Ground Water Resources of British Columbia
Chapter 3 — Ground Water Quality
Chemical Properties of Ground Water
by
W.S. Hodge
3.1 INTRODUCTION
The importance of ground water quality has become increasingly recognized as development of ground water continues to expand in British Columbia. Monitoring of ground water quality is becoming more important because of contamination concerns and development of new equipment and techniques for measuring contaminants in minute concentrations.
Although ground water is generally less susceptible to contamination than surface waters it is usually more highly mineralized in its natural state. As water moves slowly through the ground it can remain for extended periods of time in contact with minerals present in the soil and bedrock and become saturated with dissolved solids from these minerals. This dissolution process continues until chemical equilibrium is reached between the water and the minerals with which it is in contact. Important mineral constituents found in ground water and their relative abundance are listed in Table 3.1. The types and relative concentrations of the chemical constituents in ground water provide information on the evolution of ground waters, age (residence time), solubility, rates of movement, flow history and sources of recharge. Older ground waters for example are generally more mineralized than younger ground waters. Fresher ground waters are normally associated with recharge areas whereas ground waters in discharge areas are more mineralized. Ground waters can be classified according to the most dominant percentage of cations and anions being present based on concentrations in equivalents per million [epm] (e.g., calcium-magnesium bicarbonate type).
3.2 MINERAL CONSTITUENTS
The greater part of the soluble constituents in ground water comes from soluble minerals in soils and sedimentary rocks. The more common soluble constituents include calcium, sodium, bicarbonate and sulphate ions. Another common constituent is chloride ion derived from intruded sea water, connate water, evapotranspiration concentrating salts, and sewage wastes for example. Nitrate can be a natural constituent but high concentrations often suggest a source of pollution.
3.3 QUALITY CHARACTERISTICS
The measure of total dissolved solids (TDS) is a good indicator of the mineralized character of the water. Ground water having less than 500 mg/L of total dissolved solids is generally satisfactory for domestic and industrial use while ground water having greater than 1000 mg/L of total dissolved solids is generally unsatisfactory for these uses. High total dissolved solids are often indicative of other characteristics such as hardness.
Other properties that are especially useful in determining ground water character are hardness, specific conductance and pH. These constituents can be determined by simple procedures using field equipment.
In order to more precisely identify and measure the quality characteristics of ground water, chemical, physical and biological analysis are usually required. Chemical analysis require the laboratory determination of the concentrations of common ions found in ground water and are commonly reported in units of milligrams per litre (mg/L). Concentrations may also be expressed as equivalents per million (epm) which is the moles of solute multiplied by the valence of the solute species in 10,000,000 g of water. Properties of ground water often evaluated in a physical analysis include temperature, turbidity, odor, taste and colour. Biological analysis include coliform bacteria tests which indicates the sanitary quality of the water for human consumption. A list of recommended parameters for chemical analysis are given in Table 3.2. Additional parameters may be tested if a more detailed analysis is requested or where known or suspected sources of pollution exist.
Some substances even in small concentrations can be troublesome. For example, iron concentrations of 1 to 5 mg/L in ground water are common throughout British Columbia and can cause staining to plumbing fixtures and laundry, encrust well screens and clog pipes. Manganese in small concentrations can also cause staining and is even more objectionable as stains are harder to remove than those caused by iron. Chloride contamination is possible in wells located near to the sea where pumping of these wells can move sea water into the freshwater aquifer making water unpotable. Ground water containing dissolved hydrogen sulphide gas is another common problem which imparts a characteristic "rotten egg" odor and taste to the water. Hydrogen sulphide will combine with other impurities in the water to form iron sulphide (black water), calcium sulphide, sodium sulphide, and so forth. Table 3.3 lists these and other substances found naturally in some ground waters which can cause problems in operating wells.
While TDS, specific conductance, hardness and pH are good indicators of the character of ground water, tritium (3H) and carbon 14 (14C) which are radioactive isotopes are good indicators of the age of ground water. Between 1952 and 1962 large scale atmospheric testing of thermonuclear bombs was carried out and atmospheric contamination occurred. It is therefore apparent that ground water from a location in the northern hemisphere, containing tritium at levels of hundreds or thousands of TU (tritium units), entered the ground water zone after 1953. If the water has less than 5-10 TU it must have entered the ground water zone prior to 1953.
Two non radioactive isotopes which occur in water are oxygen 18 (18O) and deuterium (2H) and serve mainly as indicators of ground water source areas and as evaporation indicators in surface water bodies (Freeze and Cherry 1979).
3.4 WATER QUALITY STANDARDS
Water quality standards are needed to determine whether ground water of a certain quality is suitable for its intended use. Guidelines for Canadian Drinking Water Quality have been published by Health and Welfare Canada (1989). For irrigation water, quality is commonly expressed by classes of relative suitability, although most classification systems include units on specific conductance, sodium content and boron concentration. Table 3.4 provides a classification of water for irrigation (after Wilcox 1955).
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