The Spanish Lakes Development is located, in Geologic terms, on the Paso Robles Formation, which formed after retreat of the Tertiary Sea that covered most of Western California until the late Pliocene (2 million years ago) when geologic uplifting and folding created the Coastal Range (Santa Lucia Mountains).
The Paso Robles Formation is characterized by sedimentary deposits of sand, gravel, rock, clay and silt – both from the original deposits as well as subsequent erosion of the Coastal Range formations.
There are at least four other geologic formations below the Paso Robles Formation (see below), each characterized as deposited in a marine environment with various marine fossils present. These deposits date back as far as 20 million years.
The Paso Robles area water supply is primarily obtained from rainwater draining off the Coastal Range as well as water from aquifers below the rolling hills. Most wells produce water from the Paso Robles Formation that is up to 2,000 feet thick in some areas.
The Paso Robles Groundwater Basin has been estimated to be 900 square miles in size and to contain 6.8 million acre-feet of water.
Geologic activity has formed an 89 square mile source of heated sulfur laden groundwater in the Paso Robles area. The warm water for the springs and wells in this area is also derived from the Paso Robles Formation, enhanced by water percolating from deeper formations. Well-known warm water springs exist in downtown Paso Robles and in the Santa Ysabel Ranch Development.
Two warm water artesian springs feed the Spanish Lakes Development North Lakes, one spring on Lot 50 by Creston Road (flowing at 150 gallons per minute at 90 degrees F. – June 2008) and the other spring outside the Development by the Franklin Ponds (South on Creston Road).
The South Lake also used to be supplied with water from two warm water artesian springs, one of those springs is still located on a neighboring property, but reduced water flow eliminated that source from feeding the lake.
The other was drilled in 1955, near the current water tanks, which became a free flowing warm water source for both the South Lake as well as the other lakes. This well also stopped free flowing. A solar powered pump and piping was installed in 2007 to re-supply water to the South Lake.
The below resource provides detailed Geological/Geophysical information and highlights the Geothermal resources in the Paso Robles area.