6. How does geology affect the carbonate alkalinity of natural water samplés (2

Question

6. How does geology affect the carbonate alkalinity of natural water samplés (2 points)? olty answer this question, review the "pH and Alkalinity" factsheet by Addy et al. (2004) posted on the class D2L site. Also consider the type of minerals or rocks that contairn ions that contribute to carbonate alkalinity 7. Note any differences between the two precipitation samples in terms of pH and alkalinity. Recall that sample 1 was collected in a bucket from direct precipitation while sample 2 was collected from a roof downspout. What might explain these differences (2 points)?

Explanation / Answer

Carbonates are added to a water system if the water passes through soil and rock that contain carbonate minerals, such as calcite (CaCO3). Where limestone and sedimentary rocks and carbonate-rich soils are predominant, (such as the eastern part of the Boulder Creek watershed) waters will often have high alkalinity. Where igneous rocks (such as granite) and carbonate-poor soils are predominant (such as the western part of the Boulder Creek watershed) waters will have low alkalinity.

Carbonate-rich sedimentary rocks, principally formed from deposition of biogenic marine materials, are by far the most common geogenic source of alkalinity in stream water. Limestone contains predominantly calcite CaCO3, the commonest carbonate mineral, and Mg calcite (Ca,Mg)CO3 with trace amounts of aragonite, a polymorph of CaCO3. Diagenetic alteration of calcite, involving substitution of Mg for Ca, forms dolomite CaMg(CO3)2, another major source of carbonate. Magmatic and volcanic rocks are relatively insignificant sources of carbonate compared to sedimentary rock types. Khitarov and Rengarten (1956) quote values ranging from 200 to 900 mg kg-1 CO2 in granite, the carbonate being sourced from melt derived carbon dioxide. Intermediate rocks display very similar values to those of granite, although basaltic rock types have been quoted to contain as much as 5800 mg kg-1 CO2.

Alkalinity in most natural surface and groundwater is mainly derived from the dissolution of carbonate minerals, and from CO2 present in the atmosphere and in soil above the water table. Three carbonate species (H2CO3, HCO3- and CO32-) contribute to total alkalinity, their relative proportions being dependent on pH and temperature. At near-neutral values of pH, dissolved bicarbonate (HCO3-) is the dominant ion.

  Normal rain/ direct precipitation water has a pH of 5.6 and is slightly acidic. This is because the carbon dioxide gas that is in the air reacts with water to form a weak acid called carbonic acid. Acid rain is rain that has a pH lower than 5.6. Some reports of acid rain showed that its pH can be around 2, which is like the pH of vinegar. Thus, when rain mixes with another liquid or soaks into a material, it causes the liquid to become more acidic. This is especially the case for acid rain. Thus, rain would not increase the alkalinity of a substance, but would do the opposite and decrease it.

pH and alkalinity of water sample taken from rooftop rain water harvesting system will be higher than direct precipitation water.

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