A one‐dimensional, physically based numerical model was constructed to describe the isotopic enrichment observed in throughfall of snow intercepted on evergreens. The process of enrichment is similar to that which results in formation of depth hoar in snowpack. On‐site data were obtained at a high‐altitude (3500 m) watershed in the Colorado Rocky Mountains. The model includes the ambient atmospheric variables of temperature, relative humidity, and water vapor isotopic composition and the intercepted snow variables of temperature profile, permeability for viscous flux, and isotopic composition. Model simulations yield results similar to those observed on site and suggest that the process is dominated by diffusive flux despite the very high permeability of freshly fallen snow. Median enrichments were observed to be 2.1‰ in oxygen 18 and 13‰ in deuterium.

The Regional Aquifer-System Analysis (RASA) program was started in 1978 after a congressional mandate to develop quantitative appraisals of the major ground-water systems of the United States. The RASA program represents a systematic effort to study a number of the Nation's most important aquifer systems that, in aggregate, underlie much of the country and that represent an important component of the Nation's total water supply. In general, the boundaries of these studies are identified by the hydrologic extent of each system and accordingly transcend the political subdivisions to which investigations have often been arbitrarily limited in the past. The broad objective for each study is to assemble geologic, hydrologic, and geochemical information, to analyze and develop an understanding of the system, and to develop predictive capabilities that will contribute to the effective management of the system. The use of computer simulation is an important element of the RASA studies, both to develop an understanding of the natural, undisturbed hydrologic system, and the changes brought about in it by human activities, and to provide a means of predicting the regional effects of future pumping or other stresses.The final interpretive results of the RASA program are presented in a series of U.S. Geological Survey Professional Papers that describe the geology, hydrology, and geochemistry of each regional aquifer system. Each study within the RASA program is assigned a single Professional Paper number, and, where the volume of interpretive material warrants, separate topical chapters that consider the principal elements of the investigation may be published. The series of RASA interpretive reports begins with Professional Paper 1400 and thereafter will continue in numerical sequence as the interpretive products of subsequent studies become available.

Precambrian rocks _______________________________________ Cambrian and Lower Ordovician rocks ___________________________ Ordovician rocks ___.________________.________________.___ Silurian and Devonian rocks ____________________________._____ Mississippian rocks _______________________________________ Pennsylvanian and Permian rocks _______________________________ Triassic rocks ___________________________________________ 14 Jurassic rocks ___________________________________________ 14 Geologic structure _______________________________________ Hydrologic setting__.________________________________.______ Description of digial-simulation model _______________________________ Theory ..

The Northern Great Plains Regional Aquifer-System Analysis is the first of a series of planned nationwide regional geohydrologic studies. This summary is principally a graphic presentation of the major results of four basic facets of investigation designed to provide the best possible understanding of a large (about 300,000 square miles) and extremely complex groundwater flow system. The reader is encouraged to refer to subsequent volumes in this series for the details of treatment of components and of results of the study. The geologic framework within which the groundwater flow system operates has been defined. The study area basically consists of highland areas of sediment sources and basin areas of sediment deposition. The geologic investigation principally involved definition of the types of sediment (any rocks), the areal extent and thickness of sediment (any rocks), and the mechanisms that controlled deposition of the sediments. The spatial distribution of hydraulic pressure has been portrayed as potentiometric surfaces mapped for several aquifers. The implied groundwater flow system is one of recharge in and near the highland areas in the western and southwestern part of the study area and one of generally eastward and northeastward flow of ground water toward areas of discharge in Canada, North Dakota, and South Dakota. The distribution of chemical quality of the ground water has been defined with available data, and the mechanisms controlling changes in chemical quality have been interpreted. The chemistry of water from aquifers of Paleozoic through Mesozoic age is controlled by a variety of geochemical mechanisms, with dissolution of evaporites and mixing of water being dominant. Dedolomitization is a significant mechanism, and sulfate reduction and cation exchange are probably active mechanisms as well. The entire system of groundwater flow with all of its controlling factors has been defined as a conceptual model and has been simulated with a mathematical model. Five major aquifers have been defined and simulated, and the digital model has been used to interpret areas and rates of recharge, areas and rates of discharge, areas and rates of leakage, and rates and directions of flow. The model has been further used to simulate several hypothetical pumping alternatives to determine the cause-and-effect relationship between pumping, drawdown, and assumed conditions. CENTRAL HIGH PLAINS NEBRASKA EXPLANATION Jurassic and Triassic or older rocks exposed at the land surface Area of principally bicarbonate facies Area of principally sulfate facies Area of principally chloride facies Combinations of these numbers indicate order of dominant facies Modified from K.D. Peter, U.S. Geological Survey, written commun.. 1982 0 100 200 KILOMETERS FIGURE 37. Hydrochemical facies of water in the Lower Cretaceous aquifer system.

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