Southwest Hydrology: The Resource of Semi-Arid Hydrology Volume 4 Number 2

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ON THE GROUND interbasin Groundwater Flow at the Benson Narrows, Arizona Jeanmarie Haney — The Nature Conservancy and Jim Lombard — Consulting Hydrogeologist The Nature Conservancy's (TNC) Three Links Farm stretches six miles along the San Pedro River in southeastern Arizona and contains a perennial flow reach and a cottonwood - willow riparian forest with a large variety of birds and other wildlife. TNC is working to re- establish ecosystem and hydrologic function in appropriate reaches of the San Pedro River, with an emphasis on protecting and restoring perennial streamflow and associated riparian habitat. Understanding the hydrogeologic relations along the San Pedro River and source of groundwater underflow that supports the perennial flow at Three Links Farm is critically important to TNC's conservation efforts. The farm is located immediately downstream of the Benson Narrows, which marks the boundary between the Upper and Lower San Pedro basins. Previously, just 120 acre -feet per year of interbasin groundwater underflow were estimated to move through this area, based on the assumption that groundwater could move solely through the San Pedro River floodplain alluvium aquifer at the restricted slot canyon of the Benson Narrows (Heindl, 1952). 8 • March /April 2005 • Southwest Hydrology Extent Of Floods) Alluvium Aquifer G \Y)RTII t Z, Groundwater flow at the Benson Narrows figure from The Nature Conservancy). Subsequent work, however, identified the possibility of groundwater underflow through the 800- to 1,000 -foot thick alluvial basin -fill deposits flanking the bedrock outcrops of the Narrows (Montgomery, 1963; Halvorson, 1984). Recent hydrogeologic investigations show that during the 2003 water year, about 3,500 acre -feet of groundwater passed through the floodplain alluvium aquifer at Three Links Farm. TNC hypothesized that most of this water originates as Allow groundwater underflow moving from the Upper San Pedro Basin through the alluvial basin -fill deposits at the Benson Narrows. Direct measurement of such underflow was not possible because wells of sufficient depth are lacking. Thus, new hydrogeologic work conducted at Three Links Farm included the following tasks: • Fifteen existing wells at the farm were surveyed for location and altitude. Water level elevations were measured at each well during 2002 -2003. ;ice ✓�'�;�� :`� __.._. �� _:.� _.�. �._�..: j. f : ✓JI�jJ).: REDEFINING FULL SERVICE WATER RESOURCES