Groundwater seepage rate:
Rate of groundwater seepage across seabed was measured from the seepage meters either with the ultrasonic sensor at 1 second frequency (June and October RSM deployments) or manually within an attached collection bag (all other deployments). The ultrasonic and manual flow measurement approaches were adapted from Paulsen and others, (2001), and Russoniello and others (2013), respectively. For measurements made on 06/08/2015 and 06/09/2015 (see GroundwaterSeepage_Manual_GuineaCreek.csv), the SSM in standard (non-recirculating) mode was used to measure groundwater seepage rate alone. In all other cases, the RSM and the SSM were deployed as an adjacent pair, separated by 1 meter or less in a shoreline parallel orientation and sampled across 4 to 24 hour timeseries events (hereafter “paired timeseries”) for groundwater seepage rate, and dissolved constituents and other parameters. In either case, the procedures for deploying the seepage cylinders into the sediment, and for measuring groundwater seepage rate was the same. First, the seepage meters were deployed into the sediment in the nearshore subtidal zone at the Guinea Creek study site. Deployment location was measured with a Garmin GPSMAP 76CX to an accuracy of 2 meters or less. Water depth was observed at each site location for each deployment and was always between 0.3 and 1.5 meters. Unless otherwise stated (see Notes Attribute in GroundwaterSeepage_Manual_GuineaCreek.csv), following installation into the sediment, 45 minutes or longer was allowed to elapse before measuring groundwater seepage rate. Positive values for groundwater seepage indicate upward discharge of groundwater from the aquifer to the estuary (hereafter “discharge”), and negative values indicate downward recharge of estuarine surface water into the aquifer (hereafter “recharge”). Groundwater seepage is reported in this dataset in units of volumetric seepage (volume per time, e.g. liters per hour), and as specific seepage (length per time, e.g. centimeters per day).
For manual measurements, thin-walled plastic collection bags (40 liter capacity) were pre-filled with ~2 L of bay water, to allow measurement of both groundwater discharge and recharge, and to overcome bag resistance to flow (Rosenberry and others, 2008). The pre-filled bags were weighed to the nearest 0.05 kg with a digital scale before and after deployment, and initial and final mass measurements were converted to volumes based on calculated density (Fofonoff and Millard, 1983) given additional measured values of temperature and salinity. Volumetric seepage rate was calculated as the change in volume divided by the change in time. Specific seepage rate was calculated as the volumetric seepage rate divided by the area of bay floor occupied by the seepage meter. Deployment durations of the collection bags typically ranged from 0.5 to 2.5 hours, and in one case 15 hours, when a single measurement took place overnight (6/14/2015 paired timeseries). For manual measurements of seepage rate in this dataset we report the initial DateTime and the final DateTime. Therefore, reported seepage rate represents the average seepage rate across that time interval.
Ultrasonic flow in the RSM was measured at one second frequency with the FLEXIM F7407 sensor. Prior to deployment, the ultrasonic transducers were externally mounted onto the titanium flow tube following manufacturer recommended spacing and orientation, and the transducer/ tube assembly was encased in a water-tight rigid clear polyvinylchloride enclosure. A lead weight was secured to the exterior of the enclosure to keep the flow tube at a level and stable position resting on the bay floor. Water velocity through the flow tube (length per time, e.g. centimeters per second) was measured at one second frequency and internally logged by the sensor which was located on a nearby above-water sampling platform. The sensor was powered by two 12-volt deep cycle batteries linked in parallel. The sensor display allowed real-time monitoring of groundwater flows during measurement. Flow measurements made during times when field personnel had to inspect the flow tube for potential obstructions to flow (e.g. fish or other debris), and during discrete water sample collection (typically 5 minutes or less), are erroneous and therefore are not reported. Volumetric groundwater seepage rate was calculated as the product of the measured velocity and the inner cross-sectional area of the flow tube. Specific seepage rate was calculated by dividing the volumetric seepage rate by the area of bay floor occupied by the seepage meter. Laboratory and field measurements of instrument performance of the ultrasonic sensor are described in Brooks and others (2021). Results indicated both excellent precision (1 sigma = 0.07 centimeters per day, n = 1,200), and excellent agreement to manually measured flows (r2 = 1.00, p < 0.001, df = 19).
This process took place over a range of time in 2015. The Process Date below represents the most recent date.
Fofonoff, N.P., and Millard, R.C., 1983, Algorithms for computation of fundamental properties of seawater: UNESCO Technical Papers is Marine Science. No. 44. 53 pp.
Paulsen, R.J., Smith, C.F., O'Rourke, D., and Wong, T.F., 2001, Development and evaluation of an ultrasonic ground water seepage meter: Groundwater, 39, p. 904-911, https://doi.org/10.1111/j.1745-6584.2001.tb02478.x
Rosenberry, D.O., LaBaugh, J.W., and Hunt, R.J., 2008, Use of monitoring wells, portable piezometers, and seepage meters to quantify flow between surface water and ground water, p. 39-70. In D.O. Rosenberry and J.W. LaBaugh [Eds.], Field Techniques for estimating water fluxes between surface water and ground water: U.S. Geological Survey Techniques and Methods 4-D2.
Russoniello, C.J., Fernandez, C., Bratton, J.F., Banaszak, J.F., Krantz, D.E., Andres, A.S., Konikow, L.F., and Michael, H.A., 2013, Geologic effects on groundwater salinity and discharge into an estuary: Journal of Hydrology, 498 p. 1-12, https://doi.org/10.1016/j.jhydrol.2013.05.049