Determining the Spring Water Provenance in the Warm Springs Valley Subarea of the Silver Creek Watershed in the Harney Hydrologic Basin, Harney County, Oregon

Abstract

Due to the importance of springs as a water source for marshland habitat, which is vital for Pacific Flyway waterfowl in a portion of the Malheur National Wildlife Refuge in southeast Oregon, this work aims to determine the provenance of the groundwater that discharges from springs in the Warm Springs Valley (WSV) by qualitatively and quantitatively characterizing the area geology, spring discharge, groundwater hydraulic gradients, precipitation, geochemical properties, and spatial distributions of temperature, pH, and specific conductance in well and spring water. Along the southwestern margin of the WSV a number of slightly thermal springs discharge from the base of a faulted upland. Discharge was measured from seven springs during August of 2017. Discharges were estimated from groundwater evapotranspiration and remote sensing climate data for two springs lacking outflow channels necessary for field discharge measurements. Measured spring discharge values were on the low end of historical ranges for each spring, but none were less than the lowest historical values. Each spring shows both variability in measurements made in the same month for different years and a lack of consistent interannual measurements made in the same season, causing difficulty in evaluating seasonal variability. Fluctuations in discharge values could also indicate climatic variability or measurement error due to complex spring geometries. Potential WSV spring water sources include shallow surface water infiltration from Silver Creek and its tributaries flowing into the subsurface to the WSV from the northwest (Source A), precipitation infiltration and deep circulation from precipitation along the Brothers Fault Zone to the southwest of the WSV (Source B), and deep groundwater flow from the Steens Mountains to the southeast (Source C). Source B was found to be the most probable contributor to spring water by process of elimination. While no evidence was found to support or deny that the WSV springs are receiving water from Source C, groundwater from the Steens is expected to flow toward Malheur Lake and not through the WSV. A groundwater level contour map plotted using static water level measurements in wells taken by Oregon Water Resources Department (OWRD) staff in the fall of 2017 shows groundwater flowing southeast down the valley (Source A) and northeast from the southwest faulted upland (Source B). Horizontal hydraulic gradients calculated using a groundwater level contour map and aquifer transmissivity values based on well pump tests in and around the WSV allow for estimations of expected spring discharge that were zero to two orders of magnitude different from measured and calculated values. There is an apparent vertical gradient that, in combination with the slightly elevated temperature at the WSV warm springs, could support a hypothesis that groundwater is rising at the springs (Source B). Precipitation within the Silver Creek watershed was found to be enough to supply the springs if at least two percent of precipitation supplies spring water. No spatial trends for total dissolved constituents in the entire Harney Basin were found either by well depth or by subwatershed, but plots by relative flow path shows an evolution from calcium-sodium to sodium-calcium type water. Specific conductance values are consistent with groundwater flowpaths as determined by the gradient in hydraulic head.