Simulation of Subsurface Storage and 

Recovery of Treated Effluent Injected

in a Saline Aquifer, St. Petersburg, Florida

By Dann K. Yobbi

U.S. GEOLOGICAL SURVEY

Water-Resources investigations report 95-4271

Prepared in cooperation with the

CITY OF ST. PETERSBURG and the

SOUTHWEST FLORIDA WATER MANAGEMENT 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....................................................................................................................................................... 4

Hydrogeologic framework ................................................................................................................................................ 4

Surficial Aquifer .......................................................................................................................................................... 4

Intermediate Confining Unit........................................................................................................................................ 4

Floridan Aquifer System ............................................................................................................................................. 4

Zone A ............................................................................................................................................................... 6

Semiconfining Unit Between Zones A and B ................................................................................................... 6

Water quality.................................................................................................................................................... 6

Native Ground Water................................................................................................................................................... 7

Effluent ........................................................................................................................................................................ 7

Geochemical Interactions ............................................................................................................................................ 10

Simulation of Subsurface Storage and Recovery of Treated Effluent .............................................................................. 10

Numerical Model......................................................................................................................................................... 11

Design of Base Model ................................................................................................................................................. 11

Model Parameters ........................................................................................................................................................ 11

Effects of Hydrologic Parameter Variations on Recovery Efficiency ......................................................................... 14

Resident Fluid Dissolved-Solids Concentration of Zone A .............................................................................. 14

Dispersivity........................................................................................................................................................ 14

Porosity.............................................................................................................................................................. 16

Horizontal and Vertical Permeability of Zone A............................................................................................... 16

Ratio of Horizontal to Vertical Permeability ..................................................................................................... 16

Effects of Operational Factors on Recovery Efficiency .............................................................................................. 17

Duration of Injection and Withdrawal Cycle..................................................................................................... 17

Rate of Injection and Withdrawal...................................................................................................................... 18

Partially Penetrating Well.................................................................................................................................. 18

Summary and Conclusions................................................................................................................................................ 19

Selected References........................................................................................................................................................... 21

Appendix. Listing of Model Input File ............................................................................................................................. 24

FIGURES

1. Location of study area, injection sites, and lines of vertical sections...................................................................... 3

2. Generalized stratigraphic and hydrogeologic section, Pinellas County .................................................................. 5

3. Vertical sections A-A’ and B-B’ showing chloride and dissolved-solids concentration in ground water............... 7

4. Chemical composition of water at the city of St. Petersburg’s water reclamation facilities

(A) selected wells open to zone A, and (B) effluent ............................................................................................... 8

5. Radial-model grid, boundary conditions, and model layering ................................................................................ 12

6-9. Graphs showing:

6. Relation between recovery efficiency and variations in selected model parameters....................................... 15

7. Recovery efficiencies for five successive 60-, 180-, and 365-day injection/withdrawal 

cycles at 1.0 million gallons per day................................................................................................................ 17

8. Recovery efficiencies for five successive 1-year cycles of 121 days of injection, 91 days of 

withdrawal, 92 days of injection, and 61 days of withdrawal at rates of 0.5, 1.0, and 2.0 million 

gallons per day ................................................................................................................................................. 18

9. Recovery efficiencies after five cycles for various injection/withdrawal rates and operational schemes....... 18
IV Contents

TABLES

1. Chemical composition and saturation indices of ground water from zone A and effluent at the 

city of St.Petersburg water reclamation facilities ................................................................................................... 9

2. Selected saturation indices from PHREEQE model results .................................................................................. 10

3. Fluid and matrix properties assumed for simulation .............................................................................................. 13

CONVERSION FACTORS, VERTICAL DATUM, ABBREVIATIONS, AND ACRONYMS

Equation for temperature conversion between degrees Celsius (oC) and degrees Fahrenheit (oF): oC = 5/9 x (oF-32) oF = (9/5 oC) + 32

Sea level: In this report, “sea level” refers to the National Geodetic Vertical Datum of 1929 (NGVD 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.”

ADDITIONAL ABBREVIATIONS

ACRONYMS:

Multiply inch-pound unit By To obtain

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

foot squared (ft2) 0.0929 meter squared

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

foot squared per day (ft2/d) 0.0920 square meter

foot squared per pound (ft2/lb) 1.007x10-6 square meter per Newton

inch squared per day (in2/lb) 1.45x10-4 square meter per Newton

pounds per square inch (lb/in2) 6.895 kilopascal

pounds per cubic foot (lb/ft3) 0.0160 gram per cubic centmeter

million gallons per day (Mgal/d) 0.4381 cubic meter per second

mg/L milligrams per liter

αT transverse dispersivity

αL longitudinal dispersivity

 kh horizontal permeability

 kv vertical permeability

AIF aquifer influence function

DS dissolved solids

HST3D U.S. Geological Survey Three Dimensional Heat- and Solute-Transport 

computer program

PHREEQE U.S. Geological Survey equilibrium speciation computer program for 

geochemical calculations

SSMF scaled-solute mass fraction

SSR subsurface storage and recovery

SWFWMD Southwest Florida Water Management District

SI saturation index

USGS U.S. Geological Survey

WATEQF U.S. Geological Survey equilibrium speciation computer program for natural 

waters

WRF water reclamation facilities
Abstract 1

Simulation of Subsurface Storage and Recovery of 

Treated Effluent Injected in a Saline Aquifer, 

St. Petersburg, Florida

By Dann K. Yobbi

Abstract

The potential for subsurface storage and 

recovery of treated effluent into the uppermost 

producing zone (zone A) of the Upper Floridan 

aquifer in St. Petersburg, Florida, is being studied 

by the U.S. Geological Survey, in cooperation 

with the city of St. Petersburg and the Southwest 

Florida Water Management District. A measure 

of the success of this practice is the recovery 

efficiency, or the quantity of water relative to the 

quantity injected, that can be recovered before the 

water that is withdrawn fails to meet water-quality 

standards. The feasibility of this practice will 

depend upon the ability of the injected zone to 

receive, store, and discharge the injected fluid.

A cylindrical model of ground-water flow 

and solute transport, incorporating available data 

on aquifer properties and water quality, was 

developed to determine the relation of recovery 

efficiency to various aquifer and fluid properties 

that could prevail in the study area. The reference 

case for testing was a base model considered 

representative of the saline aquifer underlying St. 

Petersburg. Parameter variations in the tests 

represent possible variations in aquifer conditions 

in the area. The model also was used to study the 

effect of various cyclic injection and withdrawal 

schemes on the recovery efficiency of the well 

and aquifer system.

A base simulation assuming 15 days of 

injection of effluent at a rate of 1.0 million gallons 

per day and 15 days of withdrawal at a rate of 1.0 

million gallons per day was used as reference to 

compare changes in various hydraulic and 

chemical parameters on recovery efficiency. A 

recovery efficiency of 20 percent was estimated 

for the base simulation. For practical ranges of 

hydraulic and fluid properties that could prevail in 

the study area, the model analysis indicates that 

(1) the greater the density contrast between 

injected and resident formation water, the lower 

the recovery efficiency, (2) recovery efficiency 

decreases significantly as dispersion increases, (3) 

high formation permeability favors low recovery 

efficiencies, and (4) porosity and anisotropy have 

little effect on recovery efficiencies. In several 

hypothetical tests, the recovery efficiency 

fluctuated between about 4 and 76 percent.

The sensitivity of recovery efficiency to 

variations in the rate and duration of injection 

(0.25, 0.50, 1.0, and 2.0 million gallons per day) 

and withdrawal cycles (60, 180, and 365 days) 

was determined. For a given operational scheme, 

recovery efficiency increased as the injection and 

withdrawal rate is increased. Model results 

indicate that recovery efficiencies of between 

about 23 and 37 percent can be obtained for 

different subsurface storage and recovery 

schemes. Five successive injection, storage, and 

recovery cycles can increase the recovery 

efficiency to about 46 to 62 pecent. There is a 

larger rate of increase at smaller rates than at 

larger rates. Over the range of variables studied, 

recovery efficiency improved with successive 

cycles, increasing rapidly during initial cycles 

then more slowly at later cycles.
2 Simulation of Subsurface Storage and Recovery of Treated Effluent Injected in a Saline Aquifer, St. Petersburg, Florida

The operation of a single well used for 

subsurface storage and recovery appears to be 

technically feasible under moderately favorable 

conditions; however, the recovery efficiency is 

highly dependent upon local physical and 

operational parameters. A combination of 

hydraulic, chemical, and operational parameters 

that minimize dispersion and buoyancy flow, 

maximizes recovery efficiency. Recovery 

efficiency was optimal where resident formation 

water density and permeabilities were relatively 

similar and low.

INTRODUCTION

The city of St. Petersburg owns and operates 

one of the largest urban reclaimed-wastewater reuse 

systems in the world. Currently (1995), approximately 21 Mgal/d of advanced secondary-treated 

effluent is piped from the city’s four water reclamation 

facilities (WRF) through a 260-mile irrigation system 

to water residential and commercial properties. 

Demand for reclaimed water is seasonal. There is a 

deficiency of effluent for irrigation in the dry months 

and an excess in the wet months, during which the 

excess is disposed of through deep underground injection wells. The injected water from these deep permeable zones cannot be effectively recovered because the 

disposal zone is in a highly fractured dolomite saturated with saltwater (Hickey and Ehrlich, 1984). One 

solution, known as subsurface storage and recovery 

(SSR), to this dilemma involves the injection of effluent into shallow, less transmissive formations of low 

or moderate salinity, by one or more wells, for storage 

during wet periods when demand is low. The injected 

water would be withdrawn later for irrgation use during dry periods when demand is high. SSR is especially appropriate for areas like St. Petersburg where 

there is: (1) seasonal variations in water supply and 

demand, and (2) availability of moderately permeable 

aquifers near the surface that contain brackish water.

The feasibility of a SSR system will depend upon 

the ability of the injected zone to receive, store, and discharge the injected effluent. A measure of the success 

of a SSR system is the quantity of water that can be 

recovered relative to the quantity injected, or recovery 

efficiency. Recovery efficiency, usually expressed as a 

percentage per cycle of injection, storage, and recovery, 

is defined as the volume of water recovered before the 

withdrawn water fails to meet some prescribed chemical standard. In this report, the standard has been taken 

to be when the water that is withdrawn exceeds a dissolved solids (DS) concentration of 1,500 mg/L.

Two primary physical processes limit the recoverability of the injected water: (1) mixing by advection and hydrodynamic dispersion, and (2) density 

stratification. Mixing and dispersion creates a diffused zone between the injected and formation waters 

as a result of molecular diffusion and mechanical dispersion. Density stratification describes the tendency 

for the lighter fluid to rise above the denser fluid. 

Unfavorable physical and solute properties of the 

aquifer system can strengthen these deleterious processes and reduce the recoverability of injected water.

An assessment of the potential recovey efficiency for the aquifer system underlying St. Petersburg needs to be made before a commitment of 

resources is made for operational testing at a specific 

site. This requires a semi-quantitative understanding 

of the dependence of recovery efficiencies on hydrogeologic and chemical characteristics of the aquifer 

system and on system designs and management 

parameters. A cost-effective alternative to field testing is simulation modeling, in which many combinations of conditions can be investigated with relatively 

inexpensive computer simulations.

In 1994, the U.S. Geological Survey (USGS), in 

cooperation with the city of St. Petersburg and the 

Southwest Florida Water Management District (SWFWMD), began a model-based investigation of the 

hydrogeologic and operational aspects of subsurface 

injection and storage of effluent, and possible future 

retrieval for nonpotable use. The study area is the city 

of St. Petersburg, located on the southern tip of Pinellas County (fig. 1).

Throughout this report, statements are made 

concerning the salinity of water. The terminology 

used to describe the salinity is modified slightly from a 

USGS classification system of water based on dissolved solids (Heath, 1989, table 2, p. 65), as follows: 

Classification

Dissolved-solids

concentration (milligrams 

per liter)

Percent

seawater

Freshwater <500 <1.5

Slightly saline 

(brackish water) 500 to 3,000 1.5 to 8.6

Moderately saline 

(brackish water) 3,000 to 10,000 8.6 to 29

Very saline (saltwater) 10,000 to 35,000 29 to 100

Brine >35,000 >100
Introduction 3

0 2 4 MILES

0 2 4 KILOMETERS

LOCATION OF

STUDY AREA

McKAY CREEK

B

A’

SOUTH CROSS

BAYOU

NORTHWEST 

ST. PETERSBURG

BEAR

CREEK

ALBERT

WHITTED

A

SOUTHWEST

ST. PETERSBURG

NORTHEAST

ST.PETERSBURG

B’

Tampa

Bay

Gulf

of

Mexico

EXPLANATION

CITY OF ST. PETERSBURG

INJECTION SITES

WELL SITES

LINE OF VERTICAL A A’ SECTION IN FIGURE 3 

PINELLAS COUNTY

28°52′30′′

45′

27°37′30°

82°52′30° 82°45′ 82°37′30′′

Figure 1. Location of study area, injection sites, and lines of vertical sections.
4 Simulation of Subsurface Storage and Recovery of Treated Effluent Injected in a Saline Aquifer, St. Petersburg, Florida

Freshwater meets the DS concentration limit for potable water. Slightly saline water is nonpotable, but it 

may be suitable for irrigation. Moderately saline 

water is suitable for desalinization, and very saline 

water and brine are considered unusable.

Purpose and Scope

This report presents the results of a study conducted to provide a better understanding of the potential for SSR in a moderately transmissive, slightly to 

moderately saline-water bearing, limestone aquifer 

underlying St. Petersburg. Specifically, the objectives 

are (1) define the hydrogeologic framework and quality of ground water, (2) determine the geochemical 

interactions associated with SSR, and (3) illustrate the 

application of a solute-transport model as a tool for 

understanding the relative importance of individual 

physical properties and operational variables on recovery efficiency. The scope of the study is limited to a 

preliminary analysis because no data exist for comparison with model results.

Information on the hydrogeologic framework, 

hydrologic properties, matrix properties, and water 

chemistry of the aquifer system was compiled from 

files of the USGS and from engineering reports of the 

city’s four WRF injection well sites. 

The possibilities for geochemical interactions 

between the injected and aquifer waters were evaluated using aqueous speciation (WATEQF) and chemical mass-balance (PHREEQE) models (Plummer and 

others, 1976; Parkhurst and others, 1980). The models 

were used to assess the potential for both precipitation 

and dissolution of certain carbonate minerals.

The three dimensional heat-and solute-transport 

model (HST3D), developed by Kipp (1987), was used 

to simulate a typical injection/recovery well system. 

Injection and withdrawal in a prototype aquifer were 

simulated and recovery efficiency was computed. The 

model was used to assess the relation of recovery efficiency to various aquifer and fluid properties and to 

simulate recovery efficiencies under selected hypothetical operational schemes.

HYDROGEOLOGIC FRAMEWORK

The hydrogeologic units beneath the study area 

consist of a thick sequence of carbonate rock overlain 

by clastic deposits. The sediments are subdivided into a 

sequence of discrete lithologic units that form a layered 

sequence of two aquifers and one confining unit. The 

framework includes the unconfined, surficial aquifer 

and the confined, Floridan aquifer system (fig. 2). The 

units are separated by the intermediate confining unit.

Surficial Aquifer 

The surficial aquifer is the uppermost water 

bearing formation. It consists of undifferentiated 

sands and clays that vary in composition both laterally 

and vertically. The aquifer is a source of recharge to 

the Floridan aquifer system and is mainly used as a 

source of water for lawn irrigation. The surficial aquifer is generally less than 30 ft thick and is generally 

saturated to within 10 ft of land surface during dry 

weather. During wet weather, the water table in the 

surficial aquifer is generally close to land surface. The 

surficial aquifer has a horizontal hydraulic conductivity ranging from 13 to 33 ft/d and a vertical hydraulic 

conductivity ranging from 0.3 to 13 ft/d (Cherry and 

Brown; 1974; Sinclair, 1974; Hutchinson and Stewart, 

1978).

Intermediate Confining Unit

The intermediate confining unit of low-permeability lies between the surficial aquifer and the Floridan aquifer system. The unit coincides with the 

undifferentiated Arcadia Formation of the Hawthorn 

Group and consists of a clastic and carbonate 

sequence. The carbonates in the sequence are generally underlain and overlained by clays and marls of 

relatively low permeability (Hickey, 1981). Thickness 

of the intermediate confining unit averages about 90 ft 

and ranges from about 50 to 140 ft. The confining unit 

is highly variable both spatially and vertically. A wide 

range of hydraulic characteristics occurs within the 

unit due to lithologic heterogeneity within the unit. 

Vertical hydraulic conductivity for the intermediate 

confining unit as reported by Sinclair (1974) and 

Black, Crow, and Eidsness, Inc. (1978) ranges from 

1.3 x 10-4 to 6.9 x 10-3 ft/d.

Floridan Aquifer System

The Floridan aquifer system is a thick, regionally 

extensive sequence of Tertiary age carbonates. Miller 

(1986) defines the Floridan aquifer system to include 

the Upper Floridan aquifer, the middle confining unit, 

and the Lower Floridan aquifer. In the study area, the 
Hydrogeologic Framework 5

Figure 2. Generalized stratigraphic and hydrogeologic section, Pinellas County. (Modified from Hickey, 1982.) DEPTH BELOW SEA LEVEL, IN FEET HAWTHORN GROUP

TAMPA MEMBER

OF THE

ARCADIA

FORMATION

D

C

B

A

SERIES

MIOCENE

OLIGOCENE

EOCENE

OLDSMAR

LIMESTONE

AVON PARK

FORMATION

OCALA

LIMESTONE

SUWANNEE

LIMESTONE

UPPER

FLORIDAN

AQUIFER

LIMESTONE

DOLOMITE

GYPSIFEROUS

LIMESTONE

AND

DOLOMITE

MIDDLE

CONFINING

UNIT

FLORIDAN AQUIFER SYSTEM

LOWER

FLORIDAN

AQUIFER

LAYER 1

LAYER 2

LAYER 3 SAND, CLAY, AND

LIMESTONE

MODEL

UNITS

PERMEABLE

ZONE

HYDROGEOLOGIC LITHOLOGY

UNIT

GEOLOGIC

UNIT

0

250

500

750

1,000

1,250

1,500

1,750

2,000

2,250

2,500

2,750

3,000

3,250

PLEISTOCENE UNDIFFERENTIATED

SANDS AND CLAYS

SURFICIAL AQUIFER

INTERMEDIATE

CONFINING

UNIT

UNDIFFERENTIATED

ARCADIA 

FORMATION
6 Simulation of Subsurface Storage and Recovery of Treated Effluent Injected in a Saline Aquifer, St. Petersburg, Florida

middle confining unit and the Lower Floridan aquifer 

contain water comparable to seawater; the base of the 

freshwater flow system is limited to the Upper Floridan 

aquifer. The geologic formations that make up the 

Upper Floridan aquifer consists of limestone and dolomite rocks of, in ascending order, the Avon Park Formation, the Ocala and Suwannee Limestones, and the 

Tampa Member of the Arcadia Formation of the Hawthorn Group. The top of the Upper Floridan aquifer is 

defined as the first occurrence of a persistent carbonate 

sequence; the base of the Upper Floridan aquifer is 

defined as the first occurrence of interbedded gypsum 

in the carbonates below a dark-brown, microcrystalline 

dolomite in the Avon Park Formation (Hickey, 1982). 

In St. Petersburg, the Upper Floridan aquifer is subdivided into four permeable zones separated by semiconfining units (Hickey, 1982). The zones are 

alphabetically labeled with increasing depth from A to 

D (fig. 2). This study is concerned only with the uppermost producing zone of the Upper Floridan aquifer 

(zone A) , the overlying confining unit, and the underlying semiconfining unit between zone A and B, and 

further discussion will be restricted to these units.

Zone A

Zone A comprises the Tampa Member of the 

Arcadia Formation of the Hawthorn Group and the 

uppermost part of the Suwannee Limestone. Zone A 

is the shallowest and freshest of the producing zones 

and is the only potential receiving zone for SSR in St. 

Petersburg. Thickness of zone A averages about 180 

ft and ranges from about 115 to 245 ft. Thickness varies from site to site, occurring with no regional pattern 

(Hickey, 1982). Geraghty and Miller, Inc. (1976), 

Robertson and Mallory (1977), Black, Crow, and 

Eidsness, Inc. (1978), Hickey (1982), and Brown and 

Associates (1986) reported values of transmissivity for 

zone A that ranged from 2.2 x 104

 to 3.5 x 104 ft2/d 

and values of storativity that range from 4 x10-4 to 8 x 

10-4. Average values of porosity of zone A , estimated 

from geophysical logs, are 26, 32, and 41 percent 

(Hickey, 1982). 

Semiconfining Unit Between Zones A and B

Underlying zone A (the uppermost permeable 

zone) is the first of a series of poorly transmissive carbonate rock that acts as semiconfining units that separate permeable zones. The existence of the 

semiconfining units was determined during previous 

studies by Hickey (1982). The semiconfining unit 

below zone A is in the limestone unit of the Suwannee 

Limestone. Thickness of the semiconfining unit averages about 150 ft and ranges from about 125 to about 

170 ft. Average porosity of the semiconfining unit 

from geophysical logs is about 30 percent and ranges 

from 22 to 36 percent (Hickey, 1982). The semiconfining unit is considered a nonproducing zone in St. 

Petersburg.

Vertical hydraulic conductivity of the semiconfining unit between zones A and B, determined from 

laboratory and aquifer tests reported by Hickey 

(1982), ranges from 1.3 x 10-3 to 2.6 ft/d. The value of 

1.3 x 10-3 ft/d was determined from a laboratory test of 

a limestone core from the northeast St. Petersburg 

WRF where a higher percentage of clay occurred in 

the limestone matrix than at other sites in St. Petersburg. Test results at southwest St. Petersburg WRF 

and South Cross Bayou indicate that a value of less 

than 0.1 ft/d probably is not representative of general 

conditions (Hickey, 1982). Comparison of aquifer and 

laboratory tests indicate that the plausible range of 

vertical hydraulic conductivity applicable to semiconfining units that do not contain clay would be in the 

range of 0.1 to 1.0 ft/d (Hickey, 1982). For the semiconfining beds that contain clay between zones A and 

B, the vertical hydraulic conductivity could be on the 

order of 10-2 to 10-3 ft/d.

Data from Black, Crow, and Eidsness (1978) 

collected at the southwest St. Petersburg WRF injection site, show little difference between vertical and 

horizontal hydraulic conductivity in the semiconfining 

units. This data agrees with Stewart’s (1966) data 

from equivalent strata in Polk County, which also 

show little or no difference between vertical and horizontal conductivity.

WATER QUALITY

Freshwater, saltwater, and a mixture of the two 

occur in the rocks underlying St. Petersburg. Saline 

ground water predominates; freshwater typically 

occurs as a thin layer above the saltwater. Freshwater 

is maintained by recharge from rainfall that infiltrates 

the rocks underlying St. Petersburg. Sources of saline 

ground water are probably the Gulf of Mexico, Tampa 

Bay, and residual seawater from the geologic past 

(Hickey, 1982). Hydrodynamic dispersion or mechanical mixing is the dominant process associated with 

the spreading of the solute.
Water Quality 7

Middle confining unit of

the Floridan aquifer

72

150

680

16,000

19,000

20,000

21,000

ZONE A

ZONE B

ZONE C

ZONE D

Surficial aquifer

Intermediate

confining unit

Upper Floridan aquifer

ZONE A

ZONE B

ZONE C

ZONE D

Middle confining

unit of the

Floridan aquifer

B’

25

52

450

19,000

20,000

20,000

21,000

35

91

91

1,200

1,200

20,000

20,000

20,000

20,000

20,000

20,000

19,000

19,000

35

B

Vertical scale greatly

exaggerated

Vertical scale greatly

exaggerated

0 5 10 MILES

0 5 10 KILOMETERS

0 5 10 MILES

0 5 10 KILOMETERS

EXPLANATION

20,000 CHLORIDE CONCENTRATION IN MILLIGRAMS PER LITER

DISSOLVED-SOLIDS CONCENTRATION LESS THAN 10,000

MILLIGRAMS PER LITER

DISSOLVED-SOLIDS CONCENTRATION EQUAL TO OR GREATER

THAN 10,000 MILLIGRAMS PER LITER

1,400

4,700

16,000

19,000

19,000

20,000

ALTITUDE, IN FEET ABOVE AND BELOW SEA LEVEL

Semiconfining unit

Semiconfining unit

Semiconfining unit

Semiconfining unit

Semiconfining unit

Semiconfining unit

20,000

Surficial

aquifer Upper Floridan aquifer

Intermediate confining

unit

30

850

2,200

3,700

13,000

20,000

20,000

21,000

250

SEA LEVEL

250

500

750

1,000

1,250

1,500

250

SEA LEVEL

250

500

750

1,000

1,250

1,500

SOUTHWEST

ST. PETERSBURG

BEAR

CREEK

McKAY

CREEK

McKAY

CREEK

SOUTH CROSS

BAYOU

NORTHEAST

ST. PETERSBURG

A A’

Figure 3. Vertical sections A-A’ and B-B’ showing chloride and dissolvedsolids concentration in ground water. (Modified from Hickey, 1982.)

Native Ground Water

Water within the Upper Floridan 

aquifer is more mineralized than water 

from the surficial aquifer. Figure 3 shows 

vertical and lateral profiles of chloride 

and dissolved solid concentrations in 

ground water along lines A-A’ and B-B’. 

Chloride concentrations in water from the 

surficial aquifer range from 30 to 1,400 

mg/L. Chloride concentrations in water 

from zone A range from 91 to 16,000 

mg/L and in water from zones B through 

D from 13,000 to 21,000 mg/L. Water in 

zones C and D is comparable to seawater.

The salinity of water within rocks 

comprising zone A varies both vertically 

and areally. Freshwater, where present, is 

typically in a thin lens at the top of the 

zone. Dissolved-solids concentrations 

increase with depth. Ground water near 

the coastal margins of St. Petersburg have 

higher concentrations in both the upper 

and lower parts of zone A. The vertical 

variation in water quality suggests that 

water in zone A is density stratified 

(Hickey, 1982).

Water-quality characteristics of 

zone A at each of the city’s four WRF’s 

are shown in table 1 (CH2M Hill, 1993). 

Most water in zone A is slightly to moderately saline with DS concentrations 

ranging from 2,520 to 32,900 mg/L. 

Waters of zone A generally are a sodium 

choride type (fig. 4).

Effluent

Effluent at each of the city’s four 

WRF’s is aerated, filtered, and chlorinated reclaimed wastewater. The water 

quality of the effluent is variable due to 

differences in the influent (incoming 

waste) and the treatment processes at each 

WRF (table 1). The average pH is about 

7 and the effluent contains low concentrations of suspended solids (less than 3.0 

mg/L). The DS and chloride concentrations range between about 400 to 

900 mg/L and about 125 to 350 mg/L, 

respectively. All waters are a sodium 

chloride type (fig. 4).
8 Simulation of Subsurface Storage and Recovery of Treated Effluent Injected in a Saline Aquifer, St. Petersburg, Florida

CHLORIDE

CHLORIDE

CHLORIDE

CHLORIDE

CHLORIDE

CHLORIDE

CHLORIDE

CHLORIDE

BICARBONATE

BICARBONATE

BICARBONATE

BICARBONATE

BICARBONATE

BICARBONATE

BICARBONATE

BICARBONATE

SULFATE

SULFATE

SULFATE

SULFATE

SULFATE

SULFATE

SULFATE

SULFATE

CATIONS ANIONS

450 400 350 300 250 200 150 100 50 0 50 100 150 200 250 300 350 400 450 500

NORTHEAST B-9

NORTHWEST MW-1

SOUTHWEST B-5

ALBERT WHITTED MW-3

SODIUM AND

POTASSIUM

CALCIUM

MAGNESIUM

SODIUM AND POTASSIUM

CALCIUM

MAGNESIUM

SODIUM AND POTASSIUM

CALCIUM

MAGNESIUM

SODIUM AND POTASSIUM

CALCIUM

MAGNESIUM

SODIUM AND POTASSIUM

CALCIUM

MAGNESIUM

SODIUM AND POTASSIUM

CALCIUM

MAGNESIUM

SODIUM AND POTASSIUM

CALCIUM

MAGNESIUM

SODIUM AND POTASSIUM

CALCIUM

MAGNESIUM

MILLEQUIVALENTS PER LITER

MILLEQUIVALENTS PER LITER

CATIONS ANIONS

10 5 0 5 10

NORTHEAST EFFLUENT

NORTHWEST EFFLUENT

SOUTHWEST EFFLUENT

ALBERT WHITTED EFFLUENT

B. EFFLUENT

A. WELLS OPEN TO ZONE A

Figure 4. Chemical composition of water at the city of St. Petersburg’s water reclamation facilities. (A) selected wells 

open to zone A, and (B) effluent.
W

aterQuality9

Table 1. Chemical composition and saturation indices of ground water from zone A and effluent at the city of St.Petersburg wa[<, less than, --, no data. Chemical analysis expressed in milligrams per liter except as noted; oC, degrees Celsius; μS/cm, microsiemens per centimeter]

Ground water Effluen

Northeast

well B-91

1Chemical analyses from Hickey, 1982, table 5.

Northwest

well MW-12

2Chemical analyses from CH2M Hill, 1993, table 5.2.

Southwest

well B-52

Albert 

Whitted well 

MW-33

3Chemical analyses from Post, Buckley, Schuh & Jernigan, Inc., 1989, appendix 6.

Northeast Northwest 

Parameter Temperature (oC) 26.2 24.7 24.9 -- 25.2 25.5  Specific conductance (μS/cm) 44,800 17,800 22,400 4,400 1,613 1,610  Dissolved solids 32,900 11,600 11,400 2,520 888 882  pH (units) 6.7 7.03 6.88 7.17 6.92 7.15  Calcium (Ca) 1,100 623 653 229 90.3 80.8  Magnesium (Mg) 980 296 318 84.8 21.6 22.7  Sodium (Na) 9,300 2,640 2,350 222 197 187  Potassium (K) 200 34 29 10.5 9.8 12  Chloride (Cl) 16,000 6,040 5,540 1,050 345 349  Sulfate (SO4) 2,200 938 1,060 546 63.7 71.5  Bicarbonate (HCO3) 160 202 188 161 211 198  Fluoride (F) 0.0 0.29 0.35 0.23 0.64 0.60  Total nitrogen (N) 0.61 3.3 3.4 -- 12.9 11.6  Total phosphorous (P) 0.26 <0.04 <0.01 -- 1.66 0.64 Saturation indices4

4Log [ion activity proudct/equilibrium constant].

 Calcite -0.258 0.154 -0.003 0.001 -0.351 -0.339  Dolomite -0.146 0.361 0.061 -0.063 -0.966 -0.942  Anhydrite -0.441 -0.725 -0.647 -0.958 -2.005 -2.229  Aragonite -0.400 0.010 -0.147 -0.143 -0.495 -0.482  Gypsum -0.251 -0.509 -0.432 -0.741 -1.789 -2.020 
10 Simulation of Subsurface Storage and Recovery of Treated Effluent Injected in a Saline Aquifer, St. Petersburg, Florida

Geochemical Interactions

The injection of water into an aquifer having 

water of different quality than the injected water may 

change local conditions, resulting in geochemical 

interactions that lead to changes in water-quality and 

aquifer properties. Mixing of two waters of different 

chemical character may produce chemical reactions 

that could precipitate minerals, thus, reducing transmissivity and porosity; dissolve minerals, thus, 

increasing transmissivity and porosity, or the injected 

water may be in equilibrium with the aquifer minerals 

and native water, resulting in no chemical reactions.

The potential for a chemical reaction can be 

determined by calculating the chemical equilibrium of 

the injected and aquifer waters. The equilibrium state 

of the water with respect to a mineral phase can be 

determined by calculating a saturation index (SI) using 

analytical data. The SI is defined as the logarithm of 

the ratio of the ion activity product to the mineral equilibrium constant at a given temperature. If SI is equal 

to zero (0), the water is in equilibrium with the mineral; if it is less than 0, the water is undersaturated and 

if present, that mineral will dissolve; if the ratio is 

greater than 0, the water is supersaturated and mineral 

precipitation would be possible. Because of uncertainties in analytical and thermodynamic constants, a 

water with a SI value between 0.1 and -0.1 is assumed 

to represent saturation conditions or is in equilibrium 

with respect to the mineral phase (Swancar and Hutchinson, 1992).

The SI for the resident native waters and for the 

effluent, with respect to minerals commonly found in 

carbonate aquifers, was calculated using the computer 

program WATEQF (Plummer and others, 1976) and 

values are given in table 1. The values in table 1 show 

that most of the native ground water is saturated to 

supersaturated with respect to calcite and dolomite, 

and undersaturated with respect to the remaining minerals. The ground-water sample for the northeast 

WRF was undersaturated with respect to calcite and 

dolomite. This water has a very high concentration of 

DS and probably is residual seawater. The effluent is 

undersaturated with respect to all minerals; therefore, 

these waters apparently could dissolve minerals 

including calcite and dolomite, the principal minerals 

that compose carbonate aquifers.

The mass-balance model PHREEQE (Parkhurst 

and others, 1980) was used to simulate the geochemical reactions that may occur when effluent is mixed 

with native ground waters. PHREEQE is a generalized aqueous speciation, solubility, mass-transfer computer code that can simulate several types of reactions 

including the mixing of two waters of different chemical compositions. Modeled reactions were evaluated 

based on the equilibrium state of a 50 percent mix of 

the two water types with respect to selected mineral 

phases. Two simulations were made: The first model 

run simulated effluent from the Albert Whitted WRF 

entering the aquifer monitored by well MW-3. The 

second model run simulated effluent from the southwest WRF entering the aquifer monitored by well B-5. 

Results of the simulations indicate that the mixed 

effluent and ground water would be undersaturated 

with respect to all minerals (table 2). These data infer 

that the injected effluent, upon entering the aquifer and 

mixing with the aquifer waters, may dissolve calcite 

and dolomite. This process could produce a more 

porous limestone with increased transmissivity.

SIMULATION OF SUBSURFACE 

STORAGE AND RECOVERY OF 

TREATED EFFLUENT 

A finite-difference cylindrical-flow model of 

ground-water movement and solute transport was used 

as a simulation tool to determine the recovery efficiency of a prototype SSR system for a moderately 

transmissive, slightly to moderately saline aquifer, 

underlying St. Petersburg. Specific questions to be 

answered are: (1) In what quantitative fashion will 

the physical and operational variables influence recovery efficiency?, and (2) Will recovery efficiency 

improve when SSR is conducted as a multiple-cycle 

operation? 

Table 2. Selected saturation indices from PHREEQE model 

results 

Saturation indices Model 1 1

1 Effluent at Albert Whitted facility entered the aquifer monitored by 

well MW-3.

Model 22

2Effluent at Southwest facility entered the aquifer monitored by well 

B-5.

 Calcite -0.459 -0.122

 Dolomite -1.185 -0.068

 Anhydrite -2.324 -1.005

 Aragonite -0.645 -0.309

 Gypsum -2.116 -0.794
Simulation of Subsurface Storage and Recovery of Treated Effluent 11

To answer these questions, HST3D was used to 

simulate a well in a hypothetical hydrogeologic section representing St. Petersburg. The analysis implemented a conceptual modeling approach (Merritt, 

1985), in comparison to a calibration and predictive 

approach (Merritt,1994), as no data exist for comparison with model results. This type of analysis permitted many combinations of conditions that could be 

investigated using parameters generally similar to 

those of the aquifer system underlying St. Petersburg. 

Sensitivity tests were used to examine the response of 

the model to a range of hydrogeologic and fluid properties and the effect of various injection/withdrawal 

schemes on the recovery efficiency of the SSR system.

Numerical Model

The HST3D model (Kipp, 1987) is a computer 

program written in FORTRAN-77 that simulates variable density fluid movement and transport of either dissolved substances or energy in the subsurface. The 

model, as used in this report, solves for two interdependent variables: pressure and mass-fractional concentration in cylindrically symetrical coordinates under 

isothermal conditions. Backward-in-space and backward-in-time finite-difference equations were used for 

solution of ground-water flow and the solute-transport 

equations in the numerical model. The reader is 

referred to Kipp’s (1987) report for a complete discussion of the model code and numerical methods.

Design of Base Model

A base model used as a standard for comparisons for subsequent sensitivity analyses was designed 

to be representative of the slightly to moderately saline 

artesian limestone aquifer underlying St. Petersburg. 

Hydraulic and solute properties used in the model are 

representative of the city’s four WRF injection sites 

and, as such, do not represent any specific location in 

St. Petersburg. Data were obtained from previous 

studies and laboratory values reported in text books. 

To apply the HST3D model, the hydrologic and 

hydrogeologic characteristics of the aquifer system in 

St. Petersburg was simplified into the rectangular section in figure 5. The cylindrical-flow base model simulates the intermediate confining unit, permeable zone 

A, and the lower semiconfining unit. It consists of 26 

variable-width node spacings in the vertical direction 

and 88 variable-width node spacings in the radial 

direction. The model dimensions are 2,500 horizontal 

ft and 420 vertical ft. The vertical spacing ranges from 

10 to 50 ft. Radial spacing expands logarithmically 

from 0.14 ft at the well up to a maximum of 50 ft. The 

base model was used as the reference for sensitivity 

testing.

Several simplifying assumptions were made in 

the conceptualization and simulation of the flow system:

1. The aquifer system is homogeneous and isotropic 

in all directions,

2. Hydrostatic conditions initially prevail,

3. Regional horizontal and vertical flow is negligible,

4. A uniform native fluid density exists within each 

model layer, 

5. The water-quality profile is laterally homogeneous 

throughout the model area, 

6. The viscosities of the injected and native fluids are 

the same, and

7. Dispersivity is constant throughout each model 

layer.

Model Parameters

The strategy for the model analysis was to 

choose parameter values that, even though nonunique, 

would approximate conditions representative of the 

system so that the major processes affecting the mass 

fraction distribution during injection and withdrawal 

would be simulated. The parameters simulated in the 

model include boundary conditions, matrix properties, 

fluid properties, and well characteristics. A description of each model parameter follows:

1. Boundary conditions.--Boundary conditions are used to constrain the lateral and vertical 

extent of the simulated flow system providing a 

simplified representation of the flow and transport processes at the model limits. The top and 

bottom boundaries of the model are specified 

pressure boundaries because the producing zones 

are very permeable relative to the entire Floridan 

aquifer system (Knochenmus and Thompson, 

1991). Pressure on the upper boundary is set at 

12.967 lb/in2, equivalent to the pressure exerted 

from an overlying 30 ft column of freshwater 

presumed to exist in the overlying hypothetical 

surficial aquifer. Pressure on the lower boundary 

is 195.719 lb/in2

, equivalent to the pressure 

exerted from a 450-ft column of freshwater and 

saline water presumed to exist in the overlying 

formations. The inner radial edge is defined by 
12 Simulation of Subsurface Storage and Recovery of Treated Effluent Injected in a Saline Aquifer, St. Petersburg, Florida

the injection/production well in zone A and a noflow boundary condition for the confining units 

above and below the well. The outer radial edge 

is specified pressure for the single 30-day test 

cycle and hydrostatic pressures at each node are 

based on a column of water having DS concentration specified as a function with depth. Water 

that enters the section at a given node as a result 

of a pressure gradient at the boundary is of specified concentration; water that exits at such a node 

is of ambient aquifer concentration. For the multiple-test cycles, the outer radial edge is defined 

by a transient flow, aquifer influence function 

(AIF). AIF utilizes the Carter-Tracy approximation (Kipp, 1987) to compute flow rates between 

the inner gridded aquifer region and an infinite 

homogeneous outer region where aquifer properties are known only in a general sense. The 

radius of the inner region was set at 2,500 ft and 

the outer region is modeled as an infinite cylinder with a height of 420 ft. Initial hydrostatic 

pressures throughout the model were set based 

on the boundary conditions.

2. Matrix properties.--Matrix properties 

are defined for each of the three hydrogeologic 

SPECIFIED PRESSURE BOUNDARY

SPECIFIED PRESSURE BOUNDARY

SPECIFIED PRESSURE BOUNDARY

OR

AQUIFER INFLUENCE FUNCTION BOUNDARY

0 500 1,000 1,500 2,000 2,500

DEPTH BELOW LAND SURFACE, IN FEET

INJECTION/WITHDRAWAL WELL NO FLOW NO FLOW

50

100

150

200

250

300

350

400

450

RADIAL DISTANCE FROM INJECTION/WITHDRAWAL WELL, IN FEET

LAYER 3

INTERMEDIATE CONFINING UNIT

LAYER 2

PERMEABLE ZONE A

LAYER 1

LOWER SEMICONFINING UNIT

Figure 5. Radial-model grid, boundary conditions, and model layering.

layers for the model in table 3. The porous 

media properties are intrinsic permeability, 

matrix compressibility, effective porosity, longitudinal dispersivity, and transverse dispersivity. 

Intrinsic permeability was calculated from 

hydraulic conductivity values reported by 

Hickey (1982), with conversion factors from 

Freeze and Cherry (1979, p. 29). Values of 

hydraulic conductivity and matrix compressibility were based on aquifer tests and laboratory 

tests of limestone samples collected during previous studies. The value of porosity used in this 

model, 0.3, is an estimate based on geophysical 

logs at the WRF injection sites in St. Petersburg. Longitudinal (αL) and transverse (αT) dispersivities of the system were set at 12.5 ft and 

2.5 ft, respectively. Even though the spatial gridding stability rule-of-thumb criteria recommended in Voss (1984, p. 232), suggest that αL

and αT be greater than one-fourth and one-tenth 

of the radial and vertical spacings, an acceptable 

solution was computed using an αT value equal 

to 2.5 ft.
Simulation of Subsurface Storage and Recovery of Treated Effluent 13

3. Fluid Properties.-- The fluid properties 

of the native formation water assumed for the 

simulations are listed in table 3 and include temperature, specific weight, viscosity, compressibility, molecular diffusivity, and DS expressed 

as scaled-solute mass fraction (SSMF). The 

model was simplified with the assumption of a 

vertically uniform initial DS concentration distribution assigned to each layer although a vertical 

salinity gradient has been documented. Isothermal conditions at 75oF were assumed to prevail. 

Fluid densities assigned to injected and native 

waters were based on the measured or estimated 

concentration of DS in each fluid. Viscosity of 

the injected and native formation waters vary 

with temperature and solute fraction. Because 

isothermal conditions were assumed to prevail 

and because the viscosity of freshwater and saltwater differ by only 0.06 centipose, viscosity 

was assumed invariant in the simulations. The 

assigned value of viscosity was held constant at 

0.8904 centipose (viscosity of pure water at 

Table 3. Fluid and matrix properties assumed for simulation

[

oF, degrees Fahrenheit; lb/ft3, pounds per cubic foot; in2/lb, inch squared 

per pound; ft2

/d, foot squared per day; ft/d, foot per day; ft2; foot squared]

Parameter

Lower semiconfining 

unit

Zone A

Intermediate 

confining 

unit

Fluid properties

 Temperature (o

F) 75.0 75.0 75.0

 Specific weight 

(lb/ft3)

63.177 62.468 62.241

 Viscosity (centipoise) 0.8904 0.8904 0.8904

 Fluid compressibility 

(in2/lb)

3.03E-6 3.03E-6 3.03E-6

 Molecular diffusivity 

(ft2/d)

9.30E-5 9.30E-5 9.30E-5

 Scaled mass fraction 1.0 0.25 0.025

Matrix properties 

 Hydraulic conductivity 

(ft/d)

1.0 167.0 8.0E-4

 Intrinsic permeability 

(ft2)

3.877E-12 6.475E-10 3.102E-15

 Porosity (unitless) 0.3 0.3 0.3

 Longitudinal dispersivity (feet)12.5 12.5 12.5

 Transverse dispersivity (feet)2.5 2.5 2.5

 Matrix compressibility 

(in2

/lb)

6.2E-6 6.2E-6 6.2 E-6

75oF). Compressibility of water was held constant at 3.03x10-6 in2

/lb (Freeze and Cherry, 

1979, p. 52), and molecular diffusivity of the solute in the porous media was set at 9.30x10-5 ft2/d 

(Kimbler and others, 1975). The model SSMF is 

a dimensionless relative solute-concentration 

term ranging in value from 0 to 1. Any fluids 

present within the aquifer system, or entering it 

in simulation exercises, are considered to be a 

mixture of the two fluids by the appropriate specification of SSMF values. SSMF = 0 was used 

to represent pure freshwater, and SSMF = 1 represents the most saline water residing within the 

aquifer system, that in the lower confining unit. 

The assigned densities of 62.241 lb/ft3 (SSMF = 

0) and 63.177 lb/ft3 (SSMF = 1) at 75oF and 

atmospheric pressure were obtained from a standard handbook (Chemical Rubber Company, 

1982). The SSMF values of the injected effluent, 

the water of the aquifer system, and water in 

mixtures of effluent and native formation waters 

are assigned based on their salinity relative to the 

two extremes. SSMF values of 0.025 (500 mg/L 

DS) and 0.25 (5,000 mg/L DS), representing the 

composite background water quality collected at 

the WRF injection sites, were assigned to the 

intermediate confining unit and zone A, respectively, based on the ratio of DS concentration to 

the estimate DS concentration of the lower confining unit. Measured injected-water DS concentration was about 700 mg/L and was assigned a 

SSMF value of 0.035.

4. Well information.--The well occurs at 

the first column of nodes and the open-hole interval of the well is defined by row numbers. Injection and withdrawal of effluent is through 18 

nodes distributed vertically which, when combined, represent permeable zone A (180-ft interval). Injection and withdrawal flow at the well 

bore is allocated over rows 5 to 23 by mobility 

factors that are based on cell position, relative 

hydraulic conductivity, and an element completion factor. An element completion factor of 0 

means the well is cased off from the aquifer in 

that element, whereas an element completion 
14 Simulation of Subsurface Storage and Recovery of Treated Effluent Injected in a Saline Aquifer, St. Petersburg, Florida

factor of 1 means equivalent flow across the cell 

length.

Effects of Hydrologic Parameter Variations 

on Recovery Efficiency

When the results of a simulation study are 

largely based upon assumed estimates of data rather 

than measured data, as in this case, determining the 

effect of parameter variations on simulation results is 

of particular interest. To learn which properties and 

conditions substantially affect recovery efficiency, a 

series of simulation runs were made that started with 

the base model. For each simulation run, the value of 

an individual model parameter was changed by an 

amount that it might reasonably be expected to vary 

from the value used in the base simulation, then noting 

the change in recovery efficiency as a result of the 

change. The base simulation used as a standard for 

comparison consisted of a single cycle of 15 days of 

injection of effluent at a rate of 1.0 Mgal/d at a temperature of 75o

F and a maximum of 15 days of withdrawal at a rate of 1.0 Mgal/d. Thus, one 

injection/withdrawal cycle is completed in 30 days. 

Withdrawal was assumed to end when the increasingly 

saline water exceeded the maximum DS concentration 

(1,500 mg/L) deemed acceptable for irrigation use. 

For the base model, a DS concentration of 1,500 mg/L 

was reached in 3 days. The recovery efficiency calculated for the base model single 30-day cycle simulation is 20.0 percent.

The effect of parameter changes on recovery 

efficiency is presented in the following sections. Discussion of individual parameters is ordered from most 

to least sensitive.

Resident Fluid Dissolved-Solids Concentration 

of Zone A

A series of sensitivity tests were used to show 

the relation between recovery efficiency and resident 

fluid DS concentration of the injection zone (zone A). 

Two resident fluid DS concentrations of zone A were 

selected for comparison with the 5,000 mg/L base 

value: (1) 2,300 mg/L, an arbitrary value representing 

water somewhat less saline than the composite base 

value, and (2) 20,000 mg/L, representing the most 

saline layer in the model. Relations were established 

for a range of αLand αT combinations. DS concentrations simulated for the base model were 500 mg/L for 

the intermediate confining unit, 5,000 mg/L for zone 

A, and 20,000 mg/L for the lower semiconfining unit.

Because of the lack of local field data to indicate 

the degree of dispersion in the carbonate system, three 

additional dispersion values (sets of longitudinal αL

and transverse αT dispersivities representing some 

hypothetical degree of dispersion) were chosen to 

compare to the dispersion used in the base model. The 

additional dispersion values used for analysis are summarized below:

Dispersion value set 1 is a low degree of dispersion in the radial flow direction and a low degree of 

interlayer dispersion. Value set 2 introduces an appreciable degree of dispersion in the radial flow direction. 

Value set 3 represents appreciable degrees of dispersion in the radial flow direction and between layers.

Figure 6a shows the decline of recovery efficiencies with increasing resident fluid DS concentration for the various dispersivity combinations. The 

larger the DS difference between the injected water 

and resident formation water, the less efficient the SSR 

process. Recovery efficiency was shown by the model 

to be significantly less in aquifers of high salinity than 

in slightly saline aquifers. With higher resident fluid 

DS concentration, less water within the zone of mixing 

is acceptable for withdrawal. Higher DS concentrations also make density stratification of the fluids 

more likely to occur at a given permeability level, 

decreasing recovery efficiency. Recovery efficiency 

was shown to be greater when the level of dispersive 

mixing decreased, or such as when a smaller volume 

of injected fluid combines with more saline native formation water. Thus, recovery efficiency is most promising when resident fluid DS concentrations are low, 

and least promising when DS concentrations are high.

Dispersivity

Dispersivity is a scale-dependent property of the 

porous medium that controls the mixing of injected 

and resident formation fluids at their interface. When 

dispersivities are increased, there is more mixing that 

results in a widening of the transition zone between 

the injectant and native formation waters.

Dispersion value set αL(ft) αT (ft)

1 4.0 1.0

2 25.0 1.0

3 25.0 5.0
Simulation of Subsurface Storage and Recovery of Treated Effluent 15

Dispersion is a deleterious process that can 

severely limit recovery efficiency. Merritt (1985) 

reported that recovery efficiency is limited when 

the upper limit of the concentration of some constituent considered acceptable in recovered water 

is less than half of the average concentration of the 

d.

0

90

10

20

30

40

50

60

70

80

RECOVERY EFFI

CIEN

CY, IN PER

CENT

0 20,000 0 5,000 10,000 15,000

RESIDENT FLUID 

DISSOLVED SOLIDS OF ZONE A,

IN MILLIGRAMS PER LITER

Ldisp=4.0, Tdisp=1.0

Ldisp=12.5, Tdisp=2.5

Ldisp=25.0, Tdisp=5.0

Ldisp=25.0, Tdisp=1.0

0

90

10

20

30

40

50

60

70

80

RECOVERY EFFICIENCY, IN PERCENT

0 50 10 20 30 40

LONGITUDINAL DISPERSIVITY, 

IN FEET

DS=2,300 MG/L

DS=5,000 MG/L

DS=20,000 MG/L

Tdisp=1.0

EXPLANATION

Ldisp=longitudinal dispersivity, in feet

Tdisp=transverse dispersivity, in feet

DS=resident fluid dissolved solids

(zone A), in milligrams per liter

MG/L=milligrams per liter

a. b.

0

90

10

20

30

40

50

60

70

80

RECOVERY EFFICIENCY, IN PERCENT

20 25 30 35

POROSITY, 

IN PERCENT

Ldisp=4.0, Tdisp=1.0

Ldisp=12.5, Tdisp=2.5

Ldisp=25.0, Tdisp=1.0

Ldisp=25.0, Tdisp=5.0

c.

0

90

10

20

30

40

50

60

70

80

RECOVERY EFFI

CIEN

CY, IN PER

CENT

0 51234

TRANSVERSE DISPERSIVITY, 

IN FEET

DS=20,000 MG/L

DS=2,300 MG/L

DS=5,000 MG/L

Ldisp=12.5

40

Figure 6. Relation between recovery efficiency and variations in selected model parameters.

injected and native formation waters. In the hypothetical test case posed in this analysis, the acceptable limit for DS concentration (1,500 mg/L) was 

equal to or less than the average of the injected 

water (700 mg/L) and the native water (2,300 to 

20,000 mg/L). 
16 Simulation of Subsurface Storage and Recovery of Treated Effluent Injected in a Saline Aquifer, St. Petersburg, Florida

Various values of dispersion in the direction of 

flow (longitudinal) and interlayer (transverse) were 

tested. Values of αL were varied between 4 and 50 ft 

with respect to 12.5 ft in the base model, and values of

αT were varied between 0.5 and 5.0 ft with respect to 

2.5 ft in the base model, when the rate and duration of 

injection and withdrawal was kept constant.

Figure 6b and c illustrates the results from the 

analysis for the three previously cited resident fluid 

DS concentrations (2,300, 5,000, and 20,000 mg/L). 

Results show the decline of recovery efficiency as the 

dispersivity value increases. A large decrease of 

recovery efficiency is shown as αL increases from low 

values of αL, but the rate of decrease is small at larger 

values of αL. Recovery efficiency was slightly 

affected by changes in αT at low ranges of the parameter but at higher ranges, recovery efficiency was relatively unaffected by its variation. Evidently, for every 

resident fluid TDS concentration, there are values of 

αL and αT above which recovery efficiency is significantly reduced

Porosity

Porosity of the injection zone (zone A) was 

tested at 0.2 and 0.4 to bracket the base model value of 

0.3. Permeability and porosity control the velocity of 

injectant flow and, hence, the rate of solute transport. 

The greater the porosity, the slower the solute front 

will move; thus, the longer the time it takes to replace 

the volume of native formation water in a given volume of aquifer and the greater the dispersive mixing. 

Low porosity has the opposite effect.

Figure 6d illustrates the variation of recovery 

efficiency due to porosity changes in relation to the 

four previously cited dispersivity combinations. 

Increasing porosity caused recovery efficiency to 

decrease slightly for each dispersion model. Recovery 

efficiency decreased significantly, however, as the 

level of dispersive mixing increased.

Horizontal and Vertical Permeability of Zone A

The permeability of the aquifer material and the 

density contrast of the injected and native water determines whether an appreciable level of density stratification or buoyancy flow can occur. Density 

stratification refers to the physical process in which 

less dense injected fluid rises and flows over the more 

dense fluid. Generally, low permeability reduces stratification and optimizes recovery efficiency.

Values of horizontal (kh) and vertical (kv) permeability were changed simultaneously by the same 

factor that ranged from 0.1 to 10, with all other parameters remaining the same. The ratio of kh to kv was 

held to 1. Results of the simulations are as follows:

When permeability was doubled or halved, 

recovery efficiency was not affected. The major effect 

of the variation at these permeability values was the 

wellhead pressure required at the specified injection/withdrawal rate. However, when permeability 

was made 10 times greater, recovery efficiency was 

reduced by 61.5 percent. This is because simulation of 

a larger permeability allowed for easier horizontal and 

vertical transport of the injected fluid, which resulted 

in greater stratification and buoyancy flow that prevented complete mixing of the injected water with the 

native water. Upon withdrawal, a greater percentage 

of native water was immediately available to the well.

Ratio of Horizontal to Vertical Permeability

If flow conditions vary with direction in a geologic formation, the formation is anisotropic and differences in horizontal (kh) and vertical permeability 

(kv) can influence fluid movement. The greater the 

anisotropy ratio (kh/kv), the easier the salt front moves 

along the axis with the larger permeability component 

which contains the injected water within the open 

interval of the injection well. In the base model, permeability is equal in all directions. Anisotropy in the 

model only can be simulated by varying the ratio of kh 

to kv. To test for the effects of anisotropy on model 

results the ratio of kh to kv was varied from an isotropic base condition (kh/kv = 1) to kh/kv equal to 100. 

Results of the simulations are as follows:

kh and kv

(multiplier)

Recovery efficiency

(percent)

0.5 19.8

1.0 20.0

2.0 19.5

5.0 15.7

10.0 7.7

kh/kv Recovery effciency

1 20.0 percent

10 21.9 percent

100 23.6 percent
Simulation of Subsurface Storage and Recovery of Treated Effluent 17

Simulation of different values of kh/kv had a 

small affect on computed recovery efficiencies. 

Increasing kh/kv caused recovery efficiency to increase 

slightly. This is because simulation of a larger kh/kv 

causes more lateral flow and inhibits upward movement of buoyant injectant.

Effects of Operational Factors on 

Recovery Efficiency

The success of a SSR system is determined by 

the quantity of injected water that can be recovered 

from an aquifer. The sensitivity analyses indicated the 

physical and chemical parameters that substantially 

affect recovery efficiency. In addition to these parameters, operational factors also will affect the recovery 

efficiency of the SSR system. These factors include 

the duration and rate of injection and withdrawal, plus 

well construction. A series of model simulations was 

performed to evaluate the response of the model to 

changes in these factors. The base values of hydraulic 

and fluid properties were used in all simulations. 

Modifications were made to the HST3D model to simulate the shutdown of the production well when the 

solute fraction of the withdrawn water reached 0.0745, 

corresponding to a DS concentration of 1,500 mg/L. 

The production well was shutoff until the scheduled 

beginning of the next simulation. 

Duration of Injection and Withdrawal Cycle

Two series of multicycle model simulation tests 

were used to illustrate the effects of variations in the 

duration of injection/withdrawal cycles on recovery 

efficiency. The first series of tests consisted of 3 simulations of five uniform successive cycles (60, 180, and 

365 days) of injection and withdrawal of 1.0 Mgal/d. 

The following schedules were simulated:

1. 30 days of injection followed by a maximum of 

30 days withdrawal

2. 90 days of injection followed by a maximum of 

90 days of withdrawal

3. 183 days of injection followed by a maximum of 

182 days of withdrawal

 Figure 7 shows the results of the first series of 

testing cycles for five successive 60-, 180-, and 365-

day cycles. The plot indicates that recovery efficiency 

improves with each successive cycle for the various 

operational schemes even if the volume of injectant 

does not increase. This is because of the continuous 

growth of the zone of dispersion (mixing zone) and the 

retarding effect of the zone of dispersion on the rate of 

gravitational segradation (Kumar and Klimbler, 1970). 

With each cycle, the zone of dispersion is greater due 

to mixing of the injectant with the residual water that 

was not completely recovered from the previous cycle. 

The graph also shows recovery efficiency increasing 

very rapidly in intitial cycles and then more slowly at 

later cycles for each operational schedule. After five 

cycles, recovery efficiencies ranged between 46 and 

52 percent. Recovery efficiencies at the end of the 

first cycle had ranged between 23 and 24 percent. 

Higher recovery efficienies were obtained for 

simulation tests when the duration of injection and 

recovery phases was shorter. This is expected because 

of the nature of the conceptual system in which migration of the solute particles across the low permeable 

units will reduce the recoverability for tests of longer 

duration (Quinones-Aponte and Wexler, 1995).

An operational SSR system would not function 

on uniform cycles but rather on irregular cycles based 

on seasonal surplus and demand. In the second series 

of tests, a hypothetical yearly schedule of injection and 

withdrawal was simulated. The schedule consisted of 

20

65

25

30

35

40

45

50

55

60

RECOVERY EFFICIEN

CY, IN PER

CENT

CYCLE NUMBER

1 2 3 4 5

60 Day

(30,30)

180 Day

(90,90)

365 Day

(183,182)

( ) is the injection withdrawal operational cycle,

in days

Figure 7. Recovery efficiencies for five successive 60-, 

180-, and 365-day injection/withdrawal cycles at 1.0 

million gallons per day. (Withdrawal in each cycle 

ceases when the dissolved-solids concentration of 

withdrawn water exceeds 1,500 milligrams per liter.)
18 Simulation of Subsurface Storage and Recovery of Treated Effluent Injected in a Saline Aquifer, St. Petersburg, Florida

five successive 1-year cycles of 4-months of injection, 

followed by a maximum of 3-months of withdrawal, 

followed by 3-months of injection, and finally followed by a maximum of 2 months of withdrawal. In 

St. Petersburg, the injection periods correspond to 

December through March and July through September 

(months when irrigation reuse is low), and the withdrawal periods might correspond to April through 

June and October through November (months when 

irrigation reuse is high).

Figure 8 shows the results of the second series 

of tests. The model conditions are similar to those previously described for the five successive 60-, 180-, 

and 365-day cycles. Recovery efficiencies range 

between 27 and 47 percent for the 0.5 Mgal/d simulations, 31 and 55 percent for the 1.0 Mgal/d simulations, and 37 and 62 percent for the 2.0 Mgal/d 

simulations. Results indicate that recovery efficiency 

approach an asymptote after several cycles, where for 

practical purposes, no improvement of recovey efficiency occurrs. For the 2.0 Mgal/d simulation the 

maximum recovery efficiency probably will be 

between 65 and 70 percent.

Rate of Injection and Withdrawal

The rate of injection and withdrawal for a fixed 

time period and for various operational schemes was 

varied from 0.25 and 2.0 Mgal/d to determine whether 

certain injection/withdrawal rates favor higher recovery efficiencies. Zero recovery efficiency would occur 

for a sufficiently small injection volume because of 

mixing. Results of the tests suggest that recovery efficiency is directly proportional to the injected flow volume. For a given operational scheme, recovery 

efficiency increases with higher volumes (fig. 9). 

There is a larger rate of increase of recovery efficiency 

at smaller volumes than at larger volumes. Recovery 

efficiencies after five cycles ranged between 50 and 

62 percent for an injection/withdrawal rate of 

2.0 Mgal/d and between 23 and 37 percent for an 

injection/withdrawal rate of 0.25 Mgal/d.

Partially Penetrating Well

Several well-completion designs were simulated 

to evaluate the effects of partial penetration. Operational wells may be open only to part of an aquifer; 

thus, it is of interest to compare the recovery 

RECOVERY EFFICIENCY, IN PER

CENT

1 234 5

CYCLE NUMBER

Q=2.0 Mgal/D

Q=1.0 Mgal/D

Q=0.5 Mgal/D

Q is injection and withdrawal rate

Mgal/D is million gallons per day

65

60

55

50

45

40

35

30

25

20

Figure 8. Recovery efficiencies for five successive 

1-year cycles of 121 days of injection, 91 days of 

withdrawal, 92 days of injection, and 61 days of 

withdrawal at rates of 0.5, 1.0, and 2.0 million gallons 

per day. (Withdrawal in each cycle ceases when the 

dissolved-solids concentration of withdrawn water 

exceeds 1,500 milligrams per liter.)

20

65

25

30

35

40

45

50

55

60

0 2.0 0.5 1.0 1.5

365 Days (121,91,92,61)

60 Days (30,30)

180 Days (90,90)

365 Days (183,182)

( ) is the injection withdrawal operational cycle,

in days

INJECTION / WITHDRAWAL RATES,

IN MILLION GALLONS PER DAY

RECOVERY EFFICIENCY, IN PERCENT

Figure 9. Recovery efficiencies after five cycles for 

various injection/withdrawal rates and operational 

schemes.
Summary and Conclusions 19

efficiency of a partially penetrating well with recovery 

efficiency of a well open to the full thickness. The 

recovery efficiency was expected to be higher in partially penetrating wells than for fully penetrating wells 

because a buffer zone of fresher water would exist 

between the pumped zone and the underlying water of 

poor quality. The simulated well penetrations ranged 

from 100 to 25 percent. The 25-percent penetration 

well is open to the upper 45 ft of layer 2. Results of 

the simulations are shown below:

Simulations show a slight increase in recovery 

efficiency with a decrease in penetration depth. Merritt (1985) reported similiar results and suggests that 

recovery efficiencies in stratified aquifers may not be 

significantly affected if the well is open only to part of 

the aquifer. However, to generalize these results, a 

more-detailed study focusing on this aspect (injection 

into different flow zones) is needed.

SUMMARY AND CONCLUSIONS

A model-based study of the confined saline 

aquifer system underlying St. Petersburg is being conducted to analyze the hydrogeologic and operational 

aspects of underground injection, storage, and withdrawal of advanced secondary treated effluent in a single representative well. The study is specifically 

aimed toward (1) defining the hydrogeologic framework and ground-water quality in the study area, (2) 

assessing the geochemical interactions associated with 

the injection of effluent, and (3) applying a densitydependent, ground-water flow and solute-transport 

model to understand the relative importance of physical and chemical properties and operational variables 

on recoverability of injected effluent.

The sediments underlying the study area form a 

layered sequence of two aquifers and one confining 

unit. The framework includes the unconfined, surficial 

aquifer, and the confined Upper Floridan aquifer. The 

units are separated by the intermediate confining unit.

The surficial aquifer is composed of a layer of 

clastic deposits that is generally less than 30 ft thick. 

The aquifer is a source of recharge to the Floridan 

aquifer system and is primarily used as a source for 

lawn irrigation.

The intermediate confining unit is composed of 

clastic and carbonate sediments of Miocene and 

younger age. The carbonates are generally underlain 

and overlained by clays of relatively low permeability. 

Thickness of the unit averages about 90 ft and ranges 

from about 50 to 140 ft. The vertical hydraulic conductivity of the unit ranges from about 1.3 x 10-4 to 

6.9 x 10-3 ft/d.

The Upper Floridan aquifer consists of a thick, 

regionally extensive sequence of Tertiary-aged carbonate rocks that comprise the following formations 

(in ascending order): Avon Park Formation, Ocala and 

Suwannee Limestones, and the Tampa Member of the 

Arcadia Formation which is part of the Hawthorn 

Group. The Upper Floridan aquifer underlying St. 

Petersburg contains four permeable zones separated by 

semconfining units. The zones are alphabetically 

labeled with increasing depth from A to D. The proposed receiving zone is within the uppermost part of 

the Upper Floridan aquifer (zone A) in the permeable 

limestone section of the Tampa Member of the Arcadia Formation of the Hawthorn Group and the Suwannee Limestone. Thickness of zone A averages about 

180 ft and ranges from about 115 to 245 ft. Transmissivity of zone A ranges from 2.2 x 104

 to 3.5 x 

104 ft2/d.

A semiconfining unit within the Suwannee 

Limestone is below the proposed injection zone and 

has a vertical hydraulic conductivity estimated to 

range from about 0.1 to 1.0 ft/d where the beds do not 

contain clay. Average thickness of this unit below the 

proposed injection zone is about 150 ft.

Limited fresh ground-water supplies exist in the 

Upper Floridan aquifer underlying St. Petersburg. 

Fresh ground water, where it is found, typically occurs 

as a thin layer within the uppermost permeable zone. 

Most water in the aquifer is saline with chloride concentration in the proposed injection zone ranging from 

91 to 16,000 mg/L.

The chemical contrast between the injection and 

resident formation waters may lead to chemical reactions. The mix of the two water types would be understaturated with respect with calcite and dolomite. This 

might lead to dissolution of calcite and dolomite producing a more porous limestone with increased transmissivity.

Well penetration

(percent)

Recovery efficiency

(percent)

 100 20.0

75 22.3

50 23.4

25 24.1
20 Simulation of Subsurface Storage and Recovery of Treated Effluent Injected in a Saline Aquifer, St. Petersburg, Florida

A numerical model of variable density groundwater flow and solute transport (HST3D) was used to 

evaluate the importance of parameter variations affecting the recovery of effluent stored in a saline aquifer 

underlying the study area. The analyses consisted of a 

sensitivity testing approach, as opposed to the more 

familiar site-specfic calibration and prediction objective of modeling. Cyclic injection in a prototype aquifer was simulated and recovery efficiencies were 

calculated. 

Sensitivity analyses were performed to determine changes in recovery efficiency when physical 

and chemical parameters of a base model are varied. 

The simulation cycle used as a standard for comparisons consisted of 15 days of injection of effluent at a 

rate of 1.0 Mgal/d and 15 days of withdrawal at a rate 

of 1.0 Mgal/d.

On the basis of computer simulations for hypothetical aquifer conditions, sensitivity tests for individual physical and chemical properties indicate:

1. The greater the density difference 

between the injected effluent and resident formation water, the lower the recovery efficiency during the first cycle. Higher salinity makes density 

stratification of the fluids more likely to occur at 

a given permeability level, thereby causing 

recovery efficiency to decrease.

2. Recovery efficiency decreases significantly as dispersivity increases. Dispersion is a 

deleterious process that can severely limit recovery efficiency.

3. Generally, high formation permeability 

causes poor recovery efficiencies. When the 

simulated permeability was increased by a factor 

of 10, recovery efficiency was reduced by 

61.5 percent. This is because simulation of a 

larger permeability allowed for easier transport 

of the injected fluid, which resulted in greater 

stratification and buoyancy flow.

4. When the injection well was represented 

as open to only part of the aquifer, simulations 

show little difference as when the well was open 

to the entire aquifer. 

5. For hypothetical conditions studied, 

porosity and anisotropy variations do not significantly alter recovery efficiencies.

The preliminary radial-flow and solute transport 

model also was used to examine the effects on recovery efficiency of hypothetical multicycle operations. 

Simulations consisted of five injection/withdrawal 

cycles of various duration and varying injection and 

withdrawal rates. Results of the simulations are:

1. Over the range of variables studied, the 

recovery efficiency per cycle increases with total 

number of cycles, provided that each recovery 

cycle phase ends when the DS concentration of 

withdrawn water reaches some prescribed value 

less than that of the more saline formation water. 

Thus, even though conditions may be such that 

recovery would be poor on the first cycle, the 

process should improve on subsequent cycles.

2. Recovery efficiencies increase rapidly 

during initial cycles and more slowly at later 

cycles. Recovery efficiencies may approach a 

maximum value after a number of cycles.

3. For a given operational scheme, recovery efficiencies increase with higher volumes of 

injected effluent. There is a larger rate of 

increase of recovery efficiency at small injection/withdrawal volumes than at larger injection/withdrawal volumes.

The operation of a single SSR well appears to be 

technically feasible under moderately favorable conditions. Model results indicate that recovery efficiencies 

from about 23 to 37 percent can be achieve for different SSR operational schemes. Five successive injection, storage, and recovery cycles and varying 

injection and recovery rates can increase the recovery 

efficiency to about 47 to 62 pecent. The recovery efficiency that will be attained, however, is highly dependent upon local physical and operational conditions. 

A combination of hydraulic, chemical, and operational 

parameters that minimize dispersion and buoyancy 

flow, maximizes recovery efficiency. The parameters 

that favor optimum recovery efficiencies are low aquifer water density and low to moderate permeabilities. 

SSR is most promising where native fluid density is 

low. The smaller the density contrast between resident formation water and injected water the better the 

recovery efficiency. A high injection rate and a small 

difference in density between the injected and resident 

formation waters should yield better results than a low 

injection rate and a large difference in density. Recoverability improves with successive cycles of injection, 

storage, and recovery even if initial recovery efficiency is low.

The results presented are preliminary and make 

up a part of the study in progress. Development of a 

fully three-dimensional variable density-ground-water 

flow and solute transport model is presently underway 
Selected References 21

to extend the analysis to multiwell systems for testing 

of large-scale injection. In a field application of the 

SSR process, a well field likely will be needed to handle the required inflow rates and wellhead pressures. 

SELECTED REFERENCES

Black, Crow, and Eidsness, Inc., 1978, Drilling and testing 

of the monitoring and injection wells at Southwest 

Wastewater Treatment Plant for the city of St. 

Petersburg, Florida: Consultant’s report in files of the 

city of St. Petersburg, Fla.

Brown, M.P., and Associates, 1986, Hydrological 

investigation for the development and management of 

the Floridan aquifer, city of Dunedin: Consultant’s 

report in the files of the city of Dunedin, Fla.

Chemical Rubber Company, 1982, CRC Handbook of 

chemistry and physics (63rd ed): Boca Raton, Fla., 

CRC Press.

Cherry, R.N., and Brown, D.P., 1974, Hydrogeologic 

aspects of a proposed sanitary landfill near Old Tampa 

Bay, Florida: Florida Bureau of Geology Report of 

Investigations no. 68, 25 p.

CH2M Hill, 1993, Environmental risk and geochemical 

analysis related to the city of St. Petersburg’s 

underground injection and monitoring system: 

Consultant’s report in the files of the city of St. 

Petersburg, Fla.

Freeze, R.A., and Cherry, J.A., 1979, Groundwater: 

Englewood, N.J., Prentice-Hall, 604 p.

Geraghty and Miller, Inc, 1976, Management of the water 

resources of the Pinellas-Anclote and northwest 

Hillsborough basin, west-central Florida: Consultant’s 

report in the files of the Southwest Florida Water 

Management District, Brooksville, Fla.

Heath, Ralph C., 1989, Basic ground-water hydrology: 

U.S. Geological Survey Water-Supply Paper 2220, 

84 p.

Hickey, J.J., 1981, Hydrogeology, estimated impact, and 

regional well monitoring of effects of subsurface 

wastewater injection, Tampa Bay area, Florida: U.S. 

Geological Survey Water-Resources Investigations 

Report 80-118, 40 p.

———1982, Hydrogeology and results of injection tests at 

waste-injection test sites in Pinellas County, Florida: 

U.S. Geological Survey Water-Supply Paper 2183, 

42 p.

Hickey,J.J., and Ehrlich, G.G., 1984, Subsurface injection 

of treated sewage into a saline-water aquifer at St. 

Petersburg, Florida--Water quality changes and 

potential for recovery of injected sewage: Ground 

Water, v. 22, no. 4, p. 397-405. 

Hutchinson, C.B., and Stewart, J.W., 1978, Geohydrologic 

evaluation of a landfill in a coastal area, St. Petersburg, 

Florida: U.S. Geological Survey Water-Resources 

Investigations 77-78, 40 p.

Kimbler, O.K., Kazman, R.G., and Whitehead, W.R., 1975, 

Cyclic storage of freshwater in saline aquifers: 

Louisiana Water Resources Institute Bulletin 10, 78 p., 

with appendixes.

Kipp, K.L., 1987, HST: A computer code for simulation of 

heat and solute transport in three-dimensional groundwater flow systems: U.S. Geological Survey WaterResources Investigations Report 86-4095, 519 p.

Knochenmus, L.A., and Thompson, T.H., 1991, 

Hydrogeology and simulated development of the 

brackish ground-water resources in Pinellas County, 

Florida: U.S. Geological Survey Water Resources 

Investigations Report 91-4026, 20 p.

Kumar, A., and Kimbler, O.K.,1970, Efffect of dispersion, 

gravitational segregation, and formation stratification 

on the recovery of freshwater stored in saline aquifers: 

Water Resources Research, vol. 6, no. 6., p. 1689-

1700.

Merritt, M.L., 1985, Subsurface storage of freshwater in 

south Florida: A digital model analysis of 

recoverability: U.S. Geological Survey Water-Supply 

Paper 2261, 44 p.

———1994, Tests of subsurface storage of freshwater at 

Hialeah, Dade County, Florida, and digital simulation 

of the salinity of recovered water: U.S. Geological 

Survey Open-File Report 93-155, 150 p.

Miller, J.A., 1986, Hydrogeologic framework of the 

Floridan aquifer system in Florida and in parts of 

Georgia, Alabama, and South Carolina: U.S. 

Geological Survey Professional Paper 1403-B, 91 

p., 33 pl.

Parkhurst, L.P., Thorstenson, D.C., and Plummer, L.N., 

1980, PHREEQE - A computer program for 

geochemical calculations: U.S. Geologicial WaterResources Investigations Report 80-96, 193 p.

Plummer, L.N., Jones, B.F.,and Truesdell, A.H., 1976, 

WATEQF--A FORTRAN IV version of WATEQ, a 

computer program for calculating chemical 

equilibrium of natural waters: U.S. Geological Survey 

Water-Resources Investigations Report 76-173, 70 p.

Post, Buckley, Schuh & Jernigan, Inc., 1989, Reverse 

osmosis pilot studies: Consultant’s report in the files 

of the West Coast Regional Water Supply Authority, 

Clearwater, Fla.
22 Simulation of Subsurface Storage and Recovery of Treated Effluent Injected in a Saline Aquifer, St. Petersburg, Florida

Quinones-Aponte, Vicente, and Wexler, Eliezer J., 1995, 

Preliminary assessement of injection, storage, and 

recovery of freshwater in the lower Hawthorn aquifer, 

Cape Coral, Florida: U.S. Geoolgical Survey WaterResources Investigations Report 94-4121, 102 p.

Robertson, A.F., and Mallory, M.J., 1977, A digital model 

of the Floridan aquifer north of Tampa, Florida: U.S. 

Geological Survey Water-Resources Investigations 

Report 77-64, 29 p.

Seaburn and Robertson, Inc., 1983, Results of hydrologic 

testing program for the city of Clearwater water supply 

investigation: Consultant’s report in the files of the 

city of Clearwater, Fla.

Sinclair, W.C., 1974, Hydrogeologic characteristics of the 

surficial aquifer in northwest Hillsborough County, 

Florida: Florida Bureau of Geology Information 

Circular no. 86, 98 p.

Stewart, H.G., Jr., 1966, Ground-water resources of Polk 

County, Florida: Florida Geological Survey Report of 

Investigations 44, 170 p.

Swancar, A., and Hutchinson, C.B., 1992, Chemical and 

isotopic composition and potential for contamination 

of water in the Upper Floridan aquifer, west-central 

Florida: U.S. Geological Survey Open-File Report 92-

47, 47 p.

Voss, C.I., 1984, A finite-element simulation model for 

saturated-unsaturated, fluid-density-dependent groundwater flow with energy transport or chemicallyreactive single-species solute transport: U.S. 

Geological Survey Water-Resources Investigations 

Report 84-4369, p. 54-55.
APPENDIX
24 Simulation of Subsurface Storage and Recovery of Treated Effluent Injected in a Saline Aquifer, St. Petersburg, Florida

APPENDIX: LISTING OF MODEL INPUT FILE

A sample input-data listing is provide for the single cycle base simulation of 15 days of injection at 

1.0 million gallons per day followed by 15 days of withdrawal at 1.0 million gallons per day. The listing 

contains 276 lines, of which 147 lines are comments that aid construction of the data file. Critical comments are keyed to input record descriptions of Kipp (1987). The following order generally is observed for 

data input: (1) fundamental and dimensioning information, (2) spatial geometry and mesh information, (3) 

fluid properties, (4) porous medium properties, (5) source information, (6) boundary condition information, (7) initial condition information, (8) calculation parameters, and (9) out specifications.
Appendix 25

SAMPLE INPUT FILE: INJECT 1 MGAL/D FOR 15 DAYS FOLLOWED BY 15 DAYS

 OF WITHDRAWAL AT 1 MGAL/D

C.....HST Data-Input Form

C.....UNRELEASE 2.0

C------------------------------------------------------------------------------

C.....Start of the data file

C.....Specification and dimensioning data - READ1

C.1.1 .. TITLE LINE 1

Permeable Zone A simulation for St.Pete--1 Mgal/d injection followed by 1 Mgal/d

C.1.2 .. TITLE LINE 2

withdrawal--3 layers--initial mass fraction zone A = 0.25 (5000 TDS-2375 Cl)

C.1.3 .. RESTRT[T/F],TIMRST

F 0.0

C.1.4 .. HEAT[T/F],SOLUTE[T/F],EEUNIT[T/F],CYLIND[T/F],SCALMF[T/F]

F T T T T

C.1.5 .. TMUNIT[I]

4

C.1.6 .. NX,NY,NZ,NHCN

88,1,26,0

C.1.7 .. NSBC,NFBC,NAIFC,NLBC,NHCBC,NWEL

1 0 0 0 0 1 

C.1.8 ..SLMETH[I]

1

C------------------------------------------------------------------------------

C.....Static data - READ2

C.....Coordinate geometry information

C..... Cylindrical coordinates

C.2.2B.1A .. R(1),R(NR),ARGRID[T/F];(O) - CYLIND [1.4]

1. 2500. F

C.2.2B.1B .. R(I);(O) - CYLIND [1.4] and NOT ARGRID [2.2B.1A]

 1.00 1.14 1.30 1.49 1.70 1.94 2.22 2.53 2.89 3.31

 3.78 4.31 4.92 5.62 6.42 7.33 8.38 9.57 10.93 12.48

 14.25 16.28 18.59 21.23 24.24 27.69 31.62 36.12 41.25 47.11

 53.80 61.44 70.17 80.14 91.52 104.53 119.38 136.34 155.71 177.83

 203.09 231.94 264.90 302.53 350. 400. 450. 500. 550. 600.

 650. 700. 750. 800. 850. 900. 950. 1000. 1050. 1100.

1150. 1200. 1250. 1300. 1350. 1400. 1450. 1500. 1550. 1600.

1650. 1700. 1750. 1800. 1850. 1900. 1950. 2000. 2050. 2100.

2150. 2200. 2250. 2300. 2350. 2400. 2450. 2500. 

C.2.2B.2 .. UNIGRZ[T/F];(O) - CYLIND [1.4]

F

C.2.2B.3B .. Z(K);(O) - NOT UNIGRZ [2.2B.3A],CYLIND [1.4]

 0. 50. 100. 130. 150. 160. 170. 180. 190. 200. 210. 220. 230. 240. 250

260. 270. 280. 290. 300. 310. 320. 330. 350. 380. 420.

C------------------------------------------------------------------------------

C.....Fluid property information

C.2.4.1 .. BP

3.03E-6

C.2.4.2 .. P0,T0,W0,DENF0

14.7 75.0 0 62.241

C.2.4.3 .. W1,DENF1;(O) - SOLUTE [1.4]

0.020 63.1772

C.2.5.1 .. VISFAC

-0.8904

C------------------------------------------------------------------------------

C.....Reference condition information

C.2.6.1 .. PAATM

14.7

C.2.6.2 .. P0H,T0H

0,75.0

C------------------------------------------------------------------------------
26 Simulation of Subsurface Storage and Recovery of Treated Effluent Injected in a Saline Aquifer, St. Petersburg, Florida

C.....Solute information

C.2.8 .. DM,DECLAM;(O) - SOLUTE [1.4]

9.30E-5,0

C------------------------------------------------------------------------------

C.....Porous media zone information

C.2.9.1 .. IPMZ,X1Z(IPMZ),X2Z(IPMZ),Y1Z(IPMZ),Y2Z(IPMZ),Z1Z(IPMZ),Z2Z(IPMZ)

1 1 2500. 1 1 0. 150. 

2 1 2500. 1 1 150. 330. 

3 1 2500 1 1 330. 420. 

END 

C------------------------------------------------------------------------------

C.....Porous media property information

C.2.10.1 .. KXX(IPMZ),KYY(IPMZ),KZZ(IPMZ),IPMZ=1 to NPMZ [1.7]

3.877E-12,,3.877E-12

6.475E-10,,6.475E-10

3.102E-15,,3.102E-15

C.2.10.2 .. POROS(IPMZ),IPMZ=1 to NPMZ [1.7]

.3 .3 .3

C.2.10.3 .. ABPM(IPMZ),IPMZ=1 to NPMZ [1.7]

6.2E-6 6.2E-6 6.2E-6 

C------------------------------------------------------------------------------

C.....Porous media solute and thermal dispersion information

C.2.12 .. ALPHL(IPMZ),ALPHT(IPMZ),IPMZ=1 to NPMZ [1.7];(O) - SOLUTE [1.4] 

C.. and/or HEAT [1.4]

12.5 2.5

12.5 2.5

12.5 2.5

C------------------------------------------------------------------------------

C.....Porous media solute property information

C.2.13 .. DBKD(IPMZ),IPMZ=1 to NPMZ [1.7];(O) - SOLUTE [1.4]

3*0.0

C------------------------------------------------------------------------------

C.....Source-sink well information

C.2.14.1 .. IWEL,XW,YW,ZBW,ZTW,WBOD,WQMETH[I];(O) - NWEL [1.6] >0

C.2.14.2 .. WCF(L);L = 1 to NZ (EXCLUSIVE) by ELEMENT

C.2.14.3 .. WSF(L);L = 1 to NZ (EXCLUSIVE) by ELEMENT

1 1 1 150. 330. 2. 11 

0,0,0,0,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,0,0,0

25*0

END

C------------------------------------------------------------------------------

C.....Boundary condition information

C------------------------------------------------------------------------------

C..... Specified value b.c.

C.2.15 .. IBC by x,y,z range {0.1-0.3} with no IMOD parameter;(O) -

C.. NSBC [1.6] > 0

1. 2500. 1. 1. 420. 420.

101

1. 2500. 1. 1. 0. 0. 

101

2500. 2500. 1 1 50. 150.

101

2500. 2500. 1 1 150. 330.

101

2500. 2500. 1 1 330. 380.

101

END 

C------------------------------------------------------------------------------

C.....Free surface b.c.

C.2.20 .. FRESUR[T/F],PRTCCM[T/F]

F F

C------------------------------------------------------------------------------

C.....Initial condition information

C.2.21.1 .. ICHYDP,ICTPRO,ICCPRO; all [T/F]

T F F

C.2.21.3A .. ZPINIT,PINIT;(O) - ICHYDP [2.21.1] and NOT ICHWT [2.21.2]
Appendix 27

450. 0. 

C.2.21.6B .. C by x,y,z range {0.1-0.3};(O) - SOLUTE [1.4] and NOT ICCPRO

C.. [2.21.1]

1 2500. 1 1 0. 130. 

1 1 

1 2500. 1 1 150. 320. 

0.25 1

1 2500. 1 1 330. 420.

0.0250 1

END

C------------------------------------------------------------------------------

C.....Calculation information

C.2.22.1 .. FDSMTH,FDTMTH

0 1.

C.2.22.2 .. TOLDEN{.001},MAXITN{5}

.1 10 

C.2.22.3 .. EPSFS;(O) - FRESUR [2.20]

C------------------------------------------------------------------------------

C.....Output information

C.2.23.1 .. PRTPMP,PRTFP,PRTIC,PRTBC,PRTSLM,PRTWEL; all [T/F]

6*T 

C.2.23.2 .. IPRPTC,PRTDV[T/F];(O) - PRTIC [2.23.1]

201 T

C.2.23.3 .. ORENPR[I];(O) - NOT CYLIND [1.4]

C.2.23.4 .. PLTZON[T/F];(O) - PRTPMP [2.23.1]

F 

C.2.23.5 .. PLTTEM[T/F]

F

C------------------------------------------------------------------------------

C..... TRANSIENT DATA - READ3

C.3.1 .. THRU[T/F]

F

C------------------------------------------------------------------------------

C.....Source-sink well information

C.3.2.1 .. RDWTD[T/F];(O) - NWEL [1.6] > 0

T

C.3.2.2 .. IWEL,QWV,PWSUR,PWKT,TWSRKT,CWKT;(O) - RDWTD [3.2.1] 

1 133690. 0 51.87 75.0 0.035 

END

C------------------------------------------------------------------------------

C.....Boundary condition information

C------------------------------------------------------------------------------

C..... Specified value b.c.

C.3.3.1 .. RDSPBC,RDSTBC,RDSCBC; all [T/F];(O) - NSBC [1.6] > 0

T F T

C.3.3.2 .. PNP B.C. by x,y,z range {0.1-0.3};(O) - RDSPBC [3.3.1]

1 2500. 1 1 420. 420. 

12.967 1

1 2500. 1 1 0. 0. 

195.719 1

2500. 2500. 1 1 50. 380. 

3 5 

50. 173.702 380. 30.256

END

C.. SOLUTE [1.4] ALWAYS NEED THIS IF SPEC PRESS BOUNDARY

END 

C.3.3.6 .. CNP B.C. by x,y,z range {0.1-0.3};(O) - RDSCBC [3.3.1] and

C.. SOLUTE [1.4]

1 2500. 1 1 420. 420.

0.0250 1

1 2500. 1 1 0. 0.

1 1

2500. 2500. 1 1 50. 50. 

1 1

2500. 2500. 1 1 100. 100.
28 Simulation of Subsurface Storage and Recovery of Treated Effluent Injected in a Saline Aquifer, St. Petersburg, Florida

1 1

2500. 2500. 1 1 130. 130.

1 1

2500. 2500. 1 1 150. 150.

0.25 1

2500. 2500. 1 1 150. 320.

0.25 1

2500. 2500. 1 1 330. 380.

0.0250 1

END

C------------------------------------------------------------------------------

C.....Calculation information

C.3.7.1 .. RDCALC[T/F]

T

C.3.7.2 .. AUTOTS[T/F];(O) - RDCALC [3.7.1]

f

C.3.7.3.A .. DELTIM;(O) - RDCALC [3.7.1] and NOT AUTOTS [3.7.2]

0.5

C.3.7.3.B .. DPTAS{5E4},DTTAS{5.},DCTAS{.25},DTIMMN{1.E4},DTIMMX{1.E7};

C.. (O) - RDCALC [3.7.1] and AUTOTS [3.7.2]

C.3.7.4 .. TIMCHG

15. 

C------------------------------------------------------------------------------

C.....Output information

C.3.8.1 .. PRISLM,PRIKD,PRIPTC,PRIDV,PRIVEL,PRIGFB,PRIBCF,PRIWEL; all [I]

0 0 -15. 0 0 0 0 -15.

C.3.8.2 .. IPRPTC;(O) - IF PRIPTC [3.8.1] NOT = 0

201

C.3.8.3 .. CHKPTD[T/F],PRICPD,SAVLDO[T/F]

F / 

C------------------------------------------------------------------------------

C.....Contour and vector map information

C.3.9.1 .. CNTMAP[T/F],VECMAP[T/F],PRIMAP[I]

T T -15.0 

C------------------------------------------------------------------------------

C.....END OF FIRST SET OF TRANSIENT INFORMATION

C.....TRANSIENT DATA -READ3

C.....SECOND STAGE WITHDRAW 1MGAL/D FOR 15 DAYS.........................

C------------------------------------------------------------------------------

C.3.1 .. THRU[T/F]

F

C------------------------------------------------------------------------------

C.....Source-sink well information

C.3.2.1 .. RDWTD[T/F];(O) - NWEL [1.6] > 0

T

C.3.2.2 .. IWEL,QWV,PWSUR,PWKT,TWSRKT,CWKT;(O) - RDWTD [3.2.1] 

1 -133690. 0 51.87 75.0 0.035 

END

C------------------------------------------------------------------------------

C.....Boundary condition information

C------------------------------------------------------------------------------

C..... Specified value b.c.

C.3.3.1 .. RDSPBC,RDSTBC,RDSCBC; all [T/F];(O) - NSBC [1.6] > 0

T F T

C------------------------------------------------------------------------------

C.....Calculation information

C.3.7.1 .. RDCALC[T/F]

T

C.3.7.2 .. AUTOTS[T/F];(O) - RDCALC [3.7.1]

f 

C.3.7.3.A .. DELTIM;(O) - RDCALC [3.7.1] and NOT AUTOTS [3.7.2]

0.25

C.3.7.4 .. TIMCHG

30. 

C------------------------------------------------------------------------------

C.....Output information
Appendix 29

C.3.8.1 .. PRISLM,PRIKD,PRIPTC,PRIDV,PRIVEL,PRIGFB,PRIBCF,PRIWEL; all [I]

0 0 1 0 0 0 0 1

C.3.8.2 .. IPRPTC;(O) - IF PRIPTC [3.8.1] NOT = 0

201

C.3.8.3 .. CHKPTD[T/F],PRICPD,SAVLDO[T/F]

F / 

C------------------------------------------------------------------------------

C.....Contour and vector map information

C.3.9.1 .. CNTMAP[T/F],VECMAP[T/F],PRIMAP[I]

T T -30. 

C------------------------------------------------------------------------------

C.....End of second set of transient information

C.....End of simulation line follows, THRU=.TRUE.

C.3.99.1 .. THRU

T

C......End of the data file