Sea to Sky Geotechnique 2006

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1 AGUA FRIA LINEAR RECHARGE PROJECT-MODELING RECLAIMED WATER RECHARGE IMPACTS Gary E. Andres, PBS&J, Spokane, Washington, United States Frank S. Turek, PBS&J, Phoenix, Arizona, United States Aimée Conroy, City of Phoenix, Phoenix, Arizona, United States Tom Poulson, Bureau of Reclamation, Phoenix, Arizona, United States ABSTRACT The Sub-regional Operating Group in the Phoenix, Arizona area, is assessing the feasibility of an aquifer recharge project that involves recharging reclaimed water in a dry bed of the Agua Fria River. Recharge water level impacts were simulated using a groundwater flow model. Simulations without reclaimed water recharge predicted continued drastic declines in groundwater levels. Groundwater level trends in the project vicinity are reversed with reclaimed water recharge and result in a significant groundwater table rise. RÉSUMÉ La praticabilité d'un projet de recharge de couche aquifère pour plusieurs villes en Arizona actuellement est évaluée. Le projet comporte recharger les eaux usées réclamées dans un lit sec de fleuve. Des impacts sur la nappe phréatique ont été évalués en utilisant un modèle de flux d'eau souterraine régional. Les simulations sans recharge de l'eau réclamée ont prévu des déclins importants et continus dans des niveaux d'eaux souterraines. Des tendances de niveau d'eaux souterraines à proximité de projet sont renversées avec la recharge, et les niveaux d'eaux souterraines ont été sensiblement augmentés. 1. INTRODUCTION The Agua Fria Linear Recharge Project (AFLRP) will offset the increasing demands on an already overused aquifer through the reuse of reclaimed water. The project will use aquifer recharge to allow reuse of reclaimed water from wastewater treatment plants (WWTPs). Increasing the reuse of treated wastewater has been a long-term goal of the Sub-regional Operating Group (SROG), consisting of the cities of Glendale, Mesa, Phoenix, Scottsdale, and Tempe. In 1992, SROG began investigating the feasibility of recharging reclaimed water, becoming the Agua Fria Linear Recharge Project in 1996 (Figure 1). The Bureau of Reclamation (Reclamation) joined the project as the Federal sponsor in The AFLRP involves transporting reclaimed water either directly from the 91st Avenue WWTP or the 23 rd Avenue WWTP. There is also the potential for a combined project to accept reclaimed water from both WWTPs. The reclaimed water will be pumped to a series of lateral pipelines along the Agua Fria River channel where the water is recharged to the aquifer. The goal is to recharge 40,000 to100,000 acre-feet per year (AFY) in a 10-mile section of the Agua Fria River channel. The main benefits of the project are supplementing the water supply via groundwater recharge, and improving the habitat along the Agua Fria River. Implementation of the project is not expected until the year Feasibility analyses of the AFLRP is currently underway including the use of groundwater modeling which is the focus of this paper. 2. PROJECT OVERVIEW The SROG initiated studies to assess the feasibility of recharging reclaimed water from the 91 st Avenue WWTP (Figure 1) in SROG s goals were to develop a renewable, drought tolerant water resource and to eliminate WWTP discharges to the Salt River. SROG proposed the AFLRP in 1996 as a method to save reclaimed water produced in the winter when the demands are at a minimum and to allow for recovery and reuse in the summer when the water demands often exceed the reclaimed water supply. Figure 1. Agua Fria Project Location The initial SROG recharge proposal in 1992 was named the Zero Discharge Option and included delivering reclaimed water to the Buckeye Irrigation Company (BIC) 1352

2 and the Arizona Nuclear Power Project (ANPP) via the ANPP pipeline and using a recharge project to use the remaining 92,000 AFY reclaimed water. SROG proposed six regional recharge alternatives and assessed these to determine which provided the greatest water resources potential and was technically and financially feasible to implement. SROG selected the Agua Fria New River confluence north of the 91 st Avenue WWTP as the preferred regional alternative. SROG conducted additional analyses from 1994 through 1997 to assess this site. In 1996 SROG also approved the Tres Rios project to be conducted with the United States Army Corps of Engineers. The Agua Fria New River site had excellent quality groundwater and potential use of the recovered recharge would not be impacted. The depth to groundwater was estimated to be a minimum150 feet below the surface indicating there was sufficient subsurface storage capacity for the recharge. The area was easily accessible by pipeline from the 91 st Avenue WWTP and has only a moderate up hill pumping requirement. The site would allow for full recharge credits and for direct recovery by SROG. The initial recharge approach was to use recharge basins. However, analyses indicated groundwater mounding would be a problem and the three basin sites were within 0.5 mile of the end of the Glendale Airport runway. This did not meet the FAA bird aircraft strike hazard (BASH) criteria requiring 10,000 feet of separation between potential bird habitat and the runway. SROG conducted a study in 1996 to assess 91 st Avenue WWTP discharge alternatives as a part of the National Pollution Discharge Elimination System permit (NPDES). The study evaluated 64 potential alternatives including linear recharge in the Agua Fria River channel. The study found by using multiple recharge sites in the river channel, the potential groundwater mounding concern was addressed. The recharge would not result in the groundwater table getting within 30 feet of the land surface. This minimum groundwater mound depth was established to permit Soil Aquifer Treatment to enhance the quality of the reclaimed water prior to the recharge percolating to the groundwater table. Using this approach, linear recharge could be designed to eliminate the BASH concern associated with recharge basins at the Agua Fria-New River location. Designing berms in the river channel to control reclaimed water discharges at multiple locations and to enhance percolation and recharge would allow for full recharge credits. SROG decided to pursue the linear recharge concept. The AFLRP is divided into four phases to allow for a systematic approach to address issues and develop a feasible project that is a benefit to all the communities in the Agua Fria area. The first phase involved Stakeholder Coordination, Public Information and development of a Consensus Plan (PBS&J, 2003) that evaluated the project features and recommended development guidelines. The Phase 1 results showed that the project is feasible and there are no constraints that should prevent implementation. The project is currently nearing the end of Phase 2, which includes Technical Investigations, Economic Analyses, Feasibility Report Update, an Environmental Impact Statement, and computer modeling. Included were computer simulations of groundwater impacts from project implementation. Phase 3, scheduled to run from 2007 to 2009, will involve developing preliminary designs for the project. Phase 4 is planned for will include developing the final designs and implementing the project. 3. AFLRP PROJECT COMPONENTS The recharge area extends from Indian School Road north to Bell Road. A series of lateral pipelines spaced at one- mile intervals will divert water from the conveyance pipeline to the Agua Fria River channel (Figure 2). The linear recharge approach was adopted instead of using recharge basins to minimize the potential for adverse groundwater mounding. The linear recharge concept allows greater control of the quantity of reclaimed water discharged at any point and to account for local hydrogeologic conditions and land uses. Preliminary estimates of the volume of water to be routed through each of the pipe laterals were specified. These volumes were subsequently revised as a result of the groundwater modeling process. Figure 2. AFLRP Site Map The subsurface geology in the Agua Fria and New River area is divided into three alluvium formations. The Upper Alluvial Unit is approximately 300 feet thick and is a mixture of silt to gravel size sediments forming layers of 1353

3 varying thickness and horizontal extent (Reclamation, 1976). Below the Upper Alluvial Unit is the Middle Finegrained Unit, a formation dominated by clays and silts but which does contain sand layers. Below the Middle Unit is the Lower Conglomerate Unit, a cemented sand and gravel formation. These initial studies showed the groundwater table ranged from 120 to 140 feet below the proposed recharge basins site so the focus of the hydrogeologic investigations was the Upper Alluvial Unit. 3.1 Available Reclaimed Water Monthly reclaimed water availability was calculated using the projected influent quantity to the 91st Avenue WWTP and the 23 rd Avenue WWTP and subtracting existing and planned reclaimed water reuse agreement quantities. The 91 st Avenue WWTP has a current treatment capacity of 179 million gallons per day (mgd) while the 23 rd Avenue WWTP treatment capacity is about 62 mgd. Reclaimed water from the 91st Avenue WWTP is distributed through long-term contractual obligations, and thus these reclaimed water commitments must be accounted for when determining the amount of water available for the AFLRP: Tres Rios will use approximately 28,000 AFY. The Buckeye Irrigation Company (BIC) has a contract which will take effect in 2011 that allows for the purchase of 20,000 AFY as a minimum and 40,000 AFY as a maximum. The ANPP Verde Nuclear Generation Station has a contract for 105,000 AFY but typically uses no more than about 70,000 AFY. There are two commitments for reclaimed water from the 23 rd Avenue WWTP. There is an agreement with a local farmer for irrigation use and an agreement with the RID for irrigation use. A graph depicting the projected available reclaimed water for the year 2010 is shown on Figure 3. The graph shows the monthly demand associated with Tres Rios, the BIC and Palo Verde. It also combines the three reclaimed water commitments to show the total reclaimed water demand in each month. The projected monthly reclaimed water quantity produced by the 91 st Avenue WWTP includes increases associated with treatment capacity expansions which are projected to occur by There is no uncommitted flow available in June through August. SROG assessed their need for recharge credits and developed projections totalling from 40,000 AFY to 100,000 AFY. The groundwater modeling described below is based on the 100,000 AFY to project the maximum impacts. Figure 3. Anticipated Monthly Water Supply in Year Reclaimed Water Storage Credits and Recovery The AFLRP can accumulate long-term storage credits that are held for subsequent recovery in a longterm storage account. The total amount of water discharged to the recharge project (minus losses and recovery/transfer activity) is eligible for storage credits. SROG can use these credits or assign by grant, gift, sale, lease or exchange all or part of the long-term storage credits to other water users. Arizona s Underground Water Storage and Recovery program was established in 1986 by the Arizona Legislature. This program allows an entity with surplus water to recharge the water and then recover later. The Arizona Department of Water Resources (ADWR) administers the recharge program. Long-term storage credits may be issued by ADWR when eligible water is stored underground for more than one year. Long- term storage credits are held in a long-term storage account for the purpose of subsequent recovery. In the case of the AFLRP, the total amount of water discharged to the recharge project (minus losses and recovery/transfer activity) is eligible for storage credits. An entity holding long-term storage credits may recover stored water from anywhere within the same Aquifer Management Area (AMA) in which it was stored. The Agua Fria Linear Recharge Project will take advantage of the ability to recover within the Phoenix AMA, allowing SROG to maximize their use of the accumulated credits. Recovery within the area of hydrologic impact of water storage can provide water for habitat and recreation purposes along the Agua Fria River. 4. GROUNDWATER MODELING In order to assess the impact and viability of the AFLRP a groundwater flow model was used to simulate the projected reclaimed water recharge. The model development and simulations are described below. 1354

4 4.1 Previous Infiltration Impact Assessments Initial recharge impact projections were completed in The parameters incorporated several conservative assumptions into the calculations resulting in a very conservative worst case condition. Based on recharging 92,000 AFY, a groundwater mound was projected to rise to within 30 feet of the surface in two years. This projection indicated there may not be sufficient vadose zone capacity to store the recharge and the groundwater movement may be too slow to allow for dispersal of recharge horizontally and vertically throughout the underlying formations. Because this was the worst case condition, a more detailed model was needed. A multi-layer Modflow model was prepared in 1996 to provide a more detailed assessment. This Modflow model allowed incorporating historic groundwater pumping into the calculations. This was an important improvement because there was a significant amount of farmland in the area and municipal water supply systems all using groundwater as a major component of their water supplies. Model simulations showed the groundwater mound was within 30 feet of the surface after the fifth year of operation. Another modeling effort was completed by Reclamation using a model developed by the ADWR known as the West Salt River Valley Modflow model (WESTCAPS, 2001). The first series of Reclamation model runs were designed to project the expected water table impacts and did not include the AFLRP. The Reclamation model projected a decline in the regional water table to depths of 500 to 600 feet, or 300 to 400 feet within the AFLRP area. Subsequent modeling by Reclamation included the AFLRP recharging 60,000 acre-feet per year and recovering about 24,000 AFY. This showed a slight regional net benefit. This simulation projected the regional water table decline was reduced and the longterm impact was the water table would decline to 250 to 350 feet below the surface. This model confirmed the AFLRP would not produce water table groundwater mounding requiring recovery for control. This was due to recharging a lesser amount of reclaimed water, 60,000 acre-feet rather than 92,000 acre-feet and incorporating recovery. However, the Reclamation model did not incorporate the cumulative impacts produced by other groundwater recharge projects in the area. Due to the incomplete database in the other recharge model simulations, these existing models were not suitable for simulating the impacts of the AFLRP. The AFLRP model would have to incorporate the projected recharge impacts associated with other groundwater recharge projects into the model to prepare a regional impact evaluation. Phase 2 therefore included development of a more detailed model to simulate the recharge impacts along the Agua Fria River. 4.2 Groundwater Model Development A model developed by the ADWR known as the Salt River Valley (SRV) groundwater model was adopted as the basis for the AFLRP groundwater model (Figure 4). The advantage to creating a new model was the required updated regional information was already incorporated, the ADWR model was calibrated and operational, and the model is accepted by ADWR and it would not be necessary to demonstrate the calibration of a new model. The ADWR model was provided as a group of Modflow files. The initial step was to incorporate the relevant information from the ADWR Modflow files housed in a proprietary user interface into the AFLRP model which uses the Modflow interface program GMS for pre and post processing model simulations. The AFLRP model was then run and the results compared to both the groundwater elevation contours presented in an ADWR memorandum and with the output Figure 4. AFLRP Groundwater Flow Model Domain 1355

5 files generated by execution of the ADWR SRV model files provided external to GMS. The comparison revealed the AFLRP model generally reproduced the groundwater contours of the SRV model with some subtle variations. Model residuals (difference in observed and predicted heads) were compared for the two models. Over 900 well data points were used to evaluate the residuals. A plot of the residuals from the ADWR SRV model versus the AFLRP model (Figure 5) shows the residuals were similar, and in many cases identical. Calibration statistics indicate that both versions are well calibrated because the standard deviation divided by the head range is less than 5 percent (2.5 percent for the AFLRP model and 2.4 percent for the ADWR SRV model), and the residual mean divided by the head range is less than 1 percent (0.1 percent and 0.05 percent). Original Model Head Residual (ft) Calibration Statistics Statistic Original Imported Residual Mean St Dev/Head Range 2.4% 2.5% RM/Head Range 0.05% 0.1% -75 Layer 1 Layer 2 Layer 3 Match Line Imported Model Head Residual (ft) Figure 5. Original and Imported Model Comparison Lastly, water budgets for simulations from the two models were compared. Fluxes for wells and recharge were identical with only slight differences for the constant head and stream fluxes. The resulting AFLRP model compares favourably, but not exactly with the ADWR SRV model. Despite the differences, the Agua Fria model is considered well calibrated. Figure 6. Layer 1 Year 2002 Results of the existing conditions simulations indicate the water table levels in the three model layers in the West SRV area will continue to decline with time. The amount of decline varies depending upon location, but in the area of the proposed AFLRP water levels decline more than 150 feet in some locations by Thus the area of dewatering is predicted to expand, with much of the Upper Alluvial Unit aquifer becoming completely dewatered by the year 2035 (Figure 7). 4.3 No-Action Scenario ADWR groundwater recharge facilities are issued for 20 years. The period used in projecting future conditions in this study was specified as 25 years. The anticipated starting date for recharge was 2010 but could be 2015, thus the simulations extend to This incorporates the duration of the ADWR recharge permits. Initially, recharge conditions for the year 2000 were specified for future conditions in the AFLRP model. However, a review of the data indicated the 1998 conditions should be used in the model for recharge because it is more representative of average annual conditions than more recent years. The water table conditions at the start of the simulation period in 2002 for the upper layer is shown on Figure 6 (see inset on Figure 4). The small shaded squares are dry cells, Bell Road is shown as B, Cactus Road as C, Northern Avenue as N, and Indian School Road as I. Figure 7. Layer 1 Year 2035, No Action 1356

6 Hydrographs were developed for three key locations within the AFLRP area to show the projected water level trends with time (Figure 8). The hydrographs show the water levels will decline at all three locations. The Bell Road location shows the smallest amount of decline, reaching 25 feet in layers 2 and 3 (layer 1 is dry). At Northern Avenue, the decline is more severe, ranging from 77 feet in layer 1 to 88 feet in layers 2 and 3. The decline at Indian School Road is even greater, ranging from 136 feet in layer 1 to 165 feet in layers 2 and 3. Groundwater Elevation (ft MSL) Northern Ave Indian School Road Year Bell Road (dry) 1998 Recharge 2002 Recharge Figure 8. Hydrographs, Model Layer 1, No Action If recharge conditions in the future are more like the drought year of 2002, the projected water table declines by 2035 would be greater so the AFLRP model was also run using 2002 recharge conditions rather than The results are shown on Figure 8 as dashed lines. The 2002 conditions result in layer 1 water levels that are 3 feet lower at Northern Avenue. Several of the layer 1 model cells are dewatered and show up as dry in the Indian School Road area in 2035 and these cells did not go dry under the simulation using 1998 recharge. The Bell Road model cells remain dry in layer 1, and water levels are feet lower in layers 2 and 3 using the 2002 recharge conditions than with the 1998 recharge conditions. Table 1 presents the recharge volume assigned to the eight discharge locations in the AFLRP model. Recharge was simulated in the model by revising the aerial recharge specified in the Modflow recharge package for the cells where the discharge points are located. Table 1. AFLRP Recharge Values Location Percent Recharge Volume Acre-feet Bell Road 15% 15,000 Greenway Road 15% 15,000 Thunderbird Road 10% 10,000 Cactus Road 8% 8,000 Peoria Avenue 15% 15,000 Olive Avenue 15% 15,000 Northern Avenue 7% 7,000 Glendale Avenue 0% 0 Bethany Home Road 0% 0 Camelback Road 0% 0 Indian School Road 15% 15,000 The results of this AFLRP initial simulation show the groundwater table in most of the West Salt River Valley area will continue to decline, and in fact the regional groundwater table is largely unaffected by the AFLRP. However, groundwater levels in the vicinity of the AFLRP show a reversal once recharge is added to the hydrologic system and result in a significant groundwater table rise. This is evident by comparing the 25 year projection in Figure 7, the No Action alternative, to Figure 9, the projection with AFLRP recharge. Some Upper Alluvial Unit layer 1 cells do not go dry with the AFLRP recharge simulation, and some areas of model layer 1 are resaturated under this AFLRP scenario. 4.4 Initial AFLRP Projections The AFLRP model was used to project future conditions with AFLRP recharge. SROG s maximum projected need of 100,000 AFY was used as the AFLRP recharge. This was accomplished by increasing the recharge at model cells where reclaimed water would be recharged in the Agua Fria River channel. The initial modeling projections did not include recovery to permit the simulation of the maximum effects of recharge. It was determined that three of the original recharge locations could not be discharge points because the water would be attractive for birds and could increase the BASH at Glendale Municipal Airport. Additional analyses showed there could be concerns about the groundwater table rising beneath the El Mirage Landfill. Records for this landfill are sealed and the decision was made to alter the discharge to the Agua Fria River upstream from the landfill to reduce the potential size of the groundwater mound. Figure 9. Layer 1 Year 2035, AFLRP Recharge 1357

7 Hydrographs for the three locations discussed for the No Action scenario, Bell Road, Northern Avenue and Indian School Road are shown on Figure 10. A fourth point was added representing Cactus Road to evaluate the effects of the recharge in a cell near the El Mirage Landfill. Groundwater Elevation (ft MSL) Ground surface at Bell Road location is 1160 Ground surface at Northern Ave and Indian School Road is 1070 and 1020 Cactus Road Ground surface at Cactus Road location is 1120 Indian School Road Bell Road (dry) Northern Ave Year Figure 10. Hydrographs, Layer 1, AFLRP Recharge The hydrographs illustrate the impact of the recharge along the Agua Fria River (dashed lines on Figure 10). For example, at Northern Avenue where the water level is predicted to decline 77 feet for the period under the No Action Alternative, with recharge it is projected to rise 204 feet and in 2035 is projected to be roughly 160 feet higher than it was in A major feasibility assessment criteria for Agua Fria LRP is to maintain groundwater table so it is more than 30 feet below ground surface. Only at Bell Road did the projected groundwater level rise above this point, for 2015 and 2020, after that the levels drop below the 30 foot criteria. The decline is due to the future groundwater pumping increases. A revised simulation was developed to verify if the projected groundwater table rise at Bell Road could be managed by redistributing the reclaimed water discharge quantities. 4.5 Revised AFLRP Projections A revised simulation was developed using the recharge distribution shown in Table 2. Table 2 Adjusted AFLRP Recharge Values Location Percent Recharge Volume Acre-feet Bell Road 10% 10,000 Greenway Road 15% 15,000 Thunderbird Road 10% 10,000 Cactus Road 8% 8,000 Peoria Avenue 15% 15,000 Olive Avenue 15% 15,000 Northern Avenue 11% 11,000 Glendale Avenue 0% 0 Bethany Home Road 0% 0 Camelback Road 0% 0 Indian School Road 15% 15,000 The amount of reclaimed water discharged to the Agua Fria River at Bell Road was reduced and a corresponding amount was added to the discharge quantity at Northern Avenue. The purpose was to compensate for the initial AFLRP projection indicating the groundwater table rise would exceed the minimum depth of 30 feet at Bell Road. Reducing the quantity of reclaimed water at Bell Road by 5,000 acre-feet was expected to reduce the projected groundwater table rise. However, increasing the recharge at Northern Avenue was expected to increase the amount of projected groundwater table rise. The groundwater contours for this simulation with the adjusted discharge quantities are nearly identical to that of the initial recharge simulation. The exception is in the immediate vicinity around Bell Road and Northern Avenue. The hydrographs on Figure 11 for the revised discharge quantities simulation show that at no time do the water levels at any of the locations rise above the 30- foot minimum depth to groundwater criteria. At Bell Road the closest the water table gets to the surface is a depth of 40 feet in 2020, and at Northern Avenue the closest to the surface the water table reaches is 50 feet in Groundwater Elevation (ft MSL) Ground surface at Bell Road location is 1160 Ground surface at Northern Ave and Indian School Road is 1070 and 1020 Cactus Road Ground surface at Cactus Road location is 1120 Indian School Road Bell Road (dry) Northern Ave Year Figure 11. Layer 1 Hydrographs, Revised Recharge 5. AREA OF HYDROLOGIC IMPACT ADWR defines the area of hydrologic impact for a recharge project as the area where the recharge is calculated to produce at least one-foot of water table rise (Figure 12). This is the area ADWR uses to assess if the recharge project will produce adverse water table impacts in existing facilities. Because the greatest water table impacts occur beneath or adjacent to the recharge project and an operation parameter of the AFLRP is to maintain the groundwater table at least 30 feet below the land surface, the AFLRP should not produce water table impacts of concern to ADWR. The recharge area of hydrologic impact was determined by comparing the modeled groundwater table conditions projected for year 2035 for the existing conditions (No Action Alternative) and the AFLRP recharge scenarios. This information was used to develop the area of hydrologic impact related to 1-foot of groundwater table rise. 1358

8 Sea to Sky Geotechnique 2006 Figure 12. Zone of Hydrologic Impact The recharge area of impact was further refined to identify the timing when the groundwater table rise could be observable in existing wells. The area of hydrologic impact was projected for three AFLRP operation periods measured from the start of recharge activities. These periods represent 5, 10, and 20 years of recharge. These were defined by comparing modeled groundwater table levels from 2010 prior to implementation of the recharge project with simulated water levels from 2015 (5 years), 2020 (10 years), and 2030 (20 years). The area of hydrologic impact for these three different project duration periods indicated that within one year the majority of the horizontal extent of the recharge area of impact has been developed, with only slight increases in size in subsequent years. 6. CONCLUSIONS Groundwater modeling confirms the Agua Fria LRP can recharge 100,000 acre-feet of reclaimed water annually without recovery. The groundwater mound will not extend closer than 30 feet to the land surface allowing for additional water quality enhancement due to Soil Aquifer Treatment The recharge can be recovered directly by the Cities of Glendale and Phoenix using recovery wells. Remote recovery by Mesa, Phoenix, Scottsdale and Tempe is feasible and permitted by Arizona law. In addition to remote recovery, the ESRV SROG cities can exchange recharge credits with other water users in the Agua Fria River area to obtain additional surface water from SRP and CAP. Previous studies verified the reclaimed water can meet surface water quality standards and groundwater standards. There is a potential concern for the high TDS concentration in the reclaimed water to affect the regional groundwater quality and may affect some wells. References PBS&J, Sub-regional Operating Group Agua Fria Linear Recharge Project, Project No. WS Consensus Plan; Consultants Report. WESTCAPS The West Salt River Valley CAP Subcontractors Planning Process, WESTCAPS.