Environ. Sci. Technol. 1888, 23, 1402-1408
Retardatfon of Ammonium and Potassium Transport through a Contaminated Sand and Gravel Aquifer: The Role of Cation Exchange Marnle L. Ceazan" US. Geological Survey, Box 25046, MS 408, Denver Federal Center, Denver, Colorado 80225
E. Mlchael Thurman US. Geological Survey, 4821 Quail Crest Place, Lawrence, Kansas 66049
Rlchard L. SmRh US. Geological Survey, Box 25046, MS 408, Denver Federal Center, Denver, Colorado 80225
The role of cation exchange in the retardation of ammonium (NH4+)and potassium (K+) transport in a shallow sand and gravel aquifer was evaluated by use of observed distributions of NH4+and K+ within a plume of sewagecontaminated groundwater, small-scale tracer injection tests, and batch sorption experiments on aquifer material. Both NH4+and K+ were transported -2 km in the 4-kmlong contaminant plume (retardation factor, Rf = 2.0). Sediments from the NH4+-containingzone of the plume contained significant quantities of KC1-extractable NH4+ (extraction distribution coefficient, Kd,extr= 0.59-0.87 mL/g of dry sediment), and when added to uncontaminated sediments, NH4+sorption followed a linear isotherm. Small-scale tracer tests demonstrated that NH4+and K+ were retarded (Rf = 3.5) relative to a nonreactive tracer (Br-). Sorption of dissolved NH4+was accompanied by concomitant release of calcium (Ca2+),magnesium (Mg2+), and sodium (Na+)from aquifer sediments, suggesting involvement of cation exchange. In contrast, nitrate (NO,) was not retarded and cleanly separated from NH4+and K+ in the small-scale tracer tests. This study demonstrates that transport of NH4+and K+ through a sand and gravel aquifer can be markedly affected by cation-exchange processes even at a clay content less than 0.1% . Introduction Contamination of groundwater by inorganic nitrogen is recognized as a major environmental problem. Both NH4+ and NO< are common contaminants that result from pollution caused by fertilizer application and disposal of human and animal wastes (1-8). Dissolved N H 4 + ~ 8 Cn~ U W taste, odor, and health problems in drinking water supplies (9),and dissolved NO3- can cause methemoglobinemia in infants at concentrations >10 mg/L as N (10). The fate of NH4+and NO3- in groundwater is largely determined by the microbiology and geochemistry of the subsurface (11). The microbial populations in the aquifer can catalyze redox processes and, therefore, affect speciation of the inorganic nitrogen compounds (12-14). These species can be transported at different velocities based on the geochemical properties of the particular species and of the aquifer material. Although NO3- is generally absent in anaerobic contaminant plumes due to biological reduction to N2, N20, or NH4+ (14), it is persistent in oxidized groundwater and is often mobile, because of its high solubility and minimal sorption (15). The transport of NH4+, however, may be retarded by its chemical sorption properties (sorption is used here to describe both adsorptive and ion-exchange reactions) (16-20). The importance of cation exchange on clay as a mechanism of NH4+retention in surface soils is well-known (16, 21,22). However, many aquifer sediments tend to have less organic matter (0.01% vs 2% in soil), less clay, and 1402 Environ. Sci. Technol., Vol. 23, No. 11, 1989
a different hydrologic regime (saturated) than soil environments (unsaturated). Few studies have directly investigated the effect of cation exchange on NH4+transport in groundwater. For the most part, these studies were of aquifers containing a substantial proportion of clay (17, 19,20). However, results of a recent natural-gradient tracer study (23),and a study of groundwater contaminant distributions (24) indicated that the influence of cation exchange may be significant in sand and gravel aquifers that have relatively low clay content (less than 1.0%). In order to understand the role of cation exchange in transport of NH4+in an environment with low clay content, we conducted a study at a sand and gravel aquifer contaminated by secondary treated sewage that is located near Falmouth, MA. Interpretation of distributions of dissolved constituents in such an aquifer is often complicated by lack of information about the contaminant source. For example, in most studies, it is not possible to quantify the mass of contaminant that has entered the groundwater, or to determine whether the source has been constant over time. Batch experiments or small-scale tracer tests may be used in the absence of definitive on-site data. Geochemical variability in the aquifer, however, can make extrapolation of experimental results to larger scales difficult. We here describe a study, which overcame these individual difficulties by applying an integrated approach that included the following: (1)evaluation of groundwater chemistry data obtained by sampling wells located in the contaminant plume, (2) comparison of these results with batch NH4+sorption studies on aquifer material, and (3) small-scale tracer experiments, which tested the transport of NH4+,NO3-, and K+. The transport of K+ was evaluated in the tracer tests because retardation of K+ had been observed in a sandy aquifer with low clay content (23). The objective of our research was to determine the relative transport rates of NH4+and NO3- and to determine the effect of cation exchange on NH4+and K+ transport in this sand and gravel aquifer. Site Description The study site is located on a glacial outwash plain in southwestern Cape Cod, approximately 100 km southeast of Boston, MA, (Figure 1). The aquifer sediments, which were deposited as glacial outwash, contain less than 0.1 % clay (25). Since 1936, the unconfined aquifer has been contaminated by continuous disposal of secondary treated sewage effluent at Otis Air Base (26). The treated sewage is discharged onto sand infiitration beds where it percolates approximately 6.5 m to the water table. Groundwater flow is to the south-southwest (Figure 1)at an average velocity of 0.3 m/day (27). The disposal of the treated sewage has formed a plume of contaminated groundwater. The nature of the contaminant plume and the hydrogeologicalfeatures
Not subject to U S . Copyright. Published 1989 by the American Chemical Society
Massachusetts
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Figure 1. Groundwater study site on Cape Cod, MA, showing location of observation wells. Groundwater was sampled from 1985 to 1986. Arrow shows the Qectlon of regional groundwater Row. Transect A-A' used for cross section shown In Figures 2 and 3. Samples from F242 were used for background groundwater chemistry; F393 was the site of the injection tests and ammonium Isotherm study.
of the aquifer have been previously described in detail (25-30). The NO, and NH4+distributions at this site were first determined in 1978-1979 (26). Experimental Methods Groundwater Contaminant Distributions. Groundwater was obtained from monitoring wells at the study site to determine the distributions of specific conductance (SC), NH4+, NO3-, Ca2+,Mg2+,Na+, and K+. Water was collected from 3.2- and 5.1-cm-diameter polyvinyl chloride (PVC) well clusters (three to five wells, each screened at a different depth), located along a longitudinal transect through the plume (Figure 1). Samples were collected with either a peristaltic pump or a submersible pump (Model Series 2, Keck Geophysical Instruments, Inc., Okemos, MI) after SC had stabilized and at least three well volumes had been evacuated. Samples for analysis of Ca2+,Mg2+,Na+, K+, and NH4+were filtered (0.45-pin pore size) aqd preserved by acidification to pH 2 (31). Samples for NO3- analysis were filtered and preserved by
freezing (31). A retardation factor (Rf)for NH4+ was calculated from the ratio of its maximum migration distance downgradient from the infiltration beds relative to that of elevated values of SC. Analyses. Concentrations of dissolved NH4+ were measured by automated salicylate-hypochlorite analysis (32). Dissolved NO3- concentrations were analyzed by an automated cadmium-reduction-diazotization procedure (33). Dissolved major cations were assayed by atomic absorption spectroscopy (33). Dissolved Br- concentrations were measured in the field with a specific ion electrode and later in the laboratory by ion chromatography (34). In 10 separate analyses by ion chromatography, the precision for Br- a t a concentration of 2 mg/L was 0.04 mg/L. Batch Sorption Experiments. Aquifer material was collected for (1)measurement of dissolved and extractable NH4+and (2) determination of the distribution coefficient (&) for NH4+. Sediments for NH4+extraction were collected with a hollow-stem auger rig and split-spoon core barrel from within the contaminant plume a t depths ranging from 6.2 to 21.0 m below land surface at distances of 0.21 and 1.9 km (F347 and F262 in Figure 1)from the infiltration beds. Groundwater was collected from wells that were directly adjacent to the locations where the cores were collected to determine dissolved NH4+concentrations. Extractable NH4+ on the sediments was measured by mixing wet aquifer material (50 g) with 2 M KC1 (150 mL) in 250-mL plastic bottles (35). These slurries were shaken for 2 h at -23 "C (mean groundwater temperature was 10 "C) and centrifuged. The supernatant was analyzed for NH4+as described previously (32). A distribution coefficient from the extraction experiment, (ratio of the sorbed concentration to the dissolved concentration), was calculated to estimate the partitioning of NH4+between the solid (KC1 extract) and solution (NH4+ in the groundwater) phases. Sediment for the NH4+ isotherm experiment was collected from an uncontaminated location in the aquifer (F393) at a depth interval from 10.0 to 11.3 m with a piston core barrel (36). A batch sorption isotherm was determined by mixing portions (50 g) of wet sediment with 100 mL of uncontaminated groundwater (pH = 6.3) that were spiked with six different NH4+concentrations ranging from 0.25 to 25 mg/L as N. The temperature of the slurries was -23 "C. The slurries were shaken for 1h and centrifuged, and the supernatant was analyzed for NH4+. The distribution coefficient for the isotherm study (&,&,) was determined from the slope of the isotherm plot (15). The calculated R, for NH4+was obtained by use of the relation (15) (1) Rf = 1+ (pb/n)(Kd,iso) where P b is the bulk density of the aquifer solids in g/cm3, and n is the porosity. Tracer Test Experiments. Tracer tests were conducted to directly test the transport of NH4+,NO3-, and K+ within the aquifer. Two small-scale, forced-gradient tracer experiments (37) were conducted in October 1985 and June 1986 at well site F393, an uncontaminated location to the west of the contaminant plume (Figure 1). In these experiments, a radially divergent flow field was formed by continuous injection (95 L/min) of native groundwater obtained from a well -90 m away into an injection well. The injection well was screened from 10.0 to 11.2 m below land surface and was located 5.0 to 6.2 m below the water table. To establish the flow field, injection of native groundwater began 2 h prior to injection of the tracers and continued following injection of the tracers, to the end of the tests. The 190 L of injection solution was Environ. Sci. Technol., Voi. 23, No. 11, 1989
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added as a pulse during a 2-min period. The injection solution for the first tracer test consisted of 20 mequiv/L each of NH4+and Br- (as NH4Br); the injection solution for the second tracer test consisted of the same concentration of NH4+and Br- in addition to 3 mequiv/L each of K+ and NO, (as KN03). Concentrations of these constituents exceeded background concentrations by at least 2 orders of magnitude. Bromide was used as the most suitable nonreactive tracer, because it was not present in background groundwater, and sorption has not been noted in previous studies (38). Groundwater was sampled at a well located 1.5 m from the injection well at a depth opposite the top of the injection interval. Samples were preserved and later analyzed for NH4+, Br-, K+, NO3-, Ca+2,Mg+2,and Na+ by the methods mentioned previously (31-34). The R p for NH4+,NO3-, and K+ were calculated as the time required to reach maximum relative concentrations (C,/C,) at the sampling point downgradient relative to the time required for Br-. Cation exchange was evaluated during these experiments from observed concentration histories (in pequiv/L vs time) of all major cations at the sampling point downgradient.
Results Groundwater Contaminant Distributions. The extent of groundwater contamination at the study site was delineated by using SC. In 1985, the SC of the sewage effluent at the infiltration beds was 390 p S , whereas that of the uncontaminated groundwater was -50 pS. The zone where SC was >75 p S extended the entire length of the sampling transect from the infiltration beds to more than 4 km downgradient (Figure 2A). The contaminant plume (SC > 75 p S ) was overlain by uncontaminated groundwater (SC < 75 $3) and, in general, occurred at increasing depths with increasing distance downgradient. Both NH4+ and NO3- were present in the contaminant plume, but the distribution pattern of each of these ions in the groundwater was quite different (Figure 2B,C). Ammonium was present only as far south as well F262 within 2 km of the infiltration beds in the central core of the contaminant plume. On the other hand, NO3- was present throughout most of the plume, except in the central core of the plume where NH4+concentrations exceeded 6 mg/L as N (from the infiltration beds to well F411). In addition to defining the leading edge of the NH4+-containing zone, well F262 also was the location at which NO3first occurred in the central core of the plume. Contaminated groundwater also had elevated concentrations of Ca2+(Figure 3A), Mg2+(Figure 3B), Na+ (data not shown), and K+ (Figure 3C). Concentrations of Ca2+, Mg2+,and Na+ were relatively uniform along the entire length of the plume and were about the same as those in the effluent. The distribution of K+ (Figure 3C) differed from that of Ca2+,Mg2+,and Na+, but was similar to that of NH4+ (Figure 2B). Sediment Batch Experiments. Ammonium was present on contaminated aquifer sediments obtained from two locations (Figure 2B) in the NH4+-containingzone, 0.3 and 2 km downgradient from the infiltration beds. The K d e x b for NH4+ at the near-bed site (0.87 mL/g of dry sedunent) was similar to that at the downgradient site (0.59 mL/g of dry sediment). To determine the average NH4+exchange characteristics of the sediments, an NH4+isotherm study was conducted using sediments collected in an uncontaminated portion of the aquifer. No NH4+was liberated in KC1 extracts of these sediments, but cation exchange did occur when NH4+ was added. Cation exchange was characterized by a linear 1404
Environ. Sci. Technol., Vol. 23, No. 11, 1989
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Flgwo 2. Longitudinal cross sectbn of groundwater study site showlng distributions of (A) specific conductance, (B) ammonium, and (C) nitrate in the contaminated groundwater. Samples were collected In 19851986; values above arrows are the concentrations In the sewage effluent. The squares in B denote locatlons where sediment cores were collected for extraction of ammonium. Location of the cross section is shown in Figure 1.
isotherm (r2 = 0.98) for the range of dissolved NH4+concentrations tested (0.02-1.8 mequiv/L) (Figure 4). The resulting &&,,calculated from the slope of the curve, was 0.34 mL/g of dry sediment. The mean recovery of the sorbed NH4+by KC1 extraction was 80%. Tracer Tests. Relative concentration histories for the first tracer test for Br- and NH4+are shown in Figure 5. Separation occurred between NH4+ and Br-; the peak NH4+concentration arrived 5 h later than that of Br- (Rf for NH4+ = 3.5). Although not added to the injection solution, elevated concentrations of Mg2+,Ca2+,Na+, and K+ coeluted with Br- (57, 25, 16, and 3% of the total microequivalents greater than background concentrations, respectively) (Figure 6; Na+ and K+not shown). A second cation peak, consisting entirely of NH4+and K+, arrived 5 h after the first cation peak (Figure 6). The area of the first cation peak (510 pequiv/L.h) exceeded that of the second cation peak (413 pequiv/L.h) by -20%. No significant pH change occurred during the elution of either peak. The concentrations of Ca2+ and Mg2+ in the groundwater decreased to less than background concentrations (forming a negative peak) simultaneouslywith the arrival of the NH4+and K+ peak (Figure 6).\ This deficit of dissolved Ca2+and Mg2+was balanced within 4% by
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Figure 5. Time course plots of ammonium (A)and bromide (0) concentrations In the first tracer test at a sampling polnt located 1.5 m from the injection well at a depth of 10.0 m below land surface. I
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Figure 6. Time course plots of (A) calcium magnesium concenpotasslum concentrations ifi the first tratlons and (9)ammonium tracer test. Data from sampling polnt located 1.5 m from the injectkm well at a depth of 10 m below land surface. The numbers represent the areas of the shaded portions of the plots (I.lequhr1L.h).
+
the area of the simultaneous arrival of surplus dissolved NH4+ and K+ in the second cation peak (Figure 6). A second tracer test was conducted several months later at the same site with both NH4Br and KNO, included in the injection solution. Arrival times of NH4+and Br- were within 10% of those observed for the first tracer test (Figure 7A). The injected K+ exhibited a double peak similar to that of the first test (Figure 7B). Some K+ coeluted with Br-, but the bulk of the K+ separated from Br- and coeluted with NH4+. On the other hand, NO3coeluted with Br-, completely separating from NH4+ (Figure 7A,B).
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Figure 4. Sorption Isotherm of ammonium representlng sorption onto homogenized sediment, plotted as sorbed ammonlum vs aqueous ammonium. Core material was taken from a depth of 10.0-11.3 m In an uncontaminated region west of the contamlnant plume at the study site in May 1986. the line represents the best-ftt linear regression of the data points.
Discussion The distributions of NH4+and NO, in groundwater a t the study site were clearly different. Ammonium was present in only the upgradient part of the contaminant plume (Figure 2B). In contrast, NO3- was present throughout the length of the plume, although the depth at which NO,- was present varied (Figure 2C). There are three factors that could potentially account for these differences: (1)differences in rate of transport, (2) source variations, and (3) biological processes. In the first case, Environ. Scl. Technol., Vol. 23,No. 11, 1989
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Flgure 7. Time course plots (A) bromMe (0)and ammonium (A) concentrations, and (6)nitrate (A)and potassium (m) concentrations in the second tracer test. Samples taken from a sampling point located 1.5 m from the injectlon well at a depth of 10 m below land surface.
as NH4+ and NO3- were transported in the direction of regional groundwater flow, separation would clearly occur if cation exchange were retarding NH4+. Baedecker et al. (17)observed that concentrations of NH4+relative to NO, decreased with increasing distance from a landfill and attributed the observed attenuation of NH4+ to cation exchange onto aquifer clay particles. Changes in groundwater NH4+concentrations due to cation exchange have also been noted in recharge experiments (19,20). Although these studies found distribution patterns of NH4+ and NO3- similar to those at the Cape Cod study site, the aquifers contained significant amounts (2-10%) of clay (20) or clay lenses (17, 19), whereas the Cape Cod site is a sand and gravel aquifer with generally
