Water Quality Testing

This activity was written especially for the Vernier hands-on workshop. It is an introduction to three of the more popular water quality tests available in the lab manual Water Quality with Vernier. The introduction below is the introduction for the Total Dissolved Solids Test. This introduction typifies the detailed introductions that are available for all of the water quality tests, including the Temperature and pH Tests.

INTRODUCTION—Total Dissolved Solids

Solids are found in streams in two forms, suspended and dissolved. Suspended solids include silt, stirred-up bottom sediment, decaying plant matter, or sewage-treatment effluent. Suspended solids will not pass through a filter, whereas dissolved solids will. Dissolved solids in freshwater samples include soluble salts that yield ions such as sodium (Na⁺), calcium (Ca²⁺), magnesium (Mg²⁺), bicarbonate (HCO₃^–), sulfate (SO₄^2– ), or chloride (Cl^– ). Total dissolved solids, or TDS, can be determined by evaporating a pre-filtered sample to dryness, and then finding the mass of the dry residue per liter of sample. A second method uses a Vernier Conductivity Probe to determine the ability of the dissolved salts and their resulting ions in an unfiltered sample to conduct an electrical current. The conductivity is then converted to TDS. Either of these methods yields a TDS value in units of mg/L.

The TDS concentration in a body of water is affected by many different factors. A high concentration of dissolved ions is not, by itself, an indication that a stream is polluted or unhealthy. It is normal for streams to dissolve and accumulate fairly high concentrations of ions from the minerals in the rocks and soils over which they flow. If these deposits contain salts (sodium chloride or potassium chloride) or limestone (calcium carbonate), then significant concentrations of Na⁺, K⁺, Cl^– will result, as well as hard-water ions, such as Ca²⁺ and HCO₃^– from limestone.

TDS is sometimes used as a “watchdog” environmental test. Any change in the ionic composition between testing sites in a stream can quickly be detected using a Conductivity Probe. TDS values will change when ions are introduced to water from salts, acids, bases, hard-water minerals, or soluble gases that ionize in solution. However, the tests described here will not tell you the specific ion responsible for the increase or decrease in TDS. They simply give a general indication of the level of dissolved solids in the stream or lake. Further tests described in this book can then help to determine the specific ion or ions that contributed to changes in the initial TDS reading.

There are many possible manmade sources of ions that may contribute to elevated TDS readings. Fertilizers from fields and lawns can add a variety of ions to a stream. Increases in TDS can also result from runoff from roads that have been salted in the winter. Organic matter from wastewater treatment plants may contribute higher levels of nitrate or phosphate ions. Treated wastewater may also have higher TDS readings than surrounding streams if urban drinking water has been highly chlorinated. Irrigation water that is returned to a stream will often have higher concentrations of sodium or chloride ions. Acidic rainwater, with dissolved gases like CO₂, NO₂, or SO₂, often yields elevated H⁺ ion concentrations.

If TDS levels are high, especially due to dissolved salts, many forms of aquatic life are affected. The salts act to dehydrate the skin of animals. High concentrations of dissolved solids can add a laxative effect to water or cause the water to have an unpleasant mineral taste. It is also possible for dissolved ions to affect the pH of a body of water, which in turn may influence the health of aquatic species. If high TDS readings are due to hard-water ions, then soaps may be less effective, or significant boiler plating may occur in heating pipes.

Expected Levels

TDS values in lakes and streams are typically found to be in the range of 50 to 250 mg/L. In areas of especially hard water or high salinity, TDS values may be as high as 500 mg/L. Drinking water will tend to be 25 to 500 mg/L TDS. United States Drinking Water Standards¹ include a recommendation that TDS in drinking water should not exceed 500 mg/L TDS. Fresh distilled water, by comparison, will usually have a conductivity of 0.5 to 1.5 mg/L TDS.

Materials Checklist

Temperature—Testing Procedure

1. Connect the Temperature Probe to LabQuest. Choose New from the File menu.

2. Collect temperature data.

a. Place the tip of the Temperature Probe directly into the stream at Site 1 (or into a cup containing a sample taken from Site 1). Submerge the probe tip to a depth of about 10 cm and hold for 30 seconds.

b. Record the temperature value on the Data & Calculations Sheet (round to the nearest 0.1°C).

pH—Testing Procedure

1. Unplug the Temperature Probe. Connect the pH Sensor to LabQuest.

2. Collect pH data.

a. Remove the pH Sensor from the storage bottle. Rinse the tip of the sensor thoroughly with the stream water.

b. Place the tip of the sensor into the stream at Site 1, or into a cup with sample water from the stream. Submerge the sensor tip in the stream or in a cup to a depth of 3–4 cm.

c. Record the pH value on the Data & Calculations sheet.

d. Rinse the sensor with distilled water and return it to the storage bottle when you have finished collecting your data.

Total Dissolved Solids—Testing Procedure

1. Set the switch on the Conductivity Probe box to 0–2000 µS (2000 µS = 1000 mg/L TDS).

2. Unplug the pH Sensor. Connect the Conductivity Probe to LabQuest.

3. Choose Change Units►Conductivity►mg/L from the Sensors menu.

4. Collect TDS concentration data.

a. Place the tip of the electrode into a cup with sample water from the body of water you are testing. The hole near the tip of the probe should be covered completely.

b. Record the conductivity value on the Data & Calculations sheet (round to the nearest 1 mg/L TDS).

DATA & CALCULATIONS

Stream or lake:

Date:

Site name:

Time of day:

Student name:

Student name:

Student name:

Field Observations (e.g., weather, geography, vegetation along stream)

1. Established by 1986 Amendments to the Safe Drinking Water Act.