A combination of laboratory and field-based investigations were carried out to identify the mobility and persistence of the organophosphorous insecticide, chlorpyrifos, in a local soil used to build the fairways of the Guam International Country Club golf course (GICC). This facility lies directly over the island's sole-source aquifer and is regarded by some to represent a potential threat to the integrity of the underlying groundwater with respect to agrochemical contamination. Chlorpyrifos is one of several pesticides used at GICC to control troublesome weeds and insect pests.

The results of our laboratory investigations indicate that chlorpyrifos is rapidly taken up by the soil, with a sorption equilibration time in the order of 30 minutes. Sorptive partitioning between the soil and water was determined by batch equilibrium. The soil-water partition coefficient (K), determined from the Freundlich adsorption isotherm, was around 70 picomol/g, suggesting a relatively strong affinity for the soil phase. However, the desorption data implied that the adsorptive mechanisms operating were relatively weak since only 20% of the pesticide remained on spiked soil after 4 desorptive rinses.

Degradation half-lives of chlorpyrifos in unsterilized and sterilized soil samples, held at 26 º C, were estimated at 6 and 12.5 days respectively, indicating that both biotic and abiotic factors determine decay rates in this soil. Losses of chlorpyrifos from groundwater samples (dechlorinated tap water), spiked either before or after passage through unsterilized soil columns, further highlighted the importance of microbiological degradation processes, with half-lives in the order of > 4 weeks and < 1 week for each treatment respectively.

The field-based experiments were conducted in an outdoor setting using packed lysimeters. "Worst case scenario" conditions were approximated with respect to chlorpyrifos application rate, surface vegetation, and seasonal moisture regimes. To this end, two single application experiments were conducted in which the presence/absence of turf grass was the primary variable. In the first experiment, Bermuda grass vs. bare soil was irrigated with sufficient groundwater to simulate a dry season high watering regime (1.25 cm/day). In the second experiment, Zoysia grass was selected as the turf-grass representative and both treatments were watered to excess with deionized water (5 cm/day) in an attempt to simulate extreme wet season conditions.

During the first experiment, detectable levels of chlorpyrifos (3.5 to 9.6 ng/L) occurred in effluent drainage water from 4 to 24 lysimeters on days 4 to 7 and were coincident with the passing of a tropical cyclone. Thereafter, levels were below detection limits of 1.5 ng/L for the duration of the experiments. Chlorpyrifos was not detected in any effluent drainage water during the second experiment despite the increased hydraulic loading. However, the breakdown products 3,5,6-trichloro-2-pyridinol (TCP) and 3,5,6-trichloro-2-methoxypyridine (TMP) were detected from the second week onwards in both treatments.

Soil analyses indicated that chlorpyrifos was predominantly confined to the upper 2-cm portion of the soil column and was undetectable below a depth of 10 to 20 cm, depending upon hydraulic loading. The half-life of chlorpyrifos was in the order 3 days for bare soil treatments, in both experiments. Dissipation rates were about the same for the Bermudagrass turfed treatments but decreased markedly with Zoysiagrass to produce a half-life of around 7 weeks. In this particular instance, residual amounts of chlorpyrifos (up to 1.2% of the initial amount) were detected in the upper 2-cm soil section 24 weeks after application.

The implications of the data are clear. The high propensity of chlorpyrifos for GICC fairways soil, coupled with its relatively rapid degradation rate, greatly reduces the chance of it being appreciably leached into the underlying aquifer, except perhaps in very shallow soils (< 10 cm), immediately after application under conditions of prolonged, heavy rain. The possibility of this pesticide ever becoming a serious drinking water contaminant on the island, therefore, seems very unlikely under normal turf-grass management practices. However, the potential impact of the more mobile degradation products remains to be evaluated.