IN-SITU SEPARATION OF STRATIFIED GROUNDWATER CONTAMINANTS

It is common for groundwater quality to vary over the vertical section of an aquifer. This may result from the nature of sources and sinks, variation in groundwater flow patterns, temporal changes or other factors. In some cases, water of poor quality can occur within only a portion of the vertical section, and as groundwater moves it remains stratified. When water-supply wells pump from an aquifer with stratified water quality, the produced water represents a mixture of all the water in the vertical section. As a result, good-quality water that was present in the aquifer can only be recovered by costly treatment.

The author was one of the designers of a "scavenger couple", a pair of wells that achieve in-situ separation of brackish water at the bottom of the "1500-Foot Sand" aquifer in Baton Rouge, LA. The brackish water has infiltrated over recent decades in response to public and industrial supply withdrawals. A modeling study demonstrated that a properly engineered two-well design could separate the brackish water from the overlying fresh water. The system was constructed in 2013 and brought on-line in 2014 and has operated successfully ever since, protecting an important water-supply well field from intrusion and simultaneously producing over 400 gpm of high-quality water, reducing demand on other facilities.

Other contaminants tend to be present in a stratified fashion, particularly agricultural nitrate infiltration. In many cases, nitrate is present only in the upper portion of shallow aquifers. This is in part due to the dynamics of groundwater flow, but geochemistry is a factor. As infiltrated nitrate moves downward into the subsurface it can be eliminated by denitrification. The scavenger-couple approach may offer a cost-effective and reliable means of reducing nitrate contamination in water-supply wells for rural communities. This presentation will describe the Baton Rouge system and discuss the potential for nitrate removal wells in the Midwest.