Hydrogeology of Coastal Aquifers of Southeast Massachusetts: Assessment of the impacts of the combined effects of sea-level rise and terrestrial climate change

Groundwater model, salinity, and stratigraphic update

The major updates to the groundwater flow model are: (1) the grid for the groundwater flow model is finalized, and (2) a density-dependent model was run to represent the salinity distribution in the aquifer. In the zone most likely to experience saltwater intrusion (8 to -32 meters above sea level), the grid cells have a finer vertical spacing to detect precise changes in saltwater intrusion. In Figure 1, the interface between saltwater and freshwater can be seen near the coast, with more dense ocean water (red) flowing under the less dense freshwater (blue).

Figure 1: Cross-section view of the groundwater flow model with color representing salinity (red = ocean water, blue = freshwater).

With well logs, we are revising the current stratigraphic understanding of the region. When wells are drilled, information about the materials of the subsurface are collected by drillers, engineers, and geologists. We compiled wells logs in the region and are in the process of mapping a coastal confining unit, which was likely deposited from glacial lakes around 15,000 years ago (Figure 2). Because this confining unit restricts groundwater flow, it could have a large impact on saltwater intrusion.

Core locations with coastal confining unit (colored), and no confining unit (grey).

Figure 2: Map of well logs with confining unit present (colored symbol) or no confining unit (grey symbol).

Salinity data has been collected in the region from wells, lakes, and streams. We are analyzing this data to understand the salinity distribution in both the groundwater and surface water. Salinity of the groundwater aquifer is shown in Figure 3, with higher salinity in red and lower salinity in green. The units of the map are in Total Dissolved Solids (or TDS).

Shaded map, showing areas with higher salinity (red) and lower salinity (green).

Figure 3: Salinity distribution of groundwater aquifer. Red is higher salinity and green is lower salinity.

By Alexander Kirshen
March 9, 2023