Butte County Wells at Alert Stage

April 2007

In March, staff from Butte County Dept. of Water and Resource Conservation measured groundwater levels in wells across the county. Eighteen indicated water levels had declined over levels measured the same time last year. These measurements were low enough to trigger “alert levels” that had been defined by BMO stakeholders in each of 16 sub-basins identified in county Ordinance 33-A.

A larger map shows the approximate location of these 18 production wells – municipal and agricultural. Alert levels for the Basin Management Objectives (BMO) are based on historical data for each sub-basin and some of the definitions vary. The BMO reports are available online at the Butte County Water and Resource homepage. Talk with your sub-basin representative for more information.

Many factors influence groundwater level fluctuations. Decreases can indicate recent extraction from the monitoring well or other local wells. The sphere of influence varies for each well depending on the geology. Groundwater flows in both vertical and horizontal directions at different rates because of the geology.

Most wells are deep enough that seasonal changes in precipitation will not influence water levels rapidly. The most probable cause for these decreases is an accumulative effect from stresses that have occurred over many years. Changes in land use result in an increase in run-off and a decrease in infiltration. Urban and rural development has increased extraction of groundwater. At present we do not have the tools to quantify these influences, but the county and other entities are working to resolve this issue.

Geology plays a significant role in the way the system responds to the stresses mentioned above. The type of rock, how it was formed, and the overall structure of the aquifer system are the most crucial factors. While these factors are similar in origin across the valley, the depth of the well, ancient erosion and deposition events, and structural changes (for example faults) lead to a heterogeneous system. More research is necessary to completely understand this aquifer system and before causes can be assigned to the noted declines.