UNCERTAINTY IN RECHARGE ESTIMATION - IMPACT ON GROUNDWATER VULNERABILITY ASSESSMENTS FOR THE PEARL-HARBOR BASIN, OAHU, HAWAII, USA

In this paper, uncertainty in recharge estimates is investigated relat ive to its impact on assessments of groundwater contamination vulnerab ility using a relatively simple pesticide mobility index, attenuation factor (AF). We employ a combination of first-order uncertainty analys is (FOUA) and sensitivity analysis to investigate recharge uncertainti es for agricultural land on the island of O'ahu, Hawai'i, that is curr ently, or has been in the past, under sugarcane or pineapple cultivati on. Uncertainty in recharge due to recharge component uncertainties is 49% of the mean for sugarcane and 58% of the mean for pineapple. The components contributing the largest amounts of uncertainty to the rech arge estimate are irrigation in the case of sugarcane and precipitatio n in the case of pineapple, For a suite of pesticides formerly or curr ently used in the region, the contribution to AF uncertainty of rechar ge uncertainty was compared with the contributions of other AF compone nts: retardation factor (RF), a measure of the effects of sorption; so il-water content at field capacity (Theta(FC)); and pesticide half-lif e (t(1/2)). Depending upon the pesticide, the contribution of recharge to uncertainty ranks second or third among the four AF components tes ted. The natural temporal variability of recharge is another source of uncertainty in AF, because the index is calculated using the time-ave raged recharge rate. Relative to the mean, recharge variability is 10% , 44%, and 176% for the annual, monthly, and daily time scales, respec tively, under sugarcane, and 31%, 112%, and 344%, respectively, under pineapple. In general, uncertainty in AF associated with temporal vari ability in recharge at all time scales exceeds AF. For chemicals such as atrazine or diuron under sugarcane, and atrazine or bramacil under pineapple, the range of AF uncertainty due to temporal varisbility in recharge encompasses significantly higher levels of leaching potential at some locations than that indicated by the AF estimate.