WATER QUALITY PARAMETERS EXPLAINED HOME
Nitrate- and Nitrite-Nitrogen
Nitrate- and Nitrite-Nitrogen are ionic compounds of nitrogen and oxygen. These are common contaminants of drinking water and affect the health of lakes and streams. Many nitrogen compounds naturally exist in lakes, streams and groundwater, including nitrate and nitrite. Nitrate is a negative ion consisting of one nitrogen atom and three oxygen atoms. It is an essential plant nutrient. Because it is a relatively stable form of nitrogen, and because it is very water soluble, it is the most common form of nitrogen entering the stream network from farmed fields. It is also present in municipal wastewater discharge, leakage from septic tanks, and runoff from manure. Nitrate and nitrite (one nitrogen with two oxygen atoms) in drinking water pose an acute health risk for infants less than 6 months of age. The infant’s digestive system has not developed a mature assemblage of microorganisms, and as a result, nitrate and nitrite compete with hemoglobin for oxygen in the bloodstream, causing blue-baby syndrome. Drinking water nitrate and nitrite are measured “as nitrogen” in the U.S., with a limit set at 10 parts per million (mg/L) for nitrate-nitrogen and 1 mg/L for nitrite-nitrogen. Elevated nitrate in streams and lakes can upset the natural balance, leading to harmful algae blooms and poor diversity of organisms. Prior to European settlement, nitrate-nitrogen levels in Iowa streams were likely less than 2 mg/L. Nitrite usually is not stable in the environment, especially in streams, where it is quickly converted to nitrate.
Soil particles obstruct the transmittance of light through water and impart a property known as turbidity. Turbid water is cloudy water. Turbidity is measured in Nephelometric Turbidity Units (NTUs). Clear water like drinking water has a turbidity less than 1 NTU. Muddy floodwaters can have turbidity greater than 1000. A few rivers are naturally muddy. The Missouri River, for example, was very muddy prior to dam construction and channelization, and its aquatic life was adapted to that condition. Most Iowa streams, however, likely ran clear (less than 10 NTU) before the prairie was broken. Excess cloudiness reduces diversity of fish and other organisms in our lakes and streams.
Extremes of temperature, especially excess heat, are harmful to aquatic life and can lead to fish kills and other negative consequences. Because natural vegetation along the stream bank (riparian area) has been altered, many Iowa streams do not have shady areas and suffer from heat stress.
Specific Conductance (SC) is created by the positive and negative ions of dissolved salts. Ions enable water to conduct electricity. Water with high SC is salty and harmful to freshwater organisms. SC greater than about 1000 microsiemens (µS) would likely be stressful to aquatic life. Excess SC can be caused by stormwater runoff (road salts), wastewater discharges, and manure runoff.
pH is the measure of acidity or alkalinity in the water. A pH of 7 is exactly neutral; lower than 7 is acid, higher than 7 is basic (alkaline). Most Iowa waters are slightly to moderately alkaline (7.5-8.5) in their natural condition. pH tends to be slightly higher in the summer. In a lake or stream plagued by algae, pH levels can vary greatly between daytime and nighttime. This stresses the other organisms.
Dissolved oxygen (DO) is needed by fish and other aquatic life. Decomposition of organic material from wastewater discharges or manure consumes dissolved oxygen (DO). Ammonia and nitrite-nitrogen also consume oxygen. Algae blooms affect DO levels, giving off oxygen in the day but consuming it at night. When DO drops below 4-5 parts per million, fish begin to die. Fish like carp and bullhead can tolerate much lower DO levels than desirable species like smallmouth bass and walleye.
Dissolved Oxygen percent Saturation
Dissolved oxygen percent saturation indicates how much oxygen is in the water relative to what the water could hold at that temperature. The amount of oxygen that water can hold is temperature dependent. Cold water can hold much more oxygen than warm water. Water can be so warm that it is 100% saturated with oxygen, but cannot sustain aquatic life because absolute levels of oxygen are still too low. Percent saturation can exceed 100% if algae are producing oxygen so fast that it cannot “off-gas” to the atmosphere, or if the temperature of a stream or lake changes very rapidly. These conditions impart stress to aquatic life.
Chlorophyll-A is a specific type of chlorophyll used in photosynthesis and an indicator of harmful algae blooms. Less than 25 micrograms per liter (ug/L) is desirable, although some water bodies like wetlands could be expected to have far higher naturally-occurring levels.