The coarse structure of the 14C spectrum consists of a secular trend curve that may be closely fit by a sinusoidal curve with period ca 11,000 yr and half amplitude ±51. This long-term trend is the result of changes in the earth's geomagnetic dipole moment. Consequently, it modulates solar components of the 14C spectrum but does not appear to modulate a component of the spectrum of ca 2300-yr period. The ca 2300-yr period is of uncertain origin but may be due to changes in climate because it also appears in the δ18O spectrum of ice cores. This component strongly modulates the well-known ca 200-yr period of the spectrum's fine structure. The hyperfine structure consists of two components that fluctuate with the 11-yr solar cycle. One component results from solar-wind modulation of the galactic cosmic rays and has a half-amplitude of ca ±1.5. The other component is the result of 14C production by solar cosmic rays that arrive more randomly but rise and fall with the 11-yr cycle and appear to dominate the fluctuation of the galactic cosmic-ray-produced component by a factor of two.

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Carbon 14 dating of groundwater can be used to help determine the transmissivity distributions of aquifers. This method may offer significant advantages, in certain respects, over traditional aquifer pumping test techniques. We have applied 14C dating to a hydraulic analysis of a multilayer aquifer system in the central San Juan Basin of New Mexico. After corrections for geochemical evolution of the solutes, the influence of dispersive processes on the 14C distribution was investigated. A model incorporating stochastic dispersion theory indicated that macroscopic dispersion exerted only a small influence on the measured 14C activities. The •4C-derived transmissivity distribution was used to construct a numerical flow model which was applied to an analysis of interaquifer leakage. The model showed that even though vertical flow between aquifers was significant, in this case it did not cause the 14C distribution to differ significantly from that predicted by a simple piston flow model. Carbon 14 can be used as a basis for detailed hydraulic evaluations of groundwater flow in areas where traditional well hydraulics methods are not practical. PHILLIPS ET AL..' ISOTOPIC INVESTIGATION OF GROUNDWATER Study Area ß Albuquerque New Mexico 108øW 107øW

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An artificial fen environment was constructed near the municipality of Beavercreek, Ohio. The constructed wetland environment utilizes local ground‐water flow patterns to maintain a high degree of water saturation. Chemical reactions driven by the activity of plant roots and soil microorganisms affect the spatial distribution and magnitude of hydrogeochemical parameters, including alkalinity, pH, redox potential, and concentrations of Ca+2, Mg+2, total iron, NO3−, and SO4−2. The purpose of this study is to investigate the chemical interactions between wetland plants and ground water by means of characterizing the spatial variation in hydrogeochemical parameters. Nested piezometers within the artificial fen are used to monitor ground‐water chemistry within, and immediately adjacent to, the rhizosphere of cultivated plant species. The major reactions at the site include oxidation of organic matter by Fe(OH)3 and dissolution of carbonate minerals driven by high CO2 production in the root zone. The relationship between alkalinity and dissolved concentrations of Ca+2 and Mg+2 suggests that organic anions could also be major alkalinity contributors. Redox potential is buffered by the reduction of Fe (OH)3 and is typically maintained in the range of 100–200 mV. This buffering effect prevents the formation of sulfide and methane commonly found in similar wetland environments. This finding suggests that methane production associated with rice cultivation can be prevented by the addition of Fe(OH)3 to rice fields.