UNDERSTANDING THE PROCESS OF NATURAL GROUNDWATER QUALITY IMPROVEMENT AND AWARD-WINNING TASTE IN THE AMES AQUIFER

Groundwater in the Ames aquifer, a large alluvial (unconfined)/buried valley (confined) aquifer in central Iowa, is characterized by its excellent water quality and taste. Both are products of a low redox groundwater that produces CH₄ concentrations up to 2250 μmol/L and effectively removes nitrate, the primary contaminant in agricultural regions. We investigated two sites in the aquifer to understand the process of “natural water quality improvement” and award-winning taste. At Site 1, a seven piezometer nest was installed to a depth of 30 m into the unconfined alluvial aquifer and limestone below, all within 4 m of the South Skunk River – a losing reach at this location caused by pumping. Modern values of tritium occur throughout the aquifer. January and July 2009 nitrate-N concentrations of 10.5 and 11.3 mg/L in the river declined to concentrations between 3.5 and 6.0 mg/L in the adjacent groundwater to a depth of 13 m. Below this depth, nitrate-N was absent, H₂S was present, and CH₄ concentrations were 0.3 μmol/L. Constant Cl (~20 mg/L) and DOC (~2 mg/L) concentrations with depth in the aquifer, and their similarity to concentrations in the river, suggest that denitrification is occurring in the aquifer and is supported by DOC from the river. At Site 2 about 600 m south, a five piezometer nest was installed through the aquitard and into the buried valley aquifer below it at 28 m – the approximate elevation of the 13 m well at Site 1. The aquitard is composed of late Wisconsinan till, sand and gravel, and Peoria loess. Vertical hydraulic gradients are directed downward and tritium age below 5 m is pre-bomb in the aquitard, but is recent in the aquifer below. Concentrations of CH₄ and DOC increase with depth to 140 μmol/L and 14 mg/L, respectively, in the loess directly above the aquifer, suggesting that this C source could support redox reactions including methanogenesis at the aquifer-aquitard interface. We speculate at this point that continuation of the aquifer’s excellent water quality and award-winning taste depend on continued supply of C from the loess and South Skunk River, as well as the maintenance of the current microbial ecology of the aquifer.