SEEING THE SPRINGS: HYDROCHEMISTRY AND FLOW CHARACTERISTICS OF THE HOT AND COOL SPRINGS AT HENRY COE STATE PARK IN THE DIABLO RANGE

The Diablo Range is home to a multitude of hot and cool springs, with water emanating from various depths in the Franciscan Complex and other rock units. Spring water origins vary from shallow lateral flow in contrast to deeply sourced springs connected along faults and fractures where connate and meteoric water undergo exchange as evidenced by prior research from other groups. Tectonic activity along many of the active fault traces in Santa Clara County has been shown to affect spring flow as well as water chemistry. In this project we explore the major ion concentrations, flow, electrical conductivity, and pH of a subset of springs located in Henry Coe State Park in the years 2023-2024. Beyond a small number of springs that have been investigated in the early and mid 20th century, there exists scant data on the chemistry and flow of the Coe Park springs. The springs within Coe Park and the Diablo Range in general have been highly significant to the human and agricultural history of the region as well as ecological hot spots that may undergo large shifts as the climate changes. In this study we have chosen 19 springs across the park, including Gilroy Yamato Hotsprings (GYHS), to investigate the above mentioned chemical and physical characteristics. Our sampling of 19 springs during baseflow has revealed a wide range of water chemistries that reflect the diverse and complex nature of the underlying Franciscan geology. We have found that GYHS is high in Mg²⁺ and Ca²⁺ compared to other springs such as the Headquarters Spring and those that issue from near the top of the ridge along Manzanita Point Road. The elevated Mg²⁺ may reveal interaction with locally mapped serpentinites within the nearby Madrone Springs fault zone. Springs near the park headquarters and Manzanita Point reveal markedly different chemistries, with water enriched in Cl^- while also being located adjacent to the Madrone Springs Fault zone. This ongoing investigation highlights the range of water chemistry that emanates from the diverse geology within Henry Coe State Park.