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UZIG-News - February 1996 - Issue 4

edited by John R. Nimmo (jrnimmo)

Newsletter of the Unsaturated Zone Interest Group (UZIG) of the US Geological Survey. 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.

Please contact authors or other people mentioned in UZIG-News with any questions, comments, or suggestions. For contact from outside the system, append @usgs.gov to the internal USGS email addresses given.

Contents:

Line 38    News and Information

Line 79    A USGS Reference Handbook on Unsaturated-Zone 
	   Investigational Techniques, by Brian J. Andraski (andraski)

Line 149   Pre- and Post-Processing Programs for VS2DT, by Terrie Lee
           (tmlee)

Line 216   Hydraulics and Nutrient Transport beneath an Infiltration
           Basin near Orlando, Florida, by David M. Sumner (dmsumner)

Line 337   High Tritium and Carbon-14 Activities in Unsaturated-
           Zone Gases near the Beatty, Nevada Low-Level Radioactive-
           Waste Burial Site, condensed from: R.G. Striegl (rstriegl),
           D.E. Prudic, J.S. Duval, R.W. Healy, E.R. Landa, D.W. 
           Pollock, D.C. Thorstenson, and E.P. Weeks, 1996, USGS 
           Open-File Report 96-110

News and Information

++ National UZIG meeting postponed ++

Because of funding uncertainty and travel limitations the national UZIG meeting that had been scheduled for this month has been postponed until next fiscal year. Terry Rees (tfrees) will be making the arrangements for this meeting, to happen in San Diego in late November 1996.

++ Contributions to UZIG-News ++

Anyone can write an article or announcement for UZIG-News. It won't count as a publication, but is an excellent way to publicize and get feedback on your work, or to bring up an issue for email discussion and effective action. (Note examples in this issue!) One possibility is to modify an abstract, extended abstract, or short report that has already been written. Usually the modifications needed are to amplify the introductory material, lighten up or delete the more ponderous details, and put added emphasis on the most colorful highlights. Contact John Nimmo (jrnimmo) for more information or for a list of guidelines for contributors.

++ Hydrology Days ++

Hubert Morel-Seytoux calls attention to the Sixteenth Annual 'Hydrology Days', to be held April 15-18 at Colorado State University, Fort Collins. The program features talks on a wide variety of hydrologic topics. It is highlighted by a special session in honor of Donn DeCoursey with many presentations from USDA-ARS researchers dealing with measurements and modeling experimental watersheds. To receive a complete schedule by email, contact Hubert (morelsey@leland.stanford.edu).

A USGS Reference Handbook on Unsaturated-Zone Investigational Techniques

by Brian J. Andraski (andraski)

In the USGS we need a reference publication on unsaturated-zone methodologies. The techniques we use in water supply and water quality studies that involve the unsaturated zone usually require a combination of methods and technologies. These frequently need some modification for the purpose at hand, and they often do not have adequate support from commercial sources or existing handbooks.

The UZIG can take the lead in producing a report that would serve as a reference and guide for people working on unsaturated-zone problems. A likely form of publication would be a Techniques of Water-Resources Investigations Report (TWRI), the purpose of which is to present techniques used in collecting, analyzing, and processing hydrologic data to a technically-oriented audience. A possible outline of major topics:

     LABORATORY METHODS 
     Water retention relation
     Saturated hydraulic conductivity
     Unsaturated hydraulic conductivity
     Solute Transport properties--dispersion coefficient, 
          retardation factor
     Other Physical properties--bulk density, particle-size 
          distribution 
     
     FIELD MEASUREMENT and MONITORING METHODS
     Soil sampling
     Water content
     Water potential
     Saturated hydraulic conductivity
     Unsaturated hydraulic conductivity
     Soil temperature
     Soil gas pressure
     Soil gas sampling
     Soil solution sampling
MODELS for ESTIMATING WATER RETENTION and HYDRAULIC CONDUCTIVITY RELATIONS

This list is a start, intended to provide a basis for further discussion. For each topic, the report would include: (1) introduction and basic principles, (2) description of equipment and procedures, (3) comments, precautions, notes on limitations, etc., and (4) references.

If you have any interest in this report, as a user or contributor or both, please consider these questions:

1. Should the UZIG pursue development of a TWRI on unsaturated-zone methodologies?

2. Is the list of proposed topical areas appropriate? Any desirable additions or deletions?

3. Are you interested in contributing to this effort? On which topics?

Please send your answers (a simple yes or no, or more elaborate if you wish) to andraski@usgs.gov by April 1. Any additional comments and suggestions you might have also will be appreciated. I will compile the answers and summarize them in the next issue of UZIG-News.

Pre- and Post-Processing Programs for VS2DT

by Terrie Lee (tmlee)

One of the best and most widely used programs for modeling water and solute transport in the unsaturated zone is VS2DT. Created mainly by Rick Healy, this is a USGS product available to anyone.

At present, unfortunately, no programs are generally available to display output from VS2DT, or to assist in preparation of input data sets, according to recent discussions with Healy, John Nimmo, Arlen Harbaugh, and VS2DT modelers inside and outside the USGS. The output of VS2DT is in long, rigidly formatted tables. But what the user needs is a set of contour plots, hydrographs, profiles of a quantity such as flux or concentration, or some other type of visually revealing output that highlights the important results. So individual users write their own programs to meet the immediate needs of a particular project. This is a time-consuming process, especially when repeated by numerous modelers throughout the Survey and elsewhere. It takes valuable time and funding away from other vital project activities. The effort required also discourages the broader use of VS2DT by USGS and other scientists.

The solution is to have a widely available set of accessible pre- and post-processing routines that would serve the great majority of applications. There might already exist some unpublicized routines that could partly serve this purpose. Or perhaps it would be helpful to organize a pooled effort to create and distribute them.

We'd like feedback from the UZIG on this issue. Take a minute to respond to any or all of the topics listed below.

  1. What sorts of programs would you like to see generally available for use with VS2DT? Examples might include:
  2. Plot hydrographs of head or pressure at different x-z locations Contour output values
    Plot concentration profiles or time series
    Plot velocity vectors
    Plot the elevation of the water table at distance x and time t
  3. How has the current situation with pre- and post-processing software for VS2DT affected your project work?
  4. What computer do you use to run VS2DT and to display results, (e.g., Unix, PC, or both)?
  5. What graphics packages do you use and which ones would be most useful for displaying VS2DT results?
  6. What programs have you written or obtained from other users, software companies, etc., to assist in your modeling? Give a short description.
  7. Are these programs generic enough, or documented enough, to be used by another modeler? Could they be readily revised for this purpose?
  8. What might be the best way to create programs of this type and make them generally available to VS2DT users?

Send all comments before April 1 to tmlee@usgs.gov. The responses will be summarized in the next newsletter.

Hydraulics and Nutrient Transport beneath an Infiltration Basin near Orlando, Florida

by David M. Sumner (dmsumner)

What extra "clean-up" is offered by percolation of treated wastewater through the unsaturated zone beneath an infiltration basin?

What advantages are offered by investigating the unsaturated zone in a study of an infiltration basin flow system?

These are among the questions addressed by a USGS study at the infiltration basin facility used by the Reedy Creek Improvement District (RCID) for disposal of about 7 Mgal/d of treated wastewater from Disneyworld. This method of disposal has become popular worldwide because (1) it avoids streamflow eutrophication resulting from the alternative of surface-water disposal, (2) it allows for further wastewater clean-up (of viruses, nitrogen, and phosphorus) as water passes through the unsaturated zone, and (3) it produces aquifer recharge.

The 1,000-acre facility consists of 85 basins constructed in fine-to-medium sand. Treated wastewater applied to the basins contains about 4 mg/l nitrogen (primarily nitrate and organic nitrogen) and 0.3 mg/l phosphorus (orthophosphate and organic phosphorus). Infiltrating water passes through an unsaturated zone 15 to 45 feet in thickness before reaching the surficial aquifer. Water in the surficial aquifer discharges laterally to wetland areas and streams or moves downward through a semi-confining unit to the upper Floridan aquifer. Basins are loaded on a 5-week rotating schedule - 1 week loading period followed by 4 weeks "rest". During the rest period the basin floor is renovated through drying and disking to prevent development of an impeding layer. During the 1-week loading period, basin flooding occurs over about 17 hours of each day, allowing some soil reaeration during the intervening time.

One basin, located where the water table is 37 feet deep, was selected for controlled experiments. Instruments were set up to monitor (1) nutrient evolution along a vertical flowpath and (2) the hydraulic response, including water-table mounding, infiltration rate, and matric potential. Samples for water quality came from a series of short-screen (2-feet) wells below the water table and from point samplers above the water table. Use of the point samplers, rather than lysimeters, reduced headaches and afforded substantial cost savings, but limited sampling above the water table to periods of positive pressure during basin flooding. The point samplers consisted of a 1/2-in. diameter slotted aluminum point, driven to the desired depth, with a 1/8-in. diameter polypropylene tube to a peristaltic pump at land surface.

Clean-up of nitrogen (denitrification) was minimal. In fact there sometimes were nitrate "spikes", spatially limited regions of high concentration, in the subsurface. Some spikes exceeded the concentrations of the applied wastewater by an order of magnitude. Spikes appeared in two locations: high in the profile, a few feet below the surface, and also just below the water table. These two locations are collection areas for organic nitrogen during basin loading as a consequence of the effects of adsorption, straining, and sedimentation. The lower location preferentially captures the coarse fraction of organic nitrogen because of the sharp reduction in the magnitude of pore-water velocity as infiltrating water reaches the water table. Reduced fluid shear in this lower location, a consequence of the slower pore-water movement, allows for enhanced sedimentation and filtration. The finer fraction, largely colloidal and dissolved, is not subject to sedimentation with slacking pore-water velocity. The fate of this accumulated organic nitrogen is to slowly mineralize to ammonium, which produces high levels of nitrate through nitrification. This spiking effect was shown to be influenced by the schedule of basin loading--long flooding and resting periods amplified spike concentrations. Both of the observed spikes occurred after an extended period (several weeks) of basin "rest." To manage the basin so as to allow denitrification to proceed--providing real nitrogen clean-up--might be possible, making use of longer flooding periods that may provide the necessary anaerobic conditions. However, the management strategy will require a carefully chosen trade-off between enhanced net clean-up and amplification of nitrate spikes.

Unlike nitrogen, phosphorus clean-up was substantial--about 90% of the input was removed--largely because of adsorption of orthophosphate onto abundant iron and aluminum oxyhydroxides. Like organic nitrogen, however, coarse-fraction organic phosphorus that eluded the unsaturated zone "filter" accumulated just below the water table because of the slacking pore-water velocity. Yet unlike nitrogen, phosphorus did not accumulate in a spike during the rest period. Apparently the phosphorus was immobilized in a solid phase after mineralization, perhaps precipitating as calcium phosphate in the region of organic phosphorus accumulation just below the water table.

Two approaches provided different simulations of water-table mounding beneath a basin. One approach considered both the unsaturated and saturated zones using the VS2D program. The other considered only the saturated zone, using the Hantush analytical solution. The more general unsaturated/saturated formulation was used to estimate the error inherent in the computationally simpler Hantush approach. The results show that the saturated-zone-only approach can be appreciably in error, particularly as the period of flooding decreases, as the soil texture becomes finer, as the depth to the water table increases, and as the soil heterogeneity increases. Another drawback is that this approach produces no information on the hydraulic status of the unsaturated zone, leaving out, for example, the saturation status and pore-water velocity that are important to a comprehensive understanding of nutrient transport and transformation beneath the basin. The combined approach using VS2D, however, produced estimations of (1) subsurface hydraulic parameters, (2) unmeasured features of the flow system during the field experiment, and (3) hydraulic response under hypothetical scenarios of basin loading.

In summary, the clean-up offered by percolation of treated wastewater through the unsaturated zone is substantial in the case of phosphorus, but negligible for nitrogen. In fact, negative consequences-- nitrate spiking--can occur as a result of this form of treatment, although these can be mitigated through judicious basin management. Explicit inclusion of the unsaturated zone in an analysis of the flow system beneath an infiltration basin provides an improved means of estimating ground-water mounding in the saturated zone.

 

High Tritium and Carbon-14 Activities in Unsaturated-Zone Gases near the Beatty, Nevada Low-Level Radioactive-Waste Burial Site

condensed from: R.G. Striegl (rstriegl), D.E. Prudic, J.S. Duval, R.W. Healy, E.R. Landa, D.W. Pollock, D.C. Thorstenson, and E.P. Weeks, 1996, USGS Open-File Report 96-110

Deep unsaturated zones in arid regions have received much attention as potential burial sites for low-level radioactive waste because (1) the probability of contaminated water movement from such sites is thought to be extremely small, (2) the buried waste and waste containers are thought to be very stable in arid environments, and (3) arid sites can generally be located far from population centers and their water supplies.

Two radionuclides of concern in low-level waste are tritium, which incorporates in the water molecule, and carbon-14 (14C), which is long-lived and can be incorporated into living tissue. Conventional thought has been that, at arid sites, a large fraction of the tritium is in liquid water and other substances that move slowly, if at all, so that tritium movement in general is highly limited. For 14C it has been thought that too little water is available to allow extensive chemical or biological degradation of this waste. Measurements near the low-level radioactive-waste burial site near Beatty, Nevada may contradict these conceptions.

Tritium in water vapor (HTOv) and carbon-14 in gaseous carbon dioxide (14CO2) were recently measured in much greater than ambient activities near the Beatty site (Prudic and Striegl, 1995, USGS Open-File Report 95-741). At borehole UZB-2, about 100 m south of the burial site, the elevated activities prevail throughout the 108-m-thick unsaturated zone for HTOv and to at least 34 m for 14CO2. Peak activities about a hundred times greater than ambient were found for HTOv at 15 to 25 m depth and for 14CO2 at 0 to 7 m. The observations are supported by tritium activities in water transpired from six nearby creosote bushes and one Russian thistle. The activity gradients in the unsaturated zone, and comparative measurements from a test hole and a creosote bush 3 km away, identify the waste burial site as the probable origin of the increased amounts of these radionuclides.

The site is in a gently sloping part of the Amargosa River valley near the toe of an alluvial fan. The uppermost 30 m of sediments predominately are layers of moderately sorted sand and gravel deposited in ancestral Amargosa River channels, separated by intervals of a more poorly sorted mixture of silt, sand, and gravel that was deposited in the adjacent flood plains. Below 30 m the sediments become finer grained and wetter (Andraski and Prudic, In Press). The average annual precipitation is about 11 cm.

>From 1962 to 1992 radioactive wastes were buried in unlined trenches excavated 2 to 15 m into dry alluvium. The wastes were covered with the excavated materials. Hazardous chemical waste burial continues at an adjacent site. Materials buried include solid wastes in metal drums, concrete casks, and wooden and cardboard boxes. The operating license required that liquid waste be solidified with portland cement before burial. An investigation by the U.S. Nuclear Regulatory Commission Office of Inspection and Enforcement (1976, IE Inspection Report 76-02) suggests that liquid wastes delivered to the site had been put directly into the trenches. At least 600,000 gallons of liquid waste were delivered to the site before 1976.

Of the possible mechanisms for establishing the tritium and carbon-14 distributions, gas diffusion was considered first. The model of Smiles, Gardner, and Shulz (1995, WRR 31:1483-1488) was used for calculations. Even when modeled with a retardation factor of 100, 14CO2 diffuses much faster than tritium, largely because a much greater fraction of the 14CO2 is in the gas phase. (The liquid phase acts as a very large sink for the radioisotopes, inhibiting radioactive gas transport.) In 30 years 14CO2 can diffuse to lateral distances indicated by the data, though its depth distribution at UZB-2 suggests other mechanisms may also be significant. On the other hand, diffusion alone cannot explain the distribution of tritium at the Beatty site.

Gases can also move advectively along a total-pressure gradient that arises from (1) buoyant forces due to thermal or topographic effects, or (2) atmospheric pressure fluctuations. Test calculations show that for tritium completely equilibrated between vapor and pore water, a pressure difference of 1,300 Pa is required to produce the necessary gas flow in 30 years. This seems impossible to maintain for such a period. Advective gas flow, like diffusion, seems unable to transport tritium to the extent observed at Beatty.

Another possibility for HTOv transport is that liquid tritiated water moved laterally at shallow depth and then percolated downward. Liquid waste or precipitation runoff, if ponded in an open trench, could percolate rapidly through the clean sands and gravels down to a discontinuous sloping clay lens, where it could mound and move horizontally as saturated flow. At the edge of the clay lens it could spill into the sand and gravel and presumably move as unsaturated flow until it reached the next lens. However, clay lenses have not been identified at the Beatty site. An alternative mechanism would involve fingering of liquid flow through gravel and downward percolation through the finer materials until water reached the underlying contact, where it would be inhibited from entering the coarser material by a capillary barrier effect. The water might then migrate down the sloping contact until sufficient head built up to again finger through the next coarse-grained bed.

Whereas gaseous diffusion is a viable transport mechanism for 14CO2, the only possibilities that appear viable for tritium transport involve lateral liquid movement along preferential flow paths. Although the high activities of tritium in vapor samples from UZB-2 indicates the probability of liquid transport, we cannot determine, from our current state of knowledge, how such flow could have occurred. Determination of the mechanisms requires better understanding of (1) the source of the radionuclides at the disposal trenches and the relation of source strength to disposal practices, (2) probable pathways of liquid and gas movement, and (3) the distribution of tritium and carbon-14 in the unsaturated zone.


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Last modified: Mon Mar 1, 2013