“Shell Day” was a single-day regional water monitoring event coordinating simultaneous coastal carbonate chemistry observations by 59 community science programs and 7 research institutions in the northeastern United States, in which 410 total alkalinity (TA) samples from 86 stations were collected. Samples were collected at low, mid, and high tide by community science volunteers and brought to partnering research laboratories for sample processing. Minimum requirements for participation in Shell Day were the capacity to measure water temperature and salinity – some organizations used thermometers and refractometers, and others used multiparameter datasondes or handheld units. An analysis and interpretation of the temperature, salinity, and total alkalinity data can be found at Rheuban et al 2020 Environ. Res. Lett. in press https://doi.org/10.1088/1748-9326/abcb39. Included in this dataset are measurements of water temperature, salinity, total alkalinity, pH, dissolved oxygen concentration, dissolved oxygen saturation, total depth, secchi disk depth, chlorophyll, turbidity, as well as air temperature and barometric pressure, and qualitative assessments of wind, weather, and cloud cover. This dataset is associated with the following publication: Rheuban, J.E., P. Gassett, D.C. Mccorkle, C. Hunt, M. Liebman, C. Bastidas, K. O’Brien-Clayton, A. Pimenta, E. Silva, P. Vlahos, R. Woosley , J. Ries, C. Liberti, J. Grear, J. Salisbury, D. Brady, K. Guay, M. LaVigne, A. Strong, E. Stancioff, and E. Turner. Synoptic Assessment of Coastal Total Alkalinity through Community Science. Environmental Research Letters. IOP Publishing LIMITED, Bristol, UK, 16: 024009, (2021).