『Abstract
Five re-circulating tracer well tests (RCTWTs) have been conducted
in a variety of aquifer settings, at four sites across New Zealand.
The tests constitute the first practical assessment of the two-well
RCTWT methodology described by Burbery and Wang (Journal of Hufrology,
2010; 382: 163-173) and were aimed at evaluating nitrate reaction
rates in situ. The performance of the RCTWTs differed significantly
at the different sites. The RCTWT method performed well when it
was applied to determine potential nitrate reaction rates in anoxic,
electro-chemically reductive, nitrate-free aquifers of volcanic
lithology, on the North Island, New Zealand. Regional groundwater
flow was not fast-flowing in this setting. An effective first-order
nitrate reaction rate in the region of 0.09 d-1 to
0.26 d-1 was determined from two RCTWTs applied at
one site where a reaction rate of 0.37 d-1 had previously
been estimated from a push-pull test. The RCTWT method performed
poorly, however, in a fast-flowing, nitrate-impacted fluvio-glacial
gravel aquifer that was examined on the South Island, New Zealand.
This setting was more akin to the hypothetical physiochemical
problem described by Burbery and Wang (2010). although aerobic
condition were identified as the primary reason for failure to
measure any nitrate reaction in the gravel aquifer, failure to
establish significant interflow in the re-circulation cell due
to the heterogeneous nature of the aquifer structure, and natural
variability exhibited in nitrate contaminant levels of the ambient
groundwater further contributed to the poor performance of the
test. Our findings suggest that in practice, environmental conditions
are more complex than assumed by the RCTWT methodology, which
compromises the practicability of the method as one for determining
attenuation rates in groundwater based on tracing ambient contaminant
levels. although limited, there appears to be a scope for RCTWTs
to provide useful information on potential attenuation rates when
reactants are supplemented to the aquifer system under examination.
Keywords: Re-circulating tracer well test; Nitrate reaction rate
measurement; In situ; New Zealand』
1. Introduction
2. Methodology
2.1. ECTWT design considerations
2.2. Field site descriptions
2.2.1. North Island (volcanic lithology)
2.2.2. South Island (fluvio-glacial gravels)
2.3. RCTWT procedures
2.4. Determination of first-order rate coefficients
3. Experimental results
2.1. North Island (volcanic sands)
3.2. South Island (fluvio-glacial gravels)
4. Discussion
4.1. Hydraulics
4.2. Nitrate reaction
5. Conclusions
Acknowledgements
References