Ion exchange treatment of subsurface drainage water

Agricultural Water Management, 18 ( 1990 ) 121-133

121

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Ion exchange treatment of subsurface drainage water* W.L. Magette**, P.A. Pacheco and F.W. Wheaton Agricultural Engineering Department, University of Maryland, CollegePark, MD 20742-5 711 (U.S.A.) (Accepted 6 December 1989 )

ABSTRACT Magette, W.L., Pacheco, P.A. and Wheaton, F.W., 1990. Ion exchange treatment of subsurface drainage water. Agric. Water Manage., 18: 121-133. A laboratory study was conducted to investigate the single-bed anion exchange process for removal of NO3-N from agricultural subsurface drain outflow. Tests were performed in duplicate at flow rates of 103 ml/min and 512 ml/min, respectively, using both synthetic and natural drainage water. Nitrate breakthrough occurred at nearly the same column volumes for both flow rates. Sulfate competed strongly for exchange sites decreasing the efficiency of the resin to adsorb nitrate. The anion exchange process has the potential for removing NO3-N from drainage outflow, however several technical and economic problems must be solved before it can be applied in the field.

INTRODUCTION

Until recently, the use of commercial fertilizers in United States (U.S.) agriculture has increased steadily since the mid-1940's. In crop-year 1987, consumption of nitrogen fertilizers was 9.34X 106 t (Fertilizer Institute, 1988). However, in 1980, farmers in the U.S. used 11.34X 106 t of nitrogen fertilizer, whereas 6.8 X 106 t were used in 1970 and only 2.36 X 106 t in 1960 (Ritter and Manger, 1985 ). Viets ( 1971 ) and Hallberg ( 1986 ) presented circumstantial evidence indicating that water quality deterioration could be associated with increased fertilizer use. Nitrate nitrogen (NO3-N) in water can affect human and livestock health. Nitrate also contributes to nutrient enrichment and subsequent degradation of surface waters (Environmental Protection Agency, 1983). To protect human health in the United States, a water quality standard of 10 mg/1 NO3-N *Scientific article No. A-5087, Contribution No. 8147 of the Maryland Agricultural Experiment Station. **To whom correspondence should be addressed.

0378-3774/90/$03.50

© 1990 Elsevier Science Publishers B.V.

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W.L. MAGETTE ET AL.

has been set for public drinking water supplied for human consumption. In the United States, approximately 6% of rural wells contain more than 10 mg/ 1 NO3-N (Clark and Richardson, 1986). Subsurface drainage water from agricultural areas frequently contains NO3N in concentrations greater than 10 mg/1 (Baker, 1981). Willrich (1969) sampled 10 subsurface drainage outlets approximately twice monthly from 1965 through 1969 and found NO3-N concentrations ranging from 1 to 66 mg/1 with a flow weighted average of 19 mg/1. Nitrate nitrogen constituted approximately 99% of the nitrogen lost from subsurface drains. Guitjens et al. (1984) monitored 15 parallel drains and found that NO3-N concentrations exceeded 10 rag/1 in most sampling events. Kanwar et al. ( 1988 ) reported NO3-N in subsurface drainage beneath both conventional and no-till corn production fields almost always exceeded 10 mg/1, but was influenced by fertilization practices. Practices such as splitting applications of fertilizer, optimally timing and placing of fertilizer, and using appropriate application techniques, can limit the quantity of nutrients applied to the field or increase their retention in the field. However, the use of these practices does not guarantee control of NO3-N losses (Magette et al., 1989). Shirmohammadi et al. (1989) simulated the effects of 8 different cropping systems and associated management practices on nitrate losses from the root zone and predicted long-term average NO3-N concentrations in leachate from coastal plain soils to range from 13 to 15 mg/1. This study was based on two premises: nitrate leaching is inevitable from agricultural systems using currently accepted farming practices, and minimizing the loss of nitrate to ground- and surface waters is desirable. Given these two premises, the objective of this research was to determine the degree to which the ion exchange process could be used to remove NO3-N from subsurface drainage outflow. If this process could be proven to be technically feasible, and economically viable, subsurface drainage could become a more environmentally viable method by which to manage water in humid areas. METHODS AND MATERIALS A single-bed ion exchange process incorporating bypass blending of raw water was investigated (Fig. 1 ). IONAC ASB-2, a strong base Type II anion exchange resin (Sybron Chemicals Inc., 1987 )*, regenerated to the chloride form, was selected for investigation based on manufacturer's rating of the resin's nitrate and sulfate removal characteristics. Duplicate systems were established for a replicated, split-plot experimental design. Three types of water were assigned as the main plots and two different flow rates were selected for *The mentionof product names is for informationonly and does not constitutean endorsement for use by the AgriculturalEngineeringDepartment, Universityof Maryland.

123

1ON EXCHANGE TREATMENT OF SUBSURFACE DRAINAGE WATER

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m

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124

W.L. MAGETTE ET AL.

the sub-plots. These treatments were assigned randomly to the two treatment units. A variable speed, peristaltic p u m p (Randolph Co., Houston Texas; Model TX 500) driven by an electric motor (Dayton Electric Co., Chicago, IL; Model 5k282C) delivered test drainage water to the 25.4 1 D x 2 8 . 6 m m O D x 1.52 m long cast acrylic column containing the exchange resin. A return (bleeder) line and screw clamps were used to precisely control flow to the exchange column. Influent flow was measured by a calibrated flow meter (Cole Parmer Instr. Co., Chicago, IL; Model FM03439ST). The column effluent line was connected to a 500 ml flask from which periodic samples were taken by an automatic water sampler (ISCO, Lincoln, NE; Model 1680). Plastic tubing (Norton Plastics, Akron, OH; Tygon R-3603) was used to transmit all flows. Overflow from the flask was collected in a 500 ml beaker, where pH and conductance of the effluent were monitored. Conductance of effluent was determined periodically and manually by means of a conductivity cell and a conductance-resistance meter (YSI Scientific, Yellow Springs, OH; Model 34). A combination pH electrode ( C o m i n g Glass Works, Medfield, MA; Model 004176-701 ) was connected via a shielded coaxial cable and an amplifier circuit to an oscillographic recorder (Hewlett-Packard, Corvallis, OR; Model 7402A) to provide continuous pH measurements. Either a tandem diaphragm metering p u m p (Pennwalt Co., Belleville, N J; Model 44-214) or a solid state varistaltic p u m p (Manostat Co., New York, NY; Model 72-89510), (depending on the column), was used to deliver distilled water in the backwash and rinse steps, and a 5% NaC1 solution in the regeneration step. Bleeder lines and screw clamps were used to precisely control flow of rinse water and regenerant brine through calibrated flow-meters (Cole Parmer Instr. Co., Chicago, IL; Model FM092-04ST) and the exchange collected in a 500 ml beaker, in which pH and conductance were determined as described previously, and from which water quality samples were collected manually at 5 minute intervals. Three types of water were investigated (Table 1 ). Water A was actual subsurface drainage water collected from a tile-drained corn field in Maryland's Atlantic coastal plain. Waters B and C were "synthetic" drainage effluents created to represent an average of parameter concentrations reported in the literature for subsurface tile water. Water C contained approximately twice the concentration of anions as water B, but the same NO3-N concentration (21 m g / l ) . Synthetic drainage waters were made by dissolving appropriate quantities of the following chemicals in distilled water: NaHCO3, Ca (NO3) 2" 4H20, MgSO4, CaC12- 2H20, and FeSO4.7H20. Two flow rates were investigated (Table 2). A flow rate of 103 m l / m i n (2 g p m / f t 3 of resin) was selected for use based on the resin manufacturer's recommendations. A high flow rate of 512 m l / m i n (approximately 10 g p m / f t 3 of resin) was chosen arbitrarily, five times the m a x i m u m flow rate recom-

ION EXCHANGE TREATMENT OF SUBSURFACE DRAINAGE WATER

125

TABLE 1 Chemical characteristics of test waters Ion

Concentrations, meq/1 (mg/1) Water A actual drainage

Water B synthetic drainage

Water C synthetic drainage

Anions NO~SO~C1 HCO3 Total anions

1.50 (93.0) 2.29(110.0) 1.18 (42.0) 0.49 (30.0) 5.46 (275.0)

1.50 (93.0) 1.75 (84.0) 3.70(130.0) 3.40(205.0) 10.35 (512.0)

1.50 3.50 7.40 6.70 19.10

Cations Ca 2+ Mg 2+ Na +

2.61 (52.3) 1.28 (15.5) 1.23 (28.2)

5.18 (103.6) 1.75 (21.2) 3.40 (77.2

Fe 2+

Total cations Total dissolved solids a

0.0

(0.0)

5.12 (96.0) -

0.0

(0.1

10.33 (202.1 ( 714.1

(93.0) (168.0) (260.0) (410.0) (931.0)

8.83 (177.1) 3.50 (42.1) 6.70 (154.4) 0.0

19.03

(1.0)

(374.6) ( 1305.6 )

aCalculated values. The listed ions were the only constituents added to distilled water in developing the synthetic drainage water.

mended by the resin's manufacturer. Samples were collected once per hour during tests with a flow of 103 m l / m i n and once per 10 min with a flow of 512 m l / m i n . To establish a performance goal for the experimental ion exchange unit, it was assumed that, to be feasible in a field-scale application, the exchanger must treat 70% of incoming subsurface flow and bypass 30%, then blend the two flows together for discharge. Consequently, to achieve a concentration of 9 mg/1 in the blended flow from the exchange unit (i.e. raw bypassed water plus treated water), the concentration of NO3-N in the treated water would have to be 3.9 mg/1 or less. The operating conditions for the tests are given in Table 2. Each run consists of 4 steps: adsorption, backwashing, regeneration and rinsing. Analyses for nitrate, sulfate and chloride, were made using ion chromatography with a Dionex ion chromatograph (Model 2000i/SP). Bicarbonate determinations were made using potentiometric titration to end point pH (American Water Works Association, 1985). Calcium, sodium and magnesium determinations were made using a four-channel Technicon AutoAnalyzer II. Iron determinations were made using a Perkin Elmer 3030 atomic absorption spectrophotometer.

126

W.L. MAGETTE ET AL.

TABLE 2

Experimental conditions for single-bed ion exchange process Parameter

Metric units

Operating temperature Minimum bed depth Resin bed volume Adsorption step flow rates Loading rate of drainage water ( p e r v o l u m e of resin ) at 103 m l / m i n at 512 m l / m i n Backwash step flow rate loading rate bed expansion Rinse step initial flow rate loading rate sustained flow rate loading rate Regeneration step flow rate loading rate regenerant direction regenerant concentration

RESULTS

2 5 - 2 8 oC 76.0 c m 3 . 9 X 10 - 4 m 3 103 m l / m i n 512 m l / m i n

4.5 I s -1 m -3 22.01 s -l m -3 52 m l / m i n 5.6 1 s -~ m -3 50% 26 m l / m i n 1.1 l s -1 m - 3 77 m l / m i n 3.4 1 s-~ m - 3 26 m l / m i n 1.1 1 s - l m

-3

downflow 5% ( 1 . 0 N ) N a C I

AND DISCUSSION

Breakthrough capacity for nitrate (the point in an adsorption cycle at first appearance of a sharply increased effluent concentration of NO3-N) was determined in 6 replicated adsorption cycles (2 flow rates, 3 types of water, 2 replications). Of particular interest was the volume of treated drainage water and elapsed times at which effluent concentrations of NO3-N exceeded 3.9 mg/l, because the feasibility of a field-scale unit would in part depend on how frequently the resin would have to be regenerated. Breakthrough curves for three different tests are presented in Fig. 2. Curves for other tests were similar. The ratio of ion concentration in the column effluent (CE) to that in the feed water (cF) is plotted against the volume of drainage water treated. Table 3 presents data for all tests. The average breakthrough capacity with natural drainage water (water A) at a flowrate of 103 m l / m i n was 99.8 liters after 16 hours of treatment; for water B (synthetic, low ionic strength) it was 105.6 liters after 17 hours of treatment; and for water C (synthetic, high ionic strength), it was 64.9 liters

127

ION EXCHANGE TREATMENT OF SUBSURFACE DRAINAGE WATER

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Water C (TIO, C2)

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80

100

120

140

160

E f f l u e n t Volume (L)

Fig. 2. Nitrate breakthroughcurves for water A (natural subsurfacedrainagewater), water B (synthetic drainage water, low ionic strength); and water C (synthetic drainage water, high ionic strength) at an influent flow rate of 103 ml/min. CE is the concentrationof ions in the treated effluent;CFis the concentrationof ions in the untreatedinfluent (drainagewater).

after 10.5 hours of treatment. Tests conducted at 512 m l / m i n showed that average breakthrough capacities were 98.3 liters, 106.9 liters, 60.1 liters for nitrate with waters A, B and C, respectively (Table 3). These data indicated that resin performance for a given type of water was nearly identical at flow rates of 103.2 and 512 ml/min. This was expected since the exchange capacity of the resin should not change with flow rate. However, the exchange capacity should have been expended earlier at the higher flow. Such was the case: NO3-N breakthrough for waters A and B occurred approximately 13 hours sooner at the higher flow rate. For water C, breakthrough occurred 8.5 hours sooner at the higher flow. Analysis of variance (SAS, 1982 ) indicated that the type of water (and the interaction effect of water with column) had a significant influence on NO3-N breakthrough capacity (Table 4). However, the Student-Newman-Keuls multiple range comparison tests (SAS, 1982) confirmed that NO3-N removal capacities of the resin were significantly different at the 95% level of confidence only for water C. Using water B at the manufacturer's recommended flow rate of 103 ml/ min as a reference, resin removed 5% less NO3-N per unit volume of water from water A than from water B, but 38% more NO3-N from water B than from water C. The magnitudes of these differences were not expected. It was expected that, because water C contained approximately twice the concentration of anions as water B, the competition for exchange sites by anions would

128

W.L. MAGETTE ET AL.

TABLE 3 N O 3 - N b r e a k t h r o u g h capacities Test No.

Run No.

Column No.

Water~

Flow rate b

NO3-N breakthrough c capacity (1)

1 2

3 3

1 2

A A

L L

3 4

4 4

1 2

A A

H H

5 6

2 5

1 2

B B

L L

7 8

1 6

1 2

B B

H H

9 10

6 2

1 2

C C

L L

11 12

5 1

1 2

C C

H H

Mean

Mean

Mean

Mean

Mean

Mean

99.1 100.5 99.8 98.0 98.5 98.3 111.6 99.7 105.6 114.0 98.2 106.9 61.4 68.4 64.9 56.8 63.3 60.1

time ( h ) 16.0 16.0 16.0 3.2 3.2 3.2 18.0 16. i 17.0 3.7 3.2 3.5 9.9 11.0 10.5 1.8 2.1 1.9

aWater A - natural subsurface drainage water; water B - synthetic drainage water, low ionic strength; water C - synthetic drainage water, high ionic strength. ~L - 103 m l / m i n ; H - 512 m l / m i n . Clnflow N O 3 - N c o n c e n t r a t i o n = 21 m g / l ; effluent N O 3 - N c o n c e n t r a t i o n = 3.9 mg/1.

TABLE 4 Analysis o f variance for effect o f type o f water treated a n d flow rate on N O 3 - N breakthrough capacity Source o f variation

DF~

Model column type o f water c o l u m n X water type flow water type X flow

8 1 2 2 1 2

Error

3

Corrected total aDegrees o f freedom. mS= significant P < 0.05. cNS = not significant at P < 0.05.

11

M e a n square

F

Significance

577.45 12.61 2177.59 112.84 11.80 7.16

424.6 9.3 1601.2 83.0 8.7 5.3

Sb NS c S S NS NS

0.0014

IONEXCHANGETREATMENTOFSUBSURFACEDRAINAGEWATER

129

have resulted in much less NOa-N being removed from water C than from water B. This was observed. However, the same relative effect was expected with waters A and B, since water B contained twice the anionic concentration as water A. This was not observed. Apparently other anions in water A contributed to the reduction of nitrate exchange capacity of the resin. Unfortunately, water A was not analyzed for all possible anions (or cations) it might have contained, nor were total dissolved solids in the water determined. During a given adsorption cycle, there were four anion concentration plateaus corresponding to the anions measured in the test waters (Fig. 3). Each plateau was separated from the others by abrupt transition zones in accordance with the degree to which each anion was retained by the exchange resin. The most preferred anion was always S O l - , as indicated in Fig. 3 by the greatest flow volume required before breakthrough occurred; until breakthrough, virtually no SO 2- was lost from the system. Chloride in the effluent always achieved a plateau concentration of approximately twice the inflow C1- concentration as it was washed from the resin after yielding its exchange sites to NO~- and S042- . Carbonate reached a plateau concentration approximately equal to the inflow concentration, indicating little if any carbonate was retained by the exchange resin. After some initial exchange of bicarbonate for chloride, the concentrations of chloride and bicarbonate remained relatively constant until breakthrough of NO3. A significant pH change always occurred, as shown in Fig. 3, at approximately the same time that NO~- breakthrough occurred. This pH inCE/CF

pH

2.5

- 10

i

2

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1.5 ~

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,~

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Nitrate ~/~

4

Sulfate Chloride

0.5

" ,'

2

Bicarbonate pH

0

20

40

60

80

100

120

140

160

180

200

Effluent Volume (L) Fig. 3. Ion and pH breakthrough curves for water B (synthetic drainage water, low ionic strength)

during test 6 (column2) at an influentflowrate of 103 ml/min. CEis the concentrationof ions in the treated effluent;cv is the concentrationof ions in the untreatedinfluent (drainagewater).

130

W.L. MAGETTE ET AL.

crease may have occurred because carbonate started leaving the column at the same time as nitrate. Regeneration of resin after each adsorption cycle was accomplished by passing a 5% NaC1 solution downward through the resin at the rate of 26 ml/ min. Salt requirement - and hence, the required volume of regenerant - increased with higher anion concentrations in the tested waters (Table 5 ). The average regenerant volumes required were 1.3, 1.5, and 1.71, respectively, for waters A, B, and C. Analysis of variance indicated that the type of water and flow rate were significant in explaining differences in regenerant volumes (Table 6 ). The Student-Newman-Keuls multiple range comparison test confirmed that all regenerant volumes were significantly different from each other at the 95% level of confidence. Operational problems with the single-bed exchange process were minimal. However, during tests with water C, the high anion concentration and iron content of the water caused caking of the resin during the adsorption cycle. To correct this after each adsorption test, a long hard plastic rod had to be forced up and down in the resin to break up the caking; then the resin was backwashed to return it to its original nitrate removal capacity. A change in color occurred on the top layer of the resin after caking and clean up. In a practical application, caking could cause serious operational problems by rapidly decreasing the hydraulic capacity of the exchange resin. Another practical problem encountered was the retention of NO3-N by the resin after the regeneration step; from 14 to 43% of the NO3-N adsorbed reTABLE 5 Regenerant volumes and salt requirements during regeneration o f resin Test No.

Run No.

Column No.

Water a

Regenerant volume (I)

Salt requirement b

1 2 3 4

3 3 4 4

1 2 1 2

A A A A

1.29 1.29 1.42 1.42

0.65 0.64 0.72 0.72

5 6 7 8

2 5 1 6

1 2 1 2

B B B B

1.42 1.42 1.55 1.55

0.64 0.71 0.79 0.79

9 10 11 12"

6 2 5 1

1 2 1 2

C C C C

1.68 1.55 1.68 1.68

1.37 1.13 1.48 1.32

aWater A - natural subsurface drainage water; water B - synthetic drainage water, low ionic strength; water C - synthetic drainage water, high ionic strength. bkg o f salt required per 10001 o f drainage water treated.

ION EXCHANGETREATMENTOF SUBSURFACEDRAINAGEWATER

131

TABLE 6 Analysis of variance for effect of type of water treated and flow rate on volume of regenerant Source of variation Model column type of water column × water type flow water type × flow Error Corrected total

DFa

Mean square

F

Significance

8 1 2 2 1 2

0.0263 0.0048 0.0852 0.0018 0.0293 0.0013

18.8 3.4 60.9 1.3 20.9 0.9

Sb NS c S NS S NS

3

0.0014

11

aDegrees of freedom. bS = Significant P < 0.05. cNS = Not significant at P < 0.05.

mained after regeneration, but was flushed from the resin during the rinse cycle. In a field application, neither the regenerant water or the rinse water would be of suitable quality for direct discharge to receiving waters. To evaluate the potential application of the single-bed ion exchange process to a field situation, laboratory results were used to estimate the size of a hypothetical system that would treat subsurface drainage water of the quality of water A, at a flow rate of 103 m l / m i n . The m i n i m u m recommended drainage coefficient (9.5 m m ) for a mineral soil (ASAE, 1986) was assumed as a delivery rate to the treatment system, resulting in a drainage volume of 63.3 m3/ ha over a 16 h period. Laboratory results (Table 3) indicated that 258 500 1 of drainage water could be treated per cubic meter of exchange resin to an effluent N O 3 - N concentration of 3.9 mg/1 (99.8 1 + 3 . 9 × 10 - 4 m ) . Considering blending of treated and untreated drainage water at a 70:30 ratio, a total of 369 3501 of drainage water could be produced with a final NO3-N concentration of 8.1 mg/1. At the r e c o m m e n d e d drainage coefficient of 9.5 mm, 0.17 m 3 of exchange resin would be required per ha of land drained at a cost of approximately US$1000 (1987) and require a container approximately 0.5 m in diameter and 0.87 m in depth. This cost is substantial at approximately 10 times that of the most expensive soil conservation practice, terracing (Haith and Loehr, 1979 ). Although implementation costs could be shared between a farmer and a water quality management agency (as is done routinely in the United States with other agricultural best management practices), cost would probably limit the use of this system to situations in which strict controls over nitrate delivery to surface- and groundwater were required. Thus, while such a design appears technically reasonable, serious economic concerns and other design aspects (such as system hydraulics and operation procedures) would

132

W.L.MAOETTEET AL.

have to be satisfactorily evaluated before the system could be proven feasible in a field setting. CONCLUSIONS

This study investigated the potential use of ion exchange as a treatment mechanism to remove NO3-N from subsurface drainage water. Results of this study support the following conclusions. (1) The selected anion exchange resin (IONAC ASB-2) was selective for the sulfate ion; the presence of SO 2- in the waters tested decreased the ability of the resin to adsorb nitrate. (2) Exchange resin effectively removed nitrate at both low and high flow rates. At both flow ra~es, breakthrough curves were nearly identical and nitrate breakthrough occurred after approximately the same volumes of drainage water were passed through the resin. The high flow rate thus could be used for production scale-up to minimize resin requirements. (3) Where carbonate is present in drainage water, the increased pH associated with nitrate breakthrough that took place in all tests, provides a simple method with which to determine the end of a nitrate adsorption run in a practical application. (4) Practical problems such as disposal of regenerant waste and physical design and cost of a field-scale ion exchange unit must be solved before this process can be implemented on a fieldscale basis. ACKNOWLEDGEMENTS

This study was partially supported with Chesapeake Bay research funds provided by the Maryland Department of Agriculture. Appreciation is especially extended to Dr. J.J. Meisinger, USDA-Agricultural Research Service BARC Environmental Chemistry Laboratory and to the University of Maryland Departments of Agronomy and Horticulture for assistance with chemical analyses. The computer time for this project was supported in part through the facilities of the Computer Science Center of the University of Maryland.

REFERENCES American Water Works Association, 1985. Standard Methods for the Examination of Water and Wastewater. American Public Health Association, Washington, DC, 16th ed. ASAE, 1986. ASAE Standards 1986: Standards, Engineering Practices and Data Adopted by the American Society of Agricultural Engineers. American Society of Agricultural Engineers, St. Joseph, MI. Baker, J.L., 1981. Agricultural Areas as nonpoint sources of pollution. In: M.R. Overcash and

ION EXCHANGE TREATMENT OF SUBSURFACE DRAINAGE WATER

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J.M. Davidson (Editors), Environmental Impact of Nonpoint Source Pollution. Ann Arbor Science, Ann Arbor, MI, 449 pp. Clark, Jr., E.C. and Richardson, N.B., 1986. Agriculture not sole culprit. Solutions, 30 (5): 3236. Environmental Protection Agency, 1983. Chesapeak Bay program: findings and recommendations. U.S. Environmental Protection Agency, Region III, Philadelphia, PA, 48 pp. Fertilizer Institute, 1988. Fertilizer Facts and Figures. The Fertilizer Institute, Washington, DC. Guitjens, J.C., Tsui, P.S. and Thran, D.F., 1984. Quantity and quality variations in subsurface drainage. Trans. ASAE, 27 (2): 425-428. Haith, D.A. and Loehr, R.C., 1979. Effectiveness of Soil and Water Conservation Practices for Pollution Control. EPA600/3-79-106. U.S. Environmental Protection Agency, Washington, DC, 474 pp. Hallberg, G.R., 1986. From hoes to herbicides: agriculture and groundwater quality. J. Soil Water Conserv., 41 (6): 357-364. Kanwar, R.S., Baker, J.L. and Baker, D.G., 1988. Tillage and split N-fertilization effects on subsurface drainage water quality and crop yields. Trans. ASAE, 31 (2): 453-461. Magette, W.L., Weismiller, R.A., Angle, J.S. and Brinsfield, R.B., 1989. A ground water nitrate standard for the 1990 farm bill. J. Soil Water Conserv., 44 ( 5 ): 491-494. Ritter, W.F. and Manger, K.A., 1985. Effect of irrigation efficiencies on nitrogen leaching losses. J. Irrig. Drain. Eng., 111 (3): 230-240. SAS, 1982. SAS User's Guide: Basics. SAS Institute, Cary, NC, 1982 ed. Shirmohammadi, A., Magette, W.L. and Shoemaker, L.L., 1989. Reduction of nitrate loadings to ground water, in: Ground Water Issues and Solution in the Potomac River Basin/Chesapeake Bay Region. National Water Well Association, Dublin, OH, pp. 261-270. Sybron Chemicals Inc., 1987. Data sheet for IONAC ASB-2 1987; Strong Base Type II, Anion Exchange Resin. Birmingham, NJ. Viers, Jr., F.G., 1971. Fertilizers. J. Soil Water Conserv., 26: 51-53. Willrich, T.L., 1969. Properties of tile drainage water. Completion Rep. Project A-013-1A, Iowa State Water Resource Res. Inst., Iowa State University, Ames, IA, 39 pp.