Some physico-chemical measurements can be undertaken directly in the field (in rivers or subsurface waters) using data-logged monitors. These characteristics include temperature, turbidity, electrical conductivity, dissolved oxygen, redox potential and pH. Temperature is typically measured with a thermocouple. Turbidity can be measured with several different types of optical sensor. Typically, a photodiode is used to measure the degree of (coherent) light either penetrating the water or backscattered by particles in the water. The OBS-3+ turbidity probe (D&A Instrument Company, Washington, USA) uses the latter principle. Ideally, the light source and photodiode are cleaned prior to measurement, using an automatic wiper (Fig. 8.5). These turbidity measurements can be calibrated to suspended sediment concentration determined from spot samples by the gravimetric method (Section 8.4).
The electrical conductivity of water is determined by measuring the potential difference across a couple of electrodes in response to an applied current. The TDS can then be derived from these data using the calibration described in Section 8.1.1. With a dissolved oxygen probe, the oxygen diffuses through a gas-permeable plastic membrane to a platinum cathode, and it is reduced:
The oxidation taking place at the reference electrode (a silver/silver chloride anode in KCl electrolyte) is:
A current will then flow that is proportional to the rate of diffusion of the oxygen. Devices to measure redox potential (Eh) measure the ability of the water to transfer electrons to and from a reference electrode comprising a silver wire within a silver chloride solution relative to a platinum electrode (platinum because it does not readily oxidise). The pH of water requires the use of a glass electrode (which permits the passage of hydrogen ions, but no other ions) coupled to a reference electrode. The potentials created by these two electrodes connected in the same circuit gives a measure that is proportional to the H+ activity of the solution via the Nernst equation. Ion-selective electrodes (ISEs) operate on similar principles, though the ion-selective membranes are not entirely ion specific. ISEs are available to measure ions including nitrate, potassium, ammonia, cadmium and lead.
Several multi-probe sondes are now available that measure several of these characteristics in situ within rivers or subsurface wells. An example is the 6-series sonde produced by YSI Inc., which can be configured to measure eight probes (Fig. 8.6).
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