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This paper describes a direct comparison of a•parent ages derived from 3H/3He, chlorofluorocarbons (CC!3 F and CC12F2), and ø•Kr measurements in shallow groundwater. Wells chosen for this study are completed in the unconfined surficia! aquifers in late Cenozoic Atlantic Coastal Plain sediments of the Delmarva Peninsula, on the east coast of the United States. Most of the apparent tracer ages agree within 2 years of each other for recharge dates between 1965 and 1990. Discrepancies in apparent tracer ages usually can be explained by hydrological processes such as mixing in a discharge area. Recharge rate calculations based on apparent tracer age gradients at multilevel' well locations agree with previous recharge estimates. High recharge rates on the Delmarva Peninsula result in nearly complete dissolved-gas confinement in the groundwater. The remarkable agreement between the different tracer ages indicates negligible mixing of waters of different ages, insignificant dispersion, minimal gas loss to the atmosphere, and insignificant sorption-desorption processes at this location. 1. Introduction Quantification of groundwater flow requires information on flow velocities and directions, and recharge and discharge rates. The classical approach is to calculate groundwater flows from hydraulic head gradients, hydraulic conductivity, and porosity. Standard laboratory and field methods for determining hydraulic conductivity commonly give conflicting results with large errors because of heterogeneities in most aquifers. Laboratory and field methods often provide limited resolution of these heterogeneities. Most groundwater flow models are calibrated to reproduce the measured potentiometric surface. Because measurements of hydraulic head and other field data form the foundation of groundwater flow models, the model results are limited by the uncertainty of these parameters. Therefore any tracer that provides direct age information is a valuable tool for calibrating groundwater flow models. Groundwater contamination provides an unintentional "dye" experiment that can give insight into travel times along flow paths. However, often the source function of the contaminant(s) (i.e., concentration and date(s) of spill) is not known, adding uncertainty to flow calculations. Most contaminants are chemically reactive and undergo adsorptiondesorption, cation exchange, precipitation-dissolution, oxidation-reduction, or biological reactions within an aquifer. Most of these reaction rates are currently unknown. Thus even if the source of the contamination is known, large uncertainties due to nonconservative behavior exist for travel time calculations based on position of contaminant plumes. Such "dye" experiments can only be applicable on a local scale and, therefore, are not a universal dating method for use in natural systems. Transient tracers are geochemical trace substances that have source functions and/or known (radioactive) decay rates that change with time. Transient tracers provide time information that can be used to infer flo...

Researchers have quantified the contributions of industrialized and developing nations' historical emissions to global surface temperature rise. Recent findings that nearly twothirds of total industrial CO 2 and CH 4 emissions can be traced to 90 major industrial carbon producers have drawn attention to their potential climate responsibilities. Here, we use a simple climate model to quantify the contribution of historical (1880-2010) and recent emissions traced to these producers to the historical rise in global atmospheric CO 2 , surface temperature, and sea level. Emissions traced to these 90 carbon producers contributed ∼57% of the observed rise in atmospheric CO 2 , ∼42-50% of the rise in global mean surface temperature (GMST), and ∼26-32% of global sea level (GSL) rise over the historical period and ∼43% (atmospheric CO 2 ), ∼29-35% (GMST), and ∼11-14% (GSL) since 1980 (based on bestestimate parameters and accounting for uncertainty arising from the lack of data on aerosol forcings traced to producers). Emissions traced to seven investor-owned and seven majority state-owned carbon producers were consistently among the top 20 largest individual company contributors to each global impact across both time periods. This study lays the groundwork for

The Upper San Pedro River Basin (Southeastern Arizona, United States) contains one of the few desert riparian areas in the Southwest, a system that is dependent on both shallow groundwater to support phreatic vegetation and baseflow for aquatic plants and animals. Proper management decisions for sustaining this biodiversity hotspot require understanding the hydrology of the riparian system and its interaction with the basin aquifer. To meet this need and to assess whether the techniques used would be efficient for evaluating other semiarid riparian ecosystems, we addressed the following questions. What are the contributions of different water sources (e.g., local recharge during monsoon flood events versus inflow of basin groundwater) to riparian groundwater and river baseflow? How does the spatial variability in water sources relate to gaining and losing reaches along of the river? We first characterize the possible water sources to the riparian system using a suite of geochemical tracers. Results indicate that, of the possible sources, basin groundwater recharged along the Huachuca Mountains to the west and local recharge of monsoon floodwaters are the dominant riparian water sources. Then, using their geochemical composition, we quantify these sources using a two end‐member mixing model. We find that riparian groundwater composition varies between gaining and losing reaches. Locally recharged monsoon floodwater comprises 60 to 85% of riparian groundwater in losing reaches whereas that of gaining reaches contains only 10% to 40%. Baseflow, sampled year round, also contains a significant component of monsoon floodwater ranging from 80% on the upstream end and decreasing to 55% after passing though several gaining reaches. These results highlight the significance of local recharge during monsoon flood events as a water source for desert riparian systems, a fact that should be addressed when constructing and calibrating hydrologic models used to evaluate these future water management decisions.

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