Sharp Interface Models of Salt Water Intrusion and Wellhead Delineation on Nantucket Island, Massachusetts

Abstract

Inferred increases in 1981 to 1997 pumping records suggest that by the year 2020, the majority of Nantucket Island municipal wellfields will be pumping near or at capacity. A sharp‐interface model is used in this study to assess the impact of projected increases in pumping from the Wyers Valley and proposed State Forest municipal wellfields on Nantucket Island, Massachusetts. The model was first calibrated as part of a sensitivity study using a nine‐year record of monthly water level data from a monitoring well network and salinity data from a deep borehole completed near the island's center. Best‐fit values of hydraulic conductivity and specific storage obtained in the sensitivity analysis are in good agreement with those measured by single and multiple well aquifer tests across the island. The calibrated model was used to assess hydrologic conditions across the island between 1997 and 2020 due to projected increases in municipal wellfield pumping. Transient simulation results suggest that salt water will invade the well screen of the deep (32 m) Wyers Valley production well by the year 2014. Further, the width of the fresh water/salt water mixing zone under the municipal wellfields will triple by 2020 due to increasing seasonal fluctuations in pumping required to accommodate summer tourism. Using published estimates of 2020 municipal wellfield pumping, wellhead delineation zones were also calculated using a variety of assumptions regarding the island's fresh water lens geometry (uniform, variable thickness with no upconing; variable thickness with upconing). Results indicate that the calculated delineation zones are most sensitive to island geometry and, to a lesser extent, to the fresh water lens thickness.

Comparison of Different Indicators for Groundwater Contamination by Seawater Intrusion on the Lebanese Coast

“…It is often associated with over pumping in coastal regions, resulting in overdraft conditions and creating an inland gradient of seawater. The problem of seawater intrusion has been reported by many authors (Bakker, 2000;Hurswitz et al, 2000;Person et al, 1998;Langevin et al, 1998;Karahanoglu, 1997;Bonacci and Bonacci, 1997;Padilla and Sanjulian, 1997;Emekli et al, 1996;HuyaKorn et al, 1996) particularly along the southern Lebanese coast by El Moujabber et al (2004), El Moujabber et al (2003 and El Moujabber and Bou Samra (2002).…”

Investigation of submarine groundwater discharge

“…Many numerical simulations and related studies attempted to evaluate mechanisms governing this process (e.g. Segol and Pinder, 1976;Freeze and Cherry, 1979;Huyakorn et al, 1987;Reilly and Goodman, 1987;Person et al, 1998). At first glance, the processes of SGD and seawater intrusion may seem to be exactly opposite.…”

Integrating geophysical, hydrochemical, and hydrologic data to understand the freshwater resources on Nantucket Island, Massachusetts

“…The water table declines near the coastline where it is even with modern sea level. Numerical modeling, based on projected pumping increases, indicates that saltwater upconing could occur within municipal water wells before 2014 [Person et al, 1998]; these models however, did not account for the presence of the potentially thick confining glacio-lacustrine deposits from former Glacial Lake Nantucket nor the offshore freshwater. Nevertheless, these studies have led to increased interest in monitoring the distribution of freshwater and saltwater across the island Basement rocks beneath Nantucket Island consist of Triassic basalts overlain by 460 m of coastal plain sediments of Cretaceous, Tertiary, and Quaternary age [Folger et al, 1978].…”