A Method for Passive Release of Solutes from an Unpumped Well

Abstract

AbstractA new method was developed for releasing dissolved solutes into ground water which has application in both academic studies and practical efforts to stimulate in situ bioremediation, introduce chemical reactants, or understand flow and transport properties. The method involves the diffusion of solutes out of inexpensive, simple emitter devices which are installed in large diameter wells. Mass flux from an emitter depends on the geometry of the emitter, ground‐water flow rate, the imposed concentration gradient from the stock solution inside the emitter, and the diffusion coefficient of the solute through the emitter material. Diffusion coefficients are constants of a solute/tubing pair, and are established in simple experiments. If ground‐water flow rates and diffusion coefficients are known, emitter geometry and stock solution concentration can be tailored for a wide range of performance requirements.In laboratory experiments, benzene was released at or near 1.3 mg/1 for a total of 31 days, TCE for 17 days at approximately 4.5 mg/1, and bromide for 23 days at 667 mg/1. The emitted concentration of a wide range of solutes can be stabilized for much longer periods of time once the mass flux in the flow‐through system reaches steady‐state, provided the concentration gradient is maintained.A simple computer model was written to aid in the design of source emitters. The model was used to match diffusion coefficients to data for three solute‐tubing combinations. Once the diffusion coefficient of a given solute in a given type of tubing is determined from experimental data by curve‐matching, the model can be used in a predictive capacity. A second TCE release test showed that the diffusion coefficient calibrated from the model can be used to predict the steady‐state release concentration of TCE under different conditions.Simple two‐dimensional transport modeling suggests that a relatively uniform plume of solutes may be generated by emitters placed in wells spaced close together. The uniform introduction of nutrients and oxygen by this passive method may encourage more efficient in situ remediation, and in some situations, save time and money.