Sujoy B. Roy

Colloid release and transport processes in natural and model porous media

Abstract Colloid release was observed from packed columns for two natural porous media (sands) and one model system (glass beads with deposited latex colloids). Colloid release was found to occur in all cases when the ionic strength was reduced in columns that were in equilibrium with Na + ions. Most of the released colloids from the natural porous media were smaller than 1 μm in size, and comprised pure and impure forms of silica (60–70% by mass) and clay minerals (20–30% by mass). For greater reductions in ionic strength, the total mass of released colloids increased, although the shape of the effluent mass concentration profile did not change. Release rate coefficients were obtained by fitting a colloid transport model (an advection-dispersion transport model with source/sink terms for colloid release and deposition) to the column effluent data. To fit the data for different ionic strengths, the total available mass of releasable colloids had to be adjusted, and fitted release rate coefficients were not sensitive to the ionic strength. In contrast, calculations based on Derjaguin-Landau-Verwey-Overbeek (DLVO) theory indicate a strong dependence of release rate constants on ionic strength for homogeneous colloids. This discrepancy can be attributed to charge and size heterogeneity in the colloids, and to our inability to determine accurately interparticle forces at small separations. The trend of greater mass release for greater reductions in ionic strength could be explained qualitatively by computing interparticle interactions with a constant-charge boundary condition (albeit with a charge density much lower than that experimentally determined) which showed a decreasing energy barrier for particle detachment with decreasing ionic strength.

Projecting Water Withdrawal and Supply for Future Decades in the U.S. under Climate Change Scenarios

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.

Sorption nonequilibrium effects on colloid-enhanced transport of hydrophobic organic compounds in porous media

This paper presents the results of modelling and experimental investigations of factors affecting enhanced transport of hydrophobic organic compounds (HOCs) on colloids in porous media, especially effects of nonequilibrium sorption/desorption. Simulations were performed with a model that considers equilibrium and rate-limited exchange of contaminant between the true dissolved phase, the suspended and attached colloids, and the fixed solid phase as well as the advection, dispersion, deposition, and release of colloidal particles from the porous medium. The model was also applied to laboratory column data for colloid-facilitated transport of a common HOC, phenanthrene. Simulations of colloid-enhanced transport and fitting of experimental data indicated that colloid-enhancement of hydrophobic organic contaminant transport can be significant for sufficiently high colloid concentrations, a high partition coefficient of contaminant on colloids with respect to the fixed porous medium, a low deposition efficiency of the colloids on the fixed porous medium surfaces, and a slow desorption rate of contaminant from the colloids. The last of these conditions was identified as probably the most important in enhancing transport distances of contaminants in natural systems with mobile colloids which usually occur at concentrations not exceeding a few mg/l.

Mercury in the San Francisco Estuary

This review presents some of the published and other important literature on mercury contamination in San Francisco Estuary. Studies on human consumption of contaminated sportfish and on detecting ecological impacts of this contamination in wetland areas validate concerns regarding mercurys toxicity in this system. Mining, industrial, and environmental uses of mercury have occurred for more than a century, resulting in its large historic and continuing transport to the estuary. Consequently, there is a widespread distribution in the estuary, but more work is needed to show its relative chemical and biological availability from these sources. The uptake of mercury in the estuary has been shown in phytoplankton, but studies on biomagnification in local food webs have yet to draw a clear path to impairment in sportfish and waterbirds. In light of these concerns of impairment and the need for further information, large restoration activities planned for the estuary will require new technical approaches to solve important management questions, such as the location of key areas of methylmercury production.

Na+Ca2+ Exchange effects in the detachment of latex colloids deposited in glass bead porous media

Abstract The detachment of latex colloids deposited in packed glass-bead columns was observed under conditions where the ionic strength was reduced and ion exchange between Na+ and Ca2+ occurred. For identical influent solutions producing colloid release, it was found that deposited colloids in equilibrium with Ca2+ were released less readily than those in equilibrium with Na+. For influent solutions at the same reduced ionic strength with Ca2+ or Na+, colloid release was lower when Ca2+ ions were present. These phenomena can be explained by the selectivity of latex surfaces for Ca2+ over Na+ ions as observed in Na+Ca2+ exchange experiments.

Leaching of PCB compounds from untreated and biotreated sludge-soil mixtures

Column experiments were conducted to evaluate the aqueous leachability of polychlorinated biphenyls (PCBs) from sludge-soil mixtures contaminated with PCBs and hydraulic oils. Untreated and biotreated sludge-soil samples were obtained from a two-month, pilot-scale field test of PCB biodegradation in land treatment units. Column leachate comprised mainly di- and trichlorobiphenyls. In comparison to untreated samples, column leachate from biotreated samples showed 34 to 63% reduction in PCBs; average values of total PCB concentrations in the column effluent ranged from 0.36 to 0.82 μg 1−1 for untreated samples and 0.25 to 0.30 μg 1−1 for biotreated samples. Only a small fraction, less than 6%, of the PCB mass in the untreated sludge-soil mixtures was removed by prolonged flushing with up to 2400–3000 pore volumes of water, suggesting long-term, slow release of PCBs at low aqueous concentrations. It should be noted that flushing rates for the column experiments were much higher than typical ground-water flow rates and the two-month period for column experiments can be qualitatively compared to a greatly extended duration for ground-water flow. The leaching of PCBs from a nonaqueous phase liquid (NAPL) in the column experiments was modelled as a predominantly dissolution-governed process based on first-order kinetics. The estimated dissolution rate coefficients ranged from 0.02 to 0.04 min−1 for various PCB homolog groups. This investigation helps support the concept of biostabilization, in which contaminated materials may be actively biotreated to remove the potentially mobile organic contaminants leaving a residual material that is more stable against leaching. Natural attenuation processes like sorption and intrinsic biodegradation further reduce the PCB leachability potential. Additional long-term monitoring of the land treatment units under passive conditions is under progress to evaluate such reductions.

Implications of using On-Farm Flood Flow Capture to recharge groundwater and mitigate flood risks along the Kings River, CA

The agriculturally productive San Joaquin Valley faces two severe hydrologic issues: persistent groundwater overdraft and flooding risks. Capturing flood flows for groundwater recharge could help address both of these issues, yet flood flow frequency, duration, and magnitude vary greatly as upstream reservoir releases are affected by snowpack, precipitation type, reservoir volume, and flood risks. This variability makes dedicated, engineered recharge approaches expensive. Our work evaluates leveraging private farmlands in the Kings River Basin to capture flood flows for direct and in lieu recharge, calculates on-farm infiltration rates, assesses logistics, and considers potential water quality issues. The Natural Resources Conservation Service (NRCS) soil series suggested that a cementing layer would hinder recharge. The standard practice of deep ripping fractured the layer, resulting in infiltration rates averaging 2.5 in d(-1) (6 cm d(-1)) throughout the farm. Based on these rates 10 acres are needed to infiltrate 1 cfs (100 m(3) h(-1)) of flood flows. Our conceptual model predicts that salinity and nitrate pulses flush initially to the groundwater but that groundwater quality improves in the long term due to pristine flood flows low in salts or nitrate. Flood flow capture, when integrated with irrigation, is more cost-effective than groundwater pumping.

Erratum to: Climate variability and change in mountain environments: some implications for water resources and water quality in the Sierra Nevada (USA)

This article introduces this special journal issue on climate change impacts on Sierra Nevada water resources and provides a critical summary of major findings and questions that remain open, representing future research opportunities. Some of these questions are long standing, while others emerge from the new research reported in the eight research papers in this special issue. Six of the papers study Eastern Sierra watersheds, which have been under-represented in the recent literature. One of those papers presents hydrologic projections for Owens Valley, benefiting from multi-decadal streamflow records made available by the Los Angeles Department of Water and Power for hydrologic model calibration. Taken together, the eight research papers present an image of localized climatic and hydrologic specificity that allows few region-wide conclusions. A source of uncertainty across these studies concerns the inability of the (statistically downscaled) global climate model results that were used to adequately project future changes in key processes including (among others) the precipitation distribution with altitude. Greater availability of regional climate model results in the future will provide research opportunities to project altitudinal shifts in snowfall and rainfall, with important implications to snowmelt timing, streamflow temperatures, and the Eastern Sierra’s precipitation-shadow effect.

Model Prediction of the Effects of Changing Phosphorus Loads on the Everglades Protection Area

The Everglades Phosphorus and Hydrology (EPH) model was developed to simulate water movement and phosphorus transportin the Everglades Protection Area which is comprised of theEverglades National Park (ENP) and surrounding wetlands knownas the Water Conservation Areas (WCAs). Water flows from theEverglades Agricultural Area (EAA) through the WCAs intoEverglades National Park (ENP). The model is designed to represent the system as a series of cells in which water flowsfrom one cell to the next. The code allows for pumped inputsand pumped outputs of water as well as sorption and removal ofphosphorus through peat accretion. Model application involved dividing the system into twenty cellsrepresenting different segments of the WCAs. Inputs to each cellconsisted of water pumped from the EAA (where appropriate), flowfrom upgradient cells, and precipitation. Outputs included pumped outputs and flow out of each cell. Using data collectedby the South Florida Water Management District, the model wascalibrated by matching simulated and observed flows, water elevations, and phosphorus (P) concentrations for the period 1980–1988. The model was then validated for the 1988–1992 period using the same model parameters derived from the calibration process and comparing simulated and observed values.Reasonable agreement between simulated and observed values wasattained for both the calibration and validation periods. The calibrated and validated model was used to simulate the impacts on annual average total P concentrations in each cellresulting from the implementation of the management plan mandated by the Everglades Forever Act. This plan calls for the construction of six Stormwater Treatment Areas (STAs) to treat discharges from the EAA, hydrologic modifications of thesystem to promote sheet flow, and the implementation of BestManagement Practices to reduce P runoff from individual farms. In addition, the model was used to evaluate the impact of notbuilding one of the STAs (STA 3/4), and sensitivity analyseswere conducted to determine the effects of changing STA outletP concentrations throughout the system. Model results indicatethat phosphorus concentration reductions will occur in areas near EAA discharges in response to reductions in input P concentrations. However these measures will have little impacton phosphorus concentrations for 85% of the area of theWCAs and on the water entering Everglades National Park. The scenario analyses also indicate that phosphorus concentrationsthroughout most of the WCAs are similar with or without the construction of STA-3/4.

A hybrid empirical-Bayesian artificial neural network model of salinity in the San Francisco Bay-Delta estuary

This paper reports the refinement of a published empirical model of salinity in the San Francisco Bay-Delta estuary by integration with a Bayesian artificial neural network (ANN) model and incorporation of additional inputs. Performance goals established for the resulting hybrid model are based on the quality of fit to observed data (replicative and predictive validation) as well as sensitivity when compared with a priori knowledge of system behavior (structural validation). ANN model parameters were constrained to provide plausible sensitivity to coastal water level, a key input introduced in the hybrid formulation. In addition to representing observed data better than the underlying empirical model while meeting structural validation goals, the hybrid model allows for characterization of prediction uncertainty. This work demonstrates a real-world application of a general approach integration of a pre-existing model with a Bayesian ANN constrained by knowledge of system behaviorthat has broad application for environmental modeling.