Koontz, J., D.G. Huggins, C.C. Freeman, D.S. Baker - Central Plains ...

and deethylatrazine were both present. Ten sites that had no detectable herbicide concentrations,
including the statistical outlier site 7108.
Detectable levels of herbicides were found in most of the randomly selected wetlands of Phase
II. Every site had atrazine and deethylatrazine. All but one site had detectable levels of
desisopropylatrazine, all but one had metribuzine, all but four had alachlor, all but two had
metachlor, and all but nine had cyanazine. Simazine occurred in only four sites.
All samples were scored by the number of herbicides detected as a way to account for possible
combined effects and to overcome the variability in sample collection times, time of herbicide
application, and losses due to degradation and other processes. Thus herbicide hits were tallied
as present (+1) or absence (0) and the additive scores became independent of concentration.
Examination of resulting CDFs clearly indicated that random and reference populations were
different (Figure 36). However, CDFs indicated that these populations were very similar in
atrazine concentrations (Figure 37). Because reference and random sites were collected across
three summer periods there is the possibility that concentrations and detection hits were
associated with differences in annual hydrological conditions. Comparison of sites that were
sampled in both the Phase I and Phase II studies indicated yearly differences in hydrology.
CDF
100
90
80
70
60
50
40
30
20
10
0
0 1 2 3 4 5 6 7 8
Number of Herbicides Detected
Figure 36. Cumulative distribution frequency (CDF) of number of herbicides detected in
reference (Phase I) and random (Phase II) populations.
51 of 84
Phase One
Phase Two