A Micro Total Analysis System (μTAS) for the In Situ, Real-Time Tracking of Produced Water Discharges through Detection of PAHs and Other Aromatic Compounds

Abstract

Here, we present a novel micro Total Analysis System (μTAS) for the measurement of poly cyclic aromatic hydrocarbon (PAH) and other aromatic hydrocarbons (AHs) in water at ng/L levels in situ and in real time (IMiRO). The μTAS is based on in-line membrane extraction followed by detection of extracted aromatic substances with fluorescence. An offshore field demonstration of the method was conducted close to produced water (PW) discharged in the North Sea. PW was monitored with the IMiRO μTAS and compared to results from a simultaneously conducted independent tracer release experiment, where fluorescein was added to the PW as a tracer. The μTAS monitoring and fluorescein tracer experiment showed similar ability to track the dispersion of the PW plume in space, depth, and time. Moreover, the method detected the sum of phenanthrenes and the sum of heavier PAHs with limits of detection down to 6 ng/L, with a response time of 6 min. The novel μTAS system opens up for in situ real-time discharge monitoring of both permitted and accidental oil or PW releases from oil platforms as well as other sources. Such monitoring can also be used to test and verify dispersion models used for environmental risk assessment.

Keywords: PAH sensor; fluorescence; membrane extraction; produced water.

Figure 1

Outline of working principles of the IMiRO μTAS for AH-analysis at ng/L levels. (a) Schematic representation of the sensor principle. (b) Drawing of the assembled sensor. (c) Sensor submerged in water during offshore testing.

Figure 2

Emission spectra of FLU, PHE, and a mixture of PAH-16 at 1 μg/L of each PAH compound in 1-hexanol solution and 100 μg/L fluorescein in water. Recorded with excitation at 250 nm and corrected for 1-hexanol blank or water blank (fluorescein).

Figure 3

Calibration with PAH-16 standards (concentrations of individual compounds) at 19 and 3 °C in water with salinity 0 (deionized water) and salinity 35 (35 g NaCl/L). Blue dashed lines show linear fit and blue shaded areas show 95% confidence intervals for the linear regression of calibration data.

Figure 4

Calibration of the μTAS sensor at 303 nm with produced water obtained from the discharge point of DP M diluted in seawater. Calibrated with fraction of PW = 0.00246, 0.000997, 0.000486, and 0.000102.

Figure 5

Panels (a) and (b) show the fraction of PW recorded with the IMiRO μTAS (Blue line) compared with the fraction of PW calculated with the sensor recording the fluorescein (red line) in the water measured in the monitoring campaign March 25th (a) and March 27th (b), 2021. The upper plots of panels (a) and (b) show sensor depth (black line). (c) Trajectory of where the ship moved. Black square shows the position of the DP M discharge point, green arrows show the current directions measured March 25th, and blue arrows show the current measured March 27th. Currents were measured by a current meter at the field center at 10 m depth 200 m south of the discharge point. Red arrows indicate the direction of movement by the ship carrying the sensors, and the red circles show the position where the depth profiles were measured. The numbering of these corresponds with the descriptions of the sailing direction in the figure and numbering of the corresponding plot sections above the plots in panels (a) and (b). Note that the fluorescein experiment (red lines) had ended during events 10 and 11.

Figure 6

Profile of fraction of PW in the water column at two sites event 11: 140 m from DP M and event 7: 700 m from DP M determined with the IMiRO-uTAS sensor.

Figure 7

(a) FLU-equivalents measured at 303 nm and the concentration of FLU in the water column calculated from the FLU concentration in discharged PW and measured PW fraction, (b) PHE-equivalents measured at 360 nm and calculated PHE and C1–C3-methylated PHE concentrations, and (c) PAH-11-equivalents measured at 381 nm and calculated concentration of PAH-11 (ANTH, FLTH, PYR, B[a]A, CHRY, B[b]F, B[k]F, B[a]P, B[ghi]P, IND, and D[ah]A (4, 5, and 6-ring PAHs)).