Water Quality Testing Parameters 

Water quality can be assessed, directly and indirectly, by measuring a number of interrelated parameters:

Dissolved oxygen:
The concentration of dissolved oxygen (DO) in the water column provides a direct indication of the river's ability to support aquatic life such as fish and macroinvertebrates. Aquatic plants and bacteria in the sediments remove dissolved oxygen from the water when they respire. Because plants respire mainly at night, the lowest dissolved oxygen concentrations of the day occur in the early morning. During the day, plants add oxygen to the water column through photosynthesis. Both low morning DO concentrations and large changes in DO concentrations over the day (diurnal variation) are damaging to habitat.

Plants need a balance of two nutrients, phosphorus (P) and nitrogen (N), to grow. Phosphorus is measured as total phosphorus (TP) and ortho-phosphate (ortho-P; soluble inorganic phosphate, the form required by plants). In most fresh waters, the concentration of phosphorus available to plants is low enough that they cannot grow at their maximum rate. But in water bodies where human activities add phosphorus, much greater growth of aquatic plants can occur. In the Assabet, large blooms of duckweed and algae are the result of an excess of added nutrients. Algal blooms can significantly raise the pH (the balance between alkalinity and acidity) of the water. Extreme pHs are harmful to aquatic life; pH also affects how other pollutants such as heavy metals behave in the environment.

The other major nutrient supporting plant growth, nitrogen, is measured in its various forms as nitrate (NO3), ammonia (NH3), and total Kjeldahl nitrogen (TKN). Nitrate and ammonia are readily absorbed by plants and incorporated into proteins, amino acids, nucleic acids, and other molecules. Although most aquatic plant growth in rivers is limited by the availability of phosphorus, increased nitrogen availability can also lead to algal blooms. While ammonia can be readily utilized by plants, high concentrations of ammonia are toxic to fish and other aquatic life. A second effect of increased ammonia occurs when bacteria oxidize the NH3 to NO3, a process called nitrification, consuming four atoms of oxygen for every atom of nitrogen converted. This process can dramatically lower dissolved oxygen in the water.

Suspended solids and conductivity:
Excess nutrients in the Assabet come not only from wastewater treatment plant effluent, but also from non-point sources such as fertilizer runoff from lawns and golf courses and from sediments eroded at disturbed sites. Total suspended solids (TSS; the amount of silt, clay, organic material and algae in the water) gives us a measure of the inputs from erosion and the solids in effluent. Conductivity (the ability of the water to conduct a charge) increases with increasing concentrations of charged ions in the water and is a rough indicator of pollutants, such as untreated waste, entering the stream.

BOD and SOD:
Once in the water column, suspended solids are transported downstream and settle gradually, along with decaying plant matter, to form thick organic-rich sediments in the slower sections of the river. Biochemical oxygen demand (BOD) gives us a direct measure of the decomposition or oxidation processes in the water column. The more difficult-to-perform sediment oxygen demand (SOD) test measures the decomposition processes in the sediments.

Temperature affects the ecosystem in a number of ways: many organisms, especially cool water fish, are sensitive to high temperatures; the solubility of oxygen is lower in warmer water, decreasing the supply of dissolved oxygen; algae, weeds, and pathogenic microorganisms can all grow faster in warmer water.

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