Adsorption and transport of arsenic(V) in experimental subsurface systems.

The adsorption and transport of As(V) in a heterogeneous, iron oxide-containing soil was investigated in batch and column laboratory experiments. The As(V) adsorbed rapidly to the soil over the first 48 h, but continued to adsorb slowly over the next several weeks, clearly indicating the potential for rate-limited transport. The equilibrium As(V) adsorption isotherm was markedly nonlinear, further indicating the potential for nonideal transport. A model developed for the adsorption of As(V) to hydrous ferric oxide (HFO) was able to predict the pH-dependent adsorption of As(V) to the soil in batch experiments within 0.116 to 0.726 root mean square error (RMSE). Arsenic(V) was significantly retarded in column transport experiments. The column transport experiments were modeled using the one-dimensional advection-dispersion equation, considering both linear and nonlinear adsorption equilibrium. Although the nonlinear local equilibrium model (NLLE, RMSE = 0.273) predicted the data better than the linear local equilibrium model (LLE, RMSE = 0.317), As(V) breakthrough occurred more rapidly than predicted by either model due to adsorption nonequilibrium. However, due to the presence of an irreversible or slowly desorbing fraction, the peak aqueous As(V) concentration (0.624 mg L(-1)) and the total amount of As(V) recovered (44%) was lower than predicted based on the two equilibrium models (NLLE and LLE). For the conditions used in this study [1 mg L(-1) As(V), pH 4.5 and 9,0-0.25 mM PO4, 0.53-1.6 cm min(-1) pore water velocity], the effect on As(V) mobility and recovery increased in the order pH < pore water velocity < PO4.

Field Determination of Aquifer Thermal Energy Storage Parametersa.

Determination of the potential of a specific confined aquifer as an effective thermal energy storage medium requires thorough knowledge of the geochemical, thermodynamic, and hydraulic properties of the aquifer and its confining layers. A series of laboratory and field studies must be performed in order to determine the fundamental parameters. Procedures and analyses of a series of tests for a confined aquifer near Mobile, Alabama were completed prior to an aquifer thermal energy storage experiment. Parameters determined were: the regional gradient; vertical and horizontal permeabilities of the storage aquifer; horizontal dispersivity of the storage aquifer; vertical permeability of the confining layers; and thermal conductivities, heat capacities, and chemical characteristics of the aquifer matrix and native ground water.