Acidic deposition to streams A geology-based method
predicts their sensitivity .
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Owen P. Bncker U.S. Geological Survey Reston, K4 22092 Karen c. Rice U.S. Geological Survey Towson, MD 21204 All water that reaches watershed systems comes directly or indirectly from precipitation. Normally, this water contains very small amounts of dissolved solids and is only slightly acidic. As a result of chemical reactions in watersheds, however, stream water generated from precipitation normally is less acidic and contains larger concentrations of dissolved solids than does the precipitation falling onto the watershed. During the last decade, acidic deposition, the increased acidity of precipitation, and the effects of both on surface waters have caused increasing concern, particularly in the eastern United States. Emissions from the combustion of fossil fuels and from industrial proc-
esses are suspected of causing the substantially increased acidity of the dew sition in that region ( 1 , 2). Nevertheless, the response of Eastern watersheds to acidic deposition varies considerably. For a given input of acidic deposition, some surface waters have not shown any change in acidity, whereas others have become markedly acidified (3. 4). So fa& reasons for these differences in acidification are poorly understood. A reliable and efficient method for predicting the sensitivity of streams to acidification is needed
This anicle not subiecl lo US. Copyright. Published is89 American Chemical Society
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Envimn. Sci. Technol.. Vol. 23. No. 4.1989 379
FIGURE I
Ackknrutraliing capacity (ANC) at stream sampling sites and generalid geology in Frederick County, MD study area
to evaluate the environmental effect of
face waters withii each of these regions acidic deposition. provides an estimate of the sensitivity A major goal of the National Acid of surface waters in each region. Maryland's Department of Natural Precipitation Assessment Program (NAPAP) has been to assess the sensi- Resources has designed and impletivity of the nation's surface waters to mented a program to assess the acidifiacidification by acidic deposition (5). cation status of all streams withii that l b o studies-one national and the other state (13). Its approach was to suhdiconducted by the state of Marylandvide the state into physiographic provhave addressed this task. EPA, through inces and to apply to each province a NAPAP, has designed and conducted a prohabiiity sampling technique similar program on a national scale (6, 7). Its to that used by EPA. Apart from the approach is to use alkalinity maps of initial subdivision into sensitive rethe conterminous United States to iden- gions, both of thest approaches are statify those regions that are expected to tistical procedures that rely on no other contain surface waters susceptible to physical or chemical characteristics of acidification by acidic deposition (8- the system. They may provide an esti12). mate of the regional surface water acidThese maps show the acid neutraliza- ification; however, they have no prediction capacity (ANC) for all regions of tive capability for individual streams in the United States. ANC is defined as a population. An alternative method of assessing the capacity for solutes in an aqueous solution to react with and neutralize the sensitivity of the nation's surface acid; it is used as an index of the sensi- waters to acidification is through an untivity of streams to acidification. The derstanding of the fundamental procsampliig of a random subset of the sur- esses that control the chemistry of wa380 Ennmn. Sci. Technol., Vol. 23. No. 4, 1-
tershed systems. A numher of factors, such as c l i i , vegetation, soil, bedrock composition, microhial processes, and hydrology, are known to affect the chemistry of natural waters (14). In this article, we will evaluate one factor-bedrock composition-as a predictor of sensitivity of surface waters to acidification. This factor has been selected because a strong correlation between bedrock composition and the chemistry of natural waters has been well documented (15-26), and sufficiently detailed geologic information exists to apply this method to the conterminous United States. The purpose of the study on which this article is hased is to demonstrate the influence of bedrock composition on stream chemistry in a small region of the eastern United States and to introduce a geology-based method for estimating the percentage of streams in an area that are sensitive to acidic deposition. In this study, we have chosen to use alkalinity as an index of stream sensitivity to acidification. ANC is easily measured and it has been used by EPA (6, 7j, the state of Maryland ( 1 3 , and numerous other investigators (15, 2730) to characterize the sensitivity of surface waters to acidification. Other parameters such as base cations have been used as a measure of sensitivity of lakes to acidification (31, 32). These parameters, however, are more difficult to measure and have no substantial advantage over ANC for assessing the relative sensitivity of streams to acidification.
The study tuea The study area is located in Frederick County, Maryland, within the Blue Ridge and Piedmont physiographic provinces (Figure 1). Elevations range from 82-579 m above sea level; the average annual rainfall in the area is 112 cm (26). Approximately 50% of the study area is forested with deciduous trees; the other half consists of farmland and small towns. Detailed geological information is available (33-35). Geologic units exposed in the study area are the Catoctin Formation (metabasalt), the Weverton Formation (quartzite), the Loudoun and Harpers Formations (quartzites and phyllites), the Newark Supergroup (calcareous conglomerate, sandstone, and shale), and the Frederick Limestone (liiestone). Sampling and analysis Streams were sampled during a twoweek period in January 1988. Withii the study area, virtually all of the streams draining the Catoctin, Weverton, and Harpers Formations (Blue Ridge physiographic province) and
most streams draining the Newark Supergroup were sampled. Streams sampled on the Frederick Limestone all exhibited uniformly high ANC; thus, fewer streams flowing on the Frederick Limestone were sampled. The ground was covered with 8-10 cm of snow, and the average air tempxmue was below freezing during most of the sampling period. Under these. conditions, stream flow is generated primarily from groundwater contributions, and the stream chemistry should be representative of base-flow conditions (36). ANC, pH, and concentrations of base cations (Na+, K+, Ca+2, Mg+Z) should, therefore, be close to their maximum values. Streams draining two watersheds on the Catoctin Formation in the study area have been monitored for approximately six years and provide a record of seasonal variation in stream chemistry and discharge on that bedrock type (26,37). A comparison of samples collected in thii study with those from the two sites draining the Catoctin Formation suggests that the samples reflect base-flow conditions and were not influenced by snowmelt or surface run-
__ mean, and standard deviation of chemical parametem of
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Weathering reactions The neutraliition of both natural and man-made acidic inputs to watersheds is caused mainly by the reaction of hydrogen ions with primary minerals derived from the bedrock underlying the watershed. ANC in stream water draining watersheds is produced primarily through weathering of minerals by carbonic acid generated by microbid processes in the soil. If the bedrock contains a carbonate mineral such as calcite, 2 mol of bicarbonate are produced for each mol of calcite dissolved by carbonic acid CaC03 HZCO3Ca*2 2HCO; (1) The weathering of carbonate minerals by strong acids @IZSO4,HN03) produces 1 mol of bicarbonate ions per mol of carbonate mineral dissolved: 2CaC4 H2S04- 2Ca+2 2HCO; SO,-2 (2) If the silicate mineral albite is leached by a carbonic acid solution, 1 mol of bicarbonate ion is prcduced for each mol of albite dissolved: 2NaAlSi308+ 2HzC03 + (albite) AlzSi20,(OH)4 9H20-2Na+ (hOliinite) 4H.$i04 (as) 2HCO; (3) Sodium ions, dissolved silica, and the clay mineral kaoliite also are by-products of this reaction. Sodium and dis-
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58
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26
9
16-56
28
12
144-868 547
439 279
178 112
-21-119 37-153
13 94
33 28
-2-214 42-134
S2 87
85 31
1-369
103
91
