Planning and investigations for groundwater recharge using wetland-treated sewage effluent

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1 Future Groundwater Resources at Risk (Proceedings of the Helsinki Conference, June 1994). IAHS Publ. no. 222, Planning and investigations for groundwater recharge using wetland-treated sewage effluent JUDITH L. HAMILTON Groundwater Section, US Bureau of Reclamation, PO Box 25007, D-S721, Denver, Colorado , USA Abstract Increased water requirements due to expanding population coupled with a limited water supply have necessitated innovative methods for reuse of water in southern California. In the San Jacinto valley east of Los Angeles, Eastern Municipal Water District in cooperation with the U.S. Bureau of Reclamation is developing a plan for reuse of treated sewage effluent using artificial wetlands for final treatment. Pilot and demonstration wetlands have been developed with an extensive research programme underway. Additional investigations are determining localities for spreading or injection where geohydrologic conditions are favourable and where the stringent California State requirements can be met. The treated water will then be used to replenish and improve the quality of groundwater in the vicinity. This will provide reserve supplies for future growth and times of drought. When complete, the project will recharge 4000 to m 3 day" 1 treated effluent. INTRODUCTION The US Bureau of Reclamation (Reclamation) is well-known for its expertise in designing and constructing dams and irrigation projects in the western United States. The major mission of Reclamation has now shifted from development of large-scale facilities to water resources management. The historic emphasis on surface water has changed to place more importance on groundwater supplies. Groundwater projects usually involve much smaller areas than major surface water projects ; consequently local water districts and water management groups have a major role in their development. Reclamation is now involved in co-operative projects with these groups. An example is the partnership arrangement between Reclamation and Eastern Municipal Water District (EMWD) to complete a multipurpose wetlands project. One goal of this project is to evaluate wetlands treatment with respect to groundwater recharge potential. Other objectives include assessment of wildlife habitat, public values, and cost-effectiveness of constructed wetland systems. BACKGROUND History of EMWD The EMWD service area is in the San Jacinto and Santa Margarita River Basins about 110 km east of Los Angeles, California (Figs 1 and 2). It encompasses almost 1400 km 2

2 140 Judith L. Hamilton OROVtLLE DAV AND RESERVOIR SAN FRANCISCO.:^ \ SAN LUIS DA et gf» AND RESERVOIR e» - " " COLORADO RIYER \ AQUEDUCT» i." *^LA*t PER RIS f* L LOS ANGELES : : : : :'-: : \i. A K fsfrivenside COUNTY { '^MM%^\^^rfT???^^^^M\ ;:.SAN..aERNARbiNO CNTYSSS O PALM SPR1NQS tsan JACINTO HEMET EASTERN MUNICIPAL WATER DISTRICT RIVERSIDE COUNTY 7 O'ïKXftSAN DIEGO CNTY ilmperjal.cnty.:: "?» z t - Fig. 1 Location map. Virtually all of EMWD groundwater production is from the San Jacinto Basin. Groundwater levels in this and other basins within the service area have declined considerably, and recharge would aid in maintaining groundwater levels and future water supplies. EMWD has 5 regional water reclamation plants treating over m 3 of municipal wastewater per day. With the projected growth for the area, treated effluent could increase to over m 3 day" 1 by Much of the treated water is used to irrigate agricultural land. There are presently over 61 km of pipeline for reclaimed water, which is supplied to 3380 ha of irrigated land. Under the proposed facilities

3 Groundwater recharge using wetland-treated sewage effluent 141 Eastern "Water Riverside Coiiniy, Municipal District Calîlortûa BECLAIME» WATER FACILITIES EXPLANATION at Treatment Facility ^ Proposed Wetland Demonstration Site kilometers Existing Pipeline " Pipeline under construction Proposed pipeline Fig. 2 EMWD boundaries and facilities. development, potentially ha of land could be brought under irrigation. Use of reclaimed water for irrigation of these additional lands as well as for groundwater recharge would decrease the amount of imported water required.

4 142 Judith L. Hamilton EMWD-USBK, cooperation The EMWD wetlands treatment project is a part of a small loan project funded by Reclamation. Reclamation also provides technical assistance to EMWD. The entire project will involve construction of additional treatment and conveyance facilities. The wetlands aspect of the project involves a wetlands research facility for ongoing wetlands research, a demonstration wetlands to investigate the potential for treating water using wetlands, and a recharge site to replenish ground water. A saline marsh using brine from a reverse osmosis plant is also a part of the project. Purpose of wetlands programme A major goal of the proposed pilot and demonstration project is to develop design, construction and operational criteria for future multipurpose constructed wetlands for environmental enhancement and water treatment. Some of the wetland-treated water will be utilized directly for agricultural and other irrigation purposes; however, in the winter months EMWD production of reclaimed wastewater exceeds agricultural demands. At these times surplus reclaimed water can be used along with imported potable water to recharge aquifers to provide increased groundwater supplies for the future. The objectives of the wetland treatment programme are to evaluate (a) use of constructed wetlands as an alternative to conventional treatment, (b) feasibility of using constructed wetland systems for groundwater treatment, recharge, recovery and reuse operations, and 3) environmental value of the proposed wetlands for recreation and wildlife study, as well as public health and social aspects of the wetland treatment systems. Public education is also a very important aspect of the wetlands demonstration project. Public use is encouraged through access roads and a viewing area which will be partially hidden from certain areas to minimize disturbance of waterfowl. The groundwater aspects of the Multipurpose Wetlands Research and Demonstration Study can be divided into two major groups: (a) wetlands/groundwater interaction, and (b) wetlands impacts on groundwater aquifers. These aspects will be developed primarily in the later stages of the project at the recharge site. Geology and hydrology of the San Jacinto Valley The San Jacinto valley has been the subject of several research studies. Studies are currently underway by the US Geological Survey and by the Department of Earth Sciences at the University of California at Riverside. The San Jacinto Valley is located in the San Jacinto fault zone, one of the major and most currently active faults in California. The valley consists of a northwest-trending sediment-filled graben of large displacement, bounded by the Claremont fault and the San Jacinto Mountains on the east and by the Casa Loma fault and a broad alluvial mesa on the west (Fig. 3). The San Jacinto River flows northerly and northwesterly through the valley and supplies most of the recharge to the groundwater in the valley. Seismic studies indicate granite bedrock is at depths of 2100 to 2400 m beneath the surface in the graben but less than 200 m west of the graben. Consolidated and semiconsolidated sedimentary rocks of alluvial origin are exposed in Park Hill in the City of

5 Groundwater recharge using wetland-treated sewage effluent 143 Fig. 3 Geologic features of the San Jacinto Valley (Schlehuber et al., 1989). with a population of nearly EMWD was established in 1950 to provide water primarily to rural agricultural land and joined Metropolitan Water District of Southern California a year later in order to utilize imported water. Increased urbanization of the area resulted in the addition of sewer service. Increasing water demands coupled with limited water supplies has led EMWD to pursue water reclamation and reuse to assure an adequate supply of water in the future. About 80% of the present water supply to EMWD is imported from northern California through State Water Project facilities and from the Colorado River through the Colorado River Aqueduct, while about 20% is supplied by local groundwater. Hemet and underlie other areas at variable depth. Unconsolidated alluvium exists along the river and stream channels. The main water-bearing formations within the graben are the Pleistocene and Holocene alluvial deposits. The Holocene deposits are the unit most commonly drilled for water production wells. Facies changes and faulting have produced several confined aquifers in the basin. Localities suitable for recharge are limited by the composition of the surface and subsurface materials. Before extensive development of the area, water flowed in artesian wells in the northeast portion of the area and water levels in most of the valley ranged from 3 to 18 m below ground surface. Water levels in most areas have dropped considerably since that time, primarily due to groundwater pumping, and groundwater levels are now 50 m or more below ground surface in most areas.

6 144 Judith L. Hamilton California groundwater recharge requirements Recharge of groundwater in California is constrained by state requirements. As a result, water used in the recharge programme must meet stringent requirements for pathogens, trace organic compounds, and inorganic chemical composition. Presently, statewide requirements are in the status of proposed regulations. Although final regulations may differ, at present it is proposed that all wastewater used for surface spreading or direct injection be oxidized, filtered and disinfected. In addition, it is proposed that prior to recharge the treated wastewater not exceed 20 mg l 4 total organic carbon (TOC), 30 mg l" 1 suspended solids (TSS), 30 mg l" 1 biochemical oxygen demand (BOD), and an average of 2 turbidity units. Further limitations on TOC are proposed depending on the percentage contribution of reclaimed water to any affected domestic water supply well and the method used for recharge. It is further proposed that reclaimed water account for no more that 50% of the total water extracted from any domestic supply well. In addition, groundwater quality objectives have been developed for the Hemet/San Jacinto basin and adjacent areas (Fig. 4). Within the EMWD service area extensive faulting and down-dropping of blocks have created numerous fairly separate sub-basins with distinct geologic and hydrologie conditions, and different water quality objectives have been developed based on local groundwater conditions and quality. Because recharge facilities could sufficiently affect water levels to cause a change in direction of groundwater flow in some areas, groundwater recharge projects should take into account water conditions and quality in adjacent sub-basins as well as in the project sub-basin. PROGRAMME DEVELOPMENT General The present project involves the study of two potential sites within the District. These are (a) the Hemet site, and (b) the Little Valley site. At the Hemet site, not only was land available, but the location adjacent to the Hemet/San Jacinto Regional Water Reclamation Facility assured an adequate supply of water with negligible transport system requirements. The initial pilot research and demonstration facilities were developed at this location. Land at the Little Valley site had previously been acquired by the District for reservoir storage; however, the large relatively flat area with permeable soils create a good potential for an infiltration project. Investigations are now underway at this site for both wetlands treatment and groundwater recharge. Hemet site The Hemet site is in an essentially unvegetated area, as it has been repeated graded for sewage effluent storage ponds. The underlying soil is a heavy silt loam with discontinuous clay loam lenses. Geotechnical investigations indicate the soil below the site is relatively impermeable. Because the area has been used for storage of treated sewage effluent, it is not anticipated that leakage from the wetlands will be significant. The tight soils at the site also however make recharge by infiltration at the site

7 Groundwater recharge using wetland-treated sewage effluent 145 impractical. Owing to this and the unknown final quality of the wetland-treated water at this stage, no recharge facilities for reclaimed water are located at the demonstration site. Water discharged from the project is piped either to the San Jacinto Wildlife Refuge area east of Lake Perris or to agricultural users in the area. Although water will not be recharged directly to groundwater at the Hemet site, the pilot and demonstration wetlands at this location are important in the overall planning for water reuse by recharge. Information acquired on effectiveness of treatment by wetlands will be useful in estimating amounts of water which will be available for recharge.

8 146 Judith L. Hamilton Pilot/research project The pilot project is providing information on plant growth and desirable types of plants to remove nitrates and other potential contaminants. The project consists of eight research cells, each 15 by 70 m. These were planted with over 3600 bulrush clumps which had been grown in adjacent nursery cells. Four of the research cells were fully vegetated and four contain an open water area. Data obtained include paired-cell, inflow/outflow, hourly records of water temperature, ph, conductivity and dissolved oxygen concentration. The data were used in planning the vegetation for the demonstration facilities. Demonstration facilities plan Water treatment performance in constructed wetlands is dependent on the hydrodynamic configuration, i.e. conditions produced by the arrangement of physical elements combined with the vegetation established within the system. The wetlands treatment process is essentially a thin-film bioreactor, utilizing the filtration and biochemical transformations that occur when water passes through the biomass material of the wetlands vegetation. Distribution of flow through the wetlands is important both to the process functions and to maintain a healthy, thriving ecosystem. The demonstration project will provide information of effects of variation in water depth, flow rates, and island and pond configuration. The effect of different design parameters on the viability and performance of different plant resources, wildlife habitat, and water treatment Fig. 5 Demonstration wetlands design.

9 Groundwater recharge using wetland-treated sewage effluent \A1 processes at the site will be evaluated. The facilities design includes five inlet marshes, each about 0.8 ha in size. The central portion of the facilities consists of 1.2 ha of open marsh and 1.7 ha of open water. From the open water pond water flows into the 2.4 ha outlet marsh (Fig. 5). The marsh-pond-marsh configuration creates a compound system which has three distinct process phases in the direction of flow. The compound system allows subdivision into separate inlet marshes for operational control, and utilizes structural elements for conveyance between process phase units. The inlet area water depth averages 0.46 m. Water surface elevation will be controlled to within ±3 mm. Inlet water pressures will be controlled by butterfly valves, sliding weir gates, and stop logs to reduce flow turbulence. Because a major purpose of the use of wetlands for treatment is wildlife enhancement, features to attract wildlife are included in the planning. It is presently proposed to include a number of floating platforms comprising about 90 m 2 for waterfowl habitat. Research at the project will aid in determining facilities for attracting the optimum amount of waterfowl. While waterfowl are desirable from the wildlife-enhancement aspects of the wetlands, excessive numbers could adversely affect the quality of the water. Other potential facilities for wildlife include the construction of shallow shelters, bat houses, and gravel beaches. Possible future amenities include landscaping of one or two acres in the public viewing area. Little Valley site The Little Valley site is approximately 6 km southeast of the city of Hemet. Land in the main valley contains fine sandy loams that are well drained. Numerous fans with coarsetextured soils border the main valley. Planning for the Little Valley site involves a conceptual design of the pilot wetlands treatment facilities, locations for the spreading ground facilities, and monitoring. The initial concept for the pilot treatment facilities is to construct trenches and manholes to determine wetlands capabilities of the tributary valleys. At later stages of the project, wetlands-treated water from this or other sources, along with untreated potable water, will be spread on the valley floor for infiltration into the ground-water system. At the Little Valley site, the initial water treated in the wetlands will be obtained from groundwater underlying nearby citrus groves. This water is very high in nitrates (about mg l" 1 ). A pipeline to convey secondary-treated effluent from the Hemet treatment plant or tertiary-treated water from the demonstration wetlands will be built; water from the facilities at Little Valley will be blended with this water and possibly also with State Water Project water prior to spreading on the valley floor for recharge. Studies at the Little Valley site will be carried on cooperatively by EMWD and Reclamation. The US Geological Survey (USGS) has also proposed studies at the site involving organic compounds, microbes, colloid and nitrogen cycling. Preliminary analysis of the suitability of the Little Valley site indicates that the surficial materials in the valley floor will provide good recharge capability. The soils generally have moderately rapid to rapid permeability. Infiltration rates obtained at three sites ranged from 1.3 to 2.2 m day" 1. These are well below the maximum allowable rate of 0.3 in m" 1 (11m day" 1 ) proposed for the State regulations. Pilot spreading-pond

10 148 Judith L. Hamilton facilities have been installed in the northwest portion of the site and water spreading is occurring. Approximately to m 3 s" 1 water diverted from the San Jacinto River is spread over about 1.6 to 2.0 ha of bermed ponds. Monitoring facilities for this operation are being planned. Preliminary information indicates that there may be some limits to the amount of water which could be stored underground at this site. The shallow depth to granite bedrock at the west end of the site severely limits the amount of alluvium in that area. Boring data indicate a fault may pass through the west-central portion of the site. Depth to bedrock on the east side of the fault would be considerably greater. The USGS has conducted seismic investigations at the site and data are currently being interpreted. Based on the results of these investigations, drill holes/monitor holes will be constructed at the eastern end of the valley where there was little drilling done during the investigations for the previously-proposed damsite. Information from these holes will provide additional data on composition of alluvium at depth and on depth to groundwater. REFERENCE Schlehuber, M. J., Lee, T.-C. & Hall, B. S. (1989) Groundwater level and hydrochemistry in the San Jacinto basin, Riverside County, California./. Hydrol. 106,