46. Fluxes and Fate of Water and Contaminants Affected by Preferential Flow in the Unsaturated Zone

Flow in the unsaturated zone plays a crucial role in a broad spectrum of environmental problems, including ground-water contamination, sustainability of water supplies, and ecological availability of water. Frequently a major portion of unsaturated-zone water moves not by diffuse Darcian flow through a network of small pores, but much more rapidly through preferential flow channels such as root holes, animal burrows, tension cracks, fractures, faults, joints, and narrow flowpaths or fingers arising from flow focusing or instability.

Few studies have attempted to quantify the macropore contribution to aquifer recharge, yet this problem is central to the transport of water and contaminants through the unsaturated zone. Some recharge-estimation methods are primarily sensitive to episodic, preferential recharge (for example, water- table fluctuation method), some to diffuse recharge (for example, Darcian method), and some to both, though this fact is seldom acknowledged or accounted for. Moreover, recent evidence counters the widespread assumption that lateral flow in the unsaturated zone is negligible except on steep slopes (for example, Nimmo and others, 2002).

Although preferential flow has been a major research focus for more than three decades, application-directed modeling is still based largely on formulations that treat unsaturated-zone flow as diffuse flow, or that include some adaptation of diffuse-flow techniques for the preferential component. Flow through preferential paths is complicated by such effects as nonlinear initiation thresholds and alternations between diffuse and preferential flow. On the other hand, there is evidence that preferential flow is in some physical respects simpler than diffuse flow, and that exploitation of such simplifications may produce new models that are both more tractable and more accurate in predicting transport in the unsaturated zone (Nimmo, 2007).

Of many possible field sites for this research, two of particular interest in the U.S. Geological Survey (USGS) are in the San Joaquin Valley of California and the Delta region of northwestern Mississippi. Because these two sites represent contrasting end members with irrigated versus natural rainfall, and sandy soils versus clayey soils, they provide study results applicable to a large range of other sites. In the San Joaquin Valley apparent discrepancies in the water budget are likely related to preferential flow; in Mississippi preferential flow has been detected and is known to influence water quality. Preferential flow is now known to be common in most soils so there are numerous other sites that could be investigated.

Recent results from the San Joaquin Valley and Mississippi sites illustrate the unanswered questions important to management of agricultural lands. San Joaquin Valley investigations include bromide tracer studies that show faster-than-expected movement of solute, even though the soils are sandy and largely devoid of macropores or other obvious preferential flow paths (Green and others, 2005; 2008). Fluxes of agricultural chemicals in this area are much larger than other agricultural regions of the United States, leading to widespread contamination of shallow ground water. Important questions include: What is causing faster-than-expected flow in these sandy soils? How important is preferential flow in the water budget of this irrigated site and in the mass budget of agricultural chemicals reaching the water table?

In northwest Mississippi, ground-water models suggest that aquifer recharge rates are low. However, total evapotranspiration over a year cannot account for the large amount of infiltrated water that presumably does not become recharge. High nitrate concentrations have been observed in shallow portions of the saturated zone. It is likely that either there is substantial lateral flow in the unsaturated zone or that actual recharge is much greater than existing models predict. Either case has strong implications for water quality, and preferential flow is likely a key factor. Ongoing investigations include artificial infiltration of water with tracer into an instrumented subsurface. Observations to date show evidence for preferential flow to one sampler at 5 m depth, as well as considerable lateral spreading at shallow depths. Questions being addressed include: Are there observable preferential flow paths? Are there any apparently isolated parcels of tracer- affected material (connected by undetected preferential paths)? Does an apparent impeding layer discovered at ~1 m depth generate substantial lateral flow? Is lateral preferential flow extensive enough to contaminate nearby surface water?

Additional high-priority applications are numerous. Preferential flow may cause accelerated contaminant transport at nearly any unsaturated-zone waste disposal site. The effect of global climate change on aquifer recharge rates may have high sensitivity to the proportion of diffuse and preferential flow through the unsaturated zone. Ecological habitat evaluation depends on the amount and distribution of the water in the soil, for which preferential flow is generally a major controlling influence.

The Mendenhall Fellow will conduct and participate in a combination of field, lab, numerical, and theoretical investigations that address important hydrologic problems in which preferential flow plays a key role. Research questions might include: (1) What is the role of heterogeneity and geologic layering in enhancing or damping preferential fluxes? (2) How does preferential flow alter unsaturated-zone storage and influence the availability of soil water for plants? (3) What approaches might represent the physics of initiation thresholds for preferential flow? (4) How does preferential flow affect soil residence times for nitrate, contaminants, and other substances? (5) How can we adequately formulate preferential flow in numerical models?

Numerous alternative investigation goals and methods could be selected. Objectives may include prediction of travel times, evaluation of fluxes (both magnitude and direction), and separate quantification of diffuse and preferential components of recharge. Field experiments could be done with one or more contrasting modes of water application: ponded, simulated rain at controlled rates, applied water at controlled suction, and natural inputs. Various types of instrumentation could be used for detecting subsurface phenomena: tracers sampled over time and space; probes for point measurements of water content, pressure, temperature, etc.; electrical-resistance tomography or other noninvasive, spatially exhaustive method to investigate inhomogeneity and lateral flow; and water-table fluctuation as indicative of short-term recharge. Lab experiments can obtain hydraulic and other soil properties from cores and bulk samples, and also can physically model important processes in columns or chambers with instrumentation and visualization. Analytical and numerical modeling for analysis and interpretation could use advanced codes such as VS2D, MODFLOW, and UCODE, or emphasize the development of new codes, models, and theory.

References

Green, C.T., Stonestrom, D.A., Bekins, B.A., Akstin, K.C., and Schulz, M.S., 2005, Percolation and transport in a sandy soil under a natural hydraulic gradient: Water Resources Research, v. 41, W10414.

Green, C.T., Fisher, L.H., and Bekins, B.A., 2008, Nitrogen fluxes through unsaturated zones in five agricultural settings across the United States: Journal of Environmental Quality, v. 37, no. 3, p. 1073–1085, doi:10.2134/jeq2007.0010.

Nimmo, J.R., Perkins, K.S., Rose, P.A., Rousseau, J.P., Orr, B.R., Twining, B.V., and Anderson, S.R., 2002, Kilometer-scale rapid transport of naphthalene sulfonate tracer in the unsaturated zone at the Idaho National Engineering and Environmental Laboratory: Vadose Zone Journal, v. 1, p. 89–101.

Nimmo, J.R., 2007, Simple predictions of maximum transport rate in unsaturated soil and rock: Water Resources Research, v. 43, no. 5, W05426, doi:10.1029/2006WR005372, 11 p.

Proposed Duty Station: Menlo Park, CA

Areas of Ph.D.: Hydrology, hydrogeology, physics, geology, geophysics, geochemistry, soil science, environmental engineering

Qualifications: Applicants must meet one of the following qualifications: Research Hydrologist, Research Physicist, Research Chemist

(This type of research is performed by those who have backgrounds for the occupations stated above. However, other titles may be applicable depending on the applicant's background, education, and research proposal. The final classification of the position will be made by the Human Resources specialist.)

Research Advisor(s): John Nimmo, (650) 329-4537, jrnimmo@usgs.gov; Chris Green, (650) 329-4728, ctgreen@usgs.gov; Barbara Bekins, (650) 329-4691, babekins@usgs.gov

Human Resources Office contact: Candace Azevedo, (916) 278-9393, caazevedo@usgs.gov

Summary of Opportunities