Water Resources Research Act Program

Principal Investigators: Dr. Mark Lander

Abstract: The thick limestone layer that covers most of northern Guam retains an enormous amount of rain water to support most of the island’s municipal water needs. The capacity of the Northern Guam Lens Aquifer (NGLA) is enormous; tens of millions of gallons of potable fresh water are extracted daily without serious impacts on water quantity and quality. That is not to say that there are no concerns about sustainable yield, possible impacts of seawater intrusion and infiltration of pollutants (e.g., pesticides, sewage, and other problematic industrial chemicals). There are concerns for the health of the aquifer from increased human development (in particular, a major ramp-up of military infrastructure and personnel) and the possible impacts from climate change (projections show possible small decreases to annual rainfall and possible reductions in the number of typhoons affecting the island).Rainfall amounts on Guam are high. The average annual rainfall on the island is roughly 100 inches, with modest (10-15%) geographical variation (drier in the middle and on the central east coast, and wetter in the southern mountain region and in a stripe across the northern end). But, despite high average rainfall, there are large variations of rainfall at all time scales. Over the course of the year, there is typically a 6-month dry season during which only 30 inches of the annual 100 inches accrues. It is thought that very little of the dry season rainfall recharges the aquifer (Jocson et al., 1999). The amount of annual rainfall also has large variation, primarily the result of forcing by the El Niño/Southern Oscillation (ENSO) phemomenon. Annual rainfall amounts vary by a factor of three! (50 to 150 inches). Daily rainfall during a typhoon exceeds 7 inches in one hour and 30 inches in 24 hours.In past WERI research efforts, evidence was found that on top of the immediate response of the water table to rainfall and variations to sea level, the water table of the NGLA responded at an 18-month lag to long-term surpluses and deficits of rainfall (Beausoliel et al., 2001). Hormillosa (2009) outlined an elegant method (minimizing the hysteresis of modeled water table with respect to the observed water table) to elucidate the response of the aquifer to rainfall input. The findings of this research were largely exploratory, with formal investigative follow-through pending. It is the purpose of this proposal request to consolidate and summarize these past findings with a formal rigor (data consolidation, updated data, further statistical analysis, clarification of the techniques, and a detailed discussion of the results); and all this to be followed by publication.The objectives of the proposed project can be summarized as the followings;1)To consolidate all data used in the past studies of aquifer response to variations of sea level and rainfall;2)To update (through 2019) the data sets used by Beausoliel (2001) and Hormillosa (2009) in their studies of aquifer response to sea level and rainfall;3)To clarify the use of hysteresis to elucidate the character of NGLA recharge;4)To train a graduate student on statistical techniques of data analysis, to include: the use of step-wise linear regression; the parsing of vadose flow into the aquifer via a gamma distribution; and the use of hysteresis to characterize vadose flow.5)To consolidate and summarize existing research into a comprehensive Tech report6) Submit a peer-reviewed journal article.