7.6 Water Toxins

A toxin is a substance that causes injury to the health of a living thing, including the environment as a whole. Toxins that are typically found in water bodies include minerals such as lead, mercury, and arsenic; biological such as bacteria; or other pollutants such as acids, chemicals, and carbon dioxide. None of these elements have been found in hazardous concentrations in Pelican Lake or surrounding areas. Only fecal coliform is continually monitored.

7.6.1 Heavy Metals

According to one definition, the heavy metals are a group of elements between copper and lead on the periodic table of the elements. They have atomic weights between 63.546 and 200.590 and specific gravities greater than 4.0. Living organisms require trace amounts of some heavy metals, including cobalt, copper, manganese, molybdenum, vanadium, strontium, and zinc, but excessive levels can be detrimental to the organism. Other heavy metals such as mercury, lead and cadmium have no known vital or beneficial effect on organisms, and their accumulation over time in the bodies of mammals can cause serious illness.

Fig 7.12 Periodic table of elements with heavy metals highlighted.

A stricter definition restricts the term to those metals heavier than the rare earth metals, at the bottom of the periodic table. None of these are essential elements in biological systems; all of the more well-known elements with the exception of bismuth and gold are horribly toxic. Thorium and uranium are sometimes included as well, but they are more often called simply "radioactive metals".

In medical usage, the definition is considerably looser, and "heavy metal poisoning" can include excessive amounts of iron, manganese, aluminum, or beryllium (the second-lightest metal) as well as the true heavy metals.

Also, often the elements beyond mercury, e.g. the actinides such as uranium and plutonium, are not excluded from the heavy metals.

Lead

Lead is a poisonous metal that can damage nervous connections (especially in young children) and cause blood and brain disorders. Long term exposure to lead or its salts (especially soluble salts or the strong oxidant PbO 2 ) can cause nephropathy (kidney damage), colic-like abdominal pains.

Mercury

Mercury enters the environment as a pollutant from various industries:

• Coal-fired power plants are the largest source (40% of USA emissions in 1999, which have since declined by 85%).
• Industrial processes
• chlorine, steel, phosphate & gold production
• metal smelting
• manufacture & repair of weather and electronic devices
• incineration of municipal waste streams
• Medical applications, including vaccinations
• dentistry
• cosmetic industries
• Laboratory work involving mercury or sulfur compounds

Mercury also enters into the environment through the disposal (e.g., land filling, incineration) of certain products. Products containing mercury include: auto parts, batteries, fluorescent bulbs, medical products, thermometers, and thermostats. Due to health concerns toxics use reduction efforts are cutting back or eliminating mercury in such products.

One of the worst industrial disasters in history was caused by the dumping of mercury compounds into Minamata Bay , Japan . The Chisso Corporation, a fertilizer and later petrochemical company, was found responsible for polluting the bay from 1932-1968. It is estimated that over 3,000 people suffered various deformities, severe mercury poisoning symptoms or death from what became known as Minamata disease.

Mercury poisoning is the phenomenon of toxification by contact with mercury. Elemental, liquid mercury is slightly toxic, while its vapor, compounds and salts are highly toxic and have been implicated as causing brain and liver damage when ingested, inhaled or contacted. The main dangers associated with elemental mercury are that at standard conditions for temperature and pressure, mercury tends to oxidize forming mercury oxide, and that if dropped or disturbed, mercury will form microscopic drops, increasing its surface area dramatically.

Mercury is a bioaccumulative toxin that is easily absorbed through the skin, respiratory and gastrointestinal tissues. Inorganic mercury is less toxic than organic compounds (molecules containing carbon). Even though it is far less toxic than its organic compounds, elemental mercury still poses significant environmental pollution and remediation problems due to the fact that mercury forms such organic compounds inside living organisms.

Through bioaccumulation, methylmercury in the environment works its way up the food chain, reaching high concentrations among populations of some species such as tuna. Mercury poisoning in humans will result from persistent consumption of tainted foodstuffs. Larger species of fish, such as tuna or swordfish, are usually of greater concern than smaller species, since the mercury accumulates up the food chain. The MN DNR issues consumption advisories of fish for this reason.

Watersheds tend to concentrate mercury through erosion of mineral deposits and atmospheric deposition. Plants absorb mercury when wet but may emit it in dry air. Plant and sedimentary deposits in coal contain various levels of mercury.

7.6.2 Metalloids

Arsenic

Arsenic is a chemical element in the periodic table. This is a notorious poisonous metalloid. Arsenic can be naturally occurring; however it is mostly confined to wells.

Arsenic and many of its compounds are especially potent poisons. Arsenic kills by massively disrupting the digestive system, leading to death from shock.

Elemental arsenic and arsenic compounds are classified as toxic and dangerous for the environment in the European Union (EU).

The International Agency for Research on Cancer recognizes arsenic and arsenic compounds as group 1 carcinogens, and the EU lists arsenic trioxide, arsenic pentoxide and arsenate salts as category 1 carcinogens.

7.6.3 Pollutants

Acid mine drainage ( AMD )

Refers to the outflow of acidic water from (usually) abandoned metal mines. In many localities the liquor that drains from coal stocks, coal handling facilities, coal washeries, and even coal waste tips can be highly acidic, and in such cases it is treated as acid mine drainage.

Occurrence

Subsurface mining often progresses below the water table, in which case water must be constantly pumped out of the mine in order to prevent flooding. When a mine is abandoned, the pumping will cease and the water table will return to its former position, flooding the mine. The introduction of water is the initial step in most acid mine drainage situations. Tailings piles or ponds may also be a source of acid mine drainage.

Metal mines may generate highly acidic mine discharges where the ore is a sulfide or is associated with pyrites. In these cases the predominant metal ion may not be iron but may be zinc, copper, or nickel. The most commonly mined ore of copper, chalcopyrite, is itself a pyrite and occurs with a range of other sulfides. Thus, copper mines are often major culprits of AMD.

Metal sulfides (often pyrite) newly exposed to air and water are broken down into metal ions and sulfuric acid by colonies of bacteria and archaea. These microbes, called extremophiles for their ability to survive in harsh conditions, occur naturally in the rock, but limited water and air supplies usually keep their numbers low. Special extremophiles known as acidophiles especially favor the low pH levels of abandoned mines. Thiobacillus ferrooxidans in particular has been identified as a key contributor to the oxidation of pyrites.

Biotic processes far outpace the slower abiotic process of pyrite oxidation.

Chemistry

The chemistry of oxidation of pyrites, the production of ferrous ions and subsequently ferric ions, is very complex, and this complexity has considerably inhibited the design of effective treatment options.

Although a host of chemical processes contribute to AMD, pyrite oxidation is by far the greatest contributor. A general equation for this process is:

4FeS 2 (s) + 14O 2 (g) + 4H 2 O(l) ? 4Fe 2+ (aq) + 8SO 4 2- (aq) + 8H + (aq)

The solid pyrite, when introduced to oxygen and water, is catalyzed to form Iron(II) ions, sulfate ions, and hydrogen ions. The hydrogen ions bind to the sulfate ions to produce sulfuric acid.

Effects

In some AMD systems temperatures reach 120 degrees Fahrenheit (50 °C), and the pH can be as low as -3.6. AMD-causing organisms can thrive in waters with pH very close to zero. Negative pH occurs when water evaporates from already acidic pools thereby increasing the concentration of hydrogen ions.

When the pH of AMD is raised past 3, either through contact with fresh water or neutralizing minerals, soluble Iron(III) ions hydrolyze to form Iron(III) hydroxide, a yellow-orange solid colloquially known as Yellow boy. Yellow boy discolors water and smothers plant and animal life on the streambed, disrupting stream ecosystems. The process also produces additional hydrogen ions, which can further decrease pH.

Treatment

In the United Kingdom , many discharges from abandoned mines are exempt from regulatory control. In such cases the Environment Agency working with partners has provided some innovative solutions, including constructed wetland solutions such as on the River Pelena in the valley of the River Afan near Port Talbot.

Although abandoned underground mines produce most of the AMD, some recently mined and reclaimed surface mines have produced AMD and have degraded local ground-water and surface-water resources. Acidic water produced at active mines must be neutralized to achieve pH 6-9 before discharge from a mine site to a stream is permitted.

In Canada , work to reduce the effects of AMD is concentrated under the Mine Environment Neutral Drainage (MEND) program. Over a period of eight years, MEND claims to have reduced AMD.

Carbonate Neutralization

Generally, limestone or other calcareous strata that could neutralize acid are lacking or deficient at sites that produce acidic mine drainage. Limestone chips may be introduced into sites to create a neutralizing effect. Where limestone has been used, such as at Cwm Rheidol in mid Wales , the positive impact has been much less than anticipated because of the creation of an insoluble calcium sulfate layer on the limestone chips, blinding the material and preventing further neutralization.

Ion exchange

Cation exchange processes were investigated as a potential treatment for AMD. Not only would ion exchangers remove potentially toxic heavy metals from mine runoff, there was also the possibility of turning a profit off of the recovered metals. However, the cost of ion exchange materials compared to the relatively small returns, as well as the inability of current technology to efficiently deal with the vast amounts of mine discharge, renders this solution unrealistic at present.

Constructed Wetlands

Constructed wetlands systems have shown promise as a more cost-effective treatment alternative to artificial treatment plants. A spectrum of bacteria and archaea, in consortium with wetland plants, may be used to filter out heavy metals and raise pH. Anaerobic bacteria in particular are known to be capable of reverting sulfate ions into sulfide ions. These sulfide ions can then bind with heavy metal ions, precipitating heavy metals out of solution and effectively reversing the entire process.

Interestingly, T. ferrooxidans (the very bacteria which appears to be the problem) has also been shown to be effective in treating heavy metals in constructed wetland treatment systems.

The attractiveness of a constructed wetlands solution lies in its passivity - building an artificial wetlands is a relatively cheap one-time investment which continuously works to reduce acidity and heavy metal concentration. Although promising, constructed wetlands take much time to completely cleanse an area, and are simply not enough to deal with extensively polluted discharge. Constructed wetland effluent often requires additional treatment to completely stabilize pH. Also, the products of bacterial processes are unstable when exposed to oxygen, and require special disposal to ensure no further contamination. Other issues include seasonal variation in the activity of cleansing organisms, as well as the lack of a practical passive means of moving mine discharge through the most efficient regions of purification.

Alkalinity

Alkalinity is measured in mg/l as calcium carbonate (CaCO 3). It represents a measure of a solution's ability to buffer or neutralize acids. Lakes located in areas of calcareous glacial till (common throughout central and southern Minnesota ) will have higher alkalinity than lakes formed on non-calcareous bedrock (common in northeastern Minnesota ). Water with alkalinity less than about 75 mg/L could be considered soft, 76-150 moderately hard, 151-300 hard, and greater than 300 very hard. Alkalinity has also been used as a basis for estimating sensitivity to acid precipitation. For this purpose, lakes with alkalinity values less than 5 to 10 mg/L could be considered potentially sensitive to acid precipitation based on current levels of deposition across Minnesota. At this point we have identified no "culturally acidified" lakes in Minnesota.

Next page: Chapter 7.6.4 Biological Toxins