Origins of the MVP Grant: High Salinity at Center Hill

As part of our normal ‘stewardship’ of Center Hill Preserve, members of the Southeastern Massachusetts Pine Barrens Alliance – which is headquartered at the Preserve – had been casually monitoring what we thought of as one pond, Center Hill Pond, for several years. We began by locating its deep spot – a depth of about 4 meters – on the northern half of the pond, and using an old YSI submersible monitor, gathering 30-45 days of data (salinity, total dissolved solids, conductivity) at a time.

The first time we downloaded that accumulated data, we noticed a dramatic rise in salinity that corresponded almost directly with a major storm that had resulted in the overtopping of the dunes: dunes that, in part, protected Center Hill Pond. Take note that in the past Center Hill Pond was a freshwater pond, located a considerable distance from Cape Cod Bay. But over the years the beach has been eroded and today high tide regularly brings seawater within 100 yards or so of the pond’s edge.

[image: data from first year]

In 2017 a succession of winter storms, each separated by just a few days, breached the dunes and flooded the ponds and, eventually deposited enough sand to break Center Hill Pond into two distinct halves. At that time seawater came within a short distance of two homes located nearby. The water receded, but the Pond is now permanently separated into northern and southern portions.

[image: side by side map - pre 2017 ; post 2017]

The salinity levels in the northern half of the pond were actually the impetus for research into the effects of salinity on ecosystems, and to a state-funded grant (MVP) that seeks to understand the area’s vulnerability to saltwater intrusion across the world. That grant is, at this moment, in its second of two years of funding, and as part of that effort we have re-initiated our testing of the pond, and added two other bodies of water. Those three bodies of water, we were surprised to discover, are hydrogeologically connected. The salinity data indicates that the original pond (North Center), though separated from its ‘southern half’ by thousands of cubic yards of sand, is still connected to the southern portion.

Though the connection is not easily seen at Center Hill Pond, Black Pond, half a mile away, is part of the same stream system. This was made evident by the storm surge, which came ashore at the northern end, pushed through the woods and flooded Black Pond. Then, what was fascinating to watch (via the data) was the variability in the time it took for these three ponds to return to normal levels of salinity. We can only theorize at this point but it appears that Center Hill North is quick to return to normal levels of salinity after a storm event that overtops the dunes, likely due to a direct connection to the ocean. The southern half – Center Hill South – maintains a higher level of salinity for longer periods of time after a storm event, suggesting that the sand which now separates it from its northern half, creates a barrier of sorts, so that the water does not easily flow back to the ocean. Black Pond salinity levels also return to normal quickly, perhaps because the excess water – once the storm is over – can flow downstream without impediment, to Center Hill South.

The real lesson here may be a simple one: in regards to sources of freshwater, expect change. Sea level rise, coastal erosion, more frequent and intense storms, and development all appear to be increasing the pace of change along the coast.


Frank Mand
Alannah Kelliher
Sharl Heller