Monitoring Saltwater Intrusion with Robowell at Provincetown Massachusetts: Geophysical Well Logs and Preliminary Hydrogeologic and Water-Quality Interpretation

Increased land development and population growth have created concerns regarding both the quantity and the quality of ground water that is used for drinking water and that which discharges to surface water bodies of Lower Cape Cod. These concerns include the effects of increased ground-water withdrawals on the position of the interface between freshwater and saltwater and on the amount of freshwater discharge to ponds, streams, and coastal areas. The USGS investigation to analyze the water resources of the lower Cape Aquifer system is designed to to improve the understanding of ground- water flow in Lower Cape Cod Aquifer System and determine the effects of future ground-water pumping and waste-water disposal on (1) water-table altitudes, (2) position and movement of the freshwater-saltwater interface, (3) freshwater discharge to selected surface-water bodies, wetlands and coastal embayments, and (4) the areas that contribute recharge to natural and artificial receptors. These objectives will be accomplished by collecting additional hydrogeologic data and interpreting and analyzing these data within the framework of updated ground-water-flow models that have the capability of simulating both the freshwater and saltwater flow systems under different pumping and recharge conditions.

The geology, hydrology and water quality of the public supply well field is one aspect of interest to the larger Lower Cape Study to this end, monitoring wells at the site were logged and the information was used to formulate a preliminary hydrogeologic and water-quality interpretation as follows:

Well log results from the Knowles Crossing Well Field showing a gamma log and an induction log from a monitoring well in relation to the public supply well intake depth (written communication J.P. Masterson).

Conceptual schematic showing the preliminary hydrogeologic and water-quality interpretation of the Knowles Crossing Well Field the screened zones of the monitoring wells are shown in black, the screened zones of the public supply well intake are in red, and ground-water salinity is represented by the dots (written communication J.P. Masterson).

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