UZIG-News - June 1998 - Issue 8

UZIG-News June 1998 Issue 8 edited by John R. Nimmo (jrnimmo@usgs.gov) Newsletter of the Unsaturated Zone Interest Group (UZIG) of the US Geological Survey (USGS). The purpose of this newsletter is to enhance communication within the UZIG. It should not be quoted or cited as a publication. Send desired changes in the mailing list to jrnimmo@usgs.gov. Please contact authors or other people mentioned in UZIG-News with any questions, comments, or suggestions.

++ National UZIG Meeting and GW Recharge Workshop ++

Plans for the next UZIG meeting are now coming together. In addition to presentations, discussions, and possible field trips, a workshop on ground-water recharge is proposed to immediately follow the meeting. Two time frames are being considered, October-December 1998 and January, 1999. There also are two possible locations, Menlo Park, CA and Albuquerque, NM.

Feedback from potential attendees is essential -- PLEASE RESPOND by June 30 to Rick Healy (rwhealy@usgs.gov) and indicate:

1. if you have an interest in attending the UZIG meeting,
2. if so, would you plan on attending the recharge workshop, and
3. the preferred time and place for the meeting.

Related events, for coordination of timing or avoidance of conflict, include the following: October 9-11 Friends of the Pleistocene field trip to the Amargosa Desert Research Site and vicinity; October 18-23 Soil Science Society annual meeting in Baltimore; October 26-29 Geological Society of America annual meeting in Toronto; sometime during November 1-December 4 (two-day) Amargosa Desert Research Site external-internal review of research proposal; December 6-10 American Geophysical Union fall meeting in San Francisco; February 23-25 Middle Rio Grande Basin workshop in Albuquerque; March 8-12 National Toxics Substances Hydrology Program meeting in Charleston.

The last nationwide UZIG meeting was in Las Vegas in September 1996. See the UZIG home page at http://wwwmn.cr.usgs.gov/mnlocal/uzig for details.

++ New USGS Project on Ground-Water/Surface-Water Interaction ++

The Ground-Water Resources Program (GWRP) will begin a new project to evaluate water-resources issues associated with ground-water/surface-water interaction in the southwestern U.S. The project chief, who will be located in Tucson, AZ, will be selected soon. Project planning will begin this summer, and project work will commence in FY 1999.

This is a prototype for proposed studies that evaluate ground-water issues common to multistate regions of the nation as part of the GWRP. Because this project involves many states in the arid southwestern U.S., the role of the unsaturated zone in evaluating processes such as mountain-front recharge is important for several aspects of the project. If you have general questions about this or other aspects of the Ground-Water Resources Program, please contact Norm Grannemann at (517) 887-8936 (nggranne@usgs.gov).

++ Meeting with the USNRC to Discuss the Maricopa Field Study ++

The US Nuclear Regulatory Commission will hold a meeting to discuss scientific results of the Maricopa Project on July 9th at USNRC Headquarters in Rockville, Maryland. Presentations by Peter Wierenga, Art Warrick and Michael Young will discuss the unsaturated-zone monitoring and field studies at the Maricopa field site in Arizona. Interested USGS employees are invited, along with staff from EPA, USDA-ARS, DOE, and other agencies.

See the related article below by Jim Tindall on the Maricopa Technology Transfer Workshop.

++ Middle Rio Grande Workshop ++

The Second Annual Middle Rio Grande Basin Study Workshop was held February 10-12 in Albuquerque. Over 50 scientists from Geologic, National Mapping, and Water Resources Divisions, as well as from outside agencies, gave presentations on their ongoing work in the Middle Rio Grande Basin (MRGB) of New Mexico. The main purpose of the planned 5-year MRGB study is to improve the understanding of the water resources of the basin. The study is currently in its third year. Among the presentations of particular interest to UZIG members were measurements of tributary, inter-arroyo, and mountain-front recharge by Jim Constantz; streamflow loss and infiltration by Carole Thomas and Jim Constantz; environmental tracers of recharge by Dave Stonestrom and Kathy Akstin; thermal tracking of recharge by Amy Stewart, Anne Ronan, and Jim Constantz; centrifuge hydraulic property measurements by Angus Lewis and John Nimmo; and aquifer temperature profiles by Jim Bartolino. The proceedings from the workshop are once again being published as an Open-File Report and are currently in press.
--Jim Bartolino (jrbartol@usgs.gov)

by David Eckhardt (daeckhar@usgs.gov)

Much of the difficulty in studies of soil-water flow is associated with obtaining accurate values of hydraulic conductivity for heterogeneous and anisotropic field soils. However, the hydraulic conductivity of many soils can be estimated in the field under steady infiltration of ponded water, with monitoring of redistribution and drainage. Other soil properties, such as water retention characteristics, can be determined through laboratory analysis of representative soil cores that are collected to spatially coincide with the hydraulic conductivity measurements. The statistical distributions of these hydraulic properties, combined with reliable estimates of water infiltration and evapotranspiration over time, provide a basis for evaluating the spatial variability in soil-water flow.

In June 1990, the U.S. Geological Survey began a study of spatial differences in the hydraulic properties and water movement at an agronomic plot in the Kansas River Valley near Topeka, KS. The purpose was to statistically evaluate the physical and hydraulic properties of the soil and to relate these properties to the transient fluxes of soil water during drainage.

A typical vertical profile in this plot consisted of 1.2 m of fairly uniform silt loam over a 1.8-m zone of layered silty, clayey, and sandy loam, which overlies about 3 m of medium to coarse alluvial sand. The water table was in the alluvial sand about 6 m below land surface.

A series of gravity-drainage tests were conducted at each of 15 sites within the 0.75-ha plot. These entailed ponding water at a constant head within 100-cm diameter steel rings sealed 15 cm into the soil. Each ring had a neutron-probe access tube in its center for measurement of soil water content. Water infiltrated until flow was steady, which usually took about three weeks. Infiltration then was discontinued, and plastic sheeting was installed at the soil surface to prevent evaporation or additional infiltration. The water contents during drainage were measured at 15-cm intervals to a depth of 60 cm at each of the 15 access tubes during a period of up to 16 days after infiltration ended.

Two forms of the relationship between unsaturated hydraulic conductivity and soil water content, the van Genuchten-Mualem function and the Watson power function, were fitted to the measured rates of gravity drainage at the access-tube sites. The van Genuchten water-retention function was fitted to laboratory measurements of water desorption from core samples. Hydraulic parameters from measurements at each depth and site were used in eight different numerical models for simulation of soil-water flow. The eight approaches were designed to evaluate assumptions about the uniformity, spatial variability, and the interdependence of the hydraulic conductivity and retention parameters. The effectiveness of each approach was evaluated through comparisons of simulated and measured drainage rates.

For the soil studied here, the power function for hydraulic conductivity produced slightly better estimates of drainage than did the van Genuchten function. It also has the advantage of easier parameterization through linear regression of drainage-rate measurements.

Of the eight simulation approaches, those that used a global mean representation of water retention for the entire field provided better flow simulations than did those that used discrete values of retention measured at each profile. This result directly supports the assumption that gravity dominates the drainage in relatively uniform soil profiles. If this is true, emphasis can be placed more on the measurements of spatial and depth distributions of conductivity than on retention. In this way, variability from several sources is quantified within parameter estimates of the conductivity function alone.

The unit-gradient approach used here to compute hydraulic conductivity apparently has considerable robustness to random departures from the unit-gradient assumption caused by vertical variability in texture within a soil profile. In fact, because certain soil layers often control drainage, a characterization of hydraulic properties that integrates the effective properties of an entire soil profile by in situ measurement can provide a better approach than the use of detached soil cores, which may miss the critical horizons.

This study has demonstrated that hydraulic conductivity functions and their associated parameter estimates can be used to simulate gravity drainage at individual sites and at the field scale without considering variability in the water-retention characteristics. Effects of variability in water-retention properties can largely be estimated through direct measurements of gravity- drainage rates and the associated hydraulic-conductivity parameters.

by David A. Stonestrom (dastones@usgs.gov), Brian J. Andraski, and Herbert T. Buxton

In 1983, the U.S. Geological Survey (USGS) established a hydrologic research site in the northern Mojave Desert, about 20 km east of Death Valley National Park. The research site comprises a 16-ha plot adjacent to a disposal facility for low-level radioactive waste, 17 km south of Beatty, NV, and a 0.1-ha plot 3 km farther south. The site was established under the auspices of the USGS Low-Level Radioactive Waste Program to serve as a field laboratory for the study of hydrologic processes under natural and disturbed conditions.

In 1997, the site was incorporated into the USGS Toxics-Substances Hydrology Program (Toxics Program) as the Amargosa Desert Research Site (ADRS). The Toxics Program was established in 1982 to develop methods and understanding needed to predict (and if possible prevent) contaminant transport in a variety of hydrogeologic settings.

A goal of the Program at the ADRS is to provide a field laboratory that will enable scientists from different disciplines and affiliations to conduct a sustained multidisciplinary study of a complex, real-world setting. The unifying objective of research at the ADRS is to improve understanding of the mechanisms that control subsurface transport and fate of contaminants in arid environments. A large part of current research concerns developing methods for characterizing flow and transport in desert settings. Better understanding of liquid and gas movement in these regions will advance the state of the science and assist in the formulation of environmental policy and regulation.

General information about the USGS Toxic-Substances Hydrology Program is available at http://toxics.usgs.gov/toxics. This web site contains links to several intensive field investigations of the Program. The newest of these links is to the ADRS web site at http://nevada.usgs.gov/adrs/. The ADRS web site gives information on past and present research, upcoming activities, contact information, abstracts of recent publications, and an up-to-date bibliography.

by John Nimmo (jrnimmo@usgs.gov) The International Workshop on Characterization and Measurement of the Hydraulic Properties of Unsaturated Porous Media was held in Riverside, CA for three days last October. This workshop was initiated by Rien van Genuchten of the US Salinity Laboratory, and was patterned after a similar workshop held in 1989.

The USGS, with funds from the Office of Ground Water, cosponsored this workshop along with USEPA, NASA, USDA, USNRC, and other organizations. Among the 190 attendees, nine USGS scientists actively participated by giving talks and poster presentations, presiding over sessions, and contributing to workshop plans and organization. One example of USGS participation is the article by Dave Eckhardt, based on his presentation at the workshop, in this issue of UZIG-News.

The subject matter of the workshop included the theory, measurement, and indirect determination of unsaturated hydraulic properties. There was much emphasis on applications in agriculture, water resources, and environmental protection. Session topics included pore-scale models and their applications, measurement of hydraulic properties, mathematical approaches, inverse procedures, experiments, multifluid systems, scale issues, uncertainty, databases, and software. The initial session included talks by noted pioneers of pore-scale models, including Lalit Arya, Art Corey, and Hezi Mualem. A concluding panel discussion presented the comments of eight respected scientists from five countries. They reviewed the contradictions as well as the areas of progress that had been highlighted at the workshop.

At a banquet attended by nearly all participants, representatives of the sponsoring agencies spoke about the importance of unsaturated hydraulic property research in accomplishing the missions of their organizations. For the USGS this includes a huge variety of applications concerning the quality and quantity of water resources. Typical examples include pesticide contamination of rural wells, effects on air and ground water of oil spilled into the subsurface, and assessments of the fraction of precipitation that goes through the unsaturated zone to recharge aquifers. Over 300 USGS employees, most of them not unsaturated specialists, have responsibilities related to unsaturated flow. This is an unusually large number, even for an earth-science organization. It underlines the fact that unsaturated-zone techniques must be applied by people with a wide variety of backgrounds in a wide variety of locations and situations--another reason it is essential to develop practical, reliable, (and ideally, easy!) methods of determining unsaturated properties. It was also recognized at the banquet that the UZIG, with its meetings, newsletter, and other activities, helps maintain ties among the widely scattered USGS people working on these topics.

A proceedings volume, with papers based on most of the Workshop presentations, will be published in Fall 1998. The Workshop has a website at http://www.ussl.ars.usda.gov/HYPROP/hyprop.htm, which includes a listing of everyone who attended.

By Jim Tindall (jtindall@usgs.gov)

The Technology Transfer Workshop was sponsored by the Nuclear Regulatory Commission (NRC) and the University of Arizona to share information on the techniques and investigations being done at the Maricopa site in Arizona. The purpose of the coordinated studies at this site is to perform research for evaluating unsaturated-zone monitoring techniques and strategies for low level radioactive waste (LLW) and Site Decommissioning Management Plan (SDMP) locations, as deemed necessary by the NRC. This research, coordinated by Tom Nicholson of the NRC, is being done by various unsaturated-zone scientists at universities and other institutions in the Western U.S; among them are Pete Wierenga, Bridget Scanlon, and Glendon Gee. The general objectives are: (1) to assess state-of-the-art unsaturated zone monitoring systems; (2) to design and implement such a system for potential use in LLW and SDMP sites; and (3) to evaluate strategies for unsaturated-zone monitoring.

The project design was to develop a monitoring system for measuring transport processes from the meter scale to the scale of existing and proposed disposal sites. Criteria for suitability included: (1) ability to merge the monitoring devices into a coherent vadose zone monitoring framework; (2) durability of the devices; (3) flexibility of the devices in terms of installation; and (4) ability to remove the devices from service without creating preferential flow pathways or disrupting the monitoring scheme.

Monitoring at the Maricopa site includes water content measurements by neutron probe, time-domain reflectometry (TDR), electromagnetic induction, and cross-hole tomography. Matric potential was measured with tensiometers, heat dissipation sensors (HDS), and thermocouple psychrometers. To determine solute concentration, suction lysimeters, electromagnetic induction, and cross-hole tomography were employed. Gas transport measurements were made using air injection/extraction and gas pressure monitoring.

The workshop was a combination of classroom and field instruction. Five personnel from the USGS attended: William B. Scott, Kim Perkins, David Hudson, Alan Flint, and Jim Tindall. They were able to share their combined knowledge of experiences with unsaturated zone techniques and monitoring devices and to complement and enhance that of the workshop sponsors and other attendees. All of the monitoring devices listed above were described in detail, including the advantages and disadvantages of each. The attendees were thoroughly introduced to the means for monitoring and assessing the unsaturated zone using state-of-the-art equipment.