BASIN MANAGEMENT OBJECTIVES LLANO SECO SUBINVENTORY UNIT

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1 BASIN MANAGEMENT OBJECTIVES LLANO SECO SUBINVENTORY UNIT Butte County Water Advisory Committee Member Nicki Vaccaro Contact Information Phone Number: Address: Description of the Llano Seco Sub-Inventory Unit The Llano Seco Sub-Inventory Unit (SIU) covers an area of about 18,400 acres in the southwestern portion of the West Butte Inventory Unit. It is bordered by the M&T and Angel Slough SIUs to the north, Durham Dayton and Western Canal SIUs to the east, Glenn County the south, and the Sacramento River to the west. This SIU corresponds roughly to the water service area associated with Rancho Llano Seco. Land uses within the Llano Seco SIU consist of a mixture of row crops, grain, pasture and native riparian supported by both surface water and groundwater. In a normal year, about 6% of the Llano Seco SIU is in summer agricultural production supported by groundwater. Current Conditions The Llano Seco SIU has only seen one small land-use change in 2010 with the addition of 45 acres of irrigated pasture in a field that was formerly used for dry land grazing. The water applied to this pasture is surface water therefore having little effect on the area groundwater. Management Objective Observe and maintain the current and future groundwater surface elevation at a level which will provide an economical and adequate water supply to this area s crops and homes. This basin management objective is intended to be flexible to future land and water uses in this relatively undeveloped portion of Butte County. We began monitoring a well, located fairly close to the Sacramento River for water levels and water quality in 2008 which should give us an even better understanding of groundwater levels and quality in this area of Butte County. Geologic Formations Identified In Sub-Inventory Unit Geologic formations in the Llano Seco SIU, from youngest (shallowest) to oldest (deepest), include: Quaternary Alluvium Basin Deposits Tehama Formation Tuscan Unit C (Upper Tuscan) Fresh Water-bearing Units. In the Sacramento Valley Region of Butte County, fresh groundwater-bearing units include, from youngest (shallowest) to oldest (deepest), the Modesto, Riverbank, Laguna, Tehama and Tuscan Formations. Those included in the Llano Seco SIU are: Tehama Formation Tuscan Unit C (Upper Tuscan) 1

2 BMO Key Wells Selected for Groundwater Level Monitoring Well ID Aquifer Stage 1 Alert Stage 2 Alert Well Type System Elev. (ft) Elev. (ft) 20N01E18L01M Lower Tuscan Monitoring N01E18L02M Upper Tuscan Monitoring N01E18L03M Alluvial Monitoring N01W35K02M Alluvial Irrigation Well ID Aquifer Stage 1 Alert Stage 2 Alert Well Type System Elev. (ft) Elev. (ft) 20N01E18L01M Lower Tuscan Monitoring N01E18L02M Upper Tuscan Monitoring N01E18L03M Alluvial Monitoring N01W35K02M Alluvial Irrigation Geologic Aquifer Well Total Top Bottom Formation Depth Perf Perf Well ID System Type (ft) (ft) (ft) at Top Perf. 20N01E18L01M Geologic Formation at Bottom Perf. Aquifer Type Lower Tuscan Monitoring Tuscan B Tuscan B Confined 20N01E18L02M Upper Tuscan Monitoring Tuscan C Tuscan C Confined 20N01E18L03M Alluvial Monitoring Basin Dep/Tuscan C Tuscan C Semi- Confined 21N01W35K02M Alluvial Irrigation Basin Deposits Tuscan C Semi- Confined BMO Key Wells Selected for Groundwater Quality Monitoring Butte County Department of Water and Resource staff began sampling ph, temperature and EC on a well located in the Llano Seco SIU beginning in Information from that well will be presented when sufficient data has been collected for analysis. BMO Key Well(s) Selected for Land Subsidence Monitoring Land Subsidence is continuously monitored by the Department of Water Resources and Butte County Water and Resource Conservation at the closest extensometers in the Western Canal Water District and M&T Ranch sub-inventory units. Stakeholder Interaction Llano Seco interacts with its neighbors regarding pertinent water issues as they come up and we are always available to discuss issues that reach beyond the borders of the Llano Seco SIU. 2

3 Future Monitoring Recommendations Two additional wells located within the SIU were added to the monitoring network in Once both wells have a sufficient number of measurements taken that data will be presented. Supporting Data BMOIC created hydrographs depicting yearly spring and fall level measurements, including 2011 data, with established alert levels. ( 3

4 20N01E18L001 4

5 20N01E18L002 Well 20N01E18L002M was chosen to replace the original key well in the Llano Seco Subarea. This new well is part of a dedicated, multi-completion monitoring well set that was installed during The well is along the eastern margin of the sub-area, due east from the original key well. Measurements in this well represent groundwater conditions between feet in the confined portion of the Upper Tuscan aquifer system. This well data is also utilized by both the Llano Seco and the Western Canal sub-areas in the annual BMO document. Water elevations have been monitored since 2001 at this location and the historical averages, including 2011 data, are; Spring=103 feet and Fall=101 feet. The spring and fall 2011 groundwater level measurements were only slightly higher than those taken in The limited period of record for this well makes trend analysis difficult, but the available data does show a slight decline since

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7 20N01E18L003 7

8 21N01W35K002 8

9 20N01W26H002M For historic purposes, information is presented on previous monitoring activities at well 20N01W26H002M. The figure below is a hydrograph for well 20N01W26H002M, located in the southern portion of the Llano Seco Sub-area. The area surrounding this well is characterized by rural agricultural land use, supported primarily by the application of surface water. This well is an unused irrigation well constructed in the unconfined portion of the aquifer system, with a groundwater level measurement record dating back to the early 1940s. The groundwater levels in this well were monitored on a semi-annual basis until 1991 and on a monthly basis from 1991 to about 1994, when monitoring of this well was discontinued. The figure shows that the average seasonal fluctuation (spring to fall) in groundwater levels was about 1 to 5 feet during normal and drought years. Long-term comparison of spring-tospring groundwater levels show little, if any, decline in groundwater levels associated with the , and 1994 drought periods. Overall comparison of spring-to-spring groundwater levels from to 1993 showed very little change in the unconfined aquifer system within this portion of the Llano Seco Sub-area. Groundwater level monitoring was discontinued in this well in Sacramento Valley Groundwater Basin - Butte County Well Number 20N01W26H002M Spring Groundwater Elevation Ground Surface Elevation = 106' Water Surface Elevation (ft.) Fall Groundwater Elevation Summer Groundwater El ti Depth Below Ground Surface (ft.) Questionable Measurement Symbols Pumping Water Level Well Recently Pumped Nearby Well Pumping Oil in Well Other Well Use - Unused Probable Unconfined Source: DWR, Northern District Date 9