Water Quality Concepts
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What is Water Quality?
What is Water Quality?
Water quality refers to the suitability of water for human and ecological uses based on selected physical, chemical, and biological characteristics. Some physical parameters, such as temperature, acidity (pH), dissolved oxygen, electrical conductance and turbidity can be measured directly in a wetland, river or lake. Analyses of specific chemicals that can be found in the water are generally done at a laboratory.
The measured values of a water quality parameter change over time and depend on the characteristics of the water body, such as its flow, nutrient and pollutant sources, etc. The meaning of a single measurement is not always significant, whereas the changes in the measurements over time are often important.
The temperature varies with depth, time of the year and location in a lake or wetland. Temperature affects the growth of aquatic organisms. Fish and most aquatic organisms are cold-blooded. Consequently, their metabolism increases as the water warms and decreases as it cools. Aquatic plants and animals function well within preferred temperature ranges. If the temperature gets too high or too low, the local population of a species decreases. Temperature also affects water chemistry, which in turn affects biological activity.
pH is a measure of how acidic or basic water is. It is a measure of the balance of positive hydrogen ions (H+) and negative hydroxide ions (OH-) in the water. The pH range goes from 0 - 14, with 7 being neutral. Water that has more hydrogen ions is acidic (pH < 7), whereas water that has more hydroxyl ions is basic (pH > 7). pH values are on a logarithmic scale, where each successive number represents a 10-fold change in the acidity (or basicity) of the water. For example, water with a pH of 4 is ten times more acidic than water having a pH of 5.
Aquatic organisms are very sensitive to the pH of the aquatic environment. Most freshwater organisms in Western Oregon thrive best in a pH range between 6.5 and 8.5. Increased temperature or excess nutrients (phosphates and nitrates) may result in higher algal and plant growth, which may cause pH levels to increase, depending on the buffering capacity of the water.
Electrical conductivity is an estimate of the amount of total dissolved salts (TDS) or the total amount of dissolved ions in the water. Sources of dissolved ions include soil and rocks in the watershed, waste-water from sewage treatment plants, agricultural runoff, and urban runoff from roads.
Electrical conductivity is measured in units of microSiemens per centimeter (µS/cm). The conductivity of solutions with dissolved ions is highly dependent on temperature. Specific Conductance is a measurement of conductivity that is adjusted to values at 25°C for an average ionic species (e.g. KCl). Measurements at the preserve are reported as specific conductance in µS/cm.
Some Typical Conductivity Values
|Sample at 25°C
|Streams running through granite, silicon or other igneous rock
||10 - 50
|Streams running through limestone formations (eastern US)
||150 - 500
|Ocean Water (high concentration of dissolved salt)
The conductivity sensor consists of two metal electrodes that are exactly 1.0 cm apart in the water. A constant voltage is applied across the electrodes. An electrical current flows through the water due to this voltage. The measured current is proportional to the concentration of dissolved ions in the water. The greater the measured electrical current, the more ions there are in the water.
Dissolved oxygen (DO) is molecular oxygen (a gas) that is dissolved in water. Sources of DO are diffusion from the surrounding air, aeration of water from rapids in a stream or river, and as a waste product of photosynthesis. Dissolved oxygen concentrations are usually reported in units of milligrams of gas per liter of water - mg/L. (The unit mg/L is equivalent to parts per million = ppm). DO levels are an indicator of a water body's ability to support aquatic life.
- DO > 5 mg/L is considered favorable for growth and activity of most aquatic organisms.
- DO < 3 mg/L is stressful to most aquatic organisms.
- DO < 2mg/L does not support fish life.
There are many factors that cause natural variation in the concentrations of DO. Oxygen is produced during photosynthesis and consumed during respiration and decomposition. Increased temperature or excess nutrients may result in higher algal and plant growth, causing DO levels to increase. When the algae decompose, DO concentrations decline. Another variation is due to wind, which mixes the water, causing more oxygen to dissolve. Dissolved oxygen concentrations in a lake or pond may change significantly with water depth.
Dissolved oxygen is also measured as percent saturation. Note that values greater than 100% can be measured. This is because the DO probe is calibrated to 100% using only air as the source of oxygen. When photosynthesis from aquatic plants occurs, the total amount of oxygen in the water is greater than what diffusion from air provides.
Turbidity refers to how clear the water is. The greater the amount of total suspended solids (TSS) in the water, the murkier it appears and the higher the measured turbidity.
Suspended matter that may cause water to be turbid include:
- matter from decaying vegetation
- phytoplankton (suspended algae)
- microscopic organisms
- industrial wastes and sewage
Turbidity is usually measured using an optical instrument that measures how much light is scattered by the suspended solids. Turbidity is usually reported in Nephelometric Turbidity Units (NTU) or Formazin Nephelometric Units (FNU), depending on the wavelength of light used to make the measurement.
During periods of low flow, many rivers have a clear green color, and turbidities are low, usually less than 10 FNU. During a rainstorm, matter from the surrounding land is washed into the river, changing the water to a muddy brown color and increasing the turbidity. During high flows, water velocities are faster and water volumes are higher, which can stir up and suspend material from the river bed, causing higher turbidities.
Turbidity is an indicator of the concentration of suspended sediments in the water. Sediments are a natural part of streams and other water bodies and even the most pristine streams in undeveloped watersheds will run muddy during high flows. However, excessive sedimentation in streams and rivers is considered to be a major cause of surface water pollution.
The water level in wetlands, as well as streams, lakes, and oceans, is affected by the hydrologic cycle. The hydrologic cycle describes the continuous movement of water above, on, and below the surface of the earth. Water levels may increase due to direct precipitation (rain, snow, etc.), runoff due to precipitation, and groundwater discharge (flow from the ground). Water levels may decrease due to groundwater recharge (flow into the ground), evaporation and transpiration (the release of water vapor into the atmosphere from living vegetation). Evaporation and transpiration are collectively known as evapotranspiration.
Jackson Bottom is part of the floodplain of the Tualatin River. After a precipitation event (e.g. rainfall), the water level at the Preserve may increase when the Tualatin River overflows onto its floodplain. Also, Clean Water Services sometimes recycles treated wastewater into the wetland. The increased water level helps attract a diverse wildlife population.
Water levels are measured by people manually reading staff gages or automatically by sensors. Water level is often recorded in feet above Mean Sea Level (MSL), allowing the water levels from different locations to be easily compared. River stage is the level of the water surface in a river measured with reference to some datum. River stage is generally measured in feet above the datum. The datum at a gaging station on a river is a chosen level at which the river is never expected to go below. The datum is the zero level on the staff gage.
Picture is a staff gage which shows the water level of the Tualatin River.
Nick Engelfried, a volunteer, monitored macroinvertebrates at several locations at the Preserve for two years. Check out his reports:
JBWP Macroinvertebrate Report Mar-05 to Feb-06 (pdf)
JBWP Macroinvertebrate Report Apr-06 to Apr-07 (pdf)
Macroinvertebrates are aquatic invertebrates (animals without spinal columns). They include insects, crustaceans (such as crayfish), molluscs (such as aquatic snails), and aquatic worms. In moving water, the abundance and diversity of macroinvertebrates are indicators of water quality. Macroinvertebrates make up a fundamental link in the food web between organic matter resources (e.g., algae, leaf litter, detritus) and fish. Some macroinvertebrates respond in predictable ways to changes in stream environmental variables and thus are good indicators of stream quality.
Macroinvertebrates are also useful indicators of wetland health. They differ from stream macroinvertebrates in their greater tolerance of low dissolved oxygen concentrations. Wetland macroinvertebrates are sensitive to a variety of physical and chemical factors. High levels of nutrients, such as phosphorus and nitrogen, and chemicals, such as chloride, can have negative impacts on the invertebrate community.
Prong Gill Mayfly Larva - sketch by Nick Engelfried
Copepods - sketch by Nick Engelfried