Drill Cuttings - Land Farming

Land Farming

The exploration and production (E&P) industry has used land farming to treat oily petroleum industry wastes for years. Land farming is the controlled and repeated application of wastes to the soil surface, using microorganisms in the soil to naturally biodegrade hydrocarbon constituents, dilute and attenuate metals, and transform and assimilate waste constituents.

Land farming can be a relatively low-cost drilling waste management approach. Some studies indicate that land farming does not adversely affect soils and may even benefit certain sandy soils by increasing their water-retaining capacity and reducing fertilizer losses. Inorganic compounds and metals are diluted in the soil, and may also be incorporated into the matrix (through chelation, exchange reactions, covalent bonding, or other processes) or may become less soluble through oxidation, precipitation, and pH effects. The attenuation of heavy metals (or the taking up of metals by plants) can depend on clay content and cation-exchange capacity.

Optimizing Land Farm Operations: The addition of water, nutrients, and other amendments (e.g., manure, straw) can increase the biological activity and aeration of the soil, thereby preventing the development of conditions that might promote leaching and mobilization of inorganic contaminants. During periods of extended dry conditions, moisture control may also be needed to minimize dust.

Periodic tillage of the mixture (to increase aeration) and nutrient additions to the waste-soil mixture can enhance aerobic biodegradation of hydrocarbons. After applying the wastes, hydrocarbon concentrations are monitored to measure progress and determine the need for enhancing the biodegradation processes. Application rates should be controlled to minimize the potential for runoff.

Pretreating the wastes by composting and activating aerobic biodegradation by regular turning (windrows) or by forced ventilation (biopiles) can reduce the amount of acreage required for land farming (Morillon et al. 2002).

Drilling Waste Land Farm Example: In 1995, HS Resources, an oil and gas company operating in Colorado, obtained a permit for a noncommercial land farm to treat and recycle the company's nonhazardous oil field wastes, including drilling muds. At the land farm, wastes mixed with soil contaminated with hydrocarbons from other facilities are spread in a layer one foot thick or less. Natural bacterial action is enhanced through occasional addition of commercial fertilizers, monthly tilling (to add oxygen), and watering (to maintain 10-15% moisture content). Treatment is considered complete when hydrocarbon levels reach concentrations specified by regulatory agencies; not all agencies employ the same acceptability standards. Water and soil are monitored periodically to confirm that no adverse soil or groundwater impacts have occurred, and records of the source and disposition of the remediated soil are maintained. Estimated treatment costs, which include transportation, spreading, amendments, and monitoring, are about $4–5 per cubic yard. When the treated material is recycled as backfill, net costs are about $1 per cubic yard. Capital costs (not included in the treatment cost estimates) were recovered within the first eight months of operation (Cole and Mark 2000).

Implementation Considerations: Advantages of land farming include its simplicity and low capital cost, the ability to apply multiple waste loadings to the same parcel of land, and the potential to improve soil conditions. Concerns associated with land farming are its high maintenance costs (e.g., for periodic land tilling, fertilizer); potentially large land requirements; and required analysis, testing, demonstration, and monitoring. Elevated concentrations of hydrocarbons in drilling wastes can limit the application rate of a waste on a site.

Wastes containing salt must also be carefully applied to soil. Salt, unlike hydrocarbons, cannot biodegrade but may accumulate in soils, which have a limited capacity to accept salts. If salt levels become too high, the soils may be damaged and treatment of hydrocarbons can be inhibited. Salts are soluble in water and can be managed. Salt management is part of prudent operation of a land farm.

Another concern with land farming is that while lower molecular-weight petroleum compounds biodegrade efficiently, higher molecular-weight compounds biodegrade more slowly. This means that repeated applications can lead to accumulation of high molecular weight compounds. At high concentrations, these recalcitrant constituents can increase soil-water repellency, affect plant growth, reduce the ability of the soil to support a diverse community of organisms, and render the land farm no longer usable without treatment or amendment (Callahan et al. 2002). Recent studies have supported the idea that field-scale additions of earthworms with selected organic amendments may hasten the long-term recovery of conventionally treated petroleum contaminated soil. The burrowing and feeding activities of earthworms create space and allow food resources to become available to other soil organisms that would be unable to survive otherwise. The use of earthworms in Europe has improved the biological quality of soils of some large-scale land-reclamation projects.

When considering land farming as a waste management option, several items should be considered. These include site topography, site hydrology, neighboring land use, and the physical (texture and bulk density) and chemical composition of the waste and the resulting waste-soil mixture. Wastes that contain large amounts of oil and various additives may have diverse effects on parts of the food chain. Constituents of particular concern include pH, nitrogen (total mass), major soluble ions (Ca, Mg, Na, Cl), electrical conductivity, total metals, extractable organic halogens, oil content, and hydrocarbons. Oil-based muds typically utilize an emulsified phase of 20 to 35 percent by weight CaCl2 brine. This salt can be a problem in some areas, such as some parts of Canada, the mid-continent, and the Rocky Mountains. For this reason, alternative mud systems have emerged that use an environmentally preferred beneficial salt, such as calcium nitrate or potassium sulfate, as the emulsified internal water phase.

Wastes that contain significant levels of biologically available heavy metals and persistent toxic compounds are not good candidates for land farming, as these substances can accumulate in the soil to a level that renders the land unfit for further use (E&P Forum 1993). (Site monitoring can help ensure such accumulation does not occur.) Land farms may require permits or other approvals from regulatory agencies, and, depending on soil conditions, some land farms may require liners and/or groundwater monitoring wells.