Soil Stockpile Age Does Not Impact Vegetation Establishment in a Cold, Arid Natural Gas Field

Land reclamation and habitat restoration efforts are critical to ensure land surface disturbance associated with natural resource extraction does not have permanent negative effects in Wyoming (Stahl & Curran 2017). Given that there is a high risk of failure associated with reclamation in arid environments, studies to understand and improve management practices are essential (Shackelford et al. 2021). Reclamation success in Wyoming oil and natural gas fields is often measured by above-ground vegetation characteristics such as species richness or percentage vegetation cover as a proxy for erosion control (Curran et al. 2013; Curran & Stahl 2015). In some instances, regulatory criteria also require evidence of self-sustaining vegetation communities, which is judged by confirming the presence of seed heads within reclamation areas (Curran et al. 2019). Although vegetation characteristics are typically the focus of regulatory criteria, soils are critical to the success or failure of reclamation outcomes, and soil management is paramount to the success of reclamation efforts (e.g., Heneghan et al. 2008).

In Wyoming’s oil and gas fields, suitable topsoil depth is identified prior to well pad construction, and topsoil is then stripped and stockpiled adjacent to disturbed areas and subsequently respread after exploration activities are complete (United States Department of the Interior & United States Department of Agriculture 2007). Although this disturbance may significantly alter soil properties (e.g., bulk density, soil structure, microbial communities), soil management related to oil and gas well pad reclamation typically does not include challenges associated with mining such as mixing with overburden or severe recontouring during the respreading phase. Nevertheless, understanding how soil management and handling affect vegetation recovery in oil and gas fields is critical to improving reclamation success but is not well studied. In an annotated bibliography containing peer-reviewed literature, technical reports, theses and dissertations, and other gray matter that may be relevant to oil and gas reclamation in the western United States, <10% of 300 documents examined soil stockpiles (Mann et al. in press). Within those few studies, the majority examined soil characteristics or processes, and most were related to mining rather than oil and gas with little emphasis given to how vegetation responds to various soil stockpile management techniques (Mann et al. in press).

In the Jonah Infill natural gas field of Sublette County, WY (hereafter Jonah Field), extraction had been primarily in the form of single vertical well pads until the last decade. In this scenario, well pads were generally built, drilled, and brought into production within a single season, allowing reclamation to occur within a year of breaking ground. With advancements in drilling technology (i.e., directional drilling), operators in the Jonah Field have transitioned to larger multiwell pads that may take more than 1 year to develop, therefore increasing the time necessary for soils to be stockpiled. Although research suggests stockpile depth may cause seeds from the native seed bank to lose viability (Buss & Pinno 2019), operators performing reclamation associated with natural gas development typically do not rely on existing seed banks since sites are actively seeded after topsoil stockpiles are spread back into reclamation areas. Previous research in the area suggests that major alterations in soil occur during the soil-stripping phase or during the respreading phase, with negligible changes occurring while soil remains in the stockpile (Mason et al. 2011). Because soils in stockpiles may lose biota dependent on plant carbon inputs if stored for long durations (>5 years; Birnbaum et al. 2017), stockpiles in the Jonah Field are seeded with native vegetation to prevent erosion and maintain soil health during storage.

As soil handling is one of the most expensive costs of reclamation associated with natural gas development (e.g., Chenoweth et al. 2010), understanding how vegetation can establish from stockpiles of various ages is critical to operators. In this study, we examined soil stockpiles ranging from 1 to 7 years old that had been seeded within the Jonah Field and that had not yet been respread for reclamation activities. As most stockpiles in the Jonah Field are respread within 5 years, we were able to obtain only 1 replicate each from 6- and 7-year-old stockpiles. We used a reclamation seed mix common to the Jonah Field and sought to examine whether stockpile age affected vegetation establishment in a greenhouse setting by examining cover values and the presence of seed heads.

All soils in this study were collected from the Jonah Infill natural gas field, which is isolated to a ∼10-km × 10-km area in Sublette County, WY (Fig. 1). The Jonah Infill natural gas field is within a sagebrush-steppe ecosystem located at 2,100 m, has an average of 39 to 52 frost-free days per year, and receives an average of 17.8 to 22.9 cm (7-9 inches) of precipitation per year. Temperatures in the Jonah Field vary greatly, with daily fluxes between −1°C and 26°C during the growing season, with even colder temperatures common in the winter months. More information about the area’s soil and climate characteristics may be found through the United States Department of Agriculture-Natural Resources Conservation Service’s Ecological Site Information System. A total of 6,514 acres have been disturbed in the Jonah Field in the form of well pads, pipelines, and roadways, with a total of 4,817 acres (74%) being under reclamation. Well pads in the Jonah Field average ∼5.3 acres in size.

All natural gas well pads in this study are in Ecological Site Description R034AC126WY, which consists of loamy soils. In the Jonah Field, suitable topsoil is stripped according to predisturbance soil mapping (typically ∼10 cm) and stockpiled adjacent to the well pad prior to well pad construction. Stockpiles remain in place until natural gas exploration activities are complete, at which time stockpiled soil is respread to initiate interim reclamation. Stockpiles in the Jonah Field are seeded with the same seed mix as used for reclamation in an effort to prevent erosion, combat weeds, and to keep soil processes active. This seeding occurs in late fall or early winter the same year stockpiles are formed using the same seed mix as is used for reclamation (see  Appendix A). Stockpiles in the Jonah Infill may vary in width and length based on the size of the well pad being developed, although they never exceed 2.5 m in depth.

In this study, soils from 17 total stockpiles were used with three replicates each from stockpiles aged 1 to 5 years and one each of 6- and 7-year-old stockpiles. Soil from each stockpile was removed from roughly 1.5 m (5 ft) below the surface of the stockpile using a backhoe and subsequently placed and sealed into 5-gallon buckets in the Jonah Field on October 30, 2019. This depth was chosen because it is within the lower one-third of the stockpile and is representative of soil that would be at or near the surface of the reclamation area after respreading and because vegetation was established on top of the stockpiles prior to gathering our samples. After soil was collected, it was transported in sealed 5-gallon buckets to Laramie, WY, where it was placed in a freezer controlled at −6.1°C at a University of Wyoming Agriculture Experimental Station for later testing to best simulate the Jonah Field, which is typically freezing throughout winter months. The soil remained in the freezer for ∼7.5 months until 2 weeks prior to seeding, at which point the soil was removed from the freezer and placed outside to thaw out.

This study used a split-plot design in which age class was the whole-plot treatment factor (three replicates of the 1- to 5-year-old age classes and a single replicate for the 6- and 7-year age class) and the subplot factor as time. Each replicate was subdivided into four pseudo-replicates (12.7-cm [5-inch] diameter plastic pots), and the pots were randomly placed within a 68-block grid (6 rows of 10 and 1 row of 8; Figure 2) within a greenhouse controlled at 21.1°C (70°F) in the daytime and 18.3°C (65°F) at night. The temperature in the greenhouse was within the daily temperature range of the Jonah Field during the growing season. Each pot was watered during the initial placement into the greenhouse and was not watered further in the study to simulate conditions in the Jonah Field, which receives the majority of the little precipitation it gets as snow in the winter months. Each pot was seeded with a native seed mix commonly used in the Jonah Field on June 23, 2020. The seed mix contained six grasses that are commonly drill seeded as well as nine forbs and four shrubs, which are typically broadcast seeded and raked over (see  Appendix A for seed mix information; this mix is used on stockpiles and for reclamation, with all seeds being sown at the same time). To mimic this, after the pots were filled three-fourths of the way with soil, grass seed was spread over the soil, and an additional one-fourth inch of soil was added. Then, forbs and shrubs were added and lightly raked over with a plastic fork.

Digital images were taken above the vegetation starting 4 weeks after the seeding date and subsequently every 2 weeks thereafter, for a total of four sets of images, with the last set being taken slightly 10 weeks after initial seeding. Nadir images (i.e., parallel to the surface) were taken at 0.7 m above each pot with a 14-megapixel Kodak Easyshare M532 handheld camera (Eastman Kodak Corporation, Rochester, NY, USA). This resulted in a ground sample distance of ∼0.25 mm, which is similar to imagery in previous peer-reviewed studies to assess vegetation on reclaimed sites (e.g., Curran et al. 2019, 2020). All images were cropped to isolate individual pots, and four photos representing each stockpile (one per each pot) were placed into folders labeled by the stockpile name and date of photo. Photos were then analyzed with a free image analysis software called “SamplePoint,” with 25 pixels being analyzed per image using a 5 × 5 grid (Booth et al. 2006). This resulted in 25 pixels being sampled within each pot, or 100 pixels over the four pots per each stockpile during each photo period. Previous research suggests 25 pixels per image provides statistically valid results in landscape-scale studies when sample size is adequate (Ancin-Murguzur et al. 2019). Pixels within each image were classified by setting “buttons” in SamplePoint to “vegetation” or “soil” across each image set. The total values were converted into percentage cover for each category (e.g., if 24 of 25 pixels were classified as “vegetation” and 1 of 25 pixels was soil, the pot was considered to have 96% vegetation cover and 4% soil cover). The images taken on the last day of the study were also assessed for seed head appearance.

There was a statistically significant increase in percentage vegetation cover across time (p < .0001), with a mean percentage increase of 10.8% in cover every 2 weeks (Figure 3). However, the mean increase over time was not statistically different among the age-classes (p = 0.1408, Figure 3). Percentage cover did not differ significantly among the age classes at any measuring event (p > .26 for every pairwise comparison of age classes). In addition, although statistical analyses were not conducted on seed head presence, all 68 pots within the study contained vegetation with seed heads present at the 10-week sampling period.

Our results do not support our hypotheses that soil stockpile age affects vegetation establishment or ability to set seed. Vegetation establishment is critical in ensuring a critical short-term reclamation goal of preventing erosion, while seed production is necessary to ensure a self-sustaining vegetation community to achieve longer-term reclamation goals (Herrick et al. 2006). Therefore, our results are likely to have important management implications and suggest operators in the Jonah Field can establish vegetation that will set seed at similar coverage rates on reclamation sites regardless of stockpile age for at least 7 years. In addition, our results suggest that vegetation establishment and ability to set seed is not reliant on the use of fertilizers or soil amendments within the Jonah Field.

Previous research has suggested that most soil degradation associated with stockpiles in oil and gas fields is a result of initial surface disturbance and respreading soil rather than when soil is maintained in the stockpile (Mason et al. 2011). It has also been shown that additional disturbance after respreading soil from stockpiles further exacerbates soil degradation (Mason et al. 2011; Wick et al. 2009). Although vegetation cover on stockpiles within the Jonah Field was not quantified in this study, all stockpiles were covered with enough vegetation to control erosion and provide stabilization at the time soil was collected. Therefore, it is likely carbon inputs from plants help maintain soil biotic functionality near the top of the stockpiles (Valliere et al. 2022). Although we did not specifically examine soil properties in this study, it is unlikely root biomass in stockpiles was as prevalent at a 1.5-m depth as it would have been near the stockpile surface. The lower third of the stockpile is likely to be at the surface of the reclamation area after respreading. Our findings suggest vegetation establishment in the lower third of stockpiles is not limited by stockpile age in the Jonah Field. With the advent of directional drilling, it may not always be certain when drilling will be complete on a given well pad since additional wells may be placed on a given well pad if flowback rates are high during exploratory drilling. Therefore, if vegetation establishment is unaffected by soil stockpile age, it is likely more beneficial to keep soil in stockpiles until disturbances associated with drilling are complete than it is to respread soil with potential for redisturbance. Not only does redisturbance exacerbate soil degradation, but it may also lead to destruction of successful reclamation.

Like other greenhouse studies, our study has limitations of being short-term and in a controlled environment. However, our results clearly show soil from the Jonah Infill natural gas field remains fertile in stockpiles for at least 7 years. This study would be enhanced by additional, longer-term studies in field conditions. Furthermore, because stockpiles greater than 7 years old were not available, additional research is needed to determine at what age, if any, soil fertility is lost in stockpiles in this environment. Other areas of stockpile research that may benefit operators and reclamationists associated with oil and gas development would be to expand a study like this across various climate regimes, as it is possible stockpiles act as cold storage areas in the Jonah Infill due to its extreme climatic conditions. Although the purpose of this study was to examine vegetation emergence in different aged stockpiles, there may be benefit in examining soil physical and chemical properties in different-aged soil stockpiles over time. Finally, although many species were seeded in this study, this study analyzed only the percentage vegetation cover because species-specific information was difficult to obtain until seed heads emerged late in the study and because it was not possible to have an exactly equal amount of seed from each species in each pot, as seed was obtained in premixed bags. Future studies to determine if specific vegetation species are affected by soil stockpile age may be useful.

We are grateful to Jonah Energy for funding this study. We thank Douglas I. Smith and Margaret J. Smith for assisting with seeding and image analysis. We are very appreciative of two anonymous reviewers and the editorial staff at Reclamation Sciences for the comments and suggestions that have improved this manuscript.

Seed mix used for reclamation in the Jonah Infill natural gas field.