| Summary: | Shallow unconfined aquifers are prone to contamination by non-point source
nitrate as a result of agricultural and other land use practises. Management of such
susceptible groundwater resources requires a means of quantifying the transport and fate
of the nitrate from its source to the water table and its subsequent distribution through
the groundwater. A proper understanding of the dynamics associated with nitrate loading
and its effect on unconfined groundwaters is currently lacking. It has been suggested by
several investigators that in-situ management of groundwater nitrate may be achieved by
promoting the natural denitrification capacity of microbial populations present within the
aquifers. However, the feasibility of such an approach has not yet been fully evaluated.
A study involving the detailed monitoring of the changing groundwater chemistry
profiles beneath the water table of two unconfined aquifers, combined with a laboratory
column-test investigation of enhanced denitrification during saturated flow through sand,
has been performed.
The groundwater monitoring was performed using a passive sampling approach
that has enabled detailed multi-level profiling of the groundwater chemistry at and below
the water table. Measurements were made monthly, over a period of twelve months, in
order to determine the temporal variation in groundwater chemistry as related to the
surficial land use and recharge patterns. This research has included the first known
detailed measurement of the seasonal arrival of leached non-point source nitrate to the
water table beneath agricultural lands, and has provided definitive evidence of the pulsed nature of such arrival.
At the study sites, the monitoring has also shown that infiltrating recharge waters
result in the rapid delivery of leached nitrate to the water table of the unconfined
aquifers. Furthermore, the research findings suggest that, as a result of the development
of vertical hydraulic gradients at the water table during recharge, leached nitrate fronts
can be transported to a large depth below the water table within a short time period. A
conceptual model has been developed to describe the observed distribution of nitrate
below the water table. Using this conceptual model, it is also concluded that nitrate that
arrives at the water table of unconfined aquifers with seasonal recharge waters will
subsequently be transported through the aquifer in a pulsed manner in response to a
corresponding seasonal fluctuation in the groundwater flow velocities.
The laboratory column testing has found that ethanol may be preferred over
methanol as a carbon source by the subsurface microbial population during enhanced
denitrification. The findings from this testing also suggest that the effectiveness of carbon
injection systems, as well as stationary reactive barriers aimed at providing denitrifying
environments, may be compromised by clogging as a result of N2 gas bubble
accumulations.
The study results suggest that the role for carbon additions within aquifers for
promoting in-situ denitrification, on an aquifer wide basis, may be limited due to the
seasonally dynamic nature of the nitrate loading and subsequent distribution through
unconfined aquifers, as well as uncertainty associated with the efficacy of heterotrophic
denitrifying microbial populations within the aquifers.
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