USGS Groundwater Information
Flow system analysis using a surface-applied tracer at the Idaho National Engineering and Environmental Laboratory, Idaho
By Kim S. Perkins1, John R. Nimmo1,
Peter A. Rose2, Joseph P. Rousseau2,
Brennon R. Orr 2, Brian V. Twining2,
and Steve R. Anderson3
Diverse hydraulic processes control water flow through the 200-m-thick unsaturated zone at the Idaho National Engineering and Environmental Laboratory (INEEL) in southeastern Idaho. The interbedded basalts and sediments that comprise the subsurface are characterized by complex structure in terms both of preferential flow paths and of layers that contrast in thickness, permeability, porosity and other properties. Water movement in the unsaturated zone at this site may be predominantly vertical, but is likely to be significantly retarded and diverted by features of the basalts and sediments. The behavior and composition of perched water observed since the 1970s suggests that this water may sometimes move laterally or recharge the aquifer at unusually high flow rates and volumes. Several studies (Barraclough et al., 1976; Rightmire and Lewis, 1987; Anderson and Lewis, 1989) have shown that water episodically accumulates in perched layers that typically persist for a few months. By analysis of water levels and measured water content profiles, Cecil et al. (1991) showed that perching can take place within both sediments and basalts, and suggested that specific mechanisms for this might include (1) contrasts in vertical hydraulic conductivity between basalt flows and sedimentary interbeds, (2) reduced hydraulic conductivity in baked zones between basalt flows, (3) reduced vertical hydraulic conductivity in dense, unfractured basalt, and (4) reduced vertical hydraulic conductivity from sedimentary and chemical filling of fractures in basalt. Additionally, the interfaces between any adjacent layers, regardless of whether the upper or lower one is more permeable, may retard flow under unsaturated conditions (Miller and Gardner, 1962).
Large-scale flow in the unsaturated zone is commonly assumed to be slow (a few m/yr or less) and predominantly vertical, especially in arid regions. Slow transport is likely if the flow proceeds in classic diffuse fashion, limited by low hydraulic conductivity. Vertical flow is likely if the main driving force is gravity, or if the subsurface in effect is areally homogeneous. The slow-flow generalization may not hold for circumstances that include such features as unusually thick or geologically diverse unsaturated zones; large, nonuniform inputs of water over the land surface; or layers of contrasting subsurface materials that are likely to inhibit vertical flow and therefore to promote horizontal flow. Preferential flow paths can transport water horizontally to adjacent regions or vertically to the aquifer far sooner than might be predicted based on average medium properties.
In this study, a conservative tracer was applied to artificial infiltration ponds at the INEEL in order to assess travel times to the aquifer and perched water zones.During seasonal periods of high flow, water is diverted from the Big Lost River to a series of ponds in the southwestern portion of the INEEL. At this semiarid location, seasonal streamflow, local runoff, and snowmelts episodically generate large quantities of infiltrating water for periods of days or weeks. The ponds used in this study are used typically for a few weeks every two to three years. During a high flow period, 675 kilograms of a conservative chemical tracer (1,5 naphthalene disulfonate) was applied to water in the infiltration ponds. Fifty-kg bags of tracer were distributed at the water surface over the bow of a pontoon-type power boat, allowing the material to gradually spill out and the propeller to disperse it as the boat moved forward. Water inflow to the ponds continued for 10 days after the introduction of tracer, during which the cumulative flow was about 107 m3. Samples for tracer analysis were collected from nearby wells completed in the Snake River Plain aquifer and in perched water zones. A monitoring network of existing wells, both up- and down-gradient, was selected to characterize the vertical and horizontal extent of infiltrated water. Each well was sampled using a dedicated pump or bailer following USGS protocol (Mann 1996).
The tracer used in this study has been successfully tested for tracing in geothermal systems, though not in ground-water applications. 1,5 naphthalene disulfonate is environmentally benign, easily detectable by ultraviolet-fluorescence spectroscopy, and thermally stable (Rose et al., 2001). The adsorption properties of the polyaromatic sulfonates have generally not been reported, however, under low-temperature geothermal conditions 1,5-naphthalene disulfonate was shown not to adsorb (Rose et al., 1999). Because the sulfonate groups make these molecules very anionic, the polyaromatic sulfonates are not likely to adsorb on negatively charged rocks or sands. In addition, the sulfonate groups render these compounds nontoxic and very soluble in water.
The tracer was detected in the aquifer in one well 0.1 km from the nearest infiltration pond 9 days following tracer introduction, indicating average vertical movement of at least 22 m/day at that location. Tracer was detected in seven wells completed less than 80 m deep in perched water zones, two of them 0.1 km away, one 1.2 km away, and five 1.5 km away. These detections suggest an average horizontal flow rate of 13 m/day or more along at least two flow paths. The results also indicate that very discrete flow paths may exist in this environment.
This study has shown that 1,5 naphthalene disulfonate is stable and conservative in the subsurface to the degree necessary for large-scale saturated- and unsaturated-zone investigations. Our results indicate that fractured basalts allow fast, long-range flow through the unsaturated zone under high-water content conditions generated by ponded infiltration. With a large input of water, the overall geologic structure promoted rapid horizontal flow that persisted over distances greater than 1 km, entirely within the unsaturated zone, 100 m and more above the aquifer. At least under ponded infiltration, fine-textured interbeds and layers of dense basalt did not prevent the rapid recharge through fractured basalt. However, some impediments to vertical flow were sufficiently effective to cause substantial perching and horizontal diversion of flow.Because rapid flow in the unsaturated zone seems to be caused by natural features responding to a common case of ponded infiltration, these results suggest that similar phenomena may occur at other sites where the unsaturated zone is geologically complex.
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Rightmire, C.T., and B.D. Lewis. 1987. Hydrology and geochemistry of the unsaturated zone, radioactive waster management complex, Idaho National Engineering Laboratory, Idaho. Water Resources Investigations Report 87-4198. U.S. Geological Survey.
Rose, P.E., W.R. Benoit, and P.M. Kilbourn. 2001. The application of the polyaromatic sulfonates as tracers in geothermal reservoirs. Geothermics 30:[in press].
Rose, P.E., C. Goranson, D. Salls, and P.M. Kilbourn. 1999. Tracer testing at Steamboat Hills, Nevada, using fluorescein and 1,5-naphthalene disulfonate. In Proceedings, 24th Workshop on Geothermal Reservoir Engineering, Stanford University, SGP-TR-162.
In George R. Aiken and Eve L. Kuniansky, editors, 2002, U.S. Geological Survey Artificial Recharge Workshop Proceedings, Sacramento, California, April 2-4, 2002: USGS Open-File Report 02-89
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