raw water. During the rest of the year, when a significant fraction of the PCB's is present in the dissolved phase, removal is achieved by the combination of settling, coagulation, and charcoal filtration processes used for water purification.

Cleanup Plan

If PCB transport rates were to remain constant in the future, it would take nearly a century to

move the total accumulation of PCB's from the upper river. Even though covering of old contaminated sediments with new uncontaminated sediments and natural degradation of PCB's could shorten the interval, the length of time necessary for the river to clean itself by natural processes is still very long. For this reason, the U.S. Environmental Protection Agency and the New York State Department of Environmental Conservation

have decided to spend $26.7 million to remove contaminated sediments by dredging.

Dredging for removal is planned to begin in 1982 or 1983 and will be completed in 1 year. Dredging will remove sediments from about 30 "hot spots" above Schuylerville where concentration of PCB's exceeds 50 parts per million. The sediments will be stored in a clay encapsulation site on 250 acres a few miles south of Fort Edward. Removal of these hot spots will decrease total PCB's in sediments of the upper river by nearly one-half and should, therefore, result in a comparable decrease in concentrations and loads transported by the river. Water sampling on the upper Hudson is scheduled to continue through 1986 to compare "before and after" dredging concentrations.

Acid Rain-What We Know and Don't

Know

The phenomenon of "acid rain," or precipitation with low pH values was observed in Europe as early as 1966 and later was noted in the Northeastern United States in 1974. The term pH is used to describe the free hydrogen ion concentration and acidity of water. A pH of 7 is called neutral; pH's above 7 are called basic or alkaline; pH's below 7 are called acidic. The pH of water in equilibruim with atmospheric carbon dioxide (CO2) is 5.6, a value considered to be the normal condition for precipitation in the absence of strong acid-forming materials. The term acid rain applies to preciptiation with a pH of less than 5.6.

It is by no means certain that precipitation is in equilibrium with CO, all the time. Variations in the pH of precipitation above and below 5.6 may be due in part to departures from equilibrium. Oversaturation with CO2 will lower the pH below 5.6, and undersaturation will raise it. Both conditions may be caused by particular combinations of temperature, barometric pressure, and turbulence in the atmosphere during rainfall, as well as by changes in the concentrations of CO2 in the air from place to place.

Despite the present uncertainties as to the extent and importance of the effects on pH from variations in CO2, which have only partial relation to manmade air-pollution sources, scientists have tentatively ascribed the causes for pH's of less than 5.6 to strong acid-forming gases as opposed to the weak acid-forming gas CO2.

The occurrence of rainfall with a pH of below 5.6 in North America (fig. 1) is centered in the northeast corridor of the United States and through Ontario, Quebec, and the Maritime Provinces of Canada. The occurrence of pH values higher than 5.6 in Montana, North Dakota, Alberta, and Saskatchewan may be caused by dust in the rain which neutralizes the normal acidity.

Acid rain is thought to be caused by acidforming nitrogen and sulfur oxides emitted from automobiles, coal- and oil-fired powerplants, and many industrial plants (fig. 2). Emissions of nitrogen and sulfur oxides are strongly localized in the upper Midwest, Northeast, Gulf Coast, and Pacific Coast States and in southern Quebec and Ontario and parts of Alberta. These gases pass into the atmosphere where they are carried by prevailing winds and precipitated in rain and snow. These and other acid-forming materials also are produced by many natural processes such as volcanic eruptions and the decomposition of

organic material. Scientists now are attempting to determine the mechanisms that form such naturally produced acid-forming materials and the extent of their occurrence to better assess the extent and severity of man's influence.

Arctic, tropical, and Pacific airstreams tend to converge over the upper Midwest and Northeastern States and the Maritime Provinces of Canada, causing the prevailing pattern of weather fronts and normally high precipitation in this region year-round (fig. 3). In the process, airborne acid-forming materials emitted from the industrial and population centers of this country and Canada are swept into this area to be combined with precipitation to create the acid rain phenomenon. This pattern of airmass movements together with the location of manmade sources explains, in part, the localization of the effect over the northeast corridor and Maritime Provinces and the relative absence of the effect west of the Rocky Mountains. However, acid rain has been observed in the Sierra Nevada and Cascade Ranges of the West and the Rockies as well.

There the effect is thought to be due to emissions from very localized population and industrial centers, such as Los Angeles, San Francisco, and Denver.

Recent studies suggest that the acidity of precipitation has been increasing since the mid-1950's and has caused the acidification of several lakes in this country and Canada. The phenomenon apparently has been with us for at least the last 25 years but may have worsened during that period. Studies suggest a steady decrease in pH of precipitation in those most severely impacted areas of the Northeast from about 4.5 in 1955 to about 4.1 in 1975 and the present day (fig. 4). Many scientists are skeptical of the methods used to make this interpretation, and further studies and monitoring are being conducted to verify the trend.

Among the studies conducted recently to detect effects from and trends in the occurrence of acid rain was a statistical analysis of streamquality data collected over the past decade at the U.S. Geological Survey's remote Hydrologic

Bench-Mark Program stations. Analyses of the data show that there have been widespread increases in sulfate concentrations. The extent of the upward trends supports the theory that atmospheric sources (as opposed to land disturbance, which would be more localized) is the primary cause. The major source of sulfate would be sulfur oxides from combustion of fossil fuels. Concentrations of nitrate showed uptrends also, primarily at stations east of the Mississippi River. The major source of nitrate may be nitrogen oxides from internal combustion engines.

Despite the uptrends in sulfate, however, there is no clear pattern of falling pH or alkalinity levels at the Hydrologic Bench-Mark Program stations. In fact, pH levels are increasing at many stations in the West and in the Gulf Coast States. No explanation for this apparent contradiction is offered, but the existence of this contradiction suggests that further study of the effects of acid rain on stream quality is needed.

Another study recently completed by the Survey using data collected over the 13-year period from 1965 to 1978 at nine precipitation

monitoring stations in New York State suggests a mixed pattern of trends in the acidity of precipitation. The pH decreased (increased acidity) by about 0.2 pH units in the western part of the State but increased by about the same amount in the eastern part during the period of the study. The concentrations of sulfate (the ion produced by the strong acid-forming sulfur oxide gases) decreased by 0.25 percent per year, but the concentrations of nitrate (the ion produced by the strong acid-forming nitrogen oxide gases) increased by 4 to 13 percent per year. This suggests a steady shift of predominate sources of acidity from sulfur oxides emitted from fossil-fueled industries and powerplants to nitrogen oxides produced by internal combustion engines, mostly in automobiles. Other evidence suggested that the acidity was being partially neutralized to a variable degree from place to place and time to time, possibly by air-borne particulate material. Neutralization produced an increase of about 0.3 pH units in nonurban areas and 0.7 pH units in urban areas.

A statistical analysis of chemical data from several streams in New York yielded little evidence of temporal trends resulting from acid precipitation except in the Adirondack Mountains where the soils lack significant capacity to neutralize the acidity.

Studies indicate that acids can be neutralized by passage through deep well-developed chemically reactive soils. Where those soils are absent, the acids pass unaltered into streams and lakes. Areas where deep soils are absent are found throughout New England and the Canadian Shield of Ontario, Quebec, and the Maritime Provinces, and it is here that effects from acid rain are most prominent. In the Adirondack Mountains, the pH of selected lakes has decreased since the 1930's (fig. 5). The median pH value of 138 lakes decreased from 6.75 between 1930 and 1934 to 6.51 in 1979; but, more importantly, the percentage of lakes with a pH of below 6 more than tripled from 6.5 to 19.6 percent during this period. Moreover, a comparison of lakes in a different group between 1930 and 1975 showed that the occurrence of lakes with a pH of less than 5 also increased dramatically.

A loss of fish was illustrated dramatically in a comparison of fish populations between the 1930's and 1975 in 40 Adirondack lakes located at high altitudes (fig. 6). The link between reduced pH and loss of fish has not been established completely, but at least two mechanisms are thought to be at work. First, fish eggs and fry are more sensitive to low pH than are adults. Simultaneous occurrences of spawning and a sharply reduced