USING STABLE ISOTOPES OF OXYGEN AND HYDROGEN TO CORROBORATE GROUNDWATER RECHARGE/DISCHARGE RELATIONSHIPS AT ADA HAYDEN LAKE IN AMES, IOWA

Ada Hayden Lake, a 49.6 ha gravel-pit lake set in alluvial and outwash sediment, comprises the emergency water supply for the city of Ames, Iowa.  Understanding the groundwater flow system at the lake is essential to project its future as an emergency supply in terms of quantity and quality of water.  In this study, the stable isotopes of oxygen and hydrogen were used to corroborate the groundwater flow direction indicated by hydraulic head measurements.   Samples for d²H and d¹⁸O were collected from sixteen piezometers nested at five sites around the lake at depths between 3.9 m and 41.7 m.  Hydraulic head data suggest that Ada Hayden Lake is a classic flow-through lake system with respect to groundwater, which enters the lake on the southwest, west and northwest sides and leaves on the east and southeast sides, eventually discharging into the South Skunk River.  These relationships are corroborated by the stable isotope data.  Groundwater entering the lake from the southwest and northwest shows a typical groundwater signature, with d²H and d¹⁸O values ranging from -36.70‰ to -54.34‰ and -6.56‰ to -8.36‰, respectively, that plot along a local meteoric water line of d²H = 8.12 d¹⁸O + 16.94.  Lake water undergoes evaporative enrichment; hence, d ²H and d¹⁸O values in groundwater on the outflow side of the lake follow a mixing line with a slope of 4.77, intersecting with present lake water values.  On the northwest side, groundwater near the water table, enriched in ¹⁸O and ²H, shows that constructed wetlands in that area provide groundwater recharge during wetter periods.   Finally, groundwater in the Mississippian limestone aquifer, 28 m to 36 m below the lake surface, is not enriched in ¹⁸O and ²H on either side of the lake, suggesting that it has little to no interaction with the lake or wetlands.  Together, the hydraulic head, stable isotope, and water quality data from this study provide a consistent conceptual model of groundwater interaction with Ada Hayden Lake.   Results from this study will play an integral role in further assessment of the lake for use as an emergency water supply source.