nion production in the Treasure Valley region ofeastern Oregon and so - PDF document

Pit Dispo sal nion production in the Treasure Valley region ofeastern Oregon and southwestern Idaho comprises approximately one-third of the total an-nual U.S. storage onion crop and constitutes a regional farmvalue between $70 and $90 million per year. Onions areharvested from August through October. Generally, onions Cooperative Extension System Agricultural Experiment Station Pit Disposal of Cull Onions safe when using proper management practices (Hutchingset al., 1998). In this bulletin, we will summarize the experi-mental findings of this study and recommend environmen-tally sound management practices for disposal of cull on-Onions contain approximately 2 percent N on a dry-weight basis and are about 90 percent water by weight.Substantial quantities of Nand water are introducedinto burial pits as part ofonion cullage. However,less than 0.01 percent ofthe total N in onions is inthe mobile nitrate (NOform at the time the on-ions are placed in the pits.In this study, elevatednitrate concentrationslow disposal pits duringloading (Figure 2). Follow-pits with a soil cover, ni-trate concentrationsgible levels. In contrast, ammonium-N (NH-N) concentra-tions increase dramatically in the cull onion landfills fol-lowing closure (Figure 2). Ammonium-N concentrations inthe pits increase to maximum levels (about 400-500 mg/L)during the first two years following closure, then stabilizeand remain constant. Ammonium also increases in the firsttwo feet of soil region below the base of the pits, but doesnot increase substantially at deeper depths.During onion degradation, water levels increase in thecull onion landfills or burial pits even though the soilpermeability of pit bases may be adequate to allow forcomplete drainage of water. Restricted downward perco-lation is common in systems involving organic wastes suchas cattle feedlots and dairy lagoons because soil poresbecome clogged with organic waste and limit drainage;essentially, lagoons and feedlots act as self-lining systems.Organic self-lining of animal waste lagoon systems typi-cally reduces soil permeability by two to three orders ofmagnitude. Engineering applications of this process, calledartificial gleization, is used in construction of ponds andlagoons to prevent excessive water losses (see SoilConservation Service, 1979; Soil Conservation Service,1990). The continued presence of standing water in the cullonion burial pits for several years following cappingindicates that they also self-seal during degradation of theonions. This process restricts deep percolation and, conse-quently, any substantial leaching of nitrogen compoundsfrom the cull onion disposal pits.Nitrate levels in and beneath cull onion pits fall to zeroin less than three months following capping. This occursbecause the oxygen content and flow are limited in thesaturated, soil-capped systems. The low oxygen (anaero-bic) environment inhibits the formation of nitrate fromammonium (nitrification) and stimulates the conversion ofnitrate to gaseous forms ofN (denitrification). Similarinhibition of nitrate pro-duction occurs in cattlefeedlots and dairy lagoonsthat contain N-rich organicmaterial.The conversion of or-tion) is not impeded byanaerobic conditions; con-sequently, ammonium in-creases in saturated pits(low oxygen conditions)where organic N materialsare available. The pool oforganic N, deposited in thepits as cull onions, convertsto ammonium-N followingclosure in the low oxygen environment (Figure 2). Ammo-nium does not leach readily from cull onion burial pits be-cause it is a cation and strongly adsorbs to degraded or-ganic material within the pits and to soil materials under- Figure 1. The most common method for cull onions is to bury themin unlined, covered pits or landfills. Figure 2. Relative concentrations of ammonium-N and nitrate-N in a cullonion landfill during loading and following closure with a soil cap. Pit Disposal of Cull Onions In summary, the water-saturated, low oxygen conditionsthat develop within cull onion burial pits following cap-ping limit the formation of nitrate, a highly mobile N form,in disposal pits and promote the formation of ammonium,a less mobile N form, during the first several years follow-ing closure. Loss of N to the environment via nitrate leach-ing from cull onion burial pits does not occur as long asanaerobic conditions are maintained through proper clo-sure and maintenance of the pits. The Idaho State Department of Agriculture (ISDA) has es-tablished rules and guidelines for the proper disposal ofcull onions (see Idaho Administrative Procedures Act, IDAPA02.06.17). A copy of the cull onion disposal rules can beobtained from the Idaho State Department of Agricultureor from the ISDA Homepage www.state.id.us/adm/adminrules/rules/idapa02/02index.htmon the Internet.Pit or landfill disposal of cull onions does not pose a sig-nificant environmental threat if the pit site is suitably lo-cated, is well-constructed, and is loaded and maintainedproperly. To minimize environmental, disease, and odorproblems associated with cull-onion disposal using land-fill or burial pits, the following recommendations shouldbe followed: Disposal Site Selection - Landfills or burial pitsused for cull onion disposal should be established in areaswhere disposal operations will not pose an odor nuisanceto people living adjacentto the site. The site shouldbe in an area where the wa-ter table is sufficientlydeep (depths of 100 feet ordeeper) to avoid ground-water seepage into the pitduring operation and tominimize ammonium-N,nitrate-N, odors, or any un-desirable substance associ-ated with rotting onionsfrom entering the ground-water supply. Idaho law re-quires that permission forburial pit operation be ob-tained from county offi-cials. Additionally, local dis-trict health officials, theIdaho Division of Environ-mental Quality, and theIdaho Department of Agriculture should be consulted be-fore developing a burial pit disposal site to avoid violationof applicable environmental and agricultural regulations. 2 Burial Pit Development - The landfill or burial pitshould be excavated at the required dimensions to facili-tate disposal of culls generated during the shipping andprocessing season. Dimensions of currently operatingburial pits are approximately 300 feet in length, 30 feet inwidth, and 15 feet in depth. Burial pits could have smallerdimensions if less cullage is generated, but the depthshould always exceed 10 feet; this depth will allow a suffi-ciently thick soil cover after settling (approximately 6 feet)to prevent the development of adequate oxygen condi-tions for nitrate formation. Soil removed during excavationshould be piled along the sides of the pit for capping andleveling purposes. Ditches or berms should be constructedaround the pit to prevent water runoff from accumulatingsettles. 3 Loading and Disposal of Cull Onions in PitsA burial pit can be loaded (filled) in a single layer of cullonions with a soil cap or in alternate layers of onions andsoil. Filling of the pit with onions should begin at one endto facilitate coverage with soil during loading (Figure 3). Tominimize the release of nitrate during disposal, loading andcapping should be done in a timely manner to decreasethe period in which the pits are aerobic. A layer of soilshould be placed over the onions as the desired depth orlayer thickness of cull onions is achieved. A soil layer placedover the disposed onions will limit undesirable odors andprevent development of aerobic conditions (adequate oxy-gen) which may promote the conversion of organic N and/trate-N. A compacted soilcap, with a minimal thick-ness of two feet, should beestablished over the pitimmediately after loadingis completed. Regardlesscapped by March 15 tominimize insect and dis-ease problems accordingto Idaho State Depart-ment of Agriculture rules(IDAPA 02.06.17). 4 Burial Pit - The soil capsshould be actively main-tained during the courseof pit operation and pit closure to avoid excess rechargeby surface water. As cull onions degrade, the burial pit sur-face will settle. Settling will occur for three to five years, Figure 3. Filling of the pit should begin at one end in order to facilitatecoverage with soil during loading. Issued in furtherance of the cooperative extension work in agriculture and home economics, Acts of May 8 and June 30, 1914, incooperation with the U.S. Department of Agriculture, LeRoy D. Luft, Director of Cooperative Extension System, University of Idaho, Moscow,Idaho 83844. The University of Idaho provides equal opportunity in education and employment on the basis of race, color, religinational origin, gender, age, disability, or status as a Vietnam-era veteran, as required by the state and federal laws.1,200 February 2001depending on the quantity of onions disposed and pit vol-ume. Soil accumulated during pit excavation and storedalong pit boundaries can be used to maintain the pit sur-face. The soil cap should be mounded slightly above gradeto prevent water accumulation from runoff or precipita-tion. A properly capped burial pit along with a drainagesystem around the pit boundaries will provide the controlnecessary to prevent excess water accumulation. Burial Pit Closuresettling, the soil surface should be leveled or left slightlymounded. To prevent blowing dust and soil erosion, the sur-face of the pit and the surrounding area should be seededwith cover vegetation. The area should be prepared to pro-vide the best possible soil conditions for seeding. Success-ful revegetation of the disposal site depends on the climate,the fertility status of the disturbed soil, and the adaptabilityof the selected plant species. Contact the local NRCS or dis-trict conservation offices for recommended grasses or natu-ral vegetation and revegetation techniques for the geo-graphical location of the disposal site. For more informationon choosing suitable cover plant species and various reveg-etation techniques for disturbed soil surfaces, see USDA-NRCS Agricultural Handbook 339 and USDA Bulletin SCS- Burial Pit Reuse - A continuing concern where N-wastes are concentrated is the potential for nitrification tooccur in the future if suitable conditions develop. To miti-gate the threat of long-term NO leaching and to allow forrecycling of existing pits, it is recommended that residualwastes be removed from pits when dry, a process that takesabout 5 years with proper maintenance. The dried wastecan then be applied to fields or pastures at acceptable ag-ronomic N-loading rates. This would decrease the land arearequired for disposal, reduce costs associated with exca-vating new pits, and substantially alleviate future environ-mental problems associated with burial pit disposal of cullonions. To determine the appropriate land application rate,the residual wastes should be analyzed for N content andapplied to the soil accordingly. Further information on soiltesting and revegetation can be obtained from the Uni-versity of Idaho Cooperative Extension Service.See http://info.ag.uidaho.edu for the on-line catalog.Hutchings, J.J., J.E. Hammel, and J.L. Osiensky. 1998. Nitrogen Leaching fromUnlined Cull Onion Landfills. J. Environ. Qual. 27:1,254-1,260.Levi, A.E., J.K. Fellman, J.F. Guenthner, L.D. Makus, and M.K. Thornton. 1990.Market Alternatives for Treasure Valley Cull Onions. Idaho Ag. Exp. StationBUL 730.Neely, K.W., and J.K. Crockett. 1998. Groundwater Quality Characterizationand Initial Trend Analysis for the Treasure Valley Shallow and Deep Hydro-logic Subareas. Idaho Department of Water Resources Water InformationBulletin, No. 50 Part 3.Soil Conservation Service. 1979. Pond Sealing or Lining: Artificial Gleization.USDA-SCS Interim Report 521-F-1. U.S. Gov. Print. Office, Washington, DC.Soil Conservation Service. 1990. Design and Construction Guidelines forConsidering Seepage from Agricultural Waste Storage Ponds and TreatmentLagoons. USDA-SCS Technical Note 716. SCS South National TechnicalCenter, Fort Worth, TX. About the authorsJohn Hammel is a soil scientist and John Hutchings is a formerresearch assistant with the Department of Plant, Soil and Entomo-logical Sciences at the University of Idaho.