Developing a Water Quality Model

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Author: Josie, Grade 8


My problem is to correlate Gravel Run and Centreville Landing in terms of pH, dissolved oxygen, turbidity/clarity and air and water temperature. My project will help to determine the relationship between Gravel Run and Centerville Landing, or to see if the data I have been collecting moves together. It will be the first step in building a computer model of an estuary.

Background Information

Variables: Temperature - The temperature of the water is how hot or cold the water is. We measure the air and water temperature at each site using thermometers calibrated against NBS certified thermometers. The temperature of the water is especially important to calculate the dissolved oxygen saturation level.

Dissolved Oxygen - Dissolved Oxygen is essential for most organisms that live in the stream. Several things would have an effect on the DO of Centreville Landing. The first is temperature. When the temperature is warmer, the water can hold less oxygen. When the level of oxygen goes below 3 ppm it is harmful and deadly to most all life. When the temperature is colder the water can hold more oxygen, causing a higher reading. It is better to have it this way. Another factor for dissolved oxygen would be decomposition. This is because as things decompose in the stream, oxygen is used; thus there is less DO.

Something else that affects dissolved oxygen is respiration. Animals use oxygen to live. When testing for dissolved oxygen, it sometimes depends on what time of day you do it at. Levels are always the highest in early morning because plants have stopped creating oxygen at night (the sun goes away), but organisms have been using it all night long. Thus in the morning most of the oxygen produced by plants the day before is used and animals start dying.

Nitrates - Nitrates are the most common form of nitrogen. They are essential for growth; though too much from waste, decomposition, and fertilizer can be a problem. Nitrates are measured in parts per million or ppm. From past monitoring experience, I have known Gravel Run to have higher nitrates. This could be for many reasons, some being excess farm run-off, lawn fertilizers and sewage. Centreville has not had high readings since I started monitoring.

pH - The pH in water is how acidic or basic (alkaline) it is. It is measured on a scale from 0-14. Measurements from 0-7 show how acidic the water is. Zero is the most acidic and seven is neutral, or pure water. A measurement of pH from 7- 14 show how basic the water is, fourteen being the most basic. Some factors that might affect the pH are rainfall (if there is a lot of acid rain) and organism activity.

Turbidity/Clarity - Turbidity is how dirty or cloudy the water is. It is caused by bits of clay, silt, plankton and sewage suspended in water. You can use a turbidity test kit, which measures in JTU’s (Jackson Turbidity Units). Clarity is how clear the water is, measured with a secchi disk. We measure the clarity at Centreville Landing because the water is deep enough to use a secchi disk.

The Sites: Gravel Run is a freshwater stream that flows into Centreville Landing from the center of Queen Anne’s County.

Estuaries: Centreville Landing (on the Corsica River) is an estuary, where fresh water from Gravel Run (upstream) and brackish water from the Chester River (tidal water) mix.

The effects of the salt water on fresh water: When fresh and salt water mix together in an estuary, they sometimes form layers. This is called stratification. Because the density of the salt water is greater than that of the fresh, it will lie on the bottom. Physical forces such as tides, wind, waves and river runoff, contribute to the mixing of estuaries. In areas, like the Chesapeake Bay, with a smaller river flow and larger tides, more mixing takes place and less layering. The different variables from the Bay and Gravel Run determine the water quality at Centreville Landing. Most of the nitrates come from the fresh water. This is because there is more decomposition of plants and algae, more runoff from farmers’ fields and also animal and industrial waste. The turbidity is also greater in freshwater areas because of more algae, run-off, and dirt. The ocean water is more diluted, and most dirt does not get mixed up as much as it does in freshwater. All of the salinity is found in the ocean, caused by dissolved salts. The salinity of ocean water is about 35 ppt (parts per thousand). Although freshwater does contain salt, it is usually less than 1 ppt.

Correlation: Correlation is used to determine the relation between two properties. Correlation is between 1 and –1, with zero as no correlation, one as positive and – 1 as negative correlation. When the correlation is found between two sets if numbers, it is seeing how well they move together. When one number of the set goes up does the other go up (positive), does the other number go down (negative) or they don’t move together at all (zero).

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