WHY DOES SANDY HOOK HAVE THAT SINKING FEELING?

Sandy Hook is a spit extending ~8.5 km into Raritan Bay, NJ. Tide gauge data from the spit indicate a 4.0 ± 0.5 mm/yr rise in sea level between 1900 and 2012, significantly higher than the 3.0 ± 0.3 mm/yr at the Battery, NY and the global mean sea level rise of 1.6 ± 0.3 mm/yr between 1900 and 2010. Higher regional rates are due to Glacial Isostatic Adjustment of 1.3 ± 0.4 mm/yr.

The higher rate at Sandy Hook compared to the Battery and other bedrock locations suggest the difference is due to local subsidence. Atlantic City, NJ tide gauges show a relative sea level rise (3.9 ± 0.4 mm/yr) similar to Sandy Hook; the excess subsidence at Atlantic City is a result of sediment compaction due to groundwater withdrawal (>10 million gallons per day in Atlantic County). However, rates at Sandy Hook are much lower.

To determine why Sandy Hook is sinking faster than bedrock sites, three coreholes were drilled on a N-S transect at Sandy Hook in 2014 sampling Quaternary and Cretaceous strata. At the North Maintenance Yard (NMY), adjacent the tide gauge, thick (84+ m) Quaternary strata consist of uppermost Pleistocene thin (3+ m) basal gravels (?18 ka), thick (29 m) estuarine organic-rich sandy clayey silts to silty clays dated as 13.2-12.1 ka, 13 m of lower Holocene mid-estuarine muddy sands (the “Foraminiferal Clay”; 10.3-9.3 ka), 20 m of middle Holocene estuarine sands (7-5.8 ka), thick (17 m) upper Holocene shoreface deposits, and 5 m of modern barrier sands (past 300 years). The uppermost Pleistocene muds thin to 16 m at the Salt Shed 2.5 km south and coarsen out before the South Maintenance Yard 4 km south. We are conducting sedimentological analyses (grain size, organic carbon, and porosity) of the cores, providing datasets needed for backstripping to remove the effects of compaction and loading. The porosity data show a strong correlation to gamma logs suggesting the logs could be a high-resolution proxy. At the NMY, porosity sharply increases from ~40% in Holocene sands to ~50% in the Foraminiferal Clay and 55% in the uppermost Pleistocene muds. Comparison of tide gauges shows an anthropogenic signal at Sandy Hook decreasing when Ft. Hancock was decommissioned, implicating groundwater removal as a source of excess subsidence. However, preliminary decompaction of the uppermost Pleistocene muds suggests ~ 0.5 mm/yr of subsidence is due to natural compaction.