UZIG-News - June 1996 - Issue 5

++ !!! First Announcement for National UZIG Meeting !!! ++

The national Unsaturated Zone Interest Group meeting is scheduled for Las Vegas during the week of September 23, 1996. The meeting will include two days of short research presentations and general discussion on USGS and unsaturated-zone issues. Possible field trips to Yucca Mountain and the Beatty low-level radioactive waste disposal site are being planned. The Yucca Mountain trip will include a visit underground to look at current USGS unsaturated-zone studies. Past meetings have been interesting, informative, and of great value to UZIG members. Until 1992 they were held about every 18 months. Because of budget difficulties and other problems, this will be the first one since the group met in Georgia four years ago--so it will be a great and unusual opportunity! Exact dates and times will be announced.

If interested in attending, please respond by July 19 to Alan Flint (aflint) with a cc to Rick Healy (rwhealy). Tell Alan (1) that you plan to attend, (2) whether you want lodging with the group arrangements, (3) whether you would like to go on field trips to the Yucca Mountain or Beatty research sites, and (4) whether you would like to give a talk (probably about 30 minutes in length) and what the title will be.

++ USGS Ground-Water Investigations Involving the Unsaturated Zone ++

A compendium of USGS unsaturated-zone field projects, compiled by Newell Trask, is available as Office of Ground Water Technical Memorandum 95.07. This document relates the purpose and methods used in 38 current and recently finished studies. It has 23 pages of tables, explanatory notes, and references. Anyone interested can obtain Framemaker files of the memo and two attachments by the following anonymous-ftp procedure:

type "ftp 130.11.51.175"
for user id, key in "anonymous"
for password, key in your user id
type "cd pub"
type "get TechMM95.07.doc" for memo
type "get TechMM95.07.A1.doc" for attachment 1
type "get TechMM95.07.A2.doc" for attachment 2
type "exit"

++ Ideas about the Future of USGS Ground-Water Research ++

Paul Hsieh has conducted a survey of researchers in the Ground-Water Hydrology Discipline on the direction of USGS research in this discipline. The report of this survey, though not an official publication or memo, is available by email from Chris Neuzil (ceneuzil). It includes an introductory explanation as well as the detailed responses of the researchers.

by Brian J. Andraski (andraski)

The last issue of UZIG-News presented for discussion the possible development of a USGS reference publication on unsaturated-zone methodologies. Questions included (1) should we pursue such an effort, (2) are the proposed topical areas adequate, and (3) who is interested in contributing.

The responses, while not overwhelming in number, were positive. Ten people responded, six of whom expressed interest in contributing. Replies to the question of pursuing this effort ranged from a simple "yes" to "good/great/wonderful idea" and indicated that such a handbook should be of interest throughout the USGS. Bill Alley, Chief, USGS Office of Ground Water (OGW) also was positive about pursuing the idea.

What is the next step? On the basis of the positive response and indication of support from OGW, the effort is clearly worth pursuing. Immediate efforts should go toward the formulation of a mini-proposal (1-2 pages) for headquarters review that outlines development of a Field Methods Reference Handbook. Additional comments, questions, and indications of support, especially in the near future, will help immensely in getting this project launched! Anyone interested in the handbook, as a user or contributor, please send all comments and suggestions by July 1 to andraski@usgs.gov or call me at (702)887-7636.

by Terrie Lee (tmlee)

In the last issue of UZIG-News, we asked for feedback on pre- and post-processing programs to be used with VS2DT. Newsletter readers were virtually silent on this subject. Maybe VS2DT users are content with the status quo. Or, perhaps they are too busy manipulating input and output files to respond!

The two modelers that did respond both use VS2DT on a Unix platform and manipulate input by a screen editor. They process output files with their own Fortran programs linked to DISSPLA or G2 graphics. The lack of programs does inhibit the use of the model.

What turned up in our search for software? Two pre-processor routines, but not much for post-processing, and nothing for Unix for either end. First, for PC's, the International Ground Water Modeling Center (IGWMC; igwmc@mine.edu) at Colorado School of Mines in Golden sells a pre-processor for VS2DT on PC that uses a text-based DOS program. The post-processor rewrites output in a format compatible with user-supplied matrix-contouring routines. The program was released in 1992 and has not been upgraded because of low demand. (Meanwhile, IGWMC just released a sophisticated program called Hydrus-2D that interfaces with a finite element variably-saturated flow code called SWMS-2D that was developed by ARS/Salinity Lab in Riverside, CA. The program operates in MS Windows and can use meshmaker graphics.) Second, a MacIntosh preprocessor for VS2DT is being developed by Clyde O'Neal (cfoneal), USGS, Menlo Park. The program displays the model grid, and the user clicks and drags colors across the grid to designate areas with different soil properties. The program has not yet been used in a hydrologic study.

In our USGS Office of Ground Water (OGW) the creation of pre- and post-processing programs for existing USGS model codes is currently a priority, according to OGW Chief Bill Alley (walley). OGW Hydrologists Dave Pollock (dwpolloc) and Arlen Harbaugh (harbaugh) are presently working to establish the best means for generating these programs, whether by vendor contract or through efforts of USGS personnel.

Your comments are still very important! Only though comments from potential users can the need for these programs be demonstrated. This demonstration of need is essential for securing support from OGW or other sources to create user interfaces for VS2DT. Please send any comments you have, whether a simple statement of interest or a detailed potential application, to tmlee@usgs.gov. 

by H. Essaid (hiessaid), W.N. Herkelrath (wnherkel), and G. Delin (delin)

On August 20, 1979, a buried oil pipeline near Bemidji, Minnesota broke, spilling about 1.7 million liters of crude oil, about 30% of which remains after clean-up measures. The oil collected in topographic depressions where large volumes of oil infiltrated into the subsurface, forming two main bodies of oil floating on the water table.

The site is in a pitted and dissected glacial outwash plain, with depth to the water table from 0 to 8 m. Flow through the aquifer is generally horizontal towards a lake 300 m from the break.

Since 1984, this spill site has been a USGS Toxic Substances Hydrology Program research site, with about 30 USGS and university researchers involved at present. Research efforts at the site are directed towards understanding the coupled physical, chemical, and biological processes that affect the migration and fate of oil in the subsurface. These processes include the multiphase flow of air, oil, and water, the partitioning of volatile hydrocarbon components into unsaturated zone gas, the dissolution of hydrocarbon components into the water phase, the microbially mediated redox processes, and the impact of these reactions on the organic and inorganic geochemistry of the ground-water system.

Recent work at the site has characterized the subsurface oil distribution in detail. For this purpose it was necessary to develop a sampling technique to recover relatively undisturbed core samples from both the unsaturated and saturated zones while maintaining the in situ pore-fluid distribution. The technique uses a freezing-tip core barrel to hold intact samples in the barrel. More than 800 core sections from 23 boreholes penetrating the two main oil bodies have been analyzed. Although the two oil bodies are only about 150 m apart, they show different distributions of oil saturation. At the south oil pool, where the water table was about 2 m below land surface and the maximum observed oil saturation was 0.62, the oil body had an asymmetrical lens shape with very little oil in the thin unsaturated zone. At the north oil pool, where the water table was about 6.5 m below land surface and the maximum observed oil saturation was 0.74, the oil body had an irregular shape with large amounts of oil in the thick unsaturated zone. Geostatistical simulation conditioned to the field data was used to simulate the spatial variability in permeability at the two sites. These simulations were used as inputs to a cross-sectional, multiphase flow model to simulate the post-spill oil movement. South pool simulation results indicated that spatial variability and capillary pressure - saturation hysteresis had to be included in order to reproduce the general features of the observed oil distribution. On the other hand, at the north pool the presence of fine-grained lenses within the outwash was the dominant factor influencing the oil distribution, and incorporation of hysteresis into the model was not crucial.

The dissolution of hydrocarbon components depends on the rate of water flow through the oil body. Multiphase flow simulations using geostatistical simulations of spatial variability for both homogeneous and heterogeneous aquifers indicate that heterogeneity reduces the rate of water flow through the oil zone. This effect is related to high oil saturation in the coarse sediments of heterogeneous media. However, water flow through the oil body is even more sensitive to recharge rate. Simulations showed that when there was little or no recharge, the water tended to bypass the oil and flow around the oil body. Increasing recharge caused the water to flow through the oil body, resulting in higher rates of dissolution. Using our best estimates of recharge, spatial variability, and oil distribution, the simulations suggest that it will take on the order of 100, 500, and 2000 years, respectively, to dissolve all of the benzene, toluene, and xylene in the oil body.

Although there is a wealth of knowledge regarding the general behavior of the Bemidji system, many unsaturated zone processes at the site are still not well understood, including (1) the complex spatial and temporal variability of recharge at the site, (2) the response of subsurface oil flow to variable recharge and a fluctuating water table, (3) the quantitative effects of spatial variability of hydraulic properties and topography on recharge, and (4) local reductions in recharge because of soil hydrophobicity caused by exposure to the oil.

An effort to characterize recharge at the Bemidji site is being planned. The rates of ground-water recharge will be measured beneath several areas to study the effects of topography, soil hydrophobicity, and fine-grained lenses on recharge rate. Soil moisture and wetting fronts will be tracked in the unsaturated zone using time-domain reflectometry (TDR) probes installed horizontally in the walls of a shallow pit, or vertically in a borehole, at about 20-50 cm intervals to a depth of about 3 m. Tensiometers equipped with pressure transducers will be installed in each of the areas to measure soil-moisture tension and oil pressure. Thermocouples will be installed to monitor temperature changes due to recharge events and biodegradation processes. A shaft encoder will be installed at each site to monitor water-table fluctuations. At least one precipitation gage will be installed to measure precipitation. All of the monitoring equipment will be connected to a datalogger for real-time monitoring of changes in the variables. A neutron access tube may be installed in each area to supplement the TDR data with neutron probe data. The TDR and water-level data will be used to estimate ground-water recharge rates at each of the sites. These data will be used to evaluate the significance of factors affecting the magnitude and timing of recharge at the site.

Because of the challenging nature of measuring recharge, we would like to compare as many techniques as possible. We welcome the involvement of other unsaturated zone researchers. Please contact one of the authors for more information about the site or research possibilities.