Data on gas composition in headspace of borehole NB04. Contains 11 columns of data taken from the headspace of borehole NB04 (at the clay-dominated site) using a GasClam 2.0 (Salamander Group, UK) that continuously measure temperature, atmospheric pressure, borehole pressure, and the percentage of carbon dioxide, oxygen, and methane in the borehole. All data collection for the GOAM project was performed on the same sites located in the Kien Svay district of northern Kandal Province, Cambodia, southeast of Phnom Penh. These sites are referred to as "clay-dominated" and "sand-dominated" in reference to the known lithology and are known for high arsenic concentrations (Richards et al., 2017). Each site contained four 18 m boreholes installed in January 2019 using manual drilling described in Richards et al (2015), spaced evenly about ~1.5 m apart. The boreholes that were located at the clay-dominated site are referred to NB01, NB02, NB03 and NB04. The boreholes that were located at the sand-dominated site are referred to as NB05, NB06, NB07 and NB08. The data taken are from the headspace of borehole NB04 (at the clay-dominated site) This file contains part of the data produced for NERC grant NE/P01304X/1 - called Genetic and molecular basis of organic-arsenic-microbe interactions in arsenic prone aquifers ("GOAM") project. References: Richards, L. A., Lapworth, D. J., Magnone, D., Gooddy, D. C., Chambers, L., Williams, P. J., van Dongen, B. E., Polya, D. A. Dissolved Organic Matter Tracers Reveal Contrasting Characteristics across High Arsenic Aquifers in Cambodia: A Fluorescence Spectroscopy Study. Geoscience Frontiers 2019, 10 (5), 1653-1667. Richards, L. A., Magnone, D., Sovann, C., Kong, C., Uhlemann, S., Kuras, O., van Dongen, B. E., Ballentine, C. J., Polya, D. A. High Resolution Profile of Inorganic Aqueous Geochemistry and Key Redox Zones in an Arsenic Bearing Aquifer in Cambodia. Science of The Total Environment 2017, 590-591, 540-553. Richards, L. A., Magnone, D., van Dongen, B. E., Ballentine, C. J., Polya, D. A. Use of Lithium Tracers to Quantify Drilling Fluid Contamination for Groundwater Monitoring in Southeast Asia. Applied Geochemistry 2015, 63, 190-202.