U.S. Geological Survey Ground-Water Investigations Involving the Unsaturated Zone


(Note: There are 2 attachments associated with this memorandum.  
       Attachment 1 is a file containing 6 pages of FrameMaker 
       tables which can be retrieved from ftp 130.11.51.175 as
       /var/ftp/pub/MM95.07.Attach1.doc.  Attachment 2 follows
       the memorandum below.)

  

In Reply Refer To:                              July 24, 1995
Mail Stop 411


OFFICE OF GROUND WATER TECHNICAL MEMORANDUM - 95.07

Subject:  U.S. Geological Survey Ground-Water Investigations In-
          volving the Unsaturated Zone

Studies of the unsaturated zone are becoming increasingly important 
in ground-water investigations of the U.S. Geological Survey 
(USGS).  Examples are studies of the effects of application of ag-
ricultural chemicals, assessment of waste disposal in the unsatur-
ated zone or at the surface, and studies of the effects of 
accidental spills of toxic materials.  The amount of recharge to 
the water table through the unsaturated zone is a key variable in 
many ground-water studies, particularly in the arid west.  Ongoing 
studies encompass an extremely broad range of techniques and hy-
drogeologic settings.  The purpose of this memorandum is to iden-
tify current and recently completed projects in which unsaturated 
zone studies play an important role to aid in the sharing of in-
formation among projects and the development of new projects.  This 
information has been compiled by Newell Trask in the Office of 
Ground Water.


Field projects

        A compendium of current and recently completed USGS field 
projects dealing with the unsaturated zone is provided in Tables I 
- III and indicates the variety of settings and hydrologic tech-
niques involved.  These tables are accompanied by abbreviated de-
scriptions of both the techniques used (T1-T16) and the projects 
themselves (P1-P38).  An attempt has been made to provide the most 
relevant references for each project and to include key projects 
dating from the late 1980s.  Complete references can be obtained 
by consulting the papers and the authors cited.  

        For those interested in detailed descriptions of the basic 
techniques used for investigations of the unsaturated zone, the 
following reference works are available: 1)\x11standards published by 
the American Society for Testing Materials (1994); 2) Wierenga and 
others (1993), and 3)\x11chapters in the handbook "Methods of Soils 
Analysis," published by the American Society of Agronomy (Klute, 
1986).

        Research to improve field instrumentation for use in the un-
saturated zone is an important component of USGS efforts.  Among 
the highest priority needs identified by the WRD Instrumentation 
Committee in early 1994 for study of the unsaturated zone are soil-
moisture sensors including time-domain reflectometers, water po-
tential sensors, and unsaturated zone samplers and monitors.  W.N. 
Herkelrath is the National Research Program point-of-contact with 
the WRD Instrumentation Committee.


Laboratory projects

        Table IV presents a compendium of USGS laboratory-oriented 
projects in unsaturated zone hydrology.  The research topics listed 
in the left-hand column parallel in a general way the list of tech-
niques in the left-hand column of Tables I - III.  References to 
this laboratory work can be found in the annual summary of the Na-
tional Research Program of the Water Resources Division (e.g., 
Nichols, 1995).

        Please share the information contained in this memorandum with 
those persons involved in ground-water studies in your office.  We 
welcome feedback about additional USGS studies relative to unsat-
urated zone investigations, both field and laboratory.  





                                /s/ William M. Alley, Chief,
                                Office of Ground Water

Attachment

Distribution: A, B, S, FO, PO




---------------------------
ATTACHMENT 2

NOTES FOR TABLES I-III


T1 - In humid regions, temporal variations in water content moni-
tored with moisture-sensing devices can often be used to evaluate 
the movement of water pulses through the unsaturated zone; but in 
desert soils, moisture measurements may not be sufficiently accu-
rate to detect small fluxes.  In addition, water content may depend 
on soil mineralogy and be vertically discontinuous.  Under these 
conditions, variations in water content do not necessarily indicate 
the direction of water movement, and measurements of water poten-
tial become necessary.  Tensiometers can be used to measure water 
tensions (negative pressures) up to 0.8 bars; higher tensions re-
quire thermocouple psychrometers (TCPs).  Both tensiometers and 
TCPs can be used in the laboratory or the field.  Field TCPs can 
measure tensions up to 8 MPa; higher tensions can be measured in 
the laboratory.  Field installation and calibration of TCPs is dif-
ficult; and improvements in techniques are being pursued.  The USGS 
Beatty project has made extensive use of TCPs and will be publish-
ing guidance on their use.  Another useful review on the use and 
limitations of TCPs is provided in Scanlon (1994).  

T2 - Simulations of water flow in the unsaturated zone are diffi-
cult and not commonplace because of the non-linearity of the prob-
lem, the difficulty of obtaining a relation between water content 
and water potential, and the presence of numerous heterogeneities 
in typical unsaturated zone sections.  Several codes have been de-
veloped for the conditions at Yucca Mountain.  The completed study 
at Barnwell used UNSAT2, a code developed for the Nuclear Regula-
tory Commission.  A listing of codes for water flow in the unsat-
urated zone is included in a compilation by van der Heijde (1994).  
A USGS code VS2D (Lappala and others, 1987) is being used in several 
projects.  Methods for determining the water content/water poten-
tial relationship are reviewed in standard texts as well as in 
workshop notes prepared by R. W. Healy, Central Region NRP.  Rossi 
and Nimmo (1994) also review these methods and suggest a new rela-
tionship for very low water contents.  

T3 - Flow of air through Yucca Mountain occurs under natural con-
ditions, probably enhanced by boreholes (Thorstenson and others, 
1990; Weeks, 1993).  Such flow is significant as a mechanism for 
drying out the mountain and as a potential transport medium for 
released radionuclides.  Gas studies at Sheffield concentrated on 
gases released from the low-level radioactive waste (Striegl, 
1989).  The work at Galloway involves modification of the USGS code 
MODFLOW, originally written for saturated water flow, to treat flow 
of air in the unsaturated zone induced by vacuum extraction (Joss 
and Baehr, 1994).  A modfication of this program to analyze two-
dimensional radial flow to or from a single well is also being doc-
umented by Joss and Baehr.

T4 - Sampling of fluids from the unsaturated zone can be done in 
the laboratory on samples collected for that purpose or can be done 
in the field.  Both approaches have advantages and disadvantages.  
Methods for sampling oil and water in the unsaturated zone, the 
saturated zone, and the transition between them were developed by 
Hess, Herkelrath, and Essaid (1992).  The Picatinny project has 
sampled both water and vapor from the unsaturated zone by a variety 
of techniques (Smith and others, 1990, 1992).  At Galloway, gas 
chromatograph methods were used to analyze gases from the unsatur-
ated zone at a gasoline spill (Baker and others, 1991).  There are 
probably USGS projects in addition to those listed in Tables I-III 
that have the analysis of vadose zone fluids as one of their com-
ponents, especially in the Department of Defense Environmental Con-
tamination (DODEC) Program.  

T5 - The modeling of solute transport in the unsaturated zone is 
even more difficult than the simulation of flow alone.  An addition 
to the VS2D code (Healy, 1990) produced VS2DT to simulate solute 
transport in variably saturated porous media.  The code is one of 
18 considered for further development for the Yucca Mountain Pro-
gram, but it appears that more complex codes that handle thermal 
effects, vapor flow, and non-equilibrium between matrix and frac-
tures will be needed there.  The review by Reeves and others (1994) 
which contained this conclusion and the review by van der Heijde 
(1994) which includes solute transport as well as flow models are 
useful introductions to the many complexities of simulation of sol-
ute transport in the unsaturated zone.  The problem of simulating 
volatilization of solvents in unsaturated soils in both the labo-
ratory and field has been discussed by Cho and others (1993), based 
on work at the USGS experimental site at Picatinny Arsenal, New 
Jersey.  

T6 - Transport of carbon-14 from high-level radioactive waste by 
circulating gases is a specialized complex problem at Yucca Moun-
tain (Thorstenson, and others 1990).  Computer code documentation 
is being prepared to implement a model for diffusion dominated 
transport of gases in the unsaturated zone by Art Baehr of the New 
Jersey district.  The code has been applied at sites in Galloway 
Township, New Jersey, and Beaufort, South Carolina, to estimate 
rates of in situ degradation of gasoline hydrocarbons based on the 
movement of oxygen and carbon dioxide.  

T7 - Entries in Table I reflect activities in which tracer materi-
als are withdrawn from the unsaturated zone itself, not cases in 
which tracers from ground water have been used to infer properties 
about overlying unsaturated materials.  The bomb-related isotopes 
and meteoric chloride have been used in arid and semiarid areas 
where recharge rates are relatively low.  Because of vapor diffu-
sion in these dry environments, tritium appears deeper in the sec-
tions than chlorine-36 (Phillips, 1994).  Applied tracers, 
including agricultural chemicals considered as tracers, have been 
more widely used in the midwest and east.  Chlorofluorocarbons 
(CFCs), in addition to their use in dating ground water, have been 
extracted from air in the unsaturated zone and compared with con-
centrations in adjacent ground water.  Where disequilibrium occurs, 
the ground water may have been recharged rapidly and locally along 
preferential pathways (Busenberg and others, 1993).

T8 - Most projects measure some physical properties of unsaturated 
zone materials in the course of an investigation.  Complete de-
scriptions of techniques are not possible here.  The reference by 
Wierenga and others (1993) gives fairly complete descriptions of 
techniques and numerous references.

T9 - Among USGS projects, most meteorological stations have been 
installed at arid or semiarid sites in connection with water budget 
studies.  

T10 - Workshop notes prepared by R. W. Healy, central region NRP, 
provide a useful review of various water budget methods.  There is 
growing interest in such work particularly at large scales such as 
global change models.  Nichols (1987) demonstrated the difficulties 
of doing precise water budget studies in arid environments.  How-
ever, the method may be useful for placing limits on the amount of 
recharge available to the water table.  

T11 - Wierenga and others (1993) and the references included give 
a complete listing of field methods for determining saturated and 
unsaturated hydraulic conductivity by various infiltration tech-
niques.  

T12 - Construction of tunnels and shafts involves considerable ex-
pense and can be justified only where very detailed information is 
necessary for the purposes of a study.

T13 - The time domain reflectometry (TDR) method has been used to 
remotely monitor soil moisture in the upper part of the unsaturated 
zone (Herkelrath and others, 1991).  The method can be used in both 
the laboratory and the field.

T14 - Ground penetrating radar (GPR) in the proper circumstances 
can provide information on shallow stratigraphy relevant to the un-
saturated zone (Olhoeft and Lucius, 1990; Olhoeft, 1992).

T15 - Borehole geophysical logs may be somewhat underutilized in 
unsaturated zone work; they have been used primarily in solid rock 
or where the unsaturated zone is especially thick.  At Yucca Moun-
tain, they have been used to provide continuous estimates of po-
rosity and water content.  Combinations of dielectric and density 
logs (Nelson, 1993) and of epithermal neutron and density logs 
(Nelson, 1994) are effective for this purpose.

T16 - Analysis of unsaturated sediments for microorganisms is rel-
atively rare and just beginning in some studies.  The difficult 
problem of remediation of dense non-aqueous phase liquids in the 
unsaturated zone was discussed by Ehlke and Imbrigiotta (1993).  
Lahvis and others (1993) used vapor diffusion modeling to determine 
in situ microbial degradation rates of hydrocarbons at Galloway 
Township, New Jersey. 


Table I - Waste disposal and waste spill studies

P1 - The heavily funded Yucca Mountain Project embraces a broad 
range of unsaturated zone studies but is atypical of projects in-
volving the unsaturated zone in general.  The average depth to the 
water table is 600 m. and the indurated rocks above the projected 
repository level (surface to depth of 300 m) are as important as 
the rocks below.  For a complete description of studies contemplat-
ed for Yucca Mountain in 1988, see the Site Characterization Plan 
(DOE, 1988).  From the point of view of other USGS unsaturated zone 
work, the most significant investigations are those dealing with 
the uppermost several meters in which infiltration enters the shal-
low system and is redistributed.  This work has been summarized by 
Flint and Flint (1994) and by Flint and others (1994).  Tracer stud-
ies (Yang, 1992) and water chemistry (Weeks, 1995) have indicated 
the presence of preferential flow paths to depth through the bed-
rock.

P2 - Results from long-term monitoring at a site next to the Beatty 
commercial low-level radioactive waste site were summarized by Fis-
cher (1992).  In 1987, the Beatty test-trench study was begun to 
determine the effects of disturbance (trench construction, vegeta-
tion removal) on soil properties and the natural soil-water regime 
and to estimate potential rates of burial trench erosion and sub-
sidence under natural climatic conditions (Andraski and others, 
1991).  Initial results have demonstrated the interactive effects 
of climate, soils, and vegetation on the water balance and poten-
tial for deep percolation (Gee and others, 1994; Andraski, 1995).  
A study by Prudic (1995) addresses the direction of water movement 
through the deep unsaturated zone below a depth of 13 m and the 
effects of temperature and atmospheric-pressure changes on the 
movement of water vapor.  

P3 - Results of research at Sheffield were summarized by Ryan 
(1989).  The instrumentation was described by Healy and others 
(1986).  

P4 - Dennehy and McMahon (1989).

P5 - The West Valley low-level radioactive waste site involved a 
section of saturated till overlying unsaturated sand and gravel 
with flow dominantly downward.  The details are given in Prudic 
(1986).

P6 - Davis and Pittman (1990).  Like Yucca Mountain and unlike many 
other unsaturated zone sites, the site at the Idaho National Engi-
neering Laboratory (INEL) involves unsaturated solid rock below a 
veneer of unconsolidated materials.  Shakofsky (1995) made a com-
plete study of the physical properties of the disturbed and undis-
turbed materials at a simulated waste trench.  Soil and atmospheric 
conditions at the trench are being monitored.  Cecil and others 
(1992) used chlorine-36 and tritium to estimate infiltration rates.  
Through consideration of CFC concentrations in ground water, Busen-
berg and others (1993) concluded that there are preferential flow 
paths through the basalt section.  

P7 - The Pacific Northwest Laboratory has conducted numerous stud-
ies of the unsaturated zone at the Hanford Reservation (e.g. Gee 
and others, 1994).  USGS work has been limited to use of meteoric 
chlorine and chlorine-36 (Prych, 1995).

P8 - A caisson, TCPs, gas sampling ports, and a meteorological sta-
tion were established in Jackass Flats on the Nevada Test Site as 
part of the National Research Program (Weeks, 1984).  The project 
is currently suspended.

P9 - Philip Nelson, also with the Yucca Mountain Project, has been 
using electrical and density logs to determine the degree of sat-
uration in the unsaturated zone and to identify zones where clay 
minerals are especially abundant.

P10 - Rudolph and others (1993).

P11 - Studies of the fate of nitrate in a 150-foot thick unsaturated 
zone below wastewater seepage pits (Schroeder and others, 1993) may 
have application to agricultural nitrates in other areas.

P12 - The unsaturated zone is being studied in connection with con-
struction of a wetland to remove nitrogen and organic carbon from 
waste water.

P13 - Study of geochemical and microbiological transformations of 
constituents in recycled water for artificial recharge.  Emphasis 
is on unsaturated zone processes including transport of bacteria 
and viruses through the unsaturated zone.

P14 - Glynn and Busenberg (1993).  Carbon dioxide generated within 
a water-table aquifer by reaction of acidic ground water with car-
bonates is diffusing upward, while oxygen is diffusing downward to 
replace oxygen consumed by oxidation of reduced iron and manganese 
in the contaminant plume.

P15 - Chapelle and Bradley (1993).

P16 - Baehr and others (1991); Baehr and Fischer (1993).  Work on 
the unsaturated zone at a gasoline spill site has focused on esti-
mation of microbial degradation rates of hydrocarbons, based on 
rates of gas transport, and on vapor-extraction remediation design.  

P17 - A multidisciplinary study of a TCE spill with emphasis on 
ground water but includes some study of vapor movement and micro-
bial processes in the unsaturated zone (Imbrigiotta and Martin, 
1991).

P18 - Soil gas sampling is underway at several sites in the Depart-
ment of Defense Environmental Contamination (DODEC) Program.

P19 - Essaid and others (1991); Hult and others (1991).



Table II - Agriculture studies

P20 - Hicks and others (1991).

P21 - The Minnesota Management Systems Evaluation Area (MSEA) has 
been described by Delin and Landon (1993) and Delin and others 
(1993).  Studies here indicate that much of the current downward 
water flux bypasses soil as well as water already contained in the 
soil.

P22 - Tindall (1993); Blevins (1991).  Demonstrates the importance 
of macropores.

P23 - This is a study of two experimental agricultural plots in 
Maryland.  The unsaturated zone is being monitored for nitrogen 
(McFarland, 1995).

P24 - Unsaturated zone studies are part of a National Water Quality 
Assessment (NAWQA) flow-path study in an agricultural area.

P25 - Juracek (1994).

P26 - Eckhardt and Barnes (1991); Eckhardt and others (1993).




Table III - Recharge studies

P27 - One dimensional VS2D is being used to estimate the contrast-
ing effects of sprinkler and furrow irrigation.


P28 - VS2DT is being used to study vadose zone transport at Rapid 
Infiltration Basins.  

P29 - Studies here involve in-depth investigations of chemical 
weathering in a well-characterized soil sequence as well as studies 
of water percolation by Dave Stonestrom.

P30 - Nimmo is investigating techniques for estimating long-term 
average recharge rates at an experimental agricultural plot in 
southeast Washington.  The results from three different methods 
bracket the results obtained in a regional study by Bauer and Vac-
caro (P32)

P31 - Bauer and Vaccaro (1990) used water budget data calculated 
on a daily basis to estimate average long-term rates of recharge 
for the Columbia Plateau.  They indicate an accuracy of 25 percent.  

P32 - P36 - Five sites in the Water, Energy, and Biogeochemical 
Budget (WEBB) program include unsaturated zone studies: P32, Hun-
tington and others (1993);  P33, Elder and others (1992); P34, - 
Larsen and others (1993); P35, Turk and others (1993); P36, Shanley 
and others (1994)

P37 - The Owens Valley project made use of the filter paper method 
for estimating matric potentials (Sorenson and others, 1989).  This 
inexpensive method may be applicable in some other projects.

P38 - Wood and Sanford (1995) used tritium and chloride measure-
ments in both the unsaturated zone and the saturated parts of the 
aquifer to estimate recharge rates in the High Plains aquifer of 
the Southern High Plains.  



Notes for Table IV

R1 - Details of the flow of water in the vadose zone are far from 
being well-understood.  Stonestrom (1995) has raised questions 
about the applicability of Richards-based theory to even simple 
cases of isothermal flow in a homogeneous medium with negligible 
effects from gas.  Nimmo and his colleagues have explored innova-
tive ways to measure hydrologic properties in very dry materials 
(Nimmo and others, 1994).  Constantz (1993) has found a dependence 
of water retention on the rates of application of water and on tem-
perature in nonswelling porous materials.  Blain and Milly (1991) 
modeled the redistribution of moisture on hillslopes in response 
to periods of rainfall and drainage; the results suggested that 
macropores played an important role in lateral flow.  

There are many questions about the behavior of water in very dry 
materials.  Earlier formulations of the relation between water con-
tent and matric suction have not worked well at very low water con-
tents.  Rossi and Nimmo (1994) have proposed formulations for soil 
water retentivity which are physically realistic and represent, 
with the minimum possible number of parameters, the retentivity 
curve over the entire range of saturation.  

R2 - Gas flow in the unsaturated zone has been studied by Striegl, 
Healy, Thorstenson, and Weeks in a variety of settings (see 
Nichols, 1995 for references).  Thorstenson and Pollock (1989) 
questioned the adequacy of Fick's laws for describing soil-gas 
transport.  Gas flow is also under study at Yucca Mountain and at 
the Beatty experimental site.  

R3 - Ground-water contamination often involves organic chemicals 
that are immiscible with water; flow and transport models for such 
situations have been proposed but have not been extensively tested 
in the field or laboratory.  In laboratory experiments, Herkelrath 
and his associates found heretofore unreported coupling between 
liquid and vapor flow in porous media containing water, air, and 
organic liquid.  Essaid and Herkelrath are developing improved nu-
merical models for these conditions, and Essaid is addressing the 
biological component.  

R4 - Chiou has studied the sorptive and partition interactions of 
a wide variety of organic pollutants in unsaturated zone materials.  
For references, see Nichols (1995).

R5 - The code VS2DT developed by Healy has been modified to include 
energy transport through variably saturated media.  Healy is also 
developing a two-dimensional Modified Method of Characteristics 
code for the simulation of unsaturated zone transport.  

R6 - Research on tracers in the unsaturated zone is being carried 
out by Coplen (tritium, stable isotopes, nitrogen), Kendall (ni-
trogen, stable isotopes, strontium), and Plummer and Busenberg 
(CFCs).  For references see Nichols (1995).



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