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.

Transport phenomena in hydrologic systems are simulated with finite-state models (FSMs), which are similar to mixing cell models in that they utilize a mixing cell as their basic subdivision, yet are more flexible, capable of modeling more complex systems, and easier to manipulate than previous mixing cell models. The basic FSM equations are discrete, recursive forms of the continuity equation for mass transport and the storage equation for fluid transport. Different types of mixing and flow can be simulated by specifying appropriate algorithms for use in the basic equations. Finite-state models thus have a physical basis, although they avoid the use of differential equations. The FSM digital computer model can simulate systems in one, two, or three spatial dimensions with relative facility. In many important cases, transit number and age number distributions can be calculated. These distributions, and especially their means, are useful in determining fluid residence times in hydrologic systems. Two aquifer systems are modeled using finite-state models. In a portion of the Tucson Basin Aquifer of southern Arizona a three-dimensional, steady flow FSM is used to account for the observed carbon-14 age distribution in the aquifer without assuming piston flow in the aquifer and without evaluating dispersion parameters. This model provides a first approximation of the three-dimensional flow distribution, an estimate of the long-term average annual recharge, and fluid residence times in the aquifer. The second FSM, two-dimensional and non-steady flow, accounts for the transient distribution of tritium in the Edwards Limestone of south-central Texas. This aquifer is a highly anisotropic, nonhomogeneous karst aquifer that is difficult to model by traditional methods. In both models, first guesses for cell volumes and flow distributions were made on the basis of available hydrogeological data. Saturated, unsaturated, and open-channel flow also are examined. Flow algorithms for the basic FSM storage equation follow the theory of linear systems, although in certain regimes, especially those involving unsaturated flow, it may be necessary to develop nonlinear flow algorithms. This was not attempted. It is also shown that the finite-state model can simultaneously model the transport of mass and fluid in a hydrologic system. The FSM also has the potential for modeling heat transport, which may prove useful in simulating geothermal reservoirs as well as other systems involving heat transport.

Eighty years of chlorine atomic weight measurements revealed no variation of the stable isotope ratio, 37C1/ 35C1 (with precision up to 1.0%.) in natural materials. This result is not surprising because chlorine occurs in relatively few compounds, has a strong affinity for the liquid phase, occurs mostly in the -1 oxidation state and organisms don't discriminate between chlorine isotopes. Chlorine isotopes have been found to fractionate in the laboratory during kinetic reactions, equilibrium between phases and diffusion. This dissertation examined chlorine isotope composition of chloride from sea water halite, hydrothermal water samples and ground-water samples where chloride was likely moving by diffusion. The measurement method was mass spectrometry of methyl chloride gas prepared by quantitative precipitation of AgC1 from solution, and reaction of the AgC1 with methyl iodide. The precision of the technique is 0.24%. Results from sea water indicate that isotope ratios in sea water do not vary beyond measured uncertainty, thus sea water became the designated standard called SMOC (Standard mean ocean chloride). Many of the samples measured in this study vary significantly, though most are within 1.0%, of SMOC. All halite and hydrothermal samples are heavier than SMOC. Halite results may indicate isotope effects durina precipitation or time dependent variations of sea water. Hydrothermal samples may indicate source differences and/or fractionation mechanisms. Milk River aquifer samples indicate that chloride in the same aquifer may vary due to dual filtration or differina sources. Samples from a Canadian glacial clay show a total chloride variation of 2.4%,, linearly distributed with depth. chloride in the clay is known to be diffusing against ground-water movement. The isotope distribution can be approximated with a counter-current column equation. Samples from Texas and Louisiana contain an isotope range of about 1.8%, linearly distributed with depths from 6,000 to 14,000 feet. The distribution can be approximated with a simple diffusion equation. As a tracing tool, chlorine isotope measurements showed that halite AO kilometers from the ocean in south Africa probably did not precipitate from sea water mist, and that oil field brines adiacent to the Weeks Island salt dome (Louisiana) probably did not receive substantial quantities dome chloride.

Ground water in excess of one million years old may now be accurately age dated by using the radionuclide Chlorine 36 (36Cl), which has a half-life of 3.01 x 105 years. To maintain a high degree of accuracy in the resultant age it is necessary to take into account the buildup of 36Cl, which is due to thermal neutron activation of 35Cl to 36Cl. The purpose of this research is to determine the thermal neutron flux in various geochemical subsurface environments by conducting field measurements of thermal neutron production rates at discrete locations. These data are then compared with the theoretical thermal neutron flux calculated for each location. The field measurements were conducted from the surface to a maximum depth of 44 meters in a copper, silver and zinc mine. The measured thermal neutron flux was found to be larger than the theoretical thermal neutron flux by a factor of from three to six when below 17 meters depth.

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