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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 21

Management of nutrients applied to agricultural lands is a growing concern in Wisconsin as recent
sampling programs have revealed that a significant number of rural wells exceeds the state's
drinking-water standard of 10 mg/1 of nitrate-N (Kraft, 1994). Concern about nutrient
management, especially management of animal wastes, is likely to continue as the number of
farms with more than 1000 animal units is currently increasing. We need to understand the
seasonal variability of nitrate movement to the water table in order to 1) design effective
monitoring systems and 2) develop effective nutrient management plans. The objectives of this
project were 1) to determine the temporal variability in nitrate concentrations in shallow, sandy
aquifers, 2) to define the relationship between recharge and groundwater nitrate concentrations,
and 3) to develop recommendations concerning monitoring frequency for such aquifers.
        Water-quality and water-level data from the 1997 and 1998 growing seasons suggest that
variations in nitrate concentrations at the water table are a function of both 1) groundwater
recharge events and 2) crop uptake and that these factors vary seasonally. The standard practice
of quarterly groundwater sampling (4 camples/year) is inadequate in shallow sandy aquifers as
nitrate and chloride fluctuate much more rapidly. Indeed many projects designed to assess the use
of agricultural pest management practices on groundwater quality consider "frequent" menitoring
to be monthly. It is clear from this study that neighter quarterly nor monthly sampling would fully
characterize the nitrate loading to the water table in this setting. Effective monitoring strategies
need to be designed with the following considerations.
       *Jn the spring, pulses of nitrate move to the water-table with recharging snowmelt. These
       pulses are not linked to specific precipitation events but rather the timing of the frost

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