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(21) Hillman, D. C. J.; Potter, J. F.; Simon, S. J. National Surface Water Survey, Eastern Lake Survey-Phase I , Analytical Methods Manual; U.S.Environmental Pro-
tection Agency: Las Vegas, NV, 1986;EPA-60014-861009. Landers, D. H.; Overton, W. S.; Brakke, D. F.; Linthurst, R. A,; Eilen, J. M. Environ. Sci. TechnoZ. 1988,22,12&135. Mansell, R. S.; Elseftawy, A,; Hunt, P. G. Soil Sci. 1972, 15, 113. Baker, L. A.; Brezonik, P. L.; Edgerton, E. S.; Ogburn, R. W., I11 Water, Air, Soil Pollut. 1985, 25, 215. Overrein, L. N.; Seip, H. M.; Tollan, A. “Acid Precipitation-Effects on Forests and Fish”;’finalreport; SNSF Project; SNSF: Oslo, Norway, 1980. Christophersen, N.; Seip, H. M.; Wright, R. F. Water Resour. Res. 1982, 18, 977. Baker, L. A.; Pollman, C. D.; Brezonik, P. L. “Development of Interpretive Models for Lake Acidification in Florida”; Florida Acid Deposition Study, Phase IV; Environmental Engineering and Sciences, Inc.: Gainsville, FL, 1985;ESE 83-152-0602-0120.
(28) Bayley, S. E.; Schindler, D. W.; Turner, M. A. Presented at the American Society of Limnology and Oceanography Annual Meeting, Madison, WI, June 1987. (29) Junge, C. E.; Werby, R. T. J.Meterol. 1958, 15, 417. (30) Erickson, E. Tellus 1960, 12, 63. (31) Delmas, R.; Servant, J. J. Rech. Atmos. 1974, 8, 861. (32) Mairs, D. F. Water Resour. Res. 1967, 3, 1090. (33) Lag, J. Acta Agric. Scand. 1968, 18, 148. (34) Seip, H. M. Nature (London) 1986, 322, 118. (35) Sutcliffe, D. W.; Carrick, T. R.; Heron, J.; Rigg, E.; Talling, J. F.: Woof. C.: Lund. J. W. G. Freshwater Biol. 1982.12. , , 451. I
,
Received for review February 9,1987. Accepted September 14, 1987. The research described i n this article has been funded as part of the National Acid Precipitation Assessment Program by the US.Environmental Protection Agency (EPA). I t has been subjected to the Agency’s peer and administrative review, and it has been approved for publication.
Reactions That Modify Chemistry In Lakes of the National Surface Water Survey Sljln Lee and Jerald L. Schnoor”
Department of Civil and Envlronrnental Engineering, University of Iowa, Iowa City, Iowa 52242 A simple mass balance equation was used to determine the important reactions that occur in selected lake watersheds of the Adirondack Park, the Southern Blue Ridge Province of the Appalachian Mountains, and a portion of northern Florida. The mass balance requires only three input parameters: precipitation ion concentration, lake ion concentration, and an evapoconcentration factor appliable to the watershed. It was determined that the most important alkalizing reaction in all three study areas was the production of base cations (Ca2+,Mg2+,and Na+) by ion exchange and chemical weathering, followed by sulfate adsorption in the Southern Blue Ridge watersheds and nitrate assimilation/reduction in the Adirondack Park. Dry deposition of sulfate was important to the sulfate balance in the Adirondack watersheds and was of the same order of magnitude as wet precipitation inputs. Chloride ion could not be used to provide an independent check on the mass balance methodology because unmeasured sources such as dry deposition and mineral dissolution appeared to be significant, especially in the Southern Blue Ridge Province and Florida watersheds. ~
Introduction The chemistry of lake water in dilute or headwater ecosystems is an admixture of precipitation solutes, dry deposition, and biogeochemical reaction products of the land and water (1). Precipitation water is modified as it passes through the canopy by leaching of vegetation and dissolution of dry deposited materials (2). Throughfall water is further affected by decomposition and mineralization products of leaf litter and organic soil (3). In the upper soil horizons, ion-exchange reactions, ammonia assimilation by vegetation, and microbially mediated nitrification reactions are likely to occur ( 4 ) . As the water percolates into the lower soil horizons, it becomes progressively more neutralized (less acidic) by sulfate exchange reactions and chemical weathering (dissolution) of minerals in the glacial till and bedrock (5-7). Lake chemistry represents a mixture of precipitation, runoff water, in190
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terflow, and groundwater chemistry. Within the lake, further reactions modify the chemistry, including nitrate and ammonium assimilation by plants and reactions at the sediment-water interface (e.g., cation exchange and sulfate reduction; ref 8-10). Lakes that have been acidified by acid deposition are those lacking carbonate minerals and characterized by sensitive hydrological settings (1). Schnoor and Stumm (I) presented various reactions that modify the H+balance in waters. Table I shows the stoichiometry of some of these processes. The overall change in alkalinity (or acid neutralizing capacity, ANC) of a lake can be calculated from the stoichiometry of the weathering reactioq, ion exchange, redox reactions, and biological production and respiration. In some instances, acidic deposition may be sufficient to disturb an existing H+balance and cause the acidification of a lake. A quantitative example is provided by the hydrogen ion budget in the Hubbard Brook Ecosystem by Driscoll and Likens (11). The purpose of this study was to assess (on a large-scale regional basis) which reactions are most important in controlling the acid-base chemistry of lakes, on the basis of a mass balance approach for major ions. There are two data sets that were used for the study. In 1984, the Eastern Lakes Survey of the National Surface Water Survey (NSWS) was begun by the U S . EPA (12,13) in order to assess the present acidity status of surface waters and to establish a base line for temporal variability associated with these surface waters. The second data base, of the Acid Deposition System (ADS) for Statistical Reporting, is a collection of precipitation chemistry data to monitor atmospheric deposition trends in the U.S.(14). The ADS data base was used to determine the chemistry of wet deposition to lakes of the NSWS in the eastern U.S. Lake chemistry was determined from the NSWS in order to estimate the net reaction or change of major cation and anion concentrations (Ca2+,Mg2+,K+, Na+, NH4+,Nos-, C1-, S042-). A simple mass balance equation was then used to calculate the magnitude of important processes that
0013-936X/88/0922-0190$01,50/0
0 1988 American Chemical Soclety
Table I. Typical Processes Which Modify the Alkalinity Balance in Surface Waters changes in alkalinity, equiv/mol reacted
-
typical weathering reactions calcite: CaCO, ( 8 ) H+ Cazt HCO, anorthite: CaAl&3iz08(s) + 2H+ + H20 Ca2++ AI,Si,O,(OH),(s) potassium feldspar: KALSi,O,(s) + H I + 4'/*H2O K+ 2H,SiO, + 1/2A12Si205(OH)4(s) olivine: MgFeSiO,(s) + 4H+ Mg2+ Fez++ H,SiO, albite: 2NaAISiaOs(s) 2H' 9H,O 2 Na+ + 4H4Si0, + Al2Si,O5(OH),(s) ion exchange/sorption sulfate sorption: 2ROH + SO," %SO, + 20R HR + Na+ Na exchange: NaR + Hf Ca exchange: C d b + 2H+ 2HR + Ca" redox proeesk (microbial mediation) NO, + He0 + 2H' nitrification: NH,+ + 201 5C02 + 2N1 + 7Hs0 denitrification: 5CH,O + 4 N O j + 4H' S0,l- reduction: SO,*- + 2CH20 + 2H' 2C02 + H,S + HzO ammonia assimilation: NHI+ 4C02 HCO, + H20 (sun) CSH7N02+ 502 5C02 4H10 (sun) C6HIN02 + 702 + OHnitrate assimilation: NO;
+
- - ++ + -- - + + + + -
+
+
+2 +2 +1 +4
+2 +2 +1 +2
-2 +1
+2 -1 +1
mcdify chemistry for each lake, and these are summarized on a regional basis. National Surface Water Survey (NSWS) The National Surface Water Survey was begun in the fall of 1984 with the Eastern Lakes Survey (phase I). Fall was chosen because thermal stratification is small at that time and chemical variations between the surface water and bottom water of a lake are reduced. Water chemistry variables in the survey included acidity, acid neutralizing capacity (ANC), extractable aluminm, total aluminum, dissoved ammonium, diasolved inorganic carbon, dissolved organic carbon, dissolved calcium, dissolved chloride, true color, total dissolved fluoride, dissolved iron, dissolved magnesium, dissolved manganese, dissolved nitrate, pH, total phosphorus, dissolved potassium, secchi disk transparency, dissolved silica, dissolved sodium, dissolved sulfate, specific conductance, and turbidity. A description of the analytical techniques and procedures is provided by the US. Environmental Protection Agency (12, 13). Quality assurance and quality control procedures on sampling and analytical techniques were quite rigorous (12, 13).
Selection of the final target population of lakes for inclusion within the Eastern Lake Survey-Phase I was a stepwise process. Fint, a population of lakes represented on 1:250000 US. Geological Survey (USGS) maps was identified. USGS maps of this scale generally include only those lakes greater than approximately 4 ha (hectares) in area. Therefore, lakes smaller than 4 ha were not included in the survey. Some lakes were subsequently identified as 'nonlakes", such as wetlands or flowing water, and were eliminated from consideration. The remaining lakes defined the preliminary target population for the Eastern Lake Survey-Phase I. These lakes were identified within subregions. Subregions were delineated on the basis of similarities in surrounding physical geography, land-use characteristics, and available water quality information. The initial target population of lakes was further reduced to a final target population after ELS-Phase I field work began. An effort was made to eliminate lakes that were obviously impacted by anthropogenic factors (agriculture, urban development, sewage treatment discharges,or mine tailings) and those that had appeared on maps hut which were found to he no longer in existence. Also eliminated from sampling were inaccessible lakes: lakes on which it was impossible to land a helicopter, lakes to which access permission was denied, or lakes which were found to be frozen.
Flgure 1.
Location of study area.
The sampling protocol was designed to sample only lakes greater than 1.5-m depth which were mixed during fall turnover. However, a few shallow lakes (less than 1.5 m in depth) were sampled if a sample free of sediment could be obtained. Likewise, thermally stratified lakes were sampled and identified if turnover had not yet occurred. A few lakes were dropped with specific conductance values greater than 1500 pS/cm. Methodology Study Area Selection. As shown in Figure 1, three study areas were selected from the NSWS subregions: the Adirondack Park of New York, the southern Blue Ridge Province, and the northern portion of Florida and extreme southern Georgia. The Adirondack Park of New York includes some lakes which have been affected by acidic deposition (15, 16),and the Southern Blue Ridge Province has soils which are reported to adsorb/exchange sulfate and thereby to neutralize acid deposition (18). The third study area in contrast is Florida with a distinct marine influence and proximity. These areas were selected because they are quite different geologically and hydrologically and have decreasing acid deposition from north to south. All NSWS lakes were selected which were within a 100-mileradius from a National Atmospheric Deposition Program (NADP) monitoring station. It was mumed that precipitation chemistry was roughly similar within each area (Adirondack Park, Southern Blue Ridge) or within Environ. Sci. Technol., Vol. 22. No. 2, 1988
191
Table 11. Mean Concentrations of Major Ions and Their Standard Deviations for Lakes in Each Study Area
study area
Ca2+
Mg2+
K+
Adirondack Park ( n = 147) Southern Blue Ridge (n = 73) Florida (n = 61)
195 f 194 133 f 236 576 f 803
67 f 70 60 f 47 403 f 630
8.8 f 6.3 28 f 20 91 f 143
a 100-mile radius (Florida). This assumption restricts the area and number of lakes selected in each subregion. Since there is only one precipitation monitoring station (Huntington, NY) in the Adirondack Park, it was assumed that all of the lakes (located between 43’07’40’’ N < latitude < 44°50’00” N and 73’22’30’’ W < longitude < 75’15’30’’ W) receive a uniform precipitation chemistry. It is known that the actual chemistry and pH of precipitation follows a gradient from southwest or northeast through the Adirondacks, but the NADP precipitation monitoring station is near the center of the area and was taken to be an acceptable average. (The volume-weighted pH of precipitation at the NADP station in Huntington, NY, is 4.43, while that in the southwest is pH -4.2 and in the northeast pH -4.5.) The number of lakes selected in the Adirondack Park was 147. In the case of the Southern Blue Ridge Province, there were two available precipitation monitoring stations. Seventy-three lakes were selected in the Southern Blue Ridge (73 out of 112 in the NSWS data set) which were between 34”lO’OO” N < latitude < 37’10’00” N and 82’30’00” W < longitude < 85’30’00’’ W. This area includes northeastern Georgia, western North Carolina, northwestern South Carolina, and eastern Tennessee. The state of Florida has one NADP precipitation station at Bradford Forest. The study area was restricted between 28’28’30” N < latitude < 31’28‘30’‘ N and 80’42‘00” W < longitude < 83’40/00” W on the basis of 100-mile radius selection criterion. The number of lakes selected in this region was 61 out of 141 in the NSWS data set. Variables in the Data Base. The NSWS data set contains four watershed descriptors (lake surface area, lake watershed area, lake elevation,pd lake depth) and 43 water chemistry parameters, with 1619 observations in 18 states. The NSWS deposition and land-use data set contains three wet deposition parameter values (H+, SO+:, and NO3-), annual precipitation, annual runoff data from map contour, and 11 land-use variables with 769 observations in nine states. The ADS 1984 annual data summary contains nine precipitation chemistry parameters H’, and pH) (Ca2+,Mg2+,K+,Na+, NH4+,NO,, Cl-, SO,-: with 65 measuring stations in 30 states. Since both data sets contain the major eight ions (Ca2+,Mg2+,K’, Na+, NH4+, NO3-, C1-, and S042-),the NSWS data set was utilized for lake chemistry, while the NADP data set was used for wet precipitation chemistry. Major ion concentrations of precipitation for the Adirondack Park and the state of Florida were precipitation volume-weighted mean values from 1984. In the case of the Southern Blue Ridge, the volume-weighted mean of two stations (1984 volume-weighted mean precipitation chemistry values) was used. Steady-StateMass Balance. The basic mass balance equation around a control volume (the watershed) for various ion species can be written as accumulation = inputs - outputs i reactions (1) where the positive sign (+) on the reaction term indicates production of the ion and the negative sign (-) indicates consumption or uptake of the ion in the watershed. The left-hand side of eq 1 will be equal to zero at steady state. 192
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major ions, requiv/L Na+ NH4+ 62 f 97 127 f 322 494 f 1790
2.1 f 1.9 3.2 f 4.2 4.8 f 13
so42-
NO
