Water Quality Monitoring for the “Big Four Parameters”

By Adam Krumbein & Keith Bellingham

Many different parameters can be measured in natural and artificial waters with sensors or via lab analysis. Each parameter effects water quality and the organisms that depend on the water for survival. Different states, monitoring agencies, and companies also place an emphasis on different water quality parameters to be monitored, depending on local conditions and mandates.

For most water quality professionals, in-situ measurement of temperature, dissolved oxygen, electrical conductivity, and pH are considered the standards for basic water quality testing, and are referenced in many official Government guidelines, such as the Clean Water Act.

Stevens Water Monitoring Systems offers several products that make monitoring these parameters a simple task, providing accurate data for decision making. The following article investigates these different parameters, how they affect water quality, and what equipment can be used to measure these parameters effectively.

Water quality monitoring is important in both natural and artificial waters.

Temperature

Temperature is an important water quality parameter that is relatively easy to measure and has a direct impact on the organisms living in the water. Many aquatic organisms are sensitive to changes in water temperature, especially because water temperature changes will affect other water quality parameters, such as dissolved oxygen and salinity. Water bodies will naturally show changes in temperature seasonally and daily; however any changes to stream water temperature outside of the normal upper and lower bounds for that particular area will affect the ability of fish to reproduce. Many lake and rivers will exhibit vertical temperature gradients (thermal stratification) as the sun warms the upper water during the day while deeper water will remain cooler.

Fish friendly dams will have selective water releases where the temperature of the stream can be controlled by the depth of the water which is released. In the summer, the water could be released from the bottom of the dam, while in the winter the water is released from the top. This selective release will mitigate the impact the dam will have on the water temperature.

Some streams will increase in temperature as the stream water moves down stream through urban, industrial, and agricultural areas. This man made temperature loading can be characterized by the Total Maximum Daily Load (TMDL), which can be applied to temperature as well as other measureable pollutants.

For example, a stream in forested head waters will be at a suitable temperature for the native aquatic life. As the stream meanders through pasture land, the riparian vegetation will not be abundant enough to effectively shade the stream. Once the stream reaches into urban areas, the stream may become channeled to make room for housing and less overhead shade from trees may be available to help cool the water. Removing the natural meander from a stream will increase the velocity of the water which will cause erosion further degrading the quality of the water. Impervious structures such as parking lots, roads and buildings will prevent the infiltration of rainwater into the groundwater. Instead of being fed from cool groundwater, the stream will receive run off after rain events further degrading the quality of the water and increasing the temperature.

Environmental policies require the monitoring of stream water temperature. In most urban and industrial locations, environmental permits are required to help minimize the temperature loading to streams.

Stevens Products for Water Temperature Measurement:

All Hydrolab Water Quality Sondes from Stevens Water come with the water temperature sensor pre-installed, since temperature is a critical component for many other water quality measurements. The Hydrolab temperature sensor has an accuracy of ± 0.10° C, a resolution of 0.01° C, and can cover a range of -5° to 50° C.

Additional water temperature measurement sensors from Stevens:

• Greenspan CTDP 300 - Temperature, Depth, Electrical Conductivity, and pH measurements with RS-232 output and internal logging.

• Greenspan CTDP 1200 - Temperature, Depth, Electrical Conductivity, and pH measurements with SDI-12 output.

Dissolved Oxygen

Dissolved oxygen (DO) is essential to all forms of aquatic life including the organisms that break down man-made pollutants. Oxygen is soluble in water and the oxygen that is dissolved in water will equilibrate with the oxygen in atmosphere. Oxygen tends to be less soluble as temperature increases, with warmer water containing less oxygen then colder water. Water salinity also has an effect on dissolved oxygen levels, with higher saline reducing the availability of oxygen in the water.

The dissolved oxygen of fresh water at sea level will range from 15 mg/l at 0° C to 8mg/l at 25° C. Concentrations of unpolluted fresh water will be close to 10 mg/l at 15° C.

In general, the concentration of dissolved oxygen can be affected by the biological activity by marine life such as fish, aquatic plants, and bacteria found in the water.

Photosynthesis of some aquatic plants will increase the dissolved oxygen level of the water during day light hours, with the dissolved oxygen levels falling during the nighttime hours. In natural waters, man-made contamination, or natural organic material will be consumed by microorganisms.

As this microbial activity increases, oxygen will be consumed out of the water by the organisms to facilitate their digestion process. The water that is near the sediment will be depleted of oxygen for this reason.

In waters contaminated with fertilizers, suspended material, or petroleum waste, microorganisms such as bacteria will break down the contaminants. The oxygen will be consumed and the water will become anaerobic. Typically dissolved oxygen levels less than 2 mg/l will kill fish.

Other indirect laboratory tests for assessing the dissolved oxygen is the biological oxygen demand (BOD) and the chemical oxygen demand (COD). The BOD is the amount of oxygen required to biologically break down a contaminant and the COD is the amount of oxygen that will be consumed directly by an oxidizing chemical contaminant.

Stevens Products for Dissolved Oxygen Measurement:

Hydrolab Water Quality Sonde with Hach LDO (luminescent dissolved oxygen) sensor

All three Hydrolab water quality sondes available from Stevens Water Monitoring Systems can be equipped with the Hach LDO sensor, which is a high-accuracy optical sensor for measuring levels of dissolved oxygen. The Hach LDO sensor utilizes a luminescent coating on the sensor cap, combined with an LED light and a photo diode to measure the amount of ambient dissolved oxygen in the water. Because the sensor uses an optical process rather than a typical membrane measurement, passive fouling and calibration issues are minimized for more accurate data.

Electrical Conductivity

Electrical conductivity (EC) in natural waters is the normalized measure of the water’s ability to conduct electric current. This is primarily influenced by dissolved salts such as sodium chloride and potassium chloride. The common unit for electrical conductivity is Siemens per meter (S/m). Most freshwater sources will range between 0.001 to 0.1 S/m. Like many other parameters, electrical conductivity is highly temperature dependant, and can also have a negative effect on dissolved oxygen concentrations available for marine life. Most marine life also has a specific upper and lower band of dissolved salts that they will tolerate in the water. Concentration of dissolved salts that are outside of normal parameters can stress or cause die off of aquatic life.

The source of EC may be an abundance of dissolved salts due to poor irrigation management and fertilizer runoff, minerals and salts from urban rain water runoff, or other discharges. Local geology, such as the underlying rock types found within the soil nearby can also have an influence in EC levels of a water body.

EC is also the measure of the water quality parameter “Total Dissolved Solids” (TDS) or salinity. At about 0.3 S/m is the point at which the health of some crops and fresh water aquatic organisms will start to be affected by the salinity of water.

Field measurements of EC reflect the amount of total dissolved solids (TDS) in natural waters. The relationship between TDS and EC can be described by the equation;

TDS (mg/L) ˜ EC (mS/cm) X 640

Salinity refers to the presence of dissolved inorganic ions such as Mg⁺⁺, Ca⁺⁺, K⁺, Na⁺, Cl^-, SO₂^4-, HCO₃^- and CO₃^2- in the aqueous solution or soil matrix. The salinity is quantified as the total concentration of soluble salts and is expressed in terms of electrical conductivity. There exists no in-situ salinity probe (based on EC alone) that can distinguish between the different ions that may be present.

Stevens Products for Electrical Conductivity Measurement:

An Electrical Conductivity sensor is an option for all Hydrolab water quality sondes that are available from Stevens Water. The Hydrolab conductivity sensor uses four graphite electrodes in an open cell design to provide extremely accurate and reliable data with virtually no maintenance.

The EC sensor has a measurement range of 0-100 mS/cm, an accuracy of ± 0.5% of reading, and a resolution of 0.001.

Additional Electrical Conductivity measurement sensors from Stevens:

Greenspan EC3000 - Electrical Conductivity and Temperature Sensor with RS-232 or optional SDI-12 output and internal logging.
Greenspan CTDP 300 - Temperature, Depth, Electrical Conductivity, and pH measurements with RS-232 output and internal logging.
Greenspan CTDP 1200 - Temperature, Depth, Electrical Conductivity, and pH measurements with SDI-12 output.

pH

The pH of natural water can provide important information about many chemical and biological processes and provides indirect correlations to a number of different impairments. The pH is the measurement of the acid/base activity in solution; specifically it is the negative common logarithm of the activity/concentration of hydrogen ions;

pH = -log[H⁺]

The pH scale runs from 0 to 14. A pH value of 7 is neutral; a pH less than 7 is acidic and greater than 7 represents base saturation or alkalinity. A pH range of 6 to 8.5 is common for natural waters.

Pure water free of dissolved gases will naturally become ionized;

H₂O H⁺ + OH^-

The actual number of water molecules that will ionize is relatively very small with the amount of hydrogen ions [H⁺] being equal to the amount hydroxide ion [OH^-]. At room temperature the concentration of [H⁺] in pure water will be 1 x 10^-7 moles per liter. A pH of 7 is neutral because the –log(1 X 10^-7) is 7 by definition.

In unpolluted or pure waters, the pH is governed by the exchange of carbon dioxide with the atmosphere. Carbon dioxide is soluble in water and the amount of CO₂ that will dissolve in the water will be a function of temperature and the concentration of CO₂ in the air. As the gaseous CO₂ becomes aqueous, the CO₂ will be converted into H₂CO₃ which will acidify the water to a pH of about 6. If any alkaline earth metals such as sodium are present, the carbonates and bicarbonate formed from the solubilization of CO₂ will interact with sodium increasing the alkalinity shifting the pH up over 7.

Lower values in pH are indicative of high acidity, which can be caused by the deposition of acid forming substances in precipitation. A high organic content will tend to decrease the pH because of the carbonate chemistry. As microorganisms break down organic material, the by product will be CO₂ that will dissolve and equilibrate with the water forming carbonic acid (H₂CO₃). Other organic acids such as humic and fluvic acid can also result from organic decomposition.

In addition to organic acids and the carbonate chemistry, the acidity of natural waters could also be controlled by mineral acids produced by hydrolyses of salts in metals such as aluminum and iron.

Most metals will become more soluble in water as the pH decreases. For example, sulfur in the atmosphere from the burning of coal will create acid rain. The acid rain will dissolve metals such as copper, lead, zinc and cadmium as the rain runs off of manmade structures and into bodies of water. The excesses of dissolved metals in solution will negatively affect the health of the aquatic organisms.

The alkalinity of natural waters is controlled by the concentration of hydroxide and represented by a pH greater than 7. This is usually an indication of the amount of carbonates, and bicarbonates that shift the equilibrium producing [OH^-]. Other contributors to an alkaline pH include boron, phosphorous, nitrogen containing compounds and potassium.

Changes in pH can be indicative of an industrial pollutant, photosynthesis or the respiration of algae that is feeding on a contaminant. Most ecosystems are sensitive to changes in pH and the monitoring of pH has been incorporated into the environmental laws of most industrialized countries.

pH is typically monitored for assessments of aquatic ecosystem health, recreational waters, irrigation sources and discharges, live stock, drinking water sources, industrial discharges, intakes, and storm water runoff.

Stevens Products for pH Measurement:

The Hydrolab pH Sensor is an option for all Hydrolab water quality sondes that are available from Stevens Water. The Hydrolab pH sensor uses glass bulb and refillable reference electrode for reliable data from a sensor that is easily maintained and long-lasting.

The Hydrolab pH sensor offers a range of 0 to 14 pH units, with an accuracy of +/- 0.2 units and a resolution of 0.01 units.

Additional pH measurement sensors from Stevens:

Greenspan pH3000 - pH and Temperature Sensor with RS-232 or optional SDI-12 output and internal logging.
Greenspan CTDP 300 - Temperature, Depth, Electrical Conductivity, and pH measurements with RS-232 output and internal logging.
Greenspan CTDP 1200 - Temperature, Depth, Electrical Conductivity, and pH measurements with SDI-12 output.

Conclusion

While there are many important water quality parameters, pH, dissolved oxygen, electrical conductivity, and temperature represent a red flag warning for many water quality problems and are easy and economical to monitor. As more demands are placed on natural waters from land use changes, agriculture, and climate change, monitoring water quality is vital in protecting human health, our natural environment, and aquatic organisms that depend on water for habitat. Monitoring water quality will not only indentify impaired waters, but help regulators and water resources managers make decisions that will help keep our natural environment healthy. Stevens Water Monitoring Systems offers economical monitoring solutions that are important tools in the protection of endangered species, natural water ways, and maintaining a high quality of life.