Pits

The geology and hydrology across the state is extremely variable. Even across much smaller areas such as a county, large differences occur. Texas Water Development Board maps show the major and minor aquifers in Texas. The minor aquifers, while yielding water in smaller quantities or in smaller areas than the major aquifers, are in some areas the only source of water supply. While it is impossible to delineate in this manual all the geologic, hydrologic or soil conditions that exist across the State, these factors will be evaluated on a case by case basis as an application to maintain and use a particular pit is received by the Railroad Commission in Austin. The applicant will be required to determine the distance to the nearest water well within one mile of the pit, the depth of that well, and the depth to the shallowest fresh water. Depending on the type of pit construction, a description of the soil material may also be required.

As previously stated, a detailed description of all geologic and hydrologic conditions across the State is not intended. Figures 2 through 5 (pdf) show surface pits located in geologic and hydrologic environments with the least potential for ground water contamination.

In Figure 2 (pdf), the pit shown extends below the water table and will cause local contamination. The extremely slow movement of the ground water would prevent any significant contamination. Typical ground water flow rates through the clayey sediments under low hydraulic gradients are 0.1 to 0.5 foot per year. The Texas Coastal Plain is composed of clays, silts and sands, and the water table is generally within 10 feet of the surface in areas other than near cities, industrialized areas or heavily irrigated areas. The sands and silts are found in old stream and river channels. On-site investigation is necessary to avoid sand and silt zones in the clays. The primary concern is contamination of surface water from rainfall traversing the pit and washing the pit contents into surface water drainageways.

The bottom of the pit shown in Figure 3 (pdf) will also intersect the water table during wet seasons. Water movement, however, is very slow, and the dissolved mineral concentrations may be altered by ion exchange. The depth of the water table in the clay belts is difficult to measure. Formations appear to be saturated because of capillary action above the actual water table. In dry seasons, the water table will be below the bottom of the pit and the relatively impervious clays will retard any significant seepage of pit waste to the water table. Properly located pits away from surface drainage on broad upland flats and on divides are preferred.

Figure 4 (pdf) shows a locality where alternating beds of clays, mudstones, marls and shales comprise most of the sediments. These are generally good sites for placement of pits. Some of the central and north-central Texas area, however, is covered by alluvial sands and gravels or stream alluvium and terraces. Exposures of bedrock generally occur on the sides of broad valleys, between terraces and in upland areas. Locations with sandstones in the sediments, the sandy surficial deposits, stream alluvium and terrace deposits should be avoided, if possible, when selecting a pit site. Placement of unlined pits in these materials could result in contamination of surface and ground water. Much of north-central Texas uses surface water sources, although some ground water is produced from sandstones, and stream deposits. The water table is generally less than 100 feet below the surface. The excavation of thin surficial deposits, which in many areas is less than three feet thick, is easily performed and may be necessary to expose suitable material for pit placement.

The characteristics of the soil at the pit site are a factor in pit construction. Soil characteristics are classified by several methods which fall into two general categories, agriculture and engineering. The behavior of a soil as an engineering construction material is the concern in pit construction. The primary engineering properties of soils are their texture and their ability to be molded and compacted, called plasticity. For most pits, soils from the excavation will be used to build the dikes. Therefore, the native soil must be compactible to prevent seepage and erosion. The soils exposed to the bottom and sides of the pit must also be reworked either to form a suitable base for a liner or to retain the fluids in the pit if it is unlined. Often a pit may be deeper than the surface soil layer, which generally is no greater than three or four feet thick, and penetrate the subsoil or the unweathered sediments. The subsoil material may be much coarser and more permeable than the soil and be unsuitable for pit construction.

Generally, information on the occurrence of soils from an agricultural standpoint are available from the Natural Resources Conservation Service (NRCS) and the NRCS county-based office. While data from these sources contain much useful information on the thickness, composition, and permeability of undisturbed soils, site-specific data is often necessary to make good engineering judgments.

Field identification of soil samples is the best, and sometimes the only way to gain sufficient information. Again, it is important to know the type of material that will compose the pit bottom and dikes. Any on-site sample should be representative of these materials. Explanations of field techniques for determining the texture and plasticity of soils are available. When necessary, the services of a soils engineering firm may be utilized.

The pit design and construction techniques described below more frequently apply to pits proposed for long-term, continuous use that must be authorized by permit. Pits that are authorized by rule such as basic sediment pits and completion/workover pits generally are less sophisticated in design. Nevertheless, good judgment is necessary to avoid seasonal problems of flooding or high water table. If such problems are prevalent, then fabricated tanks should be utilized.

All earthen dikes surrounding pits should be constructed of soil material which is capable of achieving a permeability of 1 x 10^-7 cm/sec or less when compacted. During construction, successive lifts should not exceed nine inches in thickness, and the surface between lifts should be scarified to achieve a good seal. The dike height and width should be consistent with the volume of wastewater to be retained. Where wastewater is retained in above ground pits, which will be discussed later, it is recommended that the top width of the dike be at least four feet and the side slopes not be steeper than 3 to 1 (three feet horizontal to one foot vertical). Dikes for all pits are "keyed" into the underlying soil to achieve a good seal between the ground and the bottom of the dike to prevent lateral seepage of wastewater through the base of the dike. (see Figures 5, 6, 7, and 8) (pdf)

Two of the most common construction methods for pits are "above ground" and "below ground". The above ground pit should be used in areas where the water table is high. The above ground method consists of constructing dikes around the area without excavating below the surface (see Figure 5) (pdf). The dike material can be obtained entirely from a borrow pit as in the case of Pit A, or the area intended for the pit can be excavated slightly and leveled to obtain material for the dikes (Pit B). Pits A and B are not lined because the underlying soil is relatively impermeable (K< or = 1.0 x 10^-7 cm/sec) and of sufficient thickness (>2 feet) to inhibit seepage into the ground water. Because the wastewater level in this type of pit will be above ground level a breach in the dike might result in contamination of surface waters. Therefore, it is very important that the dikes around the above ground pit be properly engineered and constructed. Any pit designed to store waste above ground level must be designed under seal of a professional registered engineer. Pits C and D in Figure 6 (pdf) are constructed in the same manner as those shown in Figure 5 (pdf) except that they are lined. The underlying sediments are relatively permeable (K> 1.0 x 10^-7 cm/sec) and clays in the soil are not of sufficient thickness to prevent seepage of wastewater into the ground water.

The "below ground" method of constructing a pit consists of excavating an area and building dikes around the excavation (see Figure 7 (pdf). The below ground pit should be used in areas where the water table is well below the surface. The pits in Figure 7 (pdf) are not lined because the underlying material is relatively impermeable and of sufficient thickness to inhibit seepage to the water table. Pit E is the preferred method of construction. The wastewater level in this pit will under normal conditions be at ground level or below. Therefore, the probability is low of wastewater flowing into area drainage if the dike is breached. The wastewater level in Pit F normally will be above ground level, requiring greater care in the engineering and construction of the dikes. The pits shown in Figure 8 (pdf) are lined because of the permeable underlying soils.

A liner is a continuous layer of materials that serves to restrict the release or migration of oilfield fluids, oil and gas wastes, or waste constituents. A liner may be constructed of synthetic or natural materials.

A liner must:

• Have a permeability low enough to contain the material.
• Be chemically compatible with the material with which it is expected to come into contact.
• Be mechanically compatible with the operation.
• Be capable of maintaining its integrity over time.

Flexible membrane liners, or geomembranes are impervious thin sheets of synthetic polymer material. The most widely used geomembranes are high density polyethylene (HDPE), polyvinyl chloride (PVC), very low density polyethylene (VLDPE), and chlorosulfonated polyethylene (CSPE); of these, HDPE is used the most.

Commercially available geomembrane liners should all have sufficiently low permeability (typical permeability values are 0.5 x 10-10 cm/sec or less); therefore, the concern is that the liners be chemically compatible with the material to be stored and possess physical and mechanical properties which enable them to survive installation and operation. Other important considerations are resistance to ultraviolet degradation and thermal degradation. The American Society for Testing and Materials (ASTM) has adopted a number of standards for geomembrane liners. Other ASTM standards for plastics and rubber may be applicable, as well as EPA developed test methods. In general, the operator will need to rely upon testing which has been performed by the manufacturer. The manufacturer should be able to provide results of verification testing which has been performed by an independent laboratory. The verification testing should have been performed on the actual batch of liner material to be used.

Chemical Compatibility

Chemical compatibility is of prime concern when selecting a liner. Manufacturers and vendors have evaluated the resistance of their liners to many chemicals. These are available in the form of chemical resistance charts. If the fluid to be contained is not on the chart, or consists of a mixture of chemicals, compatibility testing of the specific fluid/liner combination may be warranted.

Mechanical Compatibility

The geomembrane must first survive transportation and installation, avoiding tear, puncture, and impact. Deformations caused by settlement of subgrade soils and backfilled zones beneath the liner are the prime mechanical consideration in the selection of a liner. Pit liners may also be subjected to weather, inflow and outflow of fluids, abrasion, hydrostatic pressure, thermal stress, cleaning and maintenance stress, and exposure to animals. Geomembrane manufacturers should provide results of testing for mechanical properties such as tensile properties, tear resistance, impact resistance, and puncture resistance. Mechanical properties are proportional to liner thickness, therefore, thicker liners are correspondingly more resistant to mechanical damage.

Maintain Integrity over Time

In addition to chemical and mechanical degradation as noted above, liners are also subject to thermal and ultraviolet degradation. Ultraviolet degradation occurs due to natural sunlight; thermal degradation occurs at both high and low temperatures. ASTM test methods are available to evaluate these properties and manufacturers should be able to provide test results. Exposed liners must contain carbon black or other pigments to minimize ultraviolet degradation. Liner degradation can be limited by initial selection of the proper liner and the use of appropriate operating procedures during the life of the pit. Earth covers may be used to protect liners from sunlight and mechanical damage, however, this does not allow for visual inspection of the liner.

Installation

Compaction of the subgrade soil is required to provide a firm and unyielding base for the lining materials. Generally, a fill subgrade is built-up in a series of compacted layers, while an excavated subgrade is compacted only at its surface. The regularity and texture of the surface of the uppermost layer of compacted material is critical prior to liner installation. After compaction has been completed, the surface should be finished. This is best accomplished by looking for and removing rocks, debris or irregularities with sharp edges. Drags, vibrating rollers and smooth steel rollers are common tools for fine finishing the bottom and sidewalls of the pits. The fine finishing process is dependent on care and control of water. Liner placement should be halted when rainfall is imminent, and the placement of liner materials on the fine finished slopes should take place as soon after the completion of "finishing" as possible to ensure that no surfaces are lost to the erosive effects of surface runoff (see Figure 9 (pdf)).

The most likely place for a leak to occur is at the seams. The number of seams should be minimized. Seams should be oriented up and down, not across a slope. Factory seams should be used where possible. All field seaming should be performed by qualified personnel.

Compacted soil liners are constructed of soils containing clay, which are spread and compacted in layers or lifts. These liners are not generally appropriate for the primary liner in pits containing produced water, brine, or other mineralized waters, but may be appropriate for certain applications. Soils classified as CL, CH, or SC in the Unified Soil Classification System are typically used to construct liners. To be adequate, a compacted soil liner should have a thickness of two feet or more and a hydraulic conductivity of 1 x 10-7 cm/sec or less. Compaction to 95% standard Proctor at a soil moisture content of 2 to 3% wet of optimum is appropriate. A primary advantage of compacted soil liners is their ability to self-seal when the liner is penetrated.

OTHER

Other liners may be suitable in some circumstances. These lining materials may include concrete and asphalt concrete, as well as blended or admixed liners of soil and cement, soil and asphalt, and soil and bentonite. The acceptability of any liner depends on the climate and hydrogeology of the pit location, and the intended pit use.

Leak detection systems for synthetic liners will be required for brine and brine mining pits, are recommended for disposal pits, and should also be used for other pits in a hydrologically sensitive area. Some common underdrain systems consist of a gravity flow drainfield and collection points installed either under the pit liner with above a secondary liner or outside of the pit.

Pits with leak detection systems must define what flow rate into the leak detection system constitutes a primary liner failure. This flow rate is called the Action Leakage Rate. Should the leak detection system indicate a liner failure, the appropriate District Office must be notified and the pit must be emptied, inspected, and repaired, or the pit must be closed. A primary liner failure may be defined as any presence of fluid in the leak detection system or a an exceedance of the calculated Action Leakage Rate (ALR). Calculating an Action Leakage Rate is considered the practice of engineering and must be prepared under a Professional Engineer licensed in Texas. An ALR calculation must include all potential flow bottlenecks. The ALR should not exceed 1,000 gallons per acre per day (GPAD) for surface impoundments or 100 GPAD for landfills as these rates indicate a major localized or general failure of top liner according to Action Leakage Rates for Leak Detection Systems, EPA 530-R-92-004.

Figure 10 (pdf) illustrates a sloping drainfield of coarse porous material that allows wastewater that has passed through the pit liner to accumulate at the low end of the pit above a secondary liner. A riser pipe perforated or slotted in the drain material interval extends from the collection point to the ground surface or along the sloping pit wall and berm. By extending a tape or long, small diameter, cylindrical rod into the riser pipe, the presence of wastewater in the pipe can be determined. This type of underdrain system can be used for relatively deep pits.

Figure 11 (pdf) also illustrates a sloping drainfield of coarse, porous material that allows wastewater that has passed through the pit liner to accumulate at the end of the pit above a secondary liner. A drain pipe connects the collection point to a sump or trench constructed outside of the pit. The presence of wastewater in the sump or trench indicates a leaking liner. This type of underdrain system can be used for above ground or slightly excavated pits. Manufacturers of the various artificial liners also may have information on leak detection systems.

Geonets are highly efficient at transmitting fluids in the plane of the net and can be used to replace granular drainage layers in leak detection systems. Geonets are geosynthetic drainage materials composed of layers of parallel ribs or strands overlying similar layers at various angles. Virtually all commercially available geonets are made of HDPE. Geonets are available from manufacturers and vendors of geomembrane liner systems. Manufacturers should be able to provide test results and information on mechanical and hydraulic properties. When installed, geonets must have both surfaces covered with a geomembrane, geotextile, or other suitable material to prevent intrusion of soil into its openings. Geonets occupy less vertical space in the excavation and are typically easier to install than granular drainage layers.

Regular inspections should be made of the embankment and the berms. In particular, attention should be given to possible ground movement, cracks and erosion of the earth because failure of the earthwork can result in failure of the liner. Damage to the liner can also result if weed growth begins under the liner, or if a soil cover is present, on top of the liner. The berm area around the pit should be treated with a weed killer initially and maintained in a weed-free condition.

Operational procedures which may endanger the integrity of a lined pit are:

1. discharge of high-temperature waste liquids onto exposed or unprotected liners;
2. operation of a vehicle over any portion of an exposed liner;
3. discharge of a waste to the pit that is incompatible with the liner; and/or
4. direct discharge of liquid waste under high pressure.