Radon-222 and Water Column Data Related to Submarine Groundwater Discharge Along the Western Margin of Indian River Lagoon, Florida—September 2017 and November 2017

The positional accuracy of the track lines was determined by the boat-mounted Garmin echoMAP 50s GPS receiver. The positional accuracy for the Garmin GPS ranges from better than 3 m to 10 m as outlined by the manufacture when using WAAS (Wide Area Augmentation System) coverage. All six RAD7 detector units, three per mapping system, used in the surveys received the recommended annual calibration by Durridge that ensured the overall calibration accuracy of the RAD7s were within 2%, based on inter-comparison results with their master instruments which are estimated to be within +/- 5%. A relative humidity of 10% or below was kept to maintain the high sensitivity of the instrument during radon mapping; this was sustained by placing a desiccant, Drierite drying column, in the air path of the system. Per the manufacturer’s description of factors influencing the response time of the RAD7 with RAD Aqua system to changes in radon-222 concentration in water, a radon-222 data set using a 21-minute offset is provided. The 21-minute offset for this system is based on influencing factors described in the manufacture’s manual, radioactive theory, and USGS laboratory experiments (data available upon request). The laboratory experiments involved manipulation of the system’s air volume and flow rate of water into the RAD Aqua to assess the impact to response time. Similar radon-222 mapping studies acknowledge a 10 to 20-minute delay in response time of the three RAD7s and one RAD Aqua system but have chosen to use a 10 minute time shift for their radon-222 data due to the high water flow rates in their system setups (Hosono and others, 2012; Stieglitz and others, 2010). While these studies’ response times were closer to 10-minutes, laboratory experiments suggested that 21 minutes better characterized the response time for the system used in this IRL study. Radon-222 data with a 3.5-minute offset (midpoint of the 7-minute count cycle) are provided as well. The 3.5-minute offset data have not been adjusted for a system lag but provide the most accurate data with respect to time of collection. The 7-minute measurement cycle was chosen to balance the need for higher spatial resolution along the survey transects with the 21-minute response time of the system. Mapping system USGS 1 consisted of RAD7 units 2265, 2528, and 2531 for all surveys. Mapping system USGS 2 consisted of RAD7 units 1186, 1533, and 1534 for all surveys. An outline of the RAD7 units and HOBO unit used in each mapping system for all survey dates can be found in the ‘IRL_Surveys_Equipment.pdf’ document. The barge maintained a speed of 1-1.5 knots during mapping in order to achieve the desired spatial resolution of radon-222 data points. However, during the September 2017 trip the barge traveled at an average speed of 2.5-3 knots decreasing the ideal spatial resolution of radon-222 data. The YSI Pro Plus multi-parameter water quality meter (YSI, Inc.) was calibrated for barometer, dissolved oxygen, specific conductivity, pH, and oxidation-reduction potential (ORP) on the first field day. The YSI was checked each subsequent day with pH 7 buffer solution and 50 mS/cm conductivity solution to determine if recalibration was necessary. Linear regression of the YSI recorded salinity for both mapping systems (USGS1 and USGS 2) was used to QA/QC the data. In each YSI file, a column includes a flag indicating which data passed the QA/QC process. If the YSI probe was out of the water and exposed to the air, the values used for calculations were corrected by averaging salinity readings from YSI data before and after the problem time period to acquire a salinity value. ORP values for both September 2016 and November 2017 surveys were erroneous due to an incorrect calibration of the ORP standard. The ORP values for these surveys are reported as ND for no data reported. HOBO H8 temperature probes were inter-compared twice a year to check the device. Linear regression of the HOBO recorded temperature for both mapping systems (USGS1 and USGS 2) was used to QA/QC the data. Significant outliers that broke with the pattern of the regression and had unrealistic values based on the rest of the survey were examined as potentially erroneous. If removing these significant outliers resulted in a large improvement of the r-squared value, the outliers was considered erroneous. In each HOBO file, a column includes a flag indicating which data passed the QA/QC process. Some unexpectedly high and low values in ‘c:*2’ temperature data were identified. If one HOBO malfunctioned or reported erroneous ‘c:*2’ temperature values due to a compromised connection between the HOBO and the probe, the measurements from the HOBO that recorded consistent and reliable data were used in place of the faulty HOBO. This occurred on 9/26/17, 9/27/2017, and 11/3/2017. Reported radon-222 data excludes the data at the beginning of each day where appropriate temperature data was unavailable because the HOBOs were not yet exposed to surface water at that time. Due to the depth limitations of the Garmin echosounder, the shallowest transects of the Eau Gallie North site are lacking recorded depths and have been reported as ‘NR’ for not recorded. The transducer offsets are provided in the GPS csv files for each survey trip.