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Dissolved oxygen sensor in an automated hyporheic sampling system reveals biogeochemical dynamics [1]
['Matthew H. Kaufman', 'Pacific Northwest National Laboratory', 'Richland', 'Washington', 'United States Of America', 'Run. Ghosh', 'Opti', 'Llc', 'Okemos', 'Michigan']
Date: 2022-07
Field monitoring system
The field setting for this study was the shore and near-shore environment of the Columbia River at the 300 Area of the Hanford site near Richland, Washington (Fig 1). The Hanford Reach of the Columbia River is contained within the Pasco Basin in southeast Washington State. The river is surrounded by an unconfined aquifer consisting of fluvial Ringold Formation and Hanford Formation flood sediments [29]. The hydraulic conductivity of the riverbed in this area is highly variable, ranging from 2.8x10-5 cm/s to 4.3x10-2 cm/s [30].
Subsurface biogeochemistry was monitored from April 7th to April 23rd 2018 using a single cluster of aquifer sampling tubes installed in the “dense array” automated 4-dimensional sampling system, a unique hyporheic zone water sampling system that draws from a large array of riverbed monitoring points (Fig 1). While short, the study period was chosen specifically to coincide with relatively high river stage (this reach of the Columbia River is typically highest in the spring, due to snowmelt) in order to limit the influence of groundwater discharge on hyporheic zone biogeochemistry. A central measurement system located on-shore pulled water samples from an array of tubes buried in a 3-dimensional grid array in the hyporheic zone. The central measurement system automatically cycled through a pre-programmed subset of the aquifer sampling tubes, to gather physical and chemical data from the grid of spatial locations at this field site. This system used ex-situ sensors to measure fluid electrical conductivity (measured as specific conductivity: SpC), pH, nitrate, oxidation-reduction potential (ORP), and DO, and in-situ sensors to measure temperature.
The aquifer sampling tubes were installed in September of 2016 in a grid pattern in both permanently and variably wetted hyporheic zone sediments along the shoreline. The tubes were installed in clusters and were spaced over an approximately 10 m wide (perpendicular to the shoreline) by 90 m long (parallel to the shore) grid that was 10 clusters long and 3 clusters wide. Each cluster consisted of 2 or 3 tubes installed to approximate depths of 0.5m, 1m, and 2m below the sediment-water interface, allowing for four-dimensional sampling of the hyporheic zone. Due to substrate heterogeneities some locations could not be penetrated with the hand installation tools used and as a result not all clusters contained a 2m monitoring point. The end of each tube was screened to form a mini-piezometer, with slots cut into the last 7 to 15 cm of each tube to allow water to be sampled. These slots were covered with Teflon mesh to reduce sediment clogging. The distal ends of the tubes were closed with stainless steel thermistor (Fig 2). After the full installation was complete, each tube was developed. Development occurred in September of 2017 and proceeded according to the following protocol:
Set up vacuum pump and peristaltic pump. Fill bucket with river water. Put empty bucket nearby for outflow water. Verify correct direction for pumping water into the aquifer tube using the peristaltic before starting (“forward” or “reverse”). Turn on vacuum pump. Attach aquifer tube extension to pump to pull water out of tube and start timer. Record start time for pump and amount of time until water appears. Allow flask to fill to 500mL and record amount of time from water appearance until 500 mL reached. Disconnect vaccum pump from aquifer tube. Disconnect vaccum pump from beaker and empty water. Attach tube to peristaltic pump with a filter between the aquifer tube and the pump connection. Put inflow tube of peristaltic pump into the bucket of river water and pump into the aquifer tube for one minute. Disconnect peristaltic from aquifer tube and reconnect vaccum pump to aquifer tube. Pump water out of tube for two minutes, empty flask, reconnect to aquifer tube, and record start time of vaccum and when water appears (likely immediately). Allow flask to fill to 500mL and record time. Disconnect vacuum pump. Assess if the rate of water output from the tube or the clarity changed. Repeat steps 5 to 11 until there is no improvement
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TIFF original image Download: Fig 2. (a) shows the tip of an aquifer sampling tube, with the screened section and thermistor cap. (b) shows the central on-shore measurement system with banks of solenoid valves, a series of sensors, and a datalogging system. (c) shows the hand-installation of an aquifer tube. (d) shows detail of the central on-shore measurement system. The green oval in box 5 of (d) is the OptiO2 dissolved oxygen sensor flow cell. In (d), box 1 houses the syringe pumps, boxes 2 and 4 house the solenoids and solenoid controllers, boxes 3 and 5 house the various sensors mounted in their flow-through manifolds, box 6 houses the Campell CR3000, and box 7 houses interface units that allow the CR3000 to communicate with the sensors in box 3. The equipment enclosures in (b) and (d) are in the same relative positions.
https://doi.org/10.1371/journal.pwat.0000014.g002
Typically, by the time no improvement was achieved, it took <2 minutes to withdraw 500ml of water from each tube. Water quality parameters were also monitored for stabilization. The system was operated regularly in various configurations between the development date and the start of this study.
Each of the sampling tubes was connected to its own solenoid valve, and all valves were controlled by a CR3000 datalogger through four SDM-CD16AC 16-Channel AC/DC Relay Controllers (Campbell Scientific, Logan UT). For any given sampling scheme, solenoid valves corresponding to selected sample locations were opened one at a time and two coupled Kloehn syringe pumps (IMI Precision Engineering, Las Vegas NV) alternately drew sample water from the tube and then discharged it to the monitoring manifold at a rate of 5 mL s-1. The tubing manifold volume was kept as small as possible, but was not measured. Vacuum applied to the sample lines was also not measured, however the pump was approximately 4m above the surface of the river, which requires a minimum vacuum of 392 millibar to raise the water. No bubbles were observed in the sampling lines or the syringe pumps while under vacuum. In this study, each location was sampled for 41 minutes before switching to another aquifer tube. This purges the prior sample and provides time for the current sample to achieve stable readings representative of the sampled location. The 41-minute timing was selected after brief initial observation of the system in operation. With regard to the sampled sediment volume, assuming a porosity of 0.3–0.6, 41 minutes of pumping equates to a sampled sphere with a radius of 17-21cm.
The central measurement platform relied on the transport of samples from the point of origin in the river bed to the central measurement system, while keeping the sample composition unchanged for measurement at the central location. The plastic tubing used for sample transport is made of LLDPE (linear low density polyethylene), with an outside diameter of 9.5mm (3/8 inch) and an inside diameter of 6.35mm (1/4 inch). The potential for oxygen outside the tube to diffuse into the sample within the transfer tubing was a primary design consideration, and thus was measured in the laboratory. A source of nitrogen sparged water was connected to the luminescent dissolved oxygen sensor of an MS5 Sonde (Hydrolab, Loveland CO, USA) via 4.6, 6, 30.5, and 122 m (15, 20, 100, and 400 foot) lengths of the same black LLDPE tubing type being used in the field. In all cases, at a flow rate of 1 mL s-1, the measured DO was zero at the sensor distal from the source of nitrogen sparged water. Tubing lengths in the field were 6–45.7 m with tubing volumes ranging from 190ml to 1447ml respectively, and the flow rate in this field study was five times faster than the laboratory test. Therefore, the sample residence time in the field was at least twenty times less than in the laboratory experiment with the 122 m length.
Our focus was to provide an example use of this system in a ‘hypothesis-generating’ mode. There is an inherent tradeoff between temporal resolution and number of locations monitored due to transit time of pumped water. For hypothesis generation we obtained high time resolution sampling from multiple depths at a single x-y location in the dense array (Fig 1, red triangle). An alternative design would be to sample all locations infrequently to assay spatial variation across longer timescales. By focusing on one vertical transect of 3 aquifer sampling tubes and using 41 minutes of pumping time per solenoid, each depth was sampled every two hours.
The time-resolved DO measurements were acquired and logged with a novel optical DO sensor developed by Opti O 2 , LLC (www.optio2.com). The DO probe, housed in a flow cell, was incorporated into the measurement platform of the dense array system as shown in Fig 3a. DO was recorded every 30 seconds as the solenoid valves cycled between the three selected aquifer sampling tubes (Fig 3b). This high temporal resolution was necessary to cleanly identify when the system had reached a new quasi steady state after a solenoid/aquifer sampling tube switch.
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TIFF original image Download: Fig 3. (a) shows the Opti O 2 DO sensor housed in a flow cell incorporated into the dense array manifold. (b) shows a 36-hour subset of the raw data recorded by the sensor, showing the variation in DO signal as the dense array switched between the three sampling depths.
https://doi.org/10.1371/journal.pwat.0000014.g003
The Opti O 2 probe was a self-contained optical spectrometer which output a temperature and pressure corrected DO signal. Molecular oxygen was detected by monitoring the phosphorescence emission from molybdenum chloride optical indicators [32]. Ultraviolet photons pumped the optical indicators to a spin triplet excited state, which is specifically quenched by the ground state of molecular oxygen, 3O 2 . The optical indicators were immobilized in a polymer matrix, resulting in a sensing film which was placed in direct contact with the liquid to be analyzed [33]. The sensing film had minimal cross sensitivity to organic and inorganic species, and did not suffer from photobleaching [34]. These properties enabled DO measurements with high sampling rates for an extended period from the unconditioned field water samples, with highly variable temperatures and complex chemical constituents without the need for maintenance or field calibration.
In addition to the DO instrument, the pumps delivered sample water to a series of instruments for real time measurements of water chemistry (Fig 2). Measurements were made for nitrate using a S::CAN spectro::lyser UV-Vis 35mm (S::CAN Messtechnik GmbH, Vienna, Austria), SpC with Rosemount 400 (Emerson, Irvine, CA), pH and oxidation-reduction potential (ORP) with Rosemount 3900 (Emerson, Irvine, CA). All data logging and instrument control for these instruments was provided by the CR3000. The instruments were cleaned and maintained biweekly. The S::CAN instruments used factory calibration, while the Rosemount sensors were calibrated according to the manufacturer’s instructions. The Opti O 2 device was factory calibrated pre and post field deployment.
Approximately 17 meters downstream of the downstream edge of the dense array (42 m downstream of the sampled aquifer tube cluster), a transect of five 3.81 cm inner diameter stainless steel piezometers were installed. One of the piezometers (RG3) was left open to the river with the screen at the river level. The other four were capped and had a pressure sensor installed at the screen to allow for discrete measurements of hyporheic zone hydraulic head at the different depths. Of the four capped piezometers, two were installed in the permanently inundated channel with one shallow (P1S) screened at 104.24masl and the other deeper (P1D) screened at 101.67masl. The other two were located on the ephemerally inundated bank with one shallow (P2S) screened at 105.68masl and the other deeper (P2D) screened at 103.88masl. The piezometers were each outfitted with an Aqua TROLL sonde (In-Situ, Inc. Ft. Collins CO) for continuous measurement of pressure, temperature, and SpC. The sonde measuring the river was vented and the other four sondes were not vented but were corrected for barometric pressure at the data logger. The sondes were connected to a CR1000 data logger (Campbell Scientific, Logan UT) and data was downloaded twice a month during the field season. A 10-year (2008–2018) hourly spatiotemporal dataset from a network of groundwater wells at the 300 Area was used for background SpC and hydraulic head gradient information [35].
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