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Muldoon, Maureen A.; Madison, Frederick William, 1937-; Lowery, Birl / Variability of nitrate loading and determination of monitoring frequency for a shallow sandy aquifer, Arena, Wisconsin
[DNR-123] (1998)

Implications for monitoring,   pp. 21-25 PDF (1.4 MB)


Page 22


leaving the soil profile. Elevated nitrate values are observed on the rising-limb of the
hydrograph. Nitrate concentrations increase rapidly; the 1998 data indicate a change of 10
mg/i in a 24-hour period. This rapid rise in nitrate concentrations suggests that frequent
sampling is necessary to capture the loading of nitrate due to spring recharge. The timing
of springtime monitoring efforts needs to be based on soil-temperature data rather than
either precipitation or water-level data.
*Once the crop cover has been established, nitrate appears to be less mobile,
concentrations are relatively stable, and variations in concentration are difficult to
correlate with specific precipitation events. This behavior contrasts with that of chloride,
another conservative anion. Pulses of chloride arrive at the water-table approximately 5 to
5.5 days after peak water levels are recorded in the shallow piezometers. The reduced
mobility of nitrate during the summer months is assumed to be due to uptake of nitrogen
by the actively growing crop cover. Because of its extremely conservative nature,
chloride appears to be a better indicator of the impacts of rainfall events on the saturated
zone, particularly during the active evapotranspiration period. Reliable nitrate data are
only available for the summer of 1997. The largest rainfall event during this period was
1.28 inches. While nitrate concentrations did not correlate with specific precipitation
events in that year, it is possible that very large rainstorms (>1.3 in) could move nitrate to
the water table.
* From the late summer (Aug 15) on, nitrate movement appears to be correlated with crop
activity rather than with precipitation or recharge events. Once crops mature,
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