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A Day in the Life of a Scientist
(How to take water quality and
optical measurements)
Photograph of scientists on a small boat taking water samples

Water Quality Sampling

Water quality sampling involves monitoring the physical and sometimes biological and chemical aspects of the water. Samples can be taken from a dock, but are often taken from a boat at specific locations. For example, it is important that water quality is monitored by SERC scientists at the mouth of the Rhode River, which is a rather wide river, where it enters the Chesapeake Bay, therefore, samples are taken from a boat. However, they also monitor the water quality within a small tributary called Muddy Creek, where samples can be taken from a dock.

9:00 a.m. Measurements taken from a boat, dock or the water's edge:

Temperature  A temperature sensor on a rope must be used in order to lower it to various depths. Often this is included on more technical instrumentation.
Depth  Depth can be recorded with a depth gauge, which can be separate or part of other instrumentation. However, if complicated instrumentation isn't available, often the secchi disk is used for this purpose.
Secchi disk depth  A measure of transparency and an estimate for the compensation point, a secchi disk is a plate sized disk painted black and white or just white, which is lowered into the water until it disappears. It is then pulled up and when it reappears, the depth marked is the Secchi disk depth.
Salinity  A refractometer can be used, which just requires a drop of water! Salinity probes can also be used, which digitally record the salinity. It is often important to check the salinity at different depths in coastal waters, because there is often a salinity gradient. The denser salt water sinks to the bottom while the freshwater lies on top.
pH  is recorded with a digital probe or with alkalinity strips. pH is a chemical property of water. The acidity of water can be increased by acid rain!
Light intensity  Light intensity can be measured by taking readings at different depths with a light sensor. It is important to take a reading just below the surface, because this is the baseline light intensity. Readings are taken at regular intervals, the size of which depend on the depth of the water where the readings are being taken. The light intensity in the air is also a good reference value.
Turbidity  is a measure of the transparency of the water. If a water is very turbid, it contains a high concentration of particulate matter, which is scattering light. It must be measured in the lab by a nepheolometer in NTU units, which are generic units used for turbidity.
Total Suspended Solids(TSS)  is a measure of the concentration of particulate matter in a water body. It can be measured by a probe, but can also be done in the lab through a process of filtration and heating.

Water samples can be collected at specific depths with specialized collecting apparatuses. Integrated samples, which include water from the entire water column can also be collected. Samples can then be tested for nutrients, like nitrate and phosphorus, the limiting nutrients for phytoplankton growth. They can also be tested for chlorophyll, which is a representation of phytoplankton concentration. They can also be sorted, identified and counted for the number and type of phytoplankton within the sample! Basic tests, like salinity, pH, temperature, if done immediately, and TSS can also be measured from a sample of water at a particular depth.

Depending on how many stations are being monitored and how far apart the stations are, these measurements can take a while, sometimes half the day or even all day, not including the nutrient tests and biotic counting that must be done in the laboratory.
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Hydrologic Optics
Apparent Optical Properties (AOP)

These measurements have to be taken outside, because they are dependent on both the composition of the water AND the ambient light field. The ideal time to measure Apparent Optical Properties (AOP) is around noon, when the sun is directly above. This is because the angle of the sun is an important factor in light penetration. For example, light will penetrate more deeply at noon when the rays of light are at a 90° angle to the surface of the water, than, say, 5:00 p.m., when the rays of light are hitting the surface of the water at a steep angle. More light will be reflected off the surface when there is a steep angle and the light that does penetrate the surface will not be traveling through the water at a 90° angle to the bottom of the water body, which will not result in maximal depth penetration.

11:30 am  Light attenuation readingsPhotograph of scientists taking light measurements off a dock.

The Satlantic, a spectroradiometer, is used to measure upwelling and downwelling irradiance with depth in the water column at selected wavelengths. It can also measure reflectance, another AOP. The spectroradiometer is shown above being lowered into the water. There are three canisters, each containing a set of probes which recorded light intensity at specific wavelengths.

Downwelling irradiance is recorded from two sets of probes with some differing and some similar wavelength selection. The probes face upwards and therefore capture light that is moving downward in the water column. Only one set of probes records upwelling irradiance, because much less light is scattered back towards the surface than is penetrating downward. These probes face the bottom of the estuary. It is being used here off the dock at the SERC facility on the Rhode River by a laboratory technician and an intern. It is important that the probe be lowered in waters that are unobstructed from the sun on the unshaded side of a dock or boat.

Inherent Optical Properties (IOP)Photograph of intern standing with measuring equipment in a lab.

The inherent optical properties are measured with instrumentation either from a water sample brought back to the laboratory or in situ if the instrumentation is set up in the field to continually monitor data.

2:00 p.m. Absorption, Scattering and Beam attenuation

Absorption can be directly measured by a spectrophotometer by placing a sample volume of water in a vial from which the machine passes a measured amount of light through. Absorption can also be measured by a spectral absorbance transmittance meter, shown here. This instrument also measures beam attenuation. The scattering coefficient can be calculated from the two measures. This same instrument can be semi-permanently placed in the field, where it can continuously record data.

Monitoring water quality in general and through hydrologic optics is time-consuming work, just sampling and testing water. The behind-the-scenes analysis and interpretation can take even longer, depending upon the amount of data and the questions being asked about the data. However, this work, both the collection and interpretation is crucial to finding solutions to our water quality problems and issues.
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Smithsonian Environmental Research Center
647 Contees Wharf Road, P.O. Box 28, Edgewater, MD 21037-0028
Phone: 443-482-2200
FAX: 443-482-2380