7-G 2018 Recharge Planning Study.pdf

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January 24, 2018

Recharge and Recovery Planning Document Town of Prescott Valley, Yavapai County, Arizona

January 24, 2018

REPORT

RECHARGE AND RECOVERY PLANNING DOCUMENT

TOWN OF PRESCOTT VALLEY, YAVAPAI COUNTY, ARIZONA

Contents

1 INTRODUCTION / PROJECT GOALS

2 PROJECT BACKGROUND / PREVIOUS INVESTIGATIONS

2.1 Previous Investigations

2.2 Recharge Facility Performance

2.3 Recent Efforts for Recharge Enhancement

3 PROJECT APPROACH

3.1 Site Visit and Kickoff Meeting

3.2 Key Elements for New Approach

3.2.1 Recharge Options and Assumptions

3.2.2 Recovery Considerations

4 HYDROGEOLOGIC CONDITIONS

4.1 Regional Hydrogeologic Framework

4.1.1 Upper Alluvial Unit

4.1.2 Basement Unit

4.2 Lithology and Stratigraphy of the Town’s Wellfields

4.2.1 Santa Fe and North Wellfields

4.2.2 East Upper Agua Fria Sub-basin

4.2.3 Mountain Valley Park

4.3 Groundwater Conditions

4.3.1 Occurrence and Movement of Groundwater

4.3.2 Historic Water Level Changes

5 EVALUATION CRITERIA FOR RECHARGE AND RECOVERY FACILITIES

5.1 Overview of Relevant Criteria

5.1.1 General Evaluation Criteria

5.1.2 Considerations Related to Impacts on the Town’s Wellfields

5.1.3 Criteria and Considerations Specific to Injection Wells

5.1.4 Criteria and Considerations Specific to Recharge Basins

5.1.5 Criteria and Considerations Related to Recovery

5.2 Application of Selected Criteria

5.2.1 Land Ownership

5.2.2 Land Use

5.2.3 Distance from the Town’s Reclaim Line or Potable Water System

5.2.4 Groundwater Levels and Flow Direction

5.2.5 Criteria and Considerations Related to Impacts on the Town’s Wellfields

5.2.6 Water Resource Benefits

5.2.7 Cost

5.2.8 Water Quality

PAGE III

Contents – continued

5.2.9 Regulatory Requirements and Constraints

5.3 Travel Time / Distance Analysis

5.3.1 Santa Fe Wellfield

5.3.2 North Wellfield

5.3.3 Lower System

6 RECHARGE PLAN ELEMENTS

6.1 Approach for Addressing Planning Horizons

6.1.1 Immediate Options

6.1.2 Near-Term Options

6.1.3 Long-Term Options

6.2 Surface Recharge Options

6.2.1 Potential Sites for Surface Recharge Facilities

6.2.2 Cost Analysis for Surface Recharge Facilities

6.3 Vadose Zone Injection Wells

6.3.1 Considerations for Vadose Zone Injection

6.3.2 Cost Analysis for Vadose Zone Injection Wells

6.4 Direct Injection Options

6.4.1 Key Criteria for Potential Injection Well Options

6.4.2 Use of Existing Water Supply Wells

6.4.3 New Injection Wells/Locations

6.4.4 Cost Analysis for Injection Wells

6.5 Summary of Costs for Recharge Options

7 RECOVERY PLAN ELEMENTS

7.1 ADWR Recovery Rules

7.2 Recovery Planning

7.3 Potential Recovery Wells

7.3.1 Existing Town Water Supply and Recovery Wells

7.3.2 Considerations for New Recovery Wells

8 RECHARGE AND RECOVERY SCENARIOS

8.1 Key Consideratons

8.2 Recharge and Recovery Scenarios

8.2.1 Mountain Valley Park/Windsong Well Recharge Center

8.2.2 Inter-Wellfield Recharge Center

8.2.3 Base of Glassford Hill Recharge Center

8.2.4 Additional Considerations for Recharge and Recovery

9 REFERENCES CITED

10 ACRONYMS & ABBREVIATIONS

PAGE IV

Tables

Table 1. Summary of Potential Surface Recharge Sites Identified in 2006 to 2008 Reconnaissance Study, Town of Prescott Valley, Yavapai County, Arizona

Table 2. Summary of Records for Selected Wells in Study Area, Town of Prescott Valley, Yavapai County, Arizona

Table 3. Summary of Records for Water Supply Wells Owned by Town of Prescott Valley, Yavapai County, Arizona

Table 4. Summary of Well Performance Data and Estimated Aquifer Hydraulic Properties for Town of Prescott Valley Water Supply Wells, Town of Prescott Valley, Yavapai County, Arizona

Table 5. Pumping Test Parameters and Seepage Velocity Analysis for Santa Fe Wellfield (in text)

Table 6. Pumping Test Parameters and Seepage Velocity Analysis for North Wellfield (in text)

Table 7. Pumping Test Parameters and Seepage Velocity Analysis for Lower System (in text)

Table 8. Summary of Evaluation Criteria and Findings for Potential Surface Recharge Facilities, Town of Prescott Valley, Yavapai County, Arizona

Illustrations

Figure 1. Land Ownership Map

Figure 2. Well Location Map

Figure 3. Lithologic Logs for Selected Wells

Figure 4. Lithologic Cross-Section A-A’

Figure 5. Lithologic Cross-Section B-B’

Figure 6. Groundwater Level Altitudes in 2016

Figure 7. Depth to Groundwater in 2016 and Groundwater Level Change, 2011 to 2016

Figure 8. Specific Capacity of Public Supply Wells

PAGE V

Illustrations – continued

Figure 9. Estimated Long-Term Injection Rates for Existing Supply Wells as a Function of Short-Term Pumping Specific Capacity

Appendix

Appendix A. Historic Water Level Hydrographs for Selected Wells in Study Area

1 INTRODUCTION / PROJECT GOALS

The Town of Prescott Valley (the Town) has identified near-term and long-term goals for recharge and recovery of treated effluent produced by the Town, and requested Montgomery & Associates (M&A) to prepare a conceptual level planning document.

M&A has completed a reconnaissance-level study and prepared the present planning document to summarize considerations and options for increasing recharge and recovery capacity for the Town based on three planning horizons. M&A’s understanding of the Town’s recharge and recovery goals is summarized as follows:

• Immediate goal is to increase total recharge capacity to approximately 2,400 acre-feet per year (AF/yr), equivalent to development of an additional 700 AF/yr of capacity, in order to maximize storage credits for currently available effluent.

• Near-term goal (within approximately the next 5 years) is to increase recharge capacity by an additional 1,600 AF/yr as discharge of effluent from the Town’s existing Advanced Treatment Facility (ATF) grows from 2,400 AF/yr to 4,200 AF/yr, while implementing recovery at appropriate locations as needed.

• Long-term goal (within 10 to 30 years) is to increase recharge capacity to accommodate additional effluent from expansion of the existing ATF or construction of a future ATF, which could be as much as 6,000 AF/yr in 30 years (based on assumption that treatment volumes will be approximately 65% of potable water deliveries), while implementing recovery at appropriate locations as needed.

The study area comprises lands within or near the Town’s municipal boundaries, inclusive of the Town’s water and wastewater service area and three potable supply wellfields. A site map showing land ownership and the Town’s boundaries and surrounding areas is shown on Figure 1; a well location map for all registered wells in the study area is shown on Figure 2. Selected areas outside the Town’s boundaries were initially reviewed for completeness, but were generally not appropriate for further consideration, chiefly due to the large distance from the Town’s existing reclaimed water pipeline and/or due to land ownership.

Recharge facility siting and planning is the primary focus for the present study;

maximizing recharge and accrual of long-term storage credits for currently available effluent is the highest priority. However, recovery options and opportunities (or constraints) were considered in the context of recharge facility siting and were evaluated for long-term recharge and recovery planning.

2 PROJECT BACKGROUND / PREVIOUS

INVESTIGATIONS

The Town has been discharging effluent from the ATF to the Agua Fria River channel since 1993, and has been accruing storage credits for the recharged effluent under an Underground Storage Facility (USF) permit (and series of permit modifications) starting in 2003. This recharge facility, known as the Upper Agua Fria Recharge Project (UAFRP), consists of a 2.2-mile stretch of the river channel extending downstream from the ATF (Figure 1). Due to continuing challenges meeting compliance criteria established by the Arizona Department of Water Resources (ADWR) for an in-channel constructed USF and the associated unrealized (lost) storage credits, the Town initiated reconnaissance investigations in 2006 to identify favorable off-channel recharge sites.

2.1 Previous Investigations

Previous reconnaissance investigations were conducted intermittently from 2006 through 2008, and included compilation and analysis of available data for hydrogeologic conditions, land use and ownership, regulatory constraints, potential for unreasonable harm, distance from the Town’s reclaim pipeline, water resource benefits, costs, and other relevant factors. A total of 26 potential sites were identified, and field investigations (exploration trenching and drilling) were conducted at nine of the sites.

A recharge feasibility matrix was prepared to evaluate, compare, and rank the potential sites based on criteria derived for the factors/data listed above; the recharge matrix is given in Table 1. Options highlighted in gray were removed from consideration for the present study (Section 6.2). Most of the 26 sites considered had substantial limitations and/or fatal flaws, which greatly reduced the number of potentially feasible sites. It is important to note that none of the sites were considered “very favorable” for recharge using surface infiltration basins, and direct injection of effluent into the aquifer was not an acceptable recharge option to the Town at that time. However, the Town’s goal for additional recharge capacity was modest and allowed consideration of sites with moderate to low infiltration rates. Within this context, the North Plains site, located approximately 1 mile north of the ATF, was determined to be the most favorable site with regard to all applicable criteria, and the North Plains USF (Phase 1) was designed, permitted, and constructed (surface infiltration basins); recharge operations began in 2013. The USF permit for the “expanded UAFRP” includes both the in-channel constructed facility and the North Plains site and the total permitted capacity of Phase 1 is 3,600 AF/yr.

2.2 Recharge Facility Performance

The Town’s primary goal with the expanded UAFRP was to maximize recharge at the North Plains facility and decrease reliance on the in-channel facility, especially during the wet periods of the year when this facility is often out of compliance. This would increase overall recharge capacity while minimizing the loss of storage credits associated with the in-channel facility. It is important to note that additional recovery of stored effluent in the south part of the Upper Agua Fria sub-basin (within the hydrologic area of impact of effluent recharge) is planned in order to meet contractual obligations for the Town’s auctioned effluent credits. Recharge and recovery of effluent via the expanded UAFRP is integral to maintaining the Town’s Physical Availability Determination.

Actual recharge capacity at the North Plains site has been less than the design capacity due to very small infiltration rates and excessive perched groundwater mounding, and has not significantly offset lost storage credits for the in-channel facility. This is the primary reason that the Town’s recharge goal for the “immediate” planning horizon (described in Section 1) is to increase recharge capacity by 700 AF/yr to approximately 2,400 AF/yr in order to maximize storage credits for currently available effluent. A summary of the Town’s effluent availability and UAFRP recharge volumes (storage credits) in 2016 demonstrates the shortfall:

• The ATF is currently permitted to treat 3.75 million gallons per day (annual rate of 4,200 AF/yr) of wastewater. A total of 2,934 acre-feet (AF) of treated effluent was discharged from the ATF in 2016. Of this total amount, approximately 16% was diverted for reuse and the remaining 84% (or 2,465 AF) was recharged to the aquifer via the UAFRP (63% was discharged to the Agua Fria River channel and 21% was delivered to the North Plains Site).

• Permitted capacity of the expanded UAFRP (including Phase 1 of the North Plains facility) is 3,600 AF/yr; the maximum permitted (credited) storage capacity for the in-channel facility is 1,200 AF/yr, with the remaining permitted capacity for the North Plains facility.

• In 2016 the total credited recharge for the expanded UAFRP was only 1,701 AF, which represents a loss of approximately 765 AF based on the effluent delivery volume of 2,465 AF. Only 544 AF of credits were accrued at the North Plains facility and 1,157 AF were accrued at the in-channel facility. Similar results occurred in previous years (670 AF of lost credits in 2015).

2.3 Recent Efforts for Recharge Enhancement

During the past 3 years, the Town has conducted several activities in an attempt to improve recharge capacity or evaluate a possible recharge site, including:

• conducted a “small basin” infiltration test on Glassford Hill near the water storage tank: result was an infiltration rate of less than 0.5 foot per day

• drilled a borehole near each of the three alert-level piezometers at the North Plains site to evaluate the effect of creating local conduits to promote downward movement of water (through a possible perching layer) and locally lowering the (perched) water table: result was a temporary lowering of water levels at one piezometer

• progressed toward purchase of land for Agua Fria Park, which could be considered for development of a recharge basin facility or multi-use public amenity with water features: result is that this site has small infiltration capacity and recharge basins would chiefly function as a water amenity.

These opportunistic efforts by the Town provide useful information that essentially supports removal of the investigated options/enhancements from further consideration.

3 PROJECT APPROACH

Investigations for the present recharge and recovery planning document consisted chiefly of reconnaissance-level analyses using available data and reports for: hydrogeologic conditions; land use and ownership; water conveyance systems (reclaimed water and potable water supply); location, design, and specific capacity of the Town’s water supply wells; performance of existing recharge facilities; and other pertinent factors. The Town provided much of the information and data for these elements via spreadsheets/databases and Geographic Information Systems (GIS) shape files. Other data sources, chiefly for hydrogeologic conditions, include ADWR (groundwater and well construction databases, including drillers’ and geologists’ logs and published modeling report for the Prescott Active Management Area [AMA]); and other hydrogeologic consultants’ reports (Southwest Ground-water Consultants, Inc. [SWGW] and Southwest Water and Mineral Resources [SMWR]). Evaluation of this information and identification of potential recharge sites were conducted in close coordination with the Town, especially in regard to use of the Town’s water supply wells. This study did not include field investigations such as trenching, infiltration testing, or drilling. Some of the potential recharge sites identified and evaluated during the 2006 to 2008 reconnaissance investigations were relevant to the present study and were re-evaluated.

3.1 Site Visit and Kickoff Meeting

A site visit and kickoff meeting were conducted with Town staff to:

• confirm and clarify project objectives

• review results of the previous reconnaissance investigations (build on the previous recharge matrix)

• initially identify potential sites to be further evaluated

• evaluate use of selected water supply wells for potential use as injection wells

• evaluate status and limitations of the UAFRP

• review considerations and constraints for recharge facilities for both direct injection and surface basins

The site visit included visual inspection of selected water supply wells in their current setting and conditions, and potential new recharge sites.

3.2 Key Elements for New Approach

3.2.1 Recharge Options and Assumptions

Key assumptions and considerations for the present study in relation to the 2006 to 2008 reconnaissance study are summarized below. It is important to note that the first three assumptions below represent critical differences between the two studies and are the basis for selecting many of the recharge options/sites evaluated for the present study.

• For the present study, injection wells are considered to be an acceptable recharge option whereas the previous study essentially excluded injection wells. Further, because the previous study focused on siting surface infiltration basin facilities and included a relatively comprehensive evaluation of potential sites, the present study more heavily addresses the use of injection wells. Vadose zone injection wells are also considered in the present study (and were considered to a limited degree in the 2006 to 2008 study) but are generally not believed to be a favorable option, as described further in Section 6.3.

• For the present study, effluent recharge in the vicinity of the Town’s wellfields is considered acceptable to some degree. The recharge facility siting goal is not to completely avoid recharging effluent at locations where some effluent could ultimately be drawn into the Town’s water supply wells. Perhaps the greatest water resource benefit to the Town from future recharge is to have a positive hydraulic effect in the Santa Fe wellfield, where very large groundwater level declines have occurred. M&A’s understanding is that the Town would consider effluent recharge locations as close as approximately 1 mile from the wellfields, with larger distances being more favorable. These distances would depend on the recharge locations relative to the regional and localized hydraulic gradients, as evaluated in the present study (especially Sections 5.2.5 and 5.3).

• The 2006 to 2008 reconnaissance study did not consider recovery planning. As described in the following section, recovery options were considered for the present study, especially in regard to considerations for recharge facility siting.

• The 2006 to 2008 study identified very few viable sites for constructing a surface recharge facility and resulted in selection of the North Plains site. The primary factors that limited potential sites were land use and ownership, distance of the site from the reclaim pipeline, and lithologic conditions (although no sites were identified with favorable lithologic conditions without other fatal flaws).

• Potential sites for surface recharge facilities are still addressed in the present study, based both on findings from the 2006 to 2008 reconnaissance study and on re-evaluation of the previously identified sites, together with evaluation of several new sites. In general, the surface recharge facility option is more applicable for the long-term than for near-term planning horizons and comprises a relatively minor component of the present study.

Similar to the 2006 to 2008 investigations, a matrix of recharge options/sites was prepared to assist with comparison, screening, and qualitative rating of the direct injection recharge options. Evaluation (screening) criteria were developed based on key factors affecting recharge feasibility, including:

• Regional and/or local hydrogeologic conditions

• land use and ownership

• regulatory constraints

• pumping and estimated injection capacity of the Town’s water supply wells

• water conveyance considerations (distance from the Town’s reclaim pipeline for recharge purposes and distance from the Town’s potable supply pipelines for recovery purposes)

• potential for unreasonable harm,

• costs

• water resource benefits

• other relevant site-specific factors

These criteria are described in more detail in Section 5. These criteria were applied to the identified recharge sites/options to determine advantages and disadvantages of the sites and overall relative favorability. Relevant rationale, considerations, and constraints were identified for each option to assist the Town in determining the most favorable options for the Town’s three planning horizons.

3.2.2 Recovery Considerations

Recovery was not considered for the 2006 to 2008 reconnaissance investigations, and recharge facility siting and planning is still considered the most critical and urgent goal for the present study. However, recovery options and opportunities (or constraints) were considered in the context of recharge facility siting, and were evaluated for recharge and recovery planning for the three planning horizons.

Advantages and opportunities to locate recharge and recovery facilities in relatively close proximity were evaluated by considering:

• Locations and construction of future recovery wells relative to existing recharge facilities (currently the Town has only two permitted recovery wells, located within the 1-mile “Safe Harbor” of UAFRP)

• Locations of future recharge facilities relative to existing water supply wells that could be permitted as recovery wells, considering chemical quality of recovered water

4 HYDROGEOLOGIC CONDITIONS

4.1 Regional Hydrogeologic Framework

The Town is located within the Upper Agua Fria (UAF) sub-basin of the Prescott Active Management Area (AMA). The Little Chino sub-basin is located adjacent to the north end of the UAF sub-basin and extends north/northwest to the Town of Chino Valley and beyond. These two sub-basins are part of a deep structural trough created by uplift of the surrounding mountains along steeply-dipping faults. The structural trough trends 28 miles northwest from Humboldt and is filled with alluvial, sedimentary, and volcanic rocks of Quaternary and upper Tertiary age (Corkhill and Mason, 1995). The floor and sides of the trough are formed chiefly by low-permeability basement rocks. Detailed descriptions of stratigraphy, hydrogeologic conditions, and groundwater resources in the area of the UAFRP are given by Krieger (1965), Littin (1981), Remick (1983), Wilson (1988), Nelson and Yunker (2014), and Corkhill and Mason (1995).

The principal geologic units in the UAF and Little Chino sub-basins are the Upper Alluvial Unit (UAU), consisting predominantly of sediments of Quaternary and Tertiary age, and the Basement Unit, consisting predominantly of rocks of Precambrian age (Wilson, 1988).

4.1.1 Upper Alluvial Unit

Corkhill and Mason (1995) provide a detailed summary of the geology of the UAU.

The unit includes generally deeper Quaternary and Tertiary sedimentary rocks and shallower unconsolidated to moderately consolidated alluvial sediments, including sand, gravel, silt, clay, and conglomerate. Thickness of the unit ranges from about 1,000 feet at the north end of the UAF sub-basin and east end of the Little Chino sub-basin to about 200 feet near the south end of the UAF sub-basin. In portions of the UAF sub-basin and adjacent Little Chino sub-basin, the lower part of the UAU includes layers of volcanic rocks ranging in thickness from approximately 20 to 500 feet. At the southernmost boundary of the UAF sub-basin (and Prescott AMA), the Agua Fria River exits the sub-basin where the UAU becomes progressively thinner and the Basement Unit occurs near land surface. A detailed description of hydrogeologic conditions of the UAU in the south part of the UAF sub-basin is given in the hydrogeologic reports associated with permitting the UAFRP (M&A, 2005 and 2011).

4.1.2 Basement Unit

The Basement Unit consists predominantly of rocks of Precambrian age, including a wide variety of crystalline or foliated igneous and metamorphic rocks that are generally dense, nonporous, and nearly impermeable (Wilson, 1988). Rock types commonly include granite, diorite, gabbro, schist, metavolcanics, and metasediments (Wilson, 1988). The Basement Unit is poorly permeable and forms the floor and sides of the structural groundwater basin in the UAF and Little Chino sub-basins (Corkhill and Mason, 1995).

As described above, the Basement Unit occurs near or at land surface at the southernmost boundary of the UAF sub-basin.

As mentioned above, volcanic rocks occur within and near the base of the principal UAU; the volcanic rock layers appear relatively continuous across much of the study area (especially the west and north portions) but are discontinuous or intermittent in the southeast part of the study area (i.e. south part of the UAF sub-basin). The volcanic rocks are fractured and porous, generally have high permeability, and provide substantial groundwater yield to production wells. In some areas, a relative thin layer of alluvial sediments underlies the volcanic rocks above the Basement Unit and is referred to as the Lower Alluvial Unit. Therefore, for the purposes of characterizing the general geologic stratigraphy of the study area, the UAU can be considered to consist of three units, resulting in the following delineation of four stratigraphic units:

• Upper alluvial unit (UAU)

• Volcanics/basalt unit

• Lower alluvial unit (LAU)

• Basement unit

4.2 Lithology and Stratigraphy of the Town’s Wellfields

The Town’s water supply wells are located within the UAF sub-basin and east part of the Little Chino sub-basin and comprise three primary water supply “systems”.

The Town’s Upper System includes the Santa Fe wellfield and several wells east of the primary wellfield (Grandview, Huggins, Manley, Laredo, and others), located within the northwest part of the UAF sub-basin. The Viewpoint System is essentially equivalent to the North wellfield, located in the southeast end of the Little Chino sub-basin and approximately 2 miles north from the Santa Fe wellfield. The Lower System wells are located in the south part of the UAF sub-basin, in the vicinity of the in-channel recharge facility (UAFRP). For the present study, it is more convenient and/or relevant to refer to these systems as the Santa Fe wellfield, North wellfield, and Lower System for geographic purposes. As described below, hydrogeologic conditions are generally similar in the Santa Fe and North wellfields, but differ from the Lower System where groundwater levels and depth to the Basement Unit are much shallower.

Based on review of available drillers’ logs and a limited number of geologists’ logs, hydrogeologic conditions in the study area are summarized below in the context of aquifer hydraulic properties relevant to recharge (injection) capacity.

4.2.1 Santa Fe and North Wellfields

Due to the general similarity of hydrogeologic conditions in the Santa Fe and North wellfields, the following summary addresses both wellfields:

• In the Santa Fe and North wellfields and adjacent areas, the UAU is very thick and includes substantial zones of coarse-grained sediments of estimated high permeability (although a few lithologic logs indicate or suggest significant cementation and lower permeability in some intervals). The UAU is underlain by the volcanics unit (chiefly basalt), which is relatively continuous throughout this region (most of the Town’s supply wells encountered the volcanics unit and/or were completed in the unit). Basalt was encountered at depths ranging from about 700 to 900 feet below land surface (bls). Very few of the wells fully penetrate the basalt; therefore the thickness is not well characterized. Thickness of volcanics/basalt at the Medina and Windsong wells, which do penetrate the volcanics/basalt, are approximately 260 and 240 feet, respectively. However, basalt was encountered at the Fat Chance well in the depth interval from 784 feet bls to total drilled depth of 1,220 feet bls (thickness of more than 436 feet) and in the depth interval from 750 to 1,258 feet bls at the Long Shot well (thickness of more than 508 feet). In general, the volcanics/basalt unit occurs at generally decreasing depths progressing to the north across the North wellfield; basalt occurs at a depth of about 700 feet bls or more at the south end of the North wellfield, and at a depth of about 500 feet bls at the north end.

• A relatively thin interval of LAU sediments appears to underlie the volcanics unit based on logs for two wells that fully penetrate the volcanics unit. The Medina well penetrates through the basalt at a depth of 980 feet bls into nonlithified predominantly coarse-grained alluvium (LAU is more than 120 feet thick), and the Windsong well penetrates through the basalt at a depth of 1,120 feet (LAU is more than 20 feet thick); neither of these wells encountered bedrock to the total drilled depths, so the LAU thickness at these wells is not known.

• The volcanics/basalt is fractured and porous and provides high yields to wells completed in it, and therefore would be expected to accept high injection rates for direct/deep injection wells. Specific capacities for wells in the Santa Fe wellfield are generally very large (as described further in Section 5.3). Glassford Hill is the residual volcanic core and source of the volcanic flows and ash that comprise the volcanics unit underlying the Santa Fe and North wellfields.

Graphic lithologic logs were prepared for selected Town wells, based on available drillers’ and geologists’ logs, as part of the Town’s Groundwater Representative Monitoring Plan (GRMP) proposal required by the U.S. Environmental Protection Agency (EPA) for the Fourth Cycle Unregulated Contaminant Monitoring Regulation (UCMR4). Graphic logs were prepared for selected wells for each of the Viewpoint System (North wellfield), Upper System (includes the Santa Fe wellfield and additional wells located east from the wellfield), and Lower System wellfield, which are shown on Figure 3. The graphic logs were prepared based on classifying the sediments or rock units described in the available logs into six categories based on lithologic properties and estimated relative permeability. Each “lithologic/permeability category” includes sediments or stratigraphic units with consistent or similar hydrogeologic properties, particularly those that affect permeability (chiefly particle size distribution and degree of lithification) based on M&A’s professional judgement.

This classification of sediment layers and rock units is approximate but provides a framework for evaluating and graphically displaying relative permeability and, therefore, potentially favorable versus restrictive conditions with regard to infiltration and downward movement of water. It should be noted that “permeability” as used in this report is synonymous with hydraulic conductivity. The six categories, as shown from top to bottom in the Explanation on the indicated figures, generally increase in silt and clay content and/or degree of lithification (cementation), and therefore decrease in estimated permeability. The first category is the exception because it represents lithified volcanic rocks that are fractured and porous and are assumed to have high to very high estimated permeability.

The graphic logs for Town wells compiled on Figure 3 are arranged generally from north/northwest to south/southeast across and between the three wellfields. Due to the large distance represented by this graphic log compilation, the graphic logs are positioned based on land surface altitudes of the wells. Lithologic and stratigraphic conditions represented by the graphic logs for selected wells in the Viewpoint System (North wellfield) and Upper System (includes the Santa Fe wellfield and several wells east of the wellfield) (Figure 3) are summarized in the following paragraphs.

Viewpoint System (North wellfield): Graphic lithologic logs for the Out & Out, Middle Robin, and Long Shot wells are all based on geologists’ logs and therefore are considered relatively reliable in regard to the lithologic/permeability categories. All three graphic logs indicate predominantly coarse-grained sediment intervals and medium to coarse-grained sediment intervals over most of the drilled depths. These sediments are interpreted to have high to very high estimated permeability. Very few (and relatively thin) fine-grained layers are indicated in the logs. The volcanics unit (basaltic rocks) was encountered at depths ranging from approximately 670 to 745 feet bls, and was not fully penetrated to total drilled depths ranging from 807 to 1,258 feet bls. Thickness of the volcanics unit is more than 513 feet at the Long Shot well (deepest of the three wells).

The geologists’ logs for these wells did indicate significant cementation in the sediments, but a few intervals were indicated to have minor cementation or to be poorly indurated.

Overall, these conditions indicate very permeable vadose zone sediments and very large transmissivity of the aquifer in the Viewpoint System (North wellfield). As described further in Section 5.3, three wells in the North wellfield have very large specific capacities while the other three wells have small specific capacities; the small specific capacities are not consistent with sediments described as predominantly coarse-grained and suggest a small transmissivity of the aquifer (unless the wells are very inefficient due to encrustation of well casing perforations).

Upper System (includes Santa Fe wellfield): Graphic lithologic logs were prepared based on geologists’ logs for the Santa Fe and Medina wells in the North wellfield and on drillers’ logs for the Windsong and Laredo wells near Mountain Valley Park. The graphic lithologic logs for the Santa Fe and Medina wells indicate relatively similar conditions, which consist predominantly of coarse-grained sediments of very high estimated permeability and medium to coarse-grained sediments of high estimated permeability to depths of approximately 390 to 440 feet (although no lithologic description was provided for the upper 240 feet of the Santa Fe well). For the Medina well lithologic log, some of this upper interval appears to be cemented, but the amount of this interval that is cemented and the degree of cementation is not clear. For the Santa Fe lithologic log, most of this upper interval is not cemented or has minor carbonate cementation. The underlying intervals at the Medina well consist of generally coarse-grained sediments that are indicated to be very cemented while the underlying sediments at the Santa Fe well contain increasing content of silt and/or clay. These intervals (for both wells) are estimated to have low to moderate permeability but include fine-grained layers of low to very low estimated permeability. The volcanics unit (chiefly basalt) was encountered at a depth of 710 feet bls at the Medina well and 820 feet bls at the Santa Fe well. The Medina well penetrates through the volcanics unit at a depth of 980 ft bls, which is underlain chiefly by very sandy sediments (apparently not cemented) to total drilled depth of 1,110 feet.

Graphic lithologic logs for the Windsong and Laredo wells, which are located in the east portion of the Upper System (and 1 to 2 miles from the Santa Fe and Medina wells), indicate notably different lithologic conditions in the upper half of the drilled interval (to depths of 400 feet or more). The graphic lithologic logs for these wells are based on drillers’ logs and may have questionable reliability, but indicate substantial intervals of finer-grained and/or “mixed” fine and coarse-grained sediments (e.g. “sandy clay” and “clay and gravel”) over most of the drilled interval, with some layers of medium and coarse-grained sediments. The driller’s log for the Windsong well indicates “sandstone” in the depth interval from 240 to 460 feet (likely corresponds to cemented sands indicated in the lithologic logs for other wells). Most of the subsurface profile would be interpreted to have low estimated permeability with some intervals of moderate to high estimated permeability and some intervals of very low estimated permeability. However, a geologist’s summary log for the Windsong well indicates that the upper 930 feet is generally coarse-grained (sandy gravel) with “minor interbedded clayey members”, which is not consistent with the driller’s log. Therefore, it is likely that the graphic lithologic log for the Windsong well underestimates the intervals of higher-permeability sediments. Basalt was encountered in the depth interval from 930 to 1,120 feet bls at the Windsong well and from 970 to 1,080 feet bls at the Laredo well. Bedrock was encountered below the basalt at the Laredo well, but sandy (non-cemented) sediments (LAU) were encountered below the basalt in the Windsong well.

Another relevant consideration for the Viewpoint System (North wellfield) and Upper System (especially the Santa Fe wellfield) is the potential occurrence of a confining layer or aquitard in this area. Based on ADWR’s research and model calibration for the 2014 Prescott AMA groundwater flow model, Keith Nelson (personal communication) indicated that the model is better calibrated in the vicinity of the Santa Fe and North wellfields if a very low permeability and somewhat extensive aquitard is simulated in the UAU at some depth above the basalt unit. The location and extent of this potential aquitard are not clear. Based on discussions with Keith Nelson and review of the ADWR model, it appears that the aquitard may be located chiefly northwest of the study area close to Granite Creek and the City of Prescott’s recharge project. The following considerations address the possible occurrence of this aquitard:

• It is possible that Keith Nelson based his conclusion on a very limited number of anomalous water level measurements, perhaps from a vadose zone piezometer at the Prescott recharge site.

• The occurrence of an aquitard in the vicinity of the North and Santa Fe wellfields, if not too shallow, would not be a fatal flaw. In fact, it could present the possibility that recharged effluent (via either surface basins or shallow injection wells) could effectively be separated from deeper groundwater zones with slow leakage through the aquitard, which would provide additional soil aquifer treatment.

• Based on evaluation of lithologic conditions encountered at several wells in the Santa Fe and North wellfields (described above), there is not an apparent extensive clay unit in the UAU. However, for the Santa Fe wellfield, clayey sediments (especially “clayey gravel”) are indicated more commonly below depths of 400 to 600 feet bls, and some of the logs reviewed (including wells not included on Figure 3) indicate substantial cementation or “conglomerate” below depths of 500 to 700 feet bls. Top of the volcanics/basalt unit was encountered at depths ranging from about 700 to 900 feet bls.

• Although it is possible that the aquitard could correspond to these variably cemented sediments in the Santa Fe wellfield, based on lithologic descriptions for all the geologists’ and drillers’ logs reviewed, it is unlikely that the aquitard is extensive, but rather is discontinuous. It is more likely that relatively localized perched groundwater zones may occur rather than an extensive aquitard and perched aquifer.

4.2.2 East Upper Agua Fria Sub-basin

For the purposes of describing hydrogeologic conditions, the “east UAF sub-basin” is considered to include the general area east of the Town’s residential area and south of Route 89, extending south to the Lower System wellfield. In the north part of the east UAF sub-basin (north of the Town’s ATF), hydrogeologic conditions are based on few available well logs:

• Much of the east UAF sub-basin appears to be predominantly fine-grained in the vadose zone and upper part of the aquifer. The deeper parts of the aquifer appear to include significant intervals of variably-cemented medium- to coarse-grained sediments with discontinuous layers of volcanic rocks. SWGW prepared a lithologic log for well (A-14-01)8bbb (Badger well), which is located approximately 0.7 mile northwest from the North Plains site (Figure 2). This geologist’s log indicates predominantly fine-grained sediments in the upper 360 feet, with underlying coarse-grained sediments to a depth of 420 feet bls, and “poorly cemented sandstone conglomerate” to a depth of 750 feet bls. Fractured basalt was encountered in the depth interval from 750 to 828 feet bls, underlain by schistose bedrock (Figure 4). Although the geologist’s log is considered reliable, it is difficult to interpret the potential permeability of the thick, poorly cemented sandstone conglomerate interval, but it is assumed to have moderate permeability.

• Based on lithologic logs prepared by M&A for three monitor wells at the North Plains site, sediments to depths of 200 feet or more are predominantly fine-grained, but deeper conditions are not known.

• Infiltration rates for surface basins in this north half of the east UAF sub-basin would likely be relatively small throughout the area, but based on very limited lithologic characterization of the deeper aquifer sediments, direct injection may be feasible.

The graphic lithologic logs prepared for the GRMP submittal and shown on Figure 3 also include two wells in the Lower System, the New Kurtz and Quailwood 3 wells. The graphic lithologic logs for these wells are based on geologists’ logs. Inspection of the graphic logs indicates the following:

• For the New Kurtz well, relatively heterogeneous sediments of low to very high estimated permeability occur in the upper 170 feet, and are underlain by sandy and/or clayey ”conglomerate” to the top of the volcanics unit at a depth of 665 feet. Conglomerate is typically associated with substantial lithification, but the geologist’s log indicates only minor cementation. Therefore, the estimated permeability of the entire conglomerate interval is essentially “indeterminant” but is assumed to be low to moderate. Volcanic ash was encountered at a depth of 665 feet bls and volcanic rocks were encountered at a depth of 710 feet bls;

thickness of the volcanics unit is approximately 80 feet.

• For the Quailwood 3 well, sediments in the upper 390 feet are generally heterogeneous but with a prevalence of medium to coarse-grained sediments of moderate to high estimated permeability. The underlying sediments consist of gravelly sand with very high estimated permeability to the top of basalt at a depth of 480 feet. No cementation is indicated in the geologist’s log for sediments in the upper 480 feet. The volcanics unit has a thickness of 50 feet and is underlain by 40 feet of sand and clayey sand above schistose bedrock at a depth of 570 feet bls.

A detailed description of hydrogeologic conditions of the UAU in the south part of the UAF sub-basin, essentially equivalent to the Lower System, is given in the reports associated with permitting the UAFRP (M&A, 2005 and 2011). The most important hydrogeologic conditions relevant to recharge and recovery considerations include:

• The UAU is much thinner than in the Santa Fe and North wellfields and the north half of the east UAF sub-basin; the saturated portion of the UAU aquifer is estimated to be as little as 200 feet thick near the south end of the UAF sub-basin (south end of Prescott AMA). This corresponds to relatively shallow depths to the Basement unit (bedrock).

• Depth to groundwater level is relatively shallow, ranging (in 2016) from several ten’s of feet (adjacent to the Agua Fria River channel) to approximately 150 feet.

• The UAU is generally heterogeneous but includes significant layers of fine-grained sediments.

• The volcanics unit is relatively thin and discontinuous in this area.

• Available pumping test results for the Lower System wells indicate very small specific capacities and associated hydraulic conductivities for the UAU aquifer (as described further in Section 5.3).

4.2.3 Mountain Valley Park

The Mountain Valley Park area is relevant to many recharge options evaluated in the present study involving direct/deep injection. Therefore, graphic lithologic logs were prepared for selected Town wells and several other wells and were used to construct two lithologic cross-sections, A-A’ and B-B’ (Figures 4 and 5); these cross-sections are located within the center of the study area and intersect at Mountain Valley Park; the lines of section are shown on Figure 2. It should be noted that these cross-sections do not include the Santa Fe and North wellfields but extend close to the Santa Fe wellfield.

These cross-sections were prepared chiefly to allow better evaluation of conditions in the vicinity of Mountain Valley Park. These graphic logs are based on the same lithologic/permeability categories described previously for the GRMP graphic logs but the categories representing alluvial sediments have somewhat different descriptions than for the GRMP graphic logs, which is due to integrating the descriptions provided in the relevant drillers’ and geologists’ logs. Cross-section A-A’ is aligned generally from south to northeast and cross-section B-B’ is aligned generally from southwest to northeast (Figure 2). The graphic logs for the wells shown on Figures 4 and 5 are not positioned based on the differing land surface altitudes but rather are shown in relation to depth below land surface to allow easier evaluation of depths to favorable (or unfavorable) intervals. Measured depth to groundwater level in 2016 is shown on each graphic log.

Relevant lithologic conditions and trends include the following:

• Well OBW-1 and the pilot vadose zone recharge well are located at Mountain Valley Park, and the Manley and Windsong wells are located within 0.5 to

0.75 mile of the park (Manley well is directly north and the Windsong well is southwest). The graphic logs for these four wells are relatively similar in the upper 400 feet, and indicate substantial intervals of fine to medium-grained sediments of low to moderate estimated permeability in approximately the upper 250 feet, underlain by medium to coarse-grained sediments of moderate to high estimated permeability.

• The graphic lithologic log for the Windsong well is based on a driller’s log, and is described in detail above (for the Upper System characterization). Of particular importance is that the driller’s log indicates “sandstone” in the depth interval from 240 to 460 feet (i.e. cemented sand), and overall indicates that most of the subsurface profile would be interpreted to have low to moderate estimated permeability (Figure 4). These conditions would not be conducive to high injection rates for direct injection wells. However, the available “lumped” geologist’s log indicates that the entire drilled depth (to top of basalt) consists predominantly of coarse-grained sediments (sandy gravel) with high or very high estimated permeability. Due to this discrepancy between the driller’s log (of questionable reliability) and the overly generalized geologist’s log, M&A’s interpretation of the relatively permeability of the vadose zone and aquifer is uncertain. In any event, the relatively thick basalt interval would be expected to have high permeability/transmissivity.

• The graphic lithologic log for the Manley well is based on a driller’s log of very questionable reliability, but indicates that lithologic conditions underlying the generally fine-grained upper zone consist of coarse-grained and/or “mixed” sediments (e.g. “sand with clay”) of moderate to high estimated permeability to a depth of 750 feet bls. Sediments in the interval from 750 to 910 feet bls (“clay with sand and gravel”) are interpreted to have low to moderate estimated permeability (Figure 4). The driller’s log indicates “granite and clay” in the interval from 910 feet to total drilled depth of 1,010 feet but does not indicate the presence of the volcanics/basalt unit. Overall, the (unreliable) driller’s log and interpreted relative permeability suggests that the deeper part of the aquifer at the Manley well may have limited transmissivity and may not be conducive to deep injection.

• Along cross-section A-A’ further north from Mountain Valley Park (Figure 2), the subsurface sediments encountered at the Huggins and Laredo wells (the Huggins well is projected to the line of section) are predominantly “mixed” (e.g.

“sandy clay and gravel” and “gravel and clay”) over most of the drilled depths, with some coarse-grained layers. Bedrock was encountered at a depth of 945 feet bls at the Huggins well; the volcanics/basalt unit appears to be absent. In general, the lithologic profiles at these two wells are interpreted to have low to moderate estimated permeability (Figure 4). However, a thick interval of coarse-grained sediments was encountered in the depth interval from 770 to 970 feet bls at the Laredo well, underlain by the volcanics/basalt unit (thickness of 110 feet) with bedrock encountered at 1,080 feet bls. Overall, lithologic conditions at the two wells indicate that the lower part of the aquifer at the Huggins well may not be very conducive for deep injection but that the lower part of the aquifer at the Laredo well may be favorable for deep injection.

• Along cross-section A-A’ southwest from Mountain Valley Park (Figure 2), the lithologic profile at the Lake Valley well is generally similar to the profile at the Manley well (Figure 4), including the absence of the volcanics/basalt unit and top of bedrock at a depth of approximately 900 feet bls. The unnamed well in the northeast corner of Section 23 has a substantial thickness of coarse-grained sediments of high estimated permeability in the upper 390 feet, but is underlain by bedrock (with no volcanics/basalt unit), indicating that bedrock becomes shallower to the south with a limited interval for injection.

• The relevant segment of cross-section B-B’ (that does not overlap cross-section A-A’) is east from Mountain Valley Park, extending to the Manley well (projected to the line of section) and further east to the Badger well (Figure 2).

The graphic lithologic logs for these two wells are very similar to a depth of approximately 730 feet, indicating chiefly fine-grained sediments of low estimated permeability in approximately the upper 300 feet bls, underlain by medium to coarse-grained sediments of moderate to high estimated permeability (Figure 5). Lithologic conditions at the Badger well are described in detail in Section 4.2.2.