EFFECTS OF SEA-LEVEL RISE ON GROUND-WATER FLOW IN A COASTAL AQUIFER SYSTEM, CAPE COD, MASSACHUSETTS

Concerns about the effects of sea-level rise in coastal aquifers include the mixing from above of saline waters that have either inundated low-lying coastal areas or encroached upon large riverine systems. It is believed that some coastal aquifers, such as the Cape Cod aquifer system in Massachusetts, are not susceptible to saltwater intrusion from a rising sea level because they are not dominated by large tidally-influenced riverine systems that extend inland to great distances and they are not areas of low topographic relief.

An analysis of a hypothetical aquifer consisting of a shallow, permeable, freshwater lens system was conducted to demonstrate that the nontidal portions of ground-water fed streams can affect nearby water levels and the depth to the underlying freshwater/saltwater (fw/sw) interface as sea-level rises. The effects of sea-level rise on the depth to the fw/sw interface were simulated by using SEAWAT, a density-dependent, three-dimensional, numerical ground-water-flow model. Simulations of sea-level rise from 1929 to 2050 were based on the average measured rate of rise of 2.65 millimeters per year at the Boston Harbor tide gage from 1921 to 2000.

Simulation of sea-level rise resulted in an increase in water levels relative to a fixed datum, yet a net decrease in water levels relative to the increased sea-level altitude. The net decrease in water levels was much greater near a gaining stream than farther from the stream. The difference in the change in water levels is attributed to the dampening effect of the stream on water-level changes as sea level rose.

In response to the decreased water-level altitudes relative to local sea level, the depth to the fw/sw interface decreased. This reduction in the thickness of the freshwater lens differed throughout the aquifer and was greatly affected by proximity to a ground-water fed stream and whether the stream was tidally influenced. Away from the stream, the thickness of the freshwater lens decreased by about 2% from 1929 to 2050, but by about 23 to 32% for the same period near the stream, depending on whether the stream was tidally influenced. The difference in the change in the fw/sw interface position is controlled by the difference in the net decline in water levels relative to local sea level.