Monitoring Saltwater Intrusion with Robowell at Provincetown Massachusetts

A Robowell prototype is being put in place to monitor salt-water intrusion in cooperation with the Provincetown Water Department, the USGS Technology Enterprise Office, and the USGS MA-RI Water Science Center Project for Analysis of Water Resources in the Lower Cape Cod Aquifer System.  This research project is designed to assess the suitability of the technology for monitoring saltwater 

Ground-water resources along the Atlantic coastal zone of the United States are vulnerable to saltwater intrusion, which is the movement of saline water into fresh-water aquifers. Saltwater intrusion is most commonly caused by ground-water pumping near the coast and the resulting high salinity can make ground water unfit for public consumption (Barlow, 2000).   Coastal communities in southeastern Massachusetts are especially vulnerable to saltwater intrusion because the maximum water demand occurs in the summer months when the population of coastal communities increase by thousands of visitors who come to enjoy the coast (Craig Wiegand, Provincetown Water Supply, personal communication).  The effect of increased water demand is exacerbated by the annual hydrologic cycle in Southeastern Massachusetts where over 75 percent of recharge occurs in the winter months and water use by plants, and people intercept recharge and draw water tables down to relatively low levels in the summer months (Granato and others, 1995).

The Robowell prototype is monitoring ground-water quality below the screen zone of a public supply well to help establish the position of the freshwater-saltwater interface (See Figures).  Under natural conditions, the seaward movement of freshwater prevents saltwater from encroaching coastal aquifers, and the interface between freshwater and saltwater is maintained near the coast or far below land surface. This interface is actually a diffuse zone in which freshwater and saltwater mix, and is referred to as the zone of dispersion--or transition zone (Barlow, 2000; See Figure). Ground-water pumping can reduce freshwater flow toward coastal discharge areas and cause saltwater to be drawn toward the freshwater zones of the aquifer. Saltwater intrusion decreases freshwater storage in the aquifers, and, in extreme cases, can result in the abandonment of supply wells. Saltwater intrusion occurs by many mechanisms, including lateral encroachment from coastal waters and vertical upconing near discharging wells (Barlow, 2000; See Figure).

The Robowell unit will provide water-quality readings that are comparable to results of samples from accepted manual sampling methods (See Figures) because Robowell pumps and purges the well in each measurement cycle.  It is hoped that the real-time monitoring will provide reliable information that can be used by the town, the USGS Project for Analysis of Water Resources in the Lower Cape Cod Aquifer System, and the USGS Ground-Water Resources Program Regional Assessment of the Freshwater-Saltwater Interactions along the Atlantic Coast to better understand the processes in play and to better manage the available water resources on Cape Cod.

Link to:  

• Real-Time Robowell Data for the Provincetown Water Department Wells
• 2001 Robowell Data for the Provincetown Water Department Wells   
• An Example of Real-Time Data Available to Water Managers by Telephone From the Voice Modem  
• Historical Ground-Water Levels Near the Monitoring Site
• Provincetown Water Department Web Page  
• The Town of Provincetown Web Page  
• The USGS Ground-Water Resources Program Regional Assessment of the Freshwater-Saltwater Interactions along the Atlantic Coast 
• Bibliography on the Occurrence and Intrusion of Saltwater in Aquifers along the Atlantic Coast of the United States
• USGS Project for Analysis of Water Resources in the Lower Cape Cod Aquifer System
• Other USGS Drinking-Water Programs
• Salt Net: A Scientific Resources Network for Saltwater Intrusion and Coastal Aquifers
• Provincetown Weather

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