Using s::can probes to get real-time water quality dataMartin DavisSOP – Adv. Field Methods in Hydrology

Why monitor in the field?• Allows capture of data as notable events occur

• No need to transport samples back to the laboratory

• Fewer concerns with contamination risk

• Gives ability to more easily characterize changes and trends

• Provides mobility for assessors without fixed locations

• Allows for continuous monitoring of remote locations

• Gives the ability to share data in real-time with collaborators

• Provides rapid response ability for water contamination events

Who is using this technology?• USGS: Tracking Nitrate in the Mississippi River• Vienna Waterworks: Drinking water QC• Arizona Dairy Industry: Reducing plant waste• Indian Government: Monitoring contamination

in the Ganges river during Kumbh Mela festival• Virginia Tech: BSE department using it to track

micronutrient levels in local streams

A brief primer on spectroscopy 

Protocols: Part 1 – Site Selection

SITE CHARACTERISTICS• Potential for water-quality measurements at the

site to be representative of the monitored system.• Cross-section variation and vertical stratification.• Unique constraints of channel configuration.• Range of stream stage (from low flow to flood)

that can be expected over the measurement interval

• Presence of existing monitoring or surveillance.• Water velocity.• Presence of turbulence.• Conditions that may enhance the rate of fouling,

such as excessive fine sediments or algaeThis is particularly important for spectroscopy.

• Range of values for water-quality field parameters.

• Need for protection from debris damage.• Need for protection from vandalism.

Select an appropriate site: Factors to consider for site selection include:MONITOR INSTALLATION• Type of state or local permits required • Safety hazards relevant to monitor construction • Optimal type and design of installation.• Consideration of installation challenges/costs.LOGISTICS (MAINTENANCE REQUIREMENTS)• Accessibility of site.• Safe and adequate maintenance space.• Presence of conditions that increase the frequency

of servicing intervals needed to meet data-quality.• For stream sites, proximity to an adequate location

for making cross-section measurements.• Accessibility and safety of the site during extreme

events (for example, floods or high winds).• Availability of electrical power or telephone service.• Need for real-time reporting.

Protocols: Part 2 – Instrument Choice• S::can instruments

have many probes.• All interface with a

central control unit.• Choice depends on

things like cost & desired monitoring.

• Spectrometer is the most robust, but the most difficult to configure & costly.

Protocols: Part 3 – Installation Types

• Pipes are convenient when there are existing monitoring stations in place at a survey site.

• Cages are easily transportable and inexpensive. They can provide some instrument protection, though not as much as fixed installations.

Protocols: Part 3 – Installation Types

• Fixed-site pumped deployment is ideal if there is already a long-term installation available. It provides the maximum access control, and sensor protection. Costly.

• Buoys are mobile like cages and are more robust, but can be pushed off course if anchors break away.

Protocols: Part 4 – Port Access

Mounting Options and Requirements• Water must pass evenly through the

instrument gap to be accurately measured by spectroscopy.

• Horizontal orientation decreases bubbles and sediment in the probe.

• Secure installation prevents sensors from knocking into objects.

• Vertical orientation is easier to put into place but cables must besecured against damage.

For this type of monitoring to work, sensors must be clean & exposed:Cleaning and Anti-Fouling Options• Screens may be used to prevent larger particles from

entering the sensor. Certain metals such as copper also exhibit anti-fouling properties to prevent algal growth.

• Polymer coats can inhibit deposition and microbial growth.• Wipers or brushes can be very effective at removing

stubborn deposits but require power and maintenance.• Air blasts are easily automated and often have built in

ports on the instruments.• Pumping water through a flow chamber at a gage house is

ideal for controlling fouling and cleaning, but it is costly.

Protocols: Part 5 – Power & Cables

• Power cable length, thickness, and quality of insulation are essential in achieving a steady input voltage necessary for continuous operation of the instruments.

• Degradation of battery terminals and sensor connectors is a common problem.• Solar cells can provide a means to recharge a battery, but they are not perfect.• In permanent installations, wired power systems are an option.• Depending on the probe, output can be either analog or digital. Considerations like

cable shielding are necessary for long cables with analog output.

Issues in controlling power to a field spectroscopy system

Left: An example of corroded battery terminals. Corrosion like this will impact instrument function. Periodic cleaning of contacts is necessary in the field, especially in sites with saltwater.

Protocols: Part 6 – Communications

Data from the probes is stored on a monitoring unit in one of two configurations:

con::cube• One box solution for data logging and instrument monitoring. Mini-PC with many I/O options.• Touch screen interface with plug and play options for s::can peripherals.• Data output via built-in USB, WIFI, Ethernet, or cellular modem (with SIM card and data plan).• Can interface with third party sensors through expansion ports in industrial settings

con::nect• Allows use of a standalone computer to control the instrumentation and data collection. Allows

flexibility in fixed installations and eliminates the need to transfer data to a PC for processing.

Both the con::cube and the con::nect use the moni::tool software, either running on the user’s PC or cube. An online demonstration of the moni::tool web interface is available at: http://monitool.s-can.at/index.x.

Collecting and sending data from a field unit to a central repository

Protocols: Part 7 – Data Quality

Interference from dissolved constituents• Dissolved constituents which absorb light at similar wavelengths to the desired analyte can

disrupt measurements obtained through spectroscopy. Their presence will increase absorptivity.• In nitrate, for example, dissolved organic carbon, bromide, hydrogen sulfide, and other types of

compounds can produce this type of interference. Advantages of using an s::can spectrometer over a conventional single or dual wavelength sensor allows for more complex spectral filtering.

Interference from suspended particles• Suspended particles will scatter light and can result in overestimating absorbance.• This scattering can vary and change both magnitude and shape of a curve.• High turbidity values can prevent an instrument from functioning beyond a certain threshold.

Ensuring that data is useful and filtering out noise and interference

Protocols: Part 7 – Data Quality

Interference from dissolved constituents Interference from suspended particles(Both actual nitrate concentrations are in 1mg/L as Nitrogen)

Ensuring that data is useful, and filtering out noise and other undesirable effects

Protocols: Part 7 – Data Quality

• This figure presents a spectral fingerprint example with both filtered and non-filtered spectra.

• Compensating is typically done at the hardware level on most control units.

• Algorithms are dependent on the contents of the water and must be updated for new sites.

• In the Easton Lab, our work originally used calibrations for wastewater effluent which had to be adjusted by the company to function properly in a stream.

Reading a spectral fingerprint and compensating for background values

Protocols: Part 7 – Data Quality

Sampling Interval vs. Reporting Interval• How often do you want the instrument to take each measurement? • Additionally, when reporting this data, do you want instantaneous values or a mean over a short

measurement interval (to account for moment-to-moment fluctuations in absorption values)Understanding detection limitations for various instruments, algorithms, and media• Bias can occur as discussed before due to turbidity or dissolved solids. It can also occur due to

wear on the instrument, a dirty aperture, and a lamp which no longer provides enough light.• Bias can be accounted for with correction algorithms.• For the S::CAN spectro::lyser probe, the maximum error for bias correction is:

±6% of reading plus 3/optical path length (in mm; mg/L).Ranges of data validity

Other items worthy of consideration when processing field data

Turbidity: 0–50 NTU, precision 0.05 NTU Nitrite (NO2-N): 0–7 mg/L, precision 0.005 mg/L abs/m DOC: 0–6 mg/L, precision 0.003 mg/L Ozone: 0 –10 mg/L, precision 0.005 mg/L Nitrate (NO3-N): 0–7 mg/L, precision 0.005 mg/L TOC: 0–8 mg/L, precision 0.005 mg/L Spectral Absorption (SAC254): 0–25 abs/m, precision 0.015 Color: 0–250 Hazen, precision 0.1 Hazen

Protocols: Part 9 – Troubleshooting

From the USGS guide to optical techniques for determination of nitrate in situ.

These guidelines are applicable in many cases to a variety of target analytes when using s::can units.

Protocols: Part 8 – Maintenance

Remote Access• Regular review of data collected by the instrument.• Regular checking of sensor indicators (lamp hours, noise levels, power, internal temperature).• Regular checking of system indicators (storage remaining, data transmission).• Flagging abnormal data for review.• Modifying the data collection and processing parameters as necessary.

Field Access• Inspections for damage, fouling, and corrosion on instrument and all connections.• Cleaning and servicing sensors as needed.• Checking the battery, solar panels, inverters, or other power components.• Downloading data and diagnostic information as necessary.• Manual sample collection for comparison to ensure data validity.

Performing diagnostics and maintaining quality data collection

References• Pellerin, B.A., Bergamaschi, B.A., Downing, B.D., Saraceno, J.F., Garrett, J.A., and Olsen,

L.D., 2013, Optical techniques for the determination of nitrate in environmental waters: Guidelines for instrument selection, operation, deployment, maintenance, quality assurance, and data reporting: U.S. Geological Survey Techniques and Methods 1–D5, 37 p.

• Lane, S.L., and Fay, R.G., 1997, Safety in field activities: U.S. Geological Survey Techniques of Water-Resources Investigations, book 9, chap. A9, October 1997, accessed Nov. 1, 2013, at http://pubs.water.usgs.gov/twri9A9/.

• 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.

• Langergraber, G., N. Fleischmann, F. Hofstaedter, and A. Weingartner. 2004. Monitoring of a paper mill wastewater treatment plant using UV/VIS spectroscopy. Water science and technology : A journal of the International Association on Water Pollution Research 49(1):9.

• U.S. EPA. Technology Evaluation Report, s::can Measuring Systems Spectro::lyser. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-12/065, 2012.