POLYACRYLAMIDE AND GYPSIFEROUS MATERIAL EFFECTS

Transcription

1 POLYACRYLAMIDE AND GYPSIFEROUS MATERIAL EFFECTS ON RUNOFF AND EROSION UNDER SIMULATED RAINFALL J. R. Peterson, D. C. Flanagan, J. K. Tishmack ABSTRACT. An indoor laboratory study was conducted to compare the dry versus sprayed application of polyacrylamide (PAM) and the use of different gypsiferous materials (gypsum and a class C ponded fly ash) on runoff and sediment yield. The six treatments incorporated in this study were: control, Nutra Ash (NA), gypsum (G), sprayed PAM plus NA (PAMW+NA), sprayed PAM plus G (PAMW+G), and granular PAM plus G (PAMD+G). Simulated rainfall at an intensity of 70 mm hr 1 was applied for 2 hours to a silty clay loam soil packed into erosion pans. Only one of the two liquid PAM treatments (PAMW+G) significantly reduced runoff (35%), while both liquid PAM treatments (PAMW+G and PAMW+NA) significantly reduced sediment yield (74% and 77%) compared to the control. Sprayed PAM was more effective than granular application in terms of total runoff, but there was no statistical difference with regard to total sediment yield. Differences between the effects of sprayed and granular PAM are explained by the mechanisms by which they reduce erosion. Sprayed PAM, in combination with gypsum, increases infiltration during the first part of a rainfall event until sufficient rainfall has occurred to break down the PAM treated aggregates, at which time runoff rate and sediment yield rate approach those of the control. Runoff and sediment yield rates from the granular PAM application were initially similar to those from the control. However, as time increased, sediment yield reached a maximum and then decreased without a corresponding decrease in runoff. This likely occurred because the PAM particles became activated during the rainfall and acted as a mortar to stabilize the soil matrix. Gypsum was a better source of electrolyte than a class C ponded fly ash, commercially known as Nutra Ash (NA), likely due to its greater solubility. Addition of PAM decreased soil erodibility and may be a viable erosion control practice for soils susceptible to flow detachment. Choice of application method should be based on the expected amount and severity of precipitation before vegetation establishment. These results indicate that sprayed PAM, in combination with gypsum or Nutra Ash, provides immediate erosion control, but its effectiveness decreases over time, as indicated by steadily increasing sediment yield rate. Dry PAM application was not as effective in the beginning of the experiment, but after sufficient rainfall it became activated and sediment yield continuously decreased over the remainder of the experiment. Keywords. Polyacrylamide, PAM, Soil amendments, Gypsum, Soil erosion, Runoff. Agriculture is a leading source of pollution to streams, rivers, and lakes in the United States. According to the US EPA (1996), agricultural pollution affects 25% of streams and 19% of lakes and contributes to 70% of all identified water quality problems in streams and 49% in lakes. Sediments are the most common pollutant affecting rivers and streams and contribute up to 51% of water quality problems in rivers and Article was submitted for review in November 2001; approved for publication by the Soil & Water Division of ASAE in April Use of trade names does not imply an endorsement by Purdue University or the USDA ARS. The authors are Joel R. Peterson, ASAE Student Member, Graduate Research Assistant, Department of Agricultural and Biological Engineering, Purdue University, and National Soil Erosion Research Laboratory, West Lafayette, Indiana; Dennis C. Flanagan, ASAE Member Engineer, Assistant Professor, Department of Agricultural and Biological Engineering, Purdue University, and National Soil Erosion Research Laboratory, West Lafayette, Indiana; and Jody K. Tishmack, Ash Management Coordinator/Continuing Lecturer, Department of Agricultural and Biological Engineering, Purdue University, West Lafayette, Indiana. Corresponding author: Joel R. Peterson, National Soil Erosion Research Laboratory, Purdue University, 1196 Soil Building, West Lafayette, IN ; phone: ; fax: ; e mail: petejoel@ecn.purdue.edu. streams and 25% in lakes (US EPA, 1996). The problem of erosion is not only an agricultural one. Erosion rates at construction sites can far exceed those of agricultural land, approaching 163 Mg ha 1 yr 1, which is about 18 times greater than maximum tolerable rates from agricultural lands (Daniel et al., 1979). Soil erosion on upland areas is caused by the impact of raindrops, which leads to interill erosion, which in turn breaks down soil structure and leads to detachment by concentrated water flows (rill erosion). Infiltration can be hampered by sealing of the soil surface, which can result in reduced infiltration rates and increased runoff and erosion. Surface sealing may also interfere with seedling geration (Shainberg and Levy, 1994). Seal formations are caused by two primary mechanisms: (1) physical disintegration of soil aggregates by raindrop impact, and (2) physicochemical dispersion and migration of clay particles in the 0.1 to 0.5 mm depth, where they can clog pores (Levy et al., 1992; Shainberg and Levy, 1994). Seal formations can be prevented by introducing mulch or some other cover to protect the soil surface from raindrop impact, introducing electrolytes at the soil surface to reduce chemical dispersion of clay particles, and/or stabilization of aggregates at the soil surface by polymeric soil conditioners (Stern et al., 1991). Transactions of the ASAE Vol. 45(4): American Society of Agricultural Engineers ISSN

2 One such soil conditioner is polyacrylamide (PAM). Polyacrylamide is a water soluble, synthetic organic polymer high in molecular weight that primarily interacts with the clay fraction of soils (Seybold, 1994) and has been proven to be superior to other polymers in erosion control applications (Shainberg and Levy, 1994). Polyacrylamide stabilizes the soil through two major mechanisms: (1) adsorption of the polymer by clay causes a physicochemical charge at the clay surface, reducing repulsive forces among clay particles, and (2) the polymer acts as a bridge between soil particles in an aggregate by binding bridged particles together (Ben Hur, 1994). Polyacrylamide can be synthesized in cationic, nonionic, or anionic forms, although anionic PAM has been found to be the most effective for erosion control (Shainberg and Levy, 1994). Shainberg et al. (1990), Levy et al. (1992), and Smith et al. (1990) reported that the benefits of PAM were enhanced by the introduction of an electrolyte source that helps to create a cation bridge for the polymer to adsorb to the soil. Typically, the electrolytes are introduced by application of a gypsiferous material. Several types of amendments have been used to enhance the electrolyte concentration of the soil solution. These include phosphogypsum (PG), gypsum, and different types of ash from coal fired power plants. Phosphogypsum is a byproduct of the phosphate fertilizer industry (Shainberg et al., 1990). Fluidized bed combustion bottom ash (FBCBA) and flue gas desulfurization (FGD) are byproducts from coal fired power plants. Norton et al. (1993) studied the effectiveness of two types of FGD material, an FBCBA material, and PG on infiltration and erosion in a laboratory setting. The effectiveness of the different materials was related to particle size and solubility (ability to release electrolytes). They concluded that gypsiferous materials with greater solubilities were more effective, and that the coarser the material, the less soluble it was due to surface area constraints. The effect of PAM on interill and rill runoff and soil loss was exaed by Flanagan et al. (1997a, 1997b). Treatments included a liquid PAM solution applied at 20 kg ha 1 and then exposed to both deionized and tap water rainfall, a control under both deionized and tap water rainfall, and an FBCBA surface amendment under deionized rainfall. They found that interill soil loss was not significantly different between treatments. Final infiltration rates for the two PAM surface treatments were not significantly different from other treatments in the rill plots, but the authors noted that the PAM treatments did appear to reduce aggregate breakdown and enhance infiltration rates. The FBCBA significantly increased infiltration on the interill plots compared to the tap water or deionized water treatments. The FBCBA also significantly reduced sediment concentrations and sediment discharge rates on the rill subplots on initially dry soil. For the rill subplots, the 20 kg ha 1 PAM treatments reduced detachment compared to the control. Flanagan et al. (1997b) recommended using FBCBA in conjunction with PAM. Mitchell et al. (1996) reported that measured runoff from PAM treated plots was not significantly different from that from the control plots using PAM application rates of 1.1 (10 Mg mol 1 MW) and 17.6 kg ha 1 (0.25 Mg mol 1 MW). The ineffectiveness of PAM was attributed to low application rates for both PAMs, but particularly for the second PAM because of its low MW. Mitchell et al. (1996) did not apply a gypsiferous material, which most likely lessened the effectiveness of the PAM in their study. Because of the high viscosity of PAM in solution, adequate dilution with water to spray the PAM solution may require excessive amounts of water, causing runoff during application or requiring several applications. Mitchell et al. (1996) suggested that dry application of PAM should be studied. Chaudhari and Flanagan (1998) conducted a natural rainfall experiment on a steep slope (34% to 37%) to evaluate the effects of PAM on runoff, erosion, and seedling emergence. Treatments were a control, PAM sprayed at 80 kg ha 1, and PAM plus gypsum applied at 80 kg ha 1 and 5 Mg ha 1, respectively. Both the PAM and PAM plus gypsum treatments significantly reduced runoff and sediment yield over the control. Stern et al. (1991) evaluated the effectiveness of PG (5 Mg ha 1 ), mulch, and two rates of PAM (5 kg ha 1 and 20 kg ha 1 ) on reducing runoff and soil loss. Measured runoff from the mulch and PAM treatments were significantly less than that from the control and PG treatments. The cost of PAM application can be small in comparison to traditional erosion control measures such as mulch. Chaudhari and Flanagan (1998) reported the cost of traditional mulch, adhering to Indiana Department of Transportation guidelines, was $1900 ha 1 compared to $160 ha 1 for PAM. Benefits of gypsiferous material and PAM on runoff and erosion have been well documented. However, the method of PAM application (dry vs. sprayed) and the interaction of PAM with different gypsiferous materials have received little attention. Our hypotheses are that sprayed PAM significantly reduces total runoff and sediment yield compared to dry PAM application, and that total runoff and sediment yield from treatments of different gypsiferous material are not statistically different. The objectives of this study were to compare the dry versus sprayed liquid application of PAM and the use of different gypsiferous materials on runoff and sediment yield. MATERIALS The six treatments incorporated in this study were: control, Nutra Ash (NA), gypsum (G), sprayed PAM plus NA (PAMW+NA), sprayed PAM plus G (PAMW+G), and granular PAM plus G (PAMD+G). Application rates of each material are presented in table 1. The application rate of gypsum was chosen based on research conducted by Chaudhari and Flanagan (1998). The application rate of Nutra Ash (NA) was designed to provide the same amount of calcium as gypsum. The soil used was a silty clay loam (20% sand, 42% silt, 38% clay, 3.0% OM, 390 ppm Mg, 4600 ppm Ca, CEC 26.6 meq 100 g 1 ) taken from the floodplain of the Wabash River by Purdue University facilities, which uses the soil for various landscaping projects around campus. This soil was used because of its relatively high clay content, which was expected to show differences in runoff and erosion between PAM treated and untreated soil and because the soil was locally available in quantity. Estimated saturated hydraulic conductivity of the soil is 0.29 cm hr 1 based on soil texture (Saxton et al., 1986). The PAM used was anionic Percol 336, a commercially available material (Allied Colloids Inc., Suffolk, Va.), having 32% charge density and a high molecular weight, in comparison to the study conducted by Mitchell et al. (1996), 1012 TRANSACTIONS OF THE ASAE

3 Table 1. List of treatments and associated application rates. Application Rate (kg ha 1 ) Abbreviation Treatment Gypsum PAM Nutra Ash C Control G Gypsum 5000 PAMW+G PAM (wet) + gypsum PAMD+G PAM (dry) + gypsum NA Nutra Ash 8042 PAMW+NA PAM (wet) + Nutra Ash of 20 Mg mol 1. Nutra Ash is a reclaimed Class C fly ash marketed as a lig/fertilizer supplement. It was manufactured from ponded class C fly ash that has been ed, crushed, and sieved to produce various sized products. Commercial fertilizer grade gypsum was obtained from Shoals, Indiana. A fertilizer/lime analysis of each gypsiferous material is given in table 2. Erosion pans used in the study measured 32 cm wide Ü 45 cm long Ü 20 cm deep. Holes placed in the bottom of the box allowed for free drainage. A programmable rainfall simulator, described by Foster et al. (1982), was used in the experiments. Adjusting the frequency of nozzle oscillation with a programmable logic controller permits one to set the rainfall intensity. The simulator troughs used VeeJet spray nozzles (Spraying Systems Co., Wheaton, Ill.), which were operated at a noal water pressure of 41.4 kpa. The troughs were suspended from the ceiling at a height of approximately 2.5 m above the surface of the erosion pans. Kinetic energy from this simulated rainfall is about 75% that of natural rainfall in a 64 mm hr 1 event (Meyer and McCune, 1958). Deionized water, with an electrical conductivity of approximately 15 S cm 1, was used to approximate the electrolyte level of natural rainfall. METHODS Soil was gently crushed and sieved to pass through an 8 mm opening and then allowed to air dry. Sand was placed in the lower 5 cm of each erosion pan to promote percolation through the box. The remaining 15 cm of each box was packed with soil, in three 5 cm layers, to achieve a 1.35 g cm 3 bulk density. Soil amendments were then added according to the treatment specified (table 1). Polyacrylamide in solution was applied at a rate of 40 kg ha 1 using a common household plant sprayer at a concentration of 405 mg L 1. The same amount of water (1.4 L) was also Table 2. Fertilizer/lime analysis of the eral amendments used in the study. Nutra Ash Gypsum As Dry As Dry Parameter Received Basis Received Basis % calcium (Ca) % magnesium (mg) % CCE [a] Moisture (105 C) % passing U.S. #8 sieve % passing U.S. #20 sieve % passing U.S. #60 sieve Bulk density (g cm 3 ) [a] CCE = calcium carbonate equivalent. applied using the same sprayer to those treatments not requiring sprayed PAM to imize differences in treatments due to the effects of antecedent moisture content. The granular PAM (PAMD) was applied at a rate of 40 kg ha 1 to dry soil and then wetted with 1.4 L water. Erosion pans were allowed to air dry for 24 hours. Slope was set at 17%. Simulated rainfall was applied using deionized water at an intensity of 70 mm hr 1 for 2 hours after runoff initiation on any one of the boxes. Samples were collected every 5 utes after runoff initiation. Three or four erosion pans were used at a time with treatments assigned randomly. Treatments were replicated three times. Runoff rate was detered by measuring the mass of runoff collected in each 1 L bottle collected during 5 utes. Sediment concentration was detered gravimetrically. Percolate through the bottom of the erosion pans was not analyzed. Differences in total runoff, total sediment yield, and final runoff and sediment yield rates between treatments were identified using the least significant difference (LSD) method at a significance level of To assess the effectiveness of a treatment over time, the relative effectiveness, R(t), of a treatment at time t is defined as: where T i (t) C(t) ( t) C( t) R ( t) = Ti (1) ( Ti C) = sediment or runoff collected for treatment i during time increment = sediment or runoff collected for the control during time increment T i C = difference between total sediment or runoff collected. A sequential dilution dissolution analysis was performed on the gypsiferous soil amendment materials (G and NA) in order to better understand their effect on electrolyte levels over the course of the experiment. Laboratory analyses were conducted by A&L Laboratory, Inc. Fifteen grams of material was placed in a 500 ml plastic bottle. Then 300 ml of ultra pure water was added, and the bottle was mechanically shaken for 10 utes. At the end of 10 utes, an 11.0 cm Whatman No. 1 filter was placed in a Buchner funnel, under suction, and the contents of the sample bottle were filtered. All solids were recovered from the filter paper, including the filter itself, and then were placed back into the mixing bottle. Another 300 ml of ultra pure water was added, and the procedure was repeated a 2nd and 3rd time. Thus, three filtrate samples, representing 10, 20, and 30 utes were obtained. The filtrate was analyzed for Ca, Mg, Na, K, S, B, Cu, Fe, Mn, and Zn. Samples were analyzed using inductively coupled plasma mass spectrometry (ICP) water matrix standards. Linear regression was used to detere the relationship between sediment yield rate and runoff rate using the equation: q s = aq w + b (2) where q s is the sediment yield rate, and q w is the runoff rate. The data used were the runoff/sediment yield rate data pairs collected at each time increment for each replicate by treatment from the beginning of each experiment until steady state runoff had been achieved. Vol. 45(4):

4 RESULTS AND DISCUSSION GYPSIFEROUS MATERIAL ANALYSIS Nutra Ash was less soluble than gypsum, and this resulted in a lower concentration of calcium in solution (table 3). Both materials maintained fairly uniform Ca concentrations over the course of the dissolution analysis. A rapid decrease in the concentration of a given ion is often referred to as a first flush, where easily dissolvable erals are quickly depleted when water first contacts a material. Class C fly ash normally contains a number of soluble calcium containing erals when the ash comes in contact with water. Since Nutra Ash had been exposed to moisture for several years while in an ash landfill, it likely did not contain as many soluble erals as fresh ash. Grinding of the material may have exposed new surface area that had slightly higher solubility than the otherwise cemented or hydrated matrix. RUNOFF Treatments in which PAM was applied as a liquid solution (PAMW) to the soil surface and allowed to dry reduced runoff. Only the PAMW+G treatment had significantly less ( = 0.05) runoff than the control, a reduction of 35% (table 4). The PAMW+NA treatment was just outside the statistical range of significance. Comparing the two electrolyte treatments, there was no statistical difference in total runoff between the G and NA treatments. The amount of calcium applied in the G and NA treatments was equal; however, the dissolution analysis indicated that much higher concentrations of Ca, and hence electrolyte, would be present for the G treatment. Table 3. Constituent concentrations by ICP in solutions (20:1) extracted using sequential dissolution. Nutra Ash (ppm) Gypsum (ppm) Parameter Calcium (Ca) ,500 12,300 12,300 Magnesium (Mg) <1 <1 < Sodium (Na) <1 Potassium (K) Sulfur (S) Boron (B) Copper (Cu) < Iron (Fe) Manganese (Mn) <0.1 <0.1 < <0.1 <0.1 Zinc (Zn) <0.1 <0.1 < Table 4. Total average runoff volume, average sediment yield, and respective percent reductions over the control. The same letter following a treatment indicates no significant difference using the least significant difference method at = Total Runoff Total Sediment Yield Treatment mm % reduction of control kg m 2 % reduction of control Control 88.1 a 1.31 a Gyp 81.7 a b 33.6 PAMW+G 57.1 b c 73.6 PAMD+G 81.9 a c 63.2 NA 84.9 a b 21.9 PAMW+NA 77.5 a c 77.0 The addition of PAM to the electrolyte source resulted in a further significant decrease in runoff when comparing PAMW+G versus G. Total runoff for the G treatment was 81.7 mm, which declined to 57.1 mm with the addition of PAMW (table 4). During the first 35 utes, the slope of the line for the PAMW+G treatment was less in comparison to the other treatments (fig. 1). At about 35 utes, the PAMW+G line slope increased sharply and then increased at a fairly uniform rate. This suggests that the effects of the PAMW+G treatment were very pronounced early in the experiments but became slightly less so as the experiment progressed beyond 40 utes. Because the gypsum was crushed and sieved, there was some amount of fines created that were more soluble than the larger particles and would have solubilized quickly. This would result in the early release of a substantial amount of cations when the PAM was sprayed, thus providing ample divalent cations to enhance the cation bridging process. This is substantiated by the dissolution analysis, which shows high Ca concentration in the first 30 utes for G compared to NA. However, after approximately 35 utes continued raindrop impact likely began to break down soil aggregates, leading to increased surface sealing, which led to reduced infiltration and increasingly greater runoff rates. The effect of application method of PAM (PAMD+G and PAMW+G) was exaed, and there was a significant difference in treatment effect with respect to total runoff volume but no significant difference in final runoff rates (tables 4 and 5). The runoff rate for PAMD+G reached a relative steady state within the first hour of the simulation, while PAMW+G did not (fig. 1). Therefore, we conclude that the sprayed PAM, in the presence of gypsum, is able to better stabilize the soil surface. However, the experimental results also suggest that runoff rates may eventually approach those of the control, either due to saturation of the soil surface layer or through eventual degradation of the soil surface structure by continued raindrop impact. SEDIMENT All treatments significantly reduced total sediment yield compared to the control. These reductions ranged from 22% for the NA treatment to 77% for PAMW+NA (table 4, fig. 2). Comparing the gypsiferous compounds, total sediment yield was significantly different between the G and NA treatments (table 4). In all cases, the combination of PAMW and the gypsiferous material resulted in a significant reduction in total sediment yield (table 4). The G treatment had a significantly lower final sediment yield rate than the control, while the rate for NA was not significantly different from that for the control (table 5). The Ca concentration for G was nearly constant throughout the dissolution analysis. Results from the dissolution analysis, along with results plotted in figure 3, where the fraction of each treatment s sediment reduction has been plotted versus time, support the supposition that the Ca concentration for the G treatment remained fairly constant through the experiment. The relative effectiveness of G, in terms of sediment yield, is fairly uniform with time after 35 utes (fig. 3). This is also demonstrated in figure 2, where the lines for the control and G treatments are nearly parallel. The NA treatment behaved noticeably different from other treatments in terms of relative effectiveness. Between 1014 TRANSACTIONS OF THE ASAE

5 Control G PAMW+G PAMD+G NA PAMW+NA Runoff Rate (mm hr 1 ) Time () Figure 1. Average runoff rate (mm hr 1 ) versus time () for each treatment. Table 5.Final average runoff rate, average sediment yield rate, and respective percent reductions over the control. The same letter following a treatment indicates no significant difference using the least significant difference method at = Final Runoff Rate Final Sediment Yield Rate Treatment mm hr 1 of control kg m 2 hr 1 of control % reduction % reduction Control 44.4 ab 0.61 a Gyp 42.7 ab b 40.2 PAMW+G 40.7 b b 49.8 PAMD+G 45.5 ab c 71.1 NA 47.8 a a 6.5 PAMW+NA 45.6 ab bc and 45 utes into the experiment, the NA treatment realized its maximum relative effectiveness, after which the effectiveness dropped off dramatically. From table 3 one can see that the Ca concentration for the NA treatment remained fairly constant through the duration of the dissolution analysis, albeit substantially less than the G treatment. In figure 2, the increase in sediment yield rate with time for the NA treatment is approximately linear for the first 50 utes of the experiment, after which a steady state sediment yield rate, approximating the sediment yield rate of the control, is attained. Thus, the greatest reduction in sediment yield for the NA treatment as compared to the control occurred in the in 20 to 45 ute range. Apparently, the contribution of Ca from the NA treatment was imal after the first 45 to 50 utes of the experiment. All of the PAM treatments mirrored each other in terms of relative effectiveness (fig. 3). That is to say, regardless of absolute differences in sediment yield, the PAM treatments behave similarly over time with respect to sediment yield. While the PAMW+G treatment provided significantly less total runoff than did the PAMD+G treatment, there was no statistical difference between the two treatments in terms of overall sediment yield. Final sediment yield rates were significantly less for the PAMD+G treatment than for PAMW+G, although there was no difference in final runoff rates. This result may be due to the migration of dry PAM granules into pore spaces, due to either raindrop impact or the application process. When the PAM particles first became activated during wetting, they provided little benefit in terms of infiltration compared to the control. The soil may become adsorbed to the activated PAM granules, or the PAM may act as a mortar to limit erosion (Peterson et al., 2001). RUNOFF AND SEDIMENT YIELD RELATIONSHIP The relationship between sediment yield rate and runoff rate can be used as an indicator of soil erodibility (Huang and Bradford, 1993). Soil erodibility is a measure of the increase in sediment yield rate per unit increase in runoff rate. Linear regression was used to detere the relationship between sediment yield rate and runoff rate. It was common for runoff to reach steady state within the first hour, at which time a maximum sediment yield rate was measured. The sediment yield rate would subsequently decrease while the runoff rate would remain constant, an indication that the detachment of soil was being limited. Reductions in sediment yield due to the addition of amendments can be attributed to a decrease in soil erodibility or an increase in infiltration (i.e., reduction in shear by flowing water). The definition of erodibility can vary slightly depending on whether the rill or interill process is being considered. Rill erodibility is a measure of the susceptibility of soil to become detached by concentrated flow and can be defined as the increase in soil detachment per unit increase in shear stress of clear water (Flanagan and Nearing, 1995). Interill erodibility is the rate at which sediment is delivered to rills as a function of rainfall intensity and runoff rate Vol. 45(4):

6 Control G PAMW+G PAMD+G NA PAMW+NA Sediment Yield Rate (kg m 2 hr 1 ) % Time () Figure 2. Average sediment yield rate (kg m 2 hr 1 ) versus time () for each treatment. G PAMW+G PAMD+G NA PAMW+NA 12% Percent Reduction in Total Sediment Yield 10% 8% 6% 4% 2% 0% % Time () Figure 3. Percent of average total reduction in sediment yield compared to control as a function of time. (Flanagan and Nearing, 1995). What has been measured in this study is sediment yield, not detachment, but sediment yield is proportional to soil detachment under these experimental conditions. Shear stress is also proportional to flow rate. Thus, an estimate of soil erodibility can be made by regressing sediment yield rate on runoff rate. A linear model best represented the relationship between sediment yield rate and runoff rate for each treatment. Both wet PAM treatments and the PAMD+G treatment were represented by a combination of two linear functions. The control, G, and NA treatments were represented with a single linear function. Huang and Bradford (1993) stated that, under net detachment conditions, sediment yield rate (q s ) is a linear function of runoff rate (q w ). Under depositional conditions, the relationship can vary between linear and quadratic, depending on the importance of the ratio of deposition rate 1016 TRANSACTIONS OF THE ASAE

7 to erosion length scale. When deposition rate is great, q s should be a quadratic function of q w (Huang and Bradford, 1993). Figure 4 shows plots of sediment yield rate versus runoff rate for each treatment, along with the accompanying best fit lines. The models for the best fit lines and model coefficients are provided in table 6. The sprayed PAM treatments can be viewed as having two distinct linear segments: a lower region where the amendments have reduced the erodibility of the soil, and an upper region where the PAM has failed, resulting in an increased erodibility that approaches that of the control. With the PAMW+G treatment (fig. 4b, table 6), the change in slope from the lower to upper region does not appear to be great; however, the difference between the two slopes was significant at = The failure point is best exhibited in figure 4c for the PAMW+NA treatment. The erodibility (2.55 Ü 10 3 kg mm 1 m 2 ) was significantly less than the control (2.02 Ü 10 2 kg mm 1 m 2 ) until failure of the PAM occurred at a runoff rate of 37 mm hr 1. At this point, the erodibility increased dramatically for the treated soil and was not significantly different from the control. Sediment Yield Rate (kg m 2 hr 1 ) G Control Linear (G) Linear (Control) (a) (b) PAMW+G PAMD+G Linear (PAMW+G) Linear (PAMD+G) Sediment Yield Rate (kg m 2 hr 1 ) NA PAMW+NA Linear (NA) Linear (PAMW+NA) (c) Runoff Rate (mm hr 1 ) Runoff Rate (mm hr 1 ) Figure 4. Sediment yield rate versus runoff rate for each treatment. Table 6. Results of regression analysis on sediment yield rate (q s ) as a function of runoff rate (q w ) for each treatment. Treatment a [a] (kg mm 1 m 2 ) 95% Upper Limit [b] 95% Lower Limit b (kg m 2 hr 1 ) r 2 Control 2.02E E E E Gyp 1.69E E E E PAMW+G [c] 7.75E E E E E E E E PAMD+G 1.27E E E E E E E E NA 1.44E E E E PAMW+NA 2.55E E E E E E E E [a] Model coefficients found by linear regression using the model: q s = aq w + b. [b] Confidence intervals of a calculated using two sided t interval with α = [c] For treatments with more than one model, the first row indicates the model fitting the lower region, while the second row represents the model fitting the upper region. Vol. 45(4):

8 The reduction in erodibility attributed to the addition of PAM is similar to that found by Chaudhari (1999), where the reduction in erodibility over the control using a PAM and gypsum treatment was 77% for a silt loam soil. This study saw reductions of 61.6% and 87.4% for the PAMW+G and PAMW+NA treatments, respectively. These values were calculated based on the erodibility of the lower region, before failure, as shown in figure 4. The erodibility reduction suggests that PAM application may be beneficial in such applications as temporary protection of soil in grassed waterways or other applications where concentrated flow may be present. The PAMD+G treatment did not have a failure point for the duration of these experiments, but it did have a point at which it became effective. Figure 4b shows that the erodibility was initially 1.27 Ü 10 2 kg mm 1 m 2, and at a runoff rate of 28 mm hr 1 the erodibility decreased to 2.91 Ü 10 3 kg mm 1 m 2. The initial erodibility was not significantly different from that of the G treatment, while the erodibility of the upper region was significantly less than the erodibility of the lower region of the PAMW+G treatment. This supports the earlier statement that the dry PAM was initially inactive and after a sufficient amount of rainfall became active. The results discussed heretofore have been based on the 2 hour duration of this experiment. Clearly one would reach a different set of conclusions had the experiment been conducted for only one hour. The total precipitation amount over the course of the experiment was 140 mm, which is close to the 12 hour, 100 year rainfall event for West Lafayette, Indiana. The 70 mm of rain in the first hour represents the 1 hour, 100 year event for West Lafayette, Indiana. Based on these experimental results, sprayed PAM application would be expected to initially provide better erosion control over a dry application. However, for longer term erosion control, or a longer duration precipitation event, a dry PAM application may provide the same level of erosion control as the sprayed application. If the goal were to protect the soil surface from initial large runoff events prior to establishment of permanent vegetative cover, then the liquid spray application would be recommended. SUMMARY AND CONCLUSIONS A laboratory study using simulated rainfall was conducted to compare the dry versus sprayed application of PAM and the use of different gypsiferous material on runoff and sediment yield. The six treatments incorporated in this study were: control, Nutra Ash (NA), gypsum (G), sprayed PAM plus NA (PAMW+NA), sprayed PAM plus G (PAMW+G), and granular PAM plus G (PAMD+G). Deionized water was applied at a target intensity of 70 mm hr 1 for two hours to a silty clay loam soil. Only one of the two liquid PAM treatments significantly reduced runoff (35%), while both liquid PAM treatments significantly reduced sediment yield (74% and 77%) compared to the control. Only the PAMW+G treatment significantly reduced total runoff. Since the PAMW+G treatment resulted in significantly less runoff than the PAMW+NA treatment, it appears that the PAM was better able to adsorb to the soil particles in the presence of G. All amendment treatments significantly reduced total sediment yield. Both gypsiferous material treatments significantly reduced total sediment yield but were not different from each other. All PAM treatments significantly reduced total sediment yield but were not different from each other. Thus, the sprayed PAM application performed marginally better compared to dry PAM application in terms of runoff reduction but had no additional benefit with regard to total sediment yield for the storm intensity and duration used in this study. Only the PAMW+G treatment significantly reduced final runoff rate compared to the control. One explanation is that after sufficient rainfall enough of the PAM treated aggregates broke down, thus reducing the infiltration rate of the soil. All of the PAM treatments significantly reduced final sediment yield rate. This suggests that enough of the aggregates in those treatments were stable so as not to be transported in runoff, or reduced runoff through increased infiltration such that flow shear stress was reduced. There were no statistical differences in total sediment yield between PAMD+G and PAMW+G. An analysis of erodibility indicates that the mechanism by which each treatment reduces soil erosion is different. The decision as to which application method to use should be based on local climate conditions (i.e., expected amount and severity of precipitation before vegetation establishment). Gypsum was a better source of electrolyte than a class C ponded fly ash, commercially known as Nutra Ash (NA), based upon its greater solubility. REFERENCES Ben Hur, M Runoff, erosion, and polymer application in moving sprinkler irrigation. Soil Science 158(4): Chaudhari, K Polyacrylamide soil amendment effects on soil erosion from steep slopes. MS thesis. West Lafayette, Ind.: Purdue University. Chaudhari, K., and D. C. Flanagan Polyacrylamide effect on sediment yield, runoff, and seedling emergence on a steep slope. Presented at 1998 ASAE Annual International Meeting. ASAE Paper No St. Joseph, Mich. ASAE. Daniel, T. C., P. E. McGuire, D. Stoffel, and B. Miller Sediment and nutrient yield from residential construction sites. J. Environmental Qual. 8(3): Flanagan, D. C., and M. A. Nearing USDA Water Erosion Prediction Project: Hillslope profile and watershed model documentation. NSERL Report No. 10. West Lafayette, Ind.: USDA ARS National Soil Erosion Research Laboratory. Flanagan, D. C., L. D. Norton, and I. Shainberg. 1997a. Effect of water chemistry and soil amendments on a silt loam soil: Part 1. Infiltration and runoff. Trans. ASAE 40(6): b. Effect of water chemistry and soil amendments on a silt loam soil: Part 2. Soil erosion. Trans. ASAE 40(6): Foster, G. R., W. H. Neibling, and R. A. Natterman A programmable rainfall simulator. ASAE Paper No St. Joseph, Mich.: ASAE. Huang, C., and J. M. Bradford Analyses of slope and runoff factors based on the WEPP erosion model. Soil Sci. Soc. Am. J. 57(5): Levy, G. J., J. Levin, M. Gal, M. Ben Hur, and I. Shainberg Polymers effects on infiltration and soil erosion during consecutive simulated sprinkler irrigations. Soil Sci. Soc. Am. J. 56(3): Meyer, L. D., and D. L. McCune Rainfall simulator for runoff plots. Agric. Eng. 39(10): TRANSACTIONS OF THE ASAE

9 Mitchell, J. K., C. Ray, G. F. McIsaac, and J. G. O Brien Land treatment effects on soil erosion. In Proc. Managing Irrigation Induced Erosion and Infiltration with Polyacrylamide Conf., R. E. Sojka and R. D. Lentz, eds. Miscellaneous Pub. No Moscow, Idaho: University of Idaho. Norton, L. D., I. Shainberg, and K. W. King Utilization of gypsiferous amendments to reduce surface sealing in some humid soils of the eastern USA. In Soil Surface Sealing and Crusting, J. W. A. Poesen and M. A. Nearing, eds. Catena Supplement 24. Cremlingen Destedt, Germany: Catena Verlag. Peterson, J. R., D. C. Flanagan, and J. K. Tishmack Effects of PAM application method and electrolyte source on runoff and erosion. In Proc. International Symposium: Soil Erosion Research for the 21st Century, Honolulu, Hawaii. 3 5 January ASAE publication 701P0007. St. Joseph, Mich.: ASAE. Saxton, K. E., W. J. Rawls, J. S. Romberger, and R. I. Papendick Estimating generalized soil water characteristics from texture. Soil Sci. Soc. Am. J. 50(4): Seybold, C. A Polyacrylamide review: Soil conditioning and environmental fate. Commun. Soil Sci. Plant Anal. 25(11 and 12): Shainberg, I., and G. J. Levy Organic polymers and soil sealing in cultivated soils. Soil Science 158(4): Shainberg, I., D. N. Warrington, and P. Rengasamy Water quality and PAM interactions reducing surface sealing. Soil Science 149(5): Smith, H. J. C, G. J. Levy, and I. Shainberg Water droplet energy and soil amendments: Effect on infiltration and erosion. Soil Sci. Soc. Am. J. 54(4): Stern, R., M. C. Laker, and A. J. van der Merwe Field studies on effect of soil conditioners and mulch on runoff from Kaolinitic and Illitic soils. Aust. J. Soil Res. 29(2): US EPA National water quality inventory. Washington, D.C.: U.S. Environmental Protection Agency. Vol. 45(4):

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