David Schaad

The dipole-flow test with a tracer: a new single-borehole tracer test for aquifer characterization

We propose a new aquifer characterization test, the dipole-flow test with a tracer (DFTT), and develop its interpretation methodology. Combining the dipole-flow test (DFT) and a tracer test, the DFTT is a single-borehole, forced-gradient tracer test. The DFTT device isolates an injection and an extraction chamber in a well with inflatable packers and utilizes a small pump to create a dipole-flow pattern. After a steady-state flow field is reached and the pumping rate and chamber drawdowns are measured, a tracer is released into the injection chamber, and the concentration breakthrough curve is recorded in the extraction chamber. In developing the DFTT model, we assume that the aquifer is homogeneous on the scale of the test and that the well has no skin zone. We use a streamtube approach to semi-analytically simulate the tracer transport in a DFTT and determine the necessary relationships for estimating the longitudinal dispersivity as well as the radial and vertical hydraulic conductivities. The arrival time of the peak concentration is linearly related to the anisotropy ratio, and the arrival time of the tracer front is related to the longitudinal dispersivity. We present data from preliminary DFTTs conducted with Rhodamine WT (RWT) as a tracer at the Lizzie Field Site located between Farmville and Maury, NC. Our results demonstrate that this single-borehole tracer test is feasible and that its estimates of dispersivity are consistent with those reported in literature, whereas its estimates of hydraulic conductivity differ from the flowmeter-test estimates by less than an order of magnitude. This difference is most likely caused by the natural aquifer heterogeneity and different characterization scales of the two tests. The sorption of RWT and its composition of two differently sorbing isomers complicate the nature of the DFTT breakthrough curve and its interpretation. The use of a conservative tracer, such as bromide, should eliminate this complication. The skin effects readily manifest themselves in the DFTT breakthrough curve as extra and/or recirculated peaks. The presented interpretation methodology applies to cases with insignificant skin effects.

Visits to the U.S. Bureau of Reclamation from South Asia and the Middle East, 1946–1990: An indicator of changing international programs and politics

International study programs have contributed to the diffusion of modern irrigation technologies, approaches, and problems. But the patterns and processes of international travel have received little attention to date. This paper examines foreign visits to the US Bureau of Reclamation (USBR) from South Asia and the Middle East from 1946 to 1990. Using data from the Foreign Activities Branch of the USBR, we compare the participation rates from 16 countries in South Asia and the Middle East. India, Turkey, and Egypt had the highest rates of participation, followed by Pakistan, Iran, and Israel. The frequency of visits is influenced by political, economic, cultural, and institutional factors. But the most important factors appear to be foreign relations and geopolitics. Given the problems faced by national irrigation bureaucracies around the world, there is a need to focus more directly on political factors than in the past.

Protecting Off-Site Populations and Site Workers from Vapor Discharges during Shallow Soil Mixing at the North Carolina State University National Priorities List Site

Abstract Although vapor monitoring is generally a component of remedial action activities, most sites do not have routine gaseous releases or vapor clouds erupting from the soil during implementation of the cleanup process (or during cleanup of the site). At the North Carolina State University Lot 86 National Priorities List Site, over 8410 m3 (11,000 yd3) of chemical waste was disposed at the Site, including organic solvents and shock-sensitive and airand water-reactive compounds. During the Remedial Action, it was imperative to protect site workers and off-site populations from potential inhalation exposures. Engineering controls were incorporated into the shallow soil mixing process to limit the release of gaseous compounds. To quantify potential exposures to on-site and off-site receptors, modeling was conducted to evaluate potential exposure routes and migration pathways. To demonstrate acceptable levels of airborne constituents, a multifaceted air sampling and monitoring program was implemented. To ensure that potential exposures could be quantified, passive dosimeters, continuous real-time monitoring, time-weighted whole air sampling, and grab samples of vapor clouds were all critical components of the air monitoring program. After the successful completion of the Remedial Action, the pre-Resource Conservation and Recovery Act (RCRA) chemical waste generated from the University’s educational and research laboratories was entirely encapsulated and neither on-site workers nor off-site populations were exposed to analyzed compounds above any health-based action level (i.e., 15-min short-term exposure limit [STEL], 8-hr threshold limit value, or time-weighted average permissible exposure limit)