The speciation of Pb in batch experiments and its mobility under flowing conditions in column transport experiments were investigated to study Pb behavior in a soil-water system in the presence of dissolved natural organic matter (DOM), peat humic acid (PHA) and peat fulvic acid (PFA). A sandy soil having a significant intraparticle porosity was used as the sorbing media. Batch equilibrium sorption isotherms for single components (Pb, PHA, and PFA) and for Pb in the presence of PHA and PFA were generated. Batch equilibrium experiments were also performed for both PHA and PFA to investigate Pb-DOM binding in the absence of soil. Single component (Pb, PHA, and PFA) and multicomponent (Pb-PHA and Pb-PFA) laboratory-scale column transport experiments were conducted to assess transport behavior of Pb in the presence of DOM. Sorption isotherms indicated that the soil had a higher affinity for PHA than for PFA. However, single component column transport experiments showed that PHA was less retarded than PFA. This anomaly was attributed to the size exclusion of the larger PHA molecules from the intraparticle porosity of the media under the geochemical conditions in the column. Pb retardation predicted by equilibrium equations based upon nonlinear isotherm parameterization agreed well with observed retardation. However, equilibrium retardation equations overpredicted retardation of DOM, indicating sorption kinetic limitations (chemical and/or physical nonequilibrium), molecular size exclusion during column transport, or chemical heterogeneity of the DOM. In multicomponent column transport experiments, Pb retardation decreased by factors of 4-8 in the presence of DOM. Multicomponent batch equilibrium experiments suggested that Pb
The successful design of constructed wetlands requires a continuous supply of water or vegetation that can withstand drought conditions. Having a constant water source is the best alternative to insure species diversity throughout the season. Consequently, detention structure designs should be based on times between events as well as on hydrologic return periods, since between events is when most evaporation and infiltration losses are likely to occur. In arid or semi‐arid environments, this is a difficult process because of long interevent times and seasonal changes in precipitation patterns. This discussion is predicated on the assumption that phytoplankton, epiphytic algae, and emergent vegetation require moist conditions to be effective at removing nutrients, metals and other pollutants. There are drought tolerant species of vegetation that can be used in constructed wetlands but it may take several days to re‐establish the attached bacteria communities necessary for optimum pollutant removal. This paper examines a stochastic framework to examine the probability of extended dry periods based on historic rainfall data. The number of consecutive dry days is selected for a specified level of assurance. By multiplying this value by the sum of daily system losses, an overall pond volume can be determined that ensures a minimum depth of water. To illustrate the utility of the approach, the method is applied to a site in Spokane, Washington.
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