Guidelines for Revegetation of Oil and Gas Drilling Sites in the Western United States

Oil and gas extraction may result in the removal of vegetation through soil disturbances and infrastructure emplacement (Stednick et al. 2010). Because of this, regulations or landowners often require revegetation of a disturbed area. Due to the considerable number of oil and gas sites requiring revegetation, there is a need for a practical set of guidelines that is accessible to land managers with limited experience in revegetation science. This article provides such a set of guidelines, but the reader should be aware that this is an overview of relevant topics. There are many other resources for revegetation planning and implementation, including many that provide much more in-depth discussion of theory and techniques than is found here. The Society for Ecological Restoration (2004) has published numerous books concerning ecological restoration and revegetation through Island Press. Other readily available resources include Ferris et al. (1996) and Davis et al. (2004a, b, c).

Like all revegetation projects, those associated with oil and gas extraction should begin with careful planning involving all stakeholders. It is important to conduct baseline studies of soil, water, vegetation, and other environmental conditions at the site to determine which factors could hinder revegetation (Whisenant 1999). If there is a nearby area with similar ecological conditions and minimal disturbance, it can be used as a baseline reference area. Comparing conditions in the disturbed and reference areas can inform what factors may limit the disturbed site from moving towards the reference condition. Based on these constraints, begin revegetation planning by setting measurable, achievable goals that are acceptable to stakeholders. Common goals include establishing certain levels of percent cover or density for desirable species or functional groups (grasses, forbs, shrubs, etc.) to reach a target level of plant biodiversity, creating habitat for wildlife, reducing erosion, providing livestock forage, and achieving target reductions of hydrocarbon or salt levels in soils. Revegetation should place the site on a trajectory towards a stable, self-sustaining plant community that is integrated into the larger landscape and will withstand periodic natural disturbances (Society for Ecological Restoration International Science & Policy Working Group 2004). A robust monitoring and maintenance plan can enable site management to adapt to ongoing disturbances such as weed infestations, climactic variability, wildfire, and erosion. Engaging people who will use the land after revegetation may facilitate setting goals and executing the project (Couix & Gonzalo-Turpin 2015). Because of the complexity involved, revegetation planning should start as early as possible, potentially even before the disturbance occurs (Society for Ecological Restoration International Science & Policy Working Group 2004).

Before disturbed lands are reseeded, anyone designing a revegetation seed mix may want to understand which desirable and undesirable species will likely recolonize a site without intervention (GeFellers et al. 2020). One way to determine this recolonization potential is to collect soil samples from the site, pot them in a greenhouse or other controlled environment, record how many individuals of various species germinate per unit area, and calculate an estimated density of these species for the entire site. Alternatively, seed testing labs can identify seeds from soil samples. A list of certified seed testing labs can be found on the Association of Official Seed Analysts (2021) website. The ability of local plants to colonize a disturbed site can also be estimated by surveying surrounding areas and investigating dispersal potential of species in the area. Species that can readily colonize a site from the seedbank or from surrounding areas may be left out of seed mixes.

When creating a seed mix, first identify a reference model(s) for the disturbed site. A reference model describes how the plant community would look and function ecologically if it had not been disturbed (Gann et al. 2019). Reference models combine multiple sources, such as local undisturbed or minimally disturbed sites, historical documents, or Natural Resources Conservation Service Ecological Site Descriptions (Natural Resources Conservation Service 2020a). Because ecosystems are constantly changing, reference models emphasize ecosystem processes and trajectories over static goals such as specific vegetation community compositions. Depending on the reference model and project goals, seed mix species can be selected for various functional traits (Table 1). Even within a species, traits vary between plant populations from different geographic origins (different ecotypes) or different breeding programs (different cultivars).

Many revegetation seeds are sold commercially, and an online search may determine whether there is a regional supplier. Inquire directly with the supplier, as seed availability varies seasonally. Commercial sources may not carry the locally adapted ecotypes or cultivars that make revegetation most successful. Other options include field collecting seeds and propagating them in a greenhouse or seed orchard, which can be too expensive and labor-intensive for large projects (Smith 2017), or buying from regional native seed cooperatives. Salvaged topsoil can also serve as an important source of locally adapted seeds but may also be a source of weed seeds.

Although seeding is most commonly used for revegetation after oil and gas extraction, transplanting can also be successful in arid environments or with plants that have low seed germination rates, such as many trees, shrubs, and wetland plants (Azam et al. 2012; Löf et al. 2004). However, transplants will often die when there is not adequate water from precipitation or irrigation immediately after planting (Azam et al. 2012). Bainbridge (2007, Chapter 10) and Kleinman (1996) are good references for transplanting techniques and methods for improving survival.

When seed is harvested or collected, there is typically a large amount of nonseed material such as chaff or hulls present. Removing this material by seed cleaning is important. Commercial seed producers clean their seed before sale. Land managers who have collected their own seed can refer to Frischie et al. (2020) for an introduction to seed cleaning techniques. After cleaning, seed should be tested for purity and viability. A list of seed testing labs that test for germination percentage, percentage of pure seed, and percentage of weed seed can be found through the Association of Official Seed Analysts (2021).

Most seed companies will provide seeds per pound on the label of a commercial seed bag. However, many seed application rates are expressed as pure live seed (PLS) per unit area. For a given quantity of seed, %PLS is the percentage of pure seed (versus inert matter or weed seed) multiplied by the germination percentage (Boggess & Brooke 2020). Because seed germination rate declines over time, %PLS will decline as well, and seeds may need to be retested for germination percentage. PLS is used in seeding rates because seeds of a given species can vary considerably in their weight based on growing conditions and ecotype of the parent plant. Seed lots that have lower PLS per unit weight are often superior for restoration because those seeds are larger and have more reserves for better establishment. Seeding rates for revegetation are often expressed as weight of PLS per square meter or square foot. This expression allows for more accurate estimates of the resulting plant community density. Seeding based on weight of seed mix per area can lead to more variable results from year to year when using the same plant species due to yearly variation in the PLS weight/kg. Boggess and Brooke (2020) provided a primer on converting weight to PLS. Kew Royal Botanic Gardens (2021) also has information on average seed weights by species.

Most seeds of nonagricultural plants are dormant when mature (Kildisheva et al. 2020). This means that even when they are exposed to ideal moisture and temperature conditions for 4–6 weeks, they will not germinate. Dormant seeds sown onto a disturbed area may not germinate for at least several growing seasons (Broadhurst et al. 2016). Therefore, before seeding it is important to understand dormancy and germination requirements of species in the seed mix.

Seeds break dormancy when they receive environmental cues that conditions are favorable for germination (Baskin & Baskin 2014), such as specific moisture or temperature requirements (temperature stratification), physical removal or chemical dissolution of seed coats in animal digestive tracts or soil (scarification), or exposure to smoke. Dormancy requirements are species specific, and it is important to research these requirements before obtaining seeds. For example, seeds requiring cold stratification are best sown in the fall to increase establishment the following growing season. Similarly, seeds requiring scarification (many legume seeds) may establish sooner when treated prior to sowing. The Kew Royal Botanic Gardens (2021) is a reliable source of dormancy information.

With proper storage techniques, dormant seeds can be stored for years and remain viable. Seed lifespan may be maximized by storing at appropriate moisture, temperature, and oxygen levels (De Vitis et al. 2020). For 90% of seeds, lowering these three factors as much as possible is the best approach (Roberts 1973). It is recommended to dry seed to 3–7% moisture content prior to storage (Food and Agriculture Organization 2013/International Plant Genetic Resources Institution 1994). See De Vitis et al. (2020) for the procedure to calculate seed moisture content. To prevent seeds from rehydrating, they should be stored at <50% relative humidity when kept for years and <25% when kept for decades (Adams et al. 2016). For storage periods <18 months, temperatures of 0–5°C are adequate (Hong & Ellis 1995). For longer storage, temperatures of −20 to −18°C are preferable. Storage in plastic bags keeps oxygen levels low while allowing some gas exchange (Smith 1986). For an overview of equipment to maintain these conditions, see De Vitis et al. (2020, Box 1). For desiccation tolerance for seeds of various species, use the information from the Kew Royal Botanic Gardens (2021). It is also important to keep rodent and other seed pests out of the seed storage area. Before seeding, it may be necessary to store seeds on site for several weeks while equipment is staged. In these cases, moisture, temperature, oxygen, and seed predators are still the primary concerns. Store seed in a shady, sheltered location to minimize exposure to precipitation, runoff, or high temperatures and protect seed from rodents and other seed predators.

Ecologists have long recognized that biodiversity in ecosystems imparts higher productivity and resilience, and these concepts have become two of the most widely held assumptions in vegetation science (Schultze & Mooney 1994). For example, an analysis of 1,126 grassland study plots across five continents revealed a very strong and consistent link between productivity and number of unique plant species (Grace et al. 2016), and numerous studies have demonstrated that diverse grassland plant communities are more productive and more resilient and resistant to drought than are species-poor communities (Tilman & Downing 1994; Wagg et al. 2017). Driven by concerns over plant species extinctions and climate change, many recent studies are digging deeper into these assumptions, with findings suggesting that recovery of productivity in plant communities after extreme climate events is strongly dependent on initial plant community diversity. For example, a meta-analysis of 46 experiments where grassland plant community diversity was manipulated revealed that biodiversity was especially important for stabilizing ecosystem productivity after extreme climate events (Isbell et al. 2015). These well-established concepts in ecology suggest that revegetation efforts that strive to maximize plant biodiversity in reclaimed areas may avoid revegetation failures resulting from extreme climate events. Seed mixes for revegetation of oil and gas sites often lack the species diversity needed to create a resilient plant community (Barr et al. 2017). Optimum seed mix diversity to achieve restoration goals may be as high as 35 species (Barr et al. 2017). In general, revegetation seed mixes should contain as many appropriate species as is practically feasible. Often, projects avoid using species with expensive seed, but these species can and should still be seeded at very low rates without dramatically increasing the overall seed mix cost. In high-diversity mixes, seeding slowly growing forbs and shrubs in separate rows from quickly growing herbs can improve establishment of the slowly growing species (Monsen & Stevens 2004).

After creating a species list for a seed mix, the next step is to determine the application rates for each species in the mix. Seed application rates are expressed either as weight or preferably as PLS per unit area (see “Seed Quality”). In most revegetation projects, seeding rates are based on the densities of species in the reference model combined with knowledge of a species expected field emergence (establishment potential). Expected field emergence is different from PLS because environmental conditions, competition with other plants, disease, or consumption by animals will prevent some percentage of viable seeds from producing established plants. In general, expected field emergence for planted seed is 10%–30% (Whisenant 1999). Therefore, sowing a mixture of grass and forb species at a rate of 100 PLS/m² would likely result in the emergence of <30 plants per square meter. Thus, using of weight of PLS per area rather than PLS per weight allows for more careful planning of competitive interactions in the resulting plant community. In many ecosystems, there are a few dominant species with high densities and many more rare species with low densities (Sasaki & Lauenroth 2011; Smith & Knapp 2003). Mimicking these trends by seeding a variety of rarer species at low rates can increase diversity without adding much cost (Wilsey & Martin 2015).

Every project will have different optimal seeding rates based on differences in project goals and site conditions. Grassland revegetation is often more successful with higher seeding rates (Hulvey & Aigner 2014; Porensky et al. 2018; Schantz et al. 2019). High seeding rates generally lead to higher seeded species cover and lower weed cover (Monsen & Stevens 2004; Rinella et al. 2016). However, high grass seeding rates can hinder establishment of slowly growing forbs, shrubs, and trees, especially when water is scarce (Hild et al. 2006; Kleinman 1996). Drill seeding rates for rangeland revegetation projects usually are 344–2,368 PLS/m². Broadcast seeding rates should generally be 50%–100% higher than drill seeding rates (Ferris et al. 1996; Monsen & Stevens 2004; Steward 2006).

Topsoil salvage refers to the removal and storage of topsoil during disturbances and subsequent redistribution on the site before the revegetation process. Topsoil is the dark upper layer of soil that is rich in decomposed organic material (Maiti 2012). Topsoil salvage and redistribution is a drastic intervention that can damage the soil microbial community (Frouz et al. 2013), which may negatively affect plant establishment. However, when extractive activities are likely to produce high levels of topsoil compaction or removal or when soil with hydrocarbons or saline drilling fluids is present, topsoil salvage is preferable to leaving topsoil in place during disturbances (Dunn and Fryer 1996). In these cases, topsoil salvage preserves soil nutrients (Larney et al. 2012), existing seeds (Lovell et al. 2018), and beneficial microorganisms (Bainbridge 2007).

Topsoil salvage consists of topsoil identification, stripping, stockpiling, and replacement. These processes were detailed by Ferris et al. (1996, Section 1). The amount of topsoil salvaged should be based on planned replacement depths, which vary across the site according to topography (Ferris et al. 1996). Replacing salvaged topsoil as soon as possible minimizes damage to microbiota and plant propagules (Ferris et al. 1996).

Storing topsoil in a dry state (water potentials below 2 megapascals for drylands) preserves seeds and fungi (Bainbridge 2007; Golos & Dixon 2014). Covering stockpiles with opaque waterproof tarps keeps them dry without raising soil temperatures (Golos & Dixon 2014). However, when soil will be stockpiled for more than 2 years, it may be preferable to forego the tarp and instead plant a cover crop on the pile to preserve soil organic matter and reduce erosion (Bainbridge 2007). Seeds of all species will lose viability over time. However, when the stored topsoil contains short-lived weed seeds, then storing for 2–3 years may disproportionately reduce weeds rather than longer lived native perennial seeds (Golos et al. 2016). Deep storage piles can reduce oxygen levels in soil and kill microorganisms (Bainbridge 2007). To maintain healthy soil, stockpiles should be created in a way that maximizes surface area and minimizes depth.

In sites undergoing decades-long extraction, continued seed viability might be a concern in stockpiled soil. In these cases, some projects use soil removed from a nearby area with similar soil characteristics instead of salvaged topsoil to initiate revegetation. This new “borrow area” needs to be revegetated. Allowing these borrow areas to revegetate passively may result in plant communities that differ significantly from the reference (Densmore 1987; LoPiccolo 1986), although reseeding with appropriate species and incorporating compost amendments can lessen the differences (LoPiccolo 1986; Prodgers 2009). When available, salvaged topsoil is the preferred substrate for revegetation of highly disturbed sites (Prodgers 2009).

Soil amendments are materials added to soils to alter physical and chemical properties (Table 2). Soil biochemistry is complex, and an accounting of all the ways amendments can be used to reclaim deficient or toxic soils is beyond the scope of this article. The U.S. Environmental Protection Agency (2007) is a useful resource for further reference on soil amendments.

Soil testing can help determine whether soil amendments are necessary for revegetation. Soil testing labs can measure key factors such as pH, available nitrogen, soil C:N (carbon:nitrogen) ratio, phosphorus, potassium, and cation exchange capacity. Because soils are highly variable regionally, soil deficiencies may be diagnosed by comparison with a reference model (see “Seed Mixes and Seed Procurement”) rather than by any standardized measure. Revegetation projects should avoid using agronomic standards of soil fertility because wildland plant communities often flourish on low-nutrient soils. Fertilizer amendments, especially nitrogen (Perry et al. 2010), can lead to weed infestations. Commercially available mycorrhizal inocula have provided mixed results for revegetation and are generally not recommended as soil amendments (Bainbridge 2007; Ohsowski et al. 2012).

Soil amendment is not the only way to improve poor soil conditions. Sites can be planted with nurse crops such as oats (Avena fatua) and barley (Hordeum vulgare) (Vallentine 1989), which grow alongside revegetation mixes, or preparatory crops such as nonwoody legumes, which are planted first and then harvested before seeding with a revegetation mix (Whisenant 1999). When nurse and preparatory crops are not desired long-term components of the vegetation community, sterile hybrid cultivars can be used. These crops will not reproduce and will thus disappear from the site after 1 year.

Tillage (mechanical disturbance of soil) is both a method of mechanical weed control and a way of altering soil physical conditions to improve plant growth. Tillage can reduce soil compaction or improve soil surface conditions for seed germination and seedling growth. Tillage at sites with many weed seeds in the soil can lead to high weed germination and establishment (Shaw et al. 2020). Tilling equipment should not be used on steep slopes or around desirable vegetation, and elevation contours should be followed (Bainbridge 2007). Soil heterogeneity can maximize biodiversity. However, uniform tillage depth and type can reduce biodiversity, as can any uniform soil treatment. Tillage should be avoided in areas of the revegetation site with intact native plant communities.

There are two categories of tillage. Primary tillage is deeper and focuses on compaction, and secondary tillage improves soil surface conditions to prepare a seedbed. There is usually no need for primary tillage at sites where soil compaction is not an issue. Soil compaction can hinder plant root growth and water infiltration (Nawaz et al. 2013). To reduce compaction on oil and gas sites, repeated traffic across the same area should be minimized, and axle load limits, e.g., <6 metric tons (Botta et al. 2019), should be established. To determine whether soil is compacted, its bulk density can be compared with that of a reference soil. Soil testing labs can determine bulk density from soil samples, and managers can test bulk density themselves by referring to the Natural Resources Conservation Service (2020b). Primary or deep tillage methods include plowing, subsoiling, and ripping (Angel et al. 2018; Negev et al. 2020; Tekeste et al. 2019). These methods till to depths from 20 cm to 1 m or more (Dunker et al. 1995).

Secondary or shallow tillage is shallower than primary tillage, usually 5–20 cm deep (Bard et al. 2004; Romo & Grilz 2002; Visser et al. 2004). The goals of secondary tillage are to reduce erosion, improve seed-soil contact, increase soil water, and remove established plants that would compete with revegetation seedlings (Sadeghpour et al. 2015; Sheley et al. 2009; Stockfisch et al. 1999). Broadcast seeding often requires secondary tillage, but drill seeding may not (Shaw et al. 2020). Stevens and Monsen (2004) described various tillage equipment such as disks, harrows, and plows.

There are also nontillage seedbed preparation techniques. Small pits and mounds can be created using shovels, mechanical spaders, or imprinters with large teeth (Bainbridge 2007; Hough-Snee et al. 2011). Pits trap snow and water, which benefit plant establishment in arid and semiarid environments. In compacted soils, tillage may be required before creating pits.

Weed control is often necessary both before and after seeding. It is usually impossible to control all weeds. It is best to target those that have significant negative effects on ecosystems or services that ecosystems provide to humans (Hulme 2020; Whisenant 1999). Local land management agencies may provide noxious weed lists and other resources to determine which species to target and how. Many plant invasions are driven by human changes to ecosystem processes such as wildfire, grazing, flooding, groundwater recharge, and nutrient cycling. In these cases, the invasion is best addressed by recreating or mimicking those original ecosystem processes on the site (Hartfield et al. 2020; Pauchard & Shea 2006; Svejcar et al. 2017; Tanentzap et al. 2014). For weeds that reproduce primarily by seed, it is most effective to implement controls annually before weeds have set seed.

Weed control in revegetation usually consists of mechanical and/or chemical control. Targeted grazing is another option. Mechanical control includes mowing, brush cutting, weed whacking, and similar methods. Such techniques are inexpensive but often ineffective in the long term (Vicklund, 1996). Follow-up with tillage can reduce regrowth. However, these methods may not control species that reproduce clonally through roots or tillers, and they tend to equally affect both desirable and undesirable species. More selective control or control of clonal weeds can be attained by manual pulling. This approach is labor- and time-intensive but can be highly effective for smaller projects (Weidlich et al. 2020).

Chemical control of weeds using herbicides often requires less labor than does mechanical treatments, can be more selective against undesirable plants, and involves minimal soil disturbance (Vallentine 2004; Whisenant 1999). Choice of herbicide and application rate should be project specific. Use of nonselective herbicides on revegetation sites can result in greatly reduced biodiversity through loss of native forb and shrub species. Chapter 10 of Vallentine (2004) provides an overview of the benefits and disadvantages of herbicides and procedures for choosing and applying them.

Timing is critical for seeding. Seeds with no dormancy requirements (see “Seed Quality”) should be planted during or before the season of peak precipitation. Topsoil replacement, tillage, grading, or contouring should be completed just before seeding to minimize time for erosion or weed invasions before seeds germinate. For revegetation, seeding with a rangeland drill is considered superior to broadcast seeding (Bernstein et al. 2014; Knutson et al. 2014). However, drill seeding may be difficult or impossible on rough or steep terrain. Stevens and Monsen (2004) provided a good overview of options for rangeland drills. St. John et al. (2008) described how to calibrate seeding rates on a Truax Rough Rider seed drill. Some seed drills allow the operator to adjust the depth of seed planting. Each species has its own ideal seeding depth of 0.2–2.5 cm, and smaller seeds should be planted closer to the soil surface (Houck 2009). A general rule is to bury the seed twice as deep as the seed diameter.

In broadcast seeding, seeds are spread across the soil surface by hand or with various specialized tools. Broadcast seeding usually leads to lower establishment rates than drill seeding but may be cheaper and can allow seeding on sites that are too rugged for a rangeland seed drill. Broadcast seeding success can be increased by raking or pulling a chain across the soil surface after seeding to improve seed-soil contact (Hardegree et al. 2016; Stevens & Monsen 2004). Hydroseeding is a form of broadcast seeding and erosion control that uses a pressurized waterborne mixture of seed, mulch, tackifier, and fertilizer or soil amendments. Hydroseeding can lead to highly variable germination depending on species and is therefore not recommended except for steep, rocky sites or sites where rapid plant establishment is more important than biodiversity (García-Palacios et al. 2010; Stevens & Monsen 2004). Stevens and Monsen (2004) discussed broadcast seeding, hydroseeding, and interseeding (see “Maintenance and Monitoring”).

Seed carriers (inert materials of uniform size) improve success during broadcast and drill seeding with mixtures of seeds of different sizes (Shaw et al. 2020). Use of rice hulls as described by St. John et al. (2012) may be the easiest and cheapest option, but the best seed carrier is one matched to the size and shape of seeds in a mix.

Unlike soil amendments, mulch is applied to the soil surface rather than incorporated into the soil. It may be necessary to apply mulch after seeding for erosion control and to protect seeds and seedlings. Erosion control is especially important on steeper slopes or in areas or seasons with high exposure to wind or water (Bainbridge 2007). Mulch improves seedbed conditions and may be applied even when soil erosion is not a primary concern (Norland 2015).

Organic mulches such as hay, straw, wood, and compost provide nutrients for plants during revegetation (Abd El-Mageed et al. 2018; Vo & Kinoshita 2020). The C:N ratio of the compost should be at least 12:1 and preferably higher to suppress weed growth (Norland 2015). Organic mulches should be certified weed free to avoid introducing weed seeds (Bainbridge 2007). Inorganic mulches such as rock or gravel may be useful in very dry climates to reduce evaporation but may also impede plant growth (Bainbridge 2007; Fehmi 2018). Hydromulching, or broadcast application of a slurry of mulch and water, is usually used on only very steep or rocky sites (Stevens & Monsen 2004; Tamura et al. 2017). Mulch application rates will vary by project. Table 3 shows typical application rates for a variety of mulch materials. When mulch is expensive, some is better than none at all (Bainbridge 2007), especially after broadcast seeding.

In addition to mulch, various structures or tools are available for erosion control. Erosion matting consists of flat mats anchored close to the ground or three-dimensional structures filled with seeds and soil (Broda et al. 2020). Many materials are used to make erosion mats; straw and coir (coconut fiber) fabrics are effective, economical, and biodegradable (Bainbridge 2007). The Natural Resources Conservation Service (2020c) has provided information on where and how to install erosion matting. Moench and Fusaro (2012) presented an overview of additional tools, including erosion fences, culverts, water bars, and check dams.

Successful revegetation can take years to decades, so seeding is usually only the first step. In many cases, more than one round of seeding will be necessary (interseeding). An interseeder or a rangeland drill can be used to plant seeds within existing vegetation (Sheley et al. 2009; Stevens & Monsen 2004).

Continued weed control is important in most projects and will most likely be accomplished with selective herbicide applications, mechanical treatments, or grazing targeted to patches dominated by invasive plants (see “Invasive Species Control”) (Porensky et al. 2018). Herbivory can also be a problem. One method for reducing wildlife or livestock herbivory is to construct herbivore exclosures (barriers) around revegetated areas until seedlings are established (Johnson 2007). However, construction of exclosures on rugged terrain or around large areas can be prohibitively expensive. In terms of livestock management, maintaining proper stocking rates and rotating livestock between pastures so that areas are not grazed repeatedly in successive years may effectively protect vegetation communities (Davies & Boyd, 2020). In many grasslands, effectively managed livestock grazing can improve nutrient cycling and plant diversity (Davies & Boyd 2020). Livestock grazing should generally be started no earlier than the second season after seeding (Stevens 2004).

Because seeds need water to germinate and grow, supplemental water may speed revegetation during dry years or in arid regions (Bainbridge 2007). However, irrigation is expensive, and excessive use can lead to saline soils or decreased plant survival (Yadav et al. 2011). Buried systems that slowly release water can be effective, such as deep pipe irrigation, vertical porous pipes or hoses, and perforated pipe buried in trenches (Bainbridge 2007; Van Epps & Stevens 2004).

Revegetation projects should be monitored after seeding to ensure that long-term goals are met. A control treatment can be very helpful. For example, leaving a small portion of a site nonirrigated can determine whether irrigation in the other portions actually improved plant establishment. Elzinga et al. (1998) published a good reference for monitoring techniques and methodologies.

Elevated soil salinity, which can occur as a result of the release of brines during oil and gas extraction, inhibits plants’ ability to take up water (Shaygan & Baumgart 2020), which can make revegetation very difficult at saline sites. Various types of salt ions can contribute to saline soils, including chloride, sulfate, carbonate, calcium, and magnesium. Soils with high sodium ion concentrations are called sodic soils (Rengasamy 2010). Sodic soils have poor structure, are impermeable to water, and are prone to erosion (Harris et al. 2005; Rengasamy 2010). Some soils are saline-sodic, meaning they have high levels of both sodium ions and other salts (Rengasamy 2010). Soil testing labs can test soil samples for salinity and sodicity. There are various techniques to reclaim saline and sodic soils, collectively referred to as brine-impacted soils. Commonly used methods include leaching, improving drainage, soil amendments, and planting halophytes (Dornbusch et al. 2020; Harris et al. 2005). Often a combination of methods is best.

Soil salinity may be caused or exacerbated by lack of soil drainage. One solution is leaching, which involves applying water or another fluid to soil to dissolve salts, usually through drip irrigation (Burt & Isbell 2005; Li et al. 2015). Leachate can be recovered with a pump for disposal or desalinization (Robertson et al. 2006). To reduce the need for leaching, networks of connected small soil berms, ditches, or terraces can be created before seeding to collect rainwater (Ffolliott et al. 1995; Weathers et al. 1994). Soil with poor drainage due to impermeable soil layers can be temporarily improved by deep tillage or ripping (Belew 2018) often in combination with other treatments such as planting halophytes or adding organic matter (Belew 2018; Provin & Pitt 2012). Subsurface drainage systems are another option. A series of buried porous pipes surrounded by gravel can divert brine into collection pools where the brine can be removed by pumps (Harris et al. 2005). None of these solutions is permanent, and none will mitigate sodic soils (Provin & Pitt 2012). When subsoils are saline or sodic and topsoils are not, digging or tilling soil should be avoided unless topsoil is salvaged first (Whisenant 1999).

One way to address sodic soils is to replace sodium with calcium by applying a calcium source. Gypsum is a common source of soluble calcium (Beukes & Cowling 2003). Provin and Pitt (2012) discussed gypsum and a variety of other soil amendments for sodic soils and provided typical application rates. To reclaim saline-sodic soils, calcium amendments can be combined with leaching, drainage improvements, and/or halophyte plantings. Brine-impacted soils often have low organic matter content, and organic matter amendments can be added (Leogrande & Vitti 2019; Litalien & Zeeb 2020). Some compost has high concentrations of calcium, potassium, and magnesium that can exchange for sodium in sodic soils (Belew 2018). In several studies, a combination of inorganic and organic soil amendments have been recommended for the greatest improvement in soil structure and chemistry (Melgar-Ramírez et al. 2012; Nolan et al. 2020).

Because halophytes are adapted to saline and sodic conditions, they can be especially useful for revegetating brine-impacted soils. Reclamation with halophytes is less disruptive to soil than are other methods (Litalien & Zeeb 2020). Species of halophytes recommended for revegetation in various oil- and gas-producing regions in the western United States are listed in Table 4. The method by which a halophyte species tolerates salt must be understood before such species can be used properly for revegetation. Accumulator halophytes accumulate salts in their tissues and should be harvested and removed from the site when they are mature so the salt does not return to the soils after the plants die (Litalien & Zeeb 2020). Removed plants can be used for compost, fodder, or biofuel (Litalien & Zeeb 2020). Excretor halophytes exude salt through glands on the surface of their shoots (Dassanayake & Larkin 2017). Depending on site conditions, much of this salt can be removed by wind and spread over a large area. Some species of halophytes simply use biochemical barriers to exclude salt from their roots without altering the salinity of surrounding soil. Some nonhalophytes, such as many cacti, can grow in moderately brine-impacted soils because they take up very little water (Litalien & Zeeb 2020). Flowers et al. (2022) provided information about various halophytes.

Various reclamation treatments can be used for hydrocarbon-impacted soils. A few relevant examples are on-site treatment with amendments to stimulate microbial decomposition of pollutants, soil excavation and off-site treatment with organic amendments (when on-site temperature or oxygen conditions are not conducive to microbial activity), or off-site treatment with slurry bioreactors to treat clayey soils (Pollard et al. 1994). Some effective soil amendments to treat soil hydrocarbon impacts are compost and biochar (Hussain et al. 2018; Tran et al. 2021). These amendments aid plant growth by decreasing soil C:N ratio (Robson et al. 2004). In very impacted soils, addition of nitrogen directly can also be helpful (Wei et al. 2020). Kirkpatrick et al. (2006) suggested adding nitrogen to achieve a ratio of 11:1 (hydrocarbon C:added N) for growth of several warm-season grasses.

Revegetation of oil or gas extraction areas may be accomplished by seeded plants that tolerate hydrocarbon-impacted soils. Some species can interact with soil microbes that break down hydrocarbons (Olson et al. 2001). One way to identify hydrocarbon-tolerant species adapted to a particular region is to observe which species grow naturally in nearby sites that have a history of hydrocarbon impacts, such as waste disposal basins (Olson et al. 2001). Which species perform best will depend on climate, soil type, and composition and concentration of hydrocarbons spilled. Some species that can be used in these areas are listed in Table 5. Combinations of compost or biochar amendments with plantings of hydrocarbon-tolerant species have been more effective for establishing plant cover than use of these treatments individually (Han et al. 2016; Hussain et al. 2018).