Lens Dynamics

Dynamic Response of the Freshwater Lens to Natural Variations in Recharge
Northern Guam Lens Aquifer, Yigo-Tumon Basin, Guam

Bekah Dougher, Nathan C. Habana, John W. Jenson, Mark A. Lander, and Kevin Ho

Technical Report 168 | Publication

ABSTRACT

The limestone aquifer of northern Guam supplies more than 75% of the island’s drinking water. As Guam prepares for more economic growth, demand for water is a major concern. The quantity of groundwater available for extraction can be measured in terms of the freshwater lens thickness. Lens thickness can be measured directly from well salinity profiles and inferred indirectly from water levels. The amount of recharge that replenishes the aquifer depends primarily on seasonal and inter-annual changes in rainfall as well as on the porous media properties, recharge, and discharge. Time series data was analyzed to determine lens response to recharge and drought. A multi-variable time-series analysis aligned possible communicative data of ONI, rainfall, and sea level to the phreatic graphs. Lag time response to variations in recharge was determined and can be used as an indicator of lens stability. The goal of this project is to characterize the response of the lens to natural climate variations, given the ongoing withdrawal from the lens for drinking water production, as well as to observe lens and transition zone dynamics as a result of excess recharge and extensive drought. The single most important question is how much of rainfall constitutes effective recharge—recharge that actually goes into the lens and thickens the freshwater zone. The proposed research may soon provide an empirical basis for estimating the percent of rainfall that actually goes into lens storage. It will thus provide an observational baseline against which the accuracy of past, present, and future modeling studies can be evaluated and by which future modeling studies can be reliably parameterized.

BACKGROUND 

The carbonate bedrock in the northern half of Guam stores a freshwater lens, hydrogeologically referred to as the Northern Guam Lens Aquifer (NGLA). The lens is formed by recharge via rainfall infiltration, percolating and moving through aquifer pores down into the water table, and is buoyant on saltwater beneath. The freshwater-saltwater interface is a gradient transition zone, observed in deep observation wells (DOWs) and coastal caves. DOW salinity profile data reveal freshwater lens and saltwater transition zone dynamics, responding to recharge and tide. Profile data analysis reveal lens dynamics, thickening and thinning of the freshwater that range between 90-140ft (27-43m). The transition zone ranges between 40-120ft (12-37m). Lens dynamics may be influenced by tide and groundwater recharge, provided that there is communicable porosity to these influences. Tide and recharge can be influenced by natural climate cycles (e.g., El Niño, La Niña) that bring on unusual periods of prolonged low tides, drought, and irregular storm patterns in the region, affecting the normal season. Typhoons and storms bring intense rainfall that can move large runoff into surface sinks and into effective recharge that thickens the lens. In contrast, droughts can stop aquifer recharge input, discharging the lens to the coast and production, yielding a negative change in freshwater storage, thus thinning the lens.

Transition zone dynamics in the NGLA and DOWs

 

Specific conductance or salinity profiles are graphed from quarterly CTD data and show salinity changes in each well with depth. The transition zone (TZ) is divided into brackish and saline layers, as it grades into salt water. The freshwater is based on USEPA secondary water quality regulation, having less than 250 mg/L (<1100 μS/cm). The graph above shows a general specific conductance profile while the graphs at the top illustrate specific conductance profiles for the three deep observation wells in this study.

METHODS

Four graphic analyses were determined. The first analysis, salinity profiles, were graphed and examined for determining and defining the transition zones and phreatic interfaces. Second, a timeseries graph and basic statistics of observation well data, depth to scale, for the defined zones was made. Third, a multi-variable time-series analysis was made to align possible communicative data of ONI, rainfall, and sea level to the phreatic graphs. Finally, for each deep observation well, a vertical graph of frequency analysis of the phreatic interfaces was done.

RESULTS

Salinity profiles were analyzed to define four parts of the phreatic zone: freshwater, brackish, saline, and saltwater. Top right, charts for three DOWLs show the transition zone dynamics of their defined profile, to scale. To the right, is the multi-variable time-series analysis. Far right, upright depth frequency analysis of bottom of freshwater lens and bottom of transition zone. Below, table summarizing thickening and thinning, percent change, and lag time.

Specific conductance profile of three DOWs above was used to determine the lens transition
zone model. Data is available in USGS’ National Water Information System

DOW profile data analysis is arranged in a time-series, to observe lens dynamics. Ghyben-Herzberg ratio (40:1) was calculated, graphed as the dashed yellow line. This lens shows a fast response (6 months) to recharge and a much slower (12+ months) response to drought.

The phreatic interface depth frequency analysis above is a vertical graphic illustration of distribution and average of observation well water level, bottom of freshwater lens (BoFL), and end (bottom) of transition zone elevation (BoTZ). This was done for each deep observation well level (DOWL) in the study. The three phreatic interfaces of interest in this analysis are the water level (blue line), the BoFL (green line) and the BoTZ (red line). Also shown, is the production well depth zone consistent with GEPA regulations of 25 – 40ft below static level as suggested by Mink (1976), a third of the fresh water lens. The BoTZ has a poly-modal distribution for each well and the BoFL varies as well. The variations in phreatic interfaces are a result of hydrogeological and geologic distinctions, where EX-10 is close to a fault (Pugua Fault) that that reaches the coastline, bringing saltwater deep inland near it. EX-7 has a thick transition zone, while GD is the thinnest of the three with the thickest lens (average and modal), due to presence of local variations in karst porosities.

 

The multi-variable time-series analysis aligns observed groundwater dynamics with Oceanic Nino Index, Sea Surface Temperature, rainfall and 5 year running sum, and tide data. The aquifer was its thickest after Typhoon Tingting in 2004. It was soon followed by 5 years of daily rain with less than 4in, enduring 5 years at a decline rates for EX-7 and GD of about 6ft/yr, and EX-10 at 3ft/yr. The 5 year running sum of rainfall appears to rise and fall in a similar fashion to the thickness of the lens. For each DOW below, data before 2004 was sparse and consistent methodology is uncertain. Thereafter, the CWMP (G.P.L. 24-161) maintained consistent data collection and repository service from USGS for 14 years.

CONCLUSION

Lens thickening is a result of effective recharge and thinning is from little to no recharge. Effective recharge was observed during Typhoon Tingting in 2004, pouring 21.85in of rain in 24 hours. GD well CTD analysis records 143’ thick lens after several months. Soon followed was “the big nothing,” having less than 4in of rain per day, for 5 years. From Tingting to the end of the big nothing, the lens profile slowly declined, at rates of 3-6ft/yr, to 2011, when it begins to change slowly thicken again. The NGLA is robust to dry seasons and is significantly recharged with intense storm rainfall similar to Tingting.