The development of alum rates to enhance the remediation of phosphorus in fluvial systems following manure spills

Abstract

Following the remediation of animal manure spills that reach surface waters, contaminated streambed sediments are often left in place and become a source for internal phosphorus (P) loading within the stream in subsequent flow. The objective of this study was to develop treatment rates and combinations of alum and CaCO₃ to mitigate P from contaminated sediments of different particle size distributions following a manure spill. Sediment specific alum and CaCO₃ treatment rates were developed based upon the resultant alum treatment ranges established for each sediment type. Clay loam sediments required 54% more alum to mitigate P desorption relative to sediments that contain at least 60% sand. Amending sediments with the highest rates of alum/alum + CaCO₃, resulted in a 98–100% reduction in P desorption and a similar water column pH for all sediments types. Observations from this study demonstrated the effectiveness of alum/alum + CaCO₃ to increase P retention in sediments following a manure spill.

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Environmental impact

After a manure spill has reached surface waters, the most common method of remediation is to (i) contain and isolate the contaminated area using earthen or temporary dams (ii) de-water the contained area using pumping equipment, and (iii) redistribute the recovered waste into an alternative storage system or to land apply the waste in compliance with state regulations. Recent studies have examined the fate of phosphorus during a manure spill and the efficacy of the current manure spill clean-up method to remediate fluvial sediments following a spill that reached surface waters. The studies found that the current remediation of manure spills that reach surface water focuses primary on the contaminated water column, while nutrient rich sediments remain in place post treatment.

Introduction

In the U.S within the last 20 years, the number of livestock produced annually has increased significantly.¹Additionally, larger livestock herd sizes are housed on fewer individual farms and greater volumes of manure are generated and managed daily. For these reasons, Iowa, the leading swine producing state in the U.S., experienced 304 manure spills within a 10 year period (1992–2002), where 48% of those spills were caused by manure storage related issues.² Moreover, the frequency and magnitude of manure spills due to storage system failure has become an environmental issue in rural communities where CAFOs are located.

After a manure spill has occurred, the most common method of remediation is to: (i) contain and isolate the contaminated area using earthen or temporary dams; (ii) de-water the contained area using pumping equipment and; (iii) redistribute the recovered waste into an alternative storage system or to land apply the waste in compliance with state regulations.^3–5 Recent studies have examined the fate of P during a manure spill and the efficacy of the current manure spill clean-up method to remediate fluvial sediments following a spill that reached surface waters.^6–10 Findings from these studies suggested that benthic sediments are capable of storing nutrients during the manure spill and releasing nutrients to the water column at concentrations that exceed the EPA nutrient criteria for the ecoregion for weeks and months after the spill has occurred.^6,7 Studies simulating a 23 h swine manure spill and the current manure spill remediation method using fluvarium techniques demonstrated that remediated sediments were capable of releasing P to the water column at equilibrium concentrations that were up to 5 times greater that the pre-spill soluble P equilibrium concentration.⁹ In a similar study, it was determined that the first two centimetres of sediment acted as a temporary storage zone for labile P during a 23 h manure spill simulation.¹⁰ These observations demonstrated that the current method of handling a manure spill that reaches surface waters lack effectiveness in remediating the entire fluvial system (water column and sediment), therefore alternative remediation methods are needed to advance manure spill clean-up in surface water.

The use of alum (Al₂(SO₄)₃·14H₂0) has been shown to decrease the P solubility in poultry litter, swine manure, and dairy effluent.^11–14 Moreover, studies have also demonstrated that alum can enhance sediment P retention and sorption properties which could decrease the potential for internal loading and transport of P downstream. Sediments that were P enriched due to continued exposure to wastewater treatment plant effluent were treated with alum and CaCO_3.¹⁵ Results from that study indicated that the sediment treatment significantly increased the sediment P buffering capacity (the ability for sediments to adsorb P), significantly decreased the equilibrium P concentration (EPC₀) (the P concentration where net absorption and desorption of P is zero) and sediment labile P concentration.¹⁵ A similar trend was observed after amending sediments from tile fed agricultural drainage ditches, where the labile P concentration was decreased by 48–89% and the EPC₀ was significantly reduced from 0.051 to 0.004 mg P L⁻¹.¹⁶

Although the effectiveness of alum to increase the P sorption capacity of fluvial sediments has been established, data relating alum rates to differences in sediment physiochemical properties is lacking, and data supporting alum as a manure spill remediation technology is largely nonexistent. Considering the lack of effectiveness of current manure spill clean-up protocols to remediate P enriched fluvial sediments and the efficacy of alum to increase sediment P retention, there is a need to explore the benefits of incorporating alum technology to remediate internal P loading from sediments following a manure spill. Therefore, the objectives of this study are to: (1) determine the efficacy of alum to treat fluvial sediments following a manure spill; (2) determine adequate application rates of alum and CaCO₃ needed to remediate fluvial sediments following a manure spill and;(3) determine the influence of sediment physiochemical properties on alum and CaCO₃ treatment rates.

Experimental section

Sediment collection and analysis

Contaminated sediments for this study were collected from a prior study Armstrong et al., (2011)¹⁰ that determined the mixing depth of P in sediments following a manure spill. The mixing depth study simulated manure spills that occurred under the worst case scenario (base flow conditions that would only slightly dilute swine manure in the event of a manure spill), where the resultant mean initial P concentration of the water column was 41.2 mg P L⁻¹. The spill simulations were conducted by flowing contaminated water (0.065 L sec⁻¹) that consisted of a 12 : 1 ratio of swine manure to simulated ditch water (Distilled water at a CaCl₂ concentration of 2.5 mM) over ditch sediments for 23 h. Contaminated sediments for the current study were collected from the 0–2 cm depth of sediments following the mixing depth study manure spill simulation.

Ditch sediments (clay loam, loamy sand, and sand) were collected in the Cedar Creek Watershed of Northeast Indiana from drainage ditches in three sub-watersheds (Small, Medium, and Large). The Small, Medium, and Large sub-watershed and sampling locations represent drainage areas of approximately 311, 1410, and 4300 ha, respectively. Major soils surrounding the sampling locations were Blount (fine, illitic, mesic Aeric Epiaqualfs); Pewamo (fine, mixed, active, mesic Typic Argiaquolls), and Glynwood (fine, mesic Aquic Hapludalfs).¹⁷ The Blount soil CEC ranged from 9–21 meq100g⁻¹ and the pH ranged from 5.6–7.3, within the first 23 cm of the soil profile. The CEC of the Pewamo and Glynwood soils ranged from 18–39 and 10–21 meq 100g⁻¹ of soil, respectively and both soils pH ranged from 5.6–7.3.¹⁷ The most common slopes for the Blount, Pewamo and Glynwood soils were 1–4%, 0–1%, and 3–6% respectively (USDA-NRCS, 2002). Within the Small watershed, the Blount (72%) and Pewamo (22%) accounted for 94% of the drainage area and in the Medium watershed both soils made up 72% of the drainage area (Blount = 52% and Pewamo = 20%). Within the Large watershed the Blount (27%), Pewamo (21%), and Glynwood (14%) soils accounted for 62% of the drainage area. Land use surrounding each ditch was similar: agricultural row crops ranging from 83–85%, grass land and pasture 8–13% and 3–6% forest.¹⁶

To determine the particle size distribution of the sediments the hydrometer method was employed and the loss on ignition procedure was used to determine sediment organic carbon content.^18,19 The pH of the sediment samples was determined using a 1 : 1 ratio of sediment to simulated ditch water, a modification of the method mentioned in Thomas and Hargrove (1984).²⁰ Extractable P of all sediments were analyzed using the Mehlich 3 extraction procedure that contains 0.2 MCH₃COOH, 0.25 MNH₄NO₃, 0.015 MNH₄F, 0.013 MHNO₃, and 0.001 M EDTA with an extraction ratio of 1 : 10.²¹To determine the percent Al₂O₃ and Fe₂O₃ of the sediments the citrate-bicarbonate-dithionite method was employed, where five grams of oven dried sample (105 °C) were digested with 40 ml of 0.3 M sodium citrate and 5 ml of 1 M sodium bicarbonate at 80 °C with two additions of (1 g) sodium dithionite.²² The cation exchange capacity (CEC) was determined using the unbuffered salt method, where 2g (105 °C oven dry) sediment were shaken for 2 h with 1 MNH₄Cl, centrifuged at 10,000 × g for 5 min., and the decant was analyzed for Al, K, Na, Mg, and Ca. The anion exchange capacity (AEC) was determined by replacing Cl⁻ on the anion exchange sites of the sediments with NO₃⁻ by shaking 2g of sediment with 1 M KNO₃ for 2 h, centrifuging, and analyzing the decant for Cl⁻ colorimetrically using the Lachat Flow Injection Analyzer. A P isotherm experiment was used to estimate the P buffering capacity of the ditch sediments. Twenty-five grams of wet sediment (10 g dry) were shaken in a 250 mL centrifuge tube for 1 h with a 2.5 mMCaCl₂ solution at P concentrations of 0, 20, 50 and 100 mg L⁻¹. After centrifuging for 5 min at 9,820 × g, the samples were filtered using a 0.45 μm filter membrane, acidified with HCl and analyzed for dissolved P. The slope of the regression line, as a result of regressing adsorbed P against the initial P concentration (mg L⁻¹), was considered to be the P buffering capacity of the sediments.¹⁵

Determination of alum treatment range

To determine a range of alum rates required to mitigate the desorption of P from manure exposed sediments, an isotherm experiment was conducted using alum under oxic conditions for each sediment type.^23,24 This experiment involved treating 25 g of contaminated ditch sediment via the application of 5 alum rates (0, 1.6, 3.6, 5.6, and 7.6 mg g⁻¹) dissolved in 10 mL of simulated ditch water. Immediately following, the treated sediments were shaken with 90 mL of simulated ditch water for 1 h, and the samples were analyzed for pH, centrifuged for 5 min at 9,820 × g, and filtered using a 0.45μm membrane filter. Dissolved P of the treated samples was analyzed using inductive coupled plasma-optical emission spectrometry (ICP-OES). The alum rate required to totally mitigate P desorption from the sediments was considered to be the Alum_max (the maximum rate of alum needed to reduce the labile P fraction of sediments following the manure spill simulation under worst case scenario). Additionally, the determined Alum_max values were statistically correlated with the sediment chemical and physical properties using the Pearson correlation analysis and the sediment properties were used to develop models to predict optimum sediment alum amendment rates for sediments using the Backwards Elimination analysis.

The evaluation of sediment specific alum application rates

To evaluate sediment specific alum application rates and the impact of alum application treatment combinations on the pH of the water column a second test tube isotherm experiment was conducted. Twenty five grams of wet contaminated sediment were weighed into 200 ml centrifuge tubes and were treated with three levels of alum, CaCO₃, and alum + CaCO₃ (clay loam rates = 0, 2, 4, 6 mg g⁻¹ and loamy sand and sand sediment rates = 0, 1, 2, 3 mg g⁻¹). Alum application rates were determined based upon the Alum_max of each sediment type. After applying the sediment treatments, the samples were shaken with 100 ml of simulated ditch water for 1 h, centrifuged, analyzed for pH, filtered using 0.45 μm membranes, and analyzed for dissolved P using the procedures mentioned in the previous section. This experiment was performed for each sediment type (clay loam, loamy sand, and sand) and was replicated three times. The data from the alum rate and treatment combination evaluation experiment were analyzed for each sediment type separately, using a complete randomized factorial design with 3 replicates analysis of variance, and the least significant difference test for mean separation.²⁵

Results and discussion

The sediment baseline analysis (Table 1) confirmed differences in the physical and chemical properties of sediments collected from the Small, Medium, and Large drainage areas. The textural class of the Small drainage area sediment was clay loam, loamy sand for the medium drainage area, and sand for the largest drainage area. The sand content of the sediments increased as the drainage areas of the sampling location increased and the sediment clay content decreased as drainage area decreased. This observation is attributed to the increase in stream flow as drainage area increases; therefore, a greater percentage of coarser particles dominated the streambed of the Large drainage area.²⁶ Considering that the chemical properties of sediments are highly associated with the clay content of the sediments, a decrease in Mechlich 3 extractable P, Fe and Al oxides, and cation exchange capacity was observed as the drainage area of the sampling location increased.

Table 1 Baseline chemical and physical analysis of the fluvial sediments

	Ditch Sediments
	clay loam	loamy sand	Sand
a Extractible phosphorus. b Cation exchange capacity. c Anion exchange capacity.
Clay (%)	33.80	6.10	1.30
Silt (%)	48.80	8.80	6.30
Sand (%)	17.40	85.10	92.40
Organic Carbon (%)	8.02	1.72	1.12
Sediment pH	6.93	7.01	7.51
P (mg kg^−1)^a	144.00	56.60	9.26
Fe2O3 (%)	0.16	0.09	0.03
Al2O3 (%)	0.06	0.03	0.01
Buffering Capacity (L kg^−1)	10.35	4.95	3.24
CEC (cmol kg^−1)^b	20.80	13.70	10.70
AEC (cmol kg^−1)^c	2.74	1.53	1.87

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Results from the linear P isotherm experiment demonstrated that the buffering capacity (the ability of the sediments to sorb P) among sediments was significantly different and was also inversely related to drainage area. Sediment collected from the largest drainage area contained the lowest Fe and Al oxide, organic C content, and P buffering capacity (3.24 L kg⁻¹). This indicated that the Large sub-watershed sediment possessed the lowest potential to sorb P from the water column. However, sediment collected from the Small drainage area contained the largest percentage of Fe and Al oxides, organic C, and resulted in the greatest buffering capacity (10.3 L kg⁻¹).

Results from the Alum_max experiment determined that sediment physiochemical properties influenced the rate of alum needed to mitigate P desorption from manure exposed sediments following a manure spill. Sediments that were not amended with alum resulted in the greatest labile sediment P concentrations that ranged from 2.4 to 12.3 mg P kg⁻¹ (Fig. 1). The sandy sediment labile P concentration was significantly greater (12.3 mg kg⁻¹), relative to the clay loam (6.0 mg kg⁻¹) and loamy sand (2.4 mg kg⁻¹) sediments and the clay loam sediment labile P was significantly greater compared to the loamy sand sediment. A significantly greater labile P for the sandy sediments could be explained by lower Fe and Al oxide concentrations of the sand sediments relative to the clay loam and sand sediments, making the sandy sediments more prone to P release during the extraction procedure and a smaller P buffering capacity (Table 1). Amending the contaminated sediments at a rate of 1.6 mg g⁻¹ of alum mitigated the labile P of the sandy sediments and reduced the labile P of the clay loam and loamy sand sediments by 72 and 92%, respectively. Treating the sediments at rates of 3.6, 5.6, and 7.6 mg g⁻¹ of alum reduced labile P concentrations of the sandy and loamy sand sediments by 100%, relative to sediments that received no treatment. However, amending the clay loam sediments at the same rates of alum resulted in a 92, 98, and 100% reduction in labile P relative to the untreated clay loam sediments (Fig. 1). These observations suggested that different alum_max rates (the maximum rate of alum needed to mitigate P desorption) were required to mitigate labile P in manure exposed fluvial sediments of different particle size distributions and chemical properties (Table 1). A greater rate of alum was needed for the clay loam sediments (7.6 mg g⁻¹), relative to the loamy sand (3.6 mg g⁻¹) and sand sediments (1.6 mg g⁻¹). Correlations of these alum_max rates with particle size distribution components suggested that the alum_max rates for manure exposed sediments increased as the percentage clay + silt increased (Table 2). Alum_max rates were also significantly correlated with chemical properties that influence the affinity of the sediments to react with soluble P, such as Fe₂O₃ and Al₂O₃ percentages, P buffering capacity (the ability of the sediment to adsorb P from the water column), organic C, clay and silt, and CEC and AEC (Table 2). Properties such as organic carbon and Fe and Al oxides, individually and collectively increase the sorption of P via pH dependent anion exchange sites and organic acid complexes with Al and Fe that are formed on the sediment surface.²⁷ Thus, Smith et al. (2005) found a linear relationship between the sediment organic carbon and P buffering capacity.

Fig. 1 The impact of increasing alum rates on P desorption from manure contaminated sediments of different particle size distributions. Capital letters indicate significant difference (α = 0.05) between sediment types within each alum rate.

Table 2 Correlation R² values for the correlation analysis of sediment chemical and physical properties and alum_max rates

	Alummax	Clay	Silt	Buff. Cap.	Org. C	Fe2O3	Al2O3	CEC	AEC
a Organic carbon. b Cation exchange capacity. c Anion exchange capacity, Numbers that are italicized are significant at an alpha level = 0.05.
Alummax	1
Clay (%)	0.98	1
Silt (%)	0.96	0.99	1
Buff. Cap.	0.99	0.99	0.98	1
Org. C^a	0.96	0.99	0.99	0.99	1
Fe2O3	0.94	0.89	0.85	0.91	0.87	1
Al2O3	0.97	0.93	0.90	0.95	0.92	0.99	1
CEC ^b	0.98	0.97	0.95	0.98	0.97	0.94	0.97	1
AEC ^c	0.75	0.84	0.87	0.81	0.87	0.6	0.69	0.8	1

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The clay loam sediment that required the greatest rate of alum to reduce labile P desorption, contained the greatest Fe and Al content, P buffering capacity, % organic C, CEC, and AEC relative to the loamy sand and sand sediments (Table 1). Results from other studies that utilized alum to increase P retention in ditch and lake bed sediment also confirmed the influence of physiochemical sediment properties on the effectiveness of alum. Rydin and Welch (1998)²⁸ examined the influence of P fractions within lake bed sediments on the rate of alum required to adequately reduce the mobility of P. In that study two different lakes sediments were examined, both containing a similar labile P concentration and significantly different fractions of Fe and Al bound P. It was concluded that the sediment with the greater Fe and Al bound P fraction required significantly more alum to exhaust mobile inorganic P release under oxic and anoxic conditions. Smith et al. (2005)¹⁶ applied alum at an identical rate to P enriched ditch sediments and observed that sediments containing the greatest concentration of initial extractable P, percentage of clay + silt, and percentage of organic carbon resulted in significantly greater labile P after alum was amended. A similar trend was observed in the current study, both the loamy sand and sand sediments contained considerably less initial organic carbon and Mehlich 3 P than the clay loam sediment and required less alum to mitigate P desorption.

Data from this portion of the study were also used to develop models to predict the rate of alum required based upon sediment properties using the backward elimination analysis. The analysis determined that models involving the buffering capacity, % organic C, and % Al₂O₃ were the most unbiased and predictive in estimating alum rates (Table 3). As a result of these observations demonstrating the influence of sediment physical and chemical properties on the alum requirements to mitigate labile P, alum rates for each sediment type were developed and tested. Rates for the clay loam sediments ranged from 0–6 mg alum g⁻¹ and the loamy sand/sand sediment alum treatments ranged from 0–3 mg alum g⁻¹.

Table 3 Predictive Models developed to estimate sediment alum rates. (n = 18)

Variables	Regression Coef.	Models	P	R^2
Intercept (a)	−2.07	Alum rate = a + b1X1 + b2X2	0.0002	0.99
Buffering Cap. (b1)	1.16	Alum rate = a + b1X1 + b2X2 + b3X3	0.0006	0.99
Organic Carbon (b2)	−0.47
Al2O3 (b3)	21.65

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The evaluation of sediment specific treatment combinations

Alum treated fluvial sediments resulted in less desorbed P compared to untreated sediments following the manure spill simulation. Untreated contaminated sediments desorbed P at a significantly greater rate relative to sediments treated with alum or alum + CaCO₃ at all application rates (Fig. 2–4). The average desorbed P concentration for each sediment type (clay loam = 4.1 mg kg⁻¹; loamy sand = 2.8 mg kg⁻¹; sand = 17.3 mg kg⁻¹) was similar to the sediment P concentrations observed in the mixing depth study, where the average P concentration for the clay loam, loamy sand, and sand sediments were 8.4, 3.0, and 12.4 mg kg⁻¹, respectively.¹⁰

Fig. 2 Relationship of sediment desorbed P and added alum, CaCO₃, and alum + CaCO₃ in the clay loam sediment. The bars on each point represent the standard error.

Fig. 3 Relationship of sediment desorbed P and added alum, CaCO₃, and alum + CaCO₃ in the loamy sand sediment. The bars on each point represent the standard error.

Fig. 4 Relationship of sediment desorbed P and added alum, CaCO₃, and alum + CaCO₃ in the sand sediment. The bars on each point represent the standard error.

Sediments treated with CaCO₃ alone slightly increased the P desorbed from all contaminated sediments compared to the untreated sediments. This observation confirmed that alum is the effective compound that precipitates P in solution when both alum/CaCO₃ are applied together. An explanation for the increase in desorbed P after the CaCO₃ amendment could be the increase in pH that ranged from 8.03–8.21 (Table 4), compared to the pH range (6.9–7.8) of the alum and alum + CaCO₃ treatment combinations. Phosphate sorption studies have demonstrated that in oxic conditions iron bound P at high pH is released to solution, due the competitive sorption of OH⁻ ions and the solubility of iron adsorbing compounds at pH ranges of 7 to 8.²⁹ Treating the three manure exposed sediments with specific rates of alum alone and alum + CaCO₃ based upon sediment physical and chemical properties, resulted in a significant decrease in desorbed P as the rate of treatment increased (Fig. 2–4). More specifically, the clay loam sediments required greater rates of alum and alum + CaCO₃ (0, 2, 4, and 6 mg g⁻¹), which were equal in their effectiveness to decrease P desorption from the P enriched sediments at each treatment rate (Fig. 4). Amending the clay loam sediments with 2, 4, and 6 mg g⁻¹ of alum alone resulted in P desorption reductions of 77, 98, and 100% and for alum + CaCO₃ the P reductions were 81, 95, and 99%, respectively. Significant reductions in P desorption within both amendment combinations were only observed when comparing rates of 2 and 4 mg g⁻¹ and 2 and 6 mg g⁻¹. Increasing the amendment application rate from 4 to 6 mg g⁻¹ for alum alone and alum + CaCO₃ resulted in a total mitigation of P desorption and 99% reduction for alum + CaCO₃, there was no significant difference between the two application rates.

Table 4 Water column pH following sediment amendment of alum, CaCO₃, and alum + CaCO₃. Capital letters represent differences within column and lowercase letters represent differences within rows

Ditch Sediment	Treatment levels (mg g^−1)	Treatment Source
		Alum	CaCO3	Alum + CaCO3
Clay loam	0	8.0Ax	8.0Ax	8.0Ax
	2	7.4By	8.1Ax	7.4By
	4	7.0Cz	8.1Ax	7.2Cy
	6	6.8Dz	8.0Ax	6.9Dy
Loamy sand	0	8.2Ax	8.1Ax	8.1Ax
	1	7.8By	8.2Ax	7.7By
	2	7.4Cy	8.1Ax	7.4Cy
	3	7.3Dy	8.2Ax	7.2Dy
Sand	0	8.3Ax	8.0Ax	8.3Ax
	1	7.6Bx	8.0Ax	7.6Bx
	2	7.2By	8.0Ax	7.4By
	3	7.1By	8.0Ax	7.3By

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Although the loamy sand and sand sediments required 50% lower application rates of alum/alum + CaCO₃, relative to the clay loam sediments, to decrease P desorption following a manure spill, the efficacy of the treatments were similar. Treating the loamy sand sediment with three increasing levels (1, 2, 3 mg g⁻¹) of alum/alum + CaCO₃, resulted in P desorption reductions of 62, 88, and 90% (Fig. 3). The comparison of the amendment rates suggested that after the second treatment rate, there was no significant difference in treatment effectiveness within both amendment combinations. Furthermore, when comparing the amendment combinations, there was no significant difference in treatment effectiveness within each rate level. The same trends were also observed for the sand sediment, at all treatment combinations there was no significant difference in P reduction after the second level increase in treatment rate (Fig. 4). Treating the sand sediment with three rate levels decreased P desorption by an average of 61, 94, and 98%, relative to the untreated sediments and there was no difference between alum and alum + CaCO₃ at any rate level.

The effect of sediment treatment on water column pH

Treating the contaminated sediment following the manure spill with combinations of alum and CaCO₃ affected the pH of the water column. The pH of untreated contaminated sediments was greater relative to treated sediment and ranged from 8.0 to 8.3. A greater pH in the water column of untreated contaminated sediments could be a reflection of the release of basic cations (Ca²⁺, Mg²⁺, and Na⁺) that were adsorbed during the manure spill simulation. Despite the sediment type, amending sediments with the three rates of CaCO₃ alone resulted in an equal or slightly greater pH relative to the untreated contaminated sediments. However, amending each sediment type with alum/alum + CaCO₃ resulted in a significant decrease in water column pH, as the application rates increased. Within the clay loam sediments, the comparison of the amendment combinations impact on water column pH at different rates indicated that adding alum without CaCO₃ at rates greater than 2 mg g⁻¹ resulted in a significantly lower water column pH, relative to alum + CaCO₃. The same trend was observed in the water column pH of the loamy sand and sand sediments, the pH significantly decreased as the application rate of alum alone increased.

Conclusion

In light of evidence that fluvial sediments are capable of acting as a significant sink for P during a manure spill and a source of P to the water column in subsequent flow, novel in-stream treatment methods are needed to reduce the solubility of P in fluvial sediments following the current manure spill remediation protocol. We determined that amending sediments with the highest rates of alum/alum + CaCO₃, resulted in a 98–100% reduction P desorption and a similar resultant water column pH for all sediments types. Furthermore, clay loam sediments collected from the small drainage area required 50% more alum to mitigate P desorption relative to sediments that contain at least 60% sand that were collected from a larger drainage area. Also we demonstrated that clay loam sediments contained an adequate base cation reserved to buffer the expected decrease in pH, due to the production of H⁺via the hydrolysis of alum in solution. To further develop adequate rates of alum to increase P retention of contaminated sediments following a manure spill, the efficacy of the alum rates established in this study should be tested under different flow rates. Future studies should also evaluate the effect of sediment base saturation percentage on the rate of alum applied, to determine the threshold of applying alum alone before needing to amend with CaCO₃ to buffer pH change. Observations from this study demonstrated the effectiveness of alum to increase P retention in sediments following a manure spill and its potential to enhance the current manure spill remediation method.

Acknowledgements

The authors would like to express gratitude to the Indiana Sea Grant, and the Alliance for Graduate Education and Professoriate for funding the research. Also, the authors greatly appreciate Taneisha Springfield Jones and Jermy Shulman for their assistance during the execution of the research.

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