Educational Webtool Illustrating Groundwater Age Effects on Contaminant Trends in Wells

Trends in concentrations of nonpoint-source contaminants in wells, springs, and streams are related to the history of contamination in groundwater recharge and the age distribution in the groundwater discharge. The age distribution in discharge depends on the groundwater age distribution in the aquifer and the subset of flowpaths that are sampled by the discharge. Groundwater travel times from recharge to discharge are variable; consequently, responses at discharge locations to changing contaminant loading in recharge can include delayed initial responses, dilution of peak concentrations, and prolonged flushing times. These effects are well understood in principle and have important consequences for water resource management (Eberts et al. 2013), but their implications may not be easy to visualize or communicate. Here we introduce GAMACTT: Groundwater Age Mixtures and Contaminant Trends Tool (Version 1)—an interactive webtool that can be used to explore the effects of basic aquifer properties (saturated thickness, porosity, and recharge rate) and well configurations (tops and bottoms of screened intervals) on groundwater age mixtures in groundwater discharge and on contaminant trends from varying nonpoint-source contaminant input scenarios. The webtool is based on the concept of groundwater stratigraphy whereby changes in contaminant concentrations in recharge may be recorded as a vertical concentration gradient in the aquifer that is related to groundwater age