The District has five (5) wells in the Grass Valley Basin that provide approximately 150-200 acre feet of groundwater per year. The Lake Arrowhead area is comprised of approximately 4,900 acres of mountainous terrain where about 40% of the land has slopes of more than 30% grade. The ground underneath the surface is mostly dense, fractured and jointed granite. This terrain is very difficult to develop groundwater wells. Nevertheless, the District currently has five productive wells and has identified additional sites for further review and possible development.
Groundwater in Fractured Hard Rock
(information provided by California Department of Water Resources)
“In mountainous areas of California, groundwater can be found in the cracks or fractures of hard rocks, such as granite, greenstone, and basalt. The water does not actually penetrate the rocks, because there is no pore space between the grains of the rock. However, some of these rocks have fractures in them. These fractures store water and yield small amounts of water to wells that intersect the fractures. Some sedimentary rocks, like sandstone, are hard but can still absorb some water into their pores. These rocks may also have fractures that contain water. About 60 percent of California is composed of hard rocks. However, only a small quantity of groundwater is stored in the fractures of these rocks. The majority of groundwater is stored in what the average person would call “dirt” or “soil,” more accurately described as alluvium (loose gravel, sand, and silt) which has pore spaces between the grains.”