Agricultural vegetative filter strips were examined at two sites to assess the surface and subsurface effluent water quality following the application of farmstead runoff. Three filter strips were assessed and revealed significant reductions of nutrients, solids, oxygen demand, and other contaminant issues. Reduction in loading to surface water was highly effective with infiltration of all runoff for storm events within the 25-yr, 24-hr storm designation. Results for wastewater which had infiltrated had large variation and revealed soil type, influent characteristics, and other environmental factors have a large impact on performance and potential for groundwater contamination.
Farmstead runoff produced from animal waste, feed, and storage of other livestock operational necessities is a high contaminant source for surface and groundwater. In an effort to reduce the impact of farmstead animal operations, agricultural vegetative filter strips are widely used to control the outflow of pollutants through infiltration.
Engineered filter strips are designed to promote sheet flow to increase infiltration and contact time. The main mechanisms for pollutant removal include sediment trapping (where vegetation and sheet flow reduce flow velocities to capture sediment and sediment bound particles) and infiltration treatment processes. Sediment bound pollutants have greater removal rates than dissolved or soluble contaminants due to higher trapping efficiencies (Goel et al. 2004; Schmitt et al. 1999). However, infiltration is responsible for the majority of pollutant removal, in particular dissolved contaminants (Dosskey et al. 2007, Lee et al. 2003). Infiltration allows for pollutant soil assimilation, microbial degradation, and plant uptake. Removal rates by infiltration are determined by biological activity, adsorption, filtration, and oxidation, which are the primary mechanisms (Brown and Caldwell 2007). Biological activity removal rates are dependent upon environmental conditions including temperature, moisture, energy sources, and oxygen and nutrient availability (Donker et al. 1994).
Filter strip design dimensions of width, length, slope, soil type, and vegetation impact design and pollutant removal. Increases in the area available for infiltration and sediment trapping has been reported to increase pollutant removal. This can be achieved through increases in the width or length of the filter strip (Schmitt et al. 1999; Magette et al. 1989). A reduction in slope can also result in increased filter strip effectiveness (Hay et al. 2006). Sediment trapping and transport is strongly dependent upon the slope of the filter strip. An increase in the slope leads to a reduction in the trapping efficiency and an increase in pollutant transport (Jin and Romkins 2001, Dillaha et al. 1988). Soil hydraulic condictivities have been suggested from 0.27 – 0.5 in/hr in order to provide adequate infiltration and avoid ponding during wastewater application. Vegetation can uptake pollutants directly, but also impacts velocity and infiltration processes. In addition, vegetation develops dense mats of roots on the upper portions of soil profiles which can provide nutrient trapping and increases soil oxygen through respiration (Bhaskar 2003). Reported results have indicated that removal is contaminant specific with regards to differences in vegetation (Schmitt et al. 1999), and overall removal of all pollutants can be increased with the incorporation of multiple plant species to allow for numerous soil root sizes, and various stalk and leaf sizes to have the greatest overall impact on infiltration and sedimentation.
Although vegetated filter strips have been investigated over many years, reporting has indicated a large variance in efficiency of pollutant removal. Additionally, research has focused only on the removal of surface water contaminants through infiltration to limit surface discharge. With the focus on quantity concerns relating to overall infiltration, there has been a significant gap as to the fate of contaminants within the infiltration zone. Investigation as to the fate of these contaminants is critical to limiting exposure of groundwater to contaminants and maintaining a sustainable water cycle.
Determine the pollutant removal of agricultural filter strips in typical environmental and farmstead conditions. Specific objectives include the following:
• Assess the surface and subsurface water quality at two field sites.
• Assess current practice standards in regards to operation and maintenance procedures.
• Determine if agricultural filter strips are an effective agricultural treatment/management option as designed, with a particular emphasis on metal leaching into groundwater.
• Determine treatment consistency throughout season and rainfall events.
Three full-scale vegetative treatment strips were installed at two locations for performance analysis. The first site was the Michigan State University (MSA) Dairy Teaching and Research Facility (MSU dairy), a 160 head facility, the second a small MI dairy with 40 head. All design met the standards outlined in the MI NRCS 635 Wastewater Treatment Strip standard.
The MSU dairy had two filter strips installed, each 400 feet long and 40 feet wide with a 4% slope. Side slopes of 12.5% along the length of the filter strip created the channel which was backfilled with the sandy loam soil native to the site. Vegetation was planted as a mixed grass species containing 37% Tuscany II Tall Fescue, 28% Smooth Bromegrass, 20% Graze N Gro Annual Ryegrass, and 12% Chiefton Reed Canarygrass. Rock checks extended across the width of the filter strip (with a depth and width of two feet) at the flow entrance and every 100 feet downslope to redistribute flow (5 in total). Storm drains divert runoff from two drainage locations, a 1.14 acre area with feed sources and a second 1.28 acre area with manure sources, into two separate 86,774 gallon concrete storage/sedimentation basins. Wastewater was transported from the storage/sedimentation basins to two small concrete distribution basins at the top of each filter strip, which empties into a rock check to evenly disperse flow across the width of the filter strip. Surface samples were collected from the rock checks and subsurface samples from drainage tile installed 9 to 15 inches below the surface 25, 50, and 150 feet downslope.
At the second site, the small MI dairy was designed to treat runoff from from a 0.25 acre drainage area. Dairy feedlot and manure storage runoff were diverted via overland flow to a small concrete basin for sedimentation. Effluent from the concrete basin flowed over a weir to a bioretention basin lined with an impermeable membrane for storage. Effluent was transported via gravity to the first rock check for distribution to the filter strip. The filter strip was 110 ft long, 40 ft wide, with a 0.5% slope and sandy soil present at the site prior to installation. Rock checks were located at the top of the filter strip and 50 feet downslope. Surface samples were collected at rock checks, and subsurface samples at small collection wells 3 ft and 13 ft downslope.
Grab samples were collected within 24 hours after a rainfall event for all sample locations. Influent data was collected from the two concrete storage basins for the MSU dairy site and the concrete sedimentation basin and bioretention basin at the small MI dairy. Ten sampling events were obtained from the MSU dairy and six sampling events from the small MI dairy over a 2 year period. All samples were evaluated for the water quality parameters listed in Table 1.
A field study was conducted to determine pollutant removal of three agricultural filter strips at the surface and upon infiltration to the soil profile to determine the potential impact to groundwater, specific data from analysis can be found in Table 2. Sites with sand soils at the small MI dairy were more successful in reducing concentrations of COD, BOD5, ammonia, phosphorus, Total Kjeldahl Nitrogen (TKN), solids, and Total Organic Carbon (TOC). Sand soils provide characteristics to improve oxygen availability and reduce the moisture within the soil subsurface. However, these soils still resulted in leaching of nitrates and metals which posed environmental ground water concerns.
Sites with sandy loam soils had variation in performance. Poor performance of the second MSU filter strip is suspected to be due to the greater influent concentrations, as subsurface samples had higher conductivity, indicative of salt and soluble nutrient build-up over time, which is consistent with overloading. The majority of water quality parameters measured follow a linear trend in sandy loam soils where increasing influent concentration result in increased effluent concentrations indicating a limiting relationship regarding loading and sandy loam soils in terms of effluent targets. In addition, performance initially was impacted by the preferential flow to one side of the filter strip 2 resulting from improper grading, determined to be a critical design component.
Based on the results from all three sites (10 sampling events at the MSU dairy, and 6 at the small MI dairy site) subsurface samples indicated that a soil depth of 1 to 1.5 feet is not capable of eliminating the majority of pollutants to a degree suitable for groundwater protection. Concentrations of BOD5 were well above the 30 mg/L discharge limit. Concentrations for contaminants critical to human and environmental health including E. Coli, nitrate, and arsenic are well beyond concentrations that would eventually lead to contamination. Increases in nitrite concentrations occurred in all systems, which is unusual and may indicate toxicity. Nitrate values were consistently over the 10 mg/L standard. A build up of nitrate within the system, as indicated by the negative removal, may be a result of available oxygen inhibiting denitrification. Arsenic concentrations were also over the 10 ug/L drinking water standard, particularly at the small MI dairy filter strip. The greater sources of arsenic at this site were theorized to be due to influences from high groundwater arsenic levels being transported through excess plate cooler water entering the storage basin. Metal leaching was also a concern for groundwater sources as those measured in subsurface effluent were in greater concentrations than were in the waste stream prior to infiltration. Metal leaching and a reduction in nitrate at the small MI dairy site indicate that reducing conditions may have occurred. Overall, the small MI dairy had higher removal rates in 1.5 and 2.5 feet deep subsurface samples compared to the first foot in the MSU subsurface samples, but final effluent concentrations indicated potential problems for nitrate and metal leaching at both sites.
Improved performance of the MSU site can be achieved through a variety of operations and design alterations. As was indicated by the influent to effluent concentration correlations, an increase in management to reduce influent concentrations would have a significant effect on effluent concentrations. Additionally, mechanical aeration in the basin or within the field (using a lawn aerator or venting within the soil profile) can provide an increase in oxygen required to increase treatment. A further measure of backfilling sand atop native soils has potential to increase aeration and decrease moisture within the soil profile to increase performance.
Cold weather performance was evaluated at the MSU site only and was found to have no effect on performance, although the poor performance of these systems year round may influence the difference realized with seasonal variation, as a correlation may result in a more efficient system. However several deductions could be made from analysis of daily operation. Due to the transport system design requiring increased temperatures for application of waste to the treatment systems it is likely that the filter strips will have unfrozen soil subsurfaces during a runoff application in a thaw event, reducing the runoff from impermeable soils. However, there is a need to investigate the temperature effect on microbial degradation in a more efficient treatment system as microbial populations are known to be affected by temperature differentials.
Educational & Outreach Activities
Preliminary research characterizing farmstead runoff water quality has already been presented at the American Society of Agricultural and Biological Engineers (ASABE) International Conference in 2009, the information presented can be found from the article below.
Larson, R., Safferman, S.I. 2009. “Stormwater Runoff Characterization from Animal Feeding Operations.” 2009 ASABE Annual International Meeting, Reno, Nevada.
A dissertation was also published.
Larson, R. 2010. “SOURCE CHARACTERIZATION, EVALUATION, AND TREATMENT POTENTIAL OF AGRICULTURAL FILTER STRIPS.” Michigan State University, Department of Biological Systems Engineering.
Two publications are currently being drafted and submitted for peer review to a scholarly journal and projected publication in 2011 for the work presented above in addition to the corresponding laboratory investigation. Data will be provided for regulators for updates to current standards and regulations concerning application of waste. Continued presentation and education is currently scheduled and will continue with fact sheets and presentations for producers, educators, regulators, and others who may benefit from this work in an effort to promote a greater impact.
Agricultural filter strips have been implemented on two farms as a criteria of this research. Continued research is necessary for a final design recommendation which does not have pose groundwater contamination issues. The alterations in design and management proposed in this document will be incorporated in the design of standards and regulation within Michigan and Wisconsin at a minimum. Further investigation, reporting and regulation should further impact incorporation at a farmstead level.
Areas needing additional study
Additional work needs to focus on the environmental parameters within the soil environment which contribute to the leaching issues presented. Research investigating a modified filter strip with multiple components for an aerobic, then anaerobic, and finally aerobic treatment zones to promote removal of nitrates without leaching of metals is critical to maintain the viability of this treatment system. Additionally, details concerning ratios of loading application to soil volume, treatment system life, and critical treatment depth prior to reaching groundwater need to be investigated to determine continued functionality of these systems.
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Brown and Caldwell. (2007). Manual of Good Practice for Land Application of Food Processing/Rinse Water. Davis, California.
Dillaha, T.A., J.H. Sherrard, D. Lee, S. Mostaghimi, and V.O. Shanholtz. (1988). “Evaluation of Vegetative Filter Strips as a Best Management Practice for Feed Lots.” Journal of Water Pollution Control Federation, 60(7), 1231-1238.
Donker, M. H., Eijsacker, H., and Heimbach, F. (1994). Ecotoxicology of Soil Organisms, Lewis Publishers, Boca Raton.
Dosskey, M. G., Hoagland, K. D., and Brandle, J. R. (2007). “Change in filter strip performance over ten years.” Journal of soil and water conservation, 62(1), 21-32.
Goel, P. K., Rudra , R. P., Gharabaghi, B., Das, S., and Gupta, N. (2004). “Pollutants Removal by Vegetative Filter Strips Planted with Different Grasses.” In: 2004 ASAE/CSAE Annual International Meeting, American Society of Agricultural and Biological Engineers, Ottawa, Ontario, Canada.
Hay, V., Pittroff, W., Tooman, E. E., and Meyer, D. (2006). “Effectiveness of vegetative filter strips in attenuating nutrient and sediment runoff from irrigated pastures.” Journal of agricultural science, 144(4), 349-360.
Jin, C.-X., and Romkins, M. J. M. (2001). “Experiemntal Studies of Factors in Determining Sediment Trapping in Vegetative Filter Strips.” Transactions of the ASAE, 44(2), 277-288.
Lee, K. H., Isenhart, T. M., and Schultz, R. C. (2003). “Sediment and nutrient removal in an established multi-species riparian buffer.” Journal of Soil and Water Conservation, 58(1), 1.
Magette, W.L., R.B. Brinsfield, R.E. Palmer, J.D. Wood. (1989). “Nutrient and Sediment Removal by Vegetated Filter Strips.” Transactions of the ASAE, 32(2), 663-667.
Schmitt, T. J., Dosskey, M. G., and Hoagland, K. D. (1999). “Filter strip performance and processes for different vegetation, widths, and contaminants.” Journal of Environmental Quality, 28(5), 1479-1489.
US EPA. (1996). SW-896 On-line. Washington, D.C.: U.S. Environmental Protection Agency’s Office of Solid Waste. http://www.epa.gov/osw/hazard/testmethods/sw846/online/index.htm
US EPA. (2009). “National Primary Drinking Water Regulations.” EPA 816-f-09-004.