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TECHNICAL REPORTS

Ground Water Quality

Phosphorus Leaching in Sandy Outwash Soils following Potato-Processing Wastewater Application

^a Agriculture and Agri-Food Canada, Lethbridge Research Center, P.O. Box 3000, 5403-1st Ave. S., Lethbridge, AB T1J 4B1, Canada
^b Dep. of Soil, Water, and Climate, Univ. of Minnesota, 1991 Upper Buford Circle, Room 439, St. Paul, MN 55108-6028

^* Corresponding author (sgupta{at}umn.edu)

Received for publication October 9, 2004.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Land application of wastewater presents potential for ground water pollution if not properly managed. In situ breakthrough tests were conducted using potato (Solanum tuberosum L.)–processing wastewater and a Br tracer to characterize P leaching in seasonally frozen sandy outwash soils. In the first test, P and Br breakthrough were measured in a 7-m deep well following wastewater [2.94 mg L^–1 total P (TP); 280 mg L^–1 Br] application at the site that had 13.1 mg water-extractable P (WEP) kg^–1and 94.4 mg Bray-1 P kg^–1. Bromide was detected in the well after 0.4 pore volumes, but there was no P break-through after 7 pore volumes. In the second breakthrough test, wastewater containing 3.6 mg L^–1 TP and 259 mg L^–1 Br was applied on 1.5-m deep lysimeters at low (0.8 mg WEP kg^–1; 12.1 mg Bray-1 P kg^–1) and high soil test P sites (104 mg WEP kg^–1; 585 mg Bray-1 P kg^–1). Leachate TP concentration during the test remained constant (0.04 mg L^–1) at the low P sites but increased from 3.5 to 5.6 mg L^–1 at the high P sites. These results indicate no P leaching in low P soils, but leaching in high P soils, thus suggesting that most of the P leached at the high P sites was mainly due to desorption and dissolution of weakly adsorbed P from prior P applications. This was consistent with P transport simulations using the convective–dispersive equation. We conclude that P concentration in land-applied wastewater should be regulated based on soil test-P level plus wastewater P loading.

Abbreviations: Al_ox, Fe_ox, P_ox, aluminum, iron, and phosphorus extracted by acid ammonium oxalate • BTC, breakthrough curve • CDE, convective–dispersive equation • DOP, dissolved organic phosphorus • DPS, degree of soil phosphorus saturation • MPCA, Minnesota Pollution Control Agency • PP, particulate phosphorus • TKN, total Kjeldahl nitrogen • TP, total phosphorus • WEP, water-extractable phosphorus

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
LAND APPLICATION is commonly used by food processing facilities as a disposal method for the large quantities of wastewater generated during processing of food products. The wastewater typically contains large amounts of nutrients of environmental significance, particularly N and P. Total P (TP) concentrations exceeding 72 mg L^–1 have been measured in wastewater from a potato processing plant (Barl and MacKenzie, 1995). Irrigation with large volumes of such wastewater presents potential for ground and surface water contamination from P and other pollutants, if not properly managed. If applied in accordance with crop water and P use, land disposal works well during the growing season when nutrients in the wastewater are taken up by the crop. However, in the absence of actively growing vegetation during the cold winter months, when soils may be frozen, such a practice presents potential for nutrient leaching to ground water, especially from coarse-textured soils.

Although P is relatively immobile in many soil profiles because of its high affinity for soil components, leaching of P to ground water can be considerable when large quantities of high P wastes are applied on sandy soils or when P adsorption capacity is exceeded (Sharpley and Rekolainen, 1997; Haygarth et al., 1998). Hayes et al. (1990) reported elevated soil P levels after 16 mo of summer and winter irrigation with wastewater compared to irrigation with potable water. High extractable P has also been measured in soils that have been irrigated with wastewater, even 5 yr after the wastewater application had been discontinued (Nielsen et al., 1991). Adriano et al. (1975) reported downward movement of P to the 6.6-m depth in sandy soils after long-term spray application of food processing wastewater. Sommers et al. (1979) observed a significant TP increase in the 0.30- to 0.60-m layer in a sandy loam soil after 12 yr of irrigation with municipal wastewater.

Glacial terrain covering much of the north central USA is characterized by land-surface depressions, many of which contain lakes and wetlands that have inflow from ground water (Winter et al., 1998). Ground water is also an important source of water for some streams in the region (Winter et al., 1998). In situations where shallow aquifers are directly connected to surface water, ground water can be the major and potentially long-term contributor of contaminants to surface water.

There is a dearth of information on impacts of long-term wastewater application on P leaching from seasonally frozen outwash soils, which occur throughout large areas of the USA and Canada. There is potential for significant P leaching from these coarse-textured soils because a large part of the precipitation comes as snow, which contributes to ground water recharge during snowmelt in the spring. In many cases, recharge is greatest at the time of snowmelt in the spring than at any other time of the year (Sharratt, 2001). In these soils, the risk of ground water pollution is greatest during snowmelt and after heavy rainfall. The presence of topographic depressions further aggravates the potential pollution problem because snowmelt runs off into these depressions, which in turn act as areas of focused recharge with higher hydraulic loading (increased depth of water) than intended. Since P solubility increases under reducing environments (Sah and Mikkelsen, 1986), prolonged saturation of the soil under the depressional ponds may also decrease P adsorption, thereby increasing the potential for P leaching. Wetter soils and the lack of vegetation during spring thaw are additional factors that can enhance solute transport down the soil profile to ground water.

Understanding the mechanisms involved in P transport through soils can help in formulation of wastewater management practices that can prevent P-related ground water pollution. The principal processes influencing the transport of reactive solutes in the soil are sorption, advection, dispersion, and transformation (Mackay et al., 1986; Roberts et al., 1982).

Numerous laboratory scale experiments have been conducted to characterize solute transport processes in soils (e.g., Gerritse, 1996; Kamra et al., 2001; Mamo et al., 2005). However, few studies have been conducted on a field scale, largely due to the complexities of and the time involved in such in situ field measurements. These types of studies are needed to verify the applicability of laboratory experiments for real-world applications.

The objective of this study was to characterize the potential for P appearance in the ground water below a sandy outwash soil following winter wastewater application on frozen soils. We accomplished this objective through characterization of in situ P and Br breakthrough curves following application of potato-processing wastewater on low P and high P soils at the study site. Preliminary tests at the site indicated minimal leaching when the soils were frozen. Breakthrough tests were therefore conducted following spring thaw.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Site Description
The breakthrough experiment was conducted in sprayfields adjacent to a potato processing plant near Park Rapids, MN (46°98' N lat; 95°09' W long). The sprayfields have received potato-processing wastewater since the plant started operation in 1981. Because of the limited capacity of available storage lagoons, the Minnesota Pollution Control Agency (MPCA) in 1996 issued a permit to apply wastewater from the plant on 120 ha of adjacent land in the winter. The permit required that wastewater applied during the winter period (October–March) should contain no more than 6 mg TP L^–1, 10 mg NO₃–N L^–1, and 20 mg L^–1 total Kjeldahl N (TKN). These criteria were met by precipitating P and denitrifying N through use of alum and a sequencing batch reactor during the wastewater treatment process. Outside the winter period, wastewater nutrient concentrations were not regulated except that land application of the wastewater could not result in N application exceeding the annual N requirements of the crop being irrigated.

The soil at the site is a Verndale sandy loam (coarse-loamy over sandy, mixed, frigid Udic Argiboroll) with an average bulk density of 1.63 Mg m^–3. Chemical properties measured in composite samples (0–0.3 m depth) taken from the sprayfields in May 2002 were pH, 7.7; Bray-1 P (Sims, 2000), 108 mg kg^–1; organic matter (Nelson and Sommers, 1996), 1.9%; and water-extractable P (WEP) (Self-Davis et al., 2000), 72 mg kg^–1.

Before winter application of potato-processing wastewater in 1996, the site had received unknown amounts of wastewater, potato processing sludge, and horse manure. Since 1996 potato wastewater application added up to 100 kg TP ha^–1 annually to the sprayfields, mostly during the growing season when wastewater P concentrations were not regulated. These and the past practices contributed to the elevated P levels observed in the sprayfields, with soils in some sections exceeding 500 mg kg^–1 Bray-1 P. A preliminary survey of the sprayfields indicated that soil P levels were highest in topographic depressions, which also constituted focal points where wastewater accumulated during spring thaw.

Monitoring Well Test
A monitoring well previously installed and screened across the water table at a relatively low P site (13.1 mg WEP kg^–1; 94.4 mg Bray-1 P kg^–1) up-gradient of the sprayfields was used in this breakthrough test. The breakthrough test commenced May 2002 to determine P and Br breakthrough at the 1.5- and 3-m depths (stainless steel porous cup samplers) as well as at the water table. The well nest at the site consisted of a deep well, which was screened at the bottom of the 17-m deep aquifer and a shallow well, which was screened across the water table at the 8.2-m depth. In this test, we used the shallow monitoring well, which was also used for routine compliance monitoring. Depth to the water table averaged 7 m during the experiment.

Instrumentation
A containment [5.5 m (L) x 2.1 m (W) x 0.6 m (H)] was built using corrugated galvanized steel sheets that were pounded into the ground adjacent to a monitoring well using a front-end loader (Fig. 1) . Stainless steel porous cup samplers (model SW-071, capacity 260 mL, 5 cm OD; Soil Measurement Systems, Tucson, AZ) were installed near the well to test P breakthrough at 1.5- and 3-m depths (Fig. 1). During installation, each sampler was pushed into a silica-flour slurry at the bottom of a predrilled access hole. The silica flour was added to ensure good hydraulic contact between the sampler and the surrounding coarse soil. The hole was then backfilled with soil removed during drilling. Dry bentonite was placed at the soil surface around the sampler shaft to reduce the potential for preferential flow. Using a hand pump, a suction of 40 kPa was applied to each sampler to collect the soil solution surrounding the porous cup.

View larger version (127K): [in this window] [in a new window]   Fig. 1. Containment used in the breakthrough study at the low P monitoring well site. (Top) construction of the containment; (bottom) location of stainless steel suction samplers relative to well. Phosphorus and Br breakthrough were monitored in MW-31S monitoring well during the experiment.

Sampling
Water samples from the monitoring well were taken every 3 h commencing 2 d before the start of the experiment. When wastewater application started, sampling intervals were reduced to 30 to 60 min. The samples were collected in 500-mL plastic bottles and immediately stored in a cooler. Bromide concentration was determined within 3 h of sampling whenever possible. The samples were then stored frozen until analysis for P. Bromide and P concentrations were plotted as a function of cumulative time or pore volume to obtain breakthrough curves (BTC). Pore volume was estimated as discussed below using a mean bulk density of 1.63 Mg m^–3 obtained from split-spoon samples during well drilling.

Lysimeter Breakthrough Tests
Lysimeter breakthrough tests were conducted in June after the well site tests indicated no P breakthrough. We hypothesized that high leachate P concentrations observed in monitoring lysimeters at the high soil test P sprayfields (C.J. Rosen et al., unpublished data, 1999, 2004) originated from the soil rather than the wastewater and was mainly due to hydraulic loading (water in excess of natural precipitation). Results from a laboratory test using soils from the site suggest that this may be the case (Mamo et al., 2005).

Two drainage lysimeters from high P sprayfield sites (mean 104 mg WEP kg^–1, 585 mg Bray-1 P kg^–1 in the 0- to 0.3-m depth) and two from low P nonirrigated sites (mean 0.8 mg WEP kg^–1, 12.1 mg Bray-1 P kg^–1) were used in the experiment. The lysimeters were installed in 1996, 6 yr before the breakthrough tests. Ammonium oxalate (pH 3) extractable P (P_ox), iron (Fe_ox), and aluminum (Al_ox) (Schoumans, 2000) in the 0- to 0.3-m soil layer averaged 298, 1155, and 3699 mg kg^–1, respectively, for the high P sites and 12.4, 798, and 991 mg kg^–1, respectively, for the low P sites. Mean degree of P saturation [DPS = 100 x P_ox/0.5(Fe_ox + Al_ox), where all concentrations are expressed in mmol kg^–1] (Breeuwsma and Silva, 1992) was 12.2% for the high P sites and 1.4% for the low P sites.

Lysimeter Construction
Drainage lysimeters (28.9 cm internal diameter and 1.2 m long) were installed in 1996 at a 1.5-m depth (Fig. 2) . All lysimeters were constructed using undisturbed soil cores that were collected from areas outside the sprayfields by pushing a PVC tube into the soil with a backhoe. Soil in the 0- to 0.3-m layer, which would have been affected by past tillage and/or planting practices, was removed before collection of the 1.2-m long undisturbed core and replaced at installation with surface soil from the installation site.

View larger version (37K): [in this window] [in a new window]   Fig. 2. Schematic diagram of drainage lysimeter used in the breakthrough tests at the low P and high P lysimeter sites.

Breakthrough Test Setup
Each lysimeter was encompassed with a 2.44 m (diameter) by 0.6 m (height) galvanized steel cylinder, which was driven 0.1 m into the ground (Fig. 3) . Wastewater containing 3.6 mg TP L^–1 was hauled to each site in a 12.9-m³ tanker truck. Potassium bromide was added to the truckload at each site to give a Br concentration of approximately 280 mg L^–1. The actual Br concentration after addition of KBr was measured using a Br specific electrode as described below.

View larger version (67K): [in this window] [in a new window]   Fig. 3. Schematic diagram of the lysimeter breakthrough experiment showing the galvanized steel cylinder and the lysimeter in the center. The cylinder was pushed 0.1 m into the soil to minimize leakage along the edges.

Laboratory Analysis
Bromide concentration in the leachate samples was measured potentiometrically using a Br ion selective electrode (ISE) (Orion 94-35) in conjunction with a double-junction reference electrode (Orion 90-02) and an Orion 920A ISE meter (Thermo Electron Corp., Beverly, MA). Total P and total soluble P (TSP) were determined in unfiltered and filtered leachates, respectively, by the ascorbic acid method (Murphy and Riley, 1962) following digestion with perchloric acid on a block digester. Soluble reactive P (SRP) was similarly determined, but without digestion, following filtration of the leachates through 0.45-µm membrane filters. Dissolved organic P (DOP) was estimated as the difference between TSP and SRP, whereas particulate phosphorus (PP) was estimated as the difference between TP and TSP.

Parameter Estimation
The one-dimensional convective–dispersive equation (CDE) for steady state solute transport through a homogenous soil (Lapidus and Amundson, 1952) is:

[1]

[2]

where

The tracer (Br) used in this study was assumed to be non-reactive (R = 1).

The initial and boundary conditions used were:

[3]

[4]

[5]

In Eq. [4], C₀ (µg cm^–3) is the concentration of the applied solution.

Parameters describing one-dimensional solute transport were estimated by fitting the single-domain deterministic equilibrium CDE (Mode 1 in CXTFIT) to the observed Br BTCs using the CXTFIT program (Toride et al., 1999). CXTFIT uses a nonlinear least-squares parameter optimization method to estimate solute transport parameters from measured Br breakthrough curves.

The relative magnitude of advection to diffusion/dispersion for the low P and high P sites can be estimated by the Peclet number (P_e):

[6]

Peclet numbers much greater than unity imply that advection dominates over diffusion/dispersion, while values much less than unity indicate predominance of diffusion/dispersion.

Phosphorus Breakthrough
Phosphorus BTCs were plotted using both dimensional and dimensionless time and concentration obtained from the experiment. Dimensionless time (pore volume) is defined as follows:

[7]

In this study, the number of pore volumes that leached was calculated as the ratio of water volume leached (V) to pore volume of the soil column. Soil column pore volume was estimated from soil porosity and the bulk volume of the soil column (V_c) as follows:

[8]

where _b is the bulk density (mean 1.63 Mg m^–3 for the 0- to 0.6- and 0.6- to 1.5-m depths) and _s is the particle density (2.65 Mg m^–3) of the soil. Dimensionless or relative concentration (C/C₀) was calculated by dividing leachate TP concentration by TP concentration in the wastewater.

Langmuir Sorption Parameters
Phosphorus sorption isotherms were determined for the 0- to 0.6- and 0.6- to 1.5-m depths according to the method of Graetz and Nair (2000) using soil samples taken from the high P site during well drilling. One-gram soil samples were equilibrated with 20 mL of varying concentrations of P (0, 0.01, 0.1, 5, 10, 25, 50, and 100 mg P L^–1) in 0.01 M CaCl₂ solution in 50-mL centrifuge tubes. Following 24 h shaking on an end-over-end shaker, the supernatant was filtered through a 0.45-µm membrane filter and analyzed for SRP using the ascorbic acid–molybdate blue method of Murphy and Riley (1962). Isotherm data from both horizons (0–0.6 and 0.6–1.5 m) were pooled to derive an average isotherm for the 0- to 1.5-m depth. Sorption parameters (Table 1) were estimated from the pooled data using the Langmuir equation as follows:

[9]

where

The sorption parameters were used to estimate retardation factors (R) for P transport through the soil as follows:

[10]

where

The partition coefficient at the wastewater P concentration of 3.6 mg L^–1 was estimated from the slope of the Langmuir isotherm as follows:

[11]

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Breakthrough data from the monitoring well site and from the drainage lysimeter sites are presented separately. Wastewater was applied in pulses at the well site, with some truckloads containing no Br tracer, as outlined previously. Between the pulses, sampling continued, but the pore volume associated with each sampling could not be easily determined. Exceptions were samples taken at the well site during the initial 1.4 pore volumes of wastewater application and those taken at the end of the well site test, when the overall pore volume could be estimated from the total wastewater volume applied. In contrast, breakthrough tests at the drainage lysimeter sites were run under continuous ponding, and all the wastewater applied contained Br, with the exception of one low-P site, which received an extra wastewater truckload without Br. Therefore, side-by-side comparison of Br breakthrough results from the well, suction samplers, and drainage lysimeters was restricted to initial Br appearance, peak concentrations, and concentrations at the end of the breakthrough tests.

Monitoring Well Site
Bromide Breakthrough
Traces of Br were detected in the well samples within a day (<0.4 pore volumes) after the start of the breakthrough experiment at the monitoring well site (Fig. 4) . Background Br concentration in the well was <0.08 mg L^–1. Bromide concentration increased sharply and peaked at 27% of influent concentration (C/C₀) after 1.2 pore volumes of wastewater application. The low Br concentration was likely due to dilution as a result of snowmelt recharge from upgradient ground water and due to lateral and vertical mixing of the ground water. After addition of the second pulse of wastewater on Day 14, Br concentration increased again from a relative concentration of 0.07 on Day 16 to a maximum of 0.23 on Day 18. Since Br was not added in the second pulse, the increase in Br concentration at the water table was due to previously (during the first pulse) added Br remaining in the profile. Following application of the final pulse on Days 50 through 52, Br relative concentration peaked at 0.54 on Day 55. By that date, recharge of ground water from spring thaw had diminished, hence the higher Br concentrations.

View larger version (15K): [in this window] [in a new window]   Fig. 4. Breakthrough curve of Br measured in the monitoring well at the low P site up-gradient of the winter sprayfields. The water table was 7 m below the ground surface. Wastewater containing Br was applied in two pulses on Days 1 through 3 and on Days 50 through 52. Bromide was not added to wastewater applied on Day 4 and on Days 14 through 18.

View larger version (12K): [in this window] [in a new window]   Fig. 5. Bromide breakthrough curves measured in stainless steel suction samplers at 1.5- and 3-m depths at the low P monitoring well site up-gradient of the winter sprayfields. Wastewater containing Br was applied on Days 50 through 52 from the start of the experiment at the well site. Bromide breakthrough in the samplers was monitored from Day 50 to 56.

Wastewater used in this experiment was lower in TP than the average wastewater applied during the nongrowing season that year (4.47 mg L^–1). During the experiment, an equivalent of 342 kg P ha^–1 in 135 m³ (4.5 pore volumes) of wastewater was applied. The hydraulic loading rate in the winter sprayfields in November 2001 through April 2002 averaged 760 mm, which translates to 0.29 pore volumes. At this loading rate, the amount of wastewater used in the breakthrough experiment was equivalent to 15 nongrowing seasons with P supplied at an annual rate of 22 kg ha^–1. This does not include the much higher P loading during the growing season, which brought the average annual P loading to 100 kg ha^–1.

Although most of the hydraulic loading (wastewater application) occurs during the winter months, significant leaching occurs only after spring thaw in May. It was on this basis that the breakthrough tests were run after the onset of spring thaw.

Lysimeter Sites
Bromide Breakthrough
For the low and high P lysimeter breakthrough tests, Br concentration in the wastewater averaged 259 mg L^–1. Background leachate Br concentrations before the start of the breakthrough experiment were <8 mg L^–1. At both low P and high P lysimeter sites, Br was detected in the leachates after 0.2 pore volumes of wastewater had percolated through the lysimeters (Fig. 6) . Bromide concentration in the leachate reached 50% of Br concentration in the applied wastewater (C/C₀ = 0.5) after 0.25 pore volumes at the low P sites and 0.5 pore volumes at the high P sites.

View larger version (24K): [in this window] [in a new window]   Fig. 6. Measured and CXTFIT-fitted breakthrough curves for relative Br concentration in leachate collected at 1.5-m depth at nonirrigated, low P sites (a and b) and at wastewater irrigated, high P sites (c and d). The BTC for low site (a) is plotted only for the portion that received wastewater with Br added.

Phosphorus Breakthrough
Total P concentration in leachates from the low P sites did not change (mean 0.04 mg L^–1), even after five pore volumes of wastewater had percolated through the lysimeters (Fig. 7) . In contrast, the Satterthwaite test showed a significant increase in TP concentration in leachates from the high P sites with wastewater application from a mean background concentration of 3.5 mg L^–1 to peak at 5.62 mg L^–1 after 1.5 pore volumes of wastewater. The Satterthwaite test indicated that the difference in peak effluent P concentration between the low P sites (0.04 mg L^–1) and the high P sites (5.62 mg L^–1) was highly significant (P = 0.01). Relative P concentrations (C/C₀) for the high P lysimeters were greater than 1.0 throughout the experiment and peaked at 1.6. This indicates that P moving down the lysimeters during the experiment was coming from soil P reserves rather than from the wastewater per se.

View larger version (23K): [in this window] [in a new window]   Fig. 7. Breakthrough curves for (a) total P (TP) concentration and (b) relative P concentration measured in effluent collected at 1.5-m depth at nonirrigated, low soil test P sites and wastewater irrigated high soil test P sites. Total P concentration in the wastewater was 3.6 mg L–1.

Simulation of Phosphorus Transport
Langmuir sorption parameters for the 0- to 1.5-m depth at the high P site (Table 2) were used to estimate the retardation factor (R = 59) using Eq. [10] and [11]. Phosphorus transport was simulated (Fig. 8) for the high P site using sorption parameters in Table 1, transport parameters in Table 2, and the retardation factor estimated above. The CXTFIT simulation indicates that P applied in the wastewater at the soil surface under ponded conditions would not be expected in the leachate at the 1.5-m depth until 20 pore volumes had percolated through the profile (Fig. 8). The simulation also predicts that leachate P concentration at the 1.5-m depth would reach 50% of P concentration in the applied wastewater after 55 pore volumes. The simulation relates only to breakthrough originating from the added wastewater and does not take into account the leaching of P already present in the soil.

View larger version (11K): [in this window] [in a new window]   Fig. 8. CXTFIT-simulated breakthrough curve (BTC) for relative P concentration measured in effluent collected at the 1.5-m depth at a high P, wastewater irrigated site. Transport parameters were estimated from measured Br BTCs using the CXTFIT program.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

A recent study by Toor et al. (2004) indicated that 71–79% of leachate TP following dairy effluent application was PP, whereas SRP accounted for only 1–7% of the TP. In our study, SRP was the predominant P fraction in the leachates. In the study by Toor et al. (2004), however, the leachate P originated from dairy effluent application whereas results from our study indicated that high P soil was the source of leached P.

Phosphorus transport simulations using the CDE model suggest that high P concentrations measured in the leachate at the high P lysimeter sites originated from residual soil P, because P added in the wastewater would not be detected at 1.5 m until 20 pore volumes of the wastewater had leached through the 0- to 1.5-m soil layer (Fig. 8). The simulations also predict that it would take 55 pore volumes for the leachate P concentration at the 1.5-m depth to equal 50% of P concentration in the wastewater applied at the soil surface. These predictions corroborate results obtained at the low P lysimeter and monitoring well sites, which indicated no change in leachate P concentration following application of four to six pore volumes of wastewater.

The high WEP levels in the winter sprayfields are partly due to potato waste (wastewater, sludge) and horse (Equus caballus) manure application before 1996 when winter wastewater application commenced under a permit from the MPCA. Also, since winter irrigation at the site started in 1996, about 100 kg P ha^–1 have been added in wastewater annually, mostly during the growing season when the wastewater applied is not required to meet MPCA specifications. Attempts to minimize P buildup in the soils using plant uptake have largely been unsuccessful due to poor survivability of P-accumulating forage grass species under snow and ice. Nitrogen availability during the growing season has also been a limiting factor to forage crops (C.C. Sheaffer, unpublished data, 1999). Dry matter forage yields from a previous study at the site ranged from 0.7 to 6 Mg ha^–1 (mean 3.8 Mg ha^–1) for perennial reed canarygrass (Phalaris arundinacea L.), which is grown in the winter sprayfields (C.C. Sheaffer, unpublished data, 1999). The corresponding P uptake averaged 15.4 kg ha^–1 (range 3.4–25.8 kg P ha^–1) compared with an annual loading of 100 kg P ha^–1. Results obtained by McCollum (1991) suggest that even without further P application, it would take 16 to 18 yr of cropping corn (Zea mays L.) or soybean [Glycine max (L.) Merr.] to reduce the soil test P (Mehlich III) of a Portsmouth soil from 100 mg P kg ^–1 to the threshold agronomic level of 20 mg P kg^–1. In our study, Bray-1 P in the 0- to 0.3-m depth averaged 12.1 mg kg^–1 at the low P sites compared with 585 mg kg^–1 at the high P sites, which translate to approximately 59 and 2861 kg Bray-1 P ha^–1, respectively. Therefore, even at the TP concentration limit of 6 mg L^–1 set by the MPCA for wastewater application during the nongrowing season, it is likely that P will continue to leach down the high P soil profiles simply due to hydraulic loading (excess water over and above the natural precipitation). In the event that wastewater application at the site is discontinued, the potential environmental impact of soil P will still remain an issue for years to come because even rainwater will most likely move the soil P down the profile.

The Langmuir sorption maximum (b) determined from pooled data for the 0- to 0.6- and 0.6- to 1.5-m depths at the high P site indicates that the 0- to 1.5-m soil layer can sorb a maximum of 184 mg P kg^–1. The DPS estimated using Al_ox, Fe_ox, and P_ox for the 0- to 0.3-m depth suggests that 12.2% of the sorption sites in this layer are saturated. Recent studies have suggested that significant P leaching is likely above a threshold of 25% DPS (Breeuwsma et al., 1995; Sinaj et al., 2002). The fact that significant leaching of soil P was measured in our study at the DPS of 12.2% suggests that the threshold conditions of the study might be much lower than that observed in other studies. In addition, reducing conditions (ponded) under which P breakthrough was tested may have resulted in reduction of Fe³⁺ to Fe²⁺, thus mobilizing the P. Similar ponded conditions exist in depressions within the sprayfields during spring thaw.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Field monitoring data show that winter wastewater application on high P outwash soils that are seasonally frozen results in P leaching past the 1.5-m root zone following spring thaw. Field breakthrough curves and subsequent model simulations in this study show that the P leaching is likely the result of desorption and dissolution of P already present in the soil due to past applications of potato wastewater and manure. Phosphorus leaching from these wastewater disposal sites are probably influenced by hydraulic loading in addition to wastewater characteristics. To minimize the risk of P leaching, it is recommended that wastewater application on high P soils be avoided. Where alternative land low in soil P is not available, wastewater applied during summer months should be treated to lower P concentrations to avoid further buildup of soil P. In addition to wastewater P concentration, regulation of wastewater application should also take into account the soil P level and wastewater P loading.

	ACKNOWLEDGMENTS

 
This research was funded in part by Lamb-Weston/RDO Frozen, through the Minnesota Pollution Control Agency.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 

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