHWith “aging” of the adsorbed orthophosphate, 

the phosphorus penetrates deeper into the hydrous 

oxide by means of solid-state diffusion and forms a 

more stable bonding arrangement. In this process, the 

phosphorus becomes less available to the soil solution 
Nutrient Transport and Transformation Beneath a Rapid Infiltration Basin 43

and surface-adsorption sites are re-exposed, allowing 

additional phosphorus adsorption (Mansell and Selim, 

1981). The initial adsorption reaction is relatively fast 

compared to the following aging reaction (Syers and 

Iskandar, 1981). Because of the kinetic nature of 

phosphorus adsorption, phosphorus removal during 

rapid-water flow in sandy soils generally is less than 

estimates based on equilibrium batch sorption studies 

(Mansell and Selim, 1981).

Phosphorus Transport and Transformation 

Beneath Basin 50

Phosphorus concentrations were reduced about 

90 percent (from a concentration of about 0.25 mg/L 

to a concentration of about 0.02 mg/L) from concentrations in reclaimed water in the upper 15 ft of the soil 

profile (figs. 34, 35, and 36). Other investigators have 

reported similar phosphorus reductions during 

operation of RIBs: Bouwer and Rice (1984) reported a 

phosphorus removal of 93 percent, which was attributed to precipitation of calcium phosphates; Carlson 

and Linstedt (1982) reported 60 to 90 percent removal 

of phosphorus, which was attributed to adsorption. 

The abundance of iron and aluminum oxyhydroxides 

(table 3) in the unsaturated zone indicates that adsorption probably is an effective mechanism for phosphorus removal at the RCID RIBs. The possibility exists 

of future degradation in the ability of sub-basin soils to 

remove phosphorus. This condition will occur if rejuvenation of oxyhydroxide adsorption sites accompanying aging of adsorbed phosphorus proceeds at a rate 

sufficiently slow that adsorption site saturation is 

reached and phosphorus is therefore not retained by 

adsorption. The relatively high phosphorus concentrations that developed in the shallow saturated zone at a 

depth of 45 ft as basin loading continued (fig. 34 and 

35) probably indicated an accumulation of the small 

fraction of phosphorus that eludes “clean-up” within 

the unsaturated zone, which will be discussed later in 

this section.

Phosphorus concentrations in the upper 15 ft 

exhibited a temporal pattern in concentration (fig. 36) 

similar to that of organic nitrogen, probably due to 

Figure 34. Profiles of total phosphorus concentration at 

basin 50 during February-March 1992 loading event. Note: 

Tracer-type sampling used in upper 15 feet of profile. 

Snapshsot-type sampling used in lower profile.

0 1.0 0.2 0.4 0.6 0.8

0

90

10

20

30

40

50

60

70

80

DEPTH, IN FEET BELOW LAND SURFACE

2/25/92

3/3/92

3/10/92

measurement

pre-loading

water table

2/24/92

Following cycle of:

3 weeks loading 

20 weeks dry

TOTAL PHOSPHORUS AS P,

IN MILLIGRAMS PER LITER

Figure 35. Profiles of total phosphorus concentrations at 

basin 50 during May 1992 loading event. Note: Tracer-type 

sampling used in upper 15 feet of profile. Snapshot-type 

sampling used in lower profile.

0 1.0 0.2 0.4 0.6 0.8

TOTAL PHOSPHORUS AS P,

IN MILLIGRAMS PER LITER

0

90

10

20

30

40

50

60

70

80

5/19/92

5/26/92

measurement

pre-loading

DEPTH, IN FEET BELOW LAND SURFACE

water table

5/18/92

Following cycle of:

1 week loading 

4 weeks dry
44 Hydraulic Characteristics and Nutrient Transport, Reedy Creek Improvement District, Orange County, Florida

similar processes. As in the case of organic nitrogen, 

these processes could include variations in pore-water 

velocity, variations in straining effectiveness, and mineralization of the organic component during basin rest 

periods. Concentrations of phosphorus at a depth of 

3 ft were elevated above concentrations in reclaimed 

water on the first day of flooding following an 

extended rest period (figs. 34, 35, and 36). In a manner 

similar to the observed nitrate spiking, the magnitude 

of the phosphorus spike is affected by the length of 

basin loading and rest periods, as shown by a comparison of phosphorus profiles for the February-March 

1992 (following 3 weeks loading/20 weeks resting 

cycle) and May 1992 (following 1 week 

loading/4 weeks resting cycle) events (figs. 34 and 

35).These phosphorus spikes were limited to the 

coarser fraction of phosphorus (figs. 37 and 38). The 

fine-fraction orthophosphate, which was the dominant 

phosphorus constituent in the reclaimed water, was not 

a component of the spike, probably because of strong 

orthophosphate adsorption to immobile soil matrix. As 

with nitrogen, spatial variability was evident in 

phosphorus concentrations within the upper 15 ft of 

the profile, although the general form of the profiles 

was similar at each sampling cluster (fig. 32).

Although the reclaimed water was dominated by 

the finer fraction of orthophosphate, the coarser fraction of organic phosphorus dominated in the saturated 

2400 0400 0800 1200 1600 2000 2400 0400 0800 1200 1600 2000 2400

0

0.5

0.1

0.2

0.3

0.4

TOTAL PHOSPHORUS AS P,

IN MILLIGRAMS PER LITER

EXPLANATION

3-ft well

7-ft well

15-ft well

Reclaimed water

May 19, 1992 May 20, 1992

Loading

period

Loading

period

Figure 36. Near-surface phosphorus concentration at basin 50 during May 1992 loading event.

zone (figs. 37 and 38), indicating that orthophosphate 

was preferentially retained in the unsaturated zone. 

Coarse-fraction phosphorus concentrations in the shallow saturated zone increased during basin loading. 

This effect was analogous to that of organic nitrogen. 

The coarse-fraction phosphorus accumulated in the 

shallow saturated zone because of slacking pore-water 

velocity. The change in the nature of saturated-zone 

water samples following the commencement of basin 

loading (from clear to turbid, orange-colored) was 

indicative of movement of particulate- and colloidalsize oxyhydroxides of iron. However, the small quantities of orthophosphate measured below the water 

table indicate that particulate- or colloidal-facilitated 

transport of orthophosphate to the saturated zone was 

negligible (fig. 37). In contrast to the case of nitrogen 

in which basin rest periods allowed for mineralization 

and nitrification to produce nitrate spikes in the shallow saturated zone, phosphorus spikes in the saturated 

zone were not apparent following rest periods, even 

though mineralization of organic phosphorus probably 

occurred during this period. The elevated concentrations of phosphorus in the shallow saturated zone at 

the end of basin loading appeared to be immobilized 

during extended basin rest periods (figs. 34 and 35). 

Based on a measured pH range of 6.4 to 7.0 in the 

45-ft depth well during basin rest periods, these immobilization processes likely include adsorption reactions 
Summary and Conclusions 45

(in the hydrous oxide-rich zone above the pre-loading 

water table), fixation by silicate clays (probably 

kaolinite), and precipitation as calcium phosphates 

(fig. 33). The possibility of calcium phosphate precipitation was supported by an application of the chemical 

speciation program SOLMINEQ.88 (Kharaka and others, 1988) to a water sample from the shallow saturated zone at the beginning of a basin rest period (45-ft 

well sampled on March 10, 1992). The results of this 

analysis (G. F. Huff, USGS, written commun., 1995) 

indicate that the water sample was oversaturated with 

at least three calcium phosphate minerals: chlorapatite 

(Ca5(PO4)3Cl), flurapatite (Ca5(PO4)3F), and hydroxylapatite (Ca5(PO4)3(OH)). Although oversaturation 

does not prove the precipitation of calcium phosphate, 

it does establish the possibility of formation.

Figure 37. Profiles of fine- and coarse-fraction orthophosphate at basin 50 during May 1992 loading event.

Note: Tracer-type sampling used in upper 15 feet of profile. Snapshot-type sampling used in lower profile.

0

90

10

20

30

40

50

60

70

80

DEPTH, IN FEET BELOW LAND SURFACE

0 0.20 0.05 0.10 0.15

May 19

May 21

May 26

0

90

10

20

30

40

50

60

70

80

0 0.06 0.02 0.04

ORTHOPHOSPHATE

AS P, IN MILLIGRAMS PER LITER

measurement

pre-loading

water table

5/18/92

Following cycle of:

1 week loading 

4 weeks dry

(COARSE FRACTION) CONCENTRATION ORTHOPHOSPHATE

AS P, IN MILLIGRAMS PER LITER (COARSE FRACTION) CONCENTRATION

SUMMARY AND CONCLUSIONS

The Reedy Creek Improvement District disposed of about 7.5 million gallons per day of 

reclaimed water in 1992 through 85 1-acre rapid infiltration basins within a 1,000-acre site that consists of 

sandy soils in Orange County, Florida. The objective 

of this technique of reclaimed-water disposal is the 

further enhancement of water quality through the 

remedial action of chemical and physical processes on 

infiltrating water in the soil and aquifer. Nitrogen and 

phosphorus are the two constituents most likely to 

cause eutrophication in downstream surface-water 

bodies. In 1992, concentrations of total nitrogen and 

total phosphorus in reclaimed water were about 3 and 

0.25 milligrams per liter, respectively. Nitrogen within 
46 Hydraulic Characteristics and Nutrient Transport, Reedy Creek Improvement District, Orange County, Florida

the reclaimed water was primarily in the nitrate and 

organic forms. Phosphorus primarily occurred in 

orthophosphate and organic forms. Reclaimed water 

applied to basins moves through as much as 40 feet of 

unsaturated zone before entering the surficial aquifer 

system. Water from the surficial aquifer system moves 

laterally and discharges to wetland areas or moves 

downward through the intermediate confining unit 

into the Upper Floridan aquifer.

The U.S. Geological Survey conducted field 

experiments in 1992 at Reedy Creek Improvement 

District basin 50 to examine and better understand the 

hydraulic characteristics and nutrient transport and 

transformation beneath a rapid infiltration basin. The 

basin was flooded in a controlled fashion in FebruaryFigure 38. Profiles of fine- and coarse-fraction organic phosphorus at basin 50 during May loading event. 

Note: Tracer-type sampling used in upper 15 feet of profile. Snapshot-type sampling used in lower profile.

0

90

10

20

30

40

50

60

70

80

DEPTH, IN FEET BELOW LAND SURFACE

0 0.08 0.02 0.04 0.06

0

90

10

20

30

40

50

60

70

80

0 0.3 0.1 0.2

May 19

May 21

May 26

measurement

pre-loading

water table

5/18/92

Following cycle of:

1 week loading 

4 weeks dry

ORGANIC PHOSPHORUS (COARSE FRACTION)

CONCENTRATION AS P,

IN MILLIGRAMS PER LITER

ORGANIC PHOSPHORUS (COARSE FRACTION)

CONCENTRATION AS P,

IN MILLIGRAMS PER LITER

March and May 1992 and water samples were 

extracted from wells and point samplers to define evolution of reclaimed water as it percolates through the 

soil/aquifer system. Hydraulic and biogeochemical 

processes were postulated to explain the observed 

changes in nutrient chemistry with depth. During the 

February-March flooding event, the hydraulic 

response of the subsurface system was monitored. An 

unsaturated/saturated numerical flow model, using the 

numerical code VS2D, of the subsurface flow system 

was constructed and calibrated to approximate the 

observed hydraulic response. The model was used to 

provide a more complete description of the flow system than that available solely from field observations, 
Summary and Conclusions 47

as well as to predict system response to untested basinloading scenarios.

The basin infiltration rate was about 5.5 feet per 

day (averaged over daily flooding periods of about 

17 hours in length) throughout the 2-week period of 

basin loading during which about 10 million gallons of 

reclaimed water were applied. This flux was undiminished with time, indicating that surface clogging was 

not significant under the imposed loading and maintenance schedule. A lack of flux response to a change in 

ponded area was indicative of a largely vertical flow 

system in the shallow soil. Additionally, this phenomenon implies that a relatively fine-textured layer within 

the unsaturated zone is characterized by a field-saturated hydraulic conductivity equal to or greater than 

the overlying coarse-textured material, possibly 

caused by air entrapment within the latter. Air entrapment within the shallow soil was inferred based on 

field-estimated moisture content under basin flooding 

conditions and visual observations. The infiltration 

rate was controlled by the upper 20 feet of the soil profile, rather than by back-pressure from a ground-water 

mound. Because of the less-permeable soils above it, 

the relatively coarser-textured unsaturated zone 

(below a depth of 20 feet) was not thought to have 

become field-saturated from above by the infiltrating 

water. However, the lower part of this zone was saturated from below as a result of ground-water mounding. This coarse-textured layer was not subject to the 

effects of air-entrapment as evidenced by the relatively 

high value of specific yield (0.41).

The hydraulic characteristics (hydraulic conductivity, moisture content, and specific moisture capacity) were placed within the framework of a van 

Genuchten parameterization. Hydraulic parameters 

were inferred based on laboratory determination of 

moisture-characteristic curves for basin samples; soil 

core-based estimates of air entrapment effects; and 

field-observed tensiometric, water level, and textural 

data. The hydraulic properties of three subsurface layers were estimated: the shallow coarse zone (0 to 

7-feet depth), the fine-textured zone (7- to 20-feet 

depth), and a deeper coarse zone (below 20-feet 

depth). The effects of air entrapment were reflected in 

the hydraulic parameters of the uppermost layer, particularly in the relatively low value of field-saturated 

hydraulic conductivity (5.1 feet per day) and the 

discrepancy between field-saturation moisture content 

(0.28) and porosity (0.41). The coarser layers exhibited rapid desaturation under tension and lower residual moisture content (0.04 and 0.06) than the finetextured layer (0.13). The estimated radial and vertical 

values of saturated hydraulic conductivity for the 

deeper coarse zone are 150 and 45 feet per day, indicative of surficial aquifer system anisotropy.

About 1.5 days were required for the initial 

infiltration front to reach the water table following the 

commencement of basin loading on February 25, 

1992. Thereafter, ground-water mounding within the 

saturated zone began to develop and reached a maximum value of about 7 feet in the 35-foot depth observation well during the 2-week basin loading. Porewater velocity above the water table directly beneath 

the basin was relatively high—about 20 feet per day—

and predominantly vertical in direction. As infiltrating 

water reached the water table, pore-water velocity was 

estimated to have changed markedly in both magnitude (reduced to less than 10 feet per day) and direction (substantial radial component, particularly with 

increasing depth and radial distance). In the vicinity of 

the basin, flow in the deeper, saturated zone was determined to be relatively slow compared to the more vigorous flow in the shallow saturated zone. The large 

radial component of flow below the water table in the 

vicinity of the basin implies that reclaimed water 

moves preferentially in the shallow part of the saturated zone after reaching the water table. Therefore, 

there exists the possibility of some vertical stratification within the saturated zone, with recently-infiltrated 

reclaimed water overlying ambient water. This phenomenon has implications in regards to screen placement of monitoring wells. Water samples obtained 

from a well screened in the shallow part of the saturated zone are more likely representative of recently 

infiltrated, non-ambient water than are samples 

obtained from a well screened in a deeper part of the 

surficial aquifer system.

Rainfall, evapotranspiration, and leakage of 

water from the surficial aquifer system to the Upper 

Floridan aquifer were negligible compared to the basin 

loading rate at the space and time scales (in the vicinity 

of a single basin during and for a few weeks following 

basin loading) of consequence to the field experiment. 

However, at the scale of the 1,000-acre disposal facility, 

these terms are not negligible compared to the total 

basin loading of 101 inches per year (1992) over the 

facility area. Leakage is estimated to have been 4 to 

9 inches per year prior to basin loading and is expected 

to have increased (1.3 inches per year per foot of 

increase in the head difference between the surficial 
48 Hydraulic Characteristics and Nutrient Transport, Reedy Creek Improvement District, Orange County, Florida

aquifer system and the Upper Floridan aquifer) following the commencement of loading in 1991. Rainfall and 

evapotranspiration annual averages are 50 inches per 

year and 30 to 48 inches per year, respectively.

The flow model indicates that infiltration capacity at basin 50 will be unaffected by a small (less than 

10 ft) increase in background water-table altitude. 

However, rises in background water-table altitude of 

15 and 20 feet produced a reduction in the infiltration 

capacity of basin 50 of 8 and 25 percent, respectively. 

The altitude of the base of the dominant impeding 

layer is expected to have an important role in basin 

response to changing background water-table altitude. 

A relatively deep impeding layer allows only a small 

interval in which the ground-water mound can develop 

before intersecting the base of the impeding layer and 

creating a “back pressure” reduction of hydraulic gradient and, therefore, infiltration rate.

Variation of ponded depth within basins does not 

offer much promise for increasing basin infiltration 

capacity. Model simulations indicate that increasing the 

ponded depth from 4 to 12 inches and from 4 to 

24 inches will increase basin infiltration capacity by 

less than 6 and 11 percent, respectively. This minor 

response is the result of the small ponded depth (which 

serves as the system control on infiltration) relative to 

the thickness of the upper 20 feet of the subsurface profile.

The flow model indicates that basin 50 infiltration capacity would not be diminished, even after 

40 days of continuous loading (based on an assumption that the lack of basin disking/drying during the 

40-day period would have a negligible effect on basin 

infiltration). Therefore, a basin-loading strategy that 

relies on long, uninterrupted flooding offers the possibility of inducing a more anaerobic environment conducive to denitrification while maintaining reclaimedwater disposal capacity.

Transient, elevated concentrations of nitrate (as 

high as 33 milligrams per liter as N) were observed at 

the leading edge of the front of infiltrating water and 

in the shallow, saturated zone following a prolonged 

rest period. This phenomenon probably was the result 

of mineralization and nitrification of organic nitrogen 

retained within the subsurface during prior basinloading events. The organic nitrogen was retained in 

the shallow soil (due to adsorption/straining) and the 

shallow saturated zone (due to deposition under slacking pore-water velocity). The magnitude of the nitrate 

spikes seemed to be influenced by the scheduling of 

basin loading. Short loading (1 week) and resting 

(4 weeks) cycles reduced this effect (maximum concentration about 9 milligrams per liter as N) as compared to long loading (3 weeks) and resting 

(20 weeks) cycles (maximum concentration about 

33 milligrams per liter as N). Removal of nitrogen by 

denitrification from the percolating reclaimed water 

appeared to be minimal in the vicinity of the basin, 

probably as a result of the lack of reducing conditions 

and low amounts of organic carbon. Longer flooding 

periods could create reducing conditions favorable for 

nitrogen removal from the system, but would probably 

lead to more pronounced nitrate spiking.

Phosphorus concentrations were reduced about 

90 percent from concentrations in reclaimed water in 

the upper 15 ft of the soil profile, probably due to 

adsorption onto abundant iron and aluminum oxyhydroxides. However, some phosphorus—predominantly 

the coarse, organic fraction—was carried to the water 

table by the relatively high-velocity pore water of the 

unsaturated flow system. Phosphorus that reached the 

water table accumulated under slacking pore-water 

velocity, but was immobilized by adsorption or precipitation reactions during basin rest periods.

Selected References

American Society for Testing and Materials, 1985, Standard 

test method for classification of soils for engineering 

purposes, D 2487-83, 1985 Annual Book of ASTM 

Standards, 04.08:395-408: American Society for 

Testing and Materials, Philadelphia.

Backus, D.A., Ryan, J.N., Groher, D.M., MacFarlane, J.K., 

and Gschwend, P.M., 1993, Sampling colloids and 

colloid-associated contaminants in ground water: 

Ground Water, v. 31, no. 3, p. 466-479.

Bennet, E.R. and Leach, L.E., 1983, Field studies of rapid 

infiltration treatment of primary effluent, in

Proceedings of the ASCE National Conference on 

Environmental Engineering, July, 1983, p. 41-49. 

Bohn, H.L., McNeal, B.L., O’Connor, G.A., 1985, Soil 

Chemistry (2nd ed.): John Wiley and Sons, New York, 

341 p.

Bouwer, Herman, 1974, Infiltration-percolation systems, in

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