A single well push–pull test method for in situ determination of denitrification rates in a nitrate-contaminated groundwater aquifer

Abstract

In this study a single-well, "push- pull" test method is adapted for determination of in situ denitrification rates in groundwater aquifers. The rates of stepwise reduction of nitrate to nitrite, nitrous oxide, and molecular nitrogen were determined by performing a series of push-pull tests. The method consists of the controlled injection of a prepared test solution ("push") into an aquifer followed by the extraction of the test solution/ground water mixture ("pull") from the same location. The injected test solution consists of ground water containing a nonreactive tracer and one or more biologically reactive solutes. Reaction rate coefficients are computed from the mass of reactant consumed and/or product formed. A single Transport Test, one Biostimulation Test, and four Activity Tests were conducted for this study. Transport tests are conducted to evaluate the mobility of solutes used in subsequent tests. These included bromide (a conservative tracer), fumarate (a carbon and/or source), and nitrate (an electron acceptor). Extraction phase breakthrough curves for all solutes were similar, indicating apparent conservative transport of the solutes prior to biostimulation. Biostimulation tests were conducted to stimulate the activity of indigenous heterotrophic denitrifying microorganisms and consisted of injection of site ground water containing fumarate and nitrate. Biostimulation was detected by the simultaneous production of carbon dioxide and nitrite after each injection. Activity tests were conducted to quantify rates of nitrate, nitrite, and nitrous oxide reduction. Estimated zero-order degradation rates decreased in the order nitrate > nitrite > nitrous oxide. The series of push-pull tests developed and field tested in this study should prove useful for conducting rapid, low-cost feasibility assessments for in situ denitrification in nitrate-contaminated aquifers.

Recovery of groundwater N2O at the soil surface and its contribution to total N2O emissions

“…In contrast to push-and-pull tracer studies carried out at single groundwater wells Well et al 2003;Kim et al 2005), we performed a tracer experiment with labeling of an extended area of the surface groundwater where an even distribution of the label was approximated by using multiple injection points. Because concentrations of NO 3 -and N 2 O in the groundwater were very low before the first tracer application (Figs.…”

“…Diverse approaches to the problem were reviewed by Groffman et al (2006). In situ tracer tests were conducted in order to study the fate of nitrate (NO 3 -) and potential denitrification rates in subsoils and groundwater using 15 N labeled NO 3 - (Tobias et al 2001;Addy et al 2002;Well et al 2003;Kim et al 2005). For these ''push-pull methods'', a test solution containing 15 N-labeled NO 3 -is injected (''pushed'') into the soil matrix or groundwater, respectively.…”

Determination of Electron Donors by Comparing Reaction Rates for In Situ Bioremediation of Nitrate-Contaminated Groundwater

“…A single-well, "push-pull" test method may be adapted for determination of in situ denitrification rates in ground aquifers. [12] The method consists of the controlled injection of a prepared test solution consisting a tracer and a carbon source/electron donor ("push") into an aquifer followed by the extraction of the test solution/ground water mixture ("pull") from the same location. Degradation kinetics can be estimated by comparing concentrations of test solutions injected and extracted.…”

Nitrate reduction mechanisms and rates in an unconfined eogenetic karst aquifer in two sites with different redox potential

“…and how do changes in selected functional N transformation gene copy numbers and expression inform mechanisms of observed nitrate and carbon losses? Based upon expected favorable conditions for nitrate reduction, prior studies in the study area [ Katz , , ; Katz et al ., ; Heffernan et al ., ], basin fill aquifer PPTTs documented in literature [ Istok et al ., ; Kim et al ., , ], it is expected that nitrate and carbon loss rates would be higher at the anoxic site and negligible at the oxic site and DOC availability would be the primary driver of any observed nitrate reduction. Furthermore, it is expected that functional N transformation genes will show (1) denitrification, DNRA, and ANRA are negligible at ambient conditions, (2) an undetectable response to nitrate treatments and increased response with nitrate and carbon treatments, (3) denitrification is the dominant sink for nitrate with carbon addition at the anoxic site, and (4) if detected, DNRA gene expression will respond to carbon and nitrate treatments at the anoxic site.…”