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Department of Civil and Environmental Engineering University of California at Davis Davis, CA 95616, USA.
There has been increased interest in the use of artificial recharge of groundwater to augment groundwater supplies, particularly, with reclaimed municipal wastewater and waters of impaired quality. Proposed guidelines for groundwater recharge with reclaimed municipal wastewater are discussed with special reference to controlling microbiological and trace organic contaminants.
Background information and the rational basis for the establishment of the guidelines are presented in this paper, which have been development by the State of California Department of Health Services assisted by several expert committees. The proposed groundwater recharge guidelines are in the review process and are expected to be adopted shortly as a part of the State of California’s revised Wastewater Reclamation Criteria (1).
Essay # Introduction to Groundwater Recharge:
Groundwater is a far more important water resource than is often realized. For the United States, the significance of the role of the groundwater as a component of national water use can be gleaned from the statistical studies of the U.S. Geological Survey as reported most recently for the year 1990 by Solely, Pierce and Perlman (2).
Total groundwater withdrawals were estimated as 80,600 million gallons per day (mgd), or 305 × 106 m3/d. Groundwater is used for industrial-mining categories that amounted 40.2 per cent, 39.1 per cent in public water supply, 38.1 per cent in irrigation-livestock. The use of reclaimed wastewater averaged about 750 mgd (2.8 × 106 m3/d), or 30 per cent more than during 1985 (2).
The growing need for water necessitates even greater use of this vast water resource. Thus, there has been increased interest in the use of artificial recharge of groundwater to augment groundwater supplies, particularly with reclaimed municipal wastewater and waters of impaired quality.
Groundwater recharge with reclaimed municipal wastewater and waters of impaired quality is an approach to wastewater reuse that results in the planned augmentation of groundwater for various beneficial uses. Because, in many locations, groundwater recharge with reclaimed municipal wastewater presents a wide spectrum of health concerns as well as concerns for the sustainability of groundwater recharge operations.
It is essential that water extracted from a groundwater basin for potable use be of acceptable physical, chemical, microbiological, and radiological quality. Main concerns governing the acceptability of groundwater recharge projects are that adverse health effects could result from the introduction of pathogens or trace amounts of toxic chemicals into groundwater that is eventually consumed by the public. Because of the increasing concern for long-term health effects, every effort should be made to reduce the number of chemical species and concentration of specific organic constituents in the applied water (3, 4).
The quality of source waters used in groundwater recharge has a direct bearing on operational aspects of the recharge facilities and also on the use to be made of the extracted water. A source control program to limit potentially harmful constituents entering the sewer system must be an integral part of any groundwater recharge project.
Extreme caution is warranted because of the difficulty in restoring a groundwater basin once it is contaminated. Additional cost would be incurred if groundwater quality changes resulting from recharge necessitated the treatment of extracted groundwater and/or the development of additional water sources.
The level of municipal wastewater treatment necessary to produce a suitable reclaimed wastewater for groundwater recharge depends upon the groundwater quality objectives, hydrogeological characteristics of the groundwater basin, and the amount of reclaimed water and percentage of reclaimed water applied.
Federal requirements for groundwater recharge in the context of wastewater reclamation and reuse have not been established in the USA. As a consequence, water reclamation and reuse requirements for groundwater recharge are regulated by the state agencies with a case-by-case determination.
Considerably higher wastewater treatment prior to groundwater recharge is advocated in California because of the health concerns related to potential chronic effects of trace organics and viruses upon human health. The proposed guidelines for groundwater recharge with reclaimed municipal wastewater rightly reflect cautious attitude toward such health concerns.
Storm water quality is affected by several factors, including rainfall quantity and intensity, the natural and anthropogenic characteristics of the drainage basin, time since the last runoff event, and the time of year. Constituents of concern in storm water, according to the National Research Council (5), include trace metals, organic compounds, pathogenic organisms, suspended solids, and in northern climates in the winter sodium and chlorides caused by road de-icing practices.
Irrigation return flows exhibit the widest variation in quality of the three potential source waters, and water quality characteristics beyond salinity and concentrations of nitrate are not well studied. Salt content can be a problem in arid and semi-arid regions, and suspended solids and trace element concentration including selenium, uranium, boron, and arsenic may also be of concern.
Pesticide residues may also pose problems in irrigation return flows. Although treatment processes are available to remove the constituents of concern to acceptable levels, treatment of irrigation return flows is not generally carried out and the cost effectiveness of doing so is questionable because of their large volumes (5).
Essay # Proposed Groundwater Recharge Guidelines/Regulations:
The proposed regulations for groundwater recharge with reclaimed municipal wastewater rightly reflect cautious attitude toward such short-term as well as long-term health concerns as discussed previously. Proposed guidelines/regulations (6) are shown in Table 1.
The regulations rely on a combination of controls intended to maintain a microbiologically and chemically safe groundwater recharge operation. No single method of control would be effective in controlling the transmission and transport contaminants of concern into and through the environment. Therefore, source control, wastewater treatment processes, treatment reliability, recharge methods, recharge area, extraction well proximity, and monitoring wells are all specified.
The requirements in Table 1 are specified by ‘project category’ which identify a set of conditions that constitute an acceptable project. An equivalent level of perceived risk is inherent in each project category when all conditions are met and enforced.
Main concerns governing the acceptability of groundwater recharge projects with reclaimed municipal wastewater are that adverse health effects could result from the introduction of pathogens or trace amounts of toxic chemicals into groundwater that is eventually consumed by the public.
Of the known waterborne pathogens, enteric viruses have been considered most critical in wastewater reuse in California because of the possibility of contracting disease with relatively low doses and difficulty of routine examination of reclaimed wastewater for their presence.
Thus, essentially virus-free effluent via the full treatment process (primary/secondary, coagulation/flocculation, clarification, Alteration, and disinfection, cf. Table 1) is deemed necessary by the California Department of Health Services for reclaimed wastewater applications with higher potential exposures, e.g., spray irrigation of food crops eaten uncooked, or most of groundwater recharge applications (Project Categories I, II, and IV in Table 1).
The wastewater treatment requirement in Table 1 is designed to provide assurance that reclaimed water is essentially pathogen-free prior to extraction from the groundwater. The pathogen e.g., enteric viruses, removal capabilities of an individual or a combination of treatment processes have been estimated by Hultquist, (7), and, as an example, the virus removals achieved by various combinations of wastewater treatment is shown in Table 2.
In addition to the treatment processes, passage through an unsaturated zone of significant depth (> 3 m) reduces organic constituents and pathogens in treated effluents. At low infiltration rates of less than 5 m/day in sands and sandy loams, the rates of virus removal are approximated by a semi-log plot (k = -0.007 log/cm) against infiltration rates, resulting approximately 99.2 per cent or 2.1 logs removal for 3 m depth soils.
The overall estimates for the removal of enteric viruses by the treatment processes, unsaturated zone, and horizontal separation (retention time in groundwater) as specified in the proposed criteria are shown in Table 3.
Trace Organics Removal:
The regulations intend to control the concentration of organics of municipal wastewater origin as well as anthropogenic chemicals that have an impact on health when present in trace amounts. Thus, the dilution requirements and the organics removal that are specified in project categories I and IV is Table 1 in to limit average concentration of unregulated organics in extracted groundwater affected by the groundwater recharge operation.
The concentration of unregulated and unidentified trace organics is of great concern since other constituents and specific organics are dealt with through the established maximum contaminant levels and action levels developed by the California Department of Health Services.
Approximately 90 per cent by weight of the organics comprising the total organic carbon (TOC) in treated municipal wastewater are unidentified (4). One of the health concerns related to the unidentified organics is that an unknown but small fraction of them are mutagenic.
Regulating the presence of trace amount of organics in reclaimed water can be accomplished by dilution using surface water or groundwater of less contaminated source. When reclaimed water makes up more than 20 per cent of the water reaching any extraction well for potable water supply, treatment to remove organics must be provided.
Because of lack of ideal measure for trace amount of organics in reclaimed water as well as in the affected groundwater, total organic carbon (TOC) was chosen, as a surrogate, to represent the unregulated organics of concern. Although TOC is not a measure of specific organic compounds, it is considered to be suitable measure of gross organics content of reclaimed water as well as groundwater for the purpose of determining organics removal efficiency in practice.
However, there is insufficient basis for the establishment of a gross organics standard for the recharge water that protects public health. The proposed regulations shown in Table 1 require that the groundwater recharge projects by surface spreading resulting in a 20-50 per cent reclaimed wastewater contribution at any extraction well (Category I), and the recharge project by direct injection resulting in a 0-50 per cent contribution (Category IV), must provide organic removal step sufficient to limit the TOC concentration of wastewater origin in extracted water to 1 mg/L.
Table 4 shows the maximum TOC concentration that may be allowed in the reclaimed wastewater, for a given per cent reclaimed wastewater contribution, to achieve no more than 1 mg/L TOC of wastewater origin in the extracted water.
The numbers in Table 4 assume that a 70 per cent reduction through the unsaturated zone and no TOC removal in the aquifer. The numbers associated with the direct injection were derived by dividing 1 mg/L TOC concentration by the fractional contribution of reclaimed water to native groundwater at the extraction point.
Thus, the numbers for the direct injection are 30 per cent of those for the groundwater recharge by surface spreading. In addition, direct injection projects would have to achieve a 70 per cent TOC reduction to compensate for the lack of unsaturated zone in the overall soil-aquifer treatment capability.
Inorganic chemicals, with the exception of nitrogen in its various forms, are adequately controlled by meeting all maximum contaminant limits (MCLs) in the reclaimed wastewater. By limiting the concentration of total nitrogen in the reclaimed water, detrimental health effects such as methemoglobinemia can be controlled.
In those recharge operations where adequate nitrogen removal cannot be achieved by treatment processes or passage through unsaturated zone, the criteria provide the alternative method such as wellhead treatment to meet the allowable total nitrogen concentration of 10 mg/L as N.
By limiting the concentration of total nitrogen in the reclaimed water, detrimental effect (e.g., methemoglobinemia) can be controlled. In those recharge operations where adequate nitrogen removal cannot be achieved by treatment processes or in unsaturated zone, the proposed Groundwater Recharge Criteria provides the alternative method to meet the allowable concentration of total nitrogen concentration of 10 mg/L as N.
Essay # Cost of Pre-Treatment and Recharge:
Because requirements for pre-treatment for groundwater recharge vary considerably from location to location, a range of pre-treatment processes are covered by cost estimates, so that rough preliminary information on costs are available for planning purposes. Culp (8) summarized pre-treatment and recharge costs for five sets of recharge conditions as shown in Table 5. In addition to liquid processing costs, sludge processing costs are included.
The lowest cost system is for secondary treatment in aerated lagoons followed by surface spreading. The next set of conditions is secondary treatment by an activated sludge process followed by surface spreading. The third example is one which closely corresponds to project category I in Table 1.
It includes activated sludge process, filtration, and granular activated carbon (GAC) adsorption. For direct injection (similar to Project Category IV), it includes activated sludge process, high-lime treatment, re-carbonation, flow equalization, filtration, ammonia removal by ion exchange, breakpoint chlorination, GAC adsorption, reverse osmosis, ozonation, and chlorination.