『Abstract
A pilot-scale test of an in situ denitrification scheme was undertaken
to assess an adaptation of the nutrient injection wall (NIW) technology
for treating a deep (30-40 m) nitrate contamination problem (N-NO3- 〜10-12 mg/L). The adaptation is
called the Cross-Injection Scheme (CIS). It duplicates the NIW
method without a wall; wells are installed and operated directly
in the aquifer and high-flux zones of the aquifer are preferentially
targeted for treatment. The test was conducted on the site of
a municipal water supply well field, with the supply well pumping
between 15-80 m3/h. Acetate was periodically injected
into the aquifer between an injection-extraction well pair positioned
across the normal direction of flow. The injected pulses were
then permitted to move with the water toward the municipal wells,
providing a carbon supply to drive the desired denitrification.
The fate of nitrate, nitrite, acetate and sulphate were monitored
at multilevel wells located between the injection location and
the municipal wells. The acetate pulsing interval was approximately
weekly (9 h injections), so that the system was operating passively
95％ of the time. Previous work on the site has established that
the highest solute fluxes were associated with a 1-3 m thick zone
about 35 m below surface. This zone was found to respond to the
acetate additions as a function of the municipal pumping rate
and the carbon-to-nitrogen ratio (i.e., determined by the injected
acetate concentration). Initially, acetate was injected just below
the theoretical stoichiometric requirement for complete denitrification
and nitrate disappearance was accompanied by nitrite production.
Increasing the C:N ratio (doubling the acetate injection concentration)
increased the removal of nitrate and diminished the occurrence
of nitrite. Slowing the municipal pumping rate, with a C:N ratio
of 1.2-1.6, resulted in complete nitrate attenuation with no nitrite
production and no sulfate reduction. The experiment demonstrated
that the CIS injection scheme is a viable option for the treatment
of nitrate contamination in situ near high-capacity wells.
Keywords: Nitrate; Denitrification; remediation; Water quality;
Nutrient injection wall; Cross-injection scheme; Water supply』
1. Introduction
1.1. Background
1.1.1. Injection systems for in situ denitrification
1.1.2. PRBs for denitrification
1.1.3. Description of the field site
1.1.4. Selection of the cross-injection scheme (CIS) for denitrification
2. Methods
2.1. Site instrumentation
2.2. Sampling and analysis
2.3. Substrate additions
2.4. Evaluating changes to the microbial populations
3. Results and discussion
3.1. Determining the pulsing interval
3.2. Delineation of a high-flux zone
3.3. Acetate additions and distribution
3.4. The fate of ammonia, iron, and sulphate
3.5. The fate of nitrate and nitrite
3.6. Nitrate flux to B2
3.7. Changes in subsurface bacterial populations and activity
3.8. Localizing denitrification
3.9. Clogging
4. Conclusions
Acknowledgements
References