Patterns and Trends of Salinity in the Finegayan Basin and the Effects of Precipitation

Guam, an unincorporated territory of the United States, utilizes the Northern Guam Lens Aquifer to supply 90% of the drinking water consumed on the island. With the projected population increase due to the opening of a new Marine Corps Base, it is imperative to monitor and maintain this groundwater resource. As the aquifer is recharged through precipitation, we wanted to determine the effects of precipitation on chloride concentration present in production wells. Through statistical analysis of chloride concentration and the effects of average production rates and precipitation of the wells in the Finegayan Basin, we determined patterns and relationships that represent the health of the aquifer. As the study area itself does not contain a rain gauge, Kriging interpolation using precipitation data from nine existing USGS and Guam International Airport rain gauges was performed. Precipitation at each well site and chloride concentration were compared to determine any relationships. At a 95% confidence interval, chloride concentration over time for ten of the sixteen wells (62.5%) demonstrate a significant increasing trend, four (25%) demonstrate a non-significant increasing trend, one (6.25%) demonstrates a non-significant decreasing trend, and one (6.25%) demonstrates a significant decreasing trend. At the 95% confidence interval, analysis of the relationship between precipitation and chloride concentration demonstrates eleven of the sixteen wells (68.8%) show a non-significant decreasing trend and five (31.2%) show a non-significant increasing trend. As the results were not statistically significant, there is no sufficient evidence to suggest that an increase or decrease in precipitation is the only parameter affecting the increase or decrease in chloride concentration.