Proceedings of the International Symposium on Efficient Water Use in Urban Areas


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<Proceedings of the International Symposium on Efficient Water Use in Urban Areas
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GROUNDWATER RECHARGE WITH RECLAIMED MUNICIPAL WASTEWATER
- REGULATORY PERSPECTIVES -

Takashi Asano, Ph.D., P.E.
Department of Civil and Environmental Engineering
University of California at Davis, Davis, CA 95616-2311, U.S.A.

ABSTRACT

Groundwater recharge with reclaimed municipal wastewater presents a wide spectrum of technical and health challenges that must be carefully evaluated. In this paper, regulatory aspects of groundwater recharge with reclaimed municipal wastewater are reviewed. Rationale and scientific basis for the proposed California groundwater recharge regulations are discussed. At present, several health constraints limit expanding use of reclaimed municipal wastewater for groundwater recharge when a large portion of groundwater contains recharged reclaimed wastewater that may affect domestic water supply. Two case histories are presented for the soil aquifer treatment and non-potable water reuse with less stringent water quality requirements.

KEYWORDS

Groundwater, pathogens, planning, public health, recharge, water quality regulations, water resources, wastewater reclamation and reuse, wastewater treatment.

INTRODUCTION

Natural replenishment of the vast supply of underground water occurs very slowly; therefore, excessive continued exploitation of groundwater at a rate greater than this replenishment causes declining groundwater levels in the long term and if not corrected, leads to eventual mining of groundwater. To increase the natural supply of groundwater, artificial recharge of groundwater basins is becoming increasingly important in groundwater management and particularly in situations where the conjunctive use of surface water and groundwater resources is being considered.

Groundwater recharge with reclaimed municipal wastewater is an approach to water reuse that results in the planned augmentation of groundwater for various beneficial uses. The beneficial uses are the many ways water can be used, either directly by people, or for their overall benefit. Groundwater is used as a source of water supply, and its major beneficial uses include municipal water supply, agricultural irrigation, and industrial water supply. The purposes of artificial recharge of groundwater have been (Bouwer, 1978; Todd, 1980; Asano, 1985):

To reduce, stop, or even reverse declines of groundwater levels
To protect underground freshwater in coastal aquifers against saltwater intrusion from the ocean
To store surface water, including flood or other surplus water, and reclaimed municipal wastewater for future use.

Groundwater recharge is also incidentally achieved in land treatment and disposal of municipal and industrial wastewater via percolation and infiltration.

There are several advantages to storing water underground:

The cost of artificial recharge may be less than the cost of equivalent surface water reservoirs
The aquifer serves as an eventual distribution system in underground and may eliminate the need for transmission pipelines or canals for surface water
Water stored in surface reservoirs is subject to evaporation, potential taste and odor problems due to algae and other aquatic productivity, and to pollution; these may be avoided by underground storage
Suitable sites for surface water reservoirs may not be available or environmentally acceptable
The inclusion of groundwater recharge in a wastewater reuse project may provide psychological and esthetic benefits as a result of the transition between reclaimed municipal wastewater and groundwater. This aspect is particularly significant when a possibility exists in the wastewater reclamation and reuse plan to augment substantial portions of domestic or potable water supplies.

TECHNIQUE OF GROUNDWATER RECHARGE

Two types of groundwater recharge are commonly used with reclaimed municipal wastewater: surface spreading or percolation, and direct injection.

Groundwater Recharge by Surface Spreading. Direct surface spreading is the simplest, oldest, and most widely applied method of artificial recharge (Todd, 1980). In surface spreading, recharge waters such as treated municipal wastewater percolates from spreading basins through the unsaturated groundwater (vadose) zone. Infiltration basins are the most favored methods of recharge because they allow efficient use of space and require only simple maintenance. In general, infiltration rates are highest where soil and vegetation are undisturbed.

Where hydrogeological conditions are favorable for groundwater recharge with spreading basins, wastewater reclamation can be implemented relatively simply by the soil-aquifer treatment (SAT) process. The necessary treatment can be obtained by the filtration process as the wastewater percolates through the soil and the vadose zone, down to the groundwater and then some distance through the aquifer. Recommended pretreatment for municipal wastewater for the SAT process includes primary treatment or stabilization pond. Pretreatment processes that leave high algal concentrations in the recharge water should be avoided. Algae can severely clog the soil of infiltration basins. While renovated wastewater from the SAT process is much better water quality than the influent wastewater, it could be lower quality than the native groundwater. Thus, the SAT process should be designed and managed to avoid encroachment into the native groundwater and to use only a portion of the aquifer. The distance between infiltration basins and wells or drains should be as large as possible, usually at least 50-100 m to give adequate soil-aquifer treatment (Bouwer, 1978 and 1988).

The advantage of groundwater recharge by surface spreading is:

Groundwater supplies may be replenished in the vicinity of metropolitan and agricultural areas where groundwater overdraft is severe
Surface spreading provides the added benefits of the treatment effect of soils and transporting facilities of aquifers.

Direct Injection to Groundwater. Direct subsurface recharge is achieved when water is conveyed and placed directly into an aquifer. In direct injection, generally, highly treated wastewater is pumped directly into the groundwater zone, usually into a well-confined aquifer. Groundwater recharge by direct injection is practiced:

Where groundwater is deep or where the topography or existing land use makes surface spreading impractical or too expensive
When direct injection is particularly effective in creating freshwater barriers in coastal aquifers against intrusion of saltwater.
Both in surface spreading and direct injection, locating the extraction wells as great a distance as possible from the spreading basins or the injection wells increases the flow path length and residence time of the recharged water. These separations in space and in time contribute to the mixing of the recharged water and the other aquifer contents, and the loss of identity of the recharged water originated from municipal wastewater. The latter is an important consideration in successful wastewater reuse facilitating public acceptance.

In arid climates where the practice of groundwater recharge is most imperative, recharge will occur through such means as dry riverbeds and spreading basins, and in most situations there will be an unsaturated zone between the surface and the aquifer.

PRETREATMENT FOR GROUNDWATER RECHARGE

Four water quality factors are particularly significant in groundwater recharge with reclaimed wastewater: (1) microbiological quality, (2) total mineral content (total dissolved solids), (3) presence of toxicant of the heavy metal type, and (4) the concentration of stable organic substances. Thus, groundwater recharge with reclaimed wastewater presents a wide spectrum of technical and health challenges that must be carefully evaluated. Some basic questions that need to be addressed include (Asano and Wassermann, 1980; Roberts, 1980; NRC, 1994):

What treatment processes are available for producing water suitable for groundwater recharge?
How do these processes perform in practice?
How does water quality change during infiltration-percolation and in the groundwater zone?
What do infiltration-percolation and groundwater passage contribute to the overall treatment system performance and reliability?
What are the important health issues?
How do these issues influence groundwater recharge regulations at the points of recharge and extraction?
What benefits and problems have been experienced in practice?

Pretreatment requirements for groundwater recharge vary considerably, depending on purpose of groundwater recharge, sources of reclaimed wastewater, recharge methods, and location. Although the surface spreading method of groundwater recharge is in itself an effective form of wastewater treatment, a certain degree of pretreatment must be provided to untreated municipal wastewater before it can be used for groundwater recharge.

A variation of the SAT process in Ben Sergao (a suburb of Agadir, Morocco) is described in the following Case History 1. The pilot study is of interest not only for the Greater Agadir where water resources are limited but also for a number of cities in Morocco where reuse of treated wastewater constitutes an essential option in wastewater treatment and disposal. Case History 2 deals with infiltration percolation as a tertiary treatment to meet the World Health Organizations (WHO) microbiological standards applying to unrestricted agricultural reuse (WHO, 1989).

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