Technical Consultant Outlines Monitoring Network Density and Methodologies for Groundwater Level Stabilization

At the April 5 Board meeting, McMullin Area Groundwater Sustainability Agency (GSA) technical consultants Provost & Pritchard provided an update on Groundwater Sustainability Plan (GSP) progress. The presentation included current breadth of the groundwater level monitoring network and necessary improvements in the network to reach required network density. The consultants also covered potential methodologies for reaching groundwater level stabilization, an element of GSP sustainable management criteria.

GSP development progress outlined below.

MONITORING NETWORK

The purpose of the monitoring network discussed is specific to groundwater level measurement. The map below indicates with a blue dot actively monitored wells within the McMullin Area (outlined in red) that offer historical data on groundwater levels.

Although the map indicates a high density of groundwater level monitoring wells, to qualify for the monitoring network the well must be accompanied by a Well Completion Report. A Well Completion Report details construction information such as depth, perforations, and date established. A total of 9 wells currently qualify within the McMullin Area GSA, but 20-30 additional monitoring wells are needed to satisfy the necessary well density for the network. The map below indicates qualifying wells within the GSA.

Potential actions to fill the data gap includes one or more of the following: 1) Obtain additional Well Completion Reports for existing wells; 2) Video wells in absence of Well Completion Reports to obtain necessary construction information; 3) Establish new dedicated monitoring wells. Completion of the monitoring network density is projected for 2030.

GROUNDWATER LEVEL STABILIZATION

As a part of its sustainable management criteria, SGMA requires groundwater levels stabilize between 2020 and 2040. Groundwater level stabilization success is measured by maintaining the “measurable objective”, a specified groundwater level established by the GSA and approved by DWR.

Measurable objective: average maintained groundwater level over the long-term. Must be met by 2040. Success metric of groundwater level stabilization

Minimum threshold: the lowest possible groundwater level allowed in the worst case scenario. Level cannot cause an “undesirable result” and thus cannot be an arbitrary number. Undesirable results are defined by the GSA and approved by DWR in line with DWR’s suggested Best Management Practices.

In the report, Provost & Pritchard outlined potential methodologies to establish the measurable objective and minimum threshold groundwater levels. The methodologies presented rely on three sustainable criteria variables: rate of groundwater decline, rate of mitigation, and operational flexibility. Each sustainable criteria variable can be determined a number of ways, and once determined are used in relationship with one another to establish a methodology for setting and reaching the measurable objective to achieve groundwater level stabilization.

Rate of groundwater decline is the rate at which groundwater levels have declined over a set period of time, and can be determined using one of three frameworks:

  1. Look at last 20 years, or
  2. Look at worst 20 years, or
  3. Look at hydrologic average period

Rate of mitigation is the rate at which improvements towards the measurable objective are made, and can be determined one of three ways:

  1. Constant (small, equal improvements each year), or
  2. Phased (slow at first with increased rate of progress over time), or
  3. Deferred mitigation (no progress until last 5 years; not recommended, unlikely to be accepted by DWR)

Operational Flexibility is the range of fluctuation in groundwater level allowed that still maintains the measurable objective average, taking into consideration dry versus wet years. The lowest range level of operational flexibility allowed is equal to the minimum threshold. The range of flexibility can be determined one of three ways:

  1. Recent drought
  2. Other smaller drought
  3. Conjunctive use operations

The graph below illustrates the sustainability criteria variables and their relationship to the measurable objective and minimum threshold.

Provost & Pritchard recommends determining groundwater decline from the framework of the last 20 years, establishing phased rate mitigation, and setting operational flexibility using the recent drought. This methodology considers the most favorable and realistic conditions for the McMullin Area GSA stakeholders without comprising compliance with DWR’s guidelines.