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Sharp Interface Models of Salt Water Intrusion and Wellhead Delineation on Nantucket Island, Massachusetts
Author(s) -
Person Mark,
Taylor James Z.,
Dingman S. Lawrence
Publication year - 1998
Publication title -
groundwater
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.84
H-Index - 94
eISSN - 1745-6584
pISSN - 0017-467X
DOI - 10.1111/j.1745-6584.1998.tb02190.x
Subject(s) - wellhead , saltwater intrusion , intrusion , interface (matter) , salt water , salt (chemistry) , geology , fishery , environmental science , hydrology (agriculture) , geography , oceanography , groundwater , petroleum engineering , geotechnical engineering , chemistry , geochemistry , meteorology , biology , aquifer , capillary number , capillary action
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.