Water Chemistry

Nutrients

Nutrients such as phosphorus and nitrogen are essential to maintain stream life. However, nutrient overloading into streams and lakes is one of the major challenges facing the Fox River Basin today. A limiting nutrient is the nutrient which is in shortest supply in a particular ecosystem. Plant and animal growth will occur only to the point where that nutrient is no longer available. In freshwater systems, phosphorus is typically the limiting nutrient for plant growth, whereas nitrogen is the limiting nutrient in oceanic environments. Excessive amounts of these nutrients throw off the ecological balance of the aquatic system, with far-reaching impacts on biota. For example, phosphorus additions to Green Bay cause huge summer algal blooms. Similar conditions exist in the Mississippi River delta, except that it is nitrate causing phytoplankton blooms. Large amounts of detritus from these blooms settle to the bottom of the water column, where microbial decomposition depletes dissolved oxygen levels for fish and invertebrates.

Typical sources of excess nutrients in streams are wastewater treatment facility discharges and storm water runoff. Much progress has been made over the past 20 years in reducing phosphorus in wastewater, especially through the elimination of phosphates from laundry detergents. However, both manure and inorganic fertilizer inputs from agriculture and lawn-care are still widely uncontrolled nonpoint sources of nutrients. Fertilizers contain highly soluble compounds because plants need nutrients to be in solution for uptake by roots. However, these forms of nitrate, ammonia, and phosphorus are more able to migrate from the soil to nearby streams during precipitation events. In addition to fueling eutrophication in streams and lakes, both ammonia and nitrate can reach toxic levels under certain conditions.

pH

pH measures the hydrogen ion concentration of a solution, which controls whether a solution is acidic or alkaline. The pH scale is logarithmic, meaning that every one unit of change in pH is a ten-fold increase or decrease in acidity. The pH scale in the environment ranges from 0 (acids) to 14 (bases), with 7 considered neutral. pH must be measured directly in the stream, since changes in temperature can affect the pH value.

The pH of stream water is influenced by many sources. First, natural rainwater has a pH of approximately 5.6, attributable to mixing with carbon dioxide in the atmosphere to form carbonic acid. Nitrous oxides or sulfur oxides from fossil fuel consumption also can mix with water in the atmosphere to form acid rain, which can substantially lower the pH of streams. Acid rain may have more or less of an effect due to local geology. Calcium and magnesium ions dissolved from the surrounding limestone bedrock in the Fox River Basin increases the buffering capacity of the stream to resist changes in pH. However, volcanic bedrock weathers more slowly, so streams in igneous regions have less buffering capacity and acid rain has a greater impact on stream or lake chemistry. Photosynthesis also influences stream pH because carbon dioxide is used by that process during daylight hours and given off by respiration at night. Because carbon dioxide dissociates in water to create carbonic acid, decreased levels of CO2 during the day results in alkaline water conditions, and increased levels of CO2 at night create acidic pH values. This pH dynamic is most visible during algal blooms, with pH in some cases exceeding 9.0.

The optimum pH for most aquatic organisms is between 6.5 and 8.5. pH values outside this range may first affect reproductive processes, and then species survival. As pH controls the dissociation of substances into dissolved ions, low pH values increase the availability of certain toxic chemicals, such as mercury, lead, iron, chromium, and other heavy metals. Also, as pH increases, the ammonium ion (NH4) changes to ammonia (NH3), which is very toxic to aquatic life. Streams in Northeast Wisconsin typically have pH values between 7.0 and 9.0, depending on the time of year that sampling occurs. The water quality standard for most surface waters in Wisconsin to support fish and aquatic life is a pH between 6.0 and 9.0, with no change greater than 0.5 units outside the estimated natural seasonal maximum and minimum values.

Because pH is measured on a logarithmic scale, to correctly calculate an average value for replicate measurements, the values would have to be transformed logarithmically into hydrogen ion concentrations, averaged, and then transformed back into pH. Rather than perform these calculations, the median pH measurement should be reported to the LFRWMP database. pH results measured with the pH Testr 3+ should be reported to 2 decimal places.

Conductivity

Conductivity, or specific conductance, measures the ability of water to conduct an electrical current. A stream’s conductivity is directly proportional to the concentrations and types of positively and negatively charged ions present. Sources of ions are both naturally occurring and anthropogenic in origin, and include soil, bedrock, human and animal waste, fertilizers, pesticides, herbicides, and road salt. Specific conductance can also be used to approximate the Total Dissolved Solids (TDS) in the water.

A conductivity meter reads the conductivity directly from the stream. Conductivity is temperature-sensitive, but the meter automatically compensates for temperature differential by correcting values to a standard temperature of 25°C. The conductivity meter measures the flow of electrons over a specified distance, and is the reciprocal of resistance (ohms). Measurements are given in µS/cm (microSiemens per centimeter), which expresses the flow of electrons between two electrodes, each 1 square cm in surface area, that are 1 cm apart.

Streams in Northeast Wisconsin typically have conductivity values between 300 and 1800 µS/cm. During periods of snowmelt or in areas where there is barnyard runoff, the conductivity in a stream may exceed the detection limits for the Oakton ECTestr+ Low Conductivity Meter provided in the monitoring kits. In these situations, the conductivity should be reported as >1999 µS/cm. Although the State of Wisconsin has not established surface water quality standards for conductivity, it can be used as an indicator of excessive ion concentrations for further study.

Dissolved Oxygen

Unlike terrestrial environments, oxygen is typically a limiting factor in aquatic ecosystems. Dissolved oxygen (DO) concentrations are expressed as milligrams of oxygen per liter of water (mg/L). The amount of DO affects what types of aquatic life are present in a stream, because many species of fish and macroinvertebrates are sensitive to low DO levels. DO also regulates the availability of certain nutrients in the water. Many physical and biological factors affect the amount of dissolved oxygen in a stream.

The physical factors that influence DO are temperature, altitude, salinity, and stream structure. Temperature inversely controls the solubility of oxygen in water; as temperature increases, oxygen is less soluble. In contrast, there is a direct relationship between atmospheric pressure and DO; as the pressure increases due to weather or elevation changes, oxygen solubility increases. Salinity also reduces the solubility of oxygen in water. However, because streams in Northeast Wisconsin have relatively low salinity values, this factor is typically disregarded for our calculations. Stream structure also influences DO concentrations. Atmospheric oxygen becomes mixed into a stream at turbulent, shallow riffles, resulting in increased DO levels. Because there is less surface interaction between water and air in slow-moving water and deep sections of a stream, DO concentrations often decrease between surface and bottom measurements.

The biological processes of photosynthesis and respiration also affect dissolved oxygen concentrations in streams. As aquatic plants photosynthesize, they give off large amounts of DO during daylight hours. However, respiration from aquatic vegetation, microorganisms, and algae consume oxygen at all hours of the day and night. A stream experiencing an algal bloom exhibits large daily fluctuations in DO as extreme oxygen production during the day contrasts with the bacterial decomposition of algal detritus at night. Thus, the lowest concentrations of DO in the summer are typically observed just before dawn.

Biochemical oxygen demand (BOD) is another important factor that effects DO concentrations in streams. BOD is the amount of oxygen consumed by microbial decomposition of organic waste, and is measured by the change in DO in a sealed water sample over a five-day incubation period. High levels of organic pollution, such as that from sewage treatment plants, agricultural runoff, or industrial wastes, can significantly increase the BOD in a stream. Relatively healthy streams will have a 5-day BOD reading of less than 2 mg/L, whereas polluted streams may approach 10 mg/L.

DO must be measured directly in the stream, since concentrations change quickly once a sample is collected. A DO probe allows several measurements to be taken in a short period of time, allowing quick comparisons for different physical characteristics within the stream reach. Streams in the Fox River Basin typically have dissolved oxygen values between 2 and 14 mg/L. The State of Wisconsin has set a minimum water quality standard of 5 mg/L DO as necessary for a stream to support fish and aquatic life. Trout streams may not have a DO level of less than 6.0 mg/L at any time, and may not have less than 7.0 mg/L DO during the spawning season.