Almaden Quicksilver County Park is a I,520 hectare (ha), undeveloped parcel situated on the northeast ridge of the Santa Cruz Mountains, approximately 19 kilometers (km) south of downtown San Jose. The Park is located in the 447 square kilometer watershed of the Guadalupe River, which drains the south central portion of the Santa Clara Valley into South San Francisco Bay via Alviso Slough. Operations relating to the mining and/or processing of mercury ore containing the mineral cinnabar (mercury sulfide) were conducted from about 1845 to about 1971 along the Los Capitancillos Ridge, a line of hills which trend northwest-southeast across the Park. A series of mines, collectively referred to as the "New Almaden Mines", were located along the Los CapitancilIos Ridge. The land was originally part of several Spanish land grants, and the mercury was first discovered in 1845 by a Mexican army officer, who also established the first mining company on the site.
The largest production occurred between 1846 and 1905 from the underground workings of the New Almaden Mine (Mine Hill Area). Because of the abundance of ore in the Mine Hill Area, other mine areas were not developed until production declined in the early 1900's. By 1917, the extensive underground ore bodies in the Mine Hill Area were largely exhausted, and only small scale operations continued until World War II. Interest in the mines was renewed by the war, and limited mining or mining related operations might have continued into the early 1970's. Mining activity during this time period primarily consisted of the reworking of formerly processed material and mining shallow ore bodies by open pit methods. The process of extracting mercury from the ore involved heating the ore to a temperature of 700 to 1,200 degrees Fahrenheit in furnaces and retorts to volatilize the mercury, and then condensing the vapor. After ore was processed, the residual materials, or calcines, were typically dumped near the process area. Calcines were also spread on unpaved roads in the Mine Hill Area as a road base material.
Santa Clara County purchased property from the New Idria Mining & Chemical Company in two transactions taking place in 1973 and 1975. The County designated this property as Almaden Quicksilver County Park, and opened the park to the public in 1975. The County subsequently acquired the Hacienda Furnace Yard area from a third party and added this area to the Park. Remedial actions were completed at five former mercury ore extraction or processing areas in Almaden Quicksilver County Park from 1998-2000 in accordance with a Remedial Action Plan developed by the County of Santa Clara Parks and Recreation Department (County) and approved by the California Department of Toxic Substances Control (DTSC), which was the lead agency responsible for overseeing efforts by the County to investigate and remediate mercury-containing waste materials which remained at the Park. The large Guadalupe Mine, however, was located outside the current park boundary and was not included in the Remedial Action Plan.