Abstract
Increased ground and surface water contamination has fueled the
need for a better understanding of factors affecting
hydrogeological settings. Seasonal variability in the recharge of
an aquifer can affect the rate and amount of contamination that
can occur. Stable isotopes have been previously investigated as
tracers to determine flow paths of the water and contaminants. In
this study, oxygen isotopes were used to determine the factors
that influence seasonal variations in ground and surface water,
such as well location, aquifer type, surface water sites, and soil
type. Samples were collected from different locations around the
Canajoharie Field Site, including wells that were screened in an
unconfined aquifer, wells screened in confined aquifers, tile
drains, and surface water sites, in an attempt to determine
differences in seasonal variation.
The proximity of the wells to the Canajoharie Creek affected the
seasonal variation of both water table elevation and oxygen
isotopic composition of well water. The closer the well to the
stream, the greater the response to storm events and seasonal
changes. Wells along fence and tree lines show more of an
influence during spring snowmelts than wells located in the middle
of the fields, due to snow drifting from open areas to the fence
and tree lines, resulting in a disproportionately large amount of
recharge. The confined aquifers show little to no seasonal and
storm event response.
Samples collected from two tile drains show that soil type also
affects the recharge response. Soils with lower porosity, i.e.
clayey soils, displayed macro pore flushing following storm
events, whereas more porous sandy soils showed better water mixing
capabilities. The samples collected from the tile drains were also
analyzed for the herbicide metolachlor, and two metabolite
compounds, metolachlor ESA (ethanasulfonic acid) and metolachlor
OA (oxanilic acid), that are found in higher concentrations and
persist longer than the parent compound. The oxygen isotopic
composition of the tile drain water was compared with the ratios
of metolachlor ESA to metolachlor (SAM ratio) and metolachlor ESA
to metolachlor OA (SAO ratio) to illustrate macropore flushing and
to determine how residence time of groundwater affects the
herbicide degradation. A larger decrease in SAO ratios in the clay
soil is the result of macropore flow which causes the breakdown of
the less stable metolachlor OA in the infiltrating water. Pore
water has a longer residence time in clayey soil, resulting in a
higher SAM ratio in the water subsequently flushed out during
storm events. Sandy soil retains less water in the pores, the
briefer soil-water interaction resulting in lower metolachlor ESA
production and lower SAM values.
John M. Nadeau 2000. Oxygen isotopes and metolachlor residues in
water samples from a portion of the Canajoharie Creek watershed,
central New York. Unpublished MSc. thesis, State University
of New York at Albany. 84 pp., +xii
University at Albany Science Library call number: SCIENCE
Oversize (*) QC 869 Z899 2000 N33
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