Alkalinity: An Important Parameter in Assessing Water Chemistry

Introduction

Ground water is the source of drinking water for about 60 percent of Maine's population - 43 percent from private wells and 17 percent from public water supplies (Solley and others, 1995). In spite of this reliance on ground water, very little is currently known about natural ground-water conditions in different bedrock geological settings across the state. In response to numerous concerns about the quality of Maine's ground water, the Maine Geological Survey started investigating Maine's ambient bedrock water quality and associated geochemistry. A long-term monitoring program was initiated to create an ambient ground-water quality database with applications in public health, environmental protection and permitting, economic development, and basic research.

Thus far, 214 bedrock wells have been sampled in the Camden Hills area of coastal Maine, the Range Pond area of south-central Maine, the Presque Isle area in Aroostook County, the Mount Blue area in western Maine, and the Blue Hill, Penobscot, and Ellsworth areas. These samples were analyzed for all of the common water quality constituents as well as trace metals.

Alkalinity and pH

Alkalinity and pH are among the most common and important water quality parameters measured in both surface and ground water. Why is this important in our everyday lives? Alkalinity, the buffering capacity of water or its resistance to pH change, is critical in the proper metabolism of most forms of life, maintenance of aquatic life forms, understanding the geochemical nature of water, and how best to deal with problems associated with both drinking water and waste water. Alkalinity of surface and ground waters is directly related to the underlying sediment and bedrock (Weddle and Loiselle, 1996). For example, areas underlain by rocks and sediment rich in calcium from limestone would tend to be more resistant to changes in pH caused by acid rain. On the other hand, water bodies underlain by granite and sediment containing little calcareous material would tend to have less resistance to pH changes. Alkalinity is also an important factor in maintaining such things as aquariums and swimming pools to name just a few. The bottom line is that alkalinity plays an essential role in all aspects of aquatic chemistry.

Alkalinity is often related to hardness because the main source of alkalinity is usually from carbonate rocks (limestone) which is mostly CaCO₃. Hard water makes nearly every cleaning job in the home a nuisance since more soap and detergent are necessary. Soap used in hard water combines with the minerals to form a sticky soap curd. Dishes washed in hard water often appear spotted, clothes often appear dingy or feel scratchy, and scale deposits appear on plumbing fixtures and water-using appliances. Whenever hard water is heated, scale deposits of calcium and magnesium minerals occur which can clog pipes or adhere to heating elements. Hard water is not unsafe to drink, but practical uses may be limited. Water softeners are used to treat hard water.

Alkalinity is measured by titration. In some instances, scientists add small amounts of chemicals known as acid-base indicators to water samples to indicate end points as acid is incrementally titrated. Certain acid-base indicators result in color changes when an end point is reached. End points occur when only small amounts of acid are added to impart a significant change in pH. Knowing the volume of the sample, normality of the acid, and the volume of acid needed to reach an endpoint, the alkalinity can be calculated.

Determining alkalinity using the various acid-base indicators works reasonably well with highly buffered waters, but with low alkalinity water, full incremental titrations are the preferred approach. There are a number of methods of handling the data which include the inflection point method, Gran function plot method, and fixed endpoint method. For a detailed description of proper water quality sampling techniques as well as further information on alkalinity titration methodology, refer to the National Field Manual for the Collection of Water-Quality Data as well as Chapter 6 - Alkalinity and Acid Neutralizing Capacity.

Ground-water sampling at the Maine Geological Survey

When collecting ground water samples, the Maine Geological Survey utilizes a closed cell for measuring important field parameters such as pH, temperature, specific conductance, dissolved oxygen, and oxidation-reduction potential (Figure 1). Inflow is controlled by a needle valve and maintained at a rate of approximately 0.5 gallons per minute. All parameters are noted every five minutes over typically a 30 minute period to verify that stabilization has occurred. At this point, samples for laboratory and field analyses are collected. Most samples (including those for alkalinity analyses) are filtered using a disposable 0.45 µm inline filter (Figure 2). Samples for alkalinity analyses are typically examined both in the field and in the laboratory using the Gran function plot method.

Figure 1

Figure 2

Figure 3

In an effort to enhance collection of high quality field alkalinity data, staff at the Maine Geological Survey designed a self-contained alkalinity field titration device utilizing a digital titrator, pH meter with electrodes, and battery-operated variable speed magnetic stirrer. With this device, we are able to conduct the titration shortly after sample collection in an effort to avoid chemical and physical reactions which can alter carbonate and bicarbonate concentrations in a matter of minutes. In some instances, field titration immediately following sample collection is not possible. In such cases, samples are collected in 250 mL containers with no headspace and maintained at a temperature of 10° C until the procedure can be accomplished (as soon as possible). For an illustration of the MGS field titration device, refer to Figure 3.

Titration data are collected for analysis using the Gran function plot method. In the past, data was examined using a relatively simple spreadsheet analysis for total alkalinity, but no speciation of carbonate, bicarbonate, and hydroxide was performed. Fortunately, the U.S. Geological Survey developed what is known as the "Alkalinity Calculator" which takes titration data and performs all calculations using a variety of methods.

The collection and analysis of ambient bedrock water quality information by the Maine Geological Survey is continuing and a summary report detailing findings will be published in the future.

Sources of Additional Information

Hem, J.D., 1985, Study and interpretation of the chemical characteristics of natural water (3rd edition): U.S Geological Survey Water-Supply Paper 2254, 263 p.

Solley, W. B., Pierce, R. R., and Perlman, H.A., 1998, Estimated Use of Water in the United States, 1995: U.S. Geological Survey, Circular 1200, 71 p.

Weddle, T. K., and Loiselle, M. C., 1996, Background water quality in significant sand and gravel aquifers in Maine, in Loiselle, M., Weddle, T. K., and White, C. (editors), Selected papers on the hydrogeology of Maine: Geological Society of Maine, Bulletin 4, p. 53-80.

Text and photos by Daniel B. Locke

Last updated on January 13, 2009