Savannah River Ecology Laboratory

Tritium Irrigation Facility and Automated Vadose Monitoring System


The opportunity to study tritium movement in a natural system presents a rare opportunity for both physical and biological research. Researchers may take advantage of tritium's properties as a conservative tracer for modeling contaminant transport, as a radioactive tracer for examining biological processes involving water, or as an example of radionuclide contaminant behavior in natural ecosystems. For example, the facility has been used to examine the biological half-life of tritium in natural populations of rodents in normal field conditions; another investigation utilized tritium as a radioactive tracer to examine extent of root competition for water between taprooted and surface-rooted tree species. The facility has also proved invaluable in understanding physical transport processes in the vadose zone under field conditions by allowing researchers to quantify the influence of vegetation on water and contaminant transport. The associated Automated Vadose Monitoring System (AVMS) has been particularly useful in improving model scalability, by providing much more accurate point measurements than are usually available in the field.

 

Tritium Irrigation Facility

collecting pond

Collection pond and dam

The tritium irrigation facility was constructed in 2000 as a collaborative effort between SREL, USDA Forest Service, and SRNS to reduce tritium-contaminated groundwater release into Fourmile Branch from SRS mixed-waste burial grounds constructed in the 1950s. The facility is a contained watershed (approx. 8.9 ha), with an irrigation system and automated sampling system to monitor soil conditions (AVMS, described below). Tritiated groundwater surfacing as a seep above Fourmile Branch is impounded by a dam and used to irrigate natural forest vegetation uphill, which absorbs and transpires it. Irrigated water that is not absorbed by the vegetation sinks below the root zone and returns to the collection pond via the seep. Prior to installation of the irrigation facility, tritium activity in Fourmile Branch downstream of the seep averaged approximately 500 pCi/mL; after 1 yr of operations, tritium activity averaged less than 100 pCi/mL.

Additional information about the tritium irrigation facility is available in SREL publication #2864:
Hitchcock, D. R., C. D. Barton, K. T. Rebel, J. Singer, J. C. Seaman, J. D. Strawbridge, S. J. Riha and J. I. Blake. 2005. A containment and disposition strategy for tritium-contaminated groundwater at the Savannah River Site, South Carolina, United States. Environmental Geosciences 12:17-28. (PDF)

 

Automated Vadose Monitoring System (AVMS)

The AVMS is a remote monitoring system that couples real-time measurements of soil moisture and matric potential with soil solution sampling to increase the efficiency and accuracy of solute and contaminant monitoring. In contrast to most manual techniques, the AVMS allows sampling at regular intervals with specific knowledge of soil conditions at the time of sampling.

The system consists of soil moisture probes, soil matric potential sensors, and suction lysimeters, installed at multiple depths. Volumetric soil moisture content measurements, soil matric potential measurements, and pore water samples are collected at user-defined time intervals. Lysimeter sampling is initiated only when sensors indicate soil conditions are adequate. Pore water samples from the lysimeter are automatically transferred to a fraction collector for storage and retrieval (24-event capacity).

The system currently reads field sensors at 15-second intervals, and stores an average of those readings every 15 minutes. The data is downloaded to a Windows-compatible laptop PC over a serial link. The system also monitors the volume of water being applied to each plot in the irrigation field, via flow meters installed in the irrigation lines.

AVMS diagram

AVMS sensor placement

graph

Automated vs. manual (TDR) soil moisture data

Additional information about the AVMS is available in SREL publication #3023:
Singer, J. H., J. C. Seaman, S. A. Aburime, J. Harris, and D. Karapatakis. 2007. An improved technique for soil solution sampling in the vadose zone utilizing real-time data. Georgia Water Resources Conference, Athens, Georgia, The University of Georgia. (PDF)