CHEMICAL LEACHING NEAR THE WAIAWA SHAFT, OAHU, HAWAII .2. MODELING RESULTS

This paper is the second part of a two-part series concerned with asse ssing the potential for organic chemical leaching to a ground-water sk imming tunnel in the Pearl Harbor Basin, Oahu, Hawaii, as a direct res ult 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 thi s study may leach through the unsaturated zone to the water table from where they may eventually migrate to the skimming tunnel. The simulat ed concentrations in leachate reaching the water table were sufficient ly low, considering subsequent mixing in the ground water, to suggest no adverse health effects. The reliability of the simulated results ar e laced with enough uncertainty, however, to suggest the need for moni toring for diazinon, metribuzin, and nitrate, if development does proc eed. The methodology presented in this series is a first attempt at es tablishing a protocol for using numerical modeling, supported by field and laboratory measurements to aid in land-use change consideration i n Hawaii when nonpoint ground-water contamination from organic chemica ls is of concern.