Chapter 4 Existing Water Supply (PDF)

CHAPTER 4 EXISTING WATER SUPPLY

This chapter describes the City’s existing water supply sources which include treated surface water purchased from the MID and local groundwater pumped from City wells. A recommended integrated (conjunctive use) water supply plan for the future use of these existing water supplies, in addition to other potential future water supply sources, is presented in Chapter 5 Integrated Water Supply Plan.

4.1 BACKGROUND

The City currently uses a conjunctive water use strategy with two primary water sources to meet potable water demands within the City’s service area. These include:

• Treated Tuolumne River surface water purchased on a wholesale basis from MID; and • Local groundwater pumped from City wells located throughout the City’s service area.

City residents within the contiguous service area north of the Tuolumne River (including North Modesto, Salida, Empire and some unincorporated Stanislaus County “islands”) generally rely on treated surface water supply from MID year-round, supplemented with groundwater. Water demands for the contiguous service area located south of the Tuolumne River (South Modesto) and the City’s outlying service areas are met entirely with groundwater supply year-round. These service areas and available water supplies are summarized in Table 4-1.

Table 4-1. Water Supplies Available to the City’s Water Service Areas(a)

Service Area Treated Surface Water Groundwater Contiguous Service Areas North Modesto ✓ ✓ Salida ✓ ✓ Empire ✓ ✓ South Modesto ✓ Outlying Service Areas Del Rio ✓ Turlock ✓ North Ceres ✓ Ceres (Walnut Manor) ✓ Grayson ✓ (a) As noted elsewhere in this WMP, the Waterford and Hickman water systems are no longer owned by the City.

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Prior to January 1995, all municipal and industrial (M&I) water demands in the City were met from groundwater pumping. Beginning in the 1950s, groundwater pumping by the City and Del Este increased substantially in response to increased water demands resulting from growth, and this, along with periodic drought conditions, contributed to a reduction in groundwater levels and created a cone of depression under the City1. This cone of depression, combined with increasingly stringent federal and state water quality requirements, prompted a 1984 study of the groundwater supply2. The study recommended a conjunctive water use program that would supplement the City’s M&I groundwater supply with treated surface water from the Tuolumne River. Following the recommendations of the study, in 1986 MID and the City signed an agreement to allow MID to pursue the construction of a water treatment plant to supply treated water from the Tuolumne River to the City. In January 1995, Phase One of the MRWTP became operational, and the City began receiving an annual average of 30 mgd (33,600 af/yr) of treated surface water from MID3. The MRWTP Phase Two Expansion, completed in early 2016, will ultimately provide the City with up to an additional 30 mgd of treated surface water supply, for a total annual average supply of up to 60 mgd (67,200 af/yr).

4.2 ANNUAL WATER PRODUCTION

The City’s annual treated surface water purchases from MID and groundwater pumpage from 2000 through 2015 to serve the City’s contiguous and outlying service areas are shown in Figure 4-1. As shown, total annual water production has generally decreased since the early 2000’s, a direct result of decreased water demands resulting from the economic downturn and water conservation in response to recent drought conditions. Water production in both 2014 and 2015 was particularly low in response to conservation due to the on-going drought. Except for the last two years where MID imposed a 43 percent (2014) and 62 percent (2015) reduction in surface water deliveries to both their agricultural customers and the City, overall annual surface water supplies to the City have been relatively consistent, with groundwater pumpage varying as needed to meet demands.

4.3 SURFACE WATER SUPPLY

As discussed above, customers within the contiguous service area north of the Tuolumne River rely on a combination of treated surface water supply from MID and City-pumped groundwater. The treatment and delivery of the surface water supply to the City is described in more detail in the following sections.

1 Draft Environmental Impact Report for the Modesto Surface Water Treatment Plant, prepared by URS Consultants, Inc., October 1989. 2 Modesto-Ceres Area Water Management Study, Final Report, prepared by Montgomery Engineers, 1984. 3 As described elsewhere in this chapter, the agreed upon delivery is 33,602 af/yr; however, for practical purposes, this quantity is rounded to 33,600 af/yr throughout the balance of this WMP.

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4.3.1 Modesto Irrigation District

MID is a public agency which was formed in 1887 as the second irrigation district in California. MID provides the following services:

• Electric service for the greater Modesto area (north of the Tuolumne River), Waterford, Salida, Mountain House (northwest of Tracy) and parts of Ripon, Escalon, Oakdale and Riverbank; • Irrigation water to approximately 3,100 agricultural customers with approximately 58,000 irrigated acres; and • Treated surface water to the City of Modesto (for use in the City’s service areas listed in Table 4-1).

MID is primarily an agricultural water supplier, and though treated water is provided to the City of Modesto for urban delivery, MID does not directly serve any domestic water customers. MID’s treated water ‘place of use’ is defined by the overlap of the MID water service boundary with the City’s contiguous service area north of the Tuolumne River (see Figure 4-2). The common City and MID water service area excludes those areas served by the City exclusively with groundwater and/or which lie outside the MID water service boundary (excluded areas include the communities of Del Rio and Grayson, parts of Ceres and Turlock and other City water service areas south of the Tuolumne River). Other communities and/or areas served by the City (Salida, Empire and some unincorporated “islands”) lie within the MID service area.

Together with the Turlock Irrigation District (TID), MID holds senior rights to water from the Tuolumne River. The 1,960-square mile Tuolumne River watershed extends to the high Sierra Nevada Mountains and the river flows to its confluence with the San Joaquin River approximately ten miles west of Modesto. Most of the water in the Tuolumne River comes from snowmelt, with peak runoff flows occurring from April through July during which time over 60 percent of the annual flow takes place. Within the lower Tuolumne River Watershed, MID and TID operate the New Don Pedro Reservoir with a maximum storage capacity of 2,030,000 acre-feet. MID’s annual diversion from the Tuolumne River is 315,756 acre-feet of water. Of the total MID diversion, approximately 30 mgd (33,600 af) annually is delivered to the Modesto Reservoir for treatment and delivery to the City.

In addition, MID also maintains approximately 90 water wells that are used to supplement the surface water supply during dry years for use by MID’s agricultural customers.

4.3.2 Modesto Regional Water Treatment Plant

In the early 1990’s, the City, MID, and the former Del Este Water Company formed the Modesto Domestic Water Partnership to use a portion of MID’s surface water rights for municipal uses. They entered into a TDA (described further below) to address the design, construction, commercial operation, and financing for Phase One of the MRWTP.

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As noted above, Phase One of the MRWTP is a 30 mgd (33,600 af/yr) domestic water treatment plant. Since its completion, the plant has been operated by MID to treat Tuolumne River water which is then sold on a wholesale basis to the City. The original 30 mgd MRWTP is a conventional treatment facility providing flocculation, sedimentation, and filtration, along with ozonation for primary disinfection.

Treated water from the MRWTP is delivered to the City via MID’s terminal reservoir facilities (a booster pump station and two 5-million-gallon storage tanks located on the east side of the City) and 20 to 25 active turnouts constructed as part of the Tier 1 and Tier 2 Downstream Improvements that have the ability to control water supplied from the MID transmission mains at various points within the City’s water distribution system. The locations of the MRWTP and the turnouts are shown on Figure 2-2 in Chapter 2 Existing Water System.

The MRWTP Phase Two Expansion, completed in early 2016, will ultimately provide the City with up to an additional 30 mgd of treated surface water supply, for a total annual average supply of up to 60 mgd (67,200 af/yr). For the purposes of the ‘Existing Water Supply’ analysis in this chapter, and as directed by the City, it is assumed that the initial quantity of treated surface water available from the MRWTP Phase Two Expansion will be 10 mgd (11,200 af/yr), This quantity will increase as additional development occurs within the City’s contiguous service area and within MID’s treated water ‘place of use’ up to a total of 30 mgd. The MRWTP Phase Two Expansion is a membrane treatment facility.

The combined capacity available with the completion of the MRWTP Phase Two Expansion (60 mgd or 67,200 af/yr) is an annual average, and both the original and expanded facilities will have peaking capacities greater than the annual averages. Phase One of the MRWTP has a maximum functional capacity of 42.5 mgd that helps meet the maximum day and peak hour demands of the City’s contiguous service area north of the Tuolumne River, but has been permitted by the State to produce up to 45 mgd for the last few years. Peaking capacity for the Phase Two Expansion will be determined after start-up operations and testing protocols are completed.

4.3.3 Treatment and Delivery Agreement

The Treatment and Delivery Agreement among the Modesto Irrigation District, City of Modesto, and Del Este Water Company enacted in 1992 established the delivery of treated surface water to the City. It obligated MID to deliver 30 mgd of treated surface water to the City, commencing on May 1 and ending the following April 30 during wet or normal hydrologic years. The City assumed the rights and responsibilities for domestic treated water delivery as defined under the TDA upon the purchase by the City of the former Del Este Water Company in 19954. In October 2005, MID and the City approved the Amended and Restated Water Treatment and Delivery Agreement between Modesto Irrigation District and the City of Modesto (ARTDA). This agreement supersedes the original TDA and sets forth the terms and conditions for the

4 In 1995, the City of Modesto purchased the Del Este Water Company systems in Empire, Salida, Waterford, Hickman, Grayson, Del Rio, and portions of Ceres and Turlock. In 2015, the City of Modesto subsequently sold the Waterford and Hickman systems to the City of Waterford.

4-4 City of Modesto September 2017 Water Master Plan o\c\418\02-14-36\wp\mp\100814_4Ch4 Chapter 4 Existing Water Supply delivery of up to 60 mgd of treated surface water to the City from the expanded MRWTP. The increased water treatment capacity provided by the MRWTP Phase Two Expansion will allow the City to serve more surface water to its customers, thereby reducing its long-term dependence on groundwater.

4.3.4 Dry Year Availability

The ARTDA includes a formula to reduce deliveries to the City in drier than average years based on the number of inches allocated to agricultural customers.

The ARTDA specifies a maximum delivery of 42 inches of water, or the amount calculated as (y/42) times 33,602 af/yr5, whichever is less (where y is the actual number of inches of water per acre allocated by MID to agricultural water users for the irrigation season6). Although the ARTDA specifies a formula7 for water allocations during shortages, the reduction in supply is not determined until the time of the shortage (ARTDA, Section 17.2 Formula for Water Allocation).

The allocation formula is as follows:

y 33,602.1 = x 42 where: y is the number of inches of water per acre allocated to MID’s agricultural customers, and x is the calculated amount of water to be delivered to the City in that particular year in acre-feet.

The ARTDA also provides the opportunity for the City to purchase additional water from MID (at a higher rate) or to trade groundwater for agricultural use for treated surface water to achieve the full entitlement during drought years if such supplemental supplies are available.

In 2014, the base supply was defined as 24 inches of water per acre of the total 42-inch water allocation (equivalent to a 43 percent reduction), resulting in a treated water supply delivery to the City of approximately 17.1 mgd (19,200 af) for the 2014/2015 water year (May 1, 2014 through April 30, 2015).

In 2015, due to the on-going drought, MID further reduced surface water deliveries to both its agricultural customers and the City. The supply provided was only 16 inches of water per acre of the total 42-inch allocation (equivalent to a 62 percent reduction), resulting in a treated water

5 As described in the ARTDA, upon completion of MRWTP Phase Two Expansion, the treated water quantity shall be changed from 33,602 af/yr to 67,204 af/yr. 6 The irrigation season is defined the ARTDA to be May 1 through April 30 (e.g., the 2015/2016 irrigation season extends from May 1, 2015 through April 30, 2016). 7 This formula may be modified in the near future due to a number of factors including the effects of long-term climate change.

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During 2014 and 2015, MID did have supplemental water supplies available for delivery to the City (as an optional supply available at a higher rate than base supply); however, the City opted not to participate. Instead, the City pumped additional groundwater and implemented an aggressive water conservation program to help reduce demands to match available supplies (see Figure 4-1).

Additional discussion of the availability and reliability of the MID surface water supplies in dry years is provided in Chapter 5 Integrated Water Supply Plan.

4.3.5 Surface Water Quality

The Tuolumne River watershed covers approximately 1,960 square miles of the western slopes of the central Sierra Nevada Mountains, including portions of the Yosemite National Park. Snowmelt from the central Sierra Nevada is of excellent quality. For example, surface water diverted from the Tuolumne River at the La Grange Dam has a Total Dissolved Solids (TDS) concentration of only about 36 milligrams per liter (mg/L)8. Other water quality constituents that impact agricultural and domestic water use are also very low or negligible. Also, the quality of the river water is fairly consistent from year to year. As runoff from agricultural and developed land is introduced into the lower part of the river, the overall water quality degrades somewhat, but generally remains good.

4.4 GROUNDWATER SUPPLY

As of October 2015, the City had a total of 110 available groundwater wells located throughout the City’s entire water service area (92 wells in the contiguous service area and 18 wells in the outlying service areas). These wells are located within the San Joaquin Valley Groundwater Basin (Modesto, Turlock and Delta-Mendota subbasins). Figure 4-3 illustrates the location of the City’s water service areas and groundwater wells in relation to the boundaries of the three underlying groundwater subbasins.

As described in Chapter 2 Existing Water System, the City relied exclusively on groundwater to meet water demands until the introduction of treated surface water in 1995. The City uses groundwater conjunctively with the treated surface water supply from MID only in the contiguous service area north of the Tuolumne River. The remaining contiguous service area located south of the Tuolumne River (South Modesto) and the outlying service areas currently rely solely on groundwater.

8 This TDS level is extremely low (TDS ranges from 10 mg/L in rainwater to 35,000 mg/L in average seawater). The California State Water Resources Control Board Division of Drinking Water (DDW) has established Secondary Maximum Contaminant Level (SMCL) drinking water standards for public water supplies for TDS. SMCLs are ranges set by DDW for taste and odor thresholds. For TDS, the recommended SMCL is 500 mg/L and the upper SMCL is 1,000 mg/L.

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As shown on Figure 4-1, since 2000, the City’s total annual groundwater pumpage has ranged from a high of about 43.5 mgd (48,800 af) in 2002 to a low of about 28.1 mgd (31,500 af) in 2013. The general decrease in overall water use and groundwater pumpage in recent years is a direct result of water conservation by the City’s water customers in response to the recent drought conditions. However, in 2014, groundwater pumpage increased over 2013 quantities to 33.8 mgd (37,800 af) to supplement reduced surface water deliveries. In 2015, surface water supplies were again reduced, and groundwater pumpage was close to 29.7 mgd (33,300 af) (making up about two-thirds of the City’s total water supply in 2015).

The following sections describe the City’s groundwater resource, including a description of the basin and subbasins, estimated operational yield, management activities, recent sustainability legislation, monitoring requirements, historical flow and level trends, water quality issues and concerns and modeling efforts.

4.4.1 Groundwater Basin Description

The City’s existing water service area is located within the San Joaquin Valley Groundwater Basin (SJV Basin), and the City’s water customers have historically relied on groundwater pumped from three of the nine subbasins defined within the SJV Basin, those being the:

• Modesto Subbasin, • Turlock Subbasin, and • Delta-Mendota Subbasin.

Key characteristics of these groundwater subbasins are summarized in Table 4-2.

Table 4-2. Groundwater Subbasin Characteristics and Service Areas(a)

DWR Surface Area City Water Service Subbasin Name Subbasin No. General Location Acres (sq mi) Areas(b) North Modesto North of Tuolumne Salida Modesto 5-22.02 247,000 / (385) River Empire Del Rio South Modesto South of Tuolumne Turlock Turlock 5-22.03 347,000 / (542) River North Ceres Ceres (Walnut Manor)

West of the San Delta-Mendota 5-22.07 747,000 / (1,170) Grayson Joaquin River

(a) Based on information published in the State of California Department of Water Resources (DWR) Bulletin 118 Groundwater Subbasin Descriptions: • Modesto Subbasin (last updated February 27, 2004) • Turlock Subbasin (last updated January 20, 2006) • Delta-Mendota Subbasin (last updated January 20, 2006) (b) See Figure 4-3 for locations of City water service areas in relation to groundwater subbasin boundaries.

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Groundwater in the Modesto Subbasin occurs under unconfined, semi-confined, and confined conditions. The unconfined aquifers occur in the unconsolidated deposits above and east of the Corcoran Clay, which underlies the southwestern portion of the subbasin at depths ranging from 150 to 250 feet. Where clay lenses restrict the downward flow of groundwater, semi-confined conditions occur. The confined aquifers occur in the unconsolidated deposits below the Corcoran Clay. The estimated average specific yield of this subbasin is 8.8 percent9.

There are three aquifers in the Turlock Subbasin: the unconfined aquifer; the semi-confined and confined aquifer in the consolidated rock fractures; and the confined aquifer beneath the E-clay in the western subbasin. The estimated average specific yield of the subbasin is 10.1 percent10.

Groundwater in the Delta-Mendota subbasin occurs in three zones. These include the lower zone, which contains confined fresh water in the lower section of the Tulare Formation, an upper zone which contains confined, semi-confined, and unconfined water in the upper section of the Tulare Formation and younger deposits, and a shallow zone which contains unconfined water within about 25 feet of the land surface. The estimated specific yield of this subbasin is 11.8 percent11.

The primary sources of groundwater recharge in all three subbasins are from deep percolation of applied irrigation water and from reservoirs, canals and large stream/creek/river seepage. Lesser groundwater recharge occurs from percolation from small streams and direct percolation of precipitation. Table 4-3 provides a summary of the estimated natural and applied water recharge and groundwater extraction in each of the three subbasins under the City’s service area.

Table 4-3. Groundwater Subbasin Recharge and Extraction Estimates(a)

Natural Applied Water Groundwater Subbasin Name Recharge, af/yr Recharge, af/yr Extraction, af/yr 81,000 (urban) Modesto 86,000 92,000 145,000 (ag) 65,000 (urban) Turlock 33,000 313,000 387,000 (ag) 17,000 (urban) Delta-Mendota 8,000 74,000 491,000 (ag) (a) Based on information published in DWR Bulletin 118 Groundwater Subbasin Descriptions.

The Modesto and Turlock subbasins have not been identified by DWR as being critically overdrafted basins; however, in January 2016, the Delta-Mendota subbasin was identified by DWR as being a critically overdrafted basin. The Delta-Mendota subbasin was not previously identified as being critically overdrafted in DWR’s 1980 Bulletin 118; however, per the requirements of SGMA, DWR was directed to review and evaluate groundwater conditions from

9 Source: DWR Bulletin 118, Modesto Subbasin Description, last updated February 27, 2004. 10 Source: DWR Bulletin 118, Turlock Subbasin Description, last updated January 20, 2006. 11 Source: DWR Bulletin 118, Delta-Mendota Subbasin Description, last updated January 20, 2006.

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1989 to 2009 to develop a revised list of critically overdraft basins (it should be noted that conditions from 2011 to 2015 were not considered as the SGMA legislation requires the current drought period to be excluded from the evaluation). Based on this evaluation, the Delta-Mendota subbasin was added to the list of critically overdrafted basins in January 201612.

The Modesto, Turlock and Delta-Mendota subbasins are not adjudicated. This means that there is no court-appointed “watermaster” to resolve groundwater pumping issues, and there are no current specific limits on the amount of groundwater that individuals and agencies may extract from the basins. However, on September 16, 2014, Governor Jerry Brown signed into law a three-bill legislative package, composed of AB 1739 (Dickinson), SB 1168 (Pavley), and SB 1319 (Pavley), collectively known as the Sustainable Groundwater Management Act of 2014 (SGMA). SGMA empowers local agencies to manage groundwater basins in a sustainable manner over a long-term period. Further discussion on SGMA and its implications for future management of the groundwater subbasins underlying the City’s service areas is provided in Subsection 4.4.4 Groundwater Sustainability.

4.4.2 Groundwater Operational Yield

4.4.2.1 Yield Definitions

Definitions for operational yield, sustainable yield and safe yield were developed for the City during the development of the City’s 2005 Urban Water Management Plan (UWMP) and are summarized below:

• Operational Yield is the volume or quantity (typically in acre-feet) of local groundwater withdrawn on an annual average basis from a pre-defined area (i.e., sphere of influence) that does not exceed the long-term annual average recharge rate of the local aquifer(s) from which the groundwater is being pumped. • Sustainable Yield is similar to operational yield, but applies to an entire groundwater subbasin or basin and applies to all of the entities pumping from it as a whole, rather than just a localized area and a specific agency. • Safe Yield is everything defined for sustainable yield, but also includes other considerations beyond just a quantity of water extracted or recharged, such as its quality and resulting potential for surface subsidence. Safe yield can be defined as the maximum amount of water that can be pumped without creating any long-term undesirable results.

Operational yield is the amount (or rate) of localized groundwater extraction, on an annual average basis, that does not exceed the long-term annual average recharge rate of the localized aquifer(s) and does not cause groundwater elevations to drop below a pre-determined level based on available data, and as required for long-term sustainable use of the subbasin/basin.

12 Source: DWR Website, Sustainable Groundwater Management Webpage, List of Critically Overdrafted Basins, January 2016 (http://www.water.ca.gov/groundwater/sgm/pdfs/COD_BasinsTable.pdf).

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From the City’s 2007 Preliminary Operational Yield Estimate, discussed in more detail in the following subsection, this minimum level has been determined to be 40 feet above mean sea level (ft msl). At any given time, the quantity of water that can be pumped by the City depends on the amount of groundwater available in the subbasin/basin, the ability of the City’s wells to pump (e.g., operational capacity), as well as pumping by other users. This operational yield, or amount of groundwater extraction, represents the amount of groundwater that can be extracted without lowering groundwater levels below 40 feet above mean sea level and potentially affecting long-term sustainability of the subbasin/basin. Therefore, it can be assumed that the City could potentially increase its annual groundwater extraction rates to volumes greater than are currently being pumped without adversely affecting the subbasin/basin. However, the City’s current operational capacity for groundwater production is also limited by the reliable pumping capacity of its existing wells.

As described above, a groundwater subbasin/basin’s sustainable yield is the average annual amount of groundwater that can be extracted from the groundwater basin, while maintaining a non-overdraft condition. The sustainable yields of the Modesto and Turlock subbasins are currently unknown; however, the City is participating in a study with the United States Geological Survey (USGS) to model the Modesto and Turlock subbasins, which will hopefully lead to a better quantification of the sustainable yields for both subbasins in the future.

4.4.2.2 Preliminary Operational Yield Estimate

In 2007, the City estimated that their Preliminary Operational Yield from the three groundwater subbasins underlying the City’s service area is approximately 53,500 af/yr. This Preliminary Operational Yield was estimated based on historical groundwater pumpage by the City from the Modesto, Turlock, and Delta-Mendota subbasins, and was developed by City staff to maintain a minimum average groundwater elevation of 40 ft msl. A copy of the City’s 2007 Technical Memorandum titled “Discussion on Operational Yield for the 2005 Urban Water Management Plan” documenting this Preliminary Operational Yield is provided in Appendix E.

The general conclusion of the City’s analysis was that if the total, long-term average groundwater pumpage quantity is held at or below 53,500 af/yr, then stable groundwater levels will result at around 40 ft msl within and near the City’s contiguous service area. If groundwater pumpage is significantly less than 53,500 af/yr, groundwater levels will probably rise; thereby, increasing the quantity of available groundwater stored within the basin for later use in dry periods and/or to meet future demands, via “in-lieu” groundwater banking. Alternatively, if more than 53,500 af/yr is extracted (e.g., during dry years), groundwater levels will probably decline. Actual annual groundwater pumpage is expected to be less during normal or wet years and higher during dry years.

Table 4-4 summarizes the long-term average Preliminary Operational Yield assumed for each subbasin.

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Table 4-4. Preliminary Operational Yield Assumed for Each Subbasin(a)

Groundwater Subbasin Allocation, af/yr Modesto 48,286 Turlock 4,900 Delta-Mendota 314 Total 53,500 (a) As documented in the City’s Technical Memorandum titled “Discussion on Operational Yield for the 2005 Urban Water Management Plan” (see Appendix E).

4.4.2.3 Alternative Methods to Estimate Operational Yield

To put the City’s Preliminary Operational Yield into relative perspective, a brief review of alternative operational yield evaluations was conducted in 2015 by RMC (see Appendix F). This study evaluated four general categories of methods for developing operational yield estimates:

• Comparative Analysis, • Data Approach, • Modeling Approach, and • Hybrid Approach.

The City’s Preliminary Operational Yield, described above, has been developed using a Comparative Analysis. The Comparative Analysis is the least sophisticated method of determining operational yield; however, it is an appropriate method to use in the absence of extensive groundwater basin characteristic information and pumping data. Calculation of operational yield using this approach does not account for seasonal peak water demands, water quality restrictions, or localized water distribution and pressure issues. It also does not provide information related to growth, so the sophistication of the analysis and outputs are more limited than with a Data, Modeling or Hybrid Approach.

While a Comparative Analysis similar to that conducted in 2007 could be used for updating the City’s operational yield estimate, more extensive groundwater information has become available over the last eight years to support a more complex Data, Modeling, or Hybrid Approach. These new data include additional pumping and groundwater elevation data (since 2005 and including the last three years of groundwater basin drought response), water quality data, subsidence monitoring, updated hydrogeologic data (included in the update to the USGS MODFLOW model) and water budget information. These data can help refine the previously estimated Preliminary Operational Yield value because more information is known regarding the stability of the basin and the changes in groundwater elevations relative to changing hydrologic conditions.

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A Data Approach is commonly used in non-complex basins with available data and can be used when a detailed numerical groundwater flow model is not available for the basin. Compared to the other methods, a Data Approach is more sophisticated than a Comparative Analysis, as it utilizes numerical representations of water budget components, but is typically less sophisticated than the Modeling and Hybrid Approaches. With a Data Approach, the analytical results will have a higher degree of accuracy, confidence and defensibility, similar to Modeling and Hybrid Approaches, but will lack the operational flexibility and applicability to specific fine-grained areas. Similar to the relative level of sophistication of this approach, the Data Approach will be more costly to implement than a Comparative Analysis, but would be less costly (and less time intensive) than a Modeling or Hybrid Approach.

When an existing numerical groundwater model is available for a groundwater basin, the most common method for yield development is to utilize the groundwater model. Extracting groundwater budget information from a model can be done using post-processing software, particularly with respect to modifying the modeled study area to better match the localized study area of concern. Utilizing a Modeling Approach allows for flexibility in running various pumping scenarios, modifying groundwater budget variables in the model to determine the sensitivity to parameters, and taking a real-time look at how various pumping ranges effect groundwater flow conditions such as subsurface flow, groundwater levels, etc.

A Hybrid Approach to developing yield estimated utilizes a combination of numerical and analytical methods to estimate yield. For example, such an approach would use an existing groundwater model to provide larger, basin-wide values for the various water balance components, and then use an analytical approach to apply this on a regional or local basis. This method provides the highest degree of flexibility, accuracy, and applicability, but also bears the greatest cost of all of the methods described.

Based on this evaluation of available approaches, it is recommended that the City complete their updated operational yield analysis using either the Modeling Approach or Hybrid Approach, depending on the level of effort to be budgeted for this evaluation and the viability of the USGS MODFLOW model. The reason for recommending either of these two approaches is that the City has access to the recently updated and re-released USGS MODFLOW model. The City can utilize the water budget information that has been developed by the USGS and which will provide the most comprehensive look at the data input parameters available. These data can then be used with an analytical method to downscale and better match the boundary of the Modesto Sphere of Influence (or other study area), or the model grid itself can be discretized to provide more refined estimates of water balance components for the proposed study area, or both.

Additionally, by using the USGS MODFLOW model, the City can run multiple pumping scenarios to determine the appropriate operational yield threshold, and also evaluate the potential impacts of pumping certain well clusters more or less, as it relates to the potential creation of localized cones of depression. After running scenarios and extracting the operational yield estimate, the City can then compare the model outputs with historical water levels and pumpage, thus conducting a Hybrid Approach as a final check.

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The recommended steps to conduct such an analysis are as follows:

• Step 1: Data Collection — Obtain model and determine appropriate post-processing software • Step 2: Run Scenarios — Determine scenarios to model — Run scenarios and generate output information • Step 3: Scenario Analysis and Recommendations — Compare scenarios and evaluate against criteria such as water levels, underflow, etc. — Rank scenarios and recommend operational yield • Step 4: Historical Comparison — Compare results and recommended yield along with model output water levels to historical water levels and corresponding pumpage • Step 5: Summarize Findings

4.4.3 Groundwater Management

Groundwater Management Plans have been prepared for both the Modesto and Turlock subbasins as described below.

The City and MID participated in groundwater management studies initiated by the 1992 California State Assembly Bill 3030 (AB 3030). The goal of this bill, also referred to as the Groundwater Management Act, is to maximize the sustainable water supply while protecting the quality of the groundwater basin. The Stanislaus and Tuolumne Rivers’ Groundwater Basin Association completed the Integrated Regional Groundwater Management Plan for the Modesto Subbasin in 2005 in compliance with the Groundwater Management Planning Act of 2002 (SB 1938) and the Integrated Regional Water Management Planning Act of 2002 (SB 1672). The Association is made up of the following agencies: City of Modesto, MID, City of Oakdale, Oakdale Irrigation District, City of Riverbank, Stanislaus County, and the newly added City of Waterford. The Integrated Regional Groundwater Management Plan covers the entire Modesto Groundwater Subbasin as well as parts of the Eastern San Joaquin Groundwater Subbasin. The Plan was adopted by the City in July 2005.

4-13 City of Modesto September 2017 Water Master Plan o\c\418\02-14-36\wp\mp\100814_4Ch4 Chapter 4 Existing Water Supply

The City also participated in the preparation of the Turlock Groundwater Basin Management Plan. This Plan was prepared by the Turlock Groundwater Basin Association and was completed in 2008. Other agencies involved in this association include the City of Turlock, Turlock Irrigation District, City of Ceres, City of Hughson, Merced Irrigation District, Eastside Water District, Delhi County Water District, Ballico Community Services District, Ballico-Cortez Water District, Hillmar Water District, Denair Community Services District, the Keyes Community Water District, Stanislaus County and Merced County.

4.4.4 Groundwater Sustainability

The SGMA, a three-bill legislative package composed of AB 1739 (Dickinson), SB 1168 (Pavley), and SB 1319 (Pavley), was passed in September 2014. The legislation provides a framework for sustainable management of groundwater supplies by local authorities, with a limited role for state intervention when necessary to protect the resource. The legislation lays out a process and a timeline for local authorities to achieve sustainable management of groundwater basins. It also provides tools, authorities and deadlines to take the necessary steps to achieve the goal. For local agencies involved in implementation, the requirements are significant and can be expected to take years to accomplish. The State Water Resources Control Board may intervene if local agencies do not form a Groundwater Sustainability Agency (GSA) and/or fail to adopt and implement a Groundwater Sustainability Plan (GSP).

The SGMA implementation steps and deadlines are shown in Table 4-5.

Table 4-5. Sustainable Groundwater Management Act Implementation Steps and Deadlines

Implementation Step Implementation Measure Deadlines Local agencies must form local June 30, 2017 Step One Groundwater Sustainability Agencies (GSAs) within two years Agencies in basins deemed January 31, 2020 for critically overdrafted high- or medium-priority must basins adopt Groundwater Sustainability Step Two January 31, 2022 for high- and medium-priority Plans (GSPs) within five to seven basins not currently in overdraft years, depending on whether a basin is in critical overdraft Once plans are in place, local January 31, 2040 for critically overdrafted agencies have 20 years to fully basins Step Three implement them and achieve the January 31, 2042 for high- and medium-priority sustainability goal basins not currently in overdraft

SGMA applies to basins or subbasins designated by the DWR as high- or medium-priority basins, based on a statewide ranking that uses criteria including population and extent of irrigated agriculture dependent on groundwater. The final Basin Prioritization findings indicate that 127 of California's 515 groundwater basins and subbasins are high- and medium-priority basins. These high- and medium-priority basins account for 96 percent of California’s annual

4-14 City of Modesto September 2017 Water Master Plan o\c\418\02-14-36\wp\mp\100814_4Ch4 Chapter 4 Existing Water Supply groundwater pumping and supply 88 percent of the population with potable water. The rankings of the groundwater subbasins underlying the City’s service areas are shown in Table 4-6. As shown, all three of the City’s underlying groundwater subbasins have been designated as high-priority basins, and the Delta-Mendota subbasin has also been identified to be a critically overdrafted basin.

Table 4-6. Groundwater Basin Prioritization for Sustainable Groundwater Management Act (a)

Basin Overall Basin Rank(b) Number Subbasin Name Ranking Score Overall Basin Priority 13 5-22.02 Modesto Subbasin 23.5 High High 26 5-22.07 Delta-Mendota Subbasin 22.3 (Critically Overdrafted Basin) 38 5-22.03 Turlock Subbasin 21.5 High (a) CASGEM Groundwater Basin Prioritization Results, run version May 26, 2014. (b) Out of a total of 515 basins, of which 127 were high- or medium-priority basins. Rank is based on the Overall Basin Ranking Score from the highest score (highest priority) with a Rank of 1 to the lowest score (lowest priority) with a Rank of 515 (e.g., the lower the rank value, the higher the priority).

Although not yet finalized, it is believed that the local groundwater associations will act as the GSAs for the development of the GSPs for the Modesto, Delta-Mendota and Turlock subbasins.

4.4.5 Groundwater Monitoring

As described above, the City currently uses its surface water and groundwater supplies conjunctively to meet customer demands within the North Modesto service area. Additional future surface water supplies provided from the MRWTP Phase Two Expansion will provide the City with the opportunity to further utilize available surface water directly to meet demands in lieu of using groundwater.

To minimize the City’s vulnerability to groundwater quality issues, the City has developed strategies to maintain and enhance its groundwater extraction capacity through a combination of well monitoring for early detection, well rehabilitation, wellhead treatment and storage tank blending.

In accordance with SGMA, the development of GSPs for the subbasins underlying the City service areas will be required to include the monitoring and management of groundwater levels, water quality, groundwater quality and degradation, and inelastic land surface subsidence.

4.4.6 Groundwater Flow and Level Trends

4.4.6.1 Modesto Subbasin

According to data published and referenced by DWR, groundwater flow in the Modesto Subbasin is primarily to the southwest, following the regional dip of basement rock and sedimentary units. The lower to middle reaches of the Stanislaus and Tuolumne Rivers in the

4-15 City of Modesto September 2017 Water Master Plan o\c\418\02-14-36\wp\mp\100814_4Ch4 Chapter 4 Existing Water Supply

Subbasin appear to be gaining streams with groundwater flow into both, especially the Tuolumne River.13

Overall, as described earlier in this chapter, groundwater levels in the Modesto Subbasin have declined since the 1950s. A groundwater depression began forming underneath the City of Modesto in the early 1960s and generally worsened until the mid-1990s. Since the MRWTP became operational in 1995, which allowed for reduced groundwater pumping by the City, groundwater levels beneath the City have recovered somewhat and the pumping depression has been reduced. Groundwater elevation contours for the Modesto Subbasin from 1958 to 2010 are shown on Figure 4-4and Figure 4-5.

4.4.6.2 Turlock Subbasin

According to data published and referenced by DWR, groundwater flow in the Turlock Subbasin is generally to the southwest, following the regional dip of basement rock and sedimentary units. The lower to middle reaches of the Tuolumne River and the reach of the San Joaquin River in the subbasin appear to be gaining streams. No faults have been identified that affect the movement of fresh groundwater.14

Groundwater levels within the eastern areas of the Turlock Subbasin have declined significantly since the 1960s. Levels in the western areas of the subbasin are high to the point of requiring pumping in certain areas to keep the groundwater from encroaching into the root zone of agricultural crops. Groundwater elevation contours for the Turlock Subbasin from 1960 to 2005 are shown on Figure 4-6 and Figure 4-7.

4.4.6.3 Delta-Mendota Subbasin

According to data published and referenced by DWR, groundwater flow in the Delta-Mendota Subbasin was historically northwestward parallel to the San Joaquin River. More recent data show flow to the north and eastward, toward the San Joaquin River. Based on current and historical groundwater elevation maps, groundwater barriers do not appear to exist in the subbasin.15

4.4.7 Groundwater Quality Issues

The City has experienced some issues related to groundwater quality. As of October 2014, concentration levels of arsenic, uranium, perchloroethylene (PCE), trichloroethylene (TCE), dibromochloropropane (DBCP) or nitrate in excess of drinking water regulatory maximum contaminant levels (MCLs) have resulted in sixteen of the City’s production wells to be taken out of service (thirteen wells in the North Modesto service area, two wells in the South Modesto service area, and one well in the Turlock service area). These well outages have reduced the City’s hydraulic pumping capacity.

13 Source: DWR Bulletin 118, Modesto Subbasin Description, last updated February 27, 2004. 14 Source: DWR Bulletin 118, Turlock Subbasin Description, last updated January 20, 2006. 15 Source: DWR Bulletin 118, Delta-Mendota Subbasin Description, last updated January 20, 2006.

4-16 City of Modesto September 2017 Water Master Plan o\c\418\02-14-36\wp\mp\100814_4Ch4 Chapter 4 Existing Water Supply

In general, the quality of groundwater from the Turlock Subbasin, from which the South Modesto wells draw their water, is of relatively poor quality. In the South Modesto area, the City has a total of twelve wells. Of those twelve wells:

• Five wells are active wells that pump directly into the distribution system; • Five wells are blending wells that produce water of potable quality, but do not meet all secondary MCL criteria for aesthetics (water pumped from these blending wells is pumped into storage tanks and blended with water of better water quality so the water quality of these blended supplies meets all applicable California DDW primary and secondary MCL requirements); and • Two wells are inactive due to high nitrate concentrations.

Potential wellhead treatment options to address contaminants in the City’s wells are discussed in Chapter 5 Integrated Water Supply Plan.

4.4.8 Groundwater Modeling

There are two available regional groundwater flow models for the Modesto area from south of the Merced River to north of the Stanislaus River, and from west of the San Joaquin River to the Sierra Nevada foothills. The first model, released in 2007, is a steady-state model that simulates groundwater elevations during water 2000 using the MODFLOW 2000 code. The second model was completed in 2015 and consists of a transient-state model that simulates groundwater elevations from 1960 to 2004 using the MODFLOW One-Water Hydrologic Flow Model (OWHW) code. The transient model is also referred to as the MERSTAN model, named for the simulation area between the Merced and Stanislaus Rivers.

The City intends to apply the models to several projects including an on-going evaluation of potential managed aquifer recharge (MAR) strategies. To allow for a more detailed evaluation of wellfield hydraulics associated with MAR strategies, Todd Groundwater (Todd) was requested by the City to refine the steady-state model into a more highly-discretized local model, focusing on the City’s water service area. A description of Todd’s refinement of the steady-state model is provided in Todd’s September 2016 Technical Memorandum titled “Steady-State MODFLOW and Transient-State OWHM (MERSTAN) Model Evaluation and Application for City of Modesto Projects” a copy of which is provided in Appendix G. As described in Todd’s Technical Memorandum, both the steady-state and transient-state models can be used to evaluate City wellfield operations, feasibility and effectiveness of MAR projects, and potential water quality changes resulting from wellfield operations.

4-17 City of Modesto September 2017 Water Master Plan o\c\418\02-14-36\wp\mp\100814_4Ch4 Chapter 4 Existing Water Supply

4.5 EXISTING WATER SUPPLY SUMMARY

The MRWTP, and the associated shift to surface water supplies, has allowed the City to reduce groundwater pumpage and allow for groundwater recovery. The City currently maximizes the use of its treated surface water supply from MID in normal and wetter years. The use of this treated surface water supply gives the City the flexibility to preserve its groundwater supplies, through in-lieu banking. Now that the MRWTP Phase Two Expansion has been completed, there will be sufficient surface water treatment capacity to deliver up to an additional 30 mgd of MID supplies to the City (10 mgd initially), allowing the City (north of the Tuolumne River) to meet almost all but peak demands using treated surface water, further reducing the City’s dependence on groundwater supplies for servicing areas north of the Tuolumne River.

Additional potential future supply alternatives are described in Chapter 5 Integrated Water Supply Plan.

4-18 City of Modesto September 2017 Water Master Plan o\c\418\02-14-36\wp\mp\100814_4Ch4 Last Revised: 09-02-16; o\c\418\02-14-36\e\T120\Ch3_TablesandFigures.xlsx

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