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Geology topics

F. N. Robertson

Publications and source records attributed to F. N. Robertson.

4 recordsLinked to original sources
USGS2016

Radiocarbon dating of groundwater in a confined aquifer in southeast Arizona

Radiocarbon, δ 13 C and major-element data were used to construct a geochemical framework for interpretation of the hydrological flow system in the lower San Pedro basin, southeastern Arizona, USA. The 14 C and major-element data show a regional confined aquifer that extends throughout most of the basin. Groundwater ages, after correcting for chemistry, are greater than 10 ka bp. The groundwater ages do not increase in a downvalley direction, the assumed direction of groundwater movement in most intermontane basins in the region, but along general flow paths normal to the mountains toward the center of the basin. Recharge to the confined aquifer originates from infiltration of precipitation and runoff near the alluvium-mountain contact along the Galiuro Mountains and is discharged by evapotranspiration along the center of the basin. The hydrogeological concept of the 14 C model is supported by the water chemistry and by the mass transfer defined by the chemical model. Weathering of primary silicate minerals in the confined aquifer does not occur downvalley, but only along the direction of flow. Hydraulic conductivities calculated for the aquifer from 14 C velocities are about an order of magnitude slower than those determined through hydrological methods. The lower hydraulic-conductivity values are attributed to a thick confining layer overlying the discharge area along the San Pedro River.

Radiocarbon↗
USGS1991

Hydrologic and geochemical approaches for determining ground-water flow components

Lyman Lake is an irrigation-storage reservoir on the Little Colorado River near St. Johns, Arizona. The main sources of water for the lake are streamflow in the Little Colorado River and ground-water inflow from the underlying Coconino aquifer. Two approaches, a hydrologic analysis and a geochemical analysis, were used to compute the quantity of ground-water flow to and from Lyman Lake. Hydrologic data used to calculate a water budget were precipitation on the lake, evaporation from the lake, transpiration from dense vegetation, seepage through the dam, streamflow in and out of the lake, and changes in lake storage. Geochemical data used to calculate the ground-water flow components were major ions, trace elements, and the stable isotopes of hydrogen and oxygen. During the study, the potentiometric level of the Coconino aquifer was above the lake level at the upstream end of the lake and below the lake level at the downstream end. Hydrologic and geochemical data indicate that about 10 percent and 8 percent, respectively, of the water in the lake is ground-water inflow and that about 35 percent of the water in the Little Colorado River 6 miles downgradient from the lake near Salado Springs is ground water. These independent estimates of ground-water flow derived from each approach are in agreement and support a conceptual model of the water budget.

Conference Paper↗
USGS1982

Deep oxygenated ground water: Anomaly or common occurrence?

Contrary to the prevailing notion that oxygen-depleting reactions in the soil zone and in the aquifer rapidly reduce the dissolved oxygen content of recharge water to detection limits, 2 to 8 milligrams per liter of dissolved oxygen is present in water from a variety of deep (100 to 1000 meters) aquifers in Nevada, Arizona, and the hot springs of the folded Appalachians and Arkansas. Most of the waters sampled are several thousand to more than 10,000 years old, and some are 80 kilometers from their point of recharge.

Science↗