Chemical Leaching Near the Waiawa Shaft, Oahu, Hawaii: 2. Modeling Results

Abstract

This paper is the second part of a two‐part series concerned with assessing the potential for organic chemical leaching to a ground‐water skimming tunnel in the Pearl Harbor Basin, Oahu, Hawaii, as a direct result of proposed urban development. The Pesticide Root Zone Model (PRZM) was used, after testing with field and laboratory data described in the companion paper, to make long‐term predictions of the movement of chlorpyrifos, diazinon, metribuzin, and nitrate under various recharge scenarios. The PRZM simulations revealed that, with the exception of chlorpyrifos, detectable levels of all the chemicals considered in this study may leach through the unsaturated zone to the water table from where they may eventually migrate to the skimming tunnel. The simulated concentrations in leachate reaching the water table were sufficiently low, considering subsequent mixing in the ground water, to suggest no adverse health effects. The reliability of the simulated results are laced with enough uncertainty, however, to suggest the need for monitoring for diazinon, metribuzin, and nitrate, if development does proceed. The methodology presented in this series is a first attempt at establishing a protocol for using numerical modeling, supported by field and laboratory measurements to aid in land‐use change consideration in Hawaii when nonpoint ground‐water contamination from organic chemicals is of concern.

Field leaching of pesticides at five test sites in Hawaii: modeling flow and transport

“…Preferential flow and transport in oxisols was not considered in this study, although these soils are known to exhibit preferential flow and transport 45, 46. The reasons for neglecting possible preferential flow and transport in weathered oxisols were twofold: (i) the type of field observations (i.e.…”

Calibration of pesticide leaching models: critical review and guidance for reporting

“…However, it has been demonstrated that the use of a single set of input parameters for different agricultural seasons might not lead to a good description of results over the entire experimental period,15, 26, 35, 36 especially under circumstances where droughts are experienced 13. Other examples of limited extrapolation are the use of an application rate different from that used in the calibration11, 33 and the simulation of leaching deeper than the profile depth used during calibration 37…”

A screening tool for vulnerability assessment of pesticide leaching to groundwater for the islands of Hawaii, USA

“…These include simple index models1–6 and transient and deterministic simulation models 7–10. Models of the latter type have been used in several research studies11–15 as well as in regulatory contexts16, 17 to evaluate pesticide leaching to groundwater. Index models have been used to assess the leaching potential of pesticides18–20 and as part of decision support systems,21 and they are often considered suitable for relative vulnerability assessments.…”