Summary of Issues Strategies Benefits & Costs Key Uncertainties Additional Resources

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1 Summary of Issues Strategies Benefits & Costs Key Uncertainties Additional Resources KEY POINT: Applicability of deep well injection as a disposal option is entirely dependent on local geology. SUMMARY OF ISSUES Deep-well injection is a mature technology that involves the injection of liquid wastes into porous subsurface rock formations. Depths of the wells typically range from 1,000 to 8,000 feet. The rock formation receiving the waste must possess the natural ability to contain and isolate it. Paramount in the design and operation of an injection well is the ability to prevent movement of wastes into or between underground sources of drinking water (Mickley 2006). Desal concentrate is considered an industrial waste under the Clean Water Act. As a result, it must be injected via a Class I well per Underground Injection Control (UIC) regulations. This regulatory framework restricts the number of compliant well sites and requires more conservative well construction, which can increase the cost associated with this method of concentrate management. Site selection for deep well injection is dependent upon geologic and hydrogeologic conditions, and only certain areas are suitable for construction of Class I wells. Suitable underground strata capable of receiving the waste must be present and separated from any underground sources of drinking water by impermeable strata. Most favorable locations are generally in the midcontinental, Gulf Coast, and Great Lakes regions of the country. Further, it is essential that the well not be located in areas subject to earthquakes or in regions containing recoverable mineral resources such as ores, oil, coal, or gas. Any wells in the area in question, both operating and abandoned, must be investigated to assure that they are properly plugged to prevent migration of the waste to other aquifers. The salinity of the water in the aquifer formation should match or exceed that of the concentrate, such that the quality of the confined aquifer is not degraded. Suitable formations for injection often contain water with TDS concentrations in excess of 10,000 mg/l. Deep aquifers, which are used for deep well disposal, typically have very poor water quality and are not considered potential sources of drinking water. Deep well injection has been widely used for disposal of desal concentrate in Florida, and more recently in El Paso, Texas, which have some of the best geologic formations to 1

2 support deep well injection (Mickley 2006, Hutchison 2007). Deep-well injection is less common elsewhere in the U.S. due to combined regulatory and practical (i.e., cost) considerations. STRATEGIES In a recent USBR-sponsored report, Disposal: Practices and Regulation, Mickley et al. (2006) describes design and construction considerations for different concentrate management disposal options. The major themes associated with deep well injection are summarized below. Site selection Site selection is one of the most important steps, in developing an injection well. UIC regulations state: all Class I wells shall be sited in such a fashion that they inject into a formation which is beneath the lowermost formation containing, within ¼ mile of the well bore, an underground source of drinking water. (CFR, 1989b, p. 729) Site selection involves evaluation of many conditions; most important is the determination that the underground formations possess the natural ability to contain and confine the injected waste. The ability of properly designed and operated injection wells to provide long-term confinement makes deep well disposal an environmentally acceptable option. Rock formations such as sandstone are highly porous and are able to take in large volumes of liquid. Other rock formations such as shales and clays are essentially impermeable and act as confining layers that make it possible to dispose of liquids underground into porous strata and prevent migration of the waste water into potable water aquifers. Ground water quality usually deteriorates with increased depth. Although high purity deep aquifers do exist, water sources with low salinity and mineral content (freshwater) are typically located near the surface. Deep aquifers, which are used for deep well disposal, typically have very poor water quality and are not considered potential sources of drinking water. In addition to the existence of the necessary types of underground formation, it is essential that the well not be located in areas subject to earthquakes or in regions containing recoverable mineral resources such as ores, oil, coal, or gas. Any wells in the area in question, both operating and abandoned, must be investigated to assure that they are properly plugged to prevent migration of the waste to other aquifers. 2

3 Construction UIC regulations require that all Class I wells be cased and cemented to prevent the movement of fluids into or between underground sources of drinking water. The casing and cement used in the construction of each well should be designed for the life expectancy of the well. In determining and specifying casing and cementing requirements, the following factors should be considered (CFR 1989b): Depth to the injection zone Injection pressure, external pressure, internal pressure, and axial loading A Class I injection well is constructed in successive stages of drilling (or reaming), casing, and cementing until a well of the required depth (to reach the disposal formation) and diameter (to accommodate the required flow rate) is completed. If permitted by state regulations, existing wells from depleted oil and gas reservoirs could be used for concentrate injection, although their injection capacity would need to be evaluated, and the costs of transporting the concentrate may offset other cost savings. BENEFITS & COSTS Benefits When suitable geologic formations are available, deep-well injection can be the most viable and cost-effective option for concentrate management at inland desal facilities. The ability of properly designed and operated injection wells to provide long-term confinement makes deep well disposal an environmentally acceptable option. The availability of deep well injection as a disposal method provides potential for the implementation of large-scale desal at inland facilities. Over the long term the high capital investment associated with deep well injection may be partially offset by relatively low operating costs. Costs The process of designing and constructing a Class I injection well, including corrosion resistant materials, hydrogeologic investigations, drilling, and testing, can be very 3

4 expensive. In particular, the time and cost associated with determining feasibility of deep well injection is greater than for other disposal options. The costs associated with deep well injection are primarily labor and testing. Deep-well injection is typically employed for larger desal plants (e.g., > 3,800 m 3 /day [>1 mgd]) because the costs for developing deep-injection wells are relatively high and are not largely reduced for smaller flows. In general, this disposal method is subject to more significant site-specific availability and cost variation than any other method of concentrate management (Mickley, 2006). Regulatory restrictions limit the number of compliant well sites and require more conservative well construction, which increases the costs associated with this method of concentrate management. KEY UNCERTAINTIES Permit requirements for deep well injection are becoming more stringent due to potential adverse effects associated with leakage from the wells. If the injection aquifer is not adequately separated from the water supply aquifer in the area, the groundwater supply may be contaminated by the injected concentrated pollutants (Xu et al. 2009). The injection wells may also experience potential scaling and decrease of well discharge capacity over time (WHO 2007). ADDITIONAL RESOURCES AMTA Disposal of Desalting By-Product. Publication FS-4, February Hutchison, W El Paso Brackish Groundwater Desalination Plant: Initial Operation. In Proceedings of the Water Reuse & Desalination Conference. Sept. 9-12, 2007, Tampa, Fl. WateReuse Association. Mickley, M Review of Options. Available: < Mickley, M Membrane Concentrate Disposal: Practices and Regulation. USBR Desalination and Water Purification Research and Development Program Report No. 123, 2 nd Ed. April Available: < 4

5 NRC (National Research Council) Desalination: A National Perspective. Washington, D.C.: National Academy Press. Available: < WHO (World Health Organization) Desalination for Safe Water Supply: Guidance for the Health and Environmental Aspects. Public Health and the Environment. Applicable to Desalination. World Health Organization. Available: Xu, P., Cath, T, Wang, G., Drewes, J.E. and Dolnicar, S. (2009). Critical assessment of implementing desalination technology. AwwaRF Project Published by American Water Works Association Research Foundation, Denver, CO. 5