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PNitrogen Isotope Monitoring at Unsewered Subdivisions
John R. Tinker, Jr.
Geology Department                  Wrc-    .     en er
University of Wisconsin= Eau Claire          Liink-,,  !.."-iMS]
On-site septic-tank, soil absorption systems which serve unsewered
subdivisions may cause nitrate-nitrogen levels in ground water to exceed the
national drinking water standard of 10 mg/L of nitrate-nitrogen. Tinker, Jr.
(1991 and 1990) analyzed water samples for nitrate-nitrogen from private
water-supply wells in six subdivisions in Wisconsin. The nitrate-nitrogen
values were analyzed for their distribution within the subdivision and for
their relationship to the location of upgradient septic systems.     In addition,
Tinker, Jr. (1991) describes the application of a combined nitrogen mass
balance model of Wehrmann (1984) and the BURBS nitrogen mass balance model of
the Center of Environmental Research, Cornell University (1985) to the six
subdivisions. The results of this investigation concluded that nitrogen from
lawn fertilizer and septic-tank, soil absorption systems cause nitrate-
nitrogen values to increase in the ground water beneath and on the
downgradient side of the six subdivisions.
One other possible source of nitrate-nitrogen is agricultural fertilizer
used on upgradient cultivated land.    To evaluate this possibility, water
samples from private water-supply wells in the six subdivisions in Wisconsin
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were tested for   N isotope.   The assumption to the use of the    N isotope as
an indicator of source of nitrate-nitrogen is that there is a lack of
significant overlap of the range of    N isotope values for potential sources

*       of nitrogen, and that isotopic fractionation does not occur to mask the
original   N isotope signature of each source.