Theo A. Dillaha

Vegetative Filter Strips for Agricultural Nonpoint Source Pollution Control

ABSTRACT Arainfall simulator was used to evaluate the effective-ness of vegetative filter strips (VFS) for the removal of sediment, nitrogen (N), and phosphorus (P) from cropland runoff. Simulated rainfall was applied to nine experimental field plots on an eroded Groseclose silt loam soil (clayey, mixed, mesic Typic Hapludalt) with a 5.5 by 18.3 m bare cropland source area and either a 0, 4.6, or 9.1 m VFS located at the lower end of each plot. Fertilizers were applied to the plots at rates of 222 kg/ha of liquid N and 112 kg/ha of P2O5 and K2O. Water samples were collected from the base of each plot and analyzed for sediment and nutrient content. One set of plots was constructed to encourage concentrated rather than shallow uniform flow. The 9.1 and 4.6 m VFS with shallow uniform flow removed an average of 84 and 70% of the incoming suspended solids, 79 and 61% of the incoming P, and 73 and 54% of the incoming N, respectively. Soluble nutrients in the filter effluent were sometimes greater than the incoming soluble nutrient load, presumably due to lower removal efficiencies for soluble nutrients and the release of nutrients previously trapped in the filters. Concentrations of soluble inorganic N and P in filter strip effluent were sufficient to cause eutrophic plant growth in aquatic ecosystems. Observation of existing VFS showed that on-farm VFS were not likely to be as effective as experimental VFS because of problems with flow concentrations.

Rainwater catchment water quality in Micronesia

Abstract Rainwater catchment systems (RWCS) in Micronesia were sampled to assess their bacteriological water quality and to determine which RWCS characteristics had a significant impact on water quality. Total and fecal coliform bacteria tests were used to evaluate 203 catchments on 10 islands. Fifty-seven percent of the RWCS had no fecal coliform bacteria and 61% had fewer than 10 total coliform bacteria per 100 ml. Catchment characteristics were found to have a statistically significant effect on total coliform bacteria levels but they did not affect faecal coliform bacteria concentrations. Rainwater catchment systems were found to provide acceptable water in most cases but disinfection prior to consumption is still highly recommended.

Attachment of Escherichia coli and Enterococci to Particles in Runoff

Association of Escherichia coli and enterococci with particulates present in runoff from erodible soils has important implications for modeling the fate and transport of bacteria from agricultural sources and in the selection of management practices to reduce bacterial movement to surface waters. Three soils with different textures were collected from the Ap horizon (silty loam, silty clay loam, and loamy fine sand), placed in portable box plots, treated with standard cowpats, and placed under a rainfall simulator. Rainfall was applied to the plots until saturation-excess flow occurred for 30 min, and samples were collected 10, 20, and 30 min after initiation of the runoff event. The attachment of E. coli and enterococci to particles present in runoff was determined by a screen filtration and centrifugation procedure. Percentage of E. coli and enterococci attached to particulates in runoff ranged from 28 to 49%, with few statistically significant differences in attachment among the three soils. Similar partitioning release patterns were observed between E. coli and enterococci from the silty loam (r = 0.57) and silty clay loam soils (r = 0.60). At least 60% of all attached E. coli and enterococci were associated particles within an 8- to 62-microm particle size category. The results indicate that the majority of fecal bacteria attach to and are transported with manure colloids in sediment-laden flow regardless of the soil texture.

Effects of Residue Cover on Pesticide Losses from Conventional and No-Tillage Systems

ABSTRACT MOVEMENT of atrazine and 2,4-D in runoff and sediment v^as measured on twelve field plots under simulated rainfall. The plots were divided into conventional and no-tillage systems. Within each tillage system, three residue levels (0, 750, and 1500 kg/ha) were studied. Simulated rainfall was applied to the plots three times at a rate of 5.08 cm/hr for a total of 10.2 cm of rainfall. Runoff and sediment losses were found to increase with decreasing residue cover for both tillage systems. No-till reduced sediment loss and total runoff volume by 98 and 92%, respectively, compared to conventional tillage. Concentrations of atrazine and 2,4-D in runoff and sediment were greater from the no-till plots than from the conventional plots but total losses were less. Water was the major carrier for both herbicides, although the concentration of 2,4-D in sediment was higher than that of water. Averaged over all plots, the atrazine losses were 2.9% of applied amount for conventional tillage and 0.3% for no-till. The corresponding values for 2,4-D were 0.3% and 0.02%.

Impact of Land Application of Sewage Sludge on Runoff Water Quality

ABSTRACT Arainfall simulator was used to study the effects of tillage system and sludge application method and rate on runoff, sediment and nitrogen (N) losses from agricultural lands. Surface application and incorporation of sludge were studied. Anaerobically digested sewage sludge was applied at rates supplying 0, 75 and 150 kg/ha of plant available N. A total of 90 mm of rainfall, with an intensity of 40 to 45 mm/h, was applied to sixteen 0.01-ha plots, located on a silt loam soil. Runoff water samples were collected from plot discharge and later analyzed for sediment and nutrient contents. No-till was found to be effective in reducing runoff and sediment losses. Runoff and sediment losses decreased as sludge application rates increased, regardless of the tillage system. The surface application of sludge was more effective in reducing sediment losses than sludge incorporation. Nitrogen concentrations and yields were greater from conventional tillage plots than from no-till plots. Surface application of sludge to conventional tillage plots resulted in greater concentrations and yields of most forms of N, relative to incorporated sludge treatments. With respect to sediment and N yields in surface runoff, no-till appears to be a safer alternative for disposal of sewage sludge than conventional tillage system.

Influence of Tillage Systems and Residue Levels on Runoff, Sediment, and Phosphorus Losses

ABSTRACT Arainfall simulator was used to study the effects of conventional tillage and no-till on the losses of runoff, sediment, and phosphorus (P) from agricultural lands. Three rye crop residue levels, 0, 750, and 1,500 kg/ha, were studied within each tillage system. Phosphorus fertilizer was applied at a rate of 46 kg/ha. A total of 100 mm of rainfall, with an intensity of 50 mm/h, was applied to 12 0.01 ha plots, located on a silt loam soil. Water samples were collected from H-flumes and later analyzed for sediment and P contents. No-till was found to be effective in reducing runoff and sediment losses. Runoff and sediment losses decreased as residue level increased, regardless of the tillage system. Increasing crop residue level from 0 to 750 kg/ha caused a decrease in average orthophosphorus (PO4) concentration for both tillage systems. However, as the crop residue level increased, from 750 to 1,500 kg/ha, the PO4 concentrations increased. Both PO4 and total-P (P4) losses were greatest with the 0 kg/ha residue treatment, intermediate with 1,500 kg/ha residue, and least with 750 kg/ha residue. The greater P losses from the 1,500 kg/ha, relative to the 750 kg/ha residue treatment, were attributed to higher P leaching from the crop residue of the 1,500 kg/ha treatment and insufficient suspended sediment to sorb P from solution for this level. No-till was very effective in reducing PO4, sediment-bound P (Psb), and Pj losses. Averaged over all residue levels, no-till reduced PO4, P^b, and Pj losses by 91, 93, and 97%, respectively, compared to conventional tillage.

MOVEMENT OF FIELD-APPLIED ATRAZINE, METOLACHLOR, AND BROMIDE IN A SANDY LOAM SOIL

Atrazine, metolachlor, and bromide movement in the top 1.5 m of a Suffolk sandy loam soil (coarse-loamy, siliceous, thermic Typic Hapludult) in the Virginia Coastal Plain, were characterized under management practices typical of the region. The study was conducted during the 1990 corn growing season, from May to September, on two 18 × 27 m plots in a field that was in the second year of a two-year no-till small grain/soybean/corn rotation. One plot was moldboard plowed and disked before corn planting and chemical application while the other remained no-till. Simulated rainfall (37 mm) the second day after chemical application resulted in bromide leaching below 1.2 m, and atrazine and metolachlor were detected to the 0.9-1.2 m depth layer. Leaching was significant in the first 30 days following chemical application, after which there was no significant downward movement of chemicals. Chemical concentrations tended to be greater in the no-till profile initially, and greater in the tilled profile at the end of the season. Atrazine was more persistent than metolachlor, with 50% (NT) and 56% (TP) of the applied atrazine remaining in the 1.5 m soil profile at the end of the 157 day study period, as compared to 8% (NT) and 4% (TP) of metolachlor. Due to greater infiltration and the possible compounding effect of preferential flow, the no-till plot presented a greater potential for herbicide leaching compared to the tilled plot in the first rainfall events following chemical application.

UNCALIBRATED PERFORMANCE OF THE FINITE ELEMENT STORM HYDROGRAPH MODEL

The capability of the Finite Element Storm Hydrograph Model (FESHM) to simulate runoff events in an ungaged context was evaluated using data from an experimental watershed. The FESHM was used to simulate 30 runoff events over a 17-year period of record. The results from these simulations were evaluated using various statistical analyses. Regression methodology then was used to assess expected error and model bias. Runoff volumes were simulated more closely than peak flows. The model simulated runoff volume adequately for site-specific and screening analysis, but peak flow estimates were adequate only for screening purposes. The relationship of predicted to observed peak flows was found to be linear with low peaks being underpredicted and higher peak flows overpredicted. Although the runoff volume regression line was close to the desired equal value line, scatter or variance was great and no linear relationship was evident. Peak flow and volume simulation errors were related to rainfall intensity. These simulation biases are likely due to a combination of inadequate representation of the infiltration process by the model and inappropriate input parameters. A resolution of these biases and corrective action was beyond the scope of this study. However, a starting point is provided for future model investigations or modifications. Finally, it appears that some form of calibration is needed to use the FESHM with confidence.

Stream discharge measurement using a large-scale particle image velocimetry (LSPIV) prototype

New technologies have been developed for open-channel discharge measurement due to concerns about costs, accuracy, and safety of traditional methods. One emerging technology is large-scale particle image velocimetry (LSPIV). LSPIV is capable of measuring surface velocity by analyzing recorded images of particles added to the stream surface. LSPIV has several advantages over conventional measurement techniques: LSPIV is safer, potentially automated, and produces real-time measurements. Therefore, the goal of this study was to evaluate the accuracy and feasibility of using LSPIV to measure instantaneous discharge in low-order streams. The specific objectives were: (1) to determine optimum operating parameters for applying LSPIV under various conditions, (2) to design, develop, and test a prototype under controlled laboratory conditions, and (3) to develop and test the field equipment for a variety of stream flow conditions. The laboratory experiment results indicated that LSPIV accuracy was influenced by camera angle, surface disturbances (Froude number), and flow tracer concentration. Under field conditions, the prototype acquired consistent images and performed image processing using accepted input parameters. The accuracy of LSPIV for use in field applications was evaluated using a permanent weir. Overall, 18 discharge measurements were taken with each measuring device. The LSPIV prototype was accurate, with a mean error of -1.7%, compared to the weir measurements. The root mean square error (RMSE) was similar for LSPIV and current meter discharge measurements with the area-velocity method when compared to the weir. Finally, the LSPIV discharge measurements had an uncertainty of approximately ±14% (at a 95% confidence level). Therefore, LSPIV showed the potential to become competitive with conventional discharge measurement techniques.

Evaluation of a Filtration/Dispersion Method for Enumeration of Particle-associated Escherichia coli

The transport processes of microorganisms in storm-generated flows have not been clearly elucidated, limiting the ability of computational models to effectively design and evaluate watershed remediation plans. Although several studies have identified association with particulates as a critical factor in predicting microbial transport and fate, no generally accepted method exists for the identification of the planktonic and particle-associated fractions of microorganisms in water samples. In this study, a filtration/dispersion method proposed for quantification of these fractions was verified using laboratory-composed samples of bovine Escherichia coli and sterile topsoil. A statistical experimental design allowed quantification of potential sources of experimental error (e.g., filter retention, die-off), although no significant sources of methodological error were identified. On average, 78% of E. coli cells were particle associated after 1 h of contact time. Further application of the method was illustrated by an isotherm experiment examining the association of a bovine strain of E. coli with sterile topsoil. Further examination of this method in laboratory or field-based studies of microbial partitioning between the planktonic and particulate phases in surface runoff appears justified.