11.3 Water Analysis and Naturally Occurring Dissolved Species
Groundwater taken from any given place contains a wide variety of dissolved constituents. We have many methods for analyzing water, both in the field and in the laboratory, which characterize the concentrations of dissolved solutes in the water. Here we discuss the parameters that are typically measured.
Basic Chemical Parameters
It is common practice to measure temperature, pH, alkalinity, total dissolved solids (TDS), and electrical conductance when analyzing groundwater.
- pH is defined as the inverse log of hydrogen ion activity in the water (activity is very similar to concentration). For example, a pH of 7 means that the activity (or concentration) of hydrogen in the water is equal to [latex]1\ x\ 10^{- 7}[/latex] mol/L. A pH of 5.6 means that hydrogen activity is equal to [latex]1\ x\ 10^{- 5.6}[/latex] mol/L.
- Alkalinity refers to the ability of the water to neutralize an acid. This is directly related to specific dissolved species in the water; for most groundwater systems the predominant acid‑neutralizing species is the bicarbonate ion. Alkalinity is therefore often used as a measurement of the concentration of bicarbonate in groundwater. Alkalinity is usually measured in the field, when the samples are taken, because exposure to air can cause some of the bicarbonate in solution to form carbon dioxide and leave the solution.
- Total dissolved solids represent the total concentration of dissolved constituents in the water; usually measured by evaporating a liter of the sample and measuring the weight of the remaining solids. TDS is measured in mg/L. Shallow groundwater (within a few hundred meters of the surface) generally ranges from 100 mg/L to 10,000 mg/L TDS (seawater is 35,000 mg/L). The standard limit for drinking water is generally considered to be less than 1,000 mg/L, with less than 500 preferable—although people can tolerate up to 2000 mg/L for short periods of time.
- Electrical conductance (or electrical conductivity) is a measure of the ability of the sample to conduct electricity. The units of electrical conductance are microsiemens per centimeter (μS/cm). Conductance is a proxy for TDS, since the electrical conductivity of water is a function of the amount of dissolved material in the water. Conductance is also a function of water temperature, so it is occasionally reported as specific conductance, which is the equivalent conductance at 25 °C.
In addition to these basic chemical parameters, we also analyze for the concentration of individual constituents in the water. Most of the dissolved constituents in groundwater are inorganic cations and anions derived from the dissolution of minerals. In any natural groundwater sample about 95 to 99 percent of the dissolved constituents (by weight) will consist of the following constituents.
- Bicarbonate
- Calcium
- Chloride
- Magnesium
- Silicon
- Sodium
- Sulfate
- Carbonic acid
These are usually referred to as the major constituents or primary constituents, and generally have concentrations greater than 5 mg/L. These constituents come directly from the dissolution of mineral phases in the subsurface, and a set of laboratory analyses will generally test for them.
Minor constituents, which generally occur in concentrations ranging from 0.1 to 10 mg/L, include the following constituents.
- Boron
- Carbonate
- Fluoride
- Iron
- Nitrate
- Potassium
- Strontium
In addition to these, there are a host of trace constituents that occur in concentrations less than 0.1 mg/L. Some of these are heavy metals that are toxic in small quantities. These constituents are generally not analyzed for except when looking for contamination or for investigating some specific geochemical process.
If a water sample is analyzed for the major and minor constituents, the analytical results will identify almost all the mass dissolved in the sample. There are two ways that we can check the analytical results, to ensure the quality of the analyses.
- The sum of the concentrations of the individual species, as determined by various lab procedures, can be compared to the total dissolved solids concentration as determined by evaporation in the lab. If the two are significantly different, then most likely there is a problem with the lab analyses.
- Water is electrically neutral; for every positive ion in solution, there is an equal number of negative charges to offset the charge. A charge balance (also called electroneutrality) can be calculated to see if the sum of the cations equals the sum of the anions. The charge balance (C.B.) is calculated by dividing the difference between the sum of the cation concentrations (in meq/L) and the sum of the anion concentrations (in meq/L) by the sum of the concentrations of all ions as shown by Equation (92).
| [latex]C.B. = \frac{(\Sigma_{cations} - \ \Sigma_{anions})}{(\Sigma_{cations} + \ \Sigma_{anions})}(100)[/latex] | (92) |
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Ideally, the charge balance should be within ±5 percent—allowing for analytical uncertainty and the trace elements that were missed in the analysis. Exercise 21⮧ provides an opportunity to assess the charge balance of the sample analysis evaluated in Exercise 20. Exercise 22⮧ provides an opportunity to assess the charge balance of a different sample.