Coastal Groundwater Chemical Data from the North and South Shores of Long Island, New York

Groundwater samples were collected with AMS drive-point samplers and peristaltic pumps (Charette and Allen, 2006) from within or near the intertidal zones of the three study areas, and at depths of 0.3 to 10.1 meters below the sediment surface. Contact details are listed below for this process step and are the same for all subsequent process steps.
Charette, M. A., and Allen, M. C., 2006, Precision ground water sampling in coastal aquifers using a direct‐push, Shielded‐Screen Well‐Point System. Groundwater Monitoring & Remediation, v. 26 no. 23, p. 87-93.

Salinity, pH, oxidation/ reduction potential (ORP), and dissolved oxygen (DO) were measured in the field with either a calibrated YSI 600XLM multi-parameter sonde with a flow through cell, or calibrated InSitu Troll 9500 multi-parameter sonde with flow through cell. ORP values were converted to Eh (mV) by adding 200 to the values. DO was calibrated daily, and the remaining parameters were calibrated biweekly, with additional periodic checks in calibration standards performed throughout the sampling campaign as needed. Calibration frequency, accuracy, and precision for the two sondes were the same, therefore it is of no concern when comparing their measurements.

Samples for dissolved organic carbon (DOC) were filtered through Pall cartridge filters (0.45 micron pore size) into combusted glass vials with teflon-lined septa, and stored at 4 degrees C until analysis. Samples were analyzed at University of California Stable Isotope Facility on an O.I. Analytical 1030 TOC analyzer by persulfate digestion and nondispersive infrared detection. Average relative standard deviation of an intermediate standard was typically less than or equal to 5 percent.

Samples for nitrate and ammonium were filtered through Pall cartridge filters (0.45 micron pore size) and frozen until analysis. Samples were analyzed at the Marine Biological Laboratory (MBL), Woods Hole, MA on a Lachat FIA-6000 autoanalyzer using standard colorimetric methods (Strickland and Parsons, 1972). Nitrate and nitrite were not quantified separately and their sum is referred to here as nitrate.
Strickland, J. D., and Parsons, T. R., 1972, A practical handbook of seawater analysis.

Samples for N2 and Ar analysis were collected in duplicate into 12 mL glass exetainers with screw-cap butyl rubber septa. Exetainers were filled from the bottom, overflowed with >5 void volumes, capped immediately without headspace, and stored underwater at 4 degrees C until analysis. Samples were analyzed at the Ecosystems Center at MBL by membrane inlet mass spectrometry (MIMS), for 28N2, 29N2, 30N2, and 40Ar, to a precision of <0.5 percent (Kana and others, 1994; Kana, 2004). Samples were typically analyzed within a month or two of collection. Total N2 was obtained by summing the masses 28, 29 and 30. Total Ar was calculated based on assumption that the measured isotope of mass 40 comprised 99.6% of total Ar. Four N2 and Ar data points were eliminated because low gas concentrations indicated that the samples had degassed during collection. In this data report, no further judgements were made regarding whether a more subtle level of degassing had occurred in the remaining samples. The Szymczycha and others, 2017 analysis involved further judgements of minimum gas concentrations based on estimated recharge temperatures, and thus represent a subset of the data contained in this data report.
Kana, T. M., Darkangelo, C., Hunt, M. D., Oldham, J. B., Bennett, G. E., and Cornwell, J. C., 1994, Membrane inlet mass spectrometer for rapid high-precision determination of N2, O2, and Ar in environmental water samples: Analytical Chemistry, v. 66 no. 23, p. 4166-4170.
Kana, T. M., 2004, Comment on “Comparison of isotope pairing and N 2: Ar methods for measuring sediment denitrification” by B.D. Eyre, S. Rysgaard, T. Dalsgaard, and P. Bondo Christensen. 2002. Estuaries 25: 1077–1087. Estuaries and Coasts, v. 27 no. 1, p. 173-176.

Groundwater age, or time since recharge, was estimated based on mass spectrometric measurement of helium isotopes and tritium analysis of water samples. Sample analysis and age estimation were conducted by the Lamont-Doherty Earth Observatory Noble Gas Laboratory of Columbia University, Palisades, NY, by the 3He ingrowth method. Precisions for measurement of tritium, delta 3He, and the 4He and Ne concentrations are ±1 to 2%, ±0.15 to 0.2%, and ±0.2 to 0.3%, respectively, and reported age uncertainty for the samples analyzed ranged from 0.1 yr to 1.2 yr.

One author had an incorrect middle initial, so this was fixed in the originator list and the suggested citation. The correct authorship is S.M. Brosnahan, not S.B. Brosnahan.

Added keywords section with USGS persistent identifier as theme keyword.