Understanding the WSAC Recommendations

On Nov 10th, the City Council will study the recommendations resulting from 18 months of meetings of the Water Supply Advisory Committee and vote on those recommendations on Nov 24th.  Here are some highlights of the plan:

Q. What are the basics of the plan?

There are three major components of the Portfolio that the WSAC is recommending: conservation; aquifer recharge and storage; and a backup strategy of either recycled wastewater or desalination. In addition, the WSAC has created a decision roadmap for the next 10-15 years that will guide the City’s implementation of these components.

  1. Conservation: Increased investment in conservation will save between 200 and 250 million gallons of water a year by 2035. Conservation in normal years results in storing water that can be used in drought years. It is by far the least expensive source of water compared to other water supply options.
  2. Aquifer Recharge and Storage: Captures water from the San Lorenzo River when the river is flowing high. Stores that water in the aquifers by way of two strategies:
  • “In Lieu” Recharge, (Water Transfers Between Districts): Sends surface water (from river & streams) from the Santa Cruz treatment plant to Scotts Valley and Soquel Creek Water District. Those districts will use surface water in lieu of pumping their wells, allowing the aquifers to recharge naturally through rainwater infiltration. A portion of that water transferred to neighboring districts would be considered “banked” in the aquifer for use by Santa Cruz in drought years. The specific terms of the exchanges will be spelled out in negotiations between the City and districts following adoption of these recommendations.
  • Injection Wells (Aquifer Storage & Recovery): Putting treated surface water into one or more regional aquifers using wells designed for injection.

The technical team advising the WSAC estimated that either in lieu recharge or recharge through injection wells could individually meet the goal of storing the necessary amount of water for drought years. The WSAC included a combination of both strategies in order to provide the City with flexibility in achieving the goal. The Water Department will design the specifics of the plan.

  1. Back-up Strategy: The WSAC recommends treated wastewater in case the aquifer recharge strategy fails to meet objectives for providing adequate supply during droughts that are more severe than those in the historical record. Desalination would be a back-up to the back-up. Study of recycled water options would begin concurrently with implementation of the Aquifer Recharge and Storage strategies.
  2. Decision Roadmap: The WSAC has developed guidelines for evaluating the progress of the Aquifer Recharge and Storage strategy and modifying that strategy as needed to achieve yield, timeliness, and cost goals. If it appears that those goals cannot be met within the cost metric, a thorough assessment process will be conducted to update supply and demand projections, and analyze the cost-effectiveness of implementing backup strategies.

Q:  Why did the Committee choose Aquifer Recharge and Storage?

A: Santa Cruz gets 95% of its water from surface water sources and 5% from groundwater. This dependency on surface water exposes the City to shortfalls during drought years. Adding groundwater sources provides the City more reliability during droughts. There are a number of additional benefits of this strategy described below.

Q: Is there enough water in the river to recharge the aquifers?

Yes. In winter the City diverts a small portion of the water flowing down the San Lorenzo River.  Over the last 80 years, from mid-January to mid-April, the median flow in the San Lorenzo River is above 100 cubic feet per second (cfs). The City diverts an average of 5 cfs to satisfy its needs. Even in dry winters such as 2014-15, there are days when the City can divert additional water from the river without negatively impacting fish reproduction. State and federal fisheries agencies have indicated their support of this plan.

Q: Is there enough storage space in the aquifers?

Yes. The technical team estimates that the City would need to have on hand 2.4 billion gallons of storage, the equivalent of another Loch Lomond Reservoir. Decades of over-pumping and paving over recharge zones have caused a depletion of the aquifers in the Scotts Valley area is estimated to be over 9 billion gallons and depletion of the Purisima Aquifer over 3 billion gallons. Thanks to local geology, the ground has not subsided and much of this vacated space can store water again.

Q: What are the additional benefits of Aquifer Recharge and Storage?

  1. If the City chooses, it can continue the Aquifer Recharge and Storage operation beyond the 2.4 billion storage level, thus preparing the City for even worse climate change impacts than the one modeled by the WSAC.
    1. Recharging the Purisima Aquifer will reverse salt water intrusion that threatens City and Soquel Creek production wells.
    2. Recharging aquifers results in increased summer flow in area streams, enhancing habitat for native fish and wildlife.
    3. The cost of the plan is likely to be lower than a plant for recycling or desalination.

Q: Will the Aquifer Recharge and Storage Plan Work?

 Using surface water to recharge aquifers is a common practice throughout the country. In the 1930’s, Santa Clara County began recharging depleted groundwater with surface water impounded in the nearby hills. San Francisco has embarked on an in lieu program very similar to the one proposed for Santa Cruz. In normal rainfall years, San Francisco will provide Daly City, South San Francisco, and San Bruno with a higher-than-usual allotment of surface water. Those communities will use SF water in lieu of pumping from their own wells.

The in lieu strategy is how our community would use its water resources if Santa Cruz, Scotts Valley and Soquel Creek were all one district. It makes sense to stop pumping groundwater at times when billions of gallons of river water flow out to sea.

The injection well strategy also is a common practice. There are more unknowns about injection than there are about in lieu recharge. The chemistry of adding surface water to groundwater needs to be compatible. The geology of the aquifer needs to be suitable for receiving injected water. For these reasons it will take up to five years to better determine the productivity of the injection strategy.

Q: What are the timing advantages of the Aquifer Recharge and Storage strategy?

The City could receive groundwater from neighboring districts in less than five years. The backup strategy of direct potable recycled wastewater would take longer, in part because it is not yet legal in California.

Because of existing pipelines connecting Santa Cruz and Soquel Creek District, the in lieu exchange program can begin immediately. The timing of return transfers to Santa Cruz is a subject of negotiation. As the aquifer levels rise in response to the resting of District wells, the City will be able to receive larger quantities of water during droughts.

In the exchange with Scotts Valley, once the interconnecting pipeline and wells are built (less than 5 years), the City could draw from the aquifer in a drought, even if the water had not been fully “banked”, according to Scotts Valley Water Director, Piret Harmon.

 Q: What are the cost advantages of the Aquifer Recharge and Storage strategy?

  1. Neighboring districts will share costs. Revenues to the City from water transfers will be negotiated.
  2. Much of the infrastructure of in lieu recharge has a long life. Pipelines, for example, last 70 years or more before needing replacement, whereas the life span of recycled water plant components is significantly shorter.
  3. Investment in the strategy will be expended in stages. This advantage is not available with the choice of a recycled wastewater plant. Depending on the rate that groundwater levels rise, the City may be able to achieve its groundwater storage goal after the early stages of investment. The system has not yet been designed, but the following items are likely to comprise an initial stage:
  • In lieu recharge using the existing interconnecting pipeline to Soquel Creek District (A pilot program is about to be implemented).
  • Pipeline interconnect with Scotts Valley, pump station and six new groundwater wells to return water back to Santa Cruz.  The WSAC tech team estimates the cost at $14 million.
  • Retrofitting existing City production wells in order to be able to inject water. Cost = less than $1 million.

Other possible investments include expanding the capacity of pipelines to carry water to Soquel Creek District and constructing injection wells at suitable locations.

At some undetermined level of increased winter production, there may arise a constraint on the City’s capacity to treat water at its Graham Hill Treatment Plant. Costs for upgrading the plant have been estimated at $62 million. In the event that new treatment capacity is needed, an analysis of less expensive alternatives such as Ranney Collectors and satellite treatment facilities will be conducted.

Q: What are the energy use advantages of the Aquifer Recharge and Storage strategy?

The tech team estimates that the energy intensity (energy per gallon) of in lieu recharge is 70% of that of a wastewater recycling plant. However, the team did not include in this estimate the energy savings from resting wells in neighboring districts.

Moreover, the membrane plant needs to operate 365 days a year every year. The in lieu recharge is seasonal, with water coming back to Santa Cruz only in dry years. The tech team’s preliminary estimate for production from a recycled water facility is 1.1 billion gallons per year, compared to an average of only 90 million gallons per year from in lieu. Thus the annual energy use of recycled water would be 17 times that of in lieu. (When accounting for energy saved from resting wells, this disparity is significantly greater.)

 

 

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