INSIGHTS INTO GROUNDWATER—SURFACE-WATER INTERACTION IN OGDEN VALLEY, UTAH, FROM STABLE ISOTOPES OF WATER

Groundwater and surface water are highly connected in Ogden Valley, a mountain valley located in north-central Utah about 10 miles east of Ogden City. We used stable isotope ratios in water and stream seepage runs to quantify the interaction between streams and groundwater in the bedrock and principal valley-fill aquifers. We analyzed 307 water samples from wells, springs, streams, reservoirs, and precipitation for deuterium and oxygen-18 to characterize the system. Stable isotope ratios of surface water and groundwater are more similar to winter precipitation than summer precipitation, implicating winter precipitation as the dominant recharge source. Stream water was more enriched in heavier isotopes in spring than fall because of spring rains. The mean ratio of stream samples more closely matched valley-fill well samples in fall, as expected during base-flow conditions. Ogden Valley is subdivided into three sub-basins drained by the North, Middle, and South forks of the Ogden River, which empty into Pineview Reservoir. A continental effect on precipitation produced an observed difference of 11 per mil in the deuterium ratio from west to east across the watershed, and this difference allowed us to parse out nuances in the groundwater system between each of the three sub-basins. We used a Monte Carlo approach to calculate the relative contribution of recharge to the valley-fill aquifer from bedrock versus stream seepage in each of the three sub-basins. Overall, about half of the water in the upper few hundred feet of the aquifer is from stream seepage and half is from bedrock recharge, but differences exist between sub-basins.