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Continuous Monitoring in the San Francisco Bay and Delta
D.Schoellhamer and P.Buchanan
Data Collection Methods and Procedures
Instrument Description and Operation Water-Sample Collection Data Processing Sensor Calibration
Instrument Description and Operation
EquipmentOptical backscatterance (OBS) sensors are used to monitor concentrations of suspended solids. An optical backscatterance sensor is a cylinder approximately 7 inches long and 1 inch in diameter with an optical window at one end, a cable connection at the other end, and an encased circuit board (Downing and others, 1981; Downing, 1983). An infrared pulse of light is transmitted through the optical window and is scattered or reflected by particles in front of the window within about 4 to 8 inches in a 165° conical zone. Some of this scattered or reflected light is returned to the optical window where a receiver converts the backscattered light to a voltage output. The voltage output is proportional to the concentration of suspended solids in the water column at the depth of the sensor. Calibration of the OBS sensor voltage output to concentrations of suspended solids will vary depending on the size and optical properties of the suspended solids; therefore, the sensors must be calibrated either in the field or in a laboratory using the same suspended material that is found in the field (Levesque and Schoellhamer, 1995).
EquipmentThe OBS sensors are positioned in the water column using polyvinyl chloride (PVC) pipe carriages that are coated with an antifoulant paint to impede biological growth. These carriages are designed to align with the direction of flow and to ride along a stainless-steel or Kevlar-reinforced-nylon suspension line attached to an anchor weight, which allows the sensors to be raised and lowered easily for servicing. The plane of the optical window is positioned parallel to the direction of flow and, as the carriage and sensor move with the changing direction of flow, the plane of the window retains its position relative to the direction of flow. The salinity stations are equipped with Foxboro specific conductance monitors and Campbell Scientific 107B thermistors.  The Foxboro specific conductance sensor is a heavy-duty probe designed for caustic slurries.  Two toroidally wound coils are encapsulated in close proximity within the sensor which is immersed in a solution.  An AC signal, applied to one toroidal coil, induces a current in the second coil, which is directly proportional to the conductance of the solution  (Foxboro 1991).  The Campbell Scientific thermistor measures the voltage drop to the sensor resistance.  The thermistor resistance changes with temperature (Campbell Scientific, Inc, 1997).

The water-stage recorder at Point San Pablo is equipped with a Handar Model 436A incremental encoder attached to a float driven incremented stainless-steel tape housed in a 20-foot long section of 12-inch diameter pipe.  A wire weight reference gage is mounted on a railing 15 feet north of the gage house.

EquipmentData acquisition, data storage, and sensor timing are controlled by an electronic data logger. The logger is programmed to power the suspended-solids sensors every 15 minutes, collect data every second for 1 minute, and then average and store the output voltage for that 1-minute period. The data loggers are programmed to store the specific conductance, temperature, and gage height data every 15 minutes without averaging the data.
Water-Sample Collection
Sample collectionPoint sampler were compared and found to be virtually identical (Buchanan and others, 1996). With the sensors deployed, the sampler is lowered to the depth of the sensor by a reel and crane assembly and triggered while the sensor is collecting data. The water sample is then removed from the sampler, marked for identification, and placed in a cooler and chilled to limit biological growth. Conductance samples are analyzed in the field using an Orion 140 meter. The conductance probes are calibrated in the field using pre-established standard solutions to ensure that the full range of expected values will be accurately recorded by the probe.  The temperature probe is checked using a VWR thermistor during site visits.

SamplesSuspended-solids samples are sent to the USGS Sediment Laboratory in Salinas, California, for analysis to determine suspended-solids concentration. Each sample is filtered through a 0.45-mm membrane filter, the filter is rinsed to remove salts, and the insoluble material is dried at 103°C and weighed (Fishman and Friedman, 1989). Suspended sediment is the material that settles to the bottom of the sample bottle; whereas suspended solids include suspended sediment and buoyant particles that do not settle and are trapped on the filter. The difference between suspended-solids concentration and suspended-solids concentration for San Francisco Bay water probably is small. Suspended-sediment concentration and suspended-particulate matter are identical.
Data Processing
Data processingData loggers stored the voltage outputs from the optical sensors every 15 minutes. Recorded data were downloaded from the data logger onto a storage module during site visits by USGS personnel. Raw data from the storage modules were loaded into the USGS's Automated Data Processing System (ADAPS). The time-series suspended-solids data are retrieved and edited to remove invalid data. Invalid data included rapidly increasing voltage outputs and unusually high voltage outputs of short duration. As biological growth occurred on the optical sensors, the voltage output of the sensors increased.  After the sensors are cleaned, sensor output frequently decreases. Efforts to correct invalid data sometimes proves to be unsuccessful because the desired signal was sometimes highly variable. Thus, data collected during the period prior to sensor cleaning often are unusable and are removed from the record. Spikes in the data, which are anomalously high voltages probably caused by debris temporarily wrapping around the sensor or by large marine organisms (fish, crabs) on or near the sensor, also are removed from the raw data record. Sometimes, incomplete cleaning of a sensor would cause a small constant shift in sensor output that could be corrected using water-sample data.

The specific-conductance data collected in the field is converted to salinity using the 1985 UNESCO standard (UNESCO, 1985) in the range of 2-42 practical salinity units (PSU).  Salinities below 2 PSU were computed using the extension of the practical salinity scale proposed by Hill and others (1986).  The Foxboro Pro records specific-conductance, to convert the data into salinity the following relations are used:

Converting data into salinity

The first term represents the 1985 practical salinity scale and the second and third terms are the low salinity extension where:

x=400Rt,   y=100Rt

a0=0.0080,  a1=-0.1692,   a2=25.3851

a3=14.0941,   a4=-7.0261,   a5=2.7081

b0=0.0005,   b1=-0.0056,   b2=-0.0066

b3=-0.0375,   b4=0.0636,   b5=-0.0144

Rt is the ratio between the conductivity of some unknown sample and the standard sea water conductivity at constant temperature T=25°C and zero pressure.  Rt is computed as:

Conductivity ratio

where CS,T,p is the conductivity of water with S salinity, in practical salinity units (PSU); T is temperature, in °C; and p is pressure, in bars.

To compute salinity, Rt is computed using the measured specific conductivity CS,25,0 .  Then Rt is substituted into the initial equation and used to compute the measured salinity in PSU.

Suspended-Solids Sensor Calibration
The voltage output from the optical sensors is converted to suspended-solids concentration using linear regression equations.  Samples collected during site visits and analyzed at the Salinas Sediment Laboratory are correlated to voltage readings recorded at the same time.  A sample calibration curve from Point San Pablo during water year 1996 is shown below. The linear regression (calibration) plot includes the number of samples, correlation coefficient, squared correlation coefficient, regression significance level, and root-mean-squared error.
Sample calibration curve

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