The use of 36C1 as an environmental tracer of hydrologic processes has become increasingly popular over the last twenty years. The use of this isotope in hydrogeology has been limited, however, by uncertainty in the input function for 36C1 entering groundwater. To address this problem and to evaluate current 36C1/C1 distribution models, 36C1/C1 was measured for 183 groundwater samples collected from sites located throughout the United States. On the continental scale, values of 36C11C1 range from less than 100 x 10-15 in coastal regions to over 1,200 x 1015 in Wyoming. This distribution is dominated by the spatial variability of stable chloride fallout. The depositional flux of 36C1 is approximately constant over the United States with a mean of 21.3+2.7 atoms 111-2S-1 which is equivalent to a global flux of 13.6+1.7 atoms r62s4. A simple model developed in this study, which assumes constant 36C1 deposition, consistently yields high estimates of 36C11C1 in the southwestern part of the country. This discrepancy may indicate that the relative amount of dry to total fallout is an important factor in controlling 36C1/C1 in arid areas and requires further investigation. Variability of 36C11C1 at the local scale is primarily controlled by subsurface sources of chloride. Auxiliary data, such as bromide concentrations, are often useful for identifying and interpreting these localized processes. In this study, three schemes for correcting this effect are explored: i) binary mixing models, ii) scaling by chloride concentrations and iii) scaling by chloride-bromide ratios.hydrology collectio
