Phosphorus transport in Spodosols impacted by dairy waste

ELSEVIER Ecological Engineering 5 (1995) 281-299

ECOLOGICAL ENGINEERING

Phosphorus transport in Spodosols impacted by dairy waste

R.S. M a n s e l l *, S .A. B l o o m , P. N k e d i - K i z z a

Soil and Water Science Department, 2169 McCarty Hall, P.O. Box 110290, Uniuersity of Florida, Gainesuille, FL 32611-0290, USA

Abstract

Deposition of P-laden dairy waste onto acid, sandy Spodosols located in the Lake Ocheechobee Basin in south Florida is commonly recognized to increase levels of P in soil profiles, groundwater, and local streamflow. In the soil profile, P accumulations occur in the surface A horizon, to a lesser extent in the E horizon, and to a much greater extent in the reactive Bh or spodic horizon.

A one-dimensional mathematical model was used to demonstrate sorption and transport of P vertically through a 90-cm profile of an initially P-free Spodosol with A (0-15 cm), E (15-75 cm), and Bh (75-90 cm) horizons for conditions of steady, saturated water flow. Ten pore volumes (equivalent of 2.5 years of rainfall) of P-laden influent was applied before applying 40 volumes (equivalent of 10 years of rainfall) of P-free water. The loading of P in the influent was shown to greatly impact P transport during water flow in the soil. As expected, nonlinear sorption resulted in much greater retardation of P breakthrough in the soil effluent for the lowest influent concentration of 10 g m -3 than for either 100 or 1000 g m -3 concentrations.

An underlying assumption in the model was that multi-dimensional water flow could be ignored. However, multi-dimensional flow under field conditions may well result in partial bypass of flow ai'ound highly consolidated Bh horizons.

I. Introduct ion

Eut roph ica t ion of Lake O k e e c h o b e e in southern Florida has been at t r ibuted (Allen et al., 1982) to excessive accumula t ion of P and N nutr ients f rom dairy

* Paper presented at the workshop on Phosphorus Behavior in the Okeechobec Basin, sponsored by the South Florida Water Management District and the University of Florida. Institute of Food and Agricultural Sciences.

• Corresponding author.

Elsevier Science B.V. SSDI 0925-8574(95)00028-3

282 R.S. ,14ansell et al. /Ecological Engineering 5 (1995) 281-299

waste/wastewater deposition in the Taylor Creek/Nubbin Slough (TCNS) watershed of the Lake Okeechobee Basin. Inadequate management of animal waste, direct stream access by large numbers of dairy cattle, permeable sandy soils with limited contents of reactive mineral components (clay minerals and iron and aluminum oxides), high annual rainfall, and nearly flat terrain with shallow groundwater tables are contributing factors.

The objectives for this paper are 2-fold: (1) to review the state of knowledge concerning P transport during water flow in sandy soils of the Lake Okeechobee Basin in order to evaluate the impact of dairy waste upon P levels in local groundwaters; and (2) to demonstrate significant aspects of one-dimensional P sorption/transport in the profile of a typical Spodosol.

1.1. Characteristic soils m the Lake Okeechobee Basin of South Florida

Soils in the Lake Okeechobee Basin are typically Spodosols having acid, sandy surface A, subsurface E (albic), and underlying Bh (spodic) horizons (Burgoa et al., 1991a). The spodic horizon represents an accumulation of organic C and associated metals in a quartz sand matrix. During extended periods of the year, watertables occur above the spodic horizon (Graetz and Nair, 1995). Water flow occurs predominantly (Burgoa, 1989) through the permeable A and E horizons. Limited permeability in the partially-consolidated spodic horizon and nearly-level topography contribute to shallow water tables and lateral flow through the albic during the summer rainy season.

Phosphorus retention capacities are generally very low for the A horizon (Selim et al., 1975; Mansell et al., 1977b; Burgoa, 1989), negligible for the albic horizon, and large for the spodic horizon due to the presence of Al and Fe oxides. Although the thickness and degree of consolidation of spodic horizons are known to be spatially variable across Spodosol landscapes in South Florida, very little work has been performed to document that variability in large areas (Burgoa et al., 1991b).

1.2. P input from dairies to local streams in the Lake Okeechobee Basin

Otter Creek in the Upper Taylor Creek portion of the TCNS Watershed drains an intensive dairy area consisting of milking barns, holding pens, and improved pasture. Elevated ortho-P concentrations have been reported for stream water from Otter Creek. Phosphorus concentrations in Otter Creek as high as 18.80 g m -3 (Allen et al., 1975) were determined in 1973 and 3.26 g m -3 (Stewart et al., 1977) during 1974-1975. Concentrations were reported to be highest during summer months of higher rainfall and streamflow and lowest during winter and late spring with less rainfall.

Feed and supplements fed to dairy cows provide the original source of P found in dairy waste/wastewater. Graetz and Nair (1995) reported a mean P content for dairy manure from 4 dairies to be 6.5 g kg-~. Much of the P in the manure was potentially mobile in the soil since 42% of total P was extractable with water.

R.S. Mansell et al. /Ecological Engineering 5 (1995) 281-299 283

Recent investigations of P retention in Spodosols located on dairies and beef cattle ranches in the Taylor Creek-Nubin Slough revealed that highest levels of P occur in soil profiles located in intensive and holding areas of the dairies (Graetz and Nair, 1995). Approximately 4000 and 1800 kg P ha- t were identified as being available in the profile for transport via water flow from intensive and holding areas, respectively. Phosphorus desorption occurred readily in the surface A and subsurface E horizons of the profile for soils that had been exposed to heavy loading of manure, but net sorption onto AI and Fe oxides was the norm for the subsurface spodic horizon. Soils located in dairies within TCNS are expected to slowly release phosphorus for a long time after operations are stopped.

1.3. Pathways for P transport from dairies to local streams

Downward percolation of P-laden water through water-unsaturated soil to the saturated groundwater zone, subsequent saturated lateral subsurface groundwater flow to streams, and overland flow during periods of shallow watertables provide major pathways by which P levels in nearby streams can be increased and thus promote eutrophication. Each of these pathways involves water flow and requires that P molecules be dissolved in the moving water.

An additional pathway for P transport may occur due to lateral saturated water flow in a temporarily perched water table along the bottom of E horizon under field conditions where the primary water table occurs below the Bh horizon. If the spodic horizon has a high degree of consolidation and a large ratio of permeabilities for E and Bh horizons ( :~ 1.0), one would expect transient unsaturated water flow vertically downward through the A and E horizons to the upper interface, lateral saturated flow along the interface toward a nearby stream or pond, and extremely slow flow downward through the spodic horizon. Textural discontinuities at interfaces between two soil layers (such as the E and Bh horizons) with contrasting permeabilities are well known to limit vertical unsaturated flow and generate zones of water saturation. If the interface is inclined, lateral flow thus results in saturated zones above the interface. Concentrated preferential 'funnel flow' can be generated when such interracial surfaces are inclined within the soil (Kung, 1990a, b, 1993) and can generate nonuniform transport of solutes. Where the spodic horizon has a low degree of consolidation and a near-unity ratio (~ 1.0) of E to Bh permeabilities, one would expect transient, unsaturated water flow through all three horizons (A, E, and Bh) to an underlying groundwater table and lateral saturated flow toward a nearby stream.

For conditions where water flows vertically through the Bh horizon, the spodic horizon would be expected to act as a sorption sink for P and thus limit downward transport of ortho-P in Spodosols that receive loadings of dairy manure and wastewater. However, lower permeabilities in Bh than albic horizons may cause saturated lateral flow in the E horizon just above the Bh horizon. Such lateral flow would tend to effectively bypass the large sorption capacity of the spodic horizon and thus transport P molecules toward a nearby stream.

284 R.S. Mansell et aL / Ecological Engineering 5 (1995) 281-299

2. Mechanisms for P retention by soil components

During renovation of animal wastewater in acid, sandy soils, mobility of P is influenced primarily by inorganic oxyanion (orthophosphate) forms (Gerritse, 1981). Phosphorus sorption isotherms for Spodosois typically are characterized by convex nonlinearity which imparts a concentration-dependent retardation for P transport that is greater for low- than high-solution phase concentrations (Mansell et al., 1991). Thus, applications of wastewater with high concentrations of P tend to enhance the mobility of inorganic P in sandy soils relative to applications of influent with low P concentrations.

Relatively high pore water velocities (i.e. small residence times) combined with slow sorption reactions in sandy soils requires that P transport models consider sorption kinetics (Mansell et al., 1991). Mobility of applied P can be inhibited by the occurrence of either very slow desorption or partially-irreversible sorption by soil components. For conditions where desorption rates are orders of magnitude smaller than the forward sorption rates, sorbed phase P may appear to be irreversibly sorbed during short periods of rainfall infiltration into the soil.

2.1. P sorption-desorption reactions

Orthophosphatic acid (H3PO 4) present in water can dissociate to form HzPO£, HPO 2-, and HPO 3- inorganic oxyanions with dissociation constants (pK i) of 2.1, 7.2, and 12.3, respectively (Kafkafi, 1989). The proportions of these oxyanion species varies with the solution pH. For the pH range generally encountered in soil solutions, H2PO ~- and HPO 2- are the two species of primary concern (Syers and Iskandar, 1981). Oxides and hydroxides of iron and aluminum constitute the main reactive sites in soils for binding P oxyanions (Kafkafi, 1989). Short-range order components such as hydrous ferric oxide gel and amorphous aluminum hydroxide generally have much greater capacity to sorb P than do their crystalline analogs, and crystalline hydrous metal oxides are usually more effective than layer silicates (Syers and Iskandar, 1981). Aluminum and iron exist in soil as free ions in solution and as metal hydroxy-polymers. Soil Al and Fe occur with or without inorganic oxyanions (carbonate, nitrate, silicate, phosphate, selenate and molybdate) at- tached to them in weak or strong bonds.

Under acid conditions, P oxyanion adsorption by soil minerals involves a 2-step ligand exchange reaction (Kafkafi, 1989) where the first step involves protonation of surface hydroxyl groups

SOH(s ) + H += SOH~'(,) (1)

and the second step involves phosphate adsorption

~'-- I--Z SOH2(s) + L~aq) = SL~aq) + H20 (2)

where S is the metal cation, SOH~s ~ is one mole of the surface hydroxyl group and L z- is an oxyanion of valence z.


The ligand exchange approach of P oxyanion adsorption is supported (Kafkafi, 1989) by experimental observations of pH dependence, kinetics of 32p exchange, and adsorption hysteresis in the presence of indifferent electrolytes. Many inorganic and organic oxyanions are well known to compete with P for sorption sites in soil. Inorganic anions such as hydroxyl and sulfate compete with P oxyanions for sorption sites. Organic anions capable of forming stable complexes with the iron and aluminum of soil components are particularly effective in reducing P sorption (Syers and Iskandar, 1981). Additions of oxalate anions to batch samples of spodic horizons (Bh) from forested Spodosols has been observed to greatly enhance desorption of both inorganic and organic forms of sorbed P, as well as AI (Fox and Comerford, 1992). Fox and Comerford (1992) suggested that a continuous release of even small amounts of organic anions in forested Spodosols could solublize large amounts of P and AI on an annual basis. Increasing the ionic strength of the soil solution has been shown (Kafkafi, 1989) to enhance the competitiveness of P oxyanions over organic oxyanions for monoionic clays.

Sorption of P by soils and soil components generally proceeds in a time-dependent manner. An initial, rapid reaction involves adsorption and a slower subsequent reaction which is not well understood (Syers and Iskandar, 1981). With time, newly adsorbed P oxyanions undergo a shift from more-physically (weakly sorbed) to chemisorbed (strongly sorbed) forms, leading to regeneration of sorption sites during incubation (Syers and Iskandar, 1981). Mechanisms such as precipitation of discrete phosphates, a change from monodentate to bidentate forms of sorbed P, and diffusive penetration of surface-sorbed P into soil components have been proposed to explain this shift (Syers and Iskandar, 1981). With increasing time, a shift from more-physically sorbed P to more-chemically sorbed P limits the leachability of soil P by decreasing the rate of desorption during periods of rainfall. Observed decreases in extractability and desorption of sorbed P with time have been related to this shift in bonding of sorbed P molecules. Kafkafi (1989) has shown that the number of maximum adsorption sites for P oxyanions is not constant on silicate minerals and soils due to slow solubilization of silicate ions and that it changes with the experimental conditions.

2.2. Important characteristics of P sorption and desorption

Sorption and desorption are terms used here to represent overall effects of many complex chemical interactions that dynamically partition P oxyanions between the mobile solution and immobile sorbed phases in soils. Sorption is used in a general sense to include P retention, physical adsorption, and chelation with humic acids (AI and Fe may form bridges between organic ligands and P ions.). In contrast to cation exchange, stoichiometry is not imposed for P sorption and no consideration is given for which sorbed anions are displaced by sorbing P oxyanions. As P-laden water flows through soil pores, sorption transfers P molecules from the solution phase to the sorbed phase and desorption returns sorbed phase molecules to the solution. Thus, P mobility is inhibited by sorption and enhanced by desorption.

286 R.S. Mansell et al. / Ecological Engineering 5 (1995) 281-299

P sorption is characteristically convex nonlinear, partially kinetic with both fast and slow reactions, and largely reversible (Selim, 1987). The extent of reversibility depends upon the composition and nature of soil surfaces. Convex nonlinear sorption results in increased buffering and decreased P mobility as concentrations decrease for P-laden solutions applied to soils, Thus for high influent concentration, P molecules are more mobile and move with less retardation than for low influent concentration. When P sorption is characteristically instantaneous, reversible, and convex nonlinear, the initial presence of residual P in soil results in decreased buffering and increased mobility when P-laden solution is applied. Kinetic sorption also imposes a dependency of P mobility upon the local water pore velocity. Large pore velocities provide short contact times for P molecules, resulting in low sorption and enhanced mobility. Very slow pore velocities with very long contact times for solutes may result in local equilibrium even for slow kinetic sorption.

2.3. Models for P sorption-desorption

The Constant Capacitance Model (CCM) from electrical double-layer theory has been used to describe instantaneous P sorption-desorption by metal oxides and by soils that contain metal oxides (Goldberg and Sposito, 1984). Cederberg et al. (1985) coupled the CCM with a mass transport model to describe the transport of heavy metals which undergo complexation, ion exchange, and competitive adsorption. Although the Constant Capacitance Model is a step in the right direction, it is limited by the assumption of local chemical equilibrium.

Simple Freundlich and Langmuir equations have been used to describe nonlinear, reversible, instantaneous sorption of P by soils and soil components. A convex form of the Freundlich equation generally has been shown to fit experimental data for P sorption more successfully than the Langmuir equation (Mansell and Selim, 1981). In the Freundlich isotherm, the affinity of soil for P sorption is assumed to decrease exponentially as the amount of sorption increases. Affinity for P sorption tends to decrease with increasing sorbed amount because specific sorption of P oxyanions increases the negative charge upon the sorbing surface. The Freundlich equation assumes that soil surfaces contain heterogeneous sorption sites, and that the number of sorption sites is not necessarily constant. Freundlich-type kinetic equations have also been used to describe nonequilibrium P sorption in soils (Mansell and Selim, 1981) under conditions where the sorption process is reversible.

Mechanistic multiple-step and multiple-reaction type sorption models that involve a degree of local nonequilibrium (Selim et al., 1975; Cameron and Klute, 1977; Mansell et al., 1977a; De Camargo et al., 1979; Mansell et al., 1985) have been used with reasonable success to describe the transport of P with steady water flow in soil. Two-site sorption models with one fraction of sorption sites subject to instantaneous P sorption and another to slow, time-dependent removal of P from the soil solution have been used for soils (Mansell et al., 1976; Selim et al., 1976; Mansell and Selim, 1981; Selim et al., 1990).

R.S. Mansell et al. / Ecological Engineering 5 (1995) 281-299 287

Most sorption and transport research for P in soils has been performed over time periods restricted to days and at most weeks. Sorption distribution coefficients and kinetic sorption rate coefficients obtained over periods of days obviously may be inadequate for describing P sorption behavior over periods of months and years under field conditions. P oxyanions are very reactive with soil components and are known to slowly form stronger sorption bonds over long periods in field conditions. Thus sorption/transport models based upon short-term sorption parameters may be limited and probably should not be used to simulate long-term sorption. Such models would be expected to overestimate the leachability of P that has been in the soil for weeks and months.

3. P transport during subsurface water flow in soils

3.1. Important characteristics of P transport

Miscible displacement of P-laden influent into soil columns during steady, saturated water flow generally provides P breakthrough curves (BTCs) that are greatly retarded with respect to conservative solutes, have maximum concentrations that are often considerably less than influent concentration, are rotated clockwise, and exhibit considerable tailing (very gradual approaches to the maximum and to the minimum effluent concentrations). Each of these transport characteristics reflects the strong influence of sorption-desorption processes upon P transport in soils. The extent of retardation, delayed appearance of P in column effluent, and decrease of maximum concentration in the effluent with respect to influent concentration tends to be greatest for low concentrations of P in the influent. For a given P concentration in the influent, high buffering capacity of soil components for P sorption also contributes to increased retardation. Tailing and clockwise rotation observed in experimental BTCs for miscible displacement of P-laden influent into soil columns is often attributed to kinetic and convex nonlinear aspects of P sorption (Mansell et al., 1991). Relatively small kinetic rate coefficients for desorption with respect to sorption provides slow, delayed leaching of soil P.

The actual influence of residual soil P upon sorption-desorption and transport during miscible displacement of P-laden solutions through Spodosols is not well documented. If the sorption process is totally reversible, then the presence of residual soil P would be expected to enhance the transport of P-laden influent applied to the soil. On the other hand if the sorption process is totally irreversible, the presence of residual soil P should not be expected to enhance P transport. Presently, considerable uncertainty exists for the irreversibility and the kinetics of desorption for residual soil P.

3.2. Modeling P transport in Spodosols

Rates of P transport during unsteady flow in unsaturated soil may be many orders of magnitude slower than that for water movement. Since P oxyanions

288 R.S. Mansell et al. / Ecological Engtneering 5 (1995) 28 l - 299

typically undergo both fast and slow chemical reactions (Mansell and Selim, 1981), local nonequilibrium may occur between P in the solution and sorbed phases if the water flow velocity in the soil is relatively high. For slow flow rates, local equilibrium becomes favored with respect to nonequilibrium such that solute retardation is maximized. Transport models based solely upon instantaneous sorption theory (Cho et al., 1970) typically overestimate sorption-induced retardation of P movement as well as underestimate observed tailing during steady liquid flow in soils.

The descriptive value of model simulations for P transport with water flow in soils is quite often limited by the availability of independently-determined input parameters that are applicable for a wide range of P concentrations in the solution phase and for a range of soils (van der Zee and van Riemsdijk, 1986; van der Zee et al., 1989). An added complication involves the occurrence of both inorganic P and covalently-bound P present in soil organic matter (Gerritse, 1989). The wide range of different sorption-desorption models used by investigators in the pub- lished literature often require different types of parameters such that useful comparisons of common parameters for different soils is often not feasible.

By definition, models represent some degree of simplification for the complexi- ties of reality. Thus, input parameters such as rate coefficients which are assumed to have constant values over a specified range of environmental conditions are sometimes observed to vary with time (Gerritse, 1989) as water flow or P transport conditions change.

A convective-dispersive transport model coupled with simple 1-site, reversible, nonlinear adsorption-desorption using Freundlich-type kinetics and an irreversible sink for P fixation or chemisorption was used by Mansell et al. (1977b) and Selim et al. (1975) to simulate P transport in undisturbed cores of A, E, and Bh horizons from a Spodosol. Results demonstrated that P transport during water flow in sandy soils is greatly influenced by the degree of sorption nonequilibrium. Overall retention of applied P for topsoil (A), albic (E) and spodic horizons (Bh) were observed to be proportional to the quantity of ammonium - oxalate extractable Al present in the soil (170, 0, and 722 ~zg AI g-~ for A, E, and Bh horizons, respectively). Sorption nonlinearity provided large retardation of BTC in the effluent for low P concentrations in the influent, and sorption kinetics provided conditions of local chemical nonequilibrium which inhibited P sorption during water flow. Phosphorus BTCs were characterized by retardation, tailing, asymmetry, and maximum effluent concentrations which rarely reached the influent concentration.

Although a multicomponent mathematical model to describe P chemical reactions in soil completely does not actually exist (Avnimelech, 1984), many concep- tual models which approximate these reactions are available (Mansell and Selim, 1981). Multireaction-type sorption models constitute a class of such approxima- tions, and offer practical means to approximate P mobility during water flow in soils (Selim et al., 1990). Multi-reaction P models have in general provided improved results over the use of single-reaction models (Selim et al., 1990).

Selim (1987) used a multireaction model to investigate the mobility and reten-

R.S. Mansell et aL / Ecological Engineering 5 (I995) 281-299 289

tion of P in both batch and column experiments for a very fine sandy loam. Results indicated P retention to be time-dependent and strongly dependent upon soil temperature. Using a mass transport model coupled with a multi-step, kinetic submodel; the use of kinetic parameters obtained with a batch nonequilibrium technique resulted in gross underestimation of P retention during water flow in the columns. This underestimation was attributed to a small residence time for solute associated with high water flux used in the columns and the resulting local chemical nonequilibrium. Adequate descriptions were obtained when the parameters were estimated directly by nonlinear least square fitting of the P BTC data. The use of two types of reaction micro-sites (instantaneous and kinetic reactions) was observed to better describe the column data than the use of a single site.

More recently, Mansell et al. (1992) used a 2-site transport model with fast and slow sorption kinetics to investigate the mobility of P in columns of Bh soil material from Spodosols at two dairies in the Lake Okeechobee Basin. Phospho- rus-laden solutions with four (5, 10, 50, and 100 g m-3) initial concentrations (Co) were applied to soil columns. Shapes of BTCs were characteristic of kinetically- controlled nonlinear sorption reactions, i.e. early breakthrough, clockwise rotation, and tailing.

The model was calibrated from the BTC for one of the four C O values for each of two soils, and then used to simulate BTCs for the other three C O values. Phosphorus retardation was underestimated for simulations of Co = 11, 51 and 97 g m -3 using forward K t and reverse K 2 sorption rate coefficients obtained by calibration using the BTC for C O = 5 g m - 3 ; and was overestimated for simulation of C O = 5 g m -3 when input parameters for the model were obtained by calibrat- ing using the BTCs for the three highest Co values. Thus the sorption rate coefficients were not independent of influent C O for these two soils.

Early P breakthrough in effluent from columns that received C O = 5 g m -3 was attributed to the initial presence of residual phosphorus. Residual soil P initially present was assumed to be partially reversible.

A general mechanistic multi-reaction model for the transport of P in water- saturated or unsaturated soil during conditions of steady or unsteady flow of water was developed by Mansell et al. (1991). Components of the model include instantaneous or kinetic transfer of inorganic P in the mobile soil solution with multiple categories of reversibly and irreversibly-sorbed forms of inorganic P. Utility of the model has been demonstrated with the use of one-site, parallel two-site and sequential two-site P sorption models.

For steady, saturated flow in soil, convective-dispersive transport coupled with reversible sorption is described by

oc oc 0 -0 7- + 0 at = O D ~ z 2 - e -~z (3)

where C (z , t ) is P concentration in t.he soil solution [g m -3] for a given depth z (m) and time t (s), Sj represents concentrations of any of two reversibly sorbed forms (S t and S 2) of inorganic P [ g Mg-1], D is the hydrodynamic dispersion coefficient [m 2 s-l], q is the Darcy water flux [m s-~], p is dry soil bulk density


[Mg m-3], and 8 is volumetric water content [m 3 m-3]. Further details of the model are given in Mansell et al. (1991).

4. Simulated P transport in a spodosol profile

A parallel 2-site kinetic model for P sorption and transport (Fig. 1) is a special case for the general multireaction model where rapid and slow kinetic reactions control the distributions of P between rapidly S t and slowly S z sorbed compart- ments, respectively, and the solution compartment C(z,t). The equilibrium distribution coefficients for the rapid and slow reactions are defined as K~ - fKeq and Kz---(1 - f ) Keq, respectively, where f is the fraction of sorption sites associated with rapid kinetic reactions that result in the S t sorbed P and Keq is the sorption distribution coefficient.

Specific aspects of one-dimensional P transport were demonstrated by simulat- ing a hypothetical Spodosol profile during steady, saturated water flow conditions. The profile consisted of an unconsolidated A surface horizon between 0 and 10 cm depths, an unconsolidated E horizon between 11 and 75 cm, and a partially consolidated Bh horizon between 75 and 90 cm. Table 1 provides hydraulic and sorption characteristics of these soils. Two general sorption models were explored. The first was a 2-site system with slow (kinetic) sorption in the A and E horizons and a mixture of slow (kinetic) and fast (instantaneous) sorption in the Bh horizon. The second model was a 1-site system with fast (instantaneous) sorption in all horizons. All simulations used a nodal spacing (Az) of 0.18 cm, a maximum time step of 500 s, and an imposed flux rate of 5.57 x 10 -4 cm s-t . Initial conditions consisted of setting the sorbed P phase in all soils to equilibrium with a small dissolved phase P concentration of 10 -3 g m -3. The influence of P input concentration (C O ) upon transport was examined by performing three simulations of the 2-site model (kinetic and instantaneous) using C O of 10, 100, and 1000 g m -3. A

I n f l o w Solution

. . . . . ~ • ,, k 2 ' - -J

Outflow Solution

Fig. 1. A schematic diagram for a parallel 2-site kinetic model for P sorption and transport.

KS. ManseU et aL / Ecological Engineering 5 (1995) 281-299

Table 1 Characteristics of soil horizons comprising a 90-cm thick Spodosol profile

291

Soil A E Bh

Location (cm) 0...10 10...75 75...90 0 s (m 3 m -3) 0.36 0.38 0.45 K~ (m s- t) 5.083 x 10-s 1.106 x 10 -+ 1.083 x 10-5 O (Mg m -3) 1.51 1.61 1.5 D o (m z s - t ) 8.111 × 10 - s 1.428 × 10 -7 1.0× 10 -9

D l 0.0 0.0 1.0 m 0.345 1.0 0.25 Keq (m 3 Mg- t) 17 0.23 81 K t ( s - l ) 3.961 x 10 -a 1.39 x 10 -6 1.963 × 10 -3 K 2 ( s - 1) 5.556 x 10 -6 1.39 x 10 -6 9.898 x 10 -6 K 2 (m 3 Mg- t) 0.0 0.0 71.577

single simulation of the instantaneous 1-site model was also performed using a C O of 100 g m -3. In these four simulations, the cited C O was imposed for 10 pore volumes and then reduced to background level (10 -3 g m -3) for the next 40 pore volumes. Ten pore volumes of P-laden influent with 10, 100, and 1000 g m -3 concentrations represent P loadings of 350, 3500, and 35 000 kg h-1, respectively. Assuming an annual rainfall of 140 cm, 4 and 40 pore volumes of simulated flow through the soil profile represent the equivalent of one and 10 years of annual rainfall, respectively. Application of 10 pore volumes of P-laden influent represents the equivalent of 2.5 years of annual rainfall.

To explore the effect of residual P initially present in the soil, a two-site simulation with a C O of 10 mg 1-1, was run through five sequential cycles of 10 pore volumes of elevated P followed by 40 pore volumes of background level P for a total of 250 pore volumes. Overall mass balance errors for individual simulations did not exceed 0.001%.

Equilibrium sorption isotherms for A, E, and Bh horizons (Fig. 2) illustrate the medium, low, and high P sorptive capacities for these soil horizons, respectively. Retardation functions R(C) were calculated using instantaneous Freundlich sorption buffer capacities B(C) as functions of solution-phase concentration of P (Fig. 3) for each of the soil horizons.

R ( C ) = 1 + [ p / O ] B ( C ) (4)

where

B ( C ) = OS/OC + m K e q C m - l (5)

and m is the exponent in the Fruendlich isotherm. The highest retardation occurred for very small P concentrations for both the A and Bh horizons and decreased dramatically with increasing C. For any given concentration, retardation values for the Bh horizon exceeded that for the A horizon. A small constant value of R occurred for the E horizon. Thus one would expect downward transport of P in the profile to be moderately retarded by the A horizon, minimally retarded by

292 R.S. Mansell et aL / E c o l o g i c a l Engineering 5 (1995) 281-299

5 0 0 i

q~

O~ E ~to0

C 0 - , 5 0 0

0

C 2 0 0

C 0 0

I O 0

D

.0 k. 0 U3

t i i i i

S o l i 1

- - A 1

. . . . . . . A2 ,, . . . . . . E]t~

" ~ " I I I I I I ~ I , 0 2 0 0 4 0 0 6 0 0 8 0 0 ' 0 0 0

D i S S O I v e d C o n c e n t r a t i o n ( r a g L - 3 )

Fig. 2. Phosphorus sorption isotherms for A, E, and Bh soil horizons.

the E horizon, and maximally retarded by the Bh horizon. The capability of the entire profile to sorb P from an infiltrating solution is primarily determined by the spodic horizon which constitutes only a fraction of soil in the profile. The

1 .4 I I I I i

1 2 "~

Soil 0,~ i

C ......... Bh 0 0 . ~

"U 0 . 4

- "-~-==zZ-- ...................................... (]

4" O. 2: - " . . . . . . . . . . . . . . -i

I" ~ . . . . . . . . . . . . . . . . . . . . . . . i

o~ 0 200 4 - 0 0 600 800 IC00

0 D i s s O I v e d C o n c e n t r a t i o n ( m g L - 3 ) .J

Fig. 3. Equilibrium-sorption retardation functions for A, E, and Bh soil horizons plotted with concentration of P in the solution phase.

G) E ]

0 >

0 O.

R.S. Mansell et al. / Ecological Engineering 5 (1995) 281-299 293

f j o

\

1 0

0 . ~

0 . ~

0 . 4

o.2 t 0

' l

/ / / /

/ /

i / i i

l ;

I I I

J I 0

!t C 9 ( P p m l !, *' 10

i . . . . . . . 100 i ............. I 000

2 0 3 0 '~0 .50

P O r - e V O I u m e

Fig. 4. BTCs corresponding to displacement of 10 pore volume pulses of P solutions through a 90 cm Spodosol profile. Concentrations in the influent pulses were (A) 10, (B) 100, and (C) 1000 g m -3. Phosphorus sorption was assumed to be a 2-site system, i.e. mixture of kinetic and instantaneous in the Bh horizon and kinetic only in the A and E horizons.

moderate sorption buffering in A should provide some retention of incoming P-laden solutions with relatively low concentrations. However, P retention in the A horizon would be expected to be rapidly overwhelmed by incoming P-laden solutions with relatively high concentrations. The low sorption buffering for the E horizon obviously should render this horizon an effective transport zone for any P that moves through the surface A horizon.

The loading of P to a soil is well known to influence the mobility of P in soils. Simulations of P transport under three different loading conditions were performed by varying the P concentration (10, 100, and 1000 g m -3) in influent and maintaining the same volume of influent (10 pore volumes). Simulated P concentrations in the soil solution at the bottom of the soil profile indicate increasing retardation for P transport with decreasing P concentration in the influent (Fig. 4). For the 10 g m -3 concentration of P in the influent, P breakthrough was greatly retarded and very small maximum values of C occurred. P breakthrough occurred in the effluent earlier with 1000 g m -3 than with 100 g m -3. The P BTCs for the two highest influent concentrations were characterized by asymmetry, sharp approaches to large maximum concentrations that were close to influent concentrations, extensive tailing that resulted in non-zero concentrations in the effluent even after the equivalent of 12.5 years (50 pore volumes) of annual rainfall.

These one-dimensional simulations of P transport and steady, saturated water flow provide an idealized representation of field conditions where multi-dimen-

294 R.S. Mansell et aL /Ecological Engineering 5 (1995) 28l-299

E 0

.C

[3.

rh

O

1 0

2 0

. 3 0

4-0

5 0

6 0

7 0

8 0

9 0

! ' I r

Co: 10_ 1 rng L ~i

110 2 2 0

Co: I O 0 mg L-

" O ~ e V O I .

5 . . . . . 1 0

. . . . . . . 2 0

............. 5 0

2OO

i

I' I

O0

co ~ooR] "~g ~ - /

b

i : i, i I , ,

350 700

S T

Fig. 5. Distributions of sorbed phase (S T) phosphorus in the soil profiles corresponding to 5, 10, 20, and 50 pore volumes of water flow after application of a 10 pore volume solution pulse with 10, I00, and 1000 g m -3 P for the 2-site system.

sional water flow may actually occur. For conditions where the spodic horizon is highly consolidated with limited permeability, lateral saturated flow in the albic horizon just above the spodic horizon may transport P-laden soil solution to nearby streams, thereby bypassing the sorptive sites within the Bh horizon. Onsite experimental investigations of transient, multidimensional water flow and the transport of a conservative solute tracer in areas of Spodosols is needed in order to determine the effective impact of Bh horizons upon P sorption/transport and transport to groundwater and streams. The presence of physical/chemical hetero- geneity in these soils requires that such work be performed under field conditions.

Simulated (2-site Model) distributions of sorbed P in the soil profile corresponding to four selected pore volumes (5, 10, 20, and 50) of water flow (Fig. 5) indicate the relative importance of the A and Bh horizons to the retention and slow release of P. The selected values of 5, 10, 20, and 50 pore volumes of water flow are equivalent to 1.25, 2.5, 5.0, and 12.5 years of average annual rainfall. The highest P concentration (1000 g m -3) in the first 10 pore volumes of influent provided the highest P loading and resulted in the most rapid accumulation of P in the spodic horizon at the bottom of the profile. After 50 pore volumes of water flow, total sorbed concentrations of P at the 90 cm depth indicated a monotonic increase with influent concentration of P (Table 2). The ratios of P retention in the soil profile to the input P loading (Table 2) corresponding to the equivalent of 12.5 years of rainfall was highest (56.5%) for the lowest P loading, lowest (1.0%) for the highest P loading, and intermediate (6.2%) for intermediate P loading. Absolute quantities


Table 2 Parameters for simulated P transport in a 90-cm thick Spodosol profile comprised of A, E, and Bh horizons

C O (gm -3) lO 100 1000 100 Model 2-site 2-site 2-site 1-site S T (kg Mg -~) a 109 117 149 33 P loading 350 3500 35 000 3500 (kg h - t )

P retention (%) 56.5 6.2 1.0 1.3 (@ 50-pore volumes)

a At 90-cm and 50-pore volumes,

of P retained in the profile, however, were highest for the highest loading (350 kg h- t), lowest for the lowest loading (198 kg h- t), and intermediate for intermediate loading (2,170 kg h-t).

Incoming P-laden influent resulted in small amounts of P being retained in the A horizon. P escaping from the A horizon was easily transported through the nonsorptive E horizon to the underlying Bh horizon which initially provided a sink for P molecules. P molecules did not escape the Bh horizon until considerable P sorption had occurred in that horizon.

0 0

I.C

0.~

(_I \

0

0.4

I I l l

I M o ~ e l

- - M i x e d

. . . . . . . I ~ S ~ Q N ~ O N e O u S

I 0 1 0 5 0

\

i F ...... F ...... L ................. i,

2 0 3 0 4 0

P O ~ e V O I u m e

Fig. 6. BTCs corresponding to displacement of 10 pore volume pulses of P solutions through a 90 cm Spodosol profile with concentrations in the influent pulses of 100 g m -3 using a 2- and 1-site (phosphorus sorption being assumed to be instantaneous throughout the profile) system.


~)o \ ©

P

i

°° t 0 . 4

0 . 2

i I I

R e p e Q t e d R e p I G c e m e ~ t C y c l e s

- - C y c l e 1 ( 0 5 0 p v l l

. . . . . . ~ y c l e 2 5 { 5 , 0 - 2 5 0 ~ , v l l

0 1 0 2 0 3 0

P O r e V O I u m e Q f t e r C O 1 0 m g L

• 0 5 0 - 1

I m p o s e d

Fig. 7. BTC corresponding to five repeated cycles of 10 pore volume pulses of P solution with 100 g m-3 followed by 40 pore volume pulses of P solution with 0.001 g m-3.

For P influent concentration of 100 g m -3, a P BTC was simulated using 1-site instantaneous sorption (Fig. 6) and compared with the BTC obtained using a combination of 2-site kinetic/instantaneous and 1-site kinetic sorption mechanisms. The instantaneous BTC was characterized by very abrupt P fronts and some retardation for initial breakthrough of P in the effluent relative to the combined sorption BTC. Extensive tailing during the net desorption phase was apparent for both BTCs.

Residual levels of soil P would be expected to be higher in Spodosols located in the immediate vicinity of old dairies than near newly established dairies. Thus, in order to demonstrate the influence of residual soil P upon transport, five repetitive cycles of 10 pore volume pulses of P-laden influent followed with 40 pore volumes of water flow were simulated for influent with 100 g m -3 P concentration using 2-site sorption. P BTCs were essentially identical for the second and following cycles of P application. A comparison of BTCs for the first and second cycles (Fig. 7) provided distinct differences. Three features characterize the influence of residual soil P upon the second BTC: earlier P breakthrough, higher maximum concentration, and the initial presence of P in the effluent. After the second cycle, the BTCs were identical. Thus for influent concentration of 100 g m -3, the presence of residual P in the soil profile would tend to enhance the mobility of incoming influent. The presence of residual soil P would be expected to enhance the mobility of applied P influent for most P loading of the soil.

R.S. Mansell et aL / Ecological Engineering 5 (1995) 281-299 297

5. Conclusions

General characteristics of chemical reactions of P in acid, sandy soils which greatly influence P transport include: (1) multiple processes and heterogeneous sorption microsites, (2) convex nonlinear sorption, (3) partial irreversibility, (4) multiple rate (fast and slow) reaction kinetics, and (5) competitive sorption with other anions (organic as well as inorganic).

Chemical reactions that govern the mobility of P during water flow in soils involve multiple components and are complex. Nevertheless, multi-reaction models which assume that P can be treated as a single component, provide adequate simulations for acid, sandy soils having low capacity to retain P.

P BTCs corresponding to successive displacement of 10 pore volume pulses of P solutions and 40 pore volumes of P-free water through a 90-cm Spodosol profile with A, E, and Bh horizons were simulated. Concentrations of 10, 100, and 1000 g m -3 P in the influent pulses gave P loadings of 350, 3500 and 35 000 kg h-1, respectively, for the soil. Phosphorus sorption was assumed to occur as a combination of 2-site (kinetic and instantaneous) system in the Bh horizon and 1-site kinetic in the A and E horizons. The greatest sorption-driven retardation of P breakthrough in effluent from the soil profile occurred for the lowest P loading. Increasing the loading tended to decrease the retardation and increase P mobility.

Desorption of soil P initially occurred rapidly and then decreased very slowly. These simulated P transport results indicate that P leaching from Spodosols containing residual P from dairy waste could occur very slowly even under conditions where sorption/desorption was assumed to be reversible.

The presence of residual P in the soil also inhibited sorption and thus enhanced the mobility of P subsequently applied as influent to the soil surface. These simulated results imply that P accumulation during long-term use of Spodosols for dairies could enhance the mobility of P reaching the soil surface as animal waste. The enhancement of P mobility by residual soil P is obviously attributable to the finite capacity of the soil profile, and specifically to that of the spodic horizon, to sorb P from percolating soil solution.

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Phosphorus transport in Spodosols impacted by dairy waste