In southwestern Utah, we are collaborating with Vic Heilweil of the Utah District and Lori Flint of the California District to measure aquifer recharge rates. Subsurface materials here include sandy alluvium overlying fractured sandstone. To estimate prevailing and episodic unsaturated-zone fluxes and to develop understanding of hydraulic interactions between soil and underlying fractured rock, we have conducted hydraulic-property and related measurements on core samples of unconsolidated sand from shallow depths in and around an experimental infiltration pond. The 61-m diameter surface-water infiltration basin was constructed by the Washington County Water Conservancy District to evaluate recharge to the Navajo Sandstone aquifer at the new Sand Hollow Reservoir construction site. Various tracers were used in this recharge study to isolate possible flow mechanisms, including uranine dye, sodium bromide, compressed helium, and a naturally-occurring bacterial tracer. Preliminary results show that overall, infiltration rates are significantly lower than estimates from a priori numerical modeling based on core hydraulic property measurements. It has been assumed that fracturing within the formation would result in higher recharge rates than predicted by matrix-flow models, as has been the case in fractured-rock infiltration experiments at INEEL and Yucca Mountain. The lower infiltration rates are possibly due to trapped air within the formation or the presence of lower permeability caliche deposits or fine-grained layers. The study is a joint effort by the USBR, the University of Utah, and the USGS.
The Palouse site, in an agricultural area near Pullman, WA, was the first place we demonstrated that the Darcian-SSC (steady-state centrifuge) method can be used to indicate recharge rates. This effort was a collaboration with the WA District and the Infiltration and Drainage Project of the National Research Program (NRP). We chose the Palouse area mainly because (1) its deep (20 m) loess deposits constitute an unusually uniform unsaturated zone, thereby minimizing complexities of sampling and of layered unsaturated flow effects, and (2) as part of the Columbia Plateau Regional Aquifer Systems Analysis Program, this area has estimations of recharge by other methods, for example by Bauer and Vaccaro (1990).
The Darcian-SSC method for determination of point recharge rates requires accurately measured hydraulic conductivity (K) at the existing water content on core samples from a deep unsaturated zone with constant, gravity-driven flow. The steps required are as follows. (1) Obtain core samples from deep enough in the unsaturated zone that moisture fluctuations are negligible. (2) Determine the total downward driving force for water. In a homogeneous medium, at depths where moisture does not fluctuate, the flow is steady and driven only by gravity. In a layered profile, however, the likelihood and magnitude of significant matric potential gradients must be evaluated with a Darcy-based analysis. In either case, Darcy's law can indicate the local recharge rate if K at the water content existing at that depth is known. (3) Measure K with the steady state centrifuge method, which establishes a measurable steady flow of water in a centrifugal field as great as 2000 g. This method is well suited for the required measurements because it gives accurate K values in the low-water-content range important at arid and semiarid sites.
Our recharge indications from the Darcian-SSC method with core samples from depths of 4 to 19 m were consistent with the previously calculated value of 8 cm/year at the site. Our point estimates of recharge rate reasonably exceeded this value slightly in a swale and fell short of it on a ridge top. This is consistent with the simplest hydrologic interpretation, that water mostly runs off the ridge top but tends to collect in the swale, so that both infiltration and recharge are greater in the swale.
In the study at this site we showed that the Darcian-SSC method has a fundamentally sound physical basis independent of other methods (e.g. water balance, tracers) and can be implemented in a practical way. Our chief publication of the basic method and the Palouse study is Nimmo and others (1994).
This site at the western edge of the San Joaquin Valley of California. In 1985 we obtained materials from several meters deep in the unsaturated zone for the first core-sample measurements of unsaturated hydraulic conductivity by the SSC method. Some of the resulting data were published as part of a UZ flux-measurement study by Nimmo and others (1994).
We have obtained subsurface samples from the Amargosa Desert Research Site of the USGS Toxic Substances Hydrology Program, for the investigation of unsaturated-zone water fluxes. Proposed research of the UZ Flow Project includes an experimental study of liquid flow at extremely low water content, measurement of water fluxes, and development of energy-monitoring systems with other projects. The site is adjacent to a low-level radioactive waste site near Beatty, Nevada.
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