The purpose of the study was to develop a method to determine the coefficient of storage of a water-table aquifer by correlating change in gravitational field intensity with change in groundwater storage. In theory, this purpose may be accomplished by modifying the Bouguer slab equation to coefficient of storage equals 78.3 times the ratio of change in gravity in milligals to change in water-table elevation in feet. Errors which result from the Bouguer slab assumptions may be corrected through analysis of tilted finite slabs. Field investigations were made to test the theory. The study area is located in the northern Tucson basin, Pima County, Arizona, and lies on unconfined basin-fill deposits and flood-plain alluvium aquifers. The basin-fill aquifer overlies less permeable Rillito beds and is overlain by the flood-plain alluvium. The two upper aquifers are flat-bedded heterogeneous deposits of sand and gravel. The water table through these aquifers slopes westward at a rate of approximately 0.5 degree. Estimates of the coefficient of storage for the basin-fill deposits and the flood-plain alluvium have been previously made by others from laboratory and field tests and by model analyses. The most reliable determinations of the coefficient of storage range from 0.15. to 0.30. The significance of the gravity method lies in determination of the coefficient of storage by measuring the quantities which define it: rise or decline in head and weight of water placed into or removed from storage. Change in gravity was determined by repeated gravity surveys using the same set of field stations through the period, October 1968 to June 1970. Water levels in wells were recorded for the same period. The relationship between change in gravitational field intensity and change in head was determined using a straight line solution method, and the coefficient of storage was computed from the slope of the straight line. At the conclusion of the field investigations, coefficients of storage were computed for 17 field stations. After correction for limited area of water-level decline and for water-table slope, the values of the coefficients ranged from 0.11 to 0.41. An error analysis indicates a maximum probable error in gravity data of ± 26 microgals. This error may be reduced by modifying the survey and reduction procedures and by using a more sensitive gravimeter. Analysis of changes in gravitational field intensity resulting from change of amounts of water in the unsaturated zone indicates that the coefficient of storage computed for field stations near Rillito Creek, the source of the unsaturated-zone water, are too low. Using data from stations least affected by gravity increases after stream recharge, a probable range of 0.25 to 0.29 was determined for the coefficient of storage in the study area. The range for values of the coefficient of storage using the gravity method confirms the larger coefficient of storage estimation made by others for the same area. The study indicates that the gravity method may be used with success over aquifers which have high coefficients of storage and in which the water table rises or declines 20 feet or more. However, large changes in the water content of the unsaturated zone cause gravity data to show large scatter with respect to water-level data. For this reason the gravity method is more suitable for analysis of those portions of a water-table aquifer which are recharged by underflow than for the portions recharged by infiltration from surface sources.

This item was digitized as part of the Million Books Project led by Carnegie Mellon University and supported by grants from the National Science Foundation (NSF). Cornell University coordinated the participation of land-grant and agricultural libraries in providing historical agricultural information for the digitization project; the University of Arizona Libraries, the College of Agriculture and Life Sciences, and the Office of Arid Lands Studies collaborated in the selection and provision of material for the digitization project.

Study of the main channels of an ephemeral discontinuous stream shows the presence of pool-riffle morphology scoured into underlying Holocene muds. Pools are defined as starting at the point of maximum bed slope and reaching half the distance downstream to the next point of maximum bed slope. Pool-riffle morphology correlates well with high-stage hydraulics. Channel fill along pools is subject to hydraulicjump scour during intermediate-stage flows. Pools are preferentially scoured during high-stage discharges with flow completely in the tranquil flow regime. During large-stage flows, channel fill is almost completely washed away and the variable slope of the exposed mud subfloor is found to show negative exponential correlation to energy gradients, sediment transport, and shear. Reaches of steep bed slope and low shear become the locus of kinematic particle interference and gravel deposition during recession from high stages. During low-stage flow, relationships between bed slope and energy expenditure are the reverse of those during high-stage flow. Evidence indicates that certain discontinuous streams and their fans evolve to braid chute units along gullies comprising steep slope, braided channels upstream of a single-channeled chute of low slope. During large discharges, shear and sediment transport are higher along chutes, causing net scour of channel fill. This fill is stored along the braided channels of the next braid-chute downstream. During low-stage flows, the reverse occurs. Thus, the braid-chute functions as a morphologic unit to maintain evenness of sediment transport in an environment of variable flow sizes. Long-term sediment-budget deficit is indicated by gradual scour of channel subfloors. In all braid-chutes, the bed slope of the braid is greater than the slope of the flood surface, and both are greater than the bed slope of the chute. Along chutes, average low-stage energy slopes approximately equal bed slopes. As stages rise, energy slopes increase with increasing discharge so that at bankfull stages, they approximate the flood-surface slopes. This relationship allows the inference of a feedback link between channel slopes and flood-surface slopes along a chute. Along braided channels, energy slopes decrease with increasing stage. Hypothetically, the contrasting energy-gradient changes with discharge could produce a system in which bankfull stages are reached simultaneously along the entire braid-chute. Data for discontinuous streams indicate that they may be analogous to noncontinuous braided perennial streams. Braided reaches in such perennial streams may be correlated to the alluvial fans below discontinuous channels. Straight and sinuous perennial channels between braided reaches may be analogous to the discontinuous channel. Bed slopes of the discontinuous channel and its analogues are flatter than those of adjoining flood surfaces. This contrast in gradients brings flow close to the level of the flood plain at some point downstream where the flow can begin to spread laterally onto this surface. In the ephemeral environment, this downstream branching usually takes place over a riffle. The fan, which lies below the studied discontinuous stream, contains braided-channel flow, mixed channel flow and sheetflow, and sheetflow, respectively, in a down-fan direction. It is postulated that a low water to sediment discharge ratio over a long time span would produce net deposition in the fan as well as in the discontinuous channel upstream. This, in turn, would force the fanhead to migrate upstream over the filling channel. Valley alluviation, thus caused by a discontinuous stream, would construct an elongate, tabular body of sediments composed of coarse channel deposits at the base overlain by laterally extensive channel sands and sheetflood sands. Above these, there would be finer grained sheetflood silts.

This item was digitized as part of the Million Books Project led by Carnegie Mellon University and supported by grants from the National Science Foundation (NSF). Cornell University coordinated the participation of land-grant and agricultural libraries in providing historical agricultural information for the digitization project; the University of Arizona Libraries, the College of Agriculture and Life Sciences, and the Office of Arid Lands Studies collaborated in the selection and provision of material for the digitization project.

The usefulness of stable isotopes of dissolved sulfate (δ₃₄S and δ₁₈O) as well as sulfate concentration was evaluated in four cases of study in southern Arizona. In Tucson basin, they have been used to determine the sources of sulfate in the groundwater, to delineate groundwater flow paths and recharge process, and to identify the groundwater domain. In Sonoita Creek near Patagonia, in combination with hydrochemical analysis and other environmental isotopes they were used to identify the sources of sulfate in the groundwater and surface water, the groundwater residence time, the interaction between groundwater and surface water, and the sources of base flow in Sonoita Creek. In the Patagonia Mountains, they were used to investigate the oxidation mechanisms of sulfide minerals. In evaporite deposits, δ₃₄S and δ₁₈O were used to determine the sources of sulfur, and to estimate the contribution of different S sources. This work shows that δ₃₄S and δ₁₈O of sulfate are excellent tracers of sulfate sources in groundwater, surface water, acid mine drainage and evaporite deposits in southern Arizona. The distinctive isotope compositions of the sulfur sources permit a clear identification of sources in aqueous environment, and quantification of the contribution from respective sources in one case. The results demonstrate the lithospheric origins of sulfur in the aqueous environment and evaporite deposits, and with Permian evaporite the most important source of sulfate.

This item was digitized as part of the Million Books Project led by Carnegie Mellon University and supported by grants from the National Science Foundation (NSF). Cornell University coordinated the participation of land-grant and agricultural libraries in providing historical agricultural information for the digitization project; the University of Arizona Libraries, the College of Agriculture and Life Sciences, and the Office of Arid Lands Studies collaborated in the selection and provision of material for the digitization project.

This item was digitized as part of the Million Books Project led by Carnegie Mellon University and supported by grants from the National Science Foundation (NSF). Cornell University coordinated the participation of land-grant and agricultural libraries in providing historical agricultural information for the digitization project; the University of Arizona Libraries, the College of Agriculture and Life Sciences, and the Office of Arid Lands Studies collaborated in the selection and provision of material for the digitization project.

A chemical-isotopic equilibrium model was developed for adjustment of radiocarbon ages of groundwater from the arid Tucson basin for dilution of the initial groundwater C-14 activity by the solution of soil calcite having a C-14 of 25 ± 19% modern. Input to the model consisted of the laboratory chemical analyses for Ca⁺⁺, Mg⁺⁺, Na⁺, H₄SiO₄, SO₄⁼, HCO₃⁻, CO₃⁼, NO₃⁻, and pH, and δ C-13 for the total dissolved carbon in the groundwater. Output consisted of the equilibrium chemical composition of the groundwater, the ratio of soil CO₂ derived to total dissolved carbon, Q, and δ C-13 of total dissolved carbon, H₂CO₃, HCO₃⁻, and CO₃⁼, and δ C-13 for the soil CO₂ and calcite that initially dissolved in the surface water as it equilibrated with soil minerals. Radiocarbon age of the groundwater is computed from the equation T = 8270 ln [(Q + (1-Q) A(CaCO₃)/Am] where T is the age in years before A.D. 1950, A(CaCO₃) is the soil calcite activity and Am is the measured activity for the dissolved carbonate in the groundwater, both with respect to modern wood. The validity of the model was tested by comparing the predicted values for δ C-13 (CO₂), δ C-13 (CaCO₃) with measured values for samples from the Tucson basin. δ C-13 (CO 2) calculated = (-12.9 ± 1.9) per mil PDB. δ C-13 (CO2) measured = (-15.1 ± 2.8) per mil PDB. δ C-13 (CaCO3) calculated = (-3.9 ± 1.7) per nil PDB. δ C-13 (CaCO3) measured = (-3.6 ± 1.7) per mil PDB. On the basis of these results, the model adequately describes the natural system and may prove useful in future radiocarbon dating work in desert regions.

Geology and tunnelling was given by the U.S. Department of Energy (DOE) as the most important criteria in siting the SSC. The impact of ground water on construction and vice-versa is an important part of a sites geologic and tunnelling attributes, and its potential environmental impacts. Because of the site's undeveloped nature, only limited reconnaissance-type investigations of the regions hydrogeology had occurred prior to the siting of the SSC. To overcome this ignorance geological, geotechnical, and geophysical characteristics of the site were used to describe the hydrogeology. A hydrologically ideal site would lie above any aquifers, have little or no surface water interaction, and yet, have an adequate and reliable supply of good quality water nearby. The Maricopa site possesses all of these characteristics while apparently being absent of subsidence due to ground-water withdrawal.

Phase I Draft. Prepared for United States Army Corps of Engineers, Los Angeles District, Tucson Urban Study, Regional Flood Control Element, by The University of Arizona, College of Earth Sciences, Water Resources Research Center in cooperation with College of Agriculture.