Influence of Dissolved Oxygen Convection on Well Sampling

Convective transport of dissolved oxygen (DO) from shallow to deeper parts of wells was observed as the shallow water in wells in South Carolina became cooler than the deeper water in the wells due to seasonal changes. Transport of DO to the screened interval can adversely affect the ability of passive samplers to produce accurate concentrations of oxygen-sensitive solutes, such as iron other redox indicators and microbiological data. A comparison of passive sampling to low-flow sampling in a well undergoing convection, however, showed general agreement of VOC concentrations. During low-flow sampling, the pumped water may be a mixture of convecting water from within the well casing and aquifer water moving inward through the screen. This mixing of water during low-flow sampling can substantially increase equilibration times, can cause false stabilization of indicator parameters, can give false indications of the redox state, and can provide microbiological data that are not representative of the aquifer conditions. Wells having a relatively small depth to water were more susceptible to thermally induced convection than wells where the depth to water was greater because the shallower water levels were more influenced by air temperature. The potential for convective transport of DO to maintain oxygenated conditions in a well screened in an anaerobic aquifer was diminished as ground-water exchange through the well screen increased and as oxygen demand increased. Data from this investigation show that simple in-well devices can effectively mitigate convective transport of oxygen. The devices can range from inflatable packers to simple, inexpensive baffle systems.