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• Groundwater altitude contour maps were
<br />constructed for different seasons, and
<br />groundwater gradients in the floodplain
<br />alluvium aquifer were calculated.
<br />• Well records and new soils mapping by
<br />the USDA -NRCS Tucson Soil Survey
<br />Office were used to estimate the width
<br />and thickness of the floodplain alluvium.
<br />• Aquifer testing was conducted using
<br />multiple observation wells. Based on
<br />analysis of results, a transmissivity of
<br />40,000 square feet per day was estimated
<br />for the floodplain alluvium aquifer at
<br />one location on the farm.
<br />From these new data, TNC researchers
<br />estimated that the average annual volume
<br />of groundwater underflow through the
<br />floodplain alluvium where aquifer testing
<br />was conducted was more than 3,500 acre -
<br />feet per year. Researchers then considered
<br />potential sources of water that could
<br />replenish the floodplain alluvium aquifer
<br />at that rate. Their analyses suggest that
<br />interbasin groundwater flow from the
<br />Upper San Pedro Basin is the primary
<br />replenishment source for the groundwater
<br />underflow observed at Three Links Farm,
<br />for the following reasons:
<br />• No flood flows in the San Pedro River
<br />occurred at Three Links Farm during the
<br />monitoring period for which underflow
<br />estimates were made. Therefore,
<br />streambed recharge was ruled out as a
<br />source of groundwater.
<br />• Irrigation at the farm had ceased more
<br />than a year before the monitoring period;
<br />therefore, irrigation return flow was not
<br />a source.
<br />• Estimated average annual mountain
<br />front recharge is about 160 acre -feet
<br />for the small watersheds in the Little
<br />Rincon Mountains and the Johnny Lyon
<br />Hills flanking the southern end of the
<br />Lower San Pedro Basin (Goode and
<br />Maddock, 2000).
<br />• Steady groundwater levels in the
<br />floodplain alluvium at the farm during
<br />the monitoring period eliminated
<br />drainage from previously stored
<br />groundwater in the floodplain alluvium
<br />as a source.
<br />• Existing groundwater chemical analyses
<br />in the aquifers north and south of the
<br />Narrows indicate that groundwater types
<br />do not change there: the types found
<br />in the Upper San Pedro Basin are also
<br />found in and near Three Links Farm.
<br />This research indicates that substantial
<br />groundwater is moving through the
<br />floodplain alluvial aquifer at Three Links
<br />Farm and that most of this water originates
<br />from the Upper San Pedro Basin. This
<br />finding has significant implications,
<br />for in the neighboring Benson sub -area
<br />of the Upper San Pedro groundwater
<br />basin, deficit groundwater pumping was
<br />estimated to be 1,300 acre -feet in 2002
<br />(Arizona Department of Water Resources,
<br />personal communication). Increased
<br />groundwater pumping in the Benson sub-
<br />area could result in diminished interbasin
<br />groundwater flow to the Lower San
<br />Pedro Basin.
<br />Contact Jeanmarie Haney atjhaney@tnc.org.
<br />References .. ...............................
<br />Goode, T.C., and T. Maddock 111, 2000. Simulation
<br />of groundwater conditions in the Upper San
<br />Pedro Basin for the evaluation of alternative
<br />futures, Arizona Research Laboratory for
<br />Riparian Studies, Dept. of Hydrology and Water
<br />Resources, The University of Arizona, HWR
<br />Technical Series 00 -030, Tucson, Arizona.
<br />Halvorson, P.F., 1984. An exploration gravity
<br />survey in the San Pedro valley, southeastern
<br />Arizona. M.S. Thesis, Dept. ofGeosciences, The
<br />University of Arizona, Tucson, Arizona.
<br />Heindl, L.A., 1952. Lower San Pedro Basin. In
<br />Halpenny, L.C., et al., Groundwater in the Gila
<br />River Basin and Adjacent Areas, Arizona — A
<br />Summary. U.S. Geological Survey Open File
<br />Report, pp. 87 -100.
<br />Montgomery, E.L., 1963. The geology mid ground
<br />water investigation of the Tres Alamos Dam site
<br />area of the ,San Pedro River, Cochise County,
<br />Arizona. M.S. Thesis, Dept. of Geology, The
<br />University gfArizona, Tucson, Arizona.
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