The sustainability of water resources in future decades is likely to be affected by increases in water demand due to population growth, increases in power generation, and climate change. This study presents water withdrawal projections in the United States (U.S.) in 2050 as a result of projected population increases and power generation at the county level as well as the availability of local renewable water supplies. The growth scenario assumes the per capita water use rate for municipal withdrawals to remain at 2005 levels and the water use rates for new thermoelectric plants at levels in modern closed-loop cooling systems. In projecting renewable water supply in future years, median projected monthly precipitation and temperature by sixteen climate models were used to derive available precipitation in 2050 (averaged over 2040-2059). Withdrawals and available precipitation were compared to identify regions that use a large fraction of their renewable local water supply. A water supply sustainability risk index that takes into account additional attributes such as susceptibility to drought, growth in water withdrawal, increased need for storage, and groundwater use was developed to evaluate areas at greater risk. Based on the ranking by the index, high risk areas can be assessed in more mechanistic detail in future work.

A time-variable one-dimensional model (called ViM for Vapor Intrusion Model)to predict indoor vapor concentrations in a dwelling with a combined basement and crawl space has been developed. ViM predicts vapor concentrations in each of the three compartments. Volatile chemicals that intrude into the dwelling are assumed to originate from soil, groundwater (where an attenuating plume is simulated), or ambient air. Processes included in the model are advection, diffusion, biodecay, and adsorption in the soil column; transport by diffusion and advection into individual crawl space and basement compartments; advection from each compartment into an overlying dwelling space; and exchange of ambient air and indoor air. The time-variable concentration fields are solved by first transforming the partial and ordinary differential equations into Laplace space, solving the resulting ordinary differential equations or algebraic equations, and numerically inverting those equations. This approach was an expedient way of handling the coupling between the subsurface and the dwelling. ViM was applied to a building (Building 20) located at the former Moffett Field Naval Air Station, in Mountain View, CA. The building is a former bachelor officer's quarters. The shallow groundwater beneath the building is contaminated with a number of volatile chemicals, including trichloroethene, cis-1,2-dichloroethene, and trans-1,2-dichloroethene, all of which were simulated. Using indoor air data collected in 2003-2004, and other field data collected prior to that time, the accuracy of the model's predictions was demonstrated. ViM's results were also compared against a version of the steady-state Johnson and Ettinger model (1) that was modified to accommodate a dwelling with a combined crawl space and basement (called the JEM model in this paper). The predictions from the JEM model were consistently higher than the predictions from ViM, but still near the upper range of the observed data.

This paper summarizes topics related to colloid transport in subsurface media. The ultimate objective of the paper is to present a model that can be used to evaluate the significance of colloid facilitated transport on the mobility of metals. Field and laboratory studies are first reviewed to evaluate evidence that colloids are transported in subsurface media. Second, researchers active in the field are contacted to identify areas of ongoing research, and to solicit opinions concerning the level of understanding of mechanisms that control colloid migration. Third, the literature on colloid transport mechanisms is reviewed, with particular emphasis on colloid (and particle) filtration and on colloid stability as influenced by electrical repulsion and Van der Waals attraction. Fourth, a conceptual colloids‐metal transport model (COMET) is developed and incorporated into EPA's CML model, a model that simulates solute migration from a landfill in the unsaturated zone to a receptor (i.e., drinking‐water well) in the saturated zone. Among the major features of the COMET model are the capability to simulate multiple metal species either dissolved or adsorbed to mobile colloids (in conjunction with results from a geochemical equilibrium model), the capability to simulate the influence of multiple colloid types, and to adjust source concentration and duration in the presence of colloids that migrate from a source. These capabilities are embodied in equations (2) and (3) in the main text of this paper. As expected, results from the COMET simulations indicate that mobile phase metal concentrations (dissolved concentration plus concentration adsorbed to mobile colloids) increase as colloid concentrations increase, and arrival times of soluble metal species (solutes) to stationary receptors decrease. When the partition coefficients for solute‐colloid adsorption and solute‐soil matrix adsorption are the same, neither the increase in mobile phase concentration nor decrease in travel time is always significant. However, when partition coefficients for solute‐colloid adsorption are greater than partition coefficients for solute‐soil matrix adsorption, travel times to stationary receptors can dramatically decrease and total mobile phase concentrations dramatically increase.

Large-scale climatic indices have been used as predictors of precipitation totals and extremes in many studies and are used operationally in weather forecasts to circumvent the difficulty in obtaining robust dynamical simulations of precipitation. The authors show that the sea level pressure North Pacific high (NPH) wintertime anomaly, a component of the Northern Oscillation index (NOI), provides a superior covariate of interannual precipitation variability in Northern California, including seasonal precipitation totals, drought, and extreme precipitation intensity, compared to traditional ENSO indices such as the Southern Oscillation index (SOI), the multivariate ENSO index (MEI), Niño-3.4, and others. Furthermore, the authors show that the NPH anomaly more closely reflects the influence of Pacific basin conditions over California in general, over groups of stations used to characterize statewide precipitation in the Sierra Nevada range, and over the southern San Francisco Bay region (NASA Ames Research Center). This paper uses the term prediction to refer to the estimation of precipitation (the predictand) from a climate covariate (the predictor), such as a climate index, or atmospheric moisture. In this sense, predictor and predictand are simultaneous in time. Statistical models employed show the effectiveness of the NPH winter anomaly as a predictor of total winter precipitation and daily precipitation extremes at the Moffett Field station. NPH projected by global climate models is also used in conjunction with atmospheric humidity [atmospheric specific humidity (HUS) at the 850-hPa level] to obtain projections of mean and extreme precipitation. The authors show that future development of accurate forecasts of NPH anomalies issued several months in advance is important for forecasting total winter precipitation and is expected to directly benefit water resource management in California. Therefore, the authors suggest that investigating the lead-time predictability of NPH anomalies is an important direction for future research.

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