3.  TOXICOLOGY of WATER

    Many contaminants (both natural and man-made) find their way into water systems, either directly, as run-off (leachate) from polluted land, or in precipitation. While the specific toxicity of various agents applies (i.e. LD₅₀ / ED₅₀), there usually is ample opportunity for compounds to not only disperse over a large area, but to also undergo a range of chemical modifications in the aqueous environment.

i. Aquifers vs. Open Systems

"Groundwater" refers to waters beneath the ground and comprises less than 1% of the worlds' fresh water. When ground water is trapped (by rocks or clay) forming a permanent underground reservoir it is referred to as an aquifer. Many communities and rural residents rely on wells to tap into underground water supplies. If an aquifer is trapped beneath a layer of impermeable rock it is referred to as an artesian well. Such underground waters are often considered to be very pure, since they have been filtered through the soils and removed from microbial degradation for many years. Unless disturbed, these wells often maintain highly pristine water indefinitely.

The reality is however, that groundwater contamination from a variety of potentially harmful organic compounds and pesticides has been occurring for many years. Leaching from agricultural run-offs, chemical waste dumps, landfills, underground gas tanks and many other sources all contribute to groundwater contamination.

While many large industries are now aware of potential hazards of aquifer contamination (with respect to environmental damage and clean-up costs) many farms, smaller industries and even small municipalities continue to contribute to the problem.

Open, free-flowing water systems do not have the soil-filtration or reduced access seen in aquifers: they are susceptible to pollutants in the form of run-off, leachate and precipitation. However, open systems can also be fairly dynamic and may have levels of incoming fresh water not found in aquifers. Generally, it is important to consider that all water systems are in fact finite and there are very real limits to the presence of any toxic agent.

ii. Nitrates

Nitrates (NO3-) (and other nitrogen compounds) are a serious concern for water systems and can enter groundwater and free-flowing systems as a result of agricultural over-use of nitrogen-fertilizers, from microbial activity, as a result of the decomposition of sewage/manure, and from atmospheric conversion. Removal of excessive nitrate from groundwater is prohibitively expensive, rendering the groundwater unusable for human consumption. For example, levels of nitrate as low as 4 ppm have been associated with elevated incidence of lymphoma and stomach cancer.

iii. Sulfur Compounds

Sulfur compounds have become more and more of a concern since "acid rain" (containing acid / sulfur compounds) produced by numerous large industries readily enters water systems.Water systems near large industries with such emissions often exhibit elevated levels of sulfur compounds (i.e. Great Lakes). Decomposition of sulfur compounds and anaerobic decay produce a range of compounds including hydrogen sulfide, dimethyl sulfide, methanediol and other compounds. Stagnant waters with high levels of these compounds have a characteristic foul odour.

iv. Water Quality Measurements

There are a number ways to examine water quality. One consideration is oxygen demand which reflects the removal of available dissolved oxygen by various agents. Highly active water systems (fast flowing rivers, etc.) are highly aerated and less susceptible to a loss of available oxygen. However stagnant water systems with a high level of "organic load" (dead plant tissue and other organic matter) can be severely depleted of oxygen to the point where fish populations can be wiped out. Oxygen demand is divided into two categories: BOD (biological oxygen demand) the level of demand/removal of oxygen from a water system by the organic materials present., and COD: (chemical oxygen demand), a chemical measurement using dichromate ions to measure residual oxygen levels. Alternatively, measurements of TOC (total organic carbon) and DOC (dissolved organic carbon) can be used to indicate the level of organic loading on a water system. Consumption of oxygen is exasperated by stagnation, high microbial loads, high levels of sedimentation and through the decay / pollution from organic material.

Alkalinity is another measurement of water quality, often viewed as an indicator of potential resistance to acidification (i.e. from acid rain) or as an indicator of the ability of a water system to sustain plant growth. Acidic water systems tend to suppress certain types of microbial activity which normally result in the production of oxygen. With insufficient oxygen many systems cannot operate exist.

The level of cations present in water systems is described as hardness, which indicated the levels of ions present (Ca²⁺, Mg²⁺). Limestone and other mineral deposits are a main source of elevated water hardness.

Microbial Contamination, or the number of potentially harmful microorganisms (i.e. coliforms) is often an indicator of water quality. Different municipalities establish different safety limits for such potentially pathogenic microorganisms, often requiring <10 CFU/ml in potable water. While inappropriate microbial loading can lead to many potentially fatal diseases, (often seen in countries with little or no water treatment) it is often an indicator of the general level of water contamination with organic materials, sewage, and specific chemicals which supports such growth. It would be unrealistic to consider the simple presence of microbes in a water system as a pollution issue: microorganisms exist in virtually every ecosystem on the planet. It is the concentration and type of microorganisms and the existence of conditions promoting excessive microbial growth which should be of concern.

Reference - Canada's Freshwater Policies: http://www.ec.gc.ca/water/e_main.html

v. Water Purification

Most urban water in industrialized nations undergoes some form of treatment; either to reduce microbial load, remove high levels of dissolved solids, adjust the pH or remove unpleasant taste / odours.

Typical drinking water treatment often involves several stages: settling to allow excessive suspended solids to precipitate (often assisted through addition of ions of iron, or aluminum), aeration, to lower levels of sulfur-gases, addition of phosphate (if required) to reduce hardness, disinfection through the addition chlorine or ozone and possible PAC (activated carbon) treatment to reduce off smells and absorb trace levels of undesirable compounds. Following water treatment the levels of residual chlorine (free chlorine) are often used as an indicator of adequate treatment, since the chlorine ions will have been consumed by organic / chemical loading until all such reactions are complete. The presence of a residual chlorine level indicates a lack of further contaminants to interact with and is often measured after treatments are complete. A recent alternative treatment involves the use of ozone to kill microorganisms and inactivate many compounds. For waters that are too acidic lime may also be used to elevate the pH.

Treatment of sewage involves other challenges: there is obviously an extremely high organic load on sewage and a mixture of microbial and chemical contaminants which must be dealt with. Sewage treatment involves several major steps: primary treatment to separate grease (surface) and sludge (heavy precipitated materials) which is allowed to settle, secondary treatment with appropriate microbial degradation reducing contaminants and producing more sludge, and tertiary treatment to remove or add specific chemicals to meet biological and chemical load requirements prior to discharge into a water system.