Aquatic Chemistry

views updated May 09 2018

Aquatic chemistry

Water can exist in various forms within the environment , including: (1) liquid water of oceans, lakes and ponds, rivers and streams, soil interstices, and underground aquifers; (2) solid water of glacial ice and more-ephemeral snow, rime, and frost; and (3) vapor water of cloud, fog, and the general atmosphere . More than 97% of the total quantity of water in the hydrosphere occurs in the oceans, while about 2% is glacial ice, and less than 1% is groundwater . Only about 0.01% occurs in freshwater lakes, and the quantities in other compartments are even smaller.

Each compartment of water in the hydrosphere has its own characteristic chemistry. Seawater has a relatively large concentration of inorganic solutes (about 3.5%), dominated by the ions chloride (1.94%), sodium (1.08%), sulfate (0.27%), magnesium (0.13%), calcium (0.041%), potassium (0.049%), and bicarbonate (0.014%).

Surface waters such as lakes, ponds, rivers, and streams are highly variable in their chemical composition. Saline and soda lakes of arid regions have total salt concentrations that can substantially exceed that of seawater. Lakes such as Great Salt Lake in Utah and the Dead Sea in Israel can have salt concentrations that exceed 25%. The shores of such lakes are caked with a crystalline rime of evaporate minerals, which are sometimes mined for industrial use.

The most chemically dilute surface waters are lakes in watersheds with hard, slowly weathering bedrock and soils. Such lakes can have total salt concentrations of less than 0.001%. For example, Beaverskin Lake in Nova Scotia has very clear, dilute water that is chemically dominated by chloride, sodium, and sulfate, in concentrations of two-thirds of the norm for surface water or less, with only traces of calcium, usually most abundant, and no silica. A nearby body of water, Big Red Lake, has a similarly dilute concentration of inorganic ions but, because it receives drainage from a bog, its chemistry also includes a large concentration of dissolved organic carbon , mainly comprised of humic/fulvic acids that stain the water a dark brown and greatly inhibit the penetration of sunlight.

The water of precipitation is considerably more dilute than that of surface waters, with concentrations of sulfate, calcium, and magnesium of one-fortieth to one-hundredth of surface water levels, but adding small amounts of nitrate and ammonium. Chloride and sodium concentrations depend on proximity to salt water. For example, precipitation at a remote site in Nova Scotia, only 31 mi (50 km) from the Atlantic Ocean, will have six to 10 times as much sodium and chloride as a similarly remote location in northern Ontario.

Acid rain is associated with the presence of relatively large concentrations of sulfate and nitrate in precipitation water. If the negative electrical charges of the sulfate and nitrate anions cannot be counterbalanced by positive charges of the cations sodium, calcium, magnesium, and ammonium, then hydrogen ions go into solution, making the water acidic. Hubbard Brook Experimental Forest , New Hampshire, within an airshed of industrial, automobile , and residential emissions from the northeastern United States and eastern Canada, receives a substantially acidic precipitation, with an average pH of about 4.1. At Hubbard Brook, sulfate and nitrate together contribute 87% of the anion-equivalents in precipitation. Because cations other than the hydrogen ion can only neutralize about 29% of those anion charges, hydrogen ions must go into solution, making the precipitation acidic.

Fogwaters can have much larger chemical concentrations, mostly because the inorganic chemicals in fogwater droplets are less diluted by water than in rain and snow. For example, fogwater on Mount Moosilauke, New Hampshire, has average sulfate and nitrate concentrations about nine times more than in rainfall there, with ammonium eight times more, sodium seven times more, and potassium and the hydrogen ion three times more.

The above descriptions deal with chemicals present in relatively large concentrations in water. Often, however, chemicals that are present in much smaller concentrations can be of great environmental importance.

For example, in freshwaters phosphate is the nutrient that most frequently limits the productivity of plants, and therefore, of the aquatic ecosystem . If the average concentration of phosphate in lake water is less than about 10 μg/l, then the algae productivity will be very small, and the lake is classified as oligotrophic . Lakes with phosphate concentrations ranging from about 10–35 μg/l are mesotrophic, those with 35–100 μg/l are eutrophic, and those with more than 100 μ/l are very productive, and very green, hypertrophic waterbodies. In a few exceptional cases, the productivity of freshwater may be limited by nitrogen , silica, or carbon, and sometimes by unusual micronutrients. For example, the productivity of phytoplankton in Castle Lake, California, has been shown to be limited by the availability of the trace metal, molybdenum.

Sometimes, chemicals present in trace concentrations in water can be toxic to plants and animals, causing substantial ecological changes. An important characteristic of acidic waters is their ability to solubilize aluminum from minerals, producing ionic aluminum. In non-acidic waters, ionic aluminum is generally present in minute quantities, but in very acidic waters when pH is less than 2, attainable by acid mine drainage , soluble-aluminum concentrations can rise drastically. Although some aquatic biota are physiologically tolerant of these aluminum ions, other species , such as fish, suffer toxicity and may disappear from acidified waterbodies. Many aquatic species cannot tolerate even small quantities of ionic aluminum. Many ecologists believe that aluminum ions are responsible for most of the toxicity of acidic waters and also of acidic soils.

Some chemicals can be toxic to aquatic biota even when present in ultra trace concentrations. Many species within the class of chemicals known as chlorinated hydrocarbons are insoluble in water but are soluble in biological lipids such as animal fats. These chemicals often remain in the environment because they are not easily metabolized by microorganisms or degraded by ultraviolet radiation or other inorganic processes. Examples of chlorinated hydrocarbons are the insecticides DDT, DDD, dieldrin, and methoxychlor, the class of dielectric fluids known as PCBs, and the chlorinated dioxin , TCDD.

These chemicals are so dangerous because they collect in biological tissues, and accumulate progressively as organisms age. They also accumulate into especially large concentrations in organisms at the top of the ecosystem's food chain/web .In some cases, older individuals of top predator species have been found to have very large concentrations of chlorinated hydrocarbons in their fatty tissues. The toxicity caused to raptorial birds and other predators as a result of their accumulated doses of DDT, PCBs, and other chlorinated hydrocarbons is a well-recognized environmental problem.

Water pollution can also be caused by the presence of hydrocarbons. Accidental spills of petroleum from disabled tankers are the highest profiled causes of oil pollution , but smaller spills from tankers disposing of oily bilge waters and chronic discharges from refineries and urban runoff are also significant sources of oil pollution. Hydrocarbons can also be present naturally, as a result of the release of chemicals synthesized by algae or during decomposition processes in anaerobic sediment . In a few places, there are natural seepages from near-surface petroleum reservoirs, as occurs in the vicinity of Santa Barbara, California. In general, the typical, naturally-occurring concentration of hydrocarbons in seawater is quite small. Beneath a surface slick of spilled petroleum, however, the concentration of soluble hydrocarbons can be multiplied several times, sufficient to cause toxicity to some biota. This dissolved fraction does not include the concentration of finely suspended droplets of petroleum, which can become incorporated into an oil-in-water emulsion toxic to organisms that become coated with it. In general, within the very complex mix of hydrocarbons found in petroleum, the smallest molecules are the most soluble in water.

[Bill Freedman Ph.D. ]