Navajo Sandstone Recharge Study

• Hydrologic monitoring, geochemical analysis, hydraulic properties evaluation, and numerical flow modeling of the regional Navajo Sandstone Aquifer of western Washington County 1995-2009 [Heilweil and others, 2000; Heilweil and Hsieh, 2006].
• Investigation of spatial variability of infiltration processes and effects of preferential flow using vadose-zone sampling along 3,000 meters of sandstone trenches (fig. 4) 2001-2003 [Heilweil and Solomon, 2004].
• Estimation of long-term natural recharge rates at Sand Hollow using vadose-zone drilling and core collection (fig. 5), monitoring wells, environmental tracer analysis, atmospheric deposition, and climate monitoring 1999-2004 [Heilweil and others, 2006]
• Development of a regional net-infiltration map for the Navajo Sandstone outcrop using combined environmental tracer and GIS techniques 2004-2007 [Heilweil and others, 2007b; Heilweil and McKinney, 2007]

• Evaluation of managed aquifer recharge potential and clogging processes associated with trapped gases in fractured sandstone with the Infiltration Pond experiment in Sand Hollow basin using dissolved-gas tracers (fig. 6) 2000-2001 [Heilweil and others, 2004]
• Evaluation of the potential for managed recharge to sandstone along an ephemeral wash, based on drilling, core collection, permeability testing, and numerical modeling at Sand Cove Wash 2003-2004 [Norton and Susong, 2004; Norton 2005]
• Demonstration of the effectiveness of bypassing evaporation and accessing preferential pathways through sandstone fractures using Trench Infiltration (fig. 7) 2002-2003 [Ludwig, 2003; Heilweil and Watt, 2011]
• Water-levels, meteoric data, reservoir water temperature, geochemical tracers (dissolved gases, trace metals, isotopes), water budget, and evaporation data are continually being compiled for Sand Hollow Reservoir to evaluate managed aquifer recharge to the underlying Navajo Sandstone 2002-present [Heilweil and others, 2005; Heilweil and Susong, 2007; Heilweil and others, 2009a, Heilweil and Marston, 2011]
• Managed aquifer recharge rates beneath Sand Hollow Reservoir generally decreased from 2002 to 2008 and then increased slightly through 2009 (fig. 8). In a collaborative effort with the Geological and Mining Institute of Spain (2007-2010), clogging processes during managed aquifer recharge were investigated, including the effects silt and gas accumulation (Heilweil and others, 2009b) and investigate management alternatives to offset declines.

SIGNIFICANT RESULTS

• Natural net-infiltration rates for the Navajo Sandstone outcrop of western Washington County are estimated to range from 0.1 to 66 mm/yr. This indicates that about 5 percent of the precipitation falling in the outcrop area recharges the aquifer.
• Primary factors controlling natural recharge are soil coarseness, topographic slope, and distant to upgradient run-off generating outcrops.
• The combination of dissolved-gas tracers and permeability testing during the Infiltration Pond experiment indicate that about 10 percent of the porosity of the Navajo Sandstone was clogged with trapped air, reducing the permeability by more than an order of magnitude.
• Infiltration rates during the pilot-scale Trench experiment were more than an order of magnitude higher than for the pond experiment, indicating the importance of delivering water beneath the lower-permeability soils and caliche deposits along with minimizing biogenic gas formation and viscosity effects.
• From 2002 through 2009 about 90,000 acre-feet of water has recharged the Navajo Aquifer beneath Sand Hollow Reservoir (fig 9). An additional 40,000 acre-feet of water was lost as evaporation.