T. A., J . Air Pollut. Control Assoc., 23,277-81 (1973). (16) Katz, M., Sakuma, T., Ho, A,, Enuiron. Sci. Technol., 12,909-15 (1978). (17) Pierce, R. C., Katz, M., Anal. Chem., 47,1743-8 (1975). (18) Sawicki, E., Elbert, W. C., Hauser, T. R., Fox, F. T., Stanley, T. W., J . A m . Ind. Hyg. Assoc., 21,443-51 (1960). (19) Dong, M., Locke, D. C., Ferrand, E., Anal. Chem., 48, 368-72 (1976). (20) Gordon, R. J.,Enuiron. Sci. Technol., 10,370-3 (1976).
(21) Salamone, M. F., Heddle, J. A,, Katz, M., Enuiron. Int., 2 , 3 - 4 3 (1979). ( 2 2 ) Searle, C . E., Ed., ACS Monogr., No. 173 (1976). Received /or reuiew November 19, 1979. Accepted March 25, 1980. This uork was partially supported by a contract grant /rom the Ontario Ministry (if the Enuironment, Provincial Lottery Fund, Project No. 78-010-33.
Chemistry of Precipitation at Gainesville, Florida Charles D. Hendry and Patrick L. Brezonik' Department of Environmental Engineering Sciences, University of Florida, Gainesville, Fla. 326 1 1
Chemical analyses of wet-only and bulk precipitation collected from June 1976 to July 1977 indicate that rain is the predominant deposition mechanism for s04'-, NH4+, Nos-, Mg2+, and K+, but dry fallout is of comparable importance to rainfall for deposition of Na+, C1-, and Ca2+.Heavy metal levels (especially Zn) were highly variable in both types of samples. The (volume-weighted) average concentrations of total nitrogen, NH4+, and NOS- in bulk precipitation were 0.82, 0.12, and 0.23 mg of NIL, respectively. Averages for ortho- and total phosphate were 24 and 85 pg of PIL, respectively, in bulk precipitation and 19 and 34 pgIL in wet-only samples. Highest nutrient concentrations occurred in spring and early summer. Total loadings of N and P from bulk precipitation (1.15 g of N and 0.12 g of P/(m2.year)) are above permissible rates (relative to eutrophication) for shallow lakes. The volume-weighted mean p H of wet-only precipitation was 4.53; bulk precipitation had a slightly higher pH. Lowest pH values occurred in spring and summer. Acidity titrations and ionic balances indicate that rainfall acidity resulted mainly from sulfuric acid (69%)and nitric acid (23%). Geochemical cycles of many elements have important atmospheric pathways. Various contaminants are emitted into the atmosphere by human activities and enter these natural cycles; wet precipitation and dry fallout return these contaminants to earth where they can affect biologic processes in aquatic and terrestial ecosystems valuable to man. Since precipitation is instrumental in cleansing the atmosphere of natural and artificial contaminants, the study of precipitation chemistry is a convenient method for monitoring atmospheric contamination. The relative ease of sample collection has given rise to many precipitation studies. The chemistry and acidity of precipitation in Europe and the northeastern United States have been well studied (1-4). Nutrient levels of rainfall in temperate regions similarly have received considerable study, primarily as a result of efforts to compute nutrient budgets for lakes in eutrophication management studies (see reviews in ref 5-7). In comparison, relatively little information is available on the general chemistry, acidity, and nutrient content of rainfall1 over the southeastern United States. The results presented here are based on a year-long (June 1976-June 1977) study of bulk and wet-only precipitation in north-central Florida. This study had the following objectives: (1)to determine fluxes of nutrients (Nand P forms) and major ions to aquatic and terrestrial systems from bulk and wet precipitation; (2) to determine the acidity of precipitation and the principal acids responsible for the observed acidity; (3) to examine the importance of dry fallout as a source of mineral input by comparing bulk and wet-only precipitation fluxes; (4) to evaluate the relative importance of rainout and washout 0013-936X/80/0914-0843$01.OO/O
processes for removal of various substances from the atmosphere by rainfall; and (5) to determine ambient levels of heavy metals (Cd, Pb, Cu, and Zn) in Florida rainfall. Previous Studies Although several previous studies have reported on the chemical composition of precipitation in Florida, most studies had limited objectives and involved only a few parameters or short sampling periods. Comprehensive data on the chemical composition of rainfall in Florida thus are scarce. The most important previous studies on Florida rainfall chemistry are described below. Junge (8)and Junge and Werby (9)reported on the major ions (including NH4+ and NOB-) in wet-only rainfall collected during 1955-1956 a t four Florida locations as part of a nationwide study of precipitation. A second national sampling network (10) in operation from 1960 to 1966 included a station in Tampa and reported data on major ions, inorganic N, and heavy metals in wet-only precipitation. The nutrient content (Le., nitrogen and phosphorus forms) of Florida rainfall has been measured a t several sites in north-central Florida (11, 1 2 ) , near Lake Okeechobee (13), Fort Lauderdale ( 1 4 ) ,and Tallahassee (15).The latter authors also measured pH and major ions in rainfall a t Tallahassee during 1974-1975. Their data and those of Bourne (16) for a site near Gainesville during 1974-1975 apparently are the earliest measurements on the pH of Florida precipitation. Sampling Procedures and Analytical Methods Wet-Only Precipitation Collection. Wet-only precipitation was collected manually from July 1976 to July 1977, on the University of Florida campus, using a collector constructed of polyethylene sheeting attached to a (1m diameter) plastic ring 2 m above ground level. The sheet formed a funnel that channeled rainfall into tygon tubing leading to an 18-L polyethylene reservoir. The collector was kept covered to exclude dry fallout until near the start of a rain event, when it was uncovered and rinsed with deionized, distilled water, and the reservoir was connected to the tubing. Immediately following the rain event, the sample was collected, and the collector was covered again. A 50-mL aliquot was acidified by adding 0.5 mL of redistilled "03 for metal analyses. The remaining sample was stored a t 4 "C until chemical analyses were run. Bulk Precipitation Collection. Bulk precipitation was collected approximately 8 km northwest of Gainesville a t a cypress swamp site located on a large pine plantation. The cypress swamps are currently under study as possible sites for disposal and natural treatment of secondary sewage effluent (17).Bulk precipitation was collected from June of 1976 t o July of 1977, atop a 20-m steel tower at the center of one of the
@ 1980 American Chemical Society
Volume 14, Number 7, July 1980
843
Table 1. Average Weighted Concentrations and Annual Loadings for Rainfall Collected at Gainesville, Florida, 1976-1977
parameter
PH H+ TOC TON NH4+-N
PO4-P total P Na+ K+ Ca2+ Mg2+ CIS042-
site one a (wet only) annual loading, g/( m2.year)
av concn, mglL
4.53 0.0295 5.20 0.41 0.10 0.19 0.019 0.034 0.44 0.20 0.41 0.12 0.98 2.05
0.037 6.32 0.50 0.12 0.23 0.022 0.041 0.53 0.24 0.50 0.14 1.18 2.43
Sile I W O av concn, mg/L
4.64 0.0229 9.53 0.47 0.12 0.23 0.024 0.085 0.82 0.26 0.81 0.18 1.88 2.27
(bulk) annual loading, g/(m2*year)
0.021 1 1.53 0.57 0.15 0.28 0.029 0.102 0.99 0.31 0.98 0.22 2.27 2.74
Average weighted concentration of 64 individualrain events collected July 1976 to October 1977. Average weighted concentration of 27 weekly and biweekly samples collected June 1976 to July 1977.
small cypress swamps. The top of the tower extends above the tree canopy. Bulk precipitation collectors were constructed after the design of Likens ( 3 ) . Each collector consisted of two polypropylene funnels (14.5 cm diameter) fitted with glass wool plugs to collect the precipitation and channel it through tygon tubing into separate 1-L polyethylene containers. A vapor barrier was provided by a loop in the tubing to prevent evaporation or ammonia loss or gain. An aliquot (0.5 mL) of saturated mercuric chloride was placed in one reservoir prior to installation to preserve nutrient forms in rainfall. The other collector received no preservative, and samples from it were analyzed for all other parameters. The collectors were examined on a weekly basis. If insufficient precipitation had been collected to do all of the analyses, the collectors were left out an additional week, a t which time they were replaced with clean funnels, tubing, reservoirs, and mercuric chloride. During the rainy season, June to mid-September, 1976, samples were collected weekly, but 2-week collection periods were the norm for the rest of the study. A rain gauge located on top of the tower measured the amount of rainfall during each sample period. Analytical Methods. Specific conductance and pH were measured immediately after the samples were returned to the laboratory. Measurement of p H was done with a Corning Model 12 Research pH meter and combination electrode. Standardizations were made with Fisher Certified pH 4.00and 7.00 standards. Conductivity was measured according to the standard method (18), using a conductivity bridge standardized with 0.01 M KC1 solution. Acidity titrations were followed potentiometrically to pH 7.00 with a combination glass electrode. Samples (100 mL) were purged with Nz prior to titration with 5 X 10-3 M NaOH, and the titration vessel was purged with Nz during the titrations to eliminate atmospheric COa. The working solution of NaOH was standardized against potassium acid phthalate prior to and immediately following each set of analyses, Ca, Mg, Na, K, and Zn were analyzed by atomic absorption spectrophotometry using an air-acetylene flame and instrument settings recommended by the instrument manufacturer. Lanthanum and HCl were added to avoid interferences in the Ca and Mg determinations. A graphite furnace (Varian Model CR63) was used for Pb, Cu, and Cd analyses. Optimum drying 844
Environmental Science & Technology
time for rainfall samples was 20 s a t 1.2 V. Atomization times and voltage settings for each metal followed recommendations of the manual. Sample aliquots for metal analyses were stored in highdensity linear polyethylene bottles which had been leached for 3 days in 8 N nitric acid. The above metals were not detectable in deionized water blanks run on the acid-leached bottles. Standard procedures ( 1 8 , 1 9 )were used with a Technicon Auto-Analyzer for analysis of N03-, NH4+, total Kjeldahl nitrogen, orthophosphate, and C1-. Total phosphorus determinations were made using the single reagent molybdenum blue procedure and absorbance measurements a t 885 nm, following persulfate digestion (18).Total and inorganic carbon determinations were made by combusting samples in a Beckman Model 915 TOC analyzer. Organic carbon was determined by differences between the total carbon and inorganic carbon channels. Sulfate was measured turbidimetrically (18)using a 4-cm cell path. Interferences were eliminated by placing an aliquot of the sample (treated with the conditioning reagent but minus barium chloride) in the reference cell of the spectrophotometer. The detection limit for S042was 0.5 mg/L.
Results a n d D i s c u s s i o n Volume-weighted average concentrations in milligrams per liter and loading rates in grams per square meter per year were computed for the major ions and nutrient forms in wet and bulk precipitation collected over the study period. As shown in Table I, the concentrations of all constituents, except H+, were greater in bulk precipitation than wet-only precipitation, reflecting the influence of dry fallout. In general, solute concentrations are inversely related to the quantity of rainwater falling during a storm. Computing volume-weighted average concentrations tends to equalize the importance of light rains, in which concentrations are relatively high, and heavy rains, in which constituents are more dilute. Loading rates (g/m2-year)were calculated using the total rainfall for calendar year 1976 (121.5 cm), which is below the normal rainfall for Gainesville (70-year average = 140 cm). Thus, the fluxes listed in Table I probably underestimate the loadings for years of normal or above normal rainfall. Major Ions. Chloride and sulfate were the predominant anions in both bulk and wet-only precipitation. The average chloride level in bulk precipitation (B) was almost double that of wet-only precipitation (W) (B/W = 1.9). On the other hand, the average sulfate concentration in bulk precipitation was only about 10% higher than the wet-only value (B/W = 1.10). These results indicate that both dry fallout and rainfall are important contributors of chloride to the area, but rainfall accounts for nearly all the sulfate flux from the atmosphere. According to Junge (20),the major source of C1- in rainfall is sea salt. Historical data (9)indicates that rainfall a t coastal sites in Florida has higher C1- concentrations than that a t inland stations. The Cl-/Na+ ratio (2.3) in the wet-only and bulk samples collected in this study is higher than the ratio in seawater (1.8), indicating a slight enrichment of C1- in rainwater over that derived from sea salt. The S042-/Na+ ratio in seawater is 0.25, and the ratios in wet-only and bulk rainfall (4.7 and 2.8, respectively) reflect anthropogenic emissions of SO2 from fossil fuel combustion. The average sodium levels in bulk samples (0.82 mg/L) were nearly twice those of wet-only samples (0.44 mg/L; B/W = L9),indicating that dry fallout of Na+-containing particulates is an important deposition process. Like C1-, Na+ in rainfall has the sea as its main source. Coastal stations in Florida also have higher Na+ levels in rainfall than do inland stations (9).
Hydrogen ion was the major cation in the rainfall samples, accounting for 32% of the cation equivalents. Other cations contributed in the order: Ca2+ (23%),Na+ (21%), NH4+ (8%), and K+ (5%).According to Junge (20), calcium, magnesium, and potassium in rainwater are derived primarily from soil material. However, the combustion of fossil fuels undoubtedly contributes to a certain degree, especially in urban areas. The B/W ratios indicate that dry fallout is as important a source of Ca as is rainfall (B/W = 1.91, but rainfall is a more important source of Mg and K (B/W = 1.5 and 1.3, respectively). Ammonium and nitrate are also major ions in rainfall, comprising roughly 7 and 15% of the cations and anions, respectively. Nutrients (Nitrogen and Phosphorus). In both wet-only and bulk precipitation, nitrogen was predominantly in organic form (see Table I). Average (volume-weighted) TON in wetonly precipitation collected over the year was 0.41 mg/L (59% of the T N content), while the total inorganic N (NH4+-N N03--N) averaged 0.29 mg/L (41%). The relative proportions of TON and TIN in bulk precipitation were the same as in wet-only precipitation. Few data on TON in rainfall are available for comparison. Brezonik et al. (11)found that TON averaged 0.18 mg/L (volume-weighted basis) in bulk precipitation a t a rural site 40 km east of Gainesville, and Bourne ( 1 6 )reported an average of 0.25 mg/L for organic nitrogen in bulk precipitation collected 10 km north of Gainesville during 1974-1975. Ammonium and nitrate averaged 0.10 and 0.19 m g L (as N), respectively, in wet-only precipitation, and 0.12 and 0.23 mg/L in bulk precipitation (Table I). B/W ratios for both ions are 1.2, indicating that rainfall is the predominant deposition mechanism for these substances from the atmosphere. Historical data on both ions in Florida rainfall extend over a period of 20 years. Average ammonium levels of 0.03 mg/L and average nitrate levels of 0.04 mg/L were reported for the mid-1950s and mid-1960s (9,10). Average volume-weighted levels of 0.08 and 0.12 mg/L were reported (11)for bulk precipitation over rural north Florida in 1969. Joyner (13) reported mean average concentrations of 0.27 mg/L for ammonium and 0.11 mg/L for nitrate in rainfall collected during 1969 near Lake Okeechobee. Finally, ammonium and nitrate averaged 0.11 and 0.19 mg/L, respectively, in rainfall near Tallahassee during the mid-1970s (15). Thus, considerable spatial variability apparently exists, but the historical record suggests an increase in the inorganic nitrogen content of Florida rainfall over the past 20 years. Total phosphate averaged 34 and 85 p g L (as P) in wet-only and bulk sampleG4, respectively (B/W = 2.5), while orthophosphate averaged 19 and 24 pg/L (as P) (B/W = 1.2). Thus, dry fallout is more important as a source of total phosphate, but most of the atmospherically derived orthophosphate comes from rain. The B/W ratio for total P suggests that large dust particles that settle rapidly are a primary source of phosphate in bulk precipitation. Few historical data exist for phosphorus levels in Florida precipitation, and temporal and spatial trends are not identifiable. Most of the data on phosphorus in Florida rainfall has been gathered in connection with nutrient budget studies for lacustrine and forest ecosystems. Brezonik et al. (I 1) reported an average of 33 pg/L total P in bulk precipitation a t a rural site east of Gainesville, and Ewe1 e t al. (I:?) found 120 and 80 pg/L total P in winter and summer rainfall, respectively, a t a site near Gainesville. Joyner (13) reported a mean total P concentration of 40 pg/L in rainfall collected near Lake Okeechobee. Burton et al. (15) found total P concentrations of 37 and 60 pg/L in precipitation a t two sites near Tallahassee. Thus, levels in Florida rainfall are variable, and local influences such as agriculture likely are important factors controlling the concentrations. Significance o f Atmospheric Nutrient Loadings to Lakes,
WET ONLY
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Figure 1. Monthly volume-weighted average concentrations of nitrogen forms in wet-only and bulk precipitation at Gainesville, Fla.
Atmospheric loading of N and P can be a significant source of nutrients to lakes. The relative importance of atmospheric nutrient loadings depends, of course, on the contributions from other sources. In general, atmospheric sources are most important for lakes with low watershed/lake surface area ratios and small nutrient loadings from human activities. Bulk precipitation was estimated to contribute from 6 to 40% of the N loadings and 1 2 to 59% of the P loadings to 55 lakes in north-central Florida (21). The inputs of N and P via precipitation can be examined in the context of critical nutrient loading rates for lake eutrophication. The volume-weighted average concentration for total N in bulk precipitation in this study was 0.82 mg/L (Table I). The average annual rainfall for this area is 140 cm (55 in,),yielding an areal N loading of 1.15 g/(m2.year). This value is about half the “excessive” loading rate (2.0 g/(m2.year) associated with eutrophication and water quality degradation in shallow lakes (mean depth,
