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 18.104.22.168 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). References American Society for Testing Materials, (ASTM), 1994, ASTM Stan- dards on Ground Water and Vadose Zone Investigations, Second Edition: ASTM Publication (PCN): 03-418094-38, Philadelphia, PA, 431p. Andraski, B.J., 1995, Simulated trench studies near Beatty, Nevada- -Initial results and implications: in Stevens, P.R., and Nicholson, T.J., eds., Joint U.S. Geological Survey and U.S. Nuclear Regulatory Commission workshop on research related to low-level radioactive waste disposal, May 4-6, 1993, Reston, Virginia. (Note: This Proceedings will be available in late 1995 and will be distributed by Office of Ground Water Tech- nical Memorandum.) Andraski, B.J., Fischer, J.M., and Prudic, D.E., 1991, Beatty, Ne- vada, in Trask, N.J., and Stevens, P.R., eds., U.S. Geological Survey research in radioactive waste disposal-Fiscal years 1986-1990: U.S. Geological Survey Water Resources Investiga- tions Report 91-4084, p. 34-40. Baehr, A.L., Fischer, J.M., Lahvis, M.A., Baker, R.J., and Smith, N.P., 1991, Method for estimating rates of microbial degrada- tion of hydrocarbons based on gas transport in the unsaturated zone at a Gasoline-spill site in Galloway Township, New Jer- sey, in Mallard, G.E. and Aronson, D.A., eds., U.S. Geological Survey Toxic Substances Hydrology Program -- Proceedings of the technical meeting, Monterey, California, March 11-15, 1991: U.S. Geological Survey Water-Resources Investigations Report 91-4034. Baehr, A.L. and Fischer, J.M., 1993, Overview of research on the transport, microbial degradation, and remediation of hydro- carbons at a subsurface gasoline spill site in Galloway Town- ship, New Jersey in Morganwalp, D.W. and Aronson, D.A., compilers, U.S. Geological Survey Toxic Substances Hydrology Program -- Abstracts of the technical meeting, Colorado Springs, Colorado, September 20-24, 1993: U.S. Geological Survey Open-file Report 93-454. 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Blevins, D.W., 1991, Planned studies of movement of nitrogen spe- cies and herbicides on claypan soils at the Management Systems Evaluation Area in Missouri in Mallard, G.E. and Aronson, D.A., eds., U.S. Geological Survey Toxic Substances Hydrology Program -- Proceedings of the technical meeting, Monterey, California, March 11-15, 1991: U.S. Geological Survey Water- Resources Investigations Report 91-4034. Busenberg, E., Weeks, E.P., Plummer, L.N., and Bartholemay, R.C., 1993, Age dating ground water by use of chlorofluorocarbons (CCl3F and CCl2F2), and distribution of chlorofluorocarbons in the unsaturated zone, Snake River Plain aquifer, Idaho Nation- al Engineering Laboratory, Idaho: U.S. Geological Survey Wa- ter-Resources Investigations Report 93-4054, 47p. 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