Evidence for acid-precipitation-induced trends in stream chemistry at hydrologic bench-mark stations

JAMES G WATT, Secretary

Dallas l. Peck, Director

Free on application to Branch of Distribution, U.S. Geological Survey, 604 South Pickett Street, Alexandria, VA 22304 Abstract .. Introduction Methods

Summary . References

First year of record for (a) sulfate, alkalinity, and cation concentrations, and (b) pH at Bench-Mark stations . 2. Comparison of trends in stream sulfate concentrations at Bench-Mark stations for the period of record through 1981 with trends in S02 emissions to the atmosphere by State, 1965-1980 ..... .

3. Trends in (a) alkalinity, and (b) the ratio of alkalinity to total major cation concentration at Bench-Mark stations for the period of record through 1981 . . . . . . . . . . . . . . . .

4. Trends in pH at Bench-Mark stations for the period of record through 1981

Drainage area, mean discharge, and annual precipitation at Bench-Mark stations . . . . . . . . . . . .

3. Summary statistics for alkalinity at Bench-Mark stations, period of record through 1981 .

4. Summary statistics for the ratio of alkalinity to total major cation concentrations at Bench-Mark stations, period of record through 1981 . . . . . . . . . . . . . . . . . . . . . . .

5. Summary statistics for pH at Bench-Mark stations, period of record through 1981

6. S02 emissions by State for the period of record 1965-1980 . . . . . . . . . . . . . . Atmospheric contributions to stream sulfate .

Trends in stream sulfate . . Trends in S02 emissions . . Trends in stream alkalinity Trends in stream pH CONTENTS

TABLES By Richard A. Smith and Richard B. Alexander

Ten- to 15-year water-quality records from a network of headwater sampling stations show small declines in stream sulfate concentrations at stations in the northeastern quarter of the Nation and small increases in sulfate at most southeastern and western sites. The regional pattern of stream sulfate trends is similar to that reported for trends in 802 emissions to the atmosphere during the same period. Trends in the ratio of alkalinity to total major cation concentrations at the stations follow an inverse pattern of small increases in the Northeast and small, but widespread decreases elsewhere. The undeveloped nature of the sampled basins and the magnitude and direction of observed changes in relation to 802 emissions support the hypothesis that the observed patterns in water quality trends reflect regional changes in the rates of acid deposition.

INTRODUCTION

During the past decade evidence has accumulated suggesting that the phenomenon of acid precipitation may involve the transport of air pollutants over long distances and the deposition of acidic materials in basins with widely differing geochemical capacities for neutralizing acids (N ationa! Research Council, 1981; 1983). These two characteristics of the acid deposition problem give rise to the need for a more detailed geographic understanding of the phenomenon than presently exists for the Nation as a whole. A recent analysis of available data from eastern North America by the National Research Council (1983) links historical trends in atmospheric emissions of so2 and NOx to trends in precipitation chemistry at a single site, the Hubbard Brook Experimental Forest in New Hampshire. A major conclusion of that study is that the lack of long-term records of precipitation chemistry at sites in other regions of the country greatly hinders efforts to associate specific source areas of acid-producing materials with the sensitive areas receiving those materials. In this report we describe a geographic pattern in recorded changes in water chemistry at a nationwide network of stream sampling stations. The nature of these stations, and the geographic pattern of atmospheric so2 emissions occurring during the period of sampling, suggest that the stream chemistry changes result from regional changes in the rates of acid deposition. There are difficulties inherent in the use of stream chemistry data to indicate changing rates of acid deposition. Because of the numerous natural and human sources of materials entering streams in general, it is difficult to separate atmospheric factors from other possible causes of chemical change in the absence of simultaneous records of precipitation chemistry. This problem is greatly alleviated in the pl"esent study, however, by virtue of the predominantly undeveloped nature of the drainage basins in which the sampling stations are located. The principal advantage to the use of stream chemistry records over precipitation data in discerning trends in acid deposition is the larger number and wider geographic distribution of sampling sites for which long-term(> 10 year) records are available. A further advantage of stream data is that streams tend to carry acidic material deposited in both wet and dry forms so that effects on stream chemistry tend to reflect the total deposition rate. To date, direct quantification of dry deposition rates at precipitation monitoring sites TABLE !.-Drainage area, nz.ean discharge, and annual precipitation at Bench-Mark stations

STATION NUMBER

Estimates of annual precipitation are from Cobb and Biesecker, 1971.

has proved very problematic (Hicks, and others, 1981). Moreover, stream chemistry data provide direct evidence of the geochemical sensitivity of an area to acid deposition and are thus, in a sense, a step closer to the problem. Since 1964 the U.S. Geological Survey has operated the Hydrologic Bench-Mark Network of 47 streamflow and water quality monitoring stations in small, predominantly undeveloped stream basins (table 1; Cobb and Biesecker, 1971). The network includes stations in 37 states and was originally established to help define baseline hydrologic conditions in a variety of natural environments. Because of little or no changes in land use in these basins and the application of consistent sampling and analytical methods (Skougstad and others,

STATION NAME AND LOCATION

WILD RIVER AT GILEAD, ME ESOPUS CREEK AT SHANDAKEN, NY MCDONALDS B IN LEBANON STATE FOREST, NJ YOUNG WOMANS CREEK NEAR RENOVO, PA. HOLIDAY CREEK NEAR ANDERSONVILLE, VA. SCAPE ORE SWAMP NEAR BISHOPVILLE, SC UPPER THREE RUNS NEAR NEW EllENTON, ~C FALLING CREEK NEAR JULIETTE, GA. SOPCH~PPY RIVER NR SOPCHOPPY, FLA. SIPSEY FORK NEAR GRAYSON, AL CYPRESS CREEK NR JANICE, MS. UPPER TWIN CREEK AT MCGAW, OH SOUTH HOGAN CREEK NEAR DILLSBORO, IND. CATALOOCHEE CREEK NEAR CATALOOCHEE, NC BUFFALO RIVER NEAR FLAT WOODS, TENN. WASHINGTON CREEK AT WINDIGO, MICH. POPPLE RIVER NEAR FENCE, WI BEAVER CREEK NR FINLEY, ND KAWISHIWI RIVER NEAR ELY, MN NORTH FORK WHITEWATER RIVER NEAR ELBA, MN BEAUVAIS CREEK NEAR ST. XAVlER, MT. BEAR DEN CREEK NR MANDAREE, ND CASTLE CR ABOVE DEERFIELD RES NEAR HILL CITY, SD ENCAMPMENT RIV AB HOG PARK CR NR ENCAMPMENT, WYO DISMAL RIVER NR THEDFORD, NEBR ELK CREEK NEAR DECATUR CITY, IOWA NORTH SYLAMORE CREEK NEAR FIFTY SIX, ARK. HALFMOON CREEK NEAR MALTA, CO. BLUE BEAVER CREEK NR CACHE, OK KIAMICHI RIVER NR BIG CEDAR, OK BIG CREEK AT POLlOCK, LA SOUTH FORK ROCKY CREEK NEAR BRIGGS, TEX. RIO MORA NEAR TERRERO, NM VALLECITO CREEK NEAR BAYFIElD, CO. MOGOLLON CREEK NEAR CLIFF, NM WET BOTTOM CREEK NR CHILDS, ARIZ. RED BUTTE CREEK AT FT. DOUGLAS NR. SLC, UTAH STEPTOE C NR ELY, NV S TWIN R NR ROUND MOUNTAIN, NV MERCED R AT HAPPY ISLES BRIDGE NR YOSEMITE,CALIF ELDER CREEK NEAR BRANSCOMB, CALIF NORTH FORK QUINAULT R NEAR AMANDA PARK, WASH. HAYDEN CK BELOW N FK, NR HAYDEN LAKE, IDAHO ANDREWS CREEK NEAR MAZAMA, WASH. CACHE CREEK NEAR JACKSON, WYO BIG JACKS CREEK NEAR BRUNEAU, ID MINAM RIVER AT MINAM,OREG.

DRAINAGE

1979) for a 10-15-year period at each site, water-quality records from the network are particularly appropriate for investigating atmospheric influences on water quality during the past decade.

In this report we present the results of applying the Seasonal Kendall test for trend (Hirsch and others, 1982) to monthly records of stream sulfate, pH, alkalinity, and the ratio of alkalinity to total major cation concentration at Bench-Mark stations. Records of sulfate, alkalinity, and major cation concentrations begin generally in the mid- to late 1960's (fig. la), while pH records begin generally

METHODS

MEAN

o. 4 5

o. 76

o. 2 5

FIRST YEAR OF RECORD

FIGURE 1.-First year of record for (a) sulfate concentration, alkalinity, and total major cation concentrations, and (b) pH at Bench-Mark stations. TABLE 2.-Summa:ry statistics for sulfate concentrations at Bench-Mark stations, for the period of record through 1981

STATION NUMBER

Estimates of precipitation sulfate concentration are from 1981 N.A.D.P. data (Gibson and Baker, written communication, 1982).

in the late 1960's to the early 1970's (fig. 1b). The pH of samples collected prior to dates given in figure 1b were measured in the laboratory and are not comparable to in-stream measurements. Records of stream nitrate concentrations are available only since the mid- to late 1970's and are considered too short for comparison with the other records. The Seasonal Kendall test is nonparametric and is intended for analysis of time trends in seasonally varying water-quality data from fixed, regularly sampled monitoring sites such as those which the Bench-Mark Network comprises (Hirsch and others, 1982; see also Smith and others, 1982). In addition to a test for trend, the statistical procedure includes an estimate of the median rate of

STATION NAME AND LOCATION

WILD RIVER AT GILEAD, ME ESOPUS CREEK AT SHANDAKEN, NY MCDONALDS B IN LEBANON STATE FOREST, NJ YOUNG WOMANS CREEK NEAR RENOVO, PA. HOLIDAY CREEK NEAR ANDERSONVILLE, VA. SCAPE ORE SWAMP NEAR BISHOPVILLE, SC UPPER THREE RUNS NEAR NEW ELLENTON, SC FALLING CREEK NEAR JULIETTE, GA. SOPCHOPPY RIVER NR SOPCHOPPY, FLA. SIPSEY FORK NEAR GRAYSON, AL CYPRESS CREEK NR JANICE, MS. UPPER TWIN CREEK AT MCGAW, OH SOUTH HOGAN CREEK NEAR DILLSBORO, IND. CATALOOCHEE CREEK NEAR CATALOOCHEE, NC BUFFALO RIVER NEAR FLAT WOODS, TENN. WASHINGTJN CREEK AT WINDIGO, MICH. POPPLE RIVER NEAR FENCE, WI BEAVER CREEK NR FINLEY, ND KAWISHIWI RIVER NEAR ELY, MN NORTH FORK WHITEWATER RIVER NEAR ELBA, MN BEAUVAIS CREEK NEAR ST. XAVIER, MT. BEAR DeN CREEK NR MANDAREE, ND CASTLE CR ABOVE DEERFIELD RES NEAR HILL CITY, SD 167 ENCAMPMENT RIV AB HOG PARK CR NR ENCAMPMENT, WYO 124 DISMAL RIVER NR THEDFORD, NEBR ELK CREEK NEAR DECATUR CITY, IOWA NORTH SYLAMORE CREEK NEAR FIFTY SIX, ARK. HALFMOON CREEK NEAR MALTA, CO. BLUE BEAVER CREEK NR CACHE, OK KIAMICHI RIVER NR BIG CEDAR, OK BIG CREEK AT POLLOCK, LA SOUTH FORK RJCKY CREEK NEAR BRIGGS, TEX. RIO MORA NEAR TERRERO, NM VALLECITO CREEK NEAR BAYFIELD, CO. MOGOLLON CREEK NEAR CLIFF, NM WET BOTTOM CREEK NR CHILDS, ARIZ. RED BUTTE CREEK AT FT. DOUGLAS NR. SLC, UTAH STEPTOE C NR ELY, NV S TWIN R NR ROUND MOUNTAIN, NV MERCED R AT HAPPY ISLES BRIDGE NR YOSEMITE,CALIF 108 ELDER CREEK NEAR BRANSCOMB, CALIF NORTH FORK QUINAULT R NEAR AMANDA PARK, WASH. HAYDEN CK BELOW N FK, NR HAYDEN LAKE, IDAHO ANDREWS CREEK NEAR MAZAMA, WASH. CACHE CREEK NEAR JACKSON, WYO BIG JACKS CREEK NEAR BRUNEAU, ID MINAM RIVER AT MINAM,OREG.

NUMBER OF SAMPLES

change of quality over the sampling period (trend slope) and a method for adjusting the data to correct for effects of changing stream flow on trend in the water-quality record. Trend is defined here simply as monotonic change with time, occurring either as an abrupt or gradual change in water quality.

ATMOSPHERIC CONTRIBUTIONS

An important assumption of the present analysis is that stream sulfate concentrations at most Bench-Mark stations are low enough to be significantly influenced by changes in the rate of atmospheric deposition of sulfur. Annual average sulfate

TO STREAM SULFATE

o.o

ESTIMATED

LEVEL CONCENTRATION (p) (j.leq L"1)

o.ooo o.ooo

0.034 0.088 0.040 o. 001 0.007 0.696

o.ooo

o.ooo

o. 51 3 o. 118

0.1 57 0.032 o. 001 0.001

o.ooo

o.ooo

o.ooo so so concentrations of precipitation in the United States, based on 1981 data from the National Atmospheric Deposition Program (table 2), range from approximately 20t.J.eq L- in the West to 70t.J.eq L- or more over the Ohio Valley (J. H. Gibson and C. V. Baker, National Atmospheric Deposition Program, Fort Collins, Colorado, 1982, written communication). These data can be used to estimate the contribution of wet deposition to stream sulfate, provided that the tendency for evapotranspiration to increase the concentration of dissolved constituents in precipitation is taken into account. Correction factors for the effects of evapotranspiration are calculated as the ratio of annual precipitation to annual runoff (table 1; Cobb and Biesecker, 1971) and range from 10 or greater for much of the West to about 2 in New England and as low as 1.5 in the far Northwest. After adjusting for the effects of evapotranspiration, precipitation is estimated to contribute at least 90 percent of the mean sulfate concentration at half of the Bench-Mark stations and at least

SIGNIFICANCE LEVEL

j 0.1 <p<0.2

NO TREND p > 0.2 j p<0.01 ' 0.01 <p < 0.1

FIGURE 2.-Comparison of trends in stream sulfate concentrations at Bench-Mark stations for the period of record through

1981 with trends in S02 emissions to the atmosphere by State, 1965-1980. Triangles indicate direction and significance level of trends in stream sulfate. Numbers give percentage change in S02 emissions from 1965 to 1980 for each State. States showing increasing levels of S02 emissions are shaded; States showing decreasing levels of S02 emissions are unshaded. Source of emission data: G. Gschwandtner and K. Gschwandtner, written communication (1983). 22 percent of the mean sulfate concentration at all but six stations. The Bench-Mark stations at which precipitation is estimated to contribute less than 22 percent of stream sulfate are: South Hogan Creek near Dillsboro, Indiana (19 percent); North Fork Quinault River near Amanda Park, Washington (11.5 percent); Beaver Creek near Finley, North Dakota (9.2 percent); Red Butte Creek at Ft. Douglas near Salt Lake City, Utah (6.5 percent); Bear Den Creek near Mandaree, North Dakota (2 percent); and Beauvais Creek near St. Xavier, Montana (1.5 percent). The above estimates of the precipitation contribution to stream sulfate at Bench-Mark stations are conservative estimates of the total atmospheric contribution because dry deposition is not included. Dry deposition has proved difficult to quantify (Hicks and others, 1981) but, depending on climatic and other factors, has been estimated to contribute anywhere from a few percent to 60 or 70 percent of the total sulfate deposition (Niemann, 1983).

NO TREND p > 0.2 ' 0.1 <p< 0.2 '0.01 <p < 0.1 'p<0.01

FIGURE 3.-Trends in (a) alkalinity, and (b) the ratio of alkalinity to total major cation concentration at Bench-Mark stations for the period of record through 1981. Symbols indicate direction and significance level of trends. Dark symbols indicate stations with mean alkalinity less than 1 meq L-•

STATION NUMBER

The significance levels and directions of apparent trends in sulfate concentration for the period of record at Bench-Mark stations are shown in figure 2. Sulfate concentrations have tended to increase during the 10-15-year period over a broad area of the continental United States extending from the Southeast to the mountain States and the Northwest. By contrast, stations in the northeastern quarter of the Nation have tended to show TABLE 3.-Summary statistics for alkalinity at Bench-Mark stations, for the period of record through 1981

RESULTS AND DISCUSSION

TRENDS IN STREAM SULFATE

NUMBER OF SAMPLES (meq L-1)

either no trend or declines in sulfate concentrations. This geographic pattern occurs more or less independently of the significance criteria used in mapping the trend test results (see table 2 for test results at all stations). Although the statistical significance of trends at many of the stations is high (p < .01), the magnitude of change has been small in most cases (table 2). The median slope of 2~.~-eq L-yr-among stations showing trend in sulfate concentration corresponds to a median relative change in stream sulfate of 1. 7 percent per year or about 25 percent over the period of record.

(~eq L-yr-)

o. 73

LEVEL (p)

o. 458

o. 735 o.ooo

o.ooo o.ooo

o.ooo

o. 211

o.oos o. 875 o.ooo o.ooo o. 421

o.ooo TABLE 4.-Summary statistics for the ratio of alkalinity to total major cation concentrations at Bench-Mark stations, for

STATION NUMBER

10542.00 13621.93 14665.00 15456.00 20388.50 21353.00 21973.00 22126.00 23271.00 24502.50 24791.55 32372.80 32767.00 34oOO.OO 36040.00 40010.00 40637.00 5064 9.00 51244.80 53760.00 62882.00 63325.15 64090.00 66238.00 67759.00 68979.50 70607.10 70830.00 73112.00 73357.00 73730.00 81039.00 83779.00 93529.00 94306.00 95083.00 101722.00 102449.50 102493.00 112645.00 114755.60 120393.00 124160.00 124473.90 130183.00 131695.00 133315.00

cation) that trends in S02 emissions to the atmosphere from 1965 to 1980 followed a similar geographical pattern to that described above for trends in sulfate at Bench-Mark stations (fig. 2; see also table 6). Substantial declines in emissions occurred from 1965 to 1980 in the Northeast and northern Midwest while increases occurred in the Southeast and in most States west of the Mississippi. Based on previous literature, it is difficult to construct a comparable nationwide picture of trends in stream and precipitation sulfate concentrations due to the limited number and uneven distribution of sampling sites with adequate record lengths (Bubenick and others, 1983). For the Northeast, however, there are other recent reports of declining stream and precipitation sulfate concentrations from scattered locations (National Research Counthe period of record through 1981

STATION NAME AND LOCATION

WILD RIVER AT GILEAD, ME ESOPUS CREEK AT SHANDAKEN, NY MCOONALDS B IN LEBANON STATE FOREST, NJ YOUNG WOMANS CREEK NEAR RENOVO, PA. HOLIDAY CREEK NEAR ANDERSONVILLE, VA. SCAPE ORE SWAMP NEAR BISHOPVILLE, SC UPPER THREE RUNS NEAR NEW ELLENTON, SC FALLING CREEK NEAR JULIETTE, GA. SOPCHOPPY RIVER NR SOPCHOPPY, tLA. SIPSEY FORK NEAR GRAYSON, AL CYPRESS CREEK NR JANICE, MS. UPPER TWIN CREEK AT MCGAW, OH SOUTH HOGAN CREEK NEAR DILLSBORO, IND. CATALOOCHEE CREEK NEAR CATALOOCHEE, NC BUFFALO RIVER NEAR FLAT WOODS, TENN. WASHINGTON CREEK AT WINDIGO, MICH. POPPLE RIVER NEAR FENCE, WI BEAVER CREEK NR FINLEY, NO KAWISHIWI RIVER NEAR ELY, MN NORTH FORK WHITEWATER RIVER NEAR ELSA, MN BEAUVAIS CREEK NEAR ST. XAVIER, MT. BEAR DEN CREEK NR MANDAREE, NO CASTLE CR ABOVE DEERFIELD RES NEAR HILL CITY, SO 150 ENCAMPMENT RIV AB HOG PARK CR NR ENCAMPMENT, WYO 119 DISMAL RIVER NR THEDFORD, NEBR ELK CREEK NEAR DECATUR CITY, IOWA NORTH SYLAMORE CREEK NEAR FIFTY SIX, ARK. HALF~OON CREEK NEAR MALTA, CO. BLUE BEAVER CREEK NR CACHE, OK KIAMICHI RIVER NR BIG CEDAR, OK BIG CREEK AT POLLOCK, LA SOUTH FORK ROCKY CREEK NEAR BRIGGS, TEX. RIO MORA NEAR TERRERO, NM VALLECITO CREEK NEAR BAYFIELD, CO. MOGOLLON CREEK NEAR CLIFF, NM WET BDTTOM CREEK NR CHILDS, ARIZ. REO BUTTE CREEK AT FT. DOUGLAS NR. SLC, UTAH STEPTOE C NR ELY, NV S TWIN R NR ROUND MOUNTAIN, NV MERCED R AT HAPPY ISLES BRIDGE NR YOSEMITE,CALif ELDER CREEK NEAR BRANSCOMB, CALif NORTH FORK QUINAULT R NEAR AMANDA PARK, WASH. HAYDEN CK BELOW N FK, NR HAYDEN LAKE, IDAHO ANDREWS CREEK NEAR MAZAMA, WASH. CACHE CREEK NEAR JACKSON, WYO BIG JACKS CREEK NEAR BRUNEAU, ID MINAM RIVER AT MINAM,OREG.

cil, 1983; Peters and others, 1982; Ritter and Brown, 1981; Likens and others, 1980).

Trends in alkalinity at Bench-Mark stations (fig. 3a) display a geographic pattern that is the approximate inverse of that of sulfate trends: over a broad area from the Southeast to the Northwest, down trends in alkalinity greatly outnumber up trends, while in the Northeast, alkalinity trends are consistently up. The inverse relation with sulfate trends is somewhat stronger among stations with low average alkalinity (dark symbols in fig. 3a) although a number of important exceptions to the pattern exist (for example, stations in South

MEAN TREND SLOPE

o. 51 0.47 0.04 0.46 0.71 0.27 0.49 0.80 0.53 0.81 0.38 0.29 0.65 0.74 0.82 0.79 0.82 0.41 0.58 0.88 0.23 0.49 0.93 0.83 0.87

o. 73

o. 92

o. 91

(yr-1)

0.010 o. 001 0.003 0.009 0.007 0.009 0.019 -0.008 0.002 -0.006 0.015 o. 001

o.ooo

-o. oo1

o.ooo

-0.002 -0.004 -0.009 -0.005 -0.004 -0.002 -0.006 0.004 -0.020 -0.010 -0.001 -0.008 -0.020 -0.006 -0.001 -0.002 o. 001 0.002 -0.013 -0.003 -0.009 -0.008 -0.025 0.0.01 0.005 -0.016

LEVEL (p)

0.087 0 .. 543 o. 024 0.002 0 .. 172 0.022 0.002 0.315 0.555 o. 009 o. 006 0.603 0.855 0.434 0.002 0.524 0.091 0.074 0.027 o. 001 0.942 0.1 58

o.ooo

o.ooo

o.ooo

o.ooo o.ooo

o.ooo

o. 000 0.004 o. 770 0.251 0.801 0.045 0.008 0.188

o.ooo

o.ooo o. 138

o.ooo Carolina and Minnesota). On average, alkalinity trend slopes are of the same order of magnitude as sulfate trend slopes (see tables 2 and 3), but may differ considerably in magnitude from sulfate trends in a station-by-station comparison. An inverse relationship between sulfate and alkalinity is expected if the sulfate represents the introduction of sulfuric acid to the stream system and if that acid acts to reduce stream alkalinity rather than to dissolve minerals in the stream basin (Burns and others, 1981; Kramer and Tessier, 1982). To the extent the acid reacts with rock and soil, however, it is not available to reduce stream alkalinity. Also, because alkalinity itself occurs as a result of mineral dissolution, it follows that an inverse relationship between trends in sulfate and alkalinity will be strongest in low alkalinity waters. In fact, in basins characterized by carbonate weathering (and very high alkalinity) the introduction of strong acid may result in an increase in alkalinity (Kilham, 1982).

NO TREND p > 0.2 ' 0.1 <p<0.2 '0.01 <p < 0.1 'p< 0.01

FIGURE 4.-Trends in pH at Bench-Mark stations for the period of record through 1981. Symbols indicate direction and significance level of trends. Dark symbols indicate stations with mean alkalinity less than 1 meq L-. Several investigators (Burns and others, 1981; Kramer and Tessier, 1982) have suggested using the ratio of alkalinity to major cation concentrations as an index of acidification of surface waters in order to overcome the confounding alternative effects of acidification in different basins (that is, a loss of alkalinity versus an increase in mineral dissolution). The ratio can only decrease (slowly in the presence of carbonate minerals in the drainage basin, more rapidly in their absence) as a result of an increased acid input to the system. In accordance with the above theory, trends in the ratio of alkalinity to total major cation concentration (sum of sodium, potassium, calcium, and magnesium) at Bench-Mark stations (fig. 3b; see also table 4) follow a more consistent inverse relation to sulfate trends than do alkalinity trends: declining values have occurred over a broad region extending from the Mississippi Valley westward, while rising values have occurred at most eastern stations as far south as South Carolina. TABLE 5.-Summary statistics for pH at Bench-Mark stations, for the period of record through 1981

STATION NUMBER

10542.00 13621.98 14665.00 MCDONALDS B IN LEBANON STATE FORESTr NJ 15456.00 20388.50 HOLIDAY CREEK NEAR ANDERSONVILLE, VA. 21353.00 21973.00 22126.00 FALLING CREEK NEAR JULIETTE, GA. 23271.00 24502.50 24791.55 32372.80 UPPER TWIN CREEK AT MCGAW, OH 32767.00 34600.00 CATAlOOCHEE CREEK NEAR CATALOOCHEE, NC 36040.00 40010.00 40637.00 POPPLE RIVER NEAR FENCE, WI 50649.00 BEAVER CREEK NR FINLEY, NO 51244.80 KAWISHIWI RIVER NEAR ELY, MN 53760.00 NORTH FORK WHITEWATER RIVER NEAR ELBA, MN 62882.00 63325.15 64090.00 CASTLE CR ABOVE DE!RfiElD RES NEAR HILL CITY, SO 134 66238.00 ENCAMPMENT RIV AB HOG PARK CR NR ENCAMPMENT, WYO 95 67759.00 68979.50 70607.10 70830.00 73112.00 BLUE BEAVER CREEK NR CACHE, OK 73357.00 73730.00 81039.00 SOUTH FORK ROCKY CREEK NEAR BRIGGS, TEX. 83779.00 93529.00 94306.00 MOGOlLON CREEK NEAR CLIFF, NM 95083.00 WET BOTTOM CREEK NR CHILDS, ARIZ. 101722.00 102449.50 STEPTOE C NR ELY, NV 102493.00 S TWIN R NR ROUND MOUNTAIN, NV 112645.00 MERCED R AT HAPPY ISLES BRIDGE NR YOSEMITErCALIF 114755.60 ELDER CREEK NEAR BRANSCOMB, CALIF 120393.00 124160.00 HAYDEN CK BELOW N FK, NR HAYDEN LAKEr IDAHO 124473.90 ANDREWS CREEK NEAR MAZAMA, WASH. 130183.00 131695.00 BIG JACKS CREEK NEAR BRUNEAU, ID 133315.00 MINAM RIVER AT MINAM,OREG.

TRENDS IN STREAM pH

Trends in pH at Bench-Mark stations (fig. 4; see also table 5) do not follow a clear regional pattern and are only partly consistent with trends in sulfate and alkalinity. Approximately equal numbers of increasing and decreasing trends in pH have occurred nationally with down trends occurring much more frequently than up trends at low alkalinity stations (shaded symbols). An important divergence from the geographical pattern evident in figures 2 and 3 is that stations in New York and Maine show down trends in pH despite the fact that sulfate and alkalinity trends in those states suggest a slight lessening of acidification. Several possible explanations for the apparent inconsistencies between trends in pH and the other major ions are worth noting. First, pH re-

STATION NAME AND LOCATION

WILD RIVER AT GILEAD, ME ESOPUS CREEK AT SHANDAKEN, NY

YOUNG WOMANS CREEK NEAR RENOVO, PA.

SCAPE ORE SWAMP NEAR BISHOPVILL£, SC UPPER THREE RUNS NEAR NEW EllENTON, SC

SOPCHOPPY RIVER NR SOPC~OPPYr FLA. SIPSEY FORK NEAR GRAYSON, Al CYPRESS CREEK NR JANICE, MS.

SOUTH HOGAN CREEK NEAR DILLSBORO, IND.

BUFFALO RIVER NEAR FlAT WOODS, TENN. WASHINGTON CREEK AT WINDIGOr MICH.

BEAUVAIS CREEK NEAR ST. XAVIER, MT. BEAR DEN CREEK NR MANDAREE, NO

DISMAL RIVER NR THEDFORD, NEBR ELK CREEK NEAR DECATUR CITY, IOWA NORTH SYlAMORE CREEK NEAR FIFTY SIX, ARK. HALFMOON CREEK NEAR MALTA, CO.

KIAMICHI RIVER NR BIG CEDAR, OK BIG CREEK AT POLLOCK, LA

RIO MORA NEAR TERRERO, NM VALLECITO CREEK NEAR BAYFIELD, CO.

REO BUTTE CREEK AT FT. DOUGLAS NR. SLC, UTAH

NORTH FORK QUINAULT R NtAR AMANDA PARK, WASH.

CACHE CREEK NEAR JACKSON, WYO

NUMBER MEAN OF SAMPLES (std. units) (std. units yr-1)

cords at Bench-Mark stations are somewhat shorter than records for the other major ions (compare figs. la and 1b) and in some basins do not not cover periods when significant changes occurred in the other constituents. Second, alkalinities at most Bench-Mark stations (fig. 3a) are high enough to provide considerable resistance to changes in pH; the sulfate changes reported here are mostly small (see above) and would not be expected to cause significant changes in pH at the prevailing alkalinities of many stations. Third, the lack of comparable nitrate records makes it difficult to evaluate the role of atmospheric nitrogen in acid deposition at these stations. Precipitation data from Hubbard Brook, New Hampshire (Likens and others, 1980) show an increase in nitrate from 1964 until the early 1970's, followed .by a leveling off or slight decline since that time. Pre-

-o. 02

o. 01

o.oo

-o.oo

o. 01 o. 01

-o.oo

o. 01

o. 01

LEVEL (p)

o. 011 o. 001 0.320 0.217 0.034 0.116 0.480 0.500 0.003 0.187 0.274 0.002 0.360 0.059

o. 778

o. 573

o.ooo

o.ooo

o. 623 o. 597

o.ooo

o.ooo

o. 314

o. 681

TABLE 6.-SO~ emissions by state for period of record 1965- 1980. Data are recent revisions of SO~ emissions presented in Rivers and Riegal, 1982 (G. Gschwandtner and K. Gschwandtner, Pacific Environmental Services, Durham, NC, written communicatin, 1983).

STATE

Alabama Arizona Arkansas Ca 1iforni a Colorado Connecticut Delaware D.C. Florida Georgia Idaho Illinois Indiana Iowa Kansas Kentucky Louisiana Maine Maryland Massachusetts Michigan Minnesota Mississippi Missouri Monatana Nebraska Nevada New Hampshire New Jersey New Mexico New York North Carolina North Dakota Ohio Oklahoma Oregon Pennsylvannia Rhode Island South Carolina South Dakota Tennessee Texas Utah Vermont Virginia Washington West Virginia Wisconsin Wyoming

cipitation pH at the same site has varied considerably since 1964 but has shown no clear trend over the period.

SUMMARY

Water-quality records collected over a 10--15- year period from the Hydrologic Bench-Mark Network, a nationwide network of sampling stations in predominantly undeveloped stream basins, 1965

1219 1173 show small declines in stream sulfate at stations in the northeastern quarter of the Nation and small increases in sulfate at a number of southeastern and western sites. Stream sulfate concentrations at most Bench-Mark stations are low enough to be significantly influenced by changes in the rate of atmospheric deposition of sulfur. The geographic pattern of trends in atmospheric S02 emissions from 1965 to 1980 approximately coincides with the pattern of sulfate trends at Bench-Mark stations, and tends to support the hypothesis that the stream sulfate trends reflect regional trends in sulfur deposition rates.

tions follow a regional pattern that is the approximate inverse of that of the sulfate trends: small increases have occurred at most stations in the Northeast and small decreases have occurred at many stations in the South and West. The inverse relationship is strongest at stations with relatively low mean alkalinity (< 1 meq L-).

cation concentration can be used as an index of the geochemical effects of acidification in a stream basin because of the tendency of that ratio to decrease in response to increased acid inputs even in cases where the acid acts to dissolve minerals in the basin rather than reduce stream alkalinity. Accordingly, trends in the ratio of alkalinity to total major cation concentration at Bench-Mark stations follow a more consistent inverse relation to sulfate trends than do alkalinity trends: declining values have occurred over a broad region extending from the Mississippi Valley westward, while rising values have occurred at most eastern stations as far south as South Carolina.

low a clear regional pattern and are only partly consistent with trends in sulfate and alkalinity. Several factors make a strong relationship between pH trends and sulfate trends unlikely, however. These include shorter record lengths for pH data, possible conflicting effects of the nitrogen component of acid deposition, and sufficient alkalinity in many basins to resist significant changes in pH.

a broad regional basis the data presented in this report show a consistent relationship between trends in S02 emissions and trends in stream sulfate, alkalinity, and the ratio of alkalinity to the total major cation concentration. Trends in stream alkalinity at Bench-Mark sta-

Trends in pH at Bench-Mark stations do not fol-

Despite inconsistencies at individual stations, on In the northeastern quarter of the country, 802 emissions have decreased over the past 15 years and the trends in the cited chemical characteristics of Bench-Mark streams are consistent with a hypothesis of decreased acid deposition in that region. Throughout much of the remainder of the country, 802 emissions have increased and trends in stream sulfate, alkalinity, and alkalinity/total cation ratios are consistent with a hypothesis of increased acid deposition.

REFERENCES

Biesecker, J. E., and Leifeste, D. K., 1975, Water quality of hydrolocyc bench marks: U.S. Geological Survey Circular 460-E, 21 p. Bubenick, D. V., Record, F. A., and Kindya, R. J., 1983, Acid rain-an overview of the problem: Environmental Progress, v. 2, no. 1, p. 15-31. Burns, D. A., Galloway, J. N., and Hendrey, G. R., 1981, Acidification of surface waters in two areas of the eastern United States: Water, Air, and Soil Pollution, v. 16, p. 277--285. Cobb, E. D., and Biesecker, J. E., 1971, The national hydrologic Bench-Mark network: U.S. Geological Survey Circular, 460-D, 38 p. Hicks, B. B., Wesely, M. L., and Durham, J. L., 1981, Critique of methods to measure dry deposition; concise summary of workshop: in Energy and Environmental Chemistry, v. 2, Acid Rain; chapter 12, p. 205-223, Ann Arbor Science Publishers, Ann Arbor, Michigan Hirsch, R. M., Slack, J. R., and Smith, R. A., 1982, Techniques of trend analysis for monthly water quality data: Water Resources Research, v. 18, no. 1, p. 107-121. Kilham, P., 1982, Acid precipitation: its role in the alkalization of a lake in Michigan: Limnology and Oceanography, v. 27, no. 5, p. 856--867. Kramer, J., and Tessier, A., 1982, Acidification of aquatic systems: A critique of chemical approaches: Environmental Science and Technology, v. 16, no. 11, p. 606A-614A. Likens, G. E., Bormann, F. H., and Eaton, J. S., 1980, Variations in precipitation and streamwater chemistry at the Hubbard Brook Experimental Forest during 1964 to 1977: in Hutchinson, T. C. and Havas, M., eds., Effects of acid precipitation on terrestrial acosystems, Plenum Press, New York, N.Y., p. 443--464. National Research Council, 1981, Atmosphere-biosphere interactions: Toward a better understanding of the ecological consequences of fossil fuel combustion: Washington, D.C., National Academy Press. --, 1983, Acid deposition: atmospheric processes in eastern North America: National Academy Press, Washington, D.C., 375 p. Niemann, B. L., 1983, Sulfur, nitrogen, and ammonia emissions and depositions (wet and dry) on a state/province basis for current emissions and proposed reductions: Report to the Office of Technology Assessment, Congress of the United States, Washington, D.C. Peters, N. E, Schroeder, R. A., and Troutman, D. E., 1982, Temporal trends in the acidity of precipitation and surface waters of New York: U.S. Geological Survey Water-Supply Paper 2188, 35 p. Ritter, J. R., and Brown, A. E., 1981, An evaluation of the effects of acid rain on low conductivity headwater ljltreams in Pennsylvania: U.S. Geological Survey Open-File Report 81-1025, 53 p. Rivers, M. E., and Riegal, K. W., 1982, Report no. 3B final, prepared in fulfillment of Memorandum of Intent on Transboundary Air Pollution signed by Canada and the United States, August 5, 1980. Skougstad, M. W., Fishman, M. J., Friedman, L. C., Erdmann, D. E., and Duncan, S. S., eds, 1979, Methods for determination of inorganic substances in water and fluvial sediments: Techniques of Water-Resources Investigations of the United States Geological Survey, Book 5, Chapter Al. Smith, R. A., Hirsch, R. M., Slack, J. R., 1982, A study of trendsin total phosphorus measurements at NASQAN stations: U.S. Geological Survey Water-Supply Paper 2190, 34p.

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