18
Figure 2. Occurrence of pesticides in subunits mapped by Keefer (1995a, b) during sampling
conducted September 1998 through August 1999 (Mehnert and others, 2001) and
September 2000 through August 2002.
Aquifer
sensitivity
Soil
Pesticide
leaching
class
Depth to
uppermost
aquifer
materials
(ft)
Subunit
Subunit area
as proportion
of total area in
state with
aquifer
material
within 50 feet
of land surface
Number
of
samples
1998-99
Frequency of
occurrence
1998-99
Number
of
samples
2000-02
Frequency
of
occurrence
2000-02
Excessive < 20 1 0.027 19 0.158 25 0.0
Excessive
Moderate < 20 2 0.176 22 0.273 27 0.037
Somewhat
limited or
limited
< 20 3 0.294 22 0.227 33 0.152
High
Excessive 20 to 50 4 0.012 15 0.067 18 0.056
Very
limited < 20 5 0.218 22 0.182 25 0.040
Moderate
High or
moderate 20 to 50 6 0.274 17 0.0 30 0.067
Totals/Weighted Frequency of Occurrence 117 0.159 158 0.079
As part of a one-time sampling program from September 1998 through August 1999, 117
network wells were sampled. Samples were analyzed for the presence of 14 pesticides and 10
inorganic compounds. The overall weighted frequency of occurrence was 15.9 percent (Figure
2). The overall frequency of occurrence refers to the detection of any pesticide, or multiple
detections, from a single groundwater sample. For example, the occurrence of two pesticides
detected in a single well sample at concentrations above the minimum reporting level is
considered a single detection for the purposes of this program. Data from the time-series
sampling indicate that the overall frequency of occurrence of pesticides was greater in the period
from June to October than in the other months.
