Acid Rain and the Buffering Capacity of Soils

An Introduction to pH

The pH of a liquid, soil, or other substance indicates whether it is acidic, neutral, or basic.

The pH scale ranges from 0 (very acid) to 14 (very alkaline or basic). On the scale, a 7 is neutral, an acid is lower than 7, and a base is higher than 7. When an acid and base react, neutralization occurs. The result is a solution that is not as acidic or basic as the original substance. It is important to note that the pH scale is logarithmic, in other words, every one-unit change in pH represents a ten-fold change in acidity or alkalinity. For example, a pH of 6 is ten times more acidic than a pH of 7, while a pH of 5 is one hundred times more acidic than a pH of 7.

What is Acid Rain?

Acid rain is only one of several forms of acid precipitation. Other forms of acid precipitation include snow, sleet, hail, and even fog. Precipitation that has a pH value of less than 5.6 is considered to be abnormally acidic.

What Causes Acid Rain?

Acid Rain is formed from the air pollutants, sulfur dioxide, and nitrogen oxides, which are released as gases into the atmosphere during the refining of metal ores and the burning of fossil fuels such as coal, heating oil, and gasoline. Power plants, smelters, automobiles, and even volcanic activity can contribute to this problem. These gases may travel miles from their sources. Eventually, they combine with moisture in the atmosphere to form sulfuric acid and nitric acid solutions that fall to earth in rain, snow, sleet, fog, and mist.

What are the Effect of Acid Rain on Lakes, Streams, and Ponds?

A healthy, productive freshwater lake has a pH value of about 8, which is slightly basic. The effect of acid precipitation on an aquatic ecosystem can be devastating, including the loss of sensitive species of aquatic macro invertebrates, failure of fish eggs to hatch and develop, the loss of plankton and native aquatic vegetation, and the invasion of acid-tolerant species of vegetation. All of this leads to a dynamic alteration of the ecosystem. The diversity and abundance of existing species of plants and animals rapidly decline.

What is Buffering Capacity

The pH level of a lake, stream, or pond is maintained at a steady level by the presence of "buffering" chemicals in the water. The amount of basic buffering materials in the water is termed the "alkalinity" of the water. The alkalinity of the water does NOT refer to pH, but rather the ability of the water system to resist changes in pH. This resistance to changes in pH is otherwise known as "buffering capacity".

The buffering chemicals in a lake, stream, or pond provide an indication to the types of soils, minerals, and rocks in the area. As increasing amounts of acids are added to the aquatic ecosystem, its buffering capacity is consumed. If additional buffering material is obtained from the surrounding soils and rocks, the alkalinity level may eventually be restored.

What is the Buffering Capacity of Soil and the Effocts of Acid Rain?

The pH of soil is critical to the health of vegetation and soil microorganisms. The pH of soil determines the availability of nutrients to plants. When acid rain infiltrates the soil, important nutrients are leached away from the soil through runoff, into the aquatic environment.

The buffering capacity of a soil depends on several factors, including the soil's organic and mineral content, as well as the soil's physical properties, including density and particle size. Generally, soil can provide a neutralizing effect to acid rain, however, after prolonged exposure to acid rain, a soil will loose its buffering capacity. The ability of soil to buffer acid rain is dependent upon several factors, including soil type, the pH of acid precipitation events, and the duration and frequency of these events. Soils that contain alkaline material such as limestone or calcite, have a better ability to neutralize the acid rain, while soils containing material such as granite rocks provide a poor buffer to the acid rain.

Soil acidification can also inhibit helpful soil bacteria. With the increasing loss of this bacteria, nitrogen fixation and nutrification of the soil is limited and the breakdown of beneficial organic matter is slowed down.

An Experiment

An Investigation into the Interactions of Soil and the pH of Rainwater

GRADE RECOMMENDATION: Grades 5-12

OBJECTIVE: Students will examine the buffering capacity of soil.

NOTE: The following is an outline of a simple introductory experiment. No exact quantities or measurements are given for either the soil or rainwater samples. Sample sizes are variables and should be determined by your experiment. Several experiment ideas are presented in the HELPFUL EXPERIMENT IDEAS section.

What you will need:

• pH Test Kit*
  
• Soil sample
  
• 2L Soda Bottle
  
• Coffee Filters
  
• Scissors
  
• Rainwater or Rainwater Substitute (Distilled Water, this can be found in gallon containers in the beverage section of the supermarket), and
• Vinegar

*Recommended LaMotte pH Test Kits:

• pH Module Test Kit; code #5890
• Science Project Kit, pH; code #3-5873
• Wide Range pH Test Kit; code #2117
• Water and Soil Introductory Study Kit; code #5685 (10 tests) or code #5686 (100 tests).
• LaMotte pH 5 meter; code #5-0034

Set-up

1. Remove the label from a 2L soda bottle.
2. Use scissors to cut the 2L soda bottle into two separate halves (see diagram). The top half of the bottle is the FUNNEL. The bottom half is the COLLECTION CONTAINER. Remove the cap from the funnel. Do not discard cap.
3. Collect a soil sample from the schoolyard, backyard, garden, etc.
4. Remove any sticks, rocks, or vegetation from the soil sample.
5. Collect rainwater sample.

Rainwater Collection

1. Collect rainwater in either a clean glass or plastic container. DO NOT use an aluminum or tin container. A large, white bucket with a handle works great. If it is not possible to collect rainwater, the following Rainwater Substitute can be used:

Rainwater Substitute: Measure 5 tablespoons of vinegar and add to 1 gallon of distilled water.

Procedures

1. Follow the instructions of your pH water test kit to measure the pH of the RAINWATER SAMPLE. Record data.
2. Place two COFFEE FILTERS into the FUNNEL (see diagram).
3. Carefully pour the soil sample into the coffee filters in the FUNNEL. Be careful not to pour the soil in between the funnel and the coffee filters.
4. Hold FUNNEL directly over the COLLECTION CONTAINER.
5. Slowly pour the RAINWATER SAMPLE into the FUNNEL. Again, keep the soil/rainwater solution from pouring over the sides of the coffee filters and getting in between the filters and the funnel.
6. Allow the entire rainwater sample to flow through the filter and collect in the COLLECTION CONTAINER.
7. It may be necessary, depending on soil type, to filter twice. The filtered rainwater sample should be fairly colorless.
8. Measure the pH level of the filtered rainwater sample, record, and compare this to the pH measurement of the unfiltered rainwater sample.

Helpful Experiment Idea

There can be MANY different variations to this experiment. Below are some helpful hints to further examine the buffering capacity of soils.

• SOIL TYPES. Identify, collect, and compare different soil types and textures with their ability to buffer equal amounts of a rainwater sample (i.e., compare sand, clay, peat, silt, potting soil, etc). Record and graph the results.
• RELIABILITY/FREQUENCY. Examine the ability at which a soil sample can consistently buffer equal amounts of a rainwater sample. Repeatedly pour an equal volume of unfiltered rainwater through the SAME soil sample (you are replicating frequent acid rain events). Determine that point at which a change in the pH of the filtered rainwater occured. Did the pH decrease? Record and graph the results.
• LENGTH OF EXPOSURE. Vary the amount of time that a rainwater sample is exposed to a soil sample before uncapping the funnel. Compare the pH of each filtered rainwater sample between different exposure times. Record and graph the results.
• SOIL DEPTH. Vary the amount of soil placed into the COFFEE FILTERS in the FUNNEL. Compare the depth of the soil to its ability to buffer an equal amount of a rainwater sample. Record, graph, and compare results.
• pH SOLUTIONS. Determine the difference in the buffering capacity of a soil sample between the pH of a rainwater sample and the pH of a lemon juice solution.
• SOIL AND WATER TESTING INTEGRATION. Test different parameters of both the soil and the natural rainwater sample before and after filtration.