pH is used to indicate the acidity of a substance as ranked on a scale from 1.0 to 14.0. Acidity increases as the pH gets lower. pH affects many chemical and biological processes in the water and different organisms flourish best within different ranges of pH. Most aquatic animals prefer a range of 6.5-8.0. Having a pH outside this range reduces the diversity in the stream because it stresses the physical systems of most organisms and can reduce reproduction. Low pH can allow toxic elements and compounds to become mobile and "available" for uptake by aquatic plants and animals, producing conditions that are toxic to aquatic life.
Changes in acidity can be caused by atmospheric deposition (acid rain), surrounding rock, and certain wastewater discharges.
Since the scale is logarithmic, a drop in the pH by 1.0 unit is equivalent to a 10-fold increase in acidity. So, an increase of pH of 0.7, from 6.8 at the upstream site to 7.5 to the bridge site (shown in Figure 1) means that there was a 5-fold increase in acidity from the first site to the second. A 5-fold change from one location to another, spaced only 0.5 miles apart is adequate to cause a negative effect on fish swimming through.
Both phosphorus and nitrogen are essential nutrients for the plants and animals that make up the aquatic food web. Since phosphorus is the nutrient in short supply in most fresh waters, even a modest increase in phosphorus can, under the right conditions, set off a whole chain of undesirable events in a stream including accelerated plant growth, algae blooms, low dissolved oxygen, and the death of certain fish, invertebrates, and other aquatic animals.
There are many sources of phosphorus, both natural and human. These include soil and rocks, wastewater treatment plants, runoff from fertilized lawns and cropland, failing septic systems, runoff from animal manure storage areas, disturbed land areas, drained wetlands, water treatment, and commercial cleaning preparations.
The following chart is used to analyze phosphate values:
Phosphate Level (ppm) Description of Condition
0.01 - 0.03 ppm the level in uncontaminated lakes
0.025 - 0.1 ppm level at which plant growth is stimulated
0.1 ppm maximum acceptable to avoid accelerated eutrophication
> 0.1 ppm accelerated growth and consequent problems
(table from Water, Water Everywhere. HACH Company. Second Edition. 1983.)
The chart shows that all three measuring sites have phosphate levels that are much higher than would be ideal. Of the three values, only the upstream and the downstream values are below the maximum acceptable phosphate level to avoid accelerated eutrophication.