Recharge Processes in an Alluvial Aquifer 

Riparian Zone, Norman Landfill, Norman, 

Oklahoma, 1998–2000 

Scientific Investigations Report 2004–5238 

U.S. Department of the Interior 

U.S. Geological Survey 
Cover: Photo of the riparian zone, sandy alluvium, and slough at the Norman Landfill, Norman Oklahoma, taken by 

Scott Christenson, U.S. Geological Survey. 
Scholl, Martha and others.—Recharge Processes in an Alluvial Aquifer Riparian Zone, Norman Landfill, Norman, Oklahoma, 1998-2000—USGS/SIR 2004–5238 

Printed on recycled paper 
Recharge Processes in an Alluvial Aquifer 

Riparian Zone, Norman Landfill, Norman, 

Oklahoma, 1998–2000 

By Martha Scholl, Scott Christenson, Isabelle Cozzarelli, Dale Ferree, 

and Jeanne Jaeschke 

Scientific Investigations Report 2004–5238


U.S. Department of the Interior 

U.S. Geological Survey 
U.S. Department of the Interior 

Gale A. Norton, Secretary 

U.S. Geological Survey 

Charles G. Groat, Director 

U.S. Geological Survey, Reston, Virginia 

For sale by U.S. Geological Survey, Information Services 

Box 25286, Denver Federal Center 

Denver, CO 80225 

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U.S. Geological Survey 

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Oklahoma City, OK 73116 

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UNITED STATES GOVERNMENT PRINTING OFFICE: OKLAHOMA CITY 2005 
iii 

Contents 

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


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


Description of Norman Landfill Area ....................................................................................................................................1


Previous Investigations .............................................................................................................................................................4


Purpose and Scope ....................................................................................................................................................................4


Acknowledgments ......................................................................................................................................................................6


Sampling Methods for Precipitation, Ground Water, and Slough Water ...........................................................................6


Recharge and Evapotranspiration Estimates ................................................................................................................................8


Determination of Specific Yield ..............................................................................................................................................8


Water-level fluctuations .................................................................................................................................................8


Laboratory tests on cores ..............................................................................................................................................8


Evapotranspiration Estimates ..............................................................................................................................................10


Recharge Estimates Using Stable Isotopes of Water ................................................................................................12


Recharge Estimates From Rises in Water Table ..........................................................................................................16


Comparison of Recharge Estimates Obtained by Chemical and Physical Methods .....................................17


Recharge Estimates for the Slough Bank Well .............................................................................................................20


Recharge Rate Estimates from Storm Events ...............................................................................................................21


Recharge Rates and Seasonal Timing .............................................................................................................................23


Summary ..................................................................................................................................................................................................25


References ...............................................................................................................................................................................................26


Appendixes...............................................................................................................................................................................................29


1. Stable isotope values for biweekly rainfall at the Norman Landfill, Norman, Oklahoma,


from May 1996 through May 2000. ....................................................................................................................31


2. Isotope and chemical profiles with depth in the three test wells at the Norman Landfill, Norman,


Oklahoma. ...................................................................................................................................................................35


Figures 

1.	 Map showing location of the Norman Landfill, Norman, Oklahoma, showing the vegetated


riparian zone, slough, water level monitoring wells and multi-level cluster wells. .....................................2


2.	 Cross-sectional view of the Norman Landfill, Norman, Oklahoma. Line of section shown


in figure 1. ................................................................................................................................................................................ 3


3.	 Graph showing water-level record for the Control site at the Norman Landfill, Norman,


Oklahoma, May–June 1999, showing typical fluctuations from rain events and transpiration..............5


4.	 Diagram of the multi-level cluster wells used to obtain samples for profiles of isotopic


composition and chemistry in the shallow aquifer at the Norman Landfill. ..................................................7


Figures 5–8. Graphs showing:


5.	 Water-level rise with rainfall for selected rain events at the Plume and Control sites at the


Norman Landfill, Norman, Oklahoma. ..........................................................................................................................9


6.	 Evapotranspiration (ET) rates vary with depth of water table below land surface near the


Plume (A) and Control (B) sites at the Norman Landfill, Norman, Oklahoma. ...........................................11

iv 

7.	 Isotopic composition of water samples from all depths of the cluster wells at Plume,


Slough Bank and Control sites at the Norman Landfill, Norman, Oklahoma, from the


April 2000 sampling. ..........................................................................................................................................................13


8.	 Oxygen-18 composition of rainfall at the Norman Landfill, Norman, Oklahoma, for biweekly


samples taken May 1996 through May 2000. .........................................................................................................14


9.	 Scatter plots of chemical data from each sampling at each level of the cluster wells, showing


mixing between recharge and underlying ground water at the Norman Landfill, Norman,


Oklahoma. ............................................................................................................................................................................15


Figures 10–13. Graphs showing: 

10.	 Recharge estimated by three methods and compared to rainfall at the Plume (A) and Control (B)


sites at the Norman Landfill, Norman, Oklahoma. ................................................................................................18


11.	 Amount of offset from the local meteoric water line found in slough water and in ground water


from Slough Bank well at the Norman Landfill, Norman, Oklahoma. ...........................................................21


12.	 The fraction of slough water in the recharge layer at the Slough Bank well (Norman Landfill,


Norman, Oklahoma) was correlated with rainfall. ...............................................................................................22


13.	 Temporal variation in recharge rates for the Plume and Control sites at the Norman, Landfill,


Norman, Oklahoma. ..........................................................................................................................................................24


Tables 

1.	 Parameters obtained from analysis of sediments from the alluvial aquifer at Norman Landfill,


Norman, Oklahoma. ............................................................................................................................................................9


2.	 Criteria for distinguishing leachate or recharge water at the three study sites, Norman Landfill,


Oklahoma. ............................................................................................................................................................................16


3.	 Total recharge estimates for two sites at the Norman Landfill, Norman, Oklahoma, using three


methods of determining recharge. .............................................................................................................................20


4.	 Recharge rates calculated by several methods for the Plume and Control sites, Norman Landfill,


Oklahoma. ........................................................................................................................................................................... 24

v 

Conversion Factors and Datum 

Multiply By To obtain 

Length 

centimeter (cm) 0.3937 inch (in.) 

millimeter (mm) 0.03937 inch (in.) 

meter (m) 3.281 foot (ft) 

inch (in.) 2.54 centimeter (cm) 

Volume 

liter (L) 33.82 ounce, fluid (fl. oz) 

milliliter (mL) 0.0338 ounce, fluid (fl. oz) 

Flow rate 

centimeter per year (cm/yr) 0.3977 inch per year (in/yr) 

millimeter per day (mm/d) 0.03937 inch per day (in/d) 

milliliter per minute (mL/min) 0.0338 ounce per minute (oz/min) 

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


°F = (1.8 × °C) + 32


Vertical coordinate information is referenced to North American Vertical Datum of 1988 (NAVD


88).


Horizontal coordinate information is referenced to North American Datum of 1983 (NAD 83).


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


25 ̊C).


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


micrograms per liter (μg/L).


NOTE TO USGS USERS: Use of liter (L) as a special name for cubic decimeter (dm3) is restricted


to the measurement of liquids and gases. No prefix other than milli should be used with liter.

Recharge Processes in an Alluvial Aquifer Riparian Zone, 

Norman Landfill, Norman, Oklahoma, 1998 - 2000 

By Martha Scholl, Scott Christenson, Isabelle Cozzarelli, Dale Ferree, and Jeanne Jaeschke 

Abstract 

Analyses of stable isotope profiles (δ2H and δ18O) in the 

saturated zone, combined with water-table fluctuations, gave a 

comprehensive picture of recharge processes in an alluvial 

aquifer riparian zone. At the Norman Landfill U.S. Geological 

Survey Toxic Substances Hydrology research site in Norman, 

Oklahoma, recharge to the aquifer appears to drive biodegradation, contributing fresh supplies of electron acceptors for the 

attenuation of leachate compounds from the landfill. Quantifying recharge is a firststep in studying this processin detail. Both 

chemical and physical methods were used to estimate recharge. 

Chemical methods included measuring the increase in recharge 

water in the saturated zone, as defined by isotopic signature, 

specific conductance or chloride measurements; and infiltration 

rate estimates using storm event isotopic signatures. Physical 

methods included measurement of water-table rise after individual rain events and on an approximately monthly time scale. 

Evapotranspiration rates were estimated using diurnal watertable fluctuations; outflux of water from the alluvial aquifer 

during the growing season had a large effect on net recharge at 

the site. 

Evaporation and methanogenesis gave unique isotopic signatures to different sources of water at the site, allowing the distinction of recharge using the offset of the isotopic signature 

from the local meteoric water line. The downward movement of 

water from large, isotopically depleted rain events in the saturated zone yielded recharge rate estimates (2.2 - 3.3 mm/day), 

and rates also were determined by observing changes in thickness of the layer of infiltrated recharge water at the top of the 

saturated zone (1.5 - 1.6 mm/day). Recharge measured over 2 

years (1998-2000) in two locations at the site averaged 37 percent of rainfall, however, part of this water had only a short residence time in the aquifer. Isotopes showed recharge water 

entering the ground-water system in winter and spring, then 

being removed during the growing season by phreatophyte transpiration. Recharge timing was variable over the course of the 

study; July and August were the only months that had no 

recharge in both years. Recharge to the aquifer from the slough 

(wetland pond) was estimated at one location using the isotopic 

signature of water affected by evaporation. Recharge was correlated with the rainfall amount over the period of estimation, 

suggesting that recharge from the slough to the downgradient 

aquifer was an episodic process, corresponding to elevated 

water levels in the slough after large rain events. 

Introduction 

A municipal landfill on the alluvial plain of the Canadian 

River in Norman, Oklahoma, accepted solid waste from 1922 to 

1985, when it was closed and capped with a clayey soil (fig. 1). 

The landfill was unlined, and a leachate plume extends at least 

225 meters (m) downgradient in the unconfined, 12-m thick 

alluvial aquifer. The landfill is one of the U.S. Geological Survey Toxic Substances Hydrology Program study sites (Christenson and Cozzarelli, 2003). 

The overall objectives of research at the site are to understand the biogeochemical and hydrological processesthat affect 

the fate and transport of contaminants in the leachate. The landfill leachate plume within the aquifer hasseveral different zones 

in which biodegradation of leachate compounds may occur 

(Cozzarelli and others, 2000); of particular interest are two 

areas where recharge is entering the anaerobic aquifer. These 

areas are the top of the saturated zone between the landfill and 

the slough, and in the upper part of the aquifer downgradient 

from the slough (fig. 2). The influx of recharge has been 

hypothesized to cause oxidation of sulfides and iron near the 

water table, providing a fresh source of electron acceptors that 

may stimulate biological activity (Ulrich and others, 2003). 

This study covered the 2-year period from May 1998 to May 

2000. The objective of this study was to quantify the amount 

and timing of recharge to the alluvial aquifer from rainfall and 

from the slough. 

Description of Norman Landfill Area 

Annual precipitation at the site is approximately 96 centimeters per year (cm/year), and precipitation occurs year round. 

May and June have the most rainfall, with a secondary maximum in September and October (National Oceanic and Atmospheric Administration - National Weather Service, 2003). The 

growing season is from mid-April through October. 

The aquifer material is predominantly fine to medium 

grained sand with intermittent mud layers and lenses (Marston 

and others, 2001). The area near the landfill is densely vegetated, with at least three species of phreatophytes— willow, cot­
2 Recharge Processes in an Alluvial Aquifer Riparian Zone, Norman Landfill, Norman, Oklahoma, 1998 - 2000

 Figure 1. Location of the Norman Landfill, Norman, Oklahoma, showing the vegetated 

riparian zone, slough, water level monitoring wells and multi-level cluster wells. The 

transect A–A’ is shown in cross section in figure 2. 
Figure 2. Cross-sectional view of the Norman Landfill, Norman, Oklahoma. Line of section shown in figure 1. The leachate plume and recharge 

fer are a composite, drawn on the basis of chemistry measurements made in the ground-water at the sampling locations shown, from 1997 to 20included in this report.) 
4 Recharge Processes in an Alluvial Aquifer Riparian Zone, Norman Landfill, Norman, Oklahoma, 1998 - 2000 

tonwood, and tamarisk (salt cedar) (Christenson and others, 

1999). There is a slough (wetland pond) impounded by a beaver 

dam in a former channel of the Canadian River; the slough is 

approximately 700 m long and 15-25 m wide, and is 50-100 m 

from the edge of the landfill (fig. 1). The slough is downgradient 

from the landfill and functions as a surface expression of the 

local water table. It has some lateral flow; input from upstream 

and seepage through the beaver dam to a nearby sewage outfall 

stream are observed. The slough was dry for a period during 

both summers of 1998 and 1999 when the water table dropped 

below the slough bottom. 

The leachate plume extends from the southern edge of the 

landfill at least 225 m downgradient in the direction of the 

Canadian River and fills almost the entire 12 m thickness of the 

alluvium in the area between the landfill and the slough (fig. 2). 

The leachate emanating from the landfill sinks to the bottom of 

the aquifer as distance from the landfill increases, due to a highpermeability layer at the bottom of the alluvium. Samples from 

a multi-level well transect parallel to the flow direction have 

allowed delineation of different biogeochemical zones within 

the leachate plume (Cozzarelli and others, 2000). The layer of 

infiltrated recharge water above the leachate plume between the 

landfill and the slough and the water entering the aquifer from 

the slough can be discerned on the basis of chemistry, with sulfate concentration and isotopic composition (δ2H or δ18O) 

being the most specific parameters. 

Water-level fluctuations in monitoring wells show daily 

and seasonal changes. Ground-water levels in the wells used in 

this study change by as much as 1.4 m from winter to summer, 

rise rapidly in response to rainfall events, and diurnal fluctuations indicate transpiration processes are the dominant cause of 

water-level decline during the growing season (fig. 3). The 

depth of the water table below land surface (thickness of the 

unsaturated zone) ranged from 0.24 to 1.72 m during the study. 

The water level rises to a peak within 0.6-2 days after the start 

of precipitation, and decreases over about 4 to 15 days, depending on season and amount of recharge. 

Previous Investigations 

Methods for determining recharge using water-level fluctuations have been used at many sites with shallow, unconfined 

aquifers (Healy and Cook, 2002). There are several sources of 

uncertainty associated with using these methods, with probably 

the largest being the specific yield (Sy). Specific yield varies 

with the height of the water table, the grain size distribution of 

the aquifer materials, and time (Sophocleous, 1985, Healy and 

Cook, 2002, Nachabe, 2002). Uncertainty is also introduced by 

the fact that water-level rise is not always due to water from the 

unsaturated zone entering the saturated zone. Pressure effects 

from rain falling on the land surface over a relatively dry unsaturated zone can cause temporary water-level rises (the Lisse 

effect, see Weeks, 2002), and lateral flow can cause rises in the 

water table that are not due to infiltration. 

Many studies have used 18O and 2H in rainfall to estimate 

residence time for various types of water in catchments including soil water, streamflow, and shallow ground water (McGuire 

and others, 2002; Turner and Barnes, 1998; Vitvar and Balderer, 1997; Stewart and McDonnell, 1991). Most of these studies relied on a predictable seasonal variation of isotopic composition in precipitation to model the isotopic content of the 

infiltrating precipitation. Even in areas where the seasonal variation is distinct, there are difficulties interpreting residence 

times due to mixing processes in the unsaturated zone and mixing of new recharge with older ground water (McGuire and others, 2002). Other studies have successfully used stable isotopes 

to quantify timing or amounts of ground-water recharge from 

different sources, for example in a contaminant plume (Bohlke 

and others, 1999) and an alluvial plain (Guglielmi and Mudry, 

1996). 

A recharge study done in Kansas, in an area climatologically similar to the Norman site, found that two instrumented 

sites varied dramatically in the amounts of recharge that 

reached the saturated zone. The factors that determined 

recharge were the thickness of the unsaturated zone and antecedent moisture conditions, related to evapotranspiration (ET) 

and precipitation differences. Recharge occurred only during 

the late winter and spring, with no recharge in summer or fall 

(Sophocleous and Perry, 1985). 

Purpose and Scope 

The purpose of this report is to provide a comparison of 

several different methods for determining recharge, including 

water-level fluctuations and isotope tracer methods. The importance of recharge as a supply of electron acceptors for biodegradation of a contaminant plume in a shallow unconsolidated 

aquifer is investigated. The report describes in detail recharge 

processes in a riparian zone; recharge in these zones has not 

been studied much, asthey are generally thought of as discharge 

areas. 

The hydrologic system at the landfill is dynamic, with the 

water table close to the surface and large seasonal variations in 

water level due to transpiration. The zero-flux plane concept, 

that recharge includes all water that infiltrates below the root 

zone, is not applicable here in the traditional sense, because the 

vegetation community uses water from both the unsaturated and 

saturated zones. Infiltrated water that reaches the saturated 

zone may only be in the aquifer a short time (days) before being 

removed by transpiration. 

Healy and Cook (2002) noted that it is best to use multiple 

methods to estimate recharge at a site, as it is difficult to assess 

the accuracy of any particular method. That approach is taken 

in this study, where several methods were used to quantify 

recharge at the site. Differences in water levels measured in 

wells were used on both an event basis and between sampling 

periods(about monthly) to estimate recharge. Evaporation from 

the slough and methanogenesis in the leachate plume lend distinctive isotopic signatures to the different water types at the 
 Figure 3. Water-level record for the Control site at the Norman Landfill, Norman, Oklahoma, May–June 1999, showing typical fluctuation

events and transpiration. Water-table elevation is in meters based on the North American Vertical Datum of 1988. The top boundary of tland-surface elevation at 330.3 meters. 
6 Recharge Processes in an Alluvial Aquifer Riparian Zone, Norman Landfill, Norman, Oklahoma, 1998 - 2000 

site, allowing distinction of infiltrated recharge water from the 

underlying ground water. Recharge rates were estimated by 

observing changes in thickness of the layer of infiltrated 

recharge water, and rain events with distinct isotopic signatures 

were tracked during the infiltration process to yield rate estimates. A detailed picture of the timing and type of rain events 

that contribute to recharge, as well as the residence time of 

recharge in the system, was obtained. With these estimates of 

recharge, further work can be done to determine the importance 

of the interface between recharge zones and contaminant 

plumes in biodegradation of contaminants. 

Acknowledgments 

The authors thank the personnel of the USGS Reston Stable Isotope Laboratory for their careful analysis of the isotope 

samples. Ludmilla Aristilde (Cornell University) helped with 

the ET analyses. Richard Healy (USGS) analyzed the sediment 

cores for the study. James LaBaugh (USGS) and Richard Healy 

improved the report with their technical reviews. The authors 

thank Lyn Osburn and Barbara Cardellichio (both at USGS) for 

their help in preparing the final report for publication. 

SamplingMethodsforPrecipitation,Ground 

Water, and Slough Water 

Precipitation for stable isotope analysis was collected 

biweekly in a funnel/bottle collector containing a 1-centimeter 

(cm) layer of mineral oil to prevent evaporation. The funnel was 

17.4 cm in diameter, inserted through a stopper in a 2-liter (L) 

collection bottle. Samples for isotopic analysis were poured 

through a separatory funnel to remove mineral oil from the sample, into 60 milliliter (mL) glass bottles with polyseal caps. The 

volume in the precipitation isotope collector was measured with 

a graduated cylinder. There were a few periods that the bottle 

overfilled so either no analysis was done or the analysis represents only part of the rainfall for the 2-week period. Precipitation amounts were measured at the weather station on top of the 

landfill mound with a standard 8-inch tipping-bucket rain gage 

(University of Oklahoma Mesonet, unpub. data, May 1, 1998, 

through May 31, 2000). Precipitation isotope analysis began in 

May 1996, 2 years before the recharge study began, and is 

ongoing (August 2004). The precipitation isotope values for the 

period May 1996 through May 2000 are given in appendix 1. 

Water levels were continuously monitored at eight locations at the site, using down-well vented pressure transducer/data logger instruments. Data from three locations closest to the study sites were used in this analysis: 1) WLMPlume, 

located downgradient from the landfill; 2) WLMSouth, located 

on the alluvial plain south of the landfill and wetland; and 3) 

Slough Gage, a stage recorder in the slough (fig. 1). Water levels recorded by the pressure transducers were checked at least 

monthly using an electronic water level tape. 

The shallow ground water was sampled at time intervals 

ranging from 9 to 55 days, but usually once per month. In this 

report, the term “shallow ground water,” when used in reference 

to the Norman Landfill site, means ground water at depths of 

less than 5 meters from the land surface; and the term 

“monthly,” when used in the context of ground-water measurements, refers to the time between ground-water sampling periods. Ground water was sampled in three cluster wells (figs. 1 

and 4). Each cluster had 15-17 wells of 18-millimeter (mm) 

inside-diameter PVC pipe with 16-cm slotted screens. The cluster well sites were: 1) Plume (IC36), located 35 m from the edge 

of the landfill, where the leachate plume is about 2 m below the 

land surface and is overlain by a thin layer of uncontaminated 

water; 2) Slough Bank (IC54), located 7 m from the downgradient edge of the slough, where water from the slough enters the 

aquifer and overlies the leachate plume; and 3) Control 

(ICSouth), located on the alluvial plain 85 m downgradient 

from the slough, meant to be a control site with no influence 

from the slough or the leachate plume. 

Specific conductance, stable isotope, and anion (chloride 

is reported in this report) samples were taken from the top of the 

saturated zone and at each successive depth until the bottom of 

the well cluster was reached. The volume of standing water in 

each cluster well was calculated on the basis of the water level 

measured that day. The intake for the peristaltic pump was 

placed at the water table, and moved down while pumping at a 

relatively fast rate to evacuate the calculated volume ofstanding 

water. Because the well volume was small, the standing water 

was completely evacuated from the well before sampling. Samples were then taken with the pump intake at the screen, using a 

relatively slow flow rate (about 30 mL/minute) to avoid pulling 

water from other levels of the aquifer. Samples were taken 

approximately monthly, from May 1998 to May 2000 at the 

Slough Bank and Control sites. The first well installed at the 

Plume site in May 1998 did not function properly. Sample collection at a new well began October 1998 and ended May 2000, 

so the data record for the Plume site is shorter than for the other 

two sites. 

The slough samples were taken at a location directly north 

of the Slough Bank well, by wading to the middle of the slough 

and using a bailer to obtain a depth-averaged sample of the 

water column. If this part of the slough was dry during the late 

summer, water remaining near the gage at the deepest part of the 

slough was sampled. 

Specific conductance was measured at the time of sampling. Anion samples were filtered through a 0.45-micrometer 

filter into 30-ml high-density polyethylene (HDPE) bottles that 

were rinsed twice with filtered water, and refrigerated until 

analysis. Anions were analyzed on a Dionex ion chromatograph. Isotope samples were taken unfiltered in unrinsed 60-mL 

glass bottles with polyseal caps. All isotope samples were analyzed in the USGS Reston Stable Isotope Laboratory. Oxygen 

and hydrogen isotopic results are reported in per mil (‰) relative to Vienna Standard Mean Ocean Water (VSMOW) and 

normalized on scales such that the oxygen and hydrogen iso­
Sampling Methods for Precipitation, Ground Water, and Slough Water 7

Figure 4. Diagram of the multi-level cluster wells used to obtain samples 

for profiles of isotopic composition and chemistry in the shallow aquifer 

at the Norman Landfill. Each well screen is 15 centimeters long, and the 

screens are placed adjacent to one another so a depth interval of about 2 

meters is covered. 
8 Recharge Processes in an Alluvial Aquifer Riparian Zone, Norman Landfill, Norman, Oklahoma, 1998 - 2000 

topic values of Standard Light Antarctic Precipitation (SLAP) 

are -55.5 ‰ and -428 ‰, respectively. Deuterium (δ2H) analyses of the water samples were done by equilibration with gaseous hydrogen and automated analysis; precision is around ± 1 

‰. Oxygen-18 (δ18O) analyses were done by equilibration with 

carbon dioxide and automated analysis; precision is around 

± 0.1 ‰. 

Recharge and Evapotranspiration Estimates 

Determination of Specific Yield 

Specific yield is the parameter needed in all determinations of recharge by water-level fluctuations or identification of 

infiltrated water using chemical methods. Two methods to 

determine Sy were used in this study, a field method using water 

table rise after rainfall, and a laboratory method using sediment 

cores. 

Water-level fluctuations 

For the field determination of Sy, over the 2-year period of 

study, selected rain events were correlated with water level rise 

in two of the monitoring wells. The water-level records for 

WLMPlume, located near Plume, and WLMSouth, located at 

the same site as Control, were used. Water levels were recorded 

at 30 or 60-minute intervals, and the rainfall record from the 

weather station was condensed to daily totals. A spreadsheet 

program was written that identified all rising portions of the 

water-level record within each 6-hour period, then added the 

rises to obtain total rise for each day. There were 98 rain events 

with amounts ranging from 0.5 to 125 mm (0.02 to 4.94 inches) 

in the 2-year period, with a rain event defined as one or more 

consecutive days with rainfall ≥ 0.02 inch. Water-level rises for 

each rain event were totaled, and then rainfall was plotted 

against rise. The water-level rise in response to the rainfall varied with season; water table rise with rainfall was generally 

smaller during the growing season. Some rain events produced 

no change in the water table; most of these events were in the 

spring and summer months. This result was consistent with 

large water outflux from both the unsaturated and saturated 

zones due to transpiration. 

To make a field-data-based estimate of Sy, water table 

rises that occurred under wet conditions were selected and the 

relation 

Sy = (Precipitation / Water-level rise) 

was used (Rosenberry and Winter, 1997; Gerhart, 1986; Rasmussen and Andreasen, 1959). For this estimate, evapotranspiration (ET), interception by vegetation, and runoff were 

assumed to be negligible and all of the rainwater entered the 

unsaturated zone and displaced an equal amount of water to the 

saturated zone. Rain events that were no more than 2 weeks 

after a previous event were selected, and only events from 

November-June were included, as transpiration had a substantial effect on local water levels during July-October. For the 

measurements that were used, the water table at Control (WLM 

South) was 0.5-1 m below land surface, whereas the water table 

at WLM Plume was generally 1.0-1.4 m below land surface. 

Total rainfall amount was plotted against water-level rise for 

each rain event, and a linear regression was fit to the data set for 

all rain events. The slope of the regression for each site was 

taken as the estimate of Sy for the site. Values were similar: 0.12 

and 0.10 for the Plume and Control sites, respectively (fig. 5), 

however, there was a larger rise in water table at the Control site 

than at the Plume site for each rain event. There also was a disproportionately large rise in water level for the smallest rain 

events at the Control site. The reason for this disproportionate 

rise is unknown; but it is likely that water levels at the Control 

site were affected by the response of the slough and/or the 

nearby sewage outfall (fig. 1) to rainfall. During a period of 

repairs at the local sewage treatment plant, discharge of treated 

sewage was stopped and started daily; this action caused waterlevel fluctuations of 0.2-0.4 m in the Control well. 

Laboratory tests on cores 

Trenches were excavated to the saturated zone at four locations using a track hoe, in order to collect cores in the zone of 

water-table fluctuation (fig. 1). Coring was done in August 

when the water table was lowest and the unsaturated zone was 

relatively dry. Cores 5 cm long and 4 cm in diameter were taken 

at three depths in the unsaturated zone using brass cylinders that 

were pressed into the sand, which wassmoothed prior to coring. 

Cores were analyzed for porosity, moisture content and moisture retention using methods in Klute (1986). The Sy was calculated by subtracting the residual moisture under 50 cm pressure 

from the porosity (Richard Healy, U.S. Geological Survey, oral 

commun., 2003). Porosity for the cores ranged from 0.34 to 

0.44. The Sy values ranged from 0.19 to 0.29, with an average 

of 0.23 (table 1). 

The actual Sy of aquifer sediment is not constant in time or 

space. The Sy depends on the capillary fringe above the water 

table, the depth of the water table below land surface, inhomogeneity of the sediments, the size of the change in water level, 

whether the water table is rising or falling, and the time allowed 

for the aquifer system to reach equilibrium (Duke, 1972; Nachabe, 2002). For short-term excursions of the water table, such as 

transpiration estimates and estimates of recharge from individual rain events, the field-derived Sy value of 0.11 was used. The 

laboratory-derived Sy value of 0.23 was used for recharge estimates from monthly measurements of the infiltrated recharge 

layer and net water table rise, because the aquifer system generally had time to approach equilibrium between sampling periods(described below). Overall recharge estimates are presented 

using Sy values from both laboratory and field estimates, as the 

actual value of Sy for the aquifer depends on several factors, and 

is probably between the two values. 
1

1

1

Recharge and Evapotranspiration Estimates 9

Figure 5. Water-level rise with rainfall for selected rain events at the Plume and Control sites at the Norman 

Landfill, Norman, Oklahoma. All events were less than 2 weeks after previous rainfall and rain events during 

the growing season were excluded. The field-derived value for specific yield is the slope of the rainfall/water-level rise regression line. 

Table 1. Parameters obtained from analysis of sediments from the alluvial aquifer at Norman Landfill, Norman, 

Oklahoma. 

[cm, centimeters; <, less than] 

Range of values Average value Number of 

samples 

Porosity 0.34 - 0.44 0.40 12 

Specific yield 0.19 - 0.29 0.23 9 

Air entry pressure head, cm <1 - 30 1

25 10 

Pore size distribution index 2.35 - 3.52 2.78 9 

1

Median value 
10 Recharge Processes in an Alluvial Aquifer Riparian Zone, Norman Landfill, Norman, Oklahoma, 1998 - 2000 

Nachabe (2002) presents expressions for the change in Sy 

with depth to water table and with time. The Sy approaches its 

ultimate value as depth to water table increases and as time 

increases. The average laboratory-derived value of 0.23 is 

assumed to be the ultimate Sy for the study area. To determine 

which Sy value was appropriate for the longer-term monthly 

recharge estimates, an average depth to water table of 1 m and 

representative field values of hydraulic conductivity near the 

Plume and Control sites (Scholl and Christenson, 1998) were 

used to calculate the time to reach the ultimate Sy (Nachabe, 

2002, eq. 14). These times ranged from 34 to 57 days, so for 

processes on a shorter time scale, the Sy value would be less 

than 0.23. At the Norman site, water table rise after rain events 

occurred in 14-48 hours, so the field derived Sy value of 0.11 is 

in agreement with the assumption of a non-equilibrium condition. The 0.23 value of Sy was used for the monthly recharge 

estimates; however, actual recharge amounts may be slightly 

less than calculated, depending on the departure of actual conditions from the average parameters used for the equilibration 

time estimates. 

Evapotranspiration Estimates 

Evapotranspiration was estimated with water-level fluctuations using the method of White (1932) as described in Meyboom (1967). This method can be used where there are clear 

diurnal variations in ground-water level in periods without rainfall. The daily cycle of water-level decline during daylight 

hours and recovery of water levels at night is used to determine 

the amount of uptake by the transpiring plants, using the relation 

q= Sy (24r ± s), 

where q is the depth of water withdrawal (L), 

Sy is specific yield (dimensionless), 

r is hourly rate of ground-water inflow (L/T, from the 

rate of rise at night), and 

s is the net rise or fall of the water table during the 24­

hour period (L). 

Daily transpiration estimates were made on all suitable 

parts of the hydrograph for the Plume and Control monitoring 

wells during the growing seasons of 1998-99. Transpiration 

could be accurately measured between rain events, after the 

recession of peak water levels. The Sy used to estimate transpiration was corrected for depth of the water table below land surface, using equation (7) from Duke (1972). The parameters 

needed for this assessment include the air-entry pressure and the 

pore-size distribution index, which were estimated from plots of 

capillary pressure head versus effective saturation using pressure cell data from the core samples (table 1). The average values from all samples that had pressure cell data were used. The 

calculated Sy values using Duke's expression decrease as the 

water table approaches the land surface. As the field-observed 

Sy was calculated on a similar time scale and range of depths to 

water table as the ET fluctuations, it is a more appropriate value 

to use than the laboratory value. Therefore, the ultimate Sy 

value was set at 0.11 and corrected for depth to water table by 

Duke's expression. The range of Sy was small, from 0.094 at 0.5 

m depth to 0.11 at 2 m depth. These values were used to calculate ET depending on the baseline water level at the time they 

were measured. 

At the Control site, each summer had similar patterns. 

Diurnal fluctuations began in April, with relatively large-amplitude fluctuations (see figure 3 for example of fluctuations). By 

August, water levels in the aquifer had declined as much as 1.2 

m, and diurnal fluctuations were smaller in amplitude. Fluctuations were not discernable after the beginning of October. 

Changes in ground-water levels at the Plume site were similar 

to the Control site, but diurnal fluctuations were smaller, so that 

measurable fluctuations started later in the season, and ended 

earlier. There are several possible reasons for the different patterns: the sediments may be different; the Plume site ground 

water is predominantly landfill leachate, which may affect plant 

health; and there are mature cottonwoods and willows at the 

Plume site, whereas the trees at the Control site are saplings. At 

the San Pedro River riparian zone study site in Arizona (Snyder 

and Williams, 2000), willows used only ground water while 

cottonwoods used both soil water and ground water, so differences in numbers of those species may be important. The 

decreasing amplitude of transpiration fluctuations with water 

table decline at Norman Landfill may be due to a lower density 

of roots with increasing depth to the water table. Isotopic profilesfrom 1999 and 2000 (appendix 2, figs. 2-12, 2-13, 2-19 and 

2-20) show that as the growing season proceeded, water was 

removed from the water table downward, with the most recent 

(early summer and spring) rain events removed first. 

Calculated ET rates for the Plume site ranged from 1.6 to 

9.3 millimeters per day (mm/day), with an average of 4.0 

mm/day. For the Control site, ET ranged from 2.5 to 13 

mm/day, with an average of 5.3 mm/day. For comparison, transpiration (only) rates estimated using sap flux measurements 

from a cottonwood/willow forest near the San Pedro River in 

southeastern Arizona averaged 4.8 mm/day, with a range of 3.1 

to 5.7 mm/day (Schaeffer and others, 2000). ET rates from a 

shallow aquifer in North Dakota, estimated by the same water 

table fluctuation method, averaged 3 to 7 mm/day, based on a 

range of values from 1 to 9 mm/day (Rosenberry and Winter, 

1997). ET rates estimated by various methods, including water 

table fluctuation, in a natural wetland in Wisconsin averaged 

3.6 mm/day, with a range of 1.9 to 8.7 mm/day (Lott and Hunt, 

2001). Average ET rates estimated from weather station data at 

the Norman Landfill for May-June 1997 were 2.4 to 5.8 

mm/day using various methods (Gossard and others, 1998); 

however, in their calculations, potential evapotranspiration 

(PET) rates were adjusted using soil moisture data to obtain ET 

rates. Soil moisture may not be relevant in phreatophyte transpiration, so their ET estimates for this site may be low. 

Evapotranspiration rates varied with changes in water 

level at both the Plume (fig. 6A) and Control (fig. 6B) sites. 

The higher values for ET at the Control site may not be accu­
Recharge and Evapotranspiration Estimates 11 

Figure 6. Evapotranspiration (ET) rates vary with depth of water table below land surface near the Plume (A) and Control (B) sites 

at the Norman Landfill, Norman, Oklahoma. Each data point is one day’s ET rate, representing water loss from the saturated zone, 

determined by White’s (1932) method on days during the growing season that had obvious diurnal water-table fluctuations. 
12 Recharge Processes in an Alluvial Aquifer Riparian Zone, Norman Landfill, Norman, Oklahoma, 1998 - 2000 

rate, reflecting uncertainty in the Sy parameter, but the pattern 

of maximum ET when the water table was about 0.75 m below 

land surface was repeated during different months and years, 

suggesting that most roots are in that zone. Calculated pan evaporation from Central Oklahoma during the 2002-03 growing 

season averaged about 7 mm/day, with a maximum of 12 

mm/day (Oklahoma Evapotranspiration Model, 2003); it was 

assumed that pan evaporation would be an upper limit for 

phreatophyte ET. The decrease in ET rate with increasing depth 

to water table probably reflects two effects: first, at higher water 

table, both evaporation and transpiration contribute to water 

losses from the saturated zone, whereas at lower water tables, 

losses are mostly due to transpiration; secondly, as the water 

table gets deeper, fewer plants have root systems that reach it. 

Recharge Estimates Using Stable Isotopes of Water 

The variation in stable isotopes in water at the site was 

used to identify the different sources of water. Figure 7 illustrates the distinct compositions of ground water with different 

sources in the three cluster wells, using δ2H and δ18O values 

from the April 2000 data set. Isotopic composition of water 

from different depth levels in the wells plots along mixing lines 

between leachate-contaminated ground water,slough water that 

is isotopically enriched due to evaporation, and infiltrated precipitation. The end members of landfill leachate and volumeweighted average precipitation are plotted for comparison. 

Instead of a slough water end member, the evaporation trend 

based on all the slough samples is shown, as many of the slough 

water values plot too far off scale to fit on the figure. The landfill leachate has distinctive deuterium enrichment, averaging 31 

‰, and as much as 45 ‰ more enriched than precipitation, 

probably due to methanogenesis (Hackley and others, 1996). 

This deuterium signature can be used to identify leachate in the 

wells. The water that has been in the slough during summer has 

undergone evaporation, so has a different isotopic signature 

than native ground water or leachate. Ground water at the 

deeper levels of the Control well that is enriched in oxygen-18 

relative to the local meteoric water line (LMWL) is assumed to 

have a slough water component. The general origins of water at 

each sampling depth can be identified by these differing isotopic signatures, however, mixing processes and large outfluxes 

due to transpiration during the growing season complicate the 

interpretation of recharge rates from the data. 

Precipitation has varying isotopic compositions, depending on the temperature, type, and history of the weather system 

that produces it. Seasonal variations in precipitation isotopes 

are most pronounced at higher latitudes. A predictable sinusoidal cycle of yearly variations is not present for the isotopic 

record at Norman, Oklahoma (latitude about 35°; fig. 8); the 

most depleted and enriched isotopic values are not confined to 

winter and summer periods, respectively. The seasonal isotopic 

signal was not optimal for tracing recharge, however, large precipitation events with distinct isotopic signatures may be used 

to provide information on timing of infiltration. Several such 

events occurred during the period of this study, but only two 

were useful for tracking recharge. The event that could be 

traced in all the wells occurred in the fall of 1998, at the end of 

the growing season. This was the only time that water stayed in 

the aquifer system long enough for its transport to be followed. 

By repeated sampling over time it was possible to track the vertical movement of water from this event in the aquifer. 

Water types at the three sites (Plume, Slough Bank, and 

Control) were classified using isotopic composition, specific 

conductance, and chloride. The offset of a ground-water sample 

in either δ2H or δ18O from the local meteoric water line was 

used as a parameter along with a solute parameter (specific conductance or chloride) to distinguish recharge from underlying 

ground water. To determine the stable isotope offset parameter, 

the Local Meteoric Water Line (LMWL) [δ2H = 7.5(δ18O) 

+ 11.7] was calculated using the biweekly rainfall record during 

the period of study. The amount of offset of either 2H or 18O 

from the LMWL was calculated for ground-water samples from 

each well level. For 2H and 18O, respectively: 

2

H offset = δ

18Oa + b – δ2

H

18Ooffset = (δ 18H b – ) ⁄ a – δ

18O

where a is the slope of the local meteoric water line (7.5 for 

this site), and 

b is the intercept (11.7 for this site). 

The offsets from the LMWL were used rather than the isotopic 

values, because although precipitation and background groundwater composition varied, their isotopic composition was generally on the LMWL. Oxygen-18 in water from the slough was 

generally offset below the LMWL due to evaporation, and 

leachate deuterium was always offset above the LMWL due to 

methanogenic enrichment. Therefore, water mixed with 

leachate or slough water could be distinguished by the amount 

of offset from the LMWL, independent of the large variations in 

precipitation isotope values. The offset method was less sensitive when both water types were isotopically enriched; for 

example, in distinguishing slough water in a mixture of slough 

water and leachate, the calculated 18O offset decreased with 

increasing 2H-enrichment, but the method still worked to separate the water types. 

For the Plume site, 2H offset, specific conductance and 

chloride were used to distinguish infiltrated recharge from 

leachate (fig. 9A). For the Slough Bank site, 18O offset and specific conductance were used to distinguish fresh water recharge 

(slough water and infiltration) from the underlying leachate 

(fig. 9B). For the Control site on the alluvial plain, specific conductance was similar in the two types of water, so 18O offset and 

chloride were used to distinguish infiltrated recharge from the 

underlying ground water (fig. 9C). Nearly all the water in the 

Control well had an isotopic signature affected by evaporation 

(as shown by 18O offset >0), but the water at deeper levels in the 

well was more offset and had relatively high chloride, and 

appeared to originate partly from the slough. The 
Recharge and Evapotranspiration Estimates 13 

Figure 7. Isotopic composition of water samples from all depths of the cluster wells at Plume, Slough Bank and Control sites at the Norman Landfill, Norman, Oklahoma, from the April 2000 sampling. Local meteoric water line (δ2

H = 7.5 δ18O + 11.7), slough evaporation 

trend, volume weighted average precipitation and volume weighted average winter precipitation are shown for comparison. The slough 

evaporation trend is a regression line based on 29 samples; the values are not shown because most fall outside the scale of the plot. 
14 Recharge Processes in an Alluvial Aquifer Riparian Zone, Norman Landfill, Norman, Oklahoma, 1998 - 2000

 Figure 8. Oxygen-18 composition of rainfall at the Norman Landfill, Norman, Oklahoma, for biweekly samples taken May 1996 

through May 2000. Horizontal segments of the line represent the bulk precipitation oxygen-18 value over the length of each sampling 

period, and gray shading denotes winter (September 21 to March 21). Breaks in the line denote periods without rainfall or missing 

data; data are listed in appendix 1. 
Recharge and Evapotranspiration Estimates 15 

Figure 9. Scatter plots of chemical data from each sampling at each level of the cluster wells, showing mixing between recharge and underlying ground water at the Norman Landfill, Norman, Oklahoma. Dashed lines (-----) enclose the samples designated as recharge at 

each site. (A) Plume site, recharge determined using specific conductance and the offset of deuterium (δ2

H) from the local meteroric water line. Open symbols ◊ show additional samples designated as recharge on the basis of chloride. (B) Slough Bank site, recharge determined using specific conductance and the offset of oxygen-18 (δ18O) from the local meteoric water line. (C) Control site, recharge 

determined using chloride and the offset of oxygen-18 (δ18O) from the local meteoric water line. 
16 Recharge Processes in an Alluvial Aquifer Riparian Zone, Norman Landfill, Norman, Oklahoma, 1998 - 2000 

background chloride level for ground water on the alluvial plain 

is about 26 milligrams per liter (mg/L) (Schlottman, 2000), 

whereasthe deeper levelsin the Control well averaged 92 mg/L. 

Slough water had median chloride concentration of 214 mg/L, 

with a range of 18-2,360 mg/L (the higher value at the lowest 

water volume). Criteria that were used to distinguish recharge 

water from underlying water in each well are listed in Table 2. 

The criteria also are marked as dividing lines on figs. 9A - 9C. 

The criteria are subjective; with the mixing between the two 

water types and variations in end-member composition there 

was no consistent demarcation between recharge and underlying water. In the Plume well, for example, the 2H offset and specific conductance showed the best demarcation between water 

types, but high specific conductance was not uniquely associated with leachate. Incoming recharge added solutes from the 

unsaturated zone and from redox processes. The criteria for 

leachate, therefore, included chloride as well as 2H offset and 

specific conductance. 

Figures for each sampling period showing the profiles of 

δ2H or δ18O and either specific conductance or chloride for 

each depth interval in the well are given in appendix 2. Different 

coloring shows the classified water type defined by criteria in 

table 2, illustrating the layer of recharge water overlying the 

leachate plume or background ground water. After the water 

types were classified, a monthly estimate of recharge infiltrated 

through the unsaturated zone could be made at the Plume and 

Control sites. For the Slough Bank site, the amount of water in 

the shallow saturated zone originating from the slough could be 

quantified over time. 

The recharge layer thickness was determined using the 

midpoint of the screen altitude; the uncertainty in the calculated 

thickness is ± 7.5 cm (half the screen length in the multilevel 

samplers). Change in recharge layer thickness was calculated 

for each sampling interval (generally monthly), with the monthto-month change in thickness taken to represent either net infiltration or net loss due to transpiration. The positive changes in 

thickness were multiplied by Sy to obtain millimeters of 

recharge to compare to rainfall amounts. For the Slough Bank 

site, possible components of the fresh water layer were slough 

water, infiltration through the local unsaturated zone, and 

upgradient ground water that flowed under the slough. The fraction of the fresh water layer that had originated in the slough 

was calculated using a simple two-component mixing model. 

The slough water was assumed to be offset in 18O from the 

LMWL by the average amount for all samples (+2 ‰, with a 

range of 0 to +9.4 ‰), and the background ground water and 

infiltrated water were assumed to have no offset. The fraction 

of slough water was applied to the total thickness of the fresh 

water layer to obtain an estimate of slough recharge at the location of the well. This method may underestimate the recharge 

from the slough; underestimation is discussed further in the section “Recharge Estimates for the Slough Bank Well.” 

Recharge Estimates From Rises in Water Table 

Recharge was estimated from the rise in water levels at 

two time scales: for each rain event and the net rise between 

sampling periods. Water-level rises resulting from individual

 Table 2. Criteria for distinguishing leachate or recharge water at the three study sites, Norman Landfill, Oklahoma. 

[‰, per mil; <, less than; >, greater than; μS/cm, microsiemens per centimeter; mg/L, milligrams per liter; --, criterion not used.] 

Water type and site 18O offset 2H offset Chloride Specific 

conductance 

Leachate at Plume -- > 10 ‰ OR > 225 AND > 4,500 μS/cm 

Recharge 

at Slough Bank 

> 0 ‰ -- AND -- < 2,150 μS/cm 

Recharge at Control < 0.8 ‰ -- AND < 40 mg/L --
rain events (described in the section “Determination of Specific 

Yield”) were summed over the period between sampling dates 

(Timlin and others, 2000; Rasmussen and Andreasen, 1959). 

The water-level rise was calculated from the rising portion of 

each ground-water hydrograph, without adding the amount of 

additional recharge estimated from continuing the recession 

curve until the peak of the rise. The rise generally occurred so 

quickly that this amount was negligible. Total water-level rise 

for the sampling period was multiplied by a Sy value of 0.11 to 

obtain millimeters of recharge for comparison to rainfall. The 

estimate based on individual rain events may have a higher 

degree of error at the Control site because of the effects of local 

surface water on ground-water fluctuations. 

The longer time-scale estimate of recharge was based on 

the change in water level between sampling periods. The time 

between sampling was usually 4-5 weeks, although there were 

three shorter (9-12 day) sampling intervals to study large storm 

events in September and October 1998. The difference between 

water levels measured on consecutive sampling dates was calculated, and any increase in water level was multiplied by Sy of 

0.23. This Sy value may lead to overestimation of recharge for 

periods shorter than about 30 days. 

Comparison of Recharge Estimates Obtained by 

Chemical and Physical Methods 

Recharge estimates based on chemical (infiltrated 

recharge layer thickness) and physical (event-based and 

monthly water-level rise) measurements are shown in figures 

10A and 10B. The change in the recharge layer thickness or 

change in water level in millimeters of water are shown in bar 

graphs - positive values denote increasing layer thickness or 

water level, and negative values represent decreasing layer 

thickness or water-level decline. For monthly measurements, a 

Sy value of 0.23 was used to adjust field values to estimates of 

water gained or lost, and for estimates based on the sum of individual rain events, a Sy value of 0.11 was used. 

Recharge patterns were similar over time at the Plume and 

Control sites, but at both sites, results from the three different 

methods of estimating recharge for each sampling period generally did not agree. Recharge estimates based on the sum of 

water-level rises after individual rain events, as expected, were 

larger than estimates from monthly measurements during the 

growing season, as water may recharge the aquifer but be 

removed shortly thereafter by transpiration. However, the sum 

of event-based rises was frequently larger than rainfall amounts 

at the Control site, an indication that water-level rise was not 

entirely due to infiltration at that site. Recharge from the layer 

thickness or monthly water-level rise methods exceeded rainfall 

in three instances: the period from mid-October to mid-November 1998 for both the Plume and Control sites, and the period 

from January 5 to February 3, 1999, for the Plume site. The 

recharge amount exceeding rainfall may have represented a 

transfer of previous rainfall from the unsaturated zone to the 

saturated zone, as the unsaturated zone has its largest storage 

Recharge and Evapotranspiration Estimates 17 

capacity after a dry summer. Precipitation also may have been 

snow, or frozen at the surface, during the coldest part of winter 

so that infiltration would be delayed until melting occurred. The 

recharge layer, as defined by chemical criteria, disappeared at 

the Plume site from September to November 1999, while at the 

Control site, a layer of recharge water could be identified 

throughout the entire study period. 

The overall recharge estimates shown in table 3 were calculated from the sum of recharge layer thickness increases and 

monthly water-level rises over the study period, excluding sampling intervals with declining water level or decreasing layer 

thickness. Recharge is reported as a percentage of the total rainfall over the measurement period (regardless of rising or declining water levels), using both the field and laboratory-determined Sy values. 

For the Plume site, the recharge layer thickness method 

yielded an estimate for total recharge that was larger than that 

from monthly water table rises (46 percent and 34 percent of 

rainfall, respectively, for Sy = 0.23). As previously discussed, 

there was some uncertainty in setting criteria for fresh infiltration at this site -the degree of deuterium enrichment did not necessarily correlate with the specific conductance of a sample (fig. 

9A). The conceptual model of the recharge process at the Plume 

site that was obtained from chemical and physical methods also 

was different. Rain fell and water levels rose from September to 

mid-November 1999, but isotopic signatures and solute chemistry indicated that there was effectively no infiltration reaching 

the water table (fig. 10A and appendix 2, figs. 2-14 and 2-15). 

This suggests that water-level increases occurred due to lateral 

flow of ground water rather than infiltration, as was seen at the 

Konza Prairie site in Kansas (Macpherson and Sophocleous, 

2004). There is consistently higher head within the landfill 

mound that could cause a water-level rise without local infiltration reaching the water table. 

At the Control site, despite the fact that monthly changes 

in recharge layer thickness and water level did not correlate 

each month, over the long term the total recharge estimate was 

nearly the same by both methods (34 percent and 36 percent of 

rainfall, respectively, for Sy = 0.23). Comparing the Plume and 

Control sites, the estimates of recharge were similar for the 

water-level method, 34 and 36 percent of rainfall, respectively, 

for Sy = 0.23; whereas the layer thickness method at the Plume 

site yielded larger recharge estimatesthan at the Controlsite (46 

and 34 percent, respectively). 

An overall recharge estimate also was done using the sum 

of water-level rise from rain events at the Plume site. This estimate was not done for the Control site because event-based 

recharge totals frequently exceeded rainfall for the sampling 

period, for unknown reasons. This value was 64 percent of 

rainfall; as expected, it was higher than the estimates from the 

methods that calculated recharge on a longer time scale because 

it included recharge during the growing season that may have a 

short residence time in the aquifer. 
 Figure 10A. Recharge estimated by three methods and compared to rainfall at the Plume (A) and Control (B) sites at the Norman Landfill, Norman, Omillimeters of water added or removed from the saturated zone, as estimated by three methods: the change in thickness of the recharge layer as dechemical constituents, the net water-level rise or fall during the sampling period determined by water-level monitors, and the sum of water-table risethe sampling period. Rainfall was the total during the sampling period from the tipping-bucket gage at the weather station. A missing bar, if not othethat there was no change during the sampling period. A specific yield value of 0.23 was used for the layer thickness and net water-level rise recharof 0.11 was used for the event-based recharge estimates. 
 Figure 10B. Recharge estimated by three methods and compared to rainfall at the Plume (A) and Control (B) sites at the Norman Landfill, Norman, millimeters of water added or removed from the saturated zone, as estimated by three methods: the change in thickness of the recharge layer as dchemical constituents, the net water-level rise or fall during the sampling period determined by water-level monitors, and the sum of water-table rithe sampling period. Rainfall was the total during the sampling period from the tipping-bucket gage at the weather station. A missing bar, if not othethere was no change during the sampling period. A specific yield value of 0.23 was used for the layer thickness and net water-level rise recharge eswas used for the event-based recharge estimates. 
20 Recharge Processes in an Alluvial Aquifer Riparian Zone, Norman Landfill, Norman, Oklahoma, 1998 - 2000 

Table 3. Total recharge estimates for two sites at the Norman Landfill, Norman, Oklahoma, using three methods of determining recharge. 

Recharge was determined for the period May 1998 – May 2000 at the Control site and for October 1998 – May 2000 at the Plume site. Values are reported as the fraction of total rainfall (sum of sampling-period recharge times Sy, divided by rainfall), using the range of measured values for Sy. 

[%, percent; mm, millimeters; Sy, specific yield; --, not determined]. 

Layer thickness 

method – 

monthly net 

increases 

Water-level 

method –monthly 

net rises 

Water-level 

method – 

precipitation 

event rises 

Plume Site – October 1998 to May 2000 

Sum of measured water-level rises or layer thickness increases, mm 2,626 1,981 7,741 

1Rainfall, mm 1,322 1,322 1,322 

Recharge as% of rainfall; Sy = 0.11 22 16 64 

Recharge as % of rainfall; Sy = 0.23 46 34 --

Control Site – May 1998 to May 2000 

Sum of measured water-level rises or layer thickness increases, mm 2,294 2,454 --

1

Rainfall, mm 1,589 1,589 --

Recharge as % of rainfall; Sy = 0.11 16 17 --

Recharge as % of rainfall; Sy = 0.23 34 36 --

1

Rainfall amounts are the total for the time period noted, regardless of whether water levels were increasing or decreasing. 

Recharge Estimates for the Slough Bank Well 

The thickness of the fresh water layer overlying the 

leachate at the Slough Bank well was directly correlated with 

the water level in the slough at each monthly sampling, except 

when the slough was dry. This layer was assumed to be a mixture of three possible sources: water transported laterally from 

the slough, direct infiltration through the overlying sediments, 

and recharge from upgradient areas adjacent to the landfill. 

The component of recharge from the slough was estimated 

by finding the average 18O offset of the fresh water layer from 

the LMWL each month, then estimating the fraction of slough 

water on the basis of that offset, assuming a slough water 18O 

offset of 1.9 ‰ (the average offset measured in samples taken 

from the slough). The isotopic analyses suggest that on average, 

29 percent of the fresh water layer at the Slough Bank well originated in the slough, with a range of 7-49 percent. Recharge 

unaffected by evaporation also may enter the aquifer from the 

slough, but cannot be distinguished from other sources. Therefore, the estimate of slough recharge may be conservative. This 

estimate of recharge cannot be extrapolated beyond the immediate area of the Slough Bank well, as recharge from the slough 

to the aquifer may differ significantly along the length of the 

slough. Recharge estimated by this method also would decrease 

with increasing distance from the slough bank, because of dilution of the slough water by infiltration. 

The changes in isotopic composition found in the slough 

were not found in the ground water; the 18O offset from the 

LMWL varied between 0.14 and 0.95 ‰ in the ground water 

and showed no definite seasonal pattern, whereas the slough 

isotopic composition had offsets of 0 to 9.4 ‰, showing a clear 

seasonal pattern of no evaporative effects in winter to strong 

evaporative signature in summer (fig. 11). Seasonal isotopic 

fluctuations may not be distinguishable in the well samples 

because there is very little volume when the slough water has 

undergone the most evaporation, and the dilution factor is high 

when the slough refills and the water infiltrates. The 7-meter 
Recharge and Evapotranspiration Estimates 21 

Figure 11. Amount of offset from the local meteoric water line found in slough water and in ground water from Slough Bank 

well at the Norman Landfill, Norman, Oklahoma. The values from the well are the average from all depths that were sampled. 

Note separate y-axis for the well samples. 

distance between the slough and the well also would tend to 

smooth out variations in isotopic composition. 

The data from the Slough Bank well give a good indication 

of the temporal variation in recharge from the slough. Estimates 

of slough water input to the aquifer over time correlated with 

the rainfall rate between sampling periods (fig. 12), with minimum input during both summers of the study and peaks in input 

after periods of high rainfall. This result suggests that most 

infiltration of the slough water is episodic, rather than occurring 

as a continuous process. As previously mentioned, the slough 

occupies the former river channel and the water is ponded 

because of a beaver dam, but there is some flow down the channel, through and around the dam, and out toward the Canadian 

River (fig. 1). Stage records show that rainfall events sharply 

raise the water level in the slough, probably a result of runoff 

from the channel upstream from the slough ponding at the beaver dam. The elevated water levels decline over hours to days. 

These events would push slough water into the aquifer for as 

long as the water level in the surface-water body was higher 

than in the surrounding aquifer. The correlation of infiltration 

rate from the isotopic method with the rainfall rate suggests an 

effective ground-water flow velocity of about 0.28 m/day. This 

velocity would result from an elevated water level in the slough 

of at least 0.1 m and a hydraulic conductivity of at least 7.0 x 

10-5 m/s, as the well is about 7 meters from the slough. These 

values are reasonable for the site, although ground-water flow 

rates measured at another location at the site are on the order of 

0.02 - 0.04 m/day (U.S. Geological Survey, unpub. data, 

November 18, 2003, through August 25, 2004). This observation implies fast transport of slough water during periods of 

infiltration. 

Recharge Rate Estimates from Storm Events 

Recharge rates were estimated by using a tracer (a rain 

event with significantly different isotopic composition) and 

measuring the rate of downward movement within the saturated 

zone. A rain event on September 13, 1998, could be traced as 

recharge in all the wells in the study. The rainfall total was 85 

mm (3.35 inches) and the isotopic composition was δ18O -13.7 

‰, δ2H -99 ‰; a departure of 7.9 ‰ in δ18O and 67 ‰ in δ2H 

from the volume-weighted average rainfall at the site. 

At the Plume site, the water from that storm reached the 

saturated zone of the aquifer some time between January 6 and 

February 4, 1999, with a deuterium signature of -48 ‰ (appen­
22 Recharge Processes in an Alluvial Aquifer Riparian Zone, Norman Landfill, Norman, Oklahoma, 1998 - 2000

 Figure 12. The fraction of slough water in the recharge layer at the Slough Bank well (Norman Landfill, Norman, Oklahoma) 

was correlated with rainfall. Slough water fraction of recharge layer thickness was multiplied by Sy = 0.23 to obtain millimeters of water. Total rainfall and slough water thickness values were calculated at the end of each sampling period, and were 

normalized by dividing each quantity by the number of days in the sampling period. 
dix 2, fig. 2-8). The isotopic signature was more enriched than 

the original rain, due to mixing and evaporation processes 

during transport through the unsaturated zone, but still recognizable as rainfall from the September 13 storm as no subsequent rainfall had such a depleted isotopic signature. This water 

took approximately 4 months to move through the unsaturated 

zone. After it reached the saturated zone, subsequent samples 

showed the storm water moved downward in the aquifer until 

May 26, 1999; this movement represented a vertical velocity of 

about 9.8 mm/day. The water was at the same position June 24, 

1999 (appendix 2, figs. 2-10 and 2-11); presumably the downward movement stopped because the growing season had 

advanced to the point that recharge was insignificant. The 

recharge rate at the Plume site for February 4-May 26 can be 

estimated as the vertical velocity times Sy, which would be 9.8 

mm/day x 0.23, to yield 2.2 mm/day. This rate estimate requires 

the assumptions that recharge occurs as vertical flow and the 

rate of downward movement is constant with depth. 

In the Control well, the September 13, 1998, storm event 

was seen in the saturated zone at the October 20, 1998, sampling, a transport time of 24 to 37 days in the unsaturated zone 

(appendix 2, fig. 2-5). This isotopically depleted water stayed 

effectively stationary, without moving downward in the aquifer, 

for months. Water accumulated above it in the next month, then 

the water level remained constant and there was no isotopic evidence of infiltration for the next 1 to 1.5 months, although there 

was 195 mm of precipitation during this period. Between February 2 and March 31, 1999, there was 124 mm of rainfall and 

the storm water moved down 740 mm. This movement yields a 

recharge rate of 13 mm/day x 0.23 = 3.0 mm/day. The behavior 

of the tracer storm water in the two wells suggests there was 

some inhibition of the infiltration process during the winter. 

Although the storm water infiltrated to the saturated zone 

sooner at the Control site, active downward transport within the 

saturated zone occurred during the same time period in both 

wells, with the apparent transport rate slightly higher at the 

Control site than the Plume site. 

Water from another rain event on January 29-31, 1999, 

could be identified in the Control well, but the downward movement occurred during the growing season so the event could 

only be traced for two sampling periods (appendix 2, figs. 2-9, 

2-10 and 2-11). The rain event measured 34 mm (1.33 inches) 

with isotopic composition of δ18O -9.6 ‰, δ2H -63 ‰. Water 

from this event appeared in the Control well by March 31, was 

at the same position May 26, then moved down 470 mm by June 

28, a recharge rate of 3.3 mm/day, similar to the previous storm. 

The overlying water was then removed from the saturated zone 

by transpiration over the summer (appendix 2, figs. 2-12 and 2­

13). This rain event was not seen in the saturated zone at the 

Plume well. During the winter of 1999-2000, there was only 

one rain event with distinctive isotopic composition and the 

rainfall amount was too small to be observed in the ground 

water. 

The movement of the January 29, 1999, storm water in the 

ground water provides information about the recharge process 

at the Control site. Between March 31 and May 26, 1999, there 

Recharge and Evapotranspiration Estimates 23 

was no apparent infiltration or vertical movement in the ground 

water. There was 253 mm (10 inches) of rainfall during this 

time, occurring in evenly spaced and evenly sized events of less 

than about 25 mm (1 inch) of rainfall. Between May 26 and 

June 28, 1999, there were more small, evenly spaced rain 

events, then a 5-day rain event totaling 112 mm (4.4 inches). 

This event seems to account for all the vertical movement that 

occurred in the ground water at that time. This observation suggests that recharge during the growing season occurs during 

large rain events; the vegetation utilizes all the water from any 

smaller rain events that occur. This observation also implies 

that recharge rates obtained in this way should not be extrapolated over time scales longer than the sampling interval. 

At the Slough Bank well, the September 13, 1998, storm 

water was observed in the top screens of the well on February 

4, 1999, a delay of nearly 5 months. The water moved down 

about 300 mm by June 28, but by July 26 it was gone, transpiration had removed it from the aquifer (appendix 2, figs. 2-8 

through 2-11). Vertical transport was slower at this site than at 

the other two sites. A recharge rate was not calculated for this 

location, as flow may have been mostly horizontal, and transpiration began to have a large effect on the upper part of the saturated zone by late May, evidenced by the lack of further 

recharge above the storm water. 

Recharge Rates and Seasonal Timing 

For purposes of estimating input of fresh water and electron acceptors to the upper boundary of the contaminant plume, 

recharge rates and information on the seasonal timing of 

recharge are needed. Recharge rates estimated by several different methods are summarized in table 4. Although recharge 

amounts calculated by layer thickness and water level methods 

were often quite different for individual sampling periods, the 

average recharge rates for the entire study are similar for both 

the chemical and physical estimates, ranging from 1.3 to 1.6 

mm/day. The rates calculated from vertical movement of storm 

water in the aquifer were higher, 2.2 to 3.3 mm/day, because 

these rates were calculated during shorter periods of active 

recharge. 

The timing of recharge at the Plume and Control sites is 

shown in Figure 13 as recharge rates calculated for each sampling period from the layer thickness method. The layer thickness method of estimating recharge was the most reliable 

method for the study objectives, as the chemical evidence for 

infiltration of “new” water could be seen on a month-to-month 

basis. The recharge rates are in mm/day, obtained by dividing 

the increase in layer thickness by the time interval, and multiplying by Sy = 0.23 for each sampling period that there was 

recharge. Figure 13 covers the entire study period, however, the 

data set from the Plume site begins later, in November 1998. 

There was no recharge in July or August both years of the study, 

otherwise the rates and timing vary between sites and between 

years. Recharge rates were highest for the Control site in the fall 

of 1998. During the fall/winter of 1999-2000, recharge 
24 Recharge Processes in an Alluvial Aquifer Riparian Zone, Norman Landfill, Norman, Oklahoma, 1998 - 2000

 Table 4. Recharge rates calculated by several methods for the Plume and Control sites, Norman Landfill, Oklahoma. 

Rates are in millimeters per day, using a Sy value of 0.23. 

Average Range of rates 

Average recharge rate – from individual Recharge rate, 

Site recharge rate – sampling storm water water level layer method periods- both tracers increase method methods 

Plume 1.6 1.3 0-4.9 2.2 

Control 1.5 1.6 0-4.3 3.0 - 3.3

 Figure 13. Temporal variation in recharge rates for the Plume and Control sites at the Norman, Landfill, Norman, Oklahoma. 

Rates shown are the increase in recharge layer thickness divided by the number of days in the sampling period. Sampling periods with no bar are either missing data for the Plume site or no recharge was measured for that sampling period. 
rates were lower than in 1998, and most recharge occurred in 

the spring. During the period where there were data from both 

the Plume and Control sites, differences between the sites were 

present but were smaller than the differences between years. 

The stable isotope composition of the alluvial plain ground 

water also provides indirect evidence of the timing of recharge 

to the area. The average background ground-water isotopic 

composition from an uncontaminated well upgradient from the 

landfill is -5.1 ‰ and -31 ‰ in δ18O and δ2H, respectively, 

more similar to the volume-weighted average of all precipitation (-5.6 ‰, -30 ‰) than to volume-weighted average winter 

precipitation (-6.5 ‰, -35 ‰). This result suggests that large 

rain events during any season may contribute to recharge, not 

just late winter and spring rains as was found by Sophocleous 

and Perry (1985) in Kansas. 

Summary 

A leachate plume extends at least 225 meters downgradient from a closed municipal landfill in a shallow, unconfined 

alluvial aquifer. The landfill is located on the alluvial plain of 

the Canadian River in Norman, Oklahoma, and is one of the 

U.S. Geological Survey Toxic Substances Hydrology Program 

study sites. An investigation by the U.S. Geological Survey 

from May 1998 to May 2000 focused on quantifying the 

amount and timing of recharge to the alluvial aquifer from rainfall and from the slough near the landfill, to investigate how 

recharge may contribute to biodegradation processes in the 

leachate plume. The study was conducted at three sites near the 

landfill: 1) the Plume site, in the riparian zone between the landfill and the slough, 2) the Control site, on the alluvial plain 

downgradient from the slough, and 3) the Slough Bank site, in 

the riparian zone 7 meters downgradient from the slough. 

Annual precipitation at the landfill is approximately 96 

centimeters per year, and the growing season, when evapotranspiration affects ground-water levels, is from mid-April through 

October. Aquifer material is predominantly fine to medium 

grained sand with intermittent mud layers. The slough is a pond 

resulting from a beaver dam; this pond occupies a former channel of the Canadian River, about 50-100 meters downgradient 

from the edge of the landfill. 

Precipitation for stable isotope analysis was collected biweekly, and precipitation amounts were measured with a tipping-bucket rain gage. Ground-water and surface-water levels 

were continuously monitored near the Plume and Control sites 

and in the slough. Shallow ground water was sampled approximately once per month in cluster wells with 15-centimeter 

screens spanning a 2-meter depth interval, and slough water 

samples were taken at the same times, with a bailer to obtain a 

depth-averaged sample of the water column. Stable isotopes, 

anions and specific conductance were measured in the groundwater and surface-water samples.

 The recharge analyses, evapotranspiration analyses, and 

seasonal ground water observations for the Norman Landfill 

Summary 25 

site, Norman, Oklahoma, indicate that the major factor determining the amount and timing of infiltration is evapotranspiration during the growing season. The water table is generally 

within 2 meters of the land surface, and the vegetation includes 

the phreatophyte species willow, cottonwood, and tamarisk, so 

that water losses occur directly from the saturated zone as well 

as from the unsaturated zone. Rain percolating through the 

unsaturated zone first has to satisfy evapotranspiration demand, 

which is significant during the growing season and may be 

insignificant in winter. Infiltration then fills the pores to the specific retention value of the sediment, which may require a relatively large amount of rainfall at the end of the growing season, 

when the water table is lowest. After this saturation is reached, 

additional infiltration water pushes previous infiltration toward 

the saturated zone. Water from smaller rain events may not 

reach the water table, especially during the growing season. 

Some rainfall may directly reach the saturated zone by way of 

macropores, but these are probably minor in this unconsolidated 

sand. Once water reaches the saturated zone, it may remain in 

the ground water for some period of time (fall/winter months) 

or be removed quickly by transpiration (spring/summer 

months). 

Data from the Plume and Control sites indicated that ET 

rates decreased with increasing depth to the water table, and at 

the Control site, ET rates were at a maximum when the water 

table was about 0.75 meter below land surface. Transpiration by 

phreatophytes removed not only recent rainfall, but also water 

that had been in the saturated zone for some time. Isotopic profiles from both 1999 and 2000 showed that as the growing season proceeded, water was removed from the water table downward, with early summer and spring rain events removed first. 

At the Plume site, the chemically distinct layer of infiltrated 

recharge water was entirely removed from the aquifer by transpiration over the summer of 1999, while at the Control site, the 

layer of recharge water was present throughout the 2-year 

study. 

Recharge from the slough to the aquifer was determined at 

one location about 7 meters from the bank of the slough. Isotopic evidence from this well suggested that on average, 29 percent of the fresh water layer was slough water, but this may be 

an underestimate because the source water in the slough did not 

always have an isotopic signature affected by evaporation during the winter. A strong correlation between monthly rainfall 

rates and the amount of slough recharge to the aquifer (based on 

the isotopic signature) indicatesthat much of the input ofslough 

water to the aquifer is episodic, following rain events that temporarily raise the level of the surface water body above that of 

the aquifer. The fresh water layer at the Slough Bank site was 

always present over the 2-year study, even during the driest part 

of the summer. 

The results of this study indicate that at this riparian zone 

site with its complex hydrology, multiple methods of estimating 

recharge yield a better understanding of the process than using 

a single approach. The contrasting stable isotope signatures of 

water at this site gave a detailed picture of recharge and transpiration processes that would not have been discernable from 
26 Recharge Processes in an Alluvial Aquifer Riparian Zone, Norman Landfill, Norman, Oklahoma, 1998 - 2000 

water-level or dissolved-solute measurements alone. Chemical 

and physical methods yielded similar estimates of overall 

recharge rates for the entire study period (1.3 to 1.6 millimeters 

per day), but recharge amounts obtained by each method for 

individual sampling periods were almost never the same, and in 

a few cases, even disagreed as to whether recharge was occurring. The methods that involved delineating the infiltration 

recharge layer and tracing of infiltration from individual storms 

provided direct evidence of infiltration reaching the saturated 

zone, but the amounts of recharge did not correlate well with 

rainfall during the measurement periods. Uncertainties in this 

approach arise from seasonal differences in the thickness of the 

unsaturated zone, leading to differences in the transport time of 

rainfall to the saturated zone, and variations in chemical composition and mixing within the saturated zone that tend to blur 

boundaries between different sources of water. Measurements 

of water-level fluctuations on event and monthly time scales 

were easy to obtain, but there was evidence that fluctuations in 

the water table were not entirely due to infiltration. Uncertainties in these recharge estimates arise from the effects of lateral 

flow and redistribution of water from areas with different water 

levels, for instance from the elevated water table under the landfill mound or from the surface water bodies in the area. For all 

methods of estimating recharge, the possible spatial and temporal variations in the Sy value are a source of uncertainty. 

The data collected in this study show a distinct seasonality 

for the input of fresh electron acceptors to drive biodegradation 

processesin this contaminated ground watersystem. During the 

growing season, there is little or no recharge and a net loss of 

previously recharged water from the shallow alluvial aquifer. 

Most recharge occurs in the fall, winter, and spring, with relatively large rain events delivering the bulk of the recharge to the 

saturated zone of the aquifer from the unsaturated zone and 

from the slough. Therefore, biodegradation rates for contaminants in the upper fringes of the leachate plume may be correlated with recharge rates. 

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Appendixes

Appendixes 30 

Appendix 1. Stable isotope values for biweekly rainfall at the Norman Landfill, Norman, Oklahoma, from May 1996 through May 2000. 

[mL, milliliters; 2H, deuterium; 18O, oxygen-18; ‰, per mil; --, no data]. 

Sample Volume 

(mL) 

Delta 2H 

(‰) 

Delta 18O 

(‰) 

Start 

date 

End 

date Comment 

NLFP-1 91 -13.7 -2.90 5/10/96 5/22/96 

NLFP-2 1,400 -31.3 -5.26 5/22/96 6/3/96 

NLFP-3 900 -15.3 -3.39 6/3/96 6/19/96 

NLFP-4 149 -2.2 -0.82 6/19/96 7/3/96 

NLFP-5 890 -30.3 -5.42 7/3/96 7/12/96 

NLFP-6 1,670 -20.9 -4.22 7/12/96 7/31/96 

NLFP-7 2,340 -13.5 -3.59 7/31/96 8/15/96 

1NLFP-8 450 -7.7 -1.38 8/15/96 8/15/96 Control, initial 

NLFP-9 498 -36.7 -5.92 8/15/96 8/29/96 

1NLFP-10 450 -9.3 -1.33 8/15/96 8/29/96 Control, final 

NLFP-11 0 -- -- 8/29/96 9/10/96 

NLFP-12 570 -54.6 -8.43 9/10/96 9/18/96 

NLFP-13 950 -25.9 -5.03 9/18/96 9/25/96 

NLFP-14 900 -45.9 -7.44 9/25/96 10/11/96 

NLFP-15 250 -52.2 -8.74 10/11/96 10/25/96 

NLFP-16 2,000 -21.4 -4.77 10/25/96 11/8/96 

NLFP-17 303 -17.4 -4.46 11/8/96 11/22/96 

NLFP-18 530 -59.9 -9.87 11/22/96 12/5/96 

NLFP-19 0 -- -- 12/5/96 12/20/96 

NLFP-20 0 -- -- 12/20/96 1/3/97 

NLFP-21 74 -76.9 -11.7 1/3/97 1/16/97 

NLFP-22 0 -- -- 1/16/97 1/30/97 

NLFP-23 310 -122.0 -17.0 1/30/97 2/14/97 

NLFP-24 900 -34.8 -5.94 2/14/97 3/5/97 

NLFP-25 59 6.4 -1.44 3/5/97 3/13/97 

NLFP-26 410 -27.3 -5.34 3/13/97 3/27/97 

NLFP-27 1,200 -35.7 -6.46 3/27/97 4/10/97 

NLFP-28 1,002 -55.5 -8.43 4/10/97 4/23/97 

NLFP-29 1,595 -54.4 -8.44 4/23/97 5/7/97 

NLFP-30 1,384 -33.8 -5.76 5/7/97 5/22/97 

1NLFP-31 1,220 -32.8 -5.21 5/22/97 6/5/97 Duplicate 

NLFP-32 1,220 -31.2 -5.21 5/22/97 6/5/97 

1,2NLFP-33 2,500 -26.1 -4.91 6/5/97 6/16/97 Duplicate 

2

NLFP-34 2,500 -25.8 -4.95 6/5/97 6/16/97 

1NLFP-35 435 -33.4 -5.55 6/16/97 7/2/97 Duplicate 

NLFP-36 435 -33.2 -5.59 6/16/97 7/2/97 

NLFP-37 150 -14.9 -3.17 7/2/97 7/17/97 

NLFP-38 288 -16.4 -3.69 7/17/97 7/31/97 

NLFP-39 1,469 -9.2 -2.90 7/31/97 8/14/97 
31 Recharge Processes in an Alluvial Aquifer Riparian Zone, Norman Landfill, Norman, Oklahoma, 1998–2000 

Appendix 1. Stable isotope values for biweekly rainfall at the Norman Landfill, Norman, Oklahoma, from May 1996 through May 2000. 

—Continued 

Sample Volume 

(mL) 

Delta 2H 

(‰) 

Delta 18O 

(‰) 

Start 

date 

End 

date Comment 

NLFP-40 1,380 -19.8 -4.06 8/14/97 8/28/97 

NLFP-41 50 5.1 -0.61 8/28/97 9/11/97 

NLFP-42 1,861 -17.6 -3.74 9/11/97 9/25/97 

NLFP-43 216 0.3 -2.52 9/25/97 10/9/97 

NLFP-44 1,580 -8.6 -3.06 10/9/97 10/23/97 

NLFP-45 372 -31.8 -6.51 10/23/97 11/6/97 

NLFP-46 892 -73.8 -11.9 11/6/97 11/20/97 

NLFP-47 1,240 -26.4 -5.66 11/20/97 12/3/97 

NLFP-48 290 -51.5 -9.48 12/3/97 12/18/97 

NLFP-49 2,150 -61.8 -10.1 12/18/97 1/2/98 

NLFP-50 2,225 -43.2 -7.53 1/2/98 1/13/98 

NLFP-51 164 -61.4 -9.94 1/13/98 1/29/98 

NLFP-52 791 -25.7 -5.80 1/29/98 2/12/98 

NLFP-53 360 -83.5 -12.2 2/12/98 2/26/98 

NLFP-54 1,034 -54.8 -8.39 2/26/98 3/12/98 

NLFP-55 2,338 -23.6 -5.60 3/12/98 3/17/98 

NLFP-56 461 34.0 -8.86 3/17/98 3/26/98 

1NLFP-57 250 9.5 -0.39 3/26/98 4/9/98 Evaporated 

NLFP-58 238 -56.1 -8.93 4/9/98 4/23/98 

NLFP-59 1,647 -30.5 -6.18 4/23/98 4/27/98 

NLFP-60 44 -105 -14.1 4/27/98 5/7/98 

NLFP-61 1,330 -8.8 -3.01 5/7/98 5/21/98 

NLFP-62 592 -10.5 -2.89 5/21/98 6/4/98 

NLFP-63 735 -20.9 -3.60 6/4/98 6/18/98 

NLFP-64 0 -- -- 6/18/98 7/2/98 

NLFP-65 0 -- -- 7/2/98 7/16/98 

NLFP-66 175 -8.92 -1.78 7/16/98 7/30/98 

NLFP-67 215 -7.31 -1.50 7/30/98 8/17/98 

NLFP-68 0 -- -- 8/17/98 8/27/98 

NLFP-69 195 -12.0 -2.19 8/27/98 9/10/98 

NLFP-70 1,870 -98.6 -13.7 9/10/98 9/15/98 

NLFP-71 1,265 -34.8 -5.73 9/15/98 9/25/98 

NLFP-72 1,195 -31.4 -5.72 9/25/98 10/7/98 

NLFP-73 815 -30.3 -5.09 10/7/98 10/28/98 

NLFP-74 1,488 -28.4 -5.94 10/28/98 11/3/98 

NLFP-75 324 -24.8 -5.84 11/3/98 11/24/98 

NLFP-76 1,078 -27.7 -5.44 11/24/98 12/16/98 

NLFP-77 212 -21.4 -5.53 12/16/98 1/5/99 

NLFP-78 0 -- -- 1/5/99 1/20/99 

NLFP-79 1,104 -62.7 -9.64 1/20/99 2/4/99 
Appendixes 32 

Appendix 1. Stable isotope values for biweekly rainfall at the Norman Landfill, Norman, Oklahoma, from May 1996 through May 2000. 

—Continued 

Sample Volume 

(mL) 

Delta 2H 

(‰) 

Delta 18O 

(‰) 

Start 

date 

End 

date Comment 

NLFP-80 112 -21.3 -4.25 2/4/99 2/10/99 

NLFP-81 679 -20.3 -4.61 2/10/99 2/25/99 

NLFP-82 670 -9.9 -3.61 2/25/99 3/11/99 

NLFP-83 1,240 -24.1 -5.10 3/11/99 3/18/99 

NLFP-84 363 -11.9 -2.99 3/18/99 4/1/99 

2NLFP-85 2,370 -13.3 -3.44 4/1/99 4/28/99 

NLFP-86 1,035 -22.0 -4.02 4/28/99 5/12/99 

2NLFP-87 1,387 -3.2 -1.20 5/12/99 6/14/99 

NLFP-88 1,738 -15.0 -3.74 6/14/99 6/23/99 

NLFP-89 735 -25.7 -4.66 6/23/99 7/19/99 

NLFP-90 441 -13.0 -3.98 7/19/99 8/10/99 

NLFP-91 0 -- -- 8/10/99 8/30/99 

NLFP-92 2,377 -38.5 -6.73 8/30/99 9/14/99 

NLFP-93 375 -4.4 -1.98 9/14/99 10/5/99 

NLFP-94 67 -14.7 -2.75 10/5/99 10/18/99 

NLFP-95 1,165 -39.3 -7.69 10/18/99 11/2/99 

NLFP-96 0 -- -- 11/2/99 12/2/99 

NLFP-97 1,375 -28.5 -6.07 12/2/99 12/9/99 

NLFP-98 413 -49.3 -8.45 12/9/99 1/5/00 

NLFP-99 511 -91.9 -13.1 1/5/00 1/31/00 

NLFP-100 0 -- -- 1/31/00 2/10/00 

NLFP-101 753 -26.7 -4.78 2/10/00 2/24/00 

NLFP-102 1,104 -8.6 -2.77 2/24/00 3/14/00 

NLFP-103 825 -20.5 -4.42 3/14/00 3/27/00 

NLFP-104 499 -12.8 -3.44 3/27/00 4/6/00 

NLFP-105 748 -22.8 -4.99 4/6/00 4/19/00 

NLFP-106 1,005 -11.8 -2.82 4/19/00 5/3/00 

NLFP-107 22 -27.5 -4.35 5/3/00 5/24/00 

NLFP-108 1,758 -29.9 -5.07 5/24/00 6/8/00 

1

 Data not used because sample was a duplicate, a control, or evaporated due to lack of oil layer 

2

Collector overflowed 
33 Recharge Processes in an Alluvial Aquifer Riparian Zone, Norman Landfill, Norman, Oklahoma, 1998–2000 

Appendix 2. Isotope and chemical profiles with depth in the three test wells at the Norman Landfill, Norman, Oklahoma. Note 

that the isotopic δ values, not the calculated offsets from the LMWL, are shown in the profiles. The depth scale on the plots 

is in meters based on the North American Vertical Datum of 1988. The land surface elevation is 330.88 meters at the Plume 

site, 330.54 meters at the Slough Bank site, and 330.30 meters at the Control site. The plots show changes in the thickness of 

the recharge layer at each site. There is a set of plots for each sampling period; the Slough Bank and Control wells were 

sampled from May 1998 to May 2000, and the Plume well was sampled from October 1998 to May 2000. 
Appendixes 34 

Figure 2.1. 
35 Recharge Processes in an Alluvial Aquifer Riparian Zone, Norman Landfill, Norman, Oklahoma, 1998–2000 

Figure 2.2. 
Appendixes 36 

Figure 2.3. 
37 Recharge Processes in an Alluvial Aquifer Riparian Zone, Norman Landfill, Norman, Oklahoma, 1998–2000 

Figure 2.4. 
Figure 2.5. 
Figure 2.6. 
Figure 2.7. 
Figure 2.8. 
Figure 2.9. 
Figure 2.10. 
Figure 2.11. 
Figure 2.12. 
Figure 2.13. 
Figure 2.14. 
Figure 2.15. 
Figure 2.16. 
Figure 2.17. 
Figure 2.18. 
Figure 2.19. 
Figure 2.20.