Nearshore groundwater seepage and geochemical data measured in 2015 at Guinea Creek, Rehoboth Bay, Delaware

Recirculating oxygen-regulated and standard seepage meters:
Two different designs of recirculating seepage meters were used to measure SGD and fluxes of dissolved constituents from benthic sediments under contrasting biogeochemical conditions at the Guinea Creek field site in June, August, and October of 2015. Details pertaining to seepage meter design and operation are described in this Process Description, and in below Process Descriptions. The below detail, and additional detail is available in Brooks and others (2021).
The oxygen- and light-regulated seepage meter (RSM) consists of a stainless-steel cylindrical base with a detachable transparent 0.5 centimeter thick polycarbonate lid. Four “stops” fabricated from sections of stainless-steel angle stock are bolted to the inside of the seepage cylinder and ensure deployment at a consistent depth into the sediment each time. A 3.5 millimeter wall thickness, 9.5 millimeter inner diameter titanium pipe is attached to the side of the seepage cylinder to allow the quantification of groundwater seepage rate via an ultrasonic or manual approach. The material and geometry of the flow tube were selected for optimal precision in flow measurements by the ultrasonic sensor (FLEXIM F7407; FLEXIM Corp., Edgewood, NY, USA) based on manufacturer recommendations. Water is continuously recirculated through a recirculating flow system with a 10 liter per minute capacity 12-volt diaphragm pump, sufficient to provide thorough mixing, but not resuspend or disturb the sediment. Results of a tracer injection test on mix time indicated the water contained in the seepage meter headspace and circulation system is 95 percent mixed within 1.3 minutes of injection of the tracer. The flow system includes a length of gas permeable silicone tubing (9.5 mm inner diameter, 1.6 mm wall thickness) to allow the dissolved gas concentrations within the seepage meter, including oxygen, to achieve equilibrium with the surrounding water body. The length of the gas permeable tubing was 91 meters for June and August, and 60 meters for October. The silicone tubing was arranged in a loose coil and placed in a protective cradle fabricated from plastic lattice fencing, foam floats, and a concrete anchor to keep the tubing situated below the water surface but above the bay floor. The flow system includes an inline multi-parameter sonde in a flow through cell, and a syringe port for collection of discrete water samples for analysis of a suite of dissolved constituents, both of which are contained on a nearby above-water sampling platform. The internal volume of the RSM headspace including the recirculating flow system once deployed into the sediment was 77.9 liters (June and August deployments) and 75.8 liters (October deployments). The area of bay floor occupied by the seepage cylinder is 0.456 square meters.
The standard seepage meter (SSM) was constructed to test the hypothesis that isolating the enclosed water column from oxygen renewal and photosynthetically active radiation would result in artifacts in redox conditions and chemical flux. The SSM is constructed from the top portion of a 208 liter steel drum (Lee, 1977) and is operated in standard mode (for measurement of seepage rate alone) or in recirculating mode (for seepage rate and chemical flux). In recirculating mode, it employs a circulation system, discrete sampling port, and an inline multiparameter sonde as described above for the RSM. However, unlike the RSM, the design does not include gas-permeable tubing for oxygen regulation or a transparent lid for transmission of photosynthetically active radiation. Groundwater seepage rate is measured via a manual approach through a 9.5 millimeter inner diameter port located on the lid of the seepage meter. The SSM covers a surface area of 0.26 square meters, has a combined headspace and circulation system volume of 21.1 liters, and was operated with a 4 liter per minute capacity 12-volt diaphragm pump. A mixing test of the SSM yielded inconclusive results, however, the estimated turnover time of water within the SSM and flow system, based on volume and circulation rate, is 5.3 minutes, less than that of the RSM (7.8 minutes). Therefore, we anticipate the actual mix time of the SSM to be comparable, or perhaps slightly faster than the RSM.
The contact person listed for this process description is listed below and is the same for all subsequent process descriptions. This process took place over a range of time in 2015. The Process Date below represents the most recent date.
References cited:
Brooks, T.W., Kroeger, K.D., Michael, H.M., and York, J.K., 2021, Oxygen-controlled recirculating seepage meter reveals extent of nitrogen transformation in discharging coastal groundwater at the aquifer-estuary interface: Limnology and Oceanography, https://doi.org/10.1002/lno.11858.

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.
References cited:
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.

Sampling of seepage meters for dissolved constituents during paired timeseries events:
After the seepage cylinders were installed into the sediment, the seepage meters were flushed with bay water in an open loop configuration to ensure that chemical conditions (e.g. salinity, dissolved oxygen) closely approximated conditions in nearby estuarine bottom water prior to beginning paired timeseries measurement of dissolved constituents and other parameters. In the case of the RSM, the lid was left detached during this period, while the SSM relied on continuous flushing with the circulation pump. At the end of the equilibration period, the return tube was attached to the intake on the lid of each seepage meter, forming a closed loop.
Discrete water samples were collected through the syringe port located on the recirculating flow systems once the seepage meters were in closed loop mode (t = 0), and at regular intervals (0.5 to 1.5 hours) throughout the timeseries events. Paired timeseries typically ranged from 4 to 12 hours in duration (see DateTime_UTC Attribute). Samples were collected into acid-cleaned, sample-rinsed, 60 milliliter All-Plastic syringes and subsampled in the field into individual vials for analysis of a suite of dissolved constituents. Subsampling into separate vials, method of preservation, and analysis of the individual constituents is described in separate process steps below. The date and time in coordinated universal time (UTC) corresponding the midpoint of the syringe sample set was recorded for t=0 and all subsequent intervals (see Discrete_GeochemicalData_GuineaCreek.csv). Sampling duration at each timepoint was approximately 4 minutes. The total volume of water removed for each set of syringe samples, including that used for rinsing, was noted during sampling and recorded (typically 220 milliliters).
Notes on 6/14/2015 paired timeseries:
For the 6/14/2015 paired timeseries, the RSM and SSM were left to continue measuring groundwater seepage and field water quality parameters overnight, and a final set of syringe samples were collected the following morning (RSM1-15-June and SSM1-15-June, see Discrete_GeochemicalData_GuineaCreek.csv). Upon arrival to the field site on 6/15/2015, it was discovered that the water recirculation in the SSM had stopped at some point the previous night due to a failed pump. It was later determined by evaluating continuous water quality sensor data in the SSM (see below Process Description) that this flow interruption caused by the failed pump occurred on 06/15/2015 at approximately 2:35 UTC. The pump was replaced, and circulation was restored to the SSM, and real-time readings of field water quality parameters in the SSM (see below Process Description) were allowed to stabilize, indicating complete mixing, prior to collecting the final sample set (SSM1-15-June).
This process took place over a range of time in 2015. The Process Date below represents the most recent date.

Field water quality parameters in recirculating seepage meters and in estuarine surface water:
Water quality parameters (temperature, specific conductance, salinity, dissolved oxygen, pH, and oxidation-reduction potential) were measured in the recirculating seepage meters with calibrated YSI 600XLM multi-parameter sondes (YSI Inc., Yellow Springs, OH, USA) in water-tight YSI 696 Flow Cells located on a nearby above-water sampling platform. During the paired timeseries, parameters were measured and internally logged at 1-minute frequency. Deployment duration ranged from 4 to 24 hours. Conditions inside the flow through cells were monitored visually throughout the timeseries, and measurements were monitored in real time with a YSI 650 MDS Display. There were a few cases in which gas bubbles collected on the oxygen sensor in the RSM, most notably during periods of high oxygen saturation (200 percent or greater at times) in the RSM. The bubbles were removed by tapping the flow cell, and oxygen data corresponding to these time periods were later eliminated from the dataset (see attributes DO_percentSat and DO_mgperL in WaterQualityParameters_GuineaCreek.csv). Also note that the full suite of parameters measured from 2:33 to 13:03 Coordinated Universal Time (UTC) on 6/15/2015 in the SSM (deployment ID: June_SSM_Site_01) are not reported due to a failed pump and an interruption in water recirculation during that time (see above Process Description). The suite of parameters corresponding to the midpoint of discrete syringe sample sets (see above Process Description) for the SSM and RSM time series were extracted and are reported in Discrete_GeochemicalData_GuineaCreek.csv. Data were measured and reported in the above manner for all recirculating seepage meter deployments except for 10/20/2015 October_SSM_Site_01 and 10/22/2015 October_SSM_Site_02. For those deployments, salinity alone was measured on discrete bottle samples using a calibrated YSI 30 (see https://www.ysi.com/pro30 for instrument specifications and precision) and is reported in Discrete_GeochemicalData_GuineaCreek.csv. One-minute logged data for all other deployments are reported in WaterQualityParameters_GuineaCreek.csv.
A calibrated YSI 600XLM-V2 sonde was deployed at a height of 10 centimeters above the bay floor adjacent to the seepage meters to measure and internally log water depth, temperature, specific conductance, salinity, optical dissolved oxygen, pH, and ORP in the estuary at 3 minute frequency (June) or 2 minute frequency (all other deployments). The sensor was deployed onto a weighted crate with the sonde and sensors oriented horizontally, parallel with the bay floor. We report only the depth relative to the bay floor, which was calculated as the raw measured water depth minus the vertical distance between the bay floor and the pressure (depth) sensor. Deployment duration ranged from 1 to 6 days.
For all sondes, dissolved oxygen was calibrated the day of deployment, and all other parameters were calibrated the day prior to deployment following manufacturer specifications. In the case of the 600XLM sondes used in the recirculating seepage meters, the electrodes on the polarographic dissolved oxygen sensors were reconditioned according to manufacturer specifications the day of, or the day prior to deployment. Flow rate provided by the circulation pumps were within the acceptable range for polarographic dissolved oxygen measurement in the flow cells. In addition to the internal calibrations, the three sondes were simultaneously deployed in the field, both immediately before deployment, and upon retrieval, in a 5-gallon bucket of site bay water using a method adapted from Wagner and others (2006) to test for fouling and drift. A submersible 10 liter per minute capacity diaphragm pump was placed into the bucket to continuously circulate the water, and parameters were logged for 10 minutes or longer. The June and August post comparison results indicated ORP measured in the estuary (June_Estuary_Site_01 and August_Estuary_Site_01) were not within the acceptable range of accuracy and precision, likely due to prolonged exposure to reduced material. ORP data from those deployments are therefore not reported in this dataset. All other results were within the acceptable range of accuracy and precision. Instrument specifications including precision of the above listed parameters measured with the 600XLM and 600XLM-V2 sondes can be found at: https://www.ysi.com/File%20Library/Documents/Manuals/069300-YSI-6-Series-Manual-RevJ.pdf.
This process took place over a range of time in 2015. The Process Date below represents the most recent date.
Reference cited:
Wagner, R.J., Boulger, R.W., Jr., Oblinger, C.J., and Smith, B.A., 2006, Guidelines and standard procedures for continuous water-quality monitors—Station operation, record computation, and data reporting: U.S. Geological Survey Techniques and Methods 1–D3, 51 p.

Groundwater sampling:
Groundwater samples were collected at multiple depths (5 to 83 centimeters below sediment surface) at the Guinea Creek study site. Samples were collected as individual sample points, or at multiple depths in vertical profiles adjacent to the seepage meter deployments or at locations shoreward. All sample points are contained in a relatively small area in the nearshore, shallow (0.3 to 1.4 meter water depth), subtidal zone (see Bounding_Coordinates in Spatial_Domain section). Location was measured with a handheld Garmin GPSMAP 76Cx using the WGS84 datum to an accuracy of 3 meters or less in most cases. In the limited number of cases when location was not measured by GPS, it was measured as distance shoreward from either site 1 or site 2 with a meter tape to an accuracy of 0.5 meters (see Latitude, Longitude, and Notes Attributes in Discrete_GeochemicalData_GuineaCreek.csv). In many cases porewater sample collections were concurrent with paired seepage meter timeseries.
Samples were collected with 0.6 cm outer diameter, 4 cm long screened interval stainless steel push-point sampling devices (https://www.mheproducts.com/). Samples were either collected into acid-cleaned 60 mL syringes through Tygon tubing or with a peristaltic pump through a short length of Cole-Parmer C-Flex silicone pump tubing. Sampling depth is reported as the depth of the midpoint of the well screen below the sediment surface. Field water quality parameters (temperature, specific conductance, salinity, dissolved oxygen, pH, oxidation/reduction potential) were measured with a calibrated YSI ProPlus multiparameter sonde in a flow-through cell. Field parameters were recorded once dissolved oxygen readings stabilized. Details of instrument specifications including precision of individual parameters can be found at: https://www.ysi.com/proplus. Porewater was then subsampled into individual vials in the field for geochemical analysis of a suite of dissolved constituents. The specifics of sample collection, preservation and analysis of the individual analytes are described below in subsequent process steps.
This process took place over a range of time in 2015. The Process Date below represents the most recent date.

Nutrients:
Samples for analysis of nutrients nitrate, ammonium, and phosphate were filtered through sample-rinsed 0.45 micron pore size polyethersulfone filters into acid-cleaned polyethylene vials, kept on ice in the field, and then stored frozen until analysis.
Nitrate, ammonium, and phosphate were measured on a Seal AA3 autoanalyzer at the University of Delaware, in Lewes, DE using standard colorimetric techniques according to APHA (2005), method 4500-NO3-F, method 4500-NH3-G, and method 4500-P-F. Nitrate (NO3-) and nitrite (NO2-) were not quantified separately and their sum is referred to as nitrate (NO3-) in this report. Analytical limits of detection are 0.1 micromoles per liter (uM) for nitrate and ammonium and 0.05 uM for phosphate. for Coefficient of variation is 5% or less.
This process took place over a range of time in 2015. The Process Date below represents the most recent date.
Reference cited:
APHA, 2005, Standard methods for the examination of water and waste water, 21st edn., American Public Health Association, Washington, DC, https://doi.org/10.2105/SMWW.2882.001.

Concentrations and stable isotopic ratios of dissolved organic carbon:
Samples for dissolved organic carbon (DOC) concentration, and delta 13C of DOC (d13C-DOC) were filtered through sample-rinsed 0.45 micron pore size polyethersulfone filters into combusted borosilicate glass vials with acid-cleaned Teflon-lined septa caps. Vials were pretreated with 20 percent hydrochloric acid (12 microliters per milliliter of sample) to decrease the pH of the sample to 2 or less, and samples were stored at 4 degrees Celsius until analysis. Samples were analyzed for DOC on an O.I. Analytical Aurora 1030C auto-analyzer by high temperature catalytic oxidation and nondispersive infrared detection (HTCO-NDIR). Concentrations are reported relative to potassium hydrogen phthalate (KHP) calibration standard.
Isotopic ratio of 13C:12C was measured using a Thermo-Finnigan DELTAplus XL Isotope Ratio Mass Spectrometer interfaced to the Aurora 1030C following the method of Lalonde and others (2014). Stable carbon isotope ratios are reported in standard delta (d) notation relative to Vienna Pee Dee Belemnite (VPDB). Quality assurance included replicate analysis of natural reference materials, field samples and calibration standards. Analytical uncertainties in concentration and d13C were determined based on replicate measurement of field samples. Coefficient of variation in DOC concentration and d13C were 5 percent or less, and 2 percent or less, respectively. Analytical limit of detection for DOC concentration and d13C was determined as three times the standard deviation of the analytical blank, or LOD = 10 micromoles per liter (uM).
This process took place over a range of time in 2015. The Process Date below represents the most recent date.
Reference cited:
Lalonde, K., Middlestead, P. and Gelinas, Y., 2014, Automation of 13C/12C ratio measurement for freshwater and seawater DOC using high temperature combustion: Limnology and Oceanography: Methods, 12(12), pp. 816-829, https://doi.org/10.4319/lom.2014.12.816.

Concentrations and stable isotopic ratios of dissolved inorganic carbon:
Samples for DIC and delta 13C of DIC (d13C-DIC) were collected unfiltered and submitted to the University of California Stable Isotope Facility in Davis, CA (UC Davis SIF) for analysis. Samples were collected into pre-combusted borosilicate glass vials with Teflon-lined butyl rubber septa screw caps. Vials were filled from the bottom, overflowed with >3 void volumes, capped immediately without headspace, and stored at 4 degrees C until analysis.
Samples were analyzed on a GasBench II system interfaced to a Delta V Plus isotope ratio mass spectrometer at the UC Davis SIF (https://stableisotopefacility.ucdavis.edu/dictracegas.html). Stable isotopic ratios are reported in standard delta notation relative to Vienna Pee Dee Belemnite (VPDB). Quality assurance included replicate analysis of natural reference materials and field samples. Reported long-term standard deviation in d13C and Limit of Quantification in DIC as CO2 are 150 nanomoles, and 0.1 per mil. Note that d13C-DIC data measured in the regulated seepage meter are qualitative, since the gas-permeable tubing of the regulated seepage meter is expected to exchange carbon dioxide, a component of DIC, with ambient estuarine water. This process took place over a range of time from 2012 to 2016. The process date below represents the most recent date.
This process took place over a range of time in 2015. The Process Date below represents the most recent date.

Trace elements in porewater and surface water:
Samples for trace elements analysis were filtered through a sample-rinsed 0.45 micron pore size polyethersulfone filters into acid cleaned polyethylene vials, spiked with 8 normal (N) Optima nitric acid to a pH of less than 2, and stored at room temperature until analysis.
Samples were diluted 20-fold with 5% Optima nitric acid and analyzed on a Thermo Fisher iCAP Qc at the Woods Hole Oceanographic Institution (WHOI) in Woods Hole, MA for manganese (Mn), iron (Fe), copper (Cu), strontium (Sr), barium (Ba), and uranium (U). Count rates were normalized to an internal indium (In) standard to account for drift and matrix interference of the solution. Water sample density was calculated based on field salinity and a typical lab temperature of 25 degrees Celsius following the method of Fofonoff and Millard (1983). Molar concentration was calculated as the product of lab-measured molal concentration and density. Quality assurance included replicate analysis of natural reference materials and field samples. Detection limits were determined as 3 times the standard deviation of the analytical blank. LOD = 0.01 uM for Mn, Fe, and Sr; and 0.01 nM for Cu, Ba, and U. Coefficient of variation = 1, 12, 11, 5, 0.3, and 7 percent for Mn, Fe, Cu, Sr, Ba, and U, respectively. Dissolved Fe data from the SSM is not reported due to risk of potential contamination from the seepage meter.
This process took place over a range of time in 2015. The Process Date below represents the most recent date.
Reference cited:
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.

Total alkalinity in porewater and surface water:
Samples for total alkalinity were collected into acid-cleaned 60 mL HDPE plastic syringes with a two-way stopcock unfiltered without headspace and stored at 4 degrees Celsius until analysis. Samples were analyzed on a Hiranuma Sangyo Aquacounter COM-300A Automatic Titrator following standard protocols within 7 days of collection. Quality assurance included replicate analysis of natural reference materials and field samples. Results were within the acceptable range of precision and accuracy. Coefficient of variation = 2 percent.
This process took place over a range of time in 2015. The Process Date below represents the most recent date.

Chloride and sulfate:
Samples for chloride (Cl-) and sulfate (SO42-) were filtered through sample-rinsed 0.45 micron pore size polyethersulfone filters into acid-cleaned polyethylene vials, stored on ice in the field, and then stored frozen until analysis. Samples were analyzed on a Metrohm 850 Ion Chromatograph following standard protocols. Quality assurance included replicate analysis of field samples and calibration standards. Coefficient of variation = 1 percent for chloride and sulfate.
This process took place over a range of time in 2015. The Process Date below represents the most recent date.