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LeMasters, Gary S.; Doyle, Douglas J. / Grade A dairy farm well water quality survey
[DNR-058] (1989)

Results,   pp. 11-14 PDF (2.4 MB)

Page 11

Sixteen producers went out of business before the well could be sampled;
therefore samples from 534 wells were analyzed.
General Results
Of the 71 wells that contained detectable levels of one or more pesticides,
64 contained atrazine alone and 2 contained atrazine plus another pesticide.
The table below shows the findings:
Pesticide             Number of Wells
Atrazine alone              64
Alachlor alone               3
Metribuzin alone             1
Atrazine + alachlor          1
Atrazine + metolachlor       1
Alachlor + metribuzin        1
Total wells with pesticides  71
The median atrazine concentration was 0.45 ug/1 (The median is the number
where half of the remaining numbers lie below and half lie above) and the ES
was exceeded in only 3 wells. In contrast, the ES for alachlor was exceeded in
all 5 wells where it was detected. Metolachlor (Dual@) and metribuzin
(Sencor@) were the only other compounds detected. All four of these
compounds are herbicides, with all but metribuzin associated with corn
production (Figure 2). The complete pesticide results tabulated by county
within each Agricultural Statistics District are in the Appendix.
State Proportion Estimates
The estimators presented earlier were used to yield the proportion estimates
in Table 4. Thirteen percent of the wells on Grade A dairy farms in Wisconsin
are estimated to contain detectable levels of any pesticide. The standard error
of this proportion estimate is 1.4%. A 95% confidence interval can be
constructed as follows:
[0.13 ± (1.96)(0.014)] or [0.13 ± 0.027)]
We can say that in 100 repeated samples the estimated confidence intervals
would contain the true population proportion 95 times.

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