Real-Time Monitoring of Nitrate in Fresh Water with the ...

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Real-Time Monitoring of Nitrate in Fresh Water with the MBARI ISUS

The MBARI ISUS (In-Situ Ultraviolet

Spectrometer) is a real-time, chemical free system designed to overcome the traditional challenges associated with nitrate analyses in the sometimes highly turbid, high CDOM waters of rivers and lakes. Satlantic’s ISUS provides easy, accurate and continuous nitrate concentration measurements essential to the study of physical, chemical and biological processes in natural waters (Figure1). This document summarizes the field validation efforts for the ISUS sensors using the Water Quality Monitor (WQM), demonstrating the effectiveness of the ISUS sensor to measure nitrates in the challenging freshwater environment.

Figure 1. ISUS deployment in the Cornwallis River.

ISUS was developed by the Monterey Bay Aquarium Research Institute (MBARI) in California. The technology is based on the absorption characteristics of inorganic compounds in the UV portion of the spectrum. These compounds absorb light in the UV with a unique signature, which can be resolved very accurately using a 256 channel spectrometer inside the instrument. Using a unique algorithm developed by MBARI, the system can measure nitrate in the most challenging of conditions,

even in waters with very high concentrations of dissolved and particulate materials.

Satlantic’s ISUS sensor has been rigorously tested in various freshwater environments ranging from highly turbid rivers to town drinking supplies to agricultural drainage tiles. The algorithm developed by MBARI is effective at separating the nitrate signal from that of the relatively strong interfering species, such has suspended particulate (turbidity) and dissolved organic material (CDOM).

Figure 2. Measured spectra showing baseline.

The ISUS sensors are calibrated in the

laboratory over a wide range of nitrate concentrations and operating temperatures.

Figure 3. Corrected spectra showing computed nitrate.

This calibration information, along with the spectral molar absorptivities of nitrate, are stored

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onboard the instrument and used for real-time nitrate calculations. The instrument measures in situ absorption spectra every 0.9nm, from 200 to 400nm. The measured spectra are broken down into spectra resulting from nitrate absorption and a baseline spectra (includes everything else) using the classical least squares analysis (Johnson and Coletti, 2002). Accuracy is typically within 2uM (0.3 mg/l) or 10% of the nitrate reading, whichever is higher. Since the analysis is made statistically over a large number of spectral channels, the system can measure nitrate over a very wide range (0-2000uM or 28 mg/l) and with high levels of dissolved and particulate materials without interference in the accuracy of the results. This is all transparent to the user as the actual computation only takes the instruments internal computer a few milliseconds to calculate the concentration directly. For highly unusual conditions, the analysis algorithm can be adjusted by the user or by post processing software using the original data set.

Figure 4 – ISUS being towed as part of Water Quality Monitor package.

Extensive validation of the system has been done by Dr. Ken Johnson and Luke Coletti at MBARI (Johnson and Coletti, 2002) for open ocean and coastal applications. Satlantic has extended the range of previous validation studies to include the freshwater environment. The results include data collected from waters bodies of every type and condition. All freshwater

ISUS data are accompanied by traditional autoanalyzer data as well as coincident measurements of CDOM, turbidity, fluorescence, pH, oxygen, salinity and temperature.

This extensive data set demonstrates the solid performance of the system over a wide range of conditions. The results not only show the robustness of the nitrate measurement, but also the ease of use and ability for rapid real-time investigation of relatively large areas.

Figure 5 – Pan Creek nitrate transect in the Sacramento River.

In the Sacramento River of Northern

California, we used the WQM to map nitrate, CDOM, turbidity, oxygen and pH concentrations through Pan Creek and Brown’s Island (not shown) during the same morning (Figures 5&6).

Figure 6 – Pan Creek CDOM transect.

The two transects represent more than 1500

measurements over 2 km and were completed in less than 30 minutes each. The nitrate measurements were collected every 2 seconds

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continuously and viewed onboard a small boat in real-time.

The nitrate values ranged from 15 to 36 uM (0.2 to 0.5 mg/l) while CDOM varied between 35 to 67 QSU and turbidity measured 5 to 13 NTU. The water was so discolored at times that the instrument package was barely visible, even though it was just under the surface of the water (Figure 4).

The WQM was also used to map part of Lac La Biche in Northern Alberta. Here the town is concerned with steadily increasing concentrations of algae in the popular lake and wanted to know if a local golf course could be identified as a possible point-source of nutrients. A quick tour of the area with the ISUS demonstrated a well-mixed, homogeneous water body with no significant input from the golf course (Figure 7).

Figure 7 – Lac La Biche nitrate transect.

Extensive validation experiments with the

ISUS were also completed using water from several rivers, lakes, municipal and community water systems and private wells in Nova Scotia. Discrete nitrate samples were collected from each site and analyzed using the traditional autoanalyzer technique at Dalhousie University. The nitrate values obtained from the ISUS were in very close agreement (standard deviation 4.9 uM or 0.07 mg/l) with the validation results over

a very large range of measurements (Figures 8&9).

Figure 8 – ISUS vs. autoanalyzer nitrate for samples taken from Nova Scotia rivers, lakes and private wells. Nitrate values ranged from 0 to 120 uM (1.7 mg/l) with CDOM and turbidity readings as high as 125 QSD and 25 NTU respectively.

Figure 9 – Sampling in the Cornwallis River with the WQM.

To test the accuracy at a much higher range, we also took the package to a grain farm (heavy use of fertilizers), two dairy farms (significant nutrient loading from animal excrement) and an Agriculture Canada research station where experimentation with liquid manure fertilizers was being carried out. The relationship between the ISUS and autoanalyzer outputs remained

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robust despite the unnaturally elevated nutrient concentrations (Figure 10).

Figure 10 - ISUS vs. autoanalyzer nitrate for samples taken from various farms and an experimental site at Agriculture Candada. Although the AgCanada samples from the experimental drainage tile were very turbid due to the presence of soil sediments (Figure 11), the ISUS nitrate measurements (590 uM or 8.3 mg/l) were still very close to those measured using the autoanalyzer.

Figure 11 – Agriculture Canada drainage tile.

The ISUS performed remarkably well at all sites with a near 1:1 relationship with independent autoanalyzer results. In addition, lab tests with samples of up to 14,000 uM (196 mg/l), have demonstrated the flexibility of the UV technique used by the ISUS. Figure 12 – Water quality monitor

Extensive data sets have been collected to validate the accuracy and sensitivity of the ISUS sensor over a wide range of conditions and environments. If you are interested in seeing what the ISUS can do for your program, please contact our team today to discuss your application and learn more about how our systems can help you solve your most challenging monitoring problems.