Feb
07
2010

Vital signs in an estuary: salinity intrusions

Estuaries are coastal areas in which rivers and oceans meet. Thus, they include both fresh and salt water, each of which support different ecological communities of plants and animals, large and small. Salinity (“saltiness”) of the estuary is a measure of its health--a vital sign--for those communities.

In some cases, salt-water from the ocean side of the estuary can begin to “intrude” on an area previously dominated by fresh water. It is important to be able to measure and monitor this aspect estuary health.

CMOP has developed a remote sensing device that opens the way for scientists to better understand and predict salinity intrusions in estuaries.

Circuit analog for dipole source on river bedCourtesy CMOP

Oceanographer Thomas Sanford, Ph.D., and his team from the Applied Physics Laboratory at the University of Washington, have developed a bottom-mounted instrument for measuring electrical conductivity in the water column, which can be transformed into salinity readings.

The current process for measuring salinity involves sensors that provide “point” observations. Sanford’s instrument provides measurements of integrated salinities across the entire water column, allowing a more representative description of salinity intrusion.

How does it work?

Sanford’s approach is to produce a low-frequency electrical current and measure the resulting electric field at a nearby dipole receiver. The received electrical field is a function of the electrical conductivity of the water column and the sediments.

Quasi-static electrical analog circuit: The quasi-static electrical analog circuit for electric currents divided between the parallel resistors of saline water (Rw ) and the sediment (Rs ) is such that: Vr /Is = CRwRs /(Rw + Rs ) = C(Σw + Σs )-1 , ∴ Σw = CIs/Vr – Σs, where Σw is the conductance (vertical integral of σ) of the river, C is an empirical calibration value, Is is the source current, Vr is the receiver voltage and Σs is the conductance of the sedimentsCourtesy CMOP

Sanford’s team deployed the system in the Columbia River estuary before and during a flood tide. At the same time, they took measurements with a CTD, a standard oceanography-sampling device that reads Conductivity, Temperature and Depth. As the layer of seawater thickened, they observed the decreased resistance of the water column caused the receiver voltage to decrease.

Previous studies in the Columbia River had demonstrated a tight correlation between electrical conductivity and salinity. This correlation permits the conversion of electrical conductivity to salinity. Sanford’s team collected a time series of water-column electrical conductivity that they converted to salinity. The inferred salinity was shown to agree with the salinity readings from the CTD.

Vertically integrated salinity: Observed vertically integrated salinity from CTD casts (red dots) compared to that inferred from the electrical measurements using the electrical conductance time series ﬁtted to the equation: Sw = 8.82*(CIs/Vr -Σs ), where C is an empirical coefficient equal to 43, Is is source current and Vr is observed electric field and Σs an offset caused by leakage into the sediments equal to 8. Blue dots are observed electrical conductance and the continuous curve is the electrical conductance, both converted by salinity by the factor 8.82 psu/S/m.Courtesy CMOP

What's next?

CMOP researchers are looking at Sanford’s new sensor as an opportunity to better explain processes as diverse as internal waves, estuarine turbidity, and summer blooms of phytoplankton (tiny mobile plants that sometimes collect in massive “blooms” in surface waters in estuaries). They expect to improve computer models that are designed to depict the variable conditions of the estuary, and anticipate changes associated with climate and human impact. Once demonstrated for the Columbia River, the new sensor has the potential to be used in estuaries around the world.

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