FOREWORDMan cannot survive on this planet without utilizing its natural resources prudently. Every human action affects the world around us in some degree and the full effect is difficult to assess because of complex relations among living and nonliving things. Under the circumstances one can neither expect to restore the entire past nor preserve the entire present for future generations. However all can and should strive for proper balance between resource development and maintenance of pleasant surroundings.The Environmental Quality Act of 1969 and the reports on environmental assessment that it requires are aimed at insuring such a balanced approach. To be effective we must provide a system for relating large numbers of actions and environmental factors and for placing value judgments on impacts which are difficult to quantify.At my request the Geological Survey has developed an information matrix system that is described and modeled in this Circular. It is published with the thought that it will serve as a useful guide for environmental impact reporting and as a systematic reference. Those who share with us the desire to retain or improve the quality of our environment will recognize that this report is a sincere but still preliminary effort to fill an interim need. We hope that suggestions from others will improve this framework.

Stable isotope data for dissolved carbonate, sulfate, and sulfide are combined with water composition data to construct geochemical reaction models along eight flow paths in the Madison aquifer in parts of Wyoming, Montana, and South Dakota. The sulfur isotope data are treated as an isotope dilution problem, whereas the carbon isotope data are treated as Rayleigh distillations. All reaction models reproduce the observed chemical and carbon and sulfur isotopic composition of the final waters and are partially validated by predicting the observed carbon and sulfur isotopic compositions of dolomite and anhydrite from the Madison Limestone. The geochemical reaction models indicate that the dominant groundwater reaction in the Madison aquifer is dedolomitization (calcite precipitation and dolomite dissolution driven by anhydrite dissolution). Sulfate reduction, [Ca2+ + Mg2+]/Na+ cation exchange, and halite dissolution are locally important, particularly in central Montana. The groundwater system is treated as closed to CO2 gas from external sources such as the soil zone or cross‐formational leakage but open to CO2 from oxidation of organic matter coupled with sulfate reduction and other redox processes occurring within the aquifer. The computed mineral mass transfers and modeled sulfur isotopic composition of Madison anhydrites are mapped throughout the study area. Carbon 14 groundwater ages, adjusted for the modeled carbon mass transfer, range from modern to about 23,000 years B.P. and indicate flow velocities of 7–87 ft/yr (2.1–26.5 m/yr). Most horizontal hydraulic conductivities calculated from Darcy's Law using the average 14C flow velocities are within a factor of 5 of those based on digital simulation. The calculated mineral mass transfer and adjusted 14C groundwater ages permit determination of apparent rates of reaction in the aquifer. The apparent rate of organic matter oxidation is typically 0.12 μmol/L/yr. Sulfate and, to a lesser extent, ferric iron are the predominant electron acceptors. The (kinetic) biochemical fractionation of 34S between sulfate and hydrogen sulfide is approximately −44‰ at 25°C, with a temperature variation of −0.4‰ per °C. The rates of precipitation of calcite and dissolution of dolomite and anhydrite typically are 0.59, 0.24, and 0.95 μmol/L/yr, respectively.

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