Preliminary chemical and isotopic data for waters from springs and wells on and near Medicine Lake Volcano, Cascade Range, northern California

Abstract

Cold springs on and adjacent to Medicine Lake Volcano discharge dilute mixed cation bicarbonate waters of neutral to slightly alkaline pH. Chloride values in cold waters from springs and shallow wells on the volcano are low and generally increase radially outward from Medicine Lake. Chloride concentrations at high elevations are similar to values for precipitation. Higher chloride concentrations occur in wells within Lava Beds National Monument (11 mg/L) and at the headquarters for the Tule Lake National Wildlife Refuge (109 mg/L). Little Valley Hot Springs, near the southern edge of the study area, has 120 mg-Cl/L. Leakage from the high temperature geothermal system at Medicine Lake Volcano (~ 1,000 mg-Cl/L) has not been found. Isotopic values for cold springs and wells on and adjacent to the volcano range from -84 to -106%o 8D and from -12.2 to 14.4%o 8 18O and plot near the global meteoric water line. Water discharged by cold springs at the head of the Fall and Tule Rivers change somewhat in chemical and isotopic composition across the complex. Spring temperatures decrease from east to west. Chloride concentrations are highest in springs in the middle of the complex decreasing slowly eastward and sharply at the western edge. Deuterium values are most depleted in the middle of the complex. Deuterium versus chloride plots for these waters show no relationship to known geothermal waters. Deuterium and oxygen-18 values for the large discharge cold springs in the Fall River Valley are compatible with, but do not prove, recharge on Medicine Lake Volcano. INTRODUCTION Medicine Lake Volcano (fig. 1) is a large Pleistocene and Holocene shield volcano made up of basaltic to rhyolitic rock. The volcano is located about 35 miles ENE of Mt. Shasta in northern California and it sets on Pliocene basalts that are underlain by tuffs and basalt flows of Miocene age (Anderson, 1941). Medicine Lake, about 3/4 mi.2 in surface area, occupies the western end of a caldera-shaped depression in the top of the volcano. Anderson (1941) reported that most of Medicine Lake was less than 20 feet deep, although a depression in the eastern end of the lake was almost 150 feet deep. Medicine Lake Volcano has been active on at least 17 occasions over the last 12,000 years (Donnelly-Nolan, 1990), but no hot springs or major fiimaroles discharge on the mountain. The