New Water Policy and Practice Issue 4, Number 1, Fall 2017 | Page 50

New Water Policy & Practice Journal
tive approach as it has non-data-intensive input requirements that are readily
available, it can test a range of scenarios, and can rapidly compare multiple
pesticides at once. However, PIRI is limited to assessing a fixed area with
a uniform land use (Pollock et al. 2005). Therefore to model pesticide risk
at the catchment scale, PIRI offsite transport information was successfully
integrated into GIS. As the offsite transport information incorporates direct
runoff and soil erosion, it was important that land uses in the trial catchment
are understood as they have the ability to influence these features. This new
graphical risk-assessment approach was tested at this trial catchment. If all
the agricultural land in this catchment used glyphosate according to the label
instructions three days before a 50 mm rainfall event (a once-a-year occur-
rence), 460 kg of glyphosate would be expected to be exported in runoff. The
importance of verifying land uses was realized at this trial catchment as the
pesticide risk to water quality had changed.
As a result, areas of high-risk pesticide transport are readily identi-
fied using the pesticide risk modelling and have been used to direct further
pesticide monitoring efforts in the catchment for the second year of sam-
pling. Figure 1 demonstrates the risk of glyphosate transport across this trial
catchment. Two additional catchment sample points were established, in the
lower catchment area highlighted in Figure 1, and included in the sampling
program. No pesticides were detected in this catchment over the two years
of sampling. This is thought to reflect the established PDWSA land man-
agement planning explained above. However, improvements to the location
and times of pesticide monitoring at this catchment have been identified
through this project and will be pursued to ensure the pesticide risk is being
effectively monitored.
This pesticide-risk modelling approach is thought to be an informa-
tive desktop tool to investigate efficiencies with pesticide monitoring at other
drinking water catchments in WA. Refinement of this method is currently
being investigated to improve the detail of model outputs, particularly at the
sub-catchment scale.
The learnings of the research project show the relative complexities
of the pesticide transport processes in catchments and that the timing of
sampling is dependent on understanding what factors dominate transport
in different catchments. Currently these learnings are being consolidated to
support the update of the Corporation’s internal risk-assessment processes.
Importantly, the initial risk findings have guided the latest revision of the
Corporation’s MoU with DoH, through providing a logical, repeatable, and
consistent process for pesticide monitoring in WA. So far, results have provid-
ed the Corporation with greater confidence that pesticide risk to drinking wa-
ter quality is understood, and greater assurance that these risks are currently
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