IODP3-NSF Expedition 501: Offshore Freshened Groundwater in the New England Continental Shelf

IODP³-NSF Expedition 501 drilled three sites along a 45 km-long, NNW-SSE transect on the New England continental shelf offshore Nantucket and Martha’s Vineyard to characterise an extensive offshore freshened groundwater (OFG) system. Site M0112 (41 m water depth; 338 m below seafloor [mbsf]) is nearest to shore, Site M0111 (42 m water depth; 393 mbsf) is the middle site, and Site M0113 (54 m water depth; 325 mbsf) is farthest offshore. Each site was investigated with drilling, coring, wireline logging (including formation conductivity and nuclear magnetic resonance), and groundwater pumping. Long-term observatories were also lowered in holes at  Sites M0112 and M0113 to collect formation temperature and pressure data. The expedition was operationally successful, with 71% core recovery; 10,500 litres of water sampled through pump tests for post-cruise analyses; and the use of temporary PVC casing for wireline logging. Offshore (May-July 2025) and onshore (Bremen, Marum Onshore Operations, January- February 2026) analyses document an unconsolidated sedimentary package consisting predominantly of alternating layers of sand and mud; however, some coarser interbeds were observed at Site M0112. Beds were generally thicker in the shallow section and thinner at depth. Additional shore-based analyses will provide more detailed lithostratigraphic characterisation, sedimentary age, and stratigraphic ties between sites. Interstitial water and pumped groundwater from Sites M0111 and M0112 document a transition from seawater salinity to less than 10% of seawater salinity within the upper 125 mbsf, and salinity remains low until it increases at depths greater than 300 mbsf. At Site M0113 two freshened zones with salinity that is 50-60% of seawater salinity exist above 300 mbsf. Preliminary interpretations suggest that aquifers and confining units have similar salinity in the freshened zones. These freshening patterns are consistent with interpretations of marine-based electromagnetic surveys previously collected along the drilling transect. Compared to salinity, interstitial water alkalinity demonstrates more nuanced patterns, with similar values in sands layers but variable values in mud layers. In addition to shipboard analyses, numerous sediment, water, gas, and microbiological samples were collected for post-expedition research to understand freshwater emplacement mechanisms, timing of emplacement, and volumes of this extensive OFG system. This integrated research program not only elucidates the freshened system beneath the southern New England shelf, but also has implications for other OFGs worldwide.