Science practice 82

PASSAGE IV

When connection to a municipal water system is not feasible, wells are drilled to access ground water. Engineers employed by a company interested in developing a remote plot of land conducted studies to compare the water quality of 2 possible well locations on the land. Water quality is determined by a number of factors, including the levels of nitrates, lead, microbes, pH, "hardness" (calcium carbonate), and alkalinity. The water samples were kept at a constant temperature of 72°F throughout the study. The results in Table 1 show the readings of each test for the two different 100 mL samples of water, as well as the ideal level, or concentration, for each chemical.

The pH scale measures how acidic or basic a substance is on a scale of 0 to 14. Lower numbers indicate increasing acidity and higher numbers indicate increasing basicity. The normal pH level of groundwater systems is between 6 and 8.5. Water with a low pH (<6.5) could be acidic, soft, and corrosive, and could contain elevated levels of toxic metal that might cause premature damage to metal piping. Water with a pH >8.5 could indicate that the water is hard. Hard water does not pose a health risk, but can cause mineral deposits on fixtures and dishes and can have a bad taste and odor.

Alkalinity is the water's capacity to resist decreases in pH level. This resistance is achieved through a process called buffering (a buffered solution resists changes in pH until the buffer is used up). Alkalinity of natural water is determined by the soil and bedrock through which it passes. The main sources for natural alkalinity are rocks that contain carbonate, bicarbonate, and hydroxide compounds. These compounds, however, also cause hardness, which is less desirable in a drinking source. To illustrate the affect of alkalinity on pH stability, acid was added to two 100 milliliters sample solutions that initially had a pH of 6.5. The solution in Figure 1A had an alkalinity level of 200 mg/L while the solution in Figure 1B tested at zero alkalinity. The pH of the two solutions was recorded after every addition of acid and the results are shown in the figures below.

Figure 1