normal surface water pH - silica, clays, feldspars, and manganese oxides are negatively charged and
have a strong affinity for positively charged ions.
Mercury has received much attention lately, particularly since concentration increases up the
food chain (biomagnification). Mercury enters an aquatic system primarily from agricultural fungicides,
mining and smelting operations, industrial discharge, atmospheric input, flooded soils, and via mineral
weathering processes. Mercury can occur in concentrations that are toxic (0.03 (Scenedesmus) to 3.0
ppm for snails).
18.104.22.168 POLYCHLORINATED BIPHENYLS (PCBS)
waxes and exist in 210 possible forms. Generally, PCBs are long-lived in the environment, associated
with very fine sediments (clays) and while not readily mobilized by chemical reactions, they can move
up the food chain through biomagnification. The relationship of PCB contaminated sediments to
macroinvertebrate and fish communities have been evaluated for Lake Hartwell, a CE project on the
Georgia and South Carolina border (Alexander 1995). Study results indicate biomagnification, adverse
impacts to fish health and vitality, and a persistent problem downstream of the contaminated site.
These contaminants are chemical compounds that vary greatly in how long they remain in the
environment as problematic compounds. Chlorinated hydrocarbons (organochlorines, e.g., DDT,
dioxin, chlordane, myrex, aldrine, etc.) are very persistent. Organophosphorus compounds (parathion,
malathion, diazanon, etc.) are not very persistent and often photosensitive resulting in a limited time for
measurable concentrations to exist in the aquatic system. Carbamate compounds (e.g., carbaryl and
Seven) are not very persistent and are intended to kill pests mostly by working on the nervous system.
Others include inorganic pesticides that contain mercury, arsenic, and lead. Recently, lethal compounds
have been restricted and except for misuse, most currently used pesticides are short-lived in aquatic
systems. The most notable exception is DDT and its derivatives which are long-lived in sediments and
move up the food chain.