PRELIMINARY STABLE ISOTOPE AND PIEZOMETER DATA SUGGEST A RAPID RESPONSE OF SPRING DISCHARGE TO SEASONAL RECHARGE IN A SHALLOW SANDY AQUIFER, OTTAWA COUNTY, MICHIGAN

Shallow aquifers provide vital sources of water to stream and wetland environments and are seasonally recharged by precipitation. We monitored shallow groundwater levels, spring-fed stream stage, and measured stable isotope ratios of precipitation and springs to estimate time between recharge and spring discharge in a shallow aquifer. Our study area contains inland dunes and valleys eroded by sapping springs, which feed into the Pigeon River in Ottawa County, Michigan. Spring-fed stream stage and nested piezometer data monitored groundwater temperature, conductivity, and hydraulic head from May 2018 to present. Precipitation and spring water samples for stable isotope analysis (δ¹⁸O and δ²H) were collected nearly weekly from November 2019 to present.

Observed groundwater and spring-fed stream temperatures varied from ~7 to ~15°C with conductivity varying from ~50 to ~350 µS. Spring-fed stream stage is fairly constant, with the largest variations due to backwater flooding of spring-fed streams by the Pigeon River. Preliminary results of δ¹⁸O values of precipitation vary from -0.1 to -21.6‰, with spring-fed stream values varying from -7.1 to -9.6‰. Precipitation δ²H values vary from 4.5 to -153.6‰ with spring-fed stream values varying from -50.0 to -63.1‰. Nested piezometer water levels and gradients responded quickly to major precipitation events, suggesting a short response time of the shallow sandy aquifer. The wide range of δ¹⁸O and δ²H values of precipitation compared to the spring-fed stream values show mixing of groundwater and a time between recharge and spring discharge of less than a few months. Continued monitoring of δ¹⁸O and δ²H for an entire year will allow us to estimate the time between aquifer recharge and spring discharge. This study will enhance our ability to characterize travel time of potential pollutants in other similar shallow groundwater systems.