Simulated effect of vadose infiltration on water levels in the Northern Guam Lens Aquifer

Contractor, D. N.; Jenson, John W.

doi:10.1016/s0022-1694(00)00157-8

Cited by 34 publications

(30 citation statements)

References 9 publications

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“…, Darcy's law generally is assumed applicable to regional groundwater flow analyses in Guam (Contractor, 1983;Contractor and Srivastava, 1990;Jocson and others, 1999;Contractor and Jenson, 2000).…”

Section: Hydraulic Conductivitymentioning

confidence: 99%

“…Faults transecting the plateau complicate the structure and regional hydraulic conductivity of the aquifer rock units (Mylroie and others, 2001;Jenson and others, 2006). In previous modeling studies, the regional horizontal hydraulic conductivity of the main limestone aquifer generally is estimated to be thousands to tens of thousands of feet per day (Contractor and Srivastava, 1990;Jocson and others, 1999;Contractor and Jenson, 2000). Ayers and Clayshulte (1984) made regional hydraulicconductivity estimates ranging from 3,300-19,700 feet per day (ft/d) from analyses of tidal-induced fluctuations in five wells of varying distances inland on the central west coast of northern Guam, and they noted "shoreline rocks may be lower in hydraulic conductivity," acting "as a dam to the inland propagating tidal signal."…”

Section: Hydraulic Conductivitymentioning

confidence: 99%

“…Studies that are more recent have investigated the timing of individual rain events and the role of the unsaturated zone in recharge. From their numerical modeling study, Contractor and Jenson (2000) concluded that temporary storage of infiltrating water in the vadose zone is significant and infiltration rates are strongly dependent on the water content of the vadose zone at the time of the rain event.…”

Section: Rechargementioning

confidence: 99%

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The effects of withdrawals and drought on groundwater availability in the Northern Guam Lens Aquifer, Guam

Gingerich¹

2013

Scientific Investigations Report

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For more information on the USGS-the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment, visit http://www.usgs.gov or call 1-888-ASK-USGS.For an overview of USGS information products, including maps, imagery, and publications, visit http://www.usgs.gov/pubprodTo order this and other USGS information products, visit http://store.usgs.gov Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.Suggested citation: Gingerich, S.B., 2013, The effects of withdrawals and drought on groundwater availability in the Northern Guam Lens Aquifer, Guam: U.S. Geological Survey Scientific Investigations Report 2013-5216, 76 p., http://dx.doi.org/10.3133/ sir20135216. ISSN 2328-0328 (online) iii Executive SummaryMost of the public water supply on Guam is pumped from a freshwater lens in the Northern Guam Lens Aquifer, a limestone aquifer. Because of population growth and a potential military realignment involving the U.S. Marine Corps (USMC), groundwater withdrawals from wells are expected to increase. Currently (2013), the salinity of water pumped from some wells in the area is higher than acceptable for drinking water. The expected increase in demand for water has led to concern over the long-term sustainability of withdrawals from existing and proposed wells. In 2010, to aid in management of groundwater resources and to plan for sustainable growth on the island, the United States Marine Corps, Headquarters entered into a cooperative agreement with the U.S. Geological Survey (USGS) to study groundwater availability in northern Guam. The objective of this 3.5-year study was to estimate the effects of several hypothetical withdrawal scenarios on water levels, the transition zone between freshwater and saltwater, and salinity of pumped wells.On Guam, freshwater-lens systems are found in limestone and volcanic rocks, but the most important groundwater sources are from the freshwater parts of these systems in the high-hydraulic-conductivity limestone rocks of northern Guam. The most intensely developed groundwater system in this area comprises a lens-shaped freshwater body, an intermediate brackish-water transition zone, and underlying saltwater. For management purposes, the Guam Environmental Protection Agency (GEPA) delineated the Northern Guam Lens Aquifer (NGLA) into six hydrologically connected aquifer basins: Agafo Gumas, Andersen, Finegayan, Hagåtña, Mangilao, and Yigo-Tumon. Hydrologic DataKnowledge of the NGLA hydrology comes from compilation and analysis of data on water levels, salinity profiles in deep monitoring wells, production well pumping rates and salinities, and geologic information gleaned from decades of well drilling and hydrologic monitoring. The NGLA...

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Section: Hydraulic Conductivitymentioning

confidence: 99%

Section: Hydraulic Conductivitymentioning

confidence: 99%

Section: Rechargementioning

confidence: 99%

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The effects of withdrawals and drought on groundwater availability in the Northern Guam Lens Aquifer, Guam

Gingerich¹

2013

Scientific Investigations Report

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“…However, for some variables, in particular the van Genuchten parameters for the unsaturated zone of hard rocks, only few data and estimates can be found in the literature, e.g. Weiss (1987), Sauter (1992), Contractor and Jenson (2000) and Roulier et al (2006). Consequently, the sensitivity of these parameters was determined systematically to evaluate the degree of ambiguity of the model.…”

Section: Sensitivity Analysismentioning

confidence: 99%

Simulation of saturated and unsaturated flow in karst systems at catchment scale using a double continuum approach

Kordilla

Sauter

Reimann

et al. 2012

Hydrol. Earth Syst. Sci.

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Abstract. The objective of this work is the simulation of saturated and unsaturated flow in a karstified aquifer using a double continuum approach. The HydroGeoSphere code (Therrien et al., 2006) is employed to simulate spring discharge with the Richards equations and van Genuchten parameters to represent flow in the (1) fractured matrix and (2) conduit continuum coupled by a linear exchange term. Rapid vertical small-scale flow processes in the unsaturated conduit continuum are accounted for by applying recharge boundary conditions at the bottom of the saturated model domain. An extensive sensitivity analysis is performed on single parameters as well as parameter combinations. The transient hydraulic response of the karst spring is strongly controlled by the matrix porosity as well as the van Genuchten parameters of the unsaturated matrix, which determine the head dependent inter-continuum water transfer when the conduits are draining the matrix. Sensitivities of parameter combinations partially reveal a non-linear dependence over the parameter space. This can be observed for parameters not belonging to the same continuum as well as combinations, which involve the exchange parameter, showing that results of the double continuum model may depict a certain degree of ambiguity. The application of van Genuchten parameters for simulation of unsaturated flow in karst systems is critically discussed.

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“…In particular, in view of possible climate change, increased water demands and sea level rise, the development of freshwater lenses supplied with infiltrated rainwater on top of saline aquifers is a subject of intense research (e.g., Werner et al, 2013). A large number of contributions have focused on freshwater lenses in deltaic regions, as well as on islands, peninsulas, or sandbars located in tropical, arid, and temperate climatic zones (e.g., Comte et al, 2014;Contractor and Jenson, 2000;Chang et al, 2016;De Louw et al, 2011;Eeman et al, 2017;Holding and Allen, 2015;Houben and Post, 2016;Illangasekare et al, 2006;Jocson et al, 2002;Mahmoodzadeh et al, 2014;Sadurski et al, 1987;Sinclair et al, 2016;Stuyfzand, 2016;Sulzbacher et al, 2012).…”

Section: Introductionmentioning

confidence: 99%

Simulations of freshwater lens recharge and salt/freshwater interfaces using the HYDRUS and SWI2 packages for MODFLOW

Szymkiewicz

Gumuła-Kawęcka

Šimůnek

et al. 2018

Journal of Hydrology and Hydromechanics

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Abstract:The paper presents an evaluation of the combined use of the HYDRUS and SWI2 packages for MODFLOW as a potential tool for modeling recharge in coastal aquifers subject to saltwater intrusion. The HYDRUS package for MODFLOW solves numerically the one-dimensional form of the Richards equation describing water flow in variablysaturated media. The code computes groundwater recharge to or capillary rise from the groundwater table while considering weather, vegetation, and soil hydraulic property data. The SWI2 package represents in a simplified way variable-density flow associated with saltwater intrusion in coastal aquifers. Combining these two packages within the MODFLOW framework provides a more accurate description of vadose zone processes in subsurface systems with shallow aquifers, which strongly depend upon infiltration. The two packages were applied to a two-dimensional problem of recharge of a freshwater lens in a sandy peninsula, which is a typical geomorphologic form along the Baltic and the North Sea coasts, among other places. Results highlighted the sensitivity of calculated recharge rates to the temporal resolution of weather data. Using daily values of precipitation and potential evapotranspiration produced average recharge rates more than 20% larger than those obtained with weekly or monthly averaged weather data, leading to different trends in the evolution of freshwater-saltwater interfaces. Root water uptake significantly influenced both the recharge rate and the position of the freshwater-saltwater interface. The results were less sensitive to changes in soil hydraulic parameters, which in our study were found to affect average yearly recharge rates by up to 13%.

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Simulated effect of vadose infiltration on water levels in the Northern Guam Lens Aquifer

Cited by 34 publications

References 9 publications

The effects of withdrawals and drought on groundwater availability in the Northern Guam Lens Aquifer, Guam

The effects of withdrawals and drought on groundwater availability in the Northern Guam Lens Aquifer, Guam

Simulation of saturated and unsaturated flow in karst systems at catchment scale using a double continuum approach

Simulations of freshwater lens recharge and salt/freshwater interfaces using the HYDRUS and SWI2 packages for MODFLOW

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