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GSP Updates

Land Subsidence Reviewed for McMullin Area

Land subsidence is one of the six sustainability indicators that must be managed without causing undesirable results in a Groundwater Sustainability Plan. Technical Consultant Lynn Groundwater from Provost & Pritchard reviewed maps at the Ad Hoc Technical Committee meeting showing historic and current information on subsidence in the McMullin Area GSA territory.

Land subsidence occurs generally where the Corcoran Clay underlies the Valley, but recently land subsidence has been documented in some areas not underlain by the Corcoran clay.  The map below (click on map to enlarge) shows land subsidence from 1926 to 1970. This historical map shows subsidence as one foot in the middle of the McMullin territory and 4 feet in the upper left of McMullin’s boundary.

A more recent map from NASA shows the amount of land subsidence from May 2015 to May 2016 (click on map to enlarge). The green areas represent less subsidence. The yellow to orange areas are higher. The coloring within the McMullin Area indicates that subsidence varies across the territory. On the eastside of McMullin there has been minimal subsidence with it increasing as you go west. Overall there was less than 5 inches of subsidence from May 2015 to May 2016 in the McMullin Area GSA.

Ad Hoc Technical Advisory Committee Meeting Highlights

At the McMullin Area Groundwater Sustainability Agency’s Ad Hoc Technical Advisory Committee held on December 6, the participants heard a report from McMullin’s technical consultants, Provost & Pritchard Consulting Group (P&P). P&P provided an update on the development of the hydrogeologic conceptual model and the groundwater conditions and monitoring chapters of the Groundwater Sustainability Plan (GSP).

The GSP requires the identification of groundwater quality issues that may affect the supply and beneficial uses of groundwater, including a description and map of the location of known groundwater contamination sites and plumes. Water quality is one of the six undesirable results that must be addressed in the GSP. P&P staff members highlighted some of the groundwater quality issues that may be considered in the GSP, like the Raisin City oil fields.

Work is also being done on estimating the annual change in groundwater storage for the McMullin area over a 15-year period. The minimum threshold for reduction of groundwater storage shall be a total volume of groundwater that can be withdrawn from the basin without causing conditions that may lead to undesirable results.
Some of the elements being considered in the hydrogeologic conceptual model include understanding the structural trough of the Valley which governs the location of the sediments types. The McMullin Area service area is near the axis of the Valley on the eastside as shown in the visual below (click on map for larger image) that includes mostly San Joaquin River alluvial sediments and Dune Sands.

Using this information along with other factors, a picture of the McMullin Area’s groundwater conditions can start to emerge. One example is on the map below (click on image for larger view), where several elements are considered to determine the best locations for recharge. On this map, the dark green areas are the most desirable locations for recharge with the brown areas being the least desirable.

Technical Consultant Discusses Progress on Drafting Groundwater Sustainability Plan

The Board received a report on the progress of drafting three chapters for the McMullin Area’s Groundwater Sustainability Plan from their technical consultant, Provost & Pritchard. The Plan Area Chapter, which includes topics like a description of the geographic areas covered and other water and land use plans in the area, was submitted to McMullin Area’s Technical Advisory Committee for review.
The Groundwater Conditions Chapter is scheduled to be drafted by the end of November. This chapter will describe current and historical groundwater conditions in the McMullin Area including topics like groundwater elevation and depth, flows, storage variation, water quality and subsidence.
The Hydrogeologic Conceptual Model Chapter is scheduled to be drafted by the end of December. This chapter will characterize through a conceptual model the physical components and interaction of the surface water and groundwater systems in the area.

Board Receives Update on Groundwater Sustainability Plan

The Groundwater Sustainability Plan will cover a number of topics including the description of groundwater conditions, water budget, sustainable management criteria, monitoring, and projects. McMullin Area GSA’s technical consultant, Provost & Pritchard Consulting Group, is currently working on drafting the groundwater conditions and hydrogeologic conceptual model chapters. The groundwater conditions chapter will include topics like groundwater elevation and depth, flows, variation in storage, and water quality to name a few. The hydrogeologic conceptual model chapter will provide a visual and narrative description of groundwater conditions in the McMullin Area.

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