Our l.i\'iiii; Re.s/}iine.s — Human Injlufiues 



415 



paralleling a trend reported for U.S. rivers 

 (Smith et al. 1987). This decline has been attrib- 

 uted to reductions in the lead content of gaso- 

 line and to discharge restrictions at smelters and 

 other industrial sources (Smith et al. 1987). 



Selenium is a trace element required by 

 plants and animals; it is toxic at high concentra- 

 tions. Concentrations of selenium in fish 

 declined in some areas of the United States. In 

 some parts of the West, however, where concen- 

 trations were historically elevated, levels either 

 increased or remained unchanged (Schmitt and 

 Brumbaugh 1990). Selenium is a natural com- 

 ponent of soils and is present at high concentra- 

 tions in some arid areas of the U.S. West. The 

 dissolution of selenium and other potentially 

 toxic elements from soils and their accumula- 

 tion in ecosystems are accelerated by irrigation. 

 Elevated selenium concentrations, induced by 

 irrigation, are responsible for the widely publi- 

 cized wildlife deaths and deformities at 

 Kesterson National Wildlife Refuge in 

 California (Lemly 1993). 



In general. U.S. concentrations of persistent 

 contaminants that accumulate in fish and 

 wildlife are lower now than at any time for 

 which accurate data exist, although problem 

 areas remain. These results imply that direct 

 inputs of many toxic substances to the environ- 

 ment have been reduced through the regulation 

 of industrial discharges and pesticide use. 

 Declining concentrations of DDT and other 

 contaminants in North America have permitted 

 the return of predatory birds, such as bald 

 eagles, to some areas from which they had been 

 eliminated (Fig. 2). 



The persistence of contaminant problems, 

 despite curtailment of direct discharges to 

 waterways and restrictions on the uses of per- 

 sistent pesticides, has highlighted the impor- 

 tance of global and ecosystem processes such as 

 atmospheric transport and internal cycling. The 

 accumulation of selenium in California, and 

 mercury in the Everglades, has resulted from 

 natural processes — the leaching of elements 

 from soils and vegetation. The rates of these 

 processes have been accelerated by irrigation 

 and other activities associated with agriculture. 

 Atmospheric transport also represents an 

 important source of PCBs to the Great Lakes; it 

 has also been linked to the accumulation of 

 mercury in Lake Champlain [see Glaser, this 

 section; Baker et al. 1993) and other northeast- 

 em lakes (Driscoll et al. 1994). 



The exposure of migratory birds such as 

 peregrine falcons {Fcdco peregrimis) to contam- 

 inants on their wintering grounds outside of the 

 United States (Henny et al. 1982), where DDT 

 and other persistent compounds are still used, 

 also remains a problem. Moreover, the curtail- 

 ment of organochlorine pesticide use in Nonh 



I I <0,05ppm 



I I 05-0 07ppm 



I I 08-0 15ppm 



^1 >0.25ppm 



• Collection site 



America has led to increasing reliance on so- 

 called soft pesticides — highly toxic organo- 

 phosphate, carbamate, and synthetic pyrethroid 

 compounds — that are difficult to monitor 

 because they are short-lived and do not accu- 

 mulate. Evidence of the increasing use and 

 potential adverse effects of these chemicals is 

 highlighted by increasing occurrences of 

 wildlife mortality attributable to them (see 

 Glaser, this section). Additionally, chemical 

 analysis has demonstrated the presence of high- 

 ly toxic contaminants such as the chlorinated 

 dioxins. No long-term monitoring data exist for 

 these compounds, which may affect fish and 

 wildlife at extremely low concentrations (Giesy 

 et al. 1994). New approaches and technologies, 

 capable of detecting chemical exposure and its 

 effects at all levels of biological organization, 

 will be required to monitor and as.sess highly 

 toxic chemicals and those that do not accumu- 

 late in fish and wildhfe before concentrations 

 reach harmful levels. 



Fig. 4. Geographic distribution of 

 PCB residues in U.S. Fisli and 

 Wildlife Service monitoring net- 

 worlcs: (a) PCB concentrations in 

 fish collected in 1986 from the 

 indicated sites. Not shown are sta- 

 tions in Alaska and Hawaii, at 

 which PCB concentrations were < 

 1 .5 parts per million (ppm) at all 

 sites; (b) PCBs in starlings col- 

 lected in 1985. Also shown are 

 boundaries of the 5-degree (lati- 

 tude and longitude) sampling 

 blocks and collection sites. 



