An Inexpensive Flow‐Through Cell and Measurement System for Monitoring Selected Chemical Parameters in Ground Water

Abstract

An inexpensive flow‐through cell was designed and constructed to enable the accurate measurement of redox potential (Eh), conductivity and pH from shallow monitoring wells. The cell accepts sample water from inert gas‐operated bladder pumps. The electrodes are monitored by meters mounted in a portable rugged waterproof carrying case. The cell is useful for monitoring the purging of stagnant water from the well prior to sample collection for chemical analysis, as well as to provide pH, Eh and conductivity data under conditions as close to in situ as presently practical. In conjunction with the development of the flow‐through cell and measuring system, a portable field kit was constructed and equipped to determine the alkalinity of the water via modified Gran plot titration, immediately upon sample collection. In addition to the measurement cell system, an in‐line membrane filtration system was developed, that included valves to sequentially switch to different filter units, in case of membrane plugging.

Deconstructing the redox cascade: what role do microbial exudates (flavins) play?

“…In systems where flow velocity may not be easily controlled (e.g., measurements within rivers), the electrode surface should be shielded from changes to flow. [15] Addition of an inert electrolyte to water samples with low ionic strength less than 0.005 M The addition of 9 mM NaCl to water sample improves electrode performance likely due to a combination of the decreased internal resistance and increased exchange current facilitated by the presence of chloride ions adsorbed at the Pt surface.…”

Designing an enhanced groundwater sample collection system

“…In general, wells should be pumped at rates that do not cause substantial drawdown from the mid-screen portion of a well and that efficiently =move a consistent number of purge volumes from each well prior to sampling (Barcelona et al 1994). At the time of this evaluation, it was decided that a minimum number for all monitoring wells should be three to five equivalent volumes (that is, five SC for HSU-1 wells and three SC for HSU-2 wells), if indicator parameters [such as, turbidity, redox potential, pH, electrical conductivity, or dissolved 3.6 oxygen in-line (flow through), or actual contaminants of interest] have stabilized within approximately 10 percent over at least two measurements (Garske and Schock 1986;Gibs and Imbriogiotta 1990;Barcelona et al 1994). Although 1.2 equivalent volumes is possible, it was not considered an identifiable lower limit for the LEHR site because of insufficient data.…”

Spatial and temporal gradients in aquifer oxidation‐reduction conditions

“…The water samples contacted only PTFE tubing and stainless steel (fittings and filtration apparatus) prior to collection and preservation in appropriate vessels. Measurements of the purging parameters pH, Eh, conductivity, and temperature as well as alkalinity, Winkler titration, and Clark electrode (Orion ©) probe dissolved oxygen determinations were made according to methods described previously [Barcelona et al, 1985;Garske and Schock, 1986]. Sampling operations were conducted from a specially outfitted sampling van equipped with a generator, refrigerator, air compressor for bladder pump drive gas, and adequate bench space for maintaining the integrity of the water samples and conducting preservation or analytical procedures.…”

“…Several factors currently limit the in-depth, practical application of predictive chemical speciation and reaction models to groundwater systems. Among these factors are the heterogeneity of aquifer properties [Sudicky, 1986;Sudicky et al, 1983], the complexity of transport and dispersion processes at field scales [Freyberg, 1986;Vomvoris and Gelhar, 1987], and the fact that assumptions of local chemical equilibrium may not be justified [Reardon, 1981;Goltz and Roberts, 1986], particularly in contaminated hydrogeochemical situations. Accounting for the contribution of oxidation-reduction reactions in such models may be particularly difficult due to indications that groundwaters are not in redox equilibrium and that redox potential measurements reflect mixed potentials with little thermodynamic significance [Lindberg and RunnelIs, 1984].…”